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Zurich - Basel
Plant Science Center
PLANT RESPONSE TO STRESS
Zurich-Basel Plant Science Center
Authors: Nikolaus Amrhein, Klaus Apel, Sacha Baginsky,
Nina Buchmann, Markus Geisler, Felix Keller, Christian Körner,
Enrico Martinoia, Lutz Merbold, Christine Müller, Melanie Paschke, Bernhard Schmid
PLANT RESPONSE TO STRESS
PLANT RESPONSE TO STRESS
Zurich-Basel Plant Sciences Center
Authors: Nikolaus Amrhein, Klaus Apel, Sacha Baginsky, Nina Buchmann,
Markus Geisler, Felix Keller, Christian Körner, Enrico Martinoia, Lutz Merbold,
Christine Müller, Melanie Paschke, Bernhard Schmid
2012
Zurich-Basel Plant Science Center PSC
Universitätstrasse 2
8092 Zurich, Switzerland
Tel +41 44 632 60 22
Email [email protected]
Web http://www.plantsciences.ch
DOI: //dx.doi.org/10.3929/ethz-a-009779047
Terms of Use:
This publication is published under a Creative Commons License of “Attribution,
Non-Commercial - Share Alike”: http://creativecommons.org/licenses/by-ncsa/2.5/ch/.
Others can download, redistribute and alter this work following the terms of
license as long as there is attribution of the authors and a link back to the ZurichBasel Plant Science Center:
http://www.plantsciences.ch/education/e_learning/press_online.
Table of Contents HOW TO USE THIS BOOK ...................................................................................................................................... 8 THE STRESS CONCEPT IN BIOLOGY (C. KÖRNER) ....................................................................................... 10 TOPIC 1: DEFINITION OF STRESS ............................................................................................................................................. 10 TOPIC 2: DISTINCTION BETWEEN STRESS AND DISTURBANCE .......................................................................................... 12 TOPIC 3: LIMITATION AND STRESS ......................................................................................................................................... 12 TOPIC 4: PLANT RESPONSE TO STRESS .................................................................................................................................. 14 TOPIC 5: THE PHYSIOLOGICAL AND THE ECOLOGICAL OPTIMUM ...................................................................................... 17 TOPIC 6: HOW PLANTS COPE WITH STRESS .......................................................................................................................... 17 TOPIC 7: THE ECOLOGICAL DIMENSIONS OF STRESS ........................................................................................................... 19 TOPIC 8: THE SCIENCE OF STRESS BIOLOGY ......................................................................................................................... 21 SUMMARY .................................................................................................................................................................................... 22 STRESS RESPONSES AT THE CELLULAR LEVEL: THE EXAMPLE OF OXIDATIVE STRESS (K. APEL)
..................................................................................................................................................................................... 24 TOPIC 1: THE DEFINITION OF OXIDATIVE STRESS ............................................................................................................... 24 TOPIC 2: PLANTS CAN QUENCH EXCESSIVE LIGHT THROUGH SEVERAL PHYSIOLOGICAL REGULATIONS TOPIC 3: TYPES OF ROS ........................................................................................................................................................... 32 TOPIC 4: ROS UNDER STRESS AND ROS SCAVENGERS ........................................................................................................ 36 TOPIC 5: THE BIOLOGICAL ROLE OF ROS: ROS AS MESSENGERS IN SIGNAL TRANSDUCTION ..................................... 39 TOPIC 6: THE H2O2/SINGLET OXYGEN SIGNALING NETWORK AFTER EXCESSIVE LIGHT ............................................. 41 SUMMARY .................................................................................................................................................................................... 44 STRESS RESPONSES AT THE CELLULAR AND MOLECULAR LEVEL: GENE REGULATION AND GENE EXPRESSION AFTER DROUGHT AND TEMPERATURE STRESS (S. BAGINSKY, M. GEISLER, M. PASCHKE) ................................................................................................................................................................. 46 TOPIC 1: STRESS RESPONSE AT THE CELLULAR AND MOLECULAR LEVEL: ADJUSTING PROTEIN LEVELS TO PREVAILING CONDITIONS – REGULATORY LEVELS OF GENE EXPRESSION ....................................................................... 46 TOPIC 1A: TRANSCRIPTIONAL CONTROL ............................................................................................................................... 47 TOPIC 1B: TRANSLATIONAL CONTROL (=POST-­‐TRANSCRIPTIONAL CONTROL) ............................................................. 53 TOPIC 1C: POST-­‐TRANSLATIONAL CONTROL ......................................................................................................................... 57 TOPIC 2: THE ROLE OF PROTEINS IN STRESS RESPONSE .................................................................................................... 58 TOPIC 3: THE MOLECULAR RESPONSES OF THE PLANT CELL: PHYSIOLOGICAL REGULATION AND ACCLIMATIONS? 65 TOPIC 4: FROM RNA TO PROTEINS TO METABOLITES: THE CENTRAL ROLE OF PROTEOMICS IN SYSTEMS BIOLOGY
....................................................................................................................................................................................................... 66 TOPIC 5: PROTEOMICS CONCEPTS ........................................................................................................................................... 67 SUMMARY .................................................................................................................................................................................... 69 RESPONSES TO DROUGHT STRESS FROM THE CELLULAR TO THE WHOLE-­‐PLANT LEVEL (N. AMRHEIN, F. KELLER, E. MARTINOIA) ............................................................................................................ 72 INTRODUCTION ........................................................................................................................................................................... 72 5
TOPIC 1: PRINCIPLES OF TRANSPIRATION ............................................................................................................................. 73 TOPIC 2: THE CELLULAR LEVEL – SIGNAL PERCEPTION OF THE DROUGHT SIGNAL, ENDOGENOUS FORMATION OF ABSCISIC ACID, AND ABA-­‐MEDIATED EXPRESSION OF DROUGHT-­‐RESPONSIVE GENES ............................................... 74 TOPIC 3: COMPATIBLE SOLUTES – OSMOTIC ADJUSTMENT OF PLANT CELLS ................................................................. 76 TOPIC 4: PHYSIOLOGICAL REGULATION OF STOMATAL OPENING AND CLOSURE UNDER NORMAL CONDITIONS ...... 80 TOPIC 5: REGULATION OF STOMATAL OPENING AND CLOSURE UNDER DROUGHT CONDITIONS ................................. 84 TOPIC 6: MODIFICATIONS AND EVOLUTIONARY ADAPTATIONS IN RESPONSE TO DROUGHT STRESS AT THE WHOLE-­‐
PLANT LEVEL .............................................................................................................................................................................. 87 TOPIC 7: CAM PLANTS – EVOLUTIONARY ADAPTATION OF PHOTOSYNTHESIS TO HOT AND ARID CLIMATIC CONDITIONS ................................................................................................................................................................................ 88 TOPIC 8: FROM PHYSIOLOGICAL REGULATIONS AND ACCLIMATIONS TO MODIFICATIONS ........................................... 91 TOPIC 9: SALINITY CAUSES WATER DEFICITS IN PLANTS .................................................................................................. 92 SUMMARY .................................................................................................................................................................................... 94 STRESS AT THE POPULATION LEVEL: RESPONSES TO DROUGHT AND ‘DENSITY STRESS’ (M. PASCHKE, B. SCHMID) .......................................................................................................................................... 97 TOPIC 1: HOW CAN AN ENVIRONMENT BECOME STRESSFUL FOR PLANTS IN A POPULATION? ................................... 97 TOPIC 2: ‘DENSITY STRESS’ IN POPULATIONS ....................................................................................................................... 98 TOPIC 3: HOW DO PLANT POPULATIONS RESPOND TO ‘DENSITY STRESS’? .................................................................... 99 TOPIC 4: VARIATION BETWEEN INDIVIDUALS AS THE BASIS FOR POPULATION STRESS RESPONSES ........................ 100 TOPIC 5: PHENOTYPIC VARIATION AND GENOTYPIC VARIATION .................................................................................... 102 TOPIC 6: VARIATION IS A POPULATION PHENOMENON ..................................................................................................... 107 SUMMARY .................................................................................................................................................................................. 108 PLANT STRESS AT THE COMMUNITY LEVEL: RESPONSES TO BIOTIC FACTORS CAUSING PLANT STRESS (C. MÜLLER, M. PASCHKE) ................................................................................................................ 110 TOPIC 1: STRESS AND THE COMMUNITY BACKGROUND .................................................................................................... 110 TOPIC 2: PLANTS REACT TO BIOTIC FACTORS CAUSING PLANT STRESS -­‐ THE RESPONSE OF THE PLANT INDIVIDUAL
..................................................................................................................................................................................................... 113 TOPIC 3: IN A COMMUNITY CONTEXT, ABIOTIC STRESS CAN HAVE SECONDARY EFFECTS ON BIOTIC INTERACTIONS
..................................................................................................................................................................................................... 118 TOPIC 4: NEIGHBORS CAN HELP TO MITIGATE STRESS .................................................................................................... 121 TOPIC 5: INDIRECT INTERACTIONS ....................................................................................................................................... 121 TOPIC 6: PLANTS LIVE EMBEDDED IN FOOD WEBS ........................................................................................................... 125 TOPIC 7: PLANTS CAN ‘CALL’ FOR HELP .............................................................................................................................. 127 TOPIC 8: INVASIONS: PLANTS IN FOREIGN TERRAIN ......................................................................................................... 128 SUMMARY .................................................................................................................................................................................. 133 PLANT STRESS IMPLICATIONS AT THE ECOSYSTEM LEVEL (N. BUCHMANN, L. MERBOLD, M. PASCHKE) .............................................................................................................................................................. 136 TOPIC 1: WHAT ARE ECOSYSTEMS? ..................................................................................................................................... 136 TOPIC 2: STRESS FACTORS AT THE ECOSYSTEM LEVEL ..................................................................................................... 139 TOPIC 3: THE WATER AND ENERGY BUDGET OF TERRESTRIAL ECOSYSTEMS .............................................................. 143 6
TOPIC 4: THE RESPONSE OF EUROPEAN ECOSYSTEMS WATER BUDGET TO THE DROUGHT IN 2003 ...................... 145 TOPIC 5: HOW WILL ECOSYSTEMS AND THEIR SERVICES CHANGE DUE TO CLIMATE CHANGE? ................................. 148 INDEX ..................................................................................................................................................................... 153 7
How to Use This Book
This textbook is part of the online course ‘Plant Response to Stress’ (PRESS) offered by
the Zurich-Basel Plant Science Center (PSC). PRESS provides you with current
knowledge and research about plant responses to stress. It can be used independently
of the course. In connection with the online material, however, it is particularly beneficial.
The book contains all detailed texts and the most important figures of the course. Online,
the following additional elements are available:
•
Summary screens containing the key concepts of each topic.
•
Rich and interactive multimedia content: educational films, animations, exercises
and figures.
•
Self-assessments: Monitor your learning progress!
Access to Online Platform
https://www.olat.uzh.ch/olat/url/RepositoryEntry/6160392?guest=true&lang=en
How to Use This Book
•
Use the book and the online learning material simultaneously.
•
Use the syllabus provided to work through the course in a reasonable amount of
time. Of course, your can also work at your own pace.
•
Become familiar with the summary screens and the key concepts.
•
Look at the topics in more detail using the textbook.
•
Practice what you have learned: Interactive exercises are available online for
each topic.
•
Animations, figures, and educational films illustrate the learning content.
•
At the end of each lesson, you can test your learning progress using selfassessments.
Interdisciplinarity in the Field of Plant Sciences – the Contents of the Online
Course ‘Plant Response to Stress’
Plant Response to Stress (PRESS) is an online course offered by the Zurich-Basel Plant
Science Center as part of different Master’s studies. It allows students to acquaint
themselves with the basics of research in plant sciences and to prepare for further
studies in this field.
8
How to Use This Book
Why a Course on ‘Plant Response to Stress’?
Plants are sensitive organisms that cannot run away from unfavorable conditions. Given
this fact, it is all the more fascinating to look at the vast variety of responses with which
plants can react to an ever-changing environment. Over the past decade, our
understanding of plant responses to environmental changes has grown considerably. Not
every deviation of the environment from normal conditions is stressful for plants. This
awareness has important impacts on the whole field of plant sciences. However, there
are many different factors that can become stressful for plants: water scarcity, salt,
temperature, light, pathogens, herbivores, and density. In PRESS, these factors are
discussed within the scope of different research areas of plant sciences. The course
faces the challenging task to integrate the approaches of molecular biology, plant
physiology, and ecology outlining the basics and most recent findings on the subject.
Contents of the Course
The course consists of nine lessons for self-study and contains several multimediabased elements, such as video sequences, animations, interactive exercises, and an
innovative simulation tool with which students can design field experiments virtually. The
lessons focus on the reactions of plants at the molecular/cellular, the whole-plant, the
plant population, the plant community, and the ecosystem levels, as well as on shortterm and long-term responses.
Investigation in plant response to stress requires different research skills: To explain
techniques used to carry out experiments in plant sciences, the course provides some
examples of frequently used methods in a concrete context. Four different video
sequences show authentic examples of techniques used in plant stress research. These
movies were produced by the PSC as part of the course. They are supplemented by
animations and exercises in which the laboratory techniques are repeated and outlined in
more detail so that the students understand and remember the principles behind the
methods. In the Virtual Experiment Platform (VEP), designs of field experiments can be
built up. It is explained how appropriate hypotheses can be formulated and tested.
Furthermore, different statistical measures can be tried out with an innovative and highly
interactive simulation tool.
9
Lesson 1
The Stress Concept in Biology
The Stress Concept in Biology (Christian Körner)
Concept Map
Don’t Miss these Online-Learning Activities!
•
Exercise 1: Stressed or not Stressed? (Topic 1)
•
Exercise 2: Stress or Disturbance? (Topic 2)
•
Exercise 3: What Type of Response Are You Looking at? (Topic 4)
•
Exercise 4: The Example of UV-B (Topic 4)
•
Exercise 5: How Do Plants Cope with Stress? (Topic 6)
Topic 1: Definition of Stress
The term 'stress' originates from experimental physics, where an object is under stress,
when an external (usually mechanical) force, the stressor, is impacting it. Once stress is
perceived by the object, its body will be under a certain strain. If the stressor's impact,
and thus the strain it exerts, exceed the resistance of the body, the object will undergo
lasting deformation or break.
This stress concept had later been adopted by medical sciences and by psychologists,
from where it made its way to biology. Common stress situations in biology are drought
or anoxia resulting from water logging, excessive low or high temperatures, excessive
solar radiation loads, severe shortage of nutrients, high concentrations of certain
chemicals (salts, heavy metals), infections by pathogens, mechanical stress (bending,
tearing) etc.
10
Lesson 1
The Stress Concept in Biology
There is no common definition of what a stress is and what not. Hence, there is a need
for a convention. Here, two extreme views to start with:
An All-Inclusive Concept of Stress
If one would define stress as 'any deviation from optimal growth conditions' (maximum
growth rate) as is done in some textbooks, 'stress' would become synonymous to 'life',
because life conditions are almost never optimal for growth, i.e. permitting maximum
growth rate (a plant growing at its full capacity). Defined in this sense, stress does not
permit differentiating between 'normal' and 'stressed' conditions. With such a definition
any normal physiological regulation would become a stress response. See also the later
discussion of 'optimality'.
A Narrow Concept of Stress
Opposed to an all-inclusive stress concept, the other extreme definition would restrict
'stress' to conditions which exceed resistance and cause irreversible damage to tissues
or inhibit growth. With this definition, important plant responses which may occur over a
broad range of not-yet-damaging environmental conditions would be excluded from
'stress', neglecting their role in stress response.
A Practical Stress Convention
Obviously, neither an all-inclusive nor a narrow stress concept is helpful. Since the
transition from 'optimality' and process regulation in every day life to system failure and
damage is gradual, it is a matter of CONVENTION when „normal life“ is considered to
turn into „stressful life“. Any sharp boundary in this transient would be artificial. In other
words, the issue of whether certain conditions are stressful or not is a matter of casespecific severity and, to a certain degree, reflects subjective rating (as is the case in
psychology) and is not a scientific issue but more related to common sense. This
means, it is not worth a major scientific debate, but still worth some terminological
agreement in order to facilitate communication. 'Stress' should better not be used for
situations in which physiological regulation takes place in response to the “normal”
variations of life conditions, which do not exert any long term impact on whole plant
performance (see Topic 3).
Gradual versus Abrupt Stress
It makes a difference whether increasing stress exerts gradually increasing impact, or
whether stress comes as an abrupt event. Hence, one needs to distinguish
(1) gradual impacts of stress (e.g. increasing drought or salinity), or
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Lesson 1
The Stress Concept in Biology
(2) threshold impacts of stress (e.g. low temperature exceeding tolerance)
For (2), any condition which is not near to the “killing point” may not necessarily be
stressful (e.g. a cool night in the case of low temperature stress), but once the critical
threshold is surpassed, the stress consequence becomes disastrous. Prehistory (preconditioning) determines both, the thresholds and the gradual responses.
Topic 2: Distinction between Stress and Disturbance
Although they are often not different in the net outcome for an organism (e.g. death or
significantly reduced growth), it has become another established convention to make a
distinction between stress and disturbance. Stress is normally considered an impact that
affects the functioning of an intact organism. Disturbance is considered an impact that
physically (mostly mechanically) removes parts of an organism or destroys it.
There are some impacts which can be both a stress and a disturbance. For instance,
pathogens may first stress plants. Once their impact reaches an “outbreak” dimension
and induces a loss of biomass or destroys a complete population, they become a
disturbance.
Classical examples for disturbance are grazing (both the removal of forage as well as
trampling effects) or browsing by animals, fire or floods, or the damaging impact of strong
wind on forests. Typical ground disturbances are also associated with the activity of
burrowing animals.
Topic 3: Limitation and Stress
Limitation does not necessarily mean stress. Limitation is the wider concept; stress is a
specific part of it. Deviations from optimal conditions (e.g. for growth and photosynthesis)
which induce responses within the 'normal' physiotype are not considered stress but still
exert limitations to metabolism and growth. Hence, limitation and stress are not to be
treated as synonyms. Three examples:
1) Sunlight: The globe’s primary production by plants is in large achieved under rapidly
fluctuating light regimes in the leaf canopy, when fractions of a leaf may be exposed
to full sunlight in one minute and to full shade in the next minute. Light may be in
excess of demand in the first minute, and its shortage may strongly reduce
photosynthesis in the second minute. Neither of the two states would be related as
stress in a natural setting.
2) Nutrients: Nutrient availability determines plant growth.
a) Over a wide range of availability, plants maintain a constant tissue nutrient
concentration and fairly constant element ratios by adjusting their growth rate to
the availability of nutrients.
12
Lesson 1
The Stress Concept in Biology
b) Only at particularly low nutrient availability, plants may build up deficiency
symptoms (hence experience stress, which cannot be mitigated by adjusting
growth rate).
c) At extremely high nutrient supply, plants may accumulate excess anorganic
nutrients and often become susceptible to diseases (a new type of stress).
Type a) variation in growth would not be considered stress, b) and c) would be
stress.
3) Water: Plants control transpiration in a way that xylem cavitation is avoided and that
the ratio between CO2 uptake and water loss remains high. Over a wide range of
transpirational flux rates, water potential follows flux, just like pipe pressure follows
consumption rate in a public water system. Variations of water potential in this range
have nothing to do with stress. On the other hand, water shortage that affects turgor
in a way that growth becomes significantly reduced, or water overabundance that
induces water logging and anoxia are considered stress. Obviously, there is a grey
zone, related to the word 'significantly'.
Remember:
Not every deviation from maximum growth rate is stress. In fact, outside greenhouses,
most plants are healthiest and more robust, e.g. against wind, when growing at slightly
suboptimal rates.
Stress Syndromes
Combined Stresses
Stresses rarely come alone. One stress is often combined with a typical set of other
stresses, so these stresses do not act individually in nature. Stresses may be additive or
mitigating each other. Examples are:
1) Water shortage normally limits nutrient access and is often associated with strong light.
In fact, stress by high solar radiation often materializes only as a consequence of water
shortage, for instance, when closed stomata limit photosynthetic use of energy. What is
commonly rated as water stress in long-lived plants (in trees in particular) may in fact
predominantly represent nutrient starvation.
2) Low temperature stress is diminished when plants have experienced drought stress
before exposure to low temperature.
13
Lesson 1
The Stress Concept in Biology
3) Stress by excessive solar radiation diminishes when sufficient nutrients are available and
when water is abundant. For instance, in the wild, the 'shade adapted' cacao tree can not
tolerate full tropical sunlight, but when high fertilizer levels are applied in plantations, it
does well without shelter trees.
In 1), stresses are additive; in 2), one stress is mitigating another one; in 3), factors
which themselves do not exert any stress are decisive for a potential stress to become
stressful or not.
The study of stress, thus, needs to account for the common co-occurrence of stresses
and other environmental conditions.
The two Sides of the Coin: When Organismal Interactions Come into Play
What may be a stress to one individual may be a relief from stress to its neighbor. For
instance, thinning of stands due to drought stress may secure survival for the remaining
individuals. Death of one type of plant may open a new habitat for another, more tolerant
to this sort of stress. Infection stress by a pathogen of one plant may both open access
to more soil resources for others and induce their defense. In the long run, stress
induced replacements of species lead to assemblages of stress tolerant species. The
complete relief from stress would reverse the process and eliminate the stress tolerators
by competitive exclusion. In other words, 'stressful' conditions open habitats to
specialists, who would become stressed, i.e. eliminated, if the 'stressful' conditions
ceased. At community level, stressful conditions induce a syndrome of responses which
combine physiological responses at the level of the individual with interspecific effects
(plant-plant interaction).
This points at a fundamental limitation of the stress concept at higher levels of complexity
(the community, the ecosystem) that will be discussed in the next section (see also
Lesson 7).
Topic 4: Plant Response to Stress
A plant exposed to stressful conditions undergoes a number of characteristic responses.
1) Immediately exposed to a recently unexperienced environmental situation, a plant is
using its normal regulatory tools to mitigate the effect and prevent cells from departing
from optimal functioning. Examples for such effects are the dissipation of excess
photochemical energy by fluorescence, stomatal closure in response to dry air or
beginning soil moisture shortage, and release of base equivalents in case of sudden
exposure to acidic air pollution. Should these measures fail to control cellular conditions
in a homeostatic way, the situation becomes stressful and strain is building up.
14
Lesson 1
The Stress Concept in Biology
2) Exposed to a lasting strain, the plant adjusts its metabolism at the cost of reduced
growth: The osmotic pressure may be increased by hydrolyzing starch, the ABA level is
increased, genes coding for Rubisco are deactivated (downregulation of photosynthetic
capacity), membrane lipids are adjusted to cope with the needed fluidity at lower
temperatures, defense compounds are released to deter pathogens etc. These
adjustments are termed acclimation.
3) Should acclimation of the intact plant fail to balance the strain on its tissues, there are
three possibilities: The plant loses part or all of its sensitive tissues or organs but may
still survive (e.g. frozen leaves or flowers, induced cell death to block the spreading of a
pathogen), the plant actively sheds sensitive organs (drought induced leaf abscission,
abortion of ovaries) and/or induces compensatory growth (e.g. of roots), or, time
permitting, produces new types of organs which better suit the new demands (e.g. sun
leaves in place of shade leaves with a thicker mesophyll and stronger epidermis).
Commonly, such (largely morphological/anatomical) adjustments are not reversible
within a season and are termed modification.
4) Should these environmental conditions affect the reproductive fitness of a species
(production of diaspores, recruitment of offspring), selection will start to operate. At the
population level, more successful genotypes will be selected for. Initially, epigenetic
'memory' may contribute to offspring success. At the community level, taxa which had
already undergone selection for the given stress conditions or which carry traits that turn
out to be advantageous will be selected for. These evolutionary responses to stress are
called genetic adaptation.
Adaptive responses of plants to environmental demands thus fall into four categories:
1) short term physiological regulation
2) acclimation (reversible adjustment of cellular states and processes)
3) modification (largely irreversible adjustments of structures)
4) genetic adaptation (genotypic adjustments)
All four adaptive responses have a genetic background (the ability to respond). The time
scales involved are hours in 1), few days to weeks in 2), several months in 3), and many
years in 4). Commonly, physiological regulation which prevents significant deviations
from 'normal' operative states of cells (even at the price of periodic reduction of
metabolism and growth) is not considered a stress response, all other responses are.
It is important to distinguish these different adaptive responses. A widely accepted
convention is to restrict the use of the term adaptation to genetic adjustments. In order to
15
Lesson 1
The Stress Concept in Biology
avoid confusion, it is, however, well advised to add 'genetic' or 'evolutionary'. Acclimation
is not restricted to adjustments related to climatic demands (from where the term
originates) but applies to any reversible adjustment of the phenotype.
Remember:
1) Genetic adaptation does not imply that the genotype had been selected for in response
to a given environmental demand. It just means that a certain genetic trait is
advantageous under the given conditions. So there is no causality implied.
2) The better all these adaptive responses mitigate stress, the less a given stress impacts
a plant. In the ideal case, these responses remove the impact of stress. Once such a
status is achieved by acclimation, modification, or genetic adaptation, a given
environmental condition does not exert stress to that „adapted“ organism. So beware
of anthropocentric interpretations of what are stressful conditions. There is no absolute
stress scale! For instance, cold climates are not stressful for cold-adapted organisms.
3) Short-term excursions of environmental conditions from optimal conditions are the
'training ground' for adaptive adjustments. Hence, there is 'conditioning stress' (so
called 'eustress'), which is positive and contributes to fitness, and there is 'destructive
stress' (so-called 'disstress'), which reduces fitness (see the definition in the next
section). Of course, there is a grey zone in between. This dichotomy is similar to the
one known from human life.
An Example of Adaptation: The Ecotype Concept and Evolutionary Fitness
Genotypes (including crop cultivars) that carry traits of obvious advantage for certain
environmental conditions are called ecotypes and the advantageous traits are called
ecotypic. Ecotypes are thus, genotypes that exhibit particular fitness in a given
environment and, thus, experience less stress than other genotypes.
Fitness in biology should not be confused with fitness in human health. Fitness refers to
the production of successful offspring, hence is a reproductive, evolutionary relevant
character. Note that fitness does not refer to the mere production of diaspores but to the
production of reproductively successful offspring, thus ensuring the perpetuated
presence of specific genes in a given area over time. The individual's vigor (growth rate)
is not necessarily correlated with fitness. Tall individuals (or tall cultivars among crops)
are often at greater risk to suffer from stress or disturbance.
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Lesson 1
The Stress Concept in Biology
In the long run, fitness results from ecotypically adaptive traits. In an ecological
context, one can say that 'stressed are only the non-fit', although fitness does not
preclude short-term stress-induced growth restrictions. Hence, one cannot reverse this
argument and state that the 'fit ones' are never stressed. However, one can state that
such a stress does not have negative evolutionary consequences. As explained under
„Stress Syndromes“, the removal of regular stress may even diminish evolutionary
fitness and eliminate an ecotypically well adjusted species from a given habitat, because
it becomes overgrown by others.
Topic 5: The Physiological and the Ecological Optimum
It is central to the stress concept to define conditions which are non-stressful for plants.
We have touched upon this issue already before. What is optimal for a plant? Here, we
enter a fundamental dichotomy among biologists, those whose research is (in Liebig's
tradition) growth (yield) oriented, and those whose research is biodiversity oriented (in
Darwin’s tradition). As will be shown, the two approaches – one may call them the
agronomic versus the ecological approach – are mutually exclusive.
It is old wisdom that plant species distribution and the abundance of individuals of a
species are unreliable indicators for conditions which permit optimal, i.e. maximal growth.
A classical example is the distribution pattern of Pinus sylvestris (Scots pine), a species
found in wet and acidic soils, including mires, and in dry calcareous or nutrient poor,
sandy soils, suggesting a bimodal environmental preference. Such habitats are also
more likely passing through stressful conditions. In reality, scots pine grows best in
neutral, nutrient rich soils, but is outcompeted from such habitats, by other, more
vigorous species. In other words, the species reaches maximum abundance, i.e. its
ecological optimum, under conditions which are marginal to its own growth, but it copes
somewhat better with the limitation and stresses incurred than other species.
A certain degree of limitation and stress can thus be vital to the distribution and success
of a species. The physiological optimum of growth should not be confused with optimal
life conditions in the real world. It commonly requires certain protective measures for a
plant to survive if it is grown under conditions which permit maximum growth. Such
plants are usually very sensitive to climatic perturbations and pathogens, which means
that they are more susceptible to stress.
Topic 6: How Plants Cope with Stress
Because there are so many different stress factors, there is no common scheme plants
may adopt to cope with stress. However, the concept developed by J. Levitt and others
for drought and low temperature stress is a useful theoretical framework that covers the
major techniques by which plants may meet stressful conditions. Note that the different
17
Lesson 1
The Stress Concept in Biology
stress responses discussed in the previous lecture unit may operate at any of the
categories considered here, but in most cases through genetic adaptation.
Plants may cope with stress in 3 ways:
1) By escaping stressful situations, e.g. by shedding leaves in winter or during drought or
by passing through cold or dry periods as seed or bulb.
2) By resisting stressful conditions. This may principally be achieved in two ways:
a) By avoiding the impact of the stressor by adjustments such as freezing point
depression or reduction of water loss, or
b) By tolerating the associated impacts such as intercellular ice formation or
desiccation.
The mechanisms of escape and those of resistance by avoidance or tolerance are
manifold. The following examples illustrate the spectrum of solutions for coping with
low sub-freezing temperatures in taxa native to regions with regular occurrence of
such temperatures.
Escaping:
•
the life strategy of annuals (seed bank)
•
survival by bulbs, rhizomes, tubers (spring geophytes)
•
possession of subsurface meristems (many graminoids)
•
selection of snowbed habitats (insulating snow cover)
•
night time closure of rosettes (tropical giant rosettes)
•
seasonal shedding of leaves (deciduous trees and shrubs)
•
dense and compact growth, trapping heat (cushion plants, dense grass turf)
Resistance by Avoidance:
•
Freezing point depression by solutes (though not very effective, because it needs 1 mole
for a 1.8 K reduction of freezing point, corresponding to an additional osmotic pressure of
22.4 bar)
•
Supercooling: maintaining water in an unfrozen gel-type state by preventing nucleation
down to about -12 °C in leaves of some tropical high altitude plants and down to -40°C in
xylem (a fatal strategy, in cases where temperatures fall below the critical limit and cause
immediate freezing)
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Lesson 1
The Stress Concept in Biology
Resistance by Tolerance:
The predominant mechanism in freezing resistant plants (which tolerate ice formation in
tissues): Ice formation starts with little delay (possibly facilitated by nucleation peptides
of endophytic bacteria) in the apoplast, where solute concentration is at the minimum.
The growing ice body draws water from cells which gradually desiccate. This process
requires membranes to be functionally intact at very low temperatures (critical role of
lipids and specific proteins). For the protoplast to survive dehydration, membranes need
to be stabilized by protective compounds (sugars, proteins). Upon thawing, tissues first
infiltrate (get dark) and then resorb the water from the intercellular space. No plant is
known to tolerate protoplasmatic ice formation, but shock freezing (amorphous ice) is
tolerated by some species when fully dormant (e.g., Saxifraga oppositifolia, a nival plant).
Other examples are desiccation tolerance in plants including almost full desiccation in
terrestrial algae, mosses, lichens, some ferns, so-called resurrection plants (a few higher
plant species). Resistance to extremely high light intensities at otherwise restricted
activity of the photosynthetic machinery (e.g. in dormant evergreen leaves exposed to full
sun) includes protective pigments, alternative ways of dissipation of energy, and repair.
Topic 7: The Ecological Dimensions of Stress
Stress Is the Most Important Environmental 'Filter'
When plants occupy new terrain, they have to pass the filter of extreme
events/conditions to establish. The stress filter is the dominant of all environmental filters.
It is a centennial or millennial filter, which means very rare events become decisive for a
species to establish or not. Once passed, such extreme events may still impact growth
and reproduction, but not in such a way that the population is exterminated. In other
words, such taxa are genotypically fit to cope with the extremes involved. Those absent
from such regions commonly are not.
'Stress-Dominated Habitats'
A logic consequence of the above is that plants found in so-called stress-dominated
environments are the ones least affected by stress – a paradox that causes a lot of
confusion. Most people would intuitively address high mountain summits, hot deserts or
salt flats as stress dominated. The question is: Who is stressed? The observer or the
organisms which had been selected to live there? Most commonly the observer.
A good test to find out whether organisms in so-called stress dominated habitats suffer
from the conditions they are in, is to relieve them from the supposed stress: warming the
alpine, watering the desert, flushing the salt pan with fresh water... or just adding some
fertilizer to a presumed nutrient limited calcareous grassland. As is easy to imagine,
19
Lesson 1
The Stress Concept in Biology
these systems would be completely re-shaped by such treatment and most, if not all,
'stressed' species would die out sooner or later, so removal of ‘stress’ kills the
“stress-habitat” specialists.
This drastic example illustrates that communities of plants, i.e. the given assemblage of
taxa in a given habitat, can never be stressed although each individual may suffer from
regular stress exposure and stress induced growth limitation. Any relief from that stress
would, however, facilitate the invasion and dominance by other taxa, hence induce the
establishment of a new community at the loss of the old one (Körner 2003).
Can ecosystems be stressed? In terms of biomass production, yes. In terms of life
inventory, no. Simply, because the given inventory reflects the life conditions. Old
ecosystems may have arrived at a plant inventory which is so well suited to cope with the
given environmental demands that even natural productivity may become unaffected by
what would commonly be rated as stress. For instance, the mean productivity of late
successional vegetation in the humid parts of the globe does not differ between the
tundra, a temperate deciduous forest, and a humid tropical forest if expressed per unit of
time during the growing period – a rather surprising and impressive result of adaptation
(Körner 1998). Hence, per month of growing season, a humid tropical forest in a 12month season is similarly productive as a deciduous forest in Switzerland per month of a
6-month growing season (in both cases 200 g dry matter m-2 per month).
Stress and Global Change
Nowadays, global warming occurs in all ecosystems. A consequence is that:
•
cool habitats become warmer
•
some dry habitats become moist
•
some moist habitats become drier
•
nutrient limited ecosystems become polluted by soluble nitrogen fertilizer
•
the air becomes enriched with more CO2 (relieving C-shortage in places)
This affects the plant assemblage of an ecosystem through the removal or mitigation of a
selective stress, as discussed in the previous section.
Here at the latest, it becomes obvious that a stress concept based on biomass
production is unsuitable for explaining the fate of plant assemblages. A warm alpine
climate suddenly becomes a danger for plants which are adapted to the cold. A warmer
lowland climate may suddenly open the landscape for exotic taxa which had previously
been prevented by stress from spreading and escaping their garden 'captivity'. The
subtropical palm Trachicarpus sp. has become naturalized in the southern part of
20
Lesson 1
The Stress Concept in Biology
Switzerland and, relieved from stress (by less low winter temperatures), it now occupies
habitats of native species on the S-slopes above Lago Maggiore (Walther et al. 2001).
Remember:
1) Stress is the most important selective filter for plant habitat selection.
2) Stress dominated environments are least demanding for those selected for these
habitats. Less stress tolerant taxa have no access to such habitats. Stress relief opens
these habitats to other species and leads to competitive exclusion of the former
natives.
Stress mitigation by global change can be fatal for adapted native plants.
4)
Topic 8: The Science of Stress Biology
Stress is a concept that is intimately tied to the physiology of the individual. Stress, as
perceived by the individual, may reduce the yield of a large crop field, but it may also
increase a species' abundance in a matrix of less stress tolerants. These are the two
sides of the same phenomenon: Stress as a constraint and stress as a chance.
Biomass production is very sensitive to stress, particularly in taxa that are grown outside
their native habitat, such as many crops, but also in ornamental plants. In wild plants
whose presence and abundance had been governed by their ability to cope with
potentially stressful conditions, individual stress often is a pre-requisite for coexistence.
It is in the whole plant where stress phenomena become most obvious and effective.
Plant responses to stress (physiological as well as morphological ones), in essence, tend
to mitigate environmental impacts at cellular level. While a plant may undergo dramatic
changes in appearance and growth as a result of stress, changes within the operative
tissues may be far less dramatic. A remarkable variation in plant size may remain
almost unreflected at cell level.
One of the most dramatic documents of this are Bonsais. Forced by enormous stress,
potentially 40m tall trees are shaped into perhaps 80 year old individuals of only 60cm
size, with leaves only measuring 1-5% of normal leaves. If one explores the leaf
mesophyll or the stomata of such stress shaped dwarfs in a microscope, one will notice
with surprise that no change in size has occurred. The cell remains as a 'constant' in an
otherwise radically changed body. This is where developmental processes come into
21
Lesson 1
The Stress Concept in Biology
play. Stress changes plant development to much greater degree as it affects the
individual cell.
On the other hand, stress is the major driver of plant assembly in the wild, causing it to
become an obsolete concept at the resultant community level. Plant communities reflect
the impact of constraints and, thus, cannot be considered suffering from these very
constraints. However, the study of the individual's performance under those often
stressful, though selectively advantageous, growth conditions offers a gateway to the
understanding of community structure and dynamics. The outcome of these
individualistic responses to stress, in the long run, shapes ecosystems and their
function. Knowledge of such individualistic responses to the mitigation or enhancement
of stress by global change will help predicting vegetation change
Summary
1) What is considered a 'stress' is a convention. A differentiated terminology which excludes
conditions plants can manage by normal regulatory responses should be applied. Not
every growth limitation is a stress.
2) There are gradual and threshold stress phenomena, which should be distinguished.
The important point is that conditions above the threshold are not stressful, but they have
a signaling function.
3) Stress should be distinguished from disturbance, the latter referring to a loss in
biomass, commonly due to physical (most often mechanical) impact.
4) Stress often operates in a syndrome fashion with different stresses as well as
different individuals/taxa interacting.
5) It is key to separate stress effects on growth from stress effects on reproductive
fitness, because stress implications could go in opposite directions. What may be
defined a stress in terms of growth may in fact represent very favorable conditions in an
ecological context, when competition comes into play. While a growth oriented stress
concept applies better to isolated individuals and to agronomy, a fitness oriented stress
concept is better suited in an ecological context.
6) Plants can respond by physiological regulation (conditions not considered „stressful“),
by reversible physiological acclimation, structural (commonly irreversible) modification
of organs, or by genetic (evolutionary) adaptation to stress. The four categories should
not be confused. Combinations are common.
22
Lesson 1
The Stress Concept in Biology
7) Plants can cope with stress by escaping it, by avoiding its impact, or by tolerating it
(avoidance and tolerance are the two facets of resistance).
8) Stress has an ecological dimension; it secures habitats for resistant taxa. The stress
concept does not work at community level. Communities are never stressed; their
composition is the consequence of environmental demands. If these change, the
communities will change.
Literature Cited
KÖRNER C (1998). Alpine plants: stressed or adapted? In: Press MC, Scholes JD, Barker MG
(eds.) Physiological Plant Ecology, The 39th Symposium of the British Ecological Society held at
the University of York 7-9 September 1998. Blackwell Science Ldt., Oxford p. 297-311
KÖRNER C (2003). Alpine plant life, chapter 8. Springer, Berlin
KÖRNER C (2003). Limitation and stress – always or never? J Veg Sci 14:141-143
KÖRNER C (2006). Significance of temperature in plant life. In: Morrison JIL, Morecroft MD (eds)
Plant growth and climate change, Blackwell, Oxford, pp 48-69
KÖRNER C (2008). Limitierung, Fitness und Optimum (chapter 11.1). In: Bresinsky et al. (eds)
Strasburger, Lehrbuch der Botanik. Spektrum, Heidelberg
WALTHER GR, BURGA CA, EDWARDS PJ (2001). "Fingerprints" of Climate Change Adapted
Behaviour and Shifting Species Ranges. Kluwer Academic/Plenum Publishers, The Netherlands
23
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
Stress Responses at the Cellular Level: The Example of Oxidative Stress
(Klaus Apel)
Concept Map
Don’t Miss these Online-Learning Activities!
•
Exercise 1: Scavenger Game (Topic 1)
•
Exercise 2: Quenching (Topic 2)
•
Exercise 3: Violaxanthin (Topic 2)
•
Exercise 4: Quenching under different Environmental Conditions (Topic 2)
•
Exercise 5: Types of ROS (Topic 5)
•
Exercise 6: Remember the Scavengers! (Topic 4)
Topic 1: The Definition of Oxidative Stress
Plants have to cope with ever-changing light conditions. They have developed a number
of physiological regulations at the molecular level to protect their sensitive
photosynthesis apparatus against a short-term excess in light (see Topic 2). As a byproduct of these processes reactive oxygen species (ROS, highly reactive oxygencontaining free radicals, Topic 3) arise. Only a long-term excess in light causes
photosynthesis to stop (photoinhibition, Topic 2) and results in oxidative stress for the
plant.
Consequences of Oxidative Stress:
When the concentration in ROS exceeds the scavenger capacities (Topic 4), the ROS
damage different parts of the plant cell, such as lipids, proteins, and DNA. Several of
these damages are irreparable and cause cell death. Some can be repaired but require
much energy from the cell. Therefore, the cell tries to avoid oxidative stress from the
beginning, e.g. by quenching excessive light.
24
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
Topic 2: Plants Can Quench Excessive Light through Several Physiological
Regulations
Photosynthesis and Light Excess
Plants use solar energy to oxidize water (which results in the release of oxygen) and to
reduce carbon dioxide (which synthesizes carbohydrates). The light energy is absorbed
by chlorophyll and carotenoid molecules that form part of light-harvesting complexes
within the thylakoid membranes, resulting in singlet state excitation of the pigment
molecules. The excitation energy is transferred to the reaction centers of photosystems II
(PS II) and I (PS I) to drive the electron transfer.
The linear photosynthetic electron transport chain contains two redox components that
use light energy to drive the electron transport, photosystem II and photosystem I.
Electrons are fed into the electron transport chain during the oxidation of water (Figure
3a). Ideally, fixation of CO2, the intensity of light that is absorbed by the two
photosystems and the water-splitting reaction should reach an equilibrium. However,
such a balance between these various steps may be disturbed by a large variety of
environmental changes: When plants are exposed to light intensities that exceed the
capacity of CO2 assimilation, over-reduction of the electron transport chain may lead to
the inactivation of PS II and the inhibition of photosynthesis (photoinhibition). Within the
electron transport chain there are no more non-reduced acceptors left to accept
electrons. How can the plant cell avoid this worst-case scenario?
In a first attempt to prevent photooxidative damage, plants may use thermal dissipation
of excess excitation energy in the PS II antennae (non-photochemical quenching) and
the transfer of electrons to various acceptors within the chloroplast in PS I
(photochemical quenching), leading to the enhanced production of reactive oxygen
species (ROS), in this case hydrogen peroxide (H2O2) and superoxide (O2-) in
chloroplasts and peroxisomes. In case these light-scavenging devices are not sufficient
to prevent the hyper-reduction of the photosynthetic electron chain, inactivation of PS II
and photoinhibition of photosynthesis may occur and cause an enhanced generation
of singlet oxygen (another ROS).
You can find more details on the types of reactive oxygen species in Topic 3!
Non-Photochemical Quenching in Photosystem II
Absorption of excess photons can cause the accumulation of excitation energy within the
light-harvesting complexes and thereby increase the life-time of the excited state of
chlorophyll by converting 1Chl to triplet chlorophyll (3Chl) through the reversal of the spin
direction of the excited electron.
25
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
Normally, the excitation energy of 3Chl can be effectively quenched by energy transfer to
carotenoids that are closely associated with Chl within the light-harvesting structures.
The excitation energy of carotenoids is then converted to heat. In this way, formation of
3
Chl may help the plant to dissipate excess light energy (Figure 1a).
However, if energy transfer to carotenoids does not suffice, triplet chlorophyll is capable
of transferring excitation energy onto ground state triplet oxygen, giving rise to singlet
oxygen (Figure 1b). Photoinhibition results (see below).
Predominantly three carotenoids, called xanthophylls, have been implicated with nonphotochemical quenching of the excitation energy of 1Chl. In high light, violaxanthin is
converted into zeaxanthin via the intermediate antheraxanthin. When the light intensity
decreases, this process is reversed. Zeaxanthin acts as scavenger and helps to convert
excess light energy to heat. Violaxanthin acts as a light-harvesting pigment. Under low
light conditions, it improves the plant’s capacity to harvest light energy that will be
transferred to the chlorophyll in the reaction center and helps to support electron
transport under light-limiting conditions. There are mutants that are unable to convert
zeaxanthin to violaxanthin or vice versa (Figure 2).
Photochemical Quenching at Photosystem I
Photochemical and non-photochemical quenching occur simultaneously.
In plants exposed to higher light intensities and/or elevated temperatures and/or
under drought conditions, leaves loose water through respiration; they wilt and close
the stomata to avoid further water loss (see Lesson 4). However, with closed stomatas
the availability of CO2 within the leaf becomes restricted. Under these conditions, plants
may use two major types of photochemical quenching: The oxygenase reaction of
ribulose-1,5-bisphosphate
carboxylase-oxygenase
(Rubisco)
and
the
direct
reduction of molecular oxygen by photosystem I (PS I) electron transport (Mehler
reaction).
In
leaves
of
C3-plants,
the
photorespiratory
oxygenation
of
ribulose-1,5-
bisphosphate by Rubisco constitutes a major alternative sink for electrons, allowing to
maintain the partial oxidation of PS II acceptors and thereby countering the
photoinactivation of PS II when CO2 availability is restricted. Rubisco catalyzes a
competitive reaction in which oxygen is favored over CO2 as a substrate when
temperature increases or when the intracellular CO2 concentration declines. This reaction
leads to the oxidation of ribulose-1,5-bisphosphate and the release of glycolate that is
translocated from chloroplasts to peroxisomes. Its subsequent oxidation in peroxisomes
is catalyzed by the glycolate oxidase and leads to the formation of glyoxylate, but also to
26
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
the massive production of H2O2 that accounts for a major part of the H2O2
produced during photosynthesis (Figure 3b).
Plants may cope with these high concentrations of H2O2 by using primarily the
peroxisomal catalase as an ROS scavenger to catalyze the dismutation of H2O2 to water
and oxygen (Figure 8 in Topic 4).
Figure 1a & 1b: Energy Levels in the Chlorophyll Molecule
1a - During “normal” photosynthesis, absorption of blue or red light brings the chlorophyll into
an excited state, with blue light absorption resulting in a higher excited state because of the
greater energy of blue light relative to red light. Internal conversions of higher 2
nd
excited singlet
st
state to lower 1 excited singlet state results in the loss of energy in the form of heat.
In the first excited singlet state, the reaction center chlorophylls (and only these!) can transfer the
excited electron to an acceptor of the photosynthetic electron chain. This results in an oxidation of
the chlorophyll (in the diagram: ∆E). The chlorophyll can accept an electron from another
molecule. This is the end of a process which starts with the removal of an electron from water.
Additionally, during the return of the excited electron to the ground state, the energy may be
released as fluorescence or heat.
The energy may also be released by energy transfer to neighboring molecules (=pigments of the
light-harvesting complex, in the diagram: Transfer).
The spectra for fluorescence and absorption are shown at the right in the figure.
27
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
st
1b - When the return of the excited electron from the 1 excited singlet state to the ground state is
delayed due to overexcitation of the photosynthetic pigments or cannot be transferred from the
reaction center Chl to primary electron acceptors because of the hyperreduction of the
photosynthetic electron chain (in the diagram: red arrow for blocked electron transfer), the shortlived excited electrons of the singlet state with opposite spins will be transferred to the triplet
state with electrons that have parallel spins. At this stage, excessive energy in the form of heat
is drained – the triplet state is at a lower energy level. First, the excitation energy of this triplet
state is transferred to carotenoids; this process is called non-photochemical quenching. After
carotenoids are depleted, the excitation energy of this triplet state can easily be transferred to
3
1
ground state oxygen ( O2), giving rise to singlet oxygen ( O*2), while some exited carotenoids are
regenerated. If singlet oxygen concentration is higher than the scavenging capacity of carotenoids
photoinhibition will start.
Source: Biochemistry & Molecular Biology of Plants, CD-ROM & Online Images
Copyright: American Society of Plant Biologists
Mehler Reaction: The second major process that is involved in the formation of ROS
during photosynthesis is the direct reduction of O2 to the superoxide radical by reduced
electron transport components associated with PS I (Figure 3c).
Superoxide radicals generated by the one-electron reduction of molecular oxygen by PS
I are rapidly converted within the chloroplast to hydrogen peroxide by Cu,Znsuperoxide dismutase (Figure 8 in Topic 4). Whereas hydrogen peroxide generated
within the peroxisomes is detoxified predominantly by catalase, hydrogen peroxide
produced in chloroplasts is detoxified almost exclusively by ascorbate peroxidase (Figure
8 in Topic 4).
The rate of photoreduction of oxygen by PS I is several orders of magnitude lower than
that of the dismutation of superoxide catalyzed by superoxide dismutase and the
28
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
reduction of hydrogen peroxide to water catalyzed by ascorbate peroxidase (Figure 8).
This cycle, therefore, efficiently reduces the concentration of H2O2 and superoxide.
Figure 2: The xanthophyll cycle protects plants from high light intensities by converting
violaxanthin (that acts as a light-harvesting pigment and feeds light energy into the reaction
center) into zeaxanthin (that participates in thermal dissipation of excess absorbed light energy).
Enzymes interconvert these two carotenoids with antheraxanthin as the intermediate, in response
to changing light intensities.
What happens if photochemical and non-photochemical quenching is not
sufficient to quench the excess of light? Oxidative stress with photoinhibition
starts at photosystem II!
Under severe light-stress conditions, alternative electron sinks (described in nonphotochemical and photochemical quenching) may no longer suffice to avoid hyperreduction of the electron transport chain. Electrons can no longer be translocated from
PS II to the fully reduced acceptor site. Light energy absorbed by the reaction center Chl
of PS II gets trapped and, through transformation of the excited singlet to the excited
triplet state of Chl, leads to the enhanced production of singlet oxygen. As a result,
photosynthesis gets inhibited (Figure 3d).
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Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
Figure 3: Photochemical Quenching And Photoinhibition After Excess of Light.
3a
3b
3c
30
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
3d
The reactions at the PS II in greater detail: The reaction center complex of PS II consists
of cytochrome b559 and the heterodimer of the D1 and D2 proteins (Figure 4). The
heterodimer binds the reaction center chlorophyll P680, the pheophytin and the quinone
electron acceptors QA and QB (parts of the electron transport chain at PSII). Energy
absorption in the reaction center chlorophyll results in electron transfer from P680 to
pheophytin, QA, and QB. When these electron acceptors are over-reduced because of
excess light energy, formation of the excited triplet state of P680 is favored, leading to
the enhanced generation of singlet oxygen by energy transfer instead of electron
transfer.
Continuous 1O2 production seems to be an inherent property of PS II. (Even during
“normal” photosynthesis, it occurs easily that electron acceptors are temporarily
occupied.) The quenching of this singlet oxygen has been linked to the turnover of the
D1 protein, which is oxidized by singlet oxygen and serves as a scavenger of this ROS.
31
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
Excess amounts of singlet oxygen that cannot be quenched by the D1 protein seem to
initiate photoinhibition and trigger singlet oxygen-mediated stress responses and
damages (see Topic 6).
As mentioned earlier, also carotenoids of light-harvesting complexes effectively quench
triplet Chl and singlet oxygen. Under high light conditions however, excess light energy
within the PS II reaction center cannot be quenched by carotenoids but, instead,
stimulates singlet oxygen production.
Figure 4: Structural Model of the PS II Reaction Center. The structure includes the D1 and D2
core reaction center proteins, the CP43 and CP47 antenna proteins, cytochromes b559 and the
water-splitting complex. Source: Biochemistry & Molecular Biology of Plants, CD-ROM & Online
Images Copyright: American Society of Plant Biologists
Topic 3: Types of ROS
Generation of reactive oxygen species (ROS) in chloroplasts and peroxisomes is
intimately linked to photosynthesis. You have already learnt that, during the physiological
regulations of an excess of light, several ROS are generated.
32
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
The term “ROS” comprises several different activated oxygen species, which are
chemically distinct and may be generated in different compartments of the plant cell.
Firstly, you will learn how oxygen is transformed into several types of reactive oxygen
species.
Ground state triplet molecular oxygen is a biradical with its two outermost valence
electrons occupying separate orbitals with parallel spins (Figure 5). To oxidize a
nonradical atom or molecule, triplet oxygen would need to react with a partner that
provides a pair of electrons with parallel spins that fit into its free electron orbitals.
However, pairs of electrons typically have opposite spins, and thus impose a restriction
on the reaction of triplet molecular oxygen with most organic molecules.
Figure 5: The effect of the two outermost valence electrons on the reactivity of oxygen. Ground
state triplet oxygen would need to react with a partner that provides a pair of electrons with
parallel spin. However, normally, pairs of electrons of organic molecules have opposite spins
(shown for “X” in red). The reversal of spin direction of one of the electrons of triplet oxygen, e.g.
by energy transfer, leads to singlet oxygen. Sigma singlet oxygen is a high-energy form that
immediately decays to the delta form. The delta form of singlet oxygen has its two outer electrons
with opposite spins on the same orbital and, without the spin restriction of triplet oxygen, it may
react as a strong electrophilic oxidant.
Ground state molecular oxygen may be converted to many more ROS forms either by
energy transfer or by electron transfer reactions. The former leads to the formation of
singlet oxygen, whereas the latter results in the sequential reduction to superoxide,
hydrogen peroxide, and hydroxyl radical (Figure 6).
33
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
Singlet Oxygen – Through Energy Transfer to Ground State Oxygen
Singlet molecular oxygen (1O2) is a highly reactive form of molecular oxygen. It differs
from ground state triplet molecular oxygen in the reversal of spin direction of one electron
in the outermost valence shell. It is a nonradical and without the spin restriction of triplet
oxygen, it may react as a potent electrophilic oxidant. In vivo, singlet oxygen may be
generated photochemically during photosynthesis. It is limited to areas exposed to light
and is based on the interaction of molecular oxygen with photosensitizing molecules,
such as porphyrins, quinones and flavins: Following light absorption at a specific
wavelength, a photosensitizer in the ground state (So) is transformed to an excited
singlet state (S1*) such that an electron in the outermost valence shell attains a higher
energy level with conserved spin directions. The S1* may decay back to the ground state
with the emission of fluorescence or assume a longer lived excited triplet state (S3*) by
spontaneous spin inversion of the excited electron. Once the S3* has formed, it may react
with molecular oxygen, usually by energy transfer, to generate ground state sensitizer
and singlet oxygen. The life time of singlet oxygen is very low and has been estimated to
be approximately 200ns and the distance over which it may interact with other molecules
has been calculated to be less than 10 nm.
Figure 6: Generation of Different ROS. The following ROS are especially important in plants after
excess of light: singlet oxygen, hydrogen peroxide and superoxide radical ion.
Intermediates of Oxygen Reduction: Superoxide, Hydrogen Peroxide, Hydroxyl
Radical
The complete reduction of molecular oxygen to water requires four electrons (Figure 6).
This is a highly specialized reaction that can be catalyzed by relatively few enzymes
such as cytochrome c oxidase in mitochondria. Reduction of oxygen may also occur
34
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
sequentially, giving rise to the superoxide anion radical, hydrogen peroxide and the
hydroxyl radical (Figure 6). The intermediates vary greatly in their reactivity but are all
less stable than O2 or water.
Superoxide
The free radical superoxide anion (O2•‾) is formed by the addition of one electron to
ground state molecular oxygen (Figure 6). It is unstable with respect to O2 and H2O2 and
spontaneously transforms to these products by dismutation. The spontaneous
dismutation is most rapid at pH 4.8 and the rate decreases by a factor of 10 for each unit
increase in pH above 4.8. Thus, at physiological pH, non-enzymatic dismutation between
O2•‾ and O2•‾ is slow. Generation of superoxide has been associated with membrane
disruption, cell death and other cellular injuries. However, these damages may not be
directly caused by superoxide itself but by the generations of hydroxyl radicals (see
below).
Hydrogen Peroxide
The reduction of molecular oxygen by two electrons produces a peroxide ion (O22‾),
which, in its protonated state, forms hydrogen peroxide (Figure 6). Hydrogen peroxide is
not a free radical. It is more stable than the other reactive oxygen intermediates and can
easily cross biological membranes. Thus, it can be exchanged between different
subcellular compartments but also between different cells.
Hydroxyl Radical
A far more potent ROS, hydroxyl radical, is formed during the interaction of superoxide
with hydrogen peroxide. The production of hydroxyl radicals in cells is believed to be
mediated by the reduction of H2O2 by superoxide that is catalyzed by Fe2+ or Cu2+ ions
(Figure 7).
The tri-electron reduction product of molecular oxygen is the hydroxyl radical (Figure 6).
This extremely reactive intermediate of O2 reduction has a half life of less than a
microsecond and the distance over which it may interact is less than 30 nm. Hydroxyl
radicals are capable of splitting covalent bonds in a large variety of organic molecules.
35
Lesson 2
Stress Responses at the Cellular Level: The Example of Oxidative Stress
Figure 7: The Conversion of Superoxide Radical and H2O2 into Hydroxyl Radical. Dismutation of
superoxide radicals occurs either non-enzymatically (at lower pH) or enzymatically (through
superoxide dismutase SOD, Topic 4) and leads to the generation of oxygen and hydrogen
2+
peroxide (1). In the presence of Fe , hydroxyl radical is generated from hydrogen peroxide by the
2+
Fenton reaction (3), whereby Fe can be regenerated through the oxidation of superoxide anion
(2).
Topic 4: ROS under Stress and ROS Scavengers
ROS – A By-Product of All Metabolic Pathways
Up to now, we have discussed ROS as by-product of photosynthesis – generated after
excess of light (Topic 2). Excess of light is part of the fluctuating light conditions plants
are always confronted with – therefore, plants continuously produce ROS during
photosynthesis. However, plants also produce ROS as by-products in most oxidative
metabolic pathways that are localized in chloroplasts (photosynthesis), mitochondria
(respiration) and peroxisomes (photosynthesis).
Under steady state conditions, ROS are scavenged by various antioxidative defense
mechanisms we will learn about in this topic. However, the equilibrium between the
production and the scavenging of ROS may be perturbed by a number of adverse abiotic
stress or limitation factors such as continuous high light (or oxidative stress), drought,
low
temperature, high
temperature
and
mechanical stress. Such
unfavorable
environmental factors may result in rapid and transient increases of intracellular
concentrations of reactive oxygen species (ROS), whereby the concentration of these
ROS may exceed the capacity of the scavengers. In this case, ROS are closely
associated with the emergence of diverse disorders, such as diseases, stress responses,
cell death, and ageing:
•
They may cause irreversible damages that can lead to tissue necrosis and,
ultimately, may kill the plant.
36
Lesson 2
•
Stress Responses at the Cellular Level: The Example of Oxidative Stress
A common feature among the different ROS types is their capacity to cause oxidative
damage to proteins, DNA, and lipids.
The cytotoxic properties of ROS have necessitated the evolution of ROS scavengers in
order to minimize the harmful effects of ROS within the cell. In plant cells, several
enzymatic ROS scavengers help to convert the ROS. Some of these scavengers are
part of more complex scavenging systems: the ascorbate-glutathione cycle and the
glutathione-peroxidase cycle.
Enzymatic ROS Scavenging
Whereas several enzymes detoxify H2O2 and superoxide, no enzyme is known to
scavenge singlet oxygen.
Enzymatic ROS scavengers of superoxide in plants (generated during the Mehler
reaction, see Topic 2) include superoxide dismutase (SOD, located in the chloroplasts).
You can see the chemical reaction catalyzed by SOD in Figure 8.
Note that SOD converts superoxide to H2O2. Additional scavengers are needed to
subsequently detoxify the H2O2. These enzymatic scavengers are ascorbate peroxidase
(APX) and glutathione peroxidase (GPX), which are important enzymes in the
ascorbate-glutathione cycle and the glutathione-peroxidase cycle (Figure 8). The
detoxification processes of H2O2 with APX and GPX are mainly located in the cytosol.
H2O2 (generated during the oxygenase reaction of Rubisco, Topic 2) is scavenged
through catalase (CAT), mainly in the peroxisomes.
The Ascorbate-Glutathione Cycle (Figure 8)
In the ascorbate-glutathione cycle, the major cellular redox buffer ascorbate plays an
important role. The detoxification of H2O2 to H2O by ascorbate peroxidase (APX) occurs
by oxidation of ascorbate to monodehydroascorbate (MDA) (1). After that, several
chemical reactions to regenerate ascorbate via reduction are possible They are
enzymatically catalyzed by several reductases:
(2) MDA can be regenerated to ascorbate by MDA reductase (MDAR).
(3) MDA can also spontaneously dismutate into dehydroascorbate (DHA) from which
ascorbate can be regenerated by DHA reductase (DHAR). The reaction involves the
oxidation of the second main cellular buffer glutathione (GSH) to GSSG.
(4) Regeneration of GSH from GSSG occurs via reduction catalyzed by a glutathione
reductase (GR). Note this reaction is also part of the glutathione cycle, see below!
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The Glutathione-Peroxidase Cycle (Figure 8)
Glutathione peroxidase (GPX) also detoxifies H2O2 to H2O but uses glutathione (GSH)
directly. The GPX cycle is completed by regeneration of GSH from GSSG by GR (see 4
in ascorbate-glutathione cycle).
Note: Detoxification of H2O2 to water and regeneration of ascorbate and glutathione
results in the oxidation of large amounts of NADPH to NAD(P)+. NAD(P)+ accepts
electrons from the photosynthetic electron transport chain and, in this way, also helps to
alleviate the negative impact of excess light energy on the photosynthetic apparatus.
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Figure 8: The principal modes of enzymatic ROS scavenging by superoxide dismutase SOD),
catalase (CAT), the ascorbate-glutathione cycle including ascorbate peroxidase (APX), and the
glutathione-peroxidase (GPX) cycle. Several steps are necessary to regenerate APX and GPX –
some of these steps require NADPH, thus helping to regenerate an electron acceptor of the
electron transport chain of photosynthesis!
Topic 5: The Biological Role of ROS: ROS as Messengers in Signal
Transduction
In the last topic, you have seen that cells generate ROS after several distinct stresses,
e.g. after light excess. Within the cell, ROS can cause irreversible damage and,
therefore, have to be detoxified by scavengers. Plant cells have learnt to sense changes
in ROS concentrations (a result from metabolic disturbances) and utilize this information
to activate responses (i.e. generation of scavengers) that help the plant cope with the
stress. Plants use the ROS as messengers in signal transduction.
What Is Signal Transduction?
Plant cells need techniques to translate the stress signal (i.e. temperature, drought,
excess light) into a biochemical signal, which finally activate the cellular response of the
plant. Signal transduction is any process by which a cell converts one kind of signal into
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another. Processes referred to as signal transduction often involve a sequence of
biochemical reactions inside the cell, which are carried out by enzymes and linked
through second messengers. Such processes take place in as little time as a millisecond
or as long as a few seconds. Slower processes are rarely referred to as signal
transduction.
How Are ROS Used in Signal Transduction?
Plant cells use ROS as messengers in signal transduction: The stress or limitation
signal increases the concentration of a certain ROS. This second messenger then
triggers the expression of a number of responsive genes.
The products of these genes (proteins) will then activate the cellular response to the
respective stress (more detailed in Lesson 3).
At least three different ways of how ROS may trigger such stress responses have been
described:
•
Firstly, ROS activate a signaling cascade resulting in higher gene expression: As
second messengers, ROS modulate the activity of specific target molecules
involved in signaling or gene expression as transcription or translation factors (see
Lesson 3).
•
Secondly, many changes in gene expression that have been attributed to a signaling
role of ROS may result from their cytotoxicity. ROS decompose (among other cell
components) lipids via oxygenation: Polyunsaturated fatty acids within the lipids are a
preferred target of ROS attack. Several of the resulting oxygenation products are
biologically active and may change the expression of genes. Hence, a given ROS
may non-enzymatically generate a wide range of oxygenation products, some of
which may disseminate within the cell and act as second messengers that trigger
multiple cellular responses.
•
Thirdly, ROS may trigger stress responses in plants by modulating gene expression
in an indirect way. As described earlier, during detoxification of ROS, large amounts
of reductants such as ascorbic acid and glutathione are oxidized. They shift the redox
state from the reduced to a more oxidized one (see Topic 4). This triggers the gene
expression of several stress-responsive genes. However, the sensing and signaling
of this redox change is not known.
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Chemically Distinct ROS or ROS Generated within Different Cellular Components
Will Trigger Distinct Signal Pathways and Distinct Cellular Responses
ROS act as messengers after several stresses including excessive light, drought, high
temperature and pathogen attack. However, cellular stress responses triggered through
these messengers may be distinct. How can the same ROS lead to different responses?
Depending on the quality of the environmental stress, plants enhance the release of
ROS that are either chemically distinct or are generated within different cellular
compartments. For instance, during plant-pathogen interactions, superoxide anions may
be produced outside of the cell. They are rapidly converted to H2O2 that can cross the
plasma membrane. The same ROS are also produced in chloroplasts exposed to high
light stress, but by a different mechanism. Stress reactions of plants induced by
pathogens differ from those induced under high light conditions. This suggests that the
signal pathways and responses triggered by ROS exhibit a high degree of selectivity and
specificity that could be derived from their chemical identity and/or the cellular
compartment at which they are generated.
Topic 6: The H2O2/Singlet Oxygen Signaling Network after Excessive Light
H2O2/Singlet Oxygen Regulation
After a dark-to-light shift of the flu mutant (a mutant in which the FLU gene was mutated)
of Arabidopsis, the generation of singlet oxygen and H202 starts immediately at the
beginning of illumination in the plastids. Within the first 15 minutes of re-illumination, the
expression of distinct sets of genes that are distinct from those induced by
superoxide/hydrogen peroxide are activated. Among the genes that are up-regulated in
response to H2O2/superoxide are several genes that are responsible for the expression
of the H2O2/superoxide scavengers resulting in higher amounts of these scavengers
within the cell. Conversely, a larger proportion of genes activated by singlet oxygen was
unaffected by H2O2/superoxide. Collectively, these results suggest that there are major
inherent differences in the extent and specificity of stress-related changes in gene
expression, triggered by either singlet oxygen or superoxide/hydrogen peroxide.
Despite the specificity and selectivity of gene expression changes induced by singlet
oxygen, the signaling of this ROS does not occur independently but is modulated by
H2O2/superoxide. In flu plants that overexpress an H2O2-specific scavenger, the levels of
H2O2 are down-regulated. In these plants, singlet oxygen-induced changes in gene
expression were four- to fivefold higher than in the original flu mutant. At the same time
the extent of cell death and growth inhibition (details below) was more pronounced than
in flu, suggesting that signaling by singlet oxygen is negatively controlled by hydrogen
peroxide.
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In non-mutant plants under excess light H2O2/superoxide production occurs within PS I
(from photochemical quenching, Topic 2), whereas singlet oxygen is generated in PS II
(Topic 2). Singlet oxygen-dependent changes in gene expression are unlikely to occur as
long as excess amounts of electrons are still translocated from PS II to PS I giving rise to
H2O2, which suppresses the singlet-oxygen induced gene expression. Thus, the
biological activities of singlet oxygen and H2O2/superoxide may influence each
other and together form part of a more complex stress-related signaling network
(Figure 9a).
In Plant Cells Briefly Exposed to Changes in Light Intensities: H2O2 Signaling:
Plants may exhibit a graded signaling to different amounts of excess light. In plants that
are briefly exposed to changes in light intensity, photorespiratory oxygenation of
ribulose-1,5-biphosphate and the direct reduction of molecular oxygen by PS I serve as
alternative electron sinks. They help to maintain PS II electron acceptors in a partially
oxidized state and, in this way, may minimize the risk of photoinhibition of PS II (for
details see Topic 2: photochemical quenching). Under such moderate and/or transient
light stress conditions, enhanced levels of hydrogen peroxide are produced. They act as
signals and accelerate the synthesis of H2O2 scavengers. In this way, plants may keep
the internal H2O2 below a critical level. They adjust their scavenging capacity to
fluctuating concentrations of H2O2. At the same time, enhanced levels of H2O2 suppress
singlet oxygen-mediated stress responses.
In Plant Cells after Continuous Excess of Light and under Oxidative Stress: Singlet
Oxygen Signaling:
Once the plant’s capacity to quench absorbed light energy is no longer sufficient to avoid
hyper-reduction of the electron transport chain, photoinhibition of PS II may occur, block
electron transport from PS II to PS I, and, at the same time, reduce further production of
ROS by PS I. The plant cell suffers form oxidative stress. Enhanced levels of
singlet oxygen are released by PS II.
Singlet oxygen acts as a stress signal and initiates various responses: Low
concentrations of this ROS activate a genetic stress response program, which inhibits
cell growth (and further production of singlet oxygen) until environmental conditions
are more favorable again (Figure 9b). At high singlet oxygen concentrations, this
genetic stress program is overruled by photooxidative damage – cell death or necrotic
reactions result. At higher singlet oxygen levels, stress reactions seem to be caused
primarily by the toxicity of elevated levels of singlet oxygen.
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Figure 9a: Enhanced Levels of H2O2 Suppress Singlet Oxygen Mediated Stress Responses.
Genes that are part of the singlet oxygen-dependent genetic stress response program
have been conserved during the evolution of oxygenic photosynthetic organisms and
thus, this genetic stress program most likely offers some benefits to the plant. The singlet
oxygen-mediated growth inhibition resembles a stress tolerance strategy of higher plants
exposed to drought, heat, light or cold stress. Under these adverse environmental
conditions, plants may pass into a state of minimal metabolic activity that persists until
the stress is relieved. The release of singlet oxygen and its perception as a stress signal
seem to form part of a monitoring and signaling network that is necessary for plants to
constantly adjust their metabolism and development to fluctuating environmental
changes.
Note: The singlet oxygen response can initiate an acclimatory response of the
plant to severe light stress: The genetic response program is activated through
moderate levels of singlet oxygen and inhibits further cell growth and further production
of singlet oxygen to avoid the damaging effects of toxic levels of singlet oxygen in a more
effective way. Pre-exposure to moderate stress conditions that leads to an enhanced
production of singlet oxygen by PS II increases the plant’s resistance to subsequent and
even more severe light stress.
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Figure 9b: Genetically Controlled Stress Responses: Growth Inhibition And Cell Death
Summary
In this lesson, you have learnt that excessive light during photosynthesis is a stress
factor. You looked at the first steps of cellular stress response to excess of light – from
the ROS signal to gene expression and scavenger constitution:
•
Plants use the mechanisms of photochemical and non-photochemical quenching to
avoid damages to the chloroplasts. Enhanced generations of ROS are products of
these mechanisms. These responses are rapid physiological adaptations of the plant
to excess light.
•
Only a continued excess of light will cause oxidative stress and photoinhibition to the
chloroplast. Long-term acclimatory responses are initiated: A genetic response
program inhibits further cell growth until environmental conditions are more favorable
again.
•
ROS are toxic for the plant cell but enzymatic or non-enzymatic scavengers help to
detoxify the ROS.
•
Plants have further evolved mechanisms to use these ROS as signals in signal
transduction ROS stimulate the expression of genes responsive to excess of light
(e.g. coding for ROS scavengers) directly or indirectly. You have seen the example of
the H2O2/singlet oxygen signaling network.
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Literature Cited
•
AMERICAN SOCIETY OF PLANT BIOLOGISTS. Biochemistry & Molecular Biology of Plants. CDROM & Online Images.
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Lesson 3
Stress Responses at the Cellular and Molecular Level:
Gene Regulation and Gene Expression after Drought and Temperature Stress
Stress Responses at the Cellular and Molecular Level: Gene Regulation and
Gene Expression after Drought and Temperature Stress (Sacha Baginsky,
Markus Geisler & Melanie Paschke)
Concept Map
Don’t Miss these Online-Learning Activities!
•
Exercise 1: Mindmap of Stress Response at the Molecular Level (Topic 1)
•
Exercise 2: Write an Essay! What Happens during a Heat Stress Response at the
Molecular Level? (Topic 1)
•
Exercise 3: Name the Regions on the DNA! (Topic 1a)
•
Exercise 4: Name the Regions on the Gene! (Topic 1a)
•
Exercise 5: Define the Steps of UPR! (Topic 1b)
•
Exercise 6: Schematic Pathway of the Unfolded Protein Response (UPR) (Topic 1b)
Topic 1: Stress Response at the Cellular and Molecular Level: Adjusting
Protein Levels to Prevailing Conditions – Regulatory Levels of Gene
Expression
Stress or limitation responses in plants depend on complex regulations within the cell to
have an effect on changing environmental conditions. Plants, in particular, have
developed sophisticated mechanisms to cope with their environment, since they are
immobile and cannot simply escape non-optimal conditions. Each stress or limitation
signal reaching the cell from outside triggers a cascade of reactions (signal transduction,
see Lesson 2), which will change gene expression. The different processes involved in
transcription (DNA to RNA), translation (RNA to protein), and metabolic pathway activity
(protein to metabolite) are regulated at several points (Figure 1):
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•
Stress Responses at the Cellular and Molecular Level:
Gene Regulation and Gene Expression after Drought and Temperature Stress
Transcription is controlled by transcription factors that can interact with regulatory
DNA elements (=transcriptional regulation). Furthermore, the stability of the mRNA
is regulated by nucleases and protective RNA-binding proteins (post-transcriptional
regulation).
•
Translation is controlled by translation factors that initiate the translation
(=translational regulation also called post-transcriptional control).
•
Protein function is controlled by post-translational modification.
Figure 1: Gene expression is a complex process that involves several levels. In principle, all
levels of gene expression can be regulated. The arrows indicate common regulation parameters.
Expressed proteins adjust the metabolite levels in the cell to the prevailing conditions. These
protein functions are also regulated either by post-translational modifications or by protein
interactions (PPI).
Topic 1a: Transcriptional Control
Transcriptional regulation of gene expression adjusts transcript levels to constitute a
stress response. This is the first step to adjust protein concentrations to stress and
limitation signals. Through regulation, the plant cell ensures that only the appropriate,
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Gene Regulation and Gene Expression after Drought and Temperature Stress
i.e. stress relevant, genes are transcribed. And it ensures that transcription only
proceeds as long as necessary for the required response.
Principles of Regulation in Eukaryotic Transcription
The most complex control mechanisms observed in eukaryotic genes are those that
regulate the expression of RNA Polymerase II-transcribed genes and the corresponding
mRNA.
Transcriptional control is directed by cis-regulatory elements and corresponding
transcription factors:
•
Cis-regulatory elements (see Figure 2a) are regulatory DNA sequences such as
promoters and transcription factor binding sites that drive the specific
transcription initiation from a gene. The cis-regulatory element directs the efficiency
of transcription initiation and determines the transcription start site.
•
Transcription factors bind at transcription factor binding sites: Transcription
factors direct the transcription initiation. Distinct transcription factors have different
affinities for different transcription factor binding sites. Transcription can only start
when these proteins bind to their specific transcription factor binding sites at the
gene. Thus, a fine tuned regulation of transcription initiation from different genes is
determined by the equipment of the cell with different transcription factors.
•
Transcription factors possess several regulator regions that enable them to regulate
transcription (Figure 2b):
•
Environmental sensing: this part can sense the environmental conditions in the
cell, e.g. heat or cold, or interact with the molecules from signaling. The
environmental signal will induce several conformational changes within the
protein, which allows the protein to dock at the transcription factor binding site
and interact with other transcription factors.
•
Sequence-specific DNA-binding: this part will bind to the transcription factor
binding site of the corresponding gene.
•
Regulatory Protein Binding: this part interacts with other transcription factors
that bind to the nearby transcription factor binding sites of the corresponding
gene. Together the transcription factors form an active complex that regulate
transcription.
•
Transcriptional Apparatus Binding: this part can interact with proteins of the
transcriptional apparatus and initiates the beginning of transcription.
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Gene Regulation and Gene Expression after Drought and Temperature Stress
Figure 2a: Almost all eukaryotic genes contain a basic structure with regulatory elements and the
transcribed region of the gene. Cis-regulatory elements include transcription factor binding sites
(where transcription factors can bind) and the promoter (where proteins of the transcriptional
apparatus bind and trigger, when activated, the start of transcription); the promoter is followed by
the transcribed region of the gene.
We will now discuss some examples of transcriptional control after heat shock, oxidative
stress, and drought.
Example 1: Activation of Heat Shock Responses in the Cytosol: Transcription after
Heat Shock
Heat shock is defined as a tolerable short- or long-term increase in temperature in the
cell. Through such a heat shock, several normal housekeeping reactions of the cell are
deactivated and special heat shock responses are activated through expression of
several heat shock genes.
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Gene Regulation and Gene Expression after Drought and Temperature Stress
Figure 2b: Two-dimensional structure of a transcription factor including a site for binding to DNA,
to transcriptional apparatus, and to other regulatory proteins. The protein can sense the
environmental conditions within the cell (e.g. a stress signal) and may thus be activated.
These heat shock genes bring about so-called heat shock proteins (e.g. HSP60,
HSP100), which are responsible for the actual response of the plant cell to the heat
shock (for details about heat shock proteins, see Topic 2).
How does the plant sense a heat shock? After a heat shock, the concentration of
denaturated proteins (= misfolded proteins) increases in the cytosol. For the plant cell,
this is the signal to start the heat shock response.
How are Heat Shock Proteins Activated?
The expression of heat shock genes is activated at their cis-regulatory elements. One or
several heat shock cis-regulatory elements (HSE, transcription factor binding sites)
are located in the cis-regulatory elements of the heat shock genes. These elements
show a mirrored sequence (a so-called palindrome, e.g. CAGCGTG) in the two opposite
strands.
Furthermore, the heat shock cis-regulatory element contains some base pairs upstream
of the transcription start, the so-called TATA box (at which the transcriptional apparatus,
i.e. the RNA polymerases, is assembled).
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Stress Responses at the Cellular and Molecular Level:
Gene Regulation and Gene Expression after Drought and Temperature Stress
Figure 2c: Structure of a heat shock gene including heat shock cis-regulatory elements (HS1,
HS2, HS3) in the promotor.
Heat shock transcription factors are regulatory proteins that will bind to the
transcription factor binding sites within heat shock cis-regulatory elements and
thus activate transcription. Heat shock transcription factors consist of the following
components:
The Heat Shock Transcription Factor is activated by Heat (Figure 2d)
In mammalian systems (but also in plants), single heat shock transcription factors such
as HSF1 are inactive. Only the heat signal causes them to alter their structure and form
an active trimer together with two other heat shock factors. This trimer is built in the
cytoplasm, and is transported to the nucleus, the site of transcription, to bind to the heat
shock transcription factor binding sites within the promoter. Once bound to the
transcription factor binding site, they activate transcription via a TATA-box-bound RNA
polymerase. Now, transcription can start (Morimoto 1998).
Example 2: Regulation of Transcription after Oxidative Stress
In the last lesson, you have learnt that, after oxidative stress, reactive oxygen
species function as messengers in gene expression initiation. As messengers, ROS
modulate the activity of specific target molecules involved in signaling or gene
expression as transcription factors. Indeed, we still do not know the transcription factors
involved in the gene expression after oxidative stress.
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Gene Regulation and Gene Expression after Drought and Temperature Stress
Figure 2d: One-dimensional structure of heat shock transcription factor (HSF1) showing the
functional regions of the protein: DNA-binding domain, which can bind to heat shock
transcription factor sites; Regulatory protein binding domain with HR1, HR2, HR3: several
monomeric heat shock factors can bind at this site; NLS (= nucleus localization signal): the signal
peptide for the import into the nucleus; Transcriptional apparatus binding domain with the
activator: important for the activation of the transcriptional apparatus. See also figure 2b for a twodimensional view of a transcription factor. Modified after: “NOVER & HÖHFELD, 1996” in “SCHULZE,
E.-D., 2002, Pflanzenökologie. Heidelberg, Berlin: Spektrum Akademischer Verlag, page 65.“
Example 3: Regulation of Transcription after Drought
In the signal transduction after drought, the plant cell uses ABA (abscisic acid, a plant
hormone) as a signaling molecule (Lesson 4). ABA enhances gene expression of several
drought-related genes. How is the transcription regulated, when ABA has become
effective as a signaling molecule?
We will now discuss the most important type of transcription factor binding sites in
drought-sensitive genes (several types of transcription factor binding sites are possible).
The ABRE (ABA responsive element) transcription factor binding site is characterized
by a motive, which is highly conserved (i.e. invariable) in many plants. At its core lies the
base sequence CACGT(G,C), the so-called g box, which is always part of ABRE (Figure
2e).
The promoters of the drought-responsive genes usually contain several serially
connected ABREs. Cis-regulatory elements with only one ABRE react weakly at a
drought signal; they have a low transcription rate. An example: Within a drought-sensitive
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Stress Responses at the Cellular and Molecular Level:
Gene Regulation and Gene Expression after Drought and Temperature Stress
gene of maize, which is transcribed during drought, nine docking sites have been found.
Five of them were ABREs (Shinozaki et al. 2003).
Figure 2e: Structure of drought cis-regulatory element in Hordeum vulgare including
several ABA-responsive elements (ABRE) and further transcription factor binding sites.
Below, the frequency of nucleotides in the ABRE-elements. The g box (in AbRE2 &
ABRE3) is highlighted.
Modified after: “SHEN & HO, 1995” in “SCHULZE, E.-D., 2002, Pflanzenökologie. Heidelberg, Berlin:
Spektrum Akademischer Verlag, page 146.“
Additionally to ABA many different transcription factors (EREP/AP2, zinc finger TF,
MYB, HvDRF1) bind to these ABREs (Bartels & Sunkar 2005). You do not need to
learn the names of these transcription factors by heart! We just want to show that a
multitude of transcription factors play a role in the response of the plant cell to drought.
Now, the transcription of the drought-sensitive genes is initiated and several drought
proteins are released (e.g. dehydrins, and expansins, see topic 4).
Topic 1b: Translational Control (=Post-transcriptional Control)
Frequently, a stock of inactive mRNA exists within a cell. Translational control (=posttranscriptional control) after a stress or limitation signal, especially translational activation
that allows the translation machinery to utilize existing mRNAs, enables a cell to adjust
the level of stress proteins more rapidly by avoiding the activation of the complete
transcription machinery.
In most cases mRNAs in the cell are regulated in a mRNA-specific manner, in which the
translation of a defined group of mRNAs is modulated. mRNA-specific regulation is
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Gene Regulation and Gene Expression after Drought and Temperature Stress
driven by regulatory protein complexes (=translation factors) that recognize specific
binding sites of the target mRNA and, after binding, initiate translation. These specific
binding sites are often located in the untranslated regions (UTRs) of the target mRNA.
UTRs are regions that are not translated into proteins during translation. Often they are
located at the 3’ and 5’ ends of the mRNA.
Often, these translation factors enzymatically change the structure of the mRNA from an
inactive to an active status like in example 2, where the enzyme endoribonucleose
removes an intron to activate the mRNA.
Example 1: Translational Activation in Chlamydomonas
One example for the regulation of translation initiation is available from the green alga
Chlamydomonas, which contains a protein complex that regulates translation initiation of
the D1 protein by specific binding to the 5’-untranslated region (5’-UTR) of psbA mRNA
as described below. Although the RNA binding complex is equally abundant in light- and
dark-grown cells, its RNA-binding activity is substantially increased in light. The initial
binding of the protein complex to mRNA is essential for the initiation of translation Danon
& Mayfield (1994).
It has been shown, that the regulation of mRNA binding occurs via redox reactions.
When photosynthetic organisms are illuminated, they start to transport electrons, i.e. they
initiate redox reactions. For the mRNA binding complex, it was shown, that reduction of
oxidized thiol groups (the conversion of disulfide bridges (S-S) to their dithiol form (2x
SH)) activates the mRNA binding (Figure 3a).
Example 2: Activation of Heat Shock Response in the Endoplasmatic Reticulum:
The Unfolded Protein Response Is Activated after a Heat Shock Stress Signal
The unfolded protein response (UPR) is a molecular response, activated by plant cells
after a heat shock. UPR is a quality control mechanism that ensures that only correctly
folded proteins exit the endoplasmatic reticulum (where the translation happens) towards
the cytosol.
The unfolded protein response is an example of translational control of stress responses.
UPR is known to up-regulate a number of target genes and their protein products,
including the binding luminal protein (BIP, see also Topic 2). In unstressed cells,
enhanced BIP transcript levels do not give rise to enhanced protein levels, whereas, in
stressed cells, this is the case. This suggests that BIP protein levels are adjusted
translationally under stress conditions (reviewed in Shen et al. 2004).
The regulatory network of the UPR involves the catalytic action of a protein kinase that
phosphorylates an endoribonuclease, which activates mRNA.
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Gene Regulation and Gene Expression after Drought and Temperature Stress
Figure 3a: Mechanism of dithiol/disulfide exchange at target proteins (Target, right side) by
enzymes (left side) (1) dithiol reductants and (2) monothiol reductants (such as glutathione). In
the case of glutathione, a mixed disulfide can be formed with a single thiol group (thiol group: SH). This mechanism is a redox reaction.
The mechanism in greater detail (Figure 3b): Cells monitor and are able to sense
misfolded proteins in different cellular compartments. You have already learnt that a heat
shock, which is accompanied by the increase of unfolded proteins in the cytosol, triggers
transcription of heat shock genes. Very similar to this scenario, the accumulation of
misfolded proteins in the endoplasmatic reticulum (ER) after heat stress triggers an
unfolded protein response that also results in the activation of gene transcription of
binding luminal proteins (BIP).
The UPR is especially well understood in yeast. Here, a transmembrane kinase in the
ER is activated by misfolded proteins and the kinase autophosphorylates itself. These
two processes (molecular signals) activate an endoribonuclease activity, which is
provided by a domain contained in the same molecule (the kinase and the
endoribonuclease are identical proteins, however, the two activities are distributed to
different domains). This endoribonuclease is acting as translation factor and cleaves a
specific, cytosolic mRNA molecule at two positions to remove an intron. The two
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Stress Responses at the Cellular and Molecular Level:
Gene Regulation and Gene Expression after Drought and Temperature Stress
remaining exons are then joined by an RNA ligase, resulting in an active mRNA
molecule, which can be translated. The protein product is a regulatory protein, which
migrates to the nucleus and acts as transcription factor for the gene, which codes for the
binding luminal protein (BIP) Bernales et al. (2006).
Figure 3b: The mechanism of unfolded protein response (UPR): UPR is a quality control
mechanism that ensures that only correctly folded proteins exit the endoplasmatic reticulum after
a cell experienced a heat shock. The regulatory network of the UPR involves the catalytic action
of a protein kinase that phosphorylates a translation initiation factor (endoribonuclease) (2), which
activates mRNA by changing its structure (3).
Source: The mechanism of unfolded protein response (UPR) http://www.ncbi.nlm.nih.gov
So what does this mean in summary? Responses to stress or limitation can comprise
several linear pathways, which together build a regulatory network where the last step in
a certain response path can regulate the first step of another path. Together, these
pathways modulate the molecular response of the plant cell.
In the case of UPR, the first pathway comprises expression of the endoribonuclease (a
post-translational control, see next topic), the second pathway comprises expression of a
transcription factor (a transcriptional control), and the third pathway comprises the
expression of the chaperone, BIP (a translational control).
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The product of the first pathway controls translation in the second pathway; the product
of the second pathway controls transcription in the third pathway.
Example 3: Regulation of Translation after Drought in Craterostigma plantagineum
Craterostigma plantagineum is a resurrection plant which can tolerate to become
completely dry. Again ABA is the most important transcription factor here, inducing gene
expression of several drought-responsive genes after a drought signal. Droughtresponsive genes transcribed are for instance from the HDZIP gene family: CPHB-1 and
CPHB-2. While CPHP-2 is activated through ABA, CPHB1 is activated by another
transcription factor still unknown yet. The products of these genes are regulatory proteins
acting as translation factors in a regulatory network. Although no target genes for these
HDZIP proteins have been definitely identified, active bindig sites for HDZIP proteins
were found in the promoters of at least two dessication-regulated genes (Bartels &
Salamini 2001).
Topic 1c: Post-translational Control
Post-translational control is the last step in gene expression. You have learnt that several
regulatory
proteins,
trigger
transcription
(=transcription
factors)
or
translation
(=translation factors). Environmental stress signals (like heat) or signaling molecules (like
ROS, Lesson 2) will cause conformational changes to these regulatory proteins. They
bind to specific binding sites on the DNA (=cis-elements) or mRNA and activate
transcription or translation.
How are these conformational changes of the regulatory proteins controlled? A stress or
limitation signal within a plant cell may activate the expression of several regulatory
proteins. The enduring stress or limitation signal will then induce post-translational
modifications on these regulatory proteins: Remember the unfolded protein response
(UPR) discussed in Topic 1b. You have learnt that the phosphorylation of a kinase will
bring forth the endoribonuclease which activates an mRNA. Actually, this first step of the
UPR is an example for a post-translational modification. After these conformational
changes, the regulatory proteins can activate several other stress or limitationresponsive genes. In the case of the UPR, the endoribonuclease initiated the translation
of additional regulatory proteins, which then acted as transcription factor and upregulated the expression of the heat shock protein BIP.
Cells can store important proteins, which have key functions in stress response in an
inactive condition. After a stress or limitation signal, post-translational modifications will
activate the stored proteins, which is a shortcut in gene expression of these proteins and
therefore allows the cell to respond rapidly to the signal. The other way is also possible –
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an inactivation through post-translational modification of proteins in response to a stress
or limitation signal.
An example for such an post-translational control under drought conditions is the
phosphorylation of aquaporins. Aquaporins are a class of integral membrane proteins
which form channels in the membrane of biological cells. They are passive transporters
where water follow the water potential in and out of the cell. It has been shown, that
plants can control theses channels by changing their activity in response to e.g. drought.
In spinach leaves under high apoplastic (=cell wall and intercellulars) water potential
more than 50% of aquaporins of the class PM28A were phosphorylated, but rates were
much lower under water deficit in the apoplast. An increase in activity in water transport
from the apoplast into the cell followed phosphorylation. Under conditions of water deficit
in the apoplast the activity of these aquaporins is lower to avoid water loss from inside
the cell to the apoplast (Johannsen et al 1998).
Recent
work
has
significantly
enhanced
our
knowledge
on
plant
aquaporin
phosphorylation. A critical issue is to identify among the many predicted phosphorylation
sites those that are biochemically and functionally of relevance. Phosphoproteomic
analyses identified phosphorylatable Ser residues in the N-terminal tail of maize PIP1s
and Arabidopsis PIP2 (van Wilder et al. 2008). Similar analyses have also shown that
PIP2s of Arabidopsis roots can carry multiple (up to four) and interdependent
phosphorylations of Ser or Thr residues in their cytoplasmic C-terminal tail (Prak et al.
2008). With the exception of Ser283 of AtPIP2;1 (Prak et al. 2008) the functional role of
these new sites, in aquaporin gating or trafficking, remains yet unknown.
Topic 2: The Role of Proteins in Stress Response
Proteins play a central role in stress and limitation responses since their enzymatic
activities can adjust levels of nucleic acids and metabolites to prevailing conditions.
Several groups of proteins are well known to be stress induced and, for some of them,
the molecular function in the specific stress response is known. Interestingly, a common
set of proteins is induced by several distinct types of stresses, suggesting one or several
fundamental stress response pathways (see examples of heat shock response and
osmotin in this topic). We will highlight some examples for proteins that are induced by
different types of stress and discuss their molecular function in greater detail.
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Temperature Stress (Heat and Cold)
The Molecular Response of the Plant Cell to Heat
Heat shock is defined as a tolerable short- or long-term increase in temperature. For a
plant cell, such a heat shock can have severe consequences. The most dramatic
influence of high temperature on cells lies in its impact on protein structure, i.e. its ability
to induce protein denaturation and misfolding, which prevent the protein from functioning
properly.
To some extent, plants can regulate their temperature under heat conditions by
transpiration (see Lesson 4) but this may not be sufficient to prevent cellular damage
caused by high temperature.
The cell has to trigger a response (the so-called heat shock response) through the
activation of heat shock proteins (HSP). In their so-called chaperone function, (Figure 4),
HSPs either repair the misfolded proteins (if there are only minor damages), or, in a
process called proteolysis, decompose them (in case of irreversible denaturation). For
these enzymatic mechanisms, energy in form of ATP is needed (Figure 5).
At that point, the plant cell is almost exclusively involved in the production HSPs, which
repair or decompose the misfolded proteins needed during normal cell metabolism. Only
about 6 to 8 hours later, the production of heat shock proteins comes to an end. You
have already learnt how the expression and regulation of heat shock genes at the
transcription and translation level is regulated (Topic 1). Now, we will see how heat
shock proteins can repair or decompose other denaturated proteins. These repair
mechanisms need a lot of energy in form of ATP and involve the interaction of three heat
shock proteins (called HSP70, HSP40 and HSP23).
The best characterized HSP is the HSP70 system, which consists of HSP70, HSP40 and
HSP23. HSP40 has a special C-terminal domain that binds to the partially denatured
protein that is to be refolded. The complex of HSP40 and the denaturated protein needs
energy to do its work. Therefore, HSP40 binds ATP and splits it into ADP + P (and
energy). After this ATP hydrolysis, the complex recruits HSP70, which also binds to the
denaturated protein. Then, HSP23 is recruited by interaction with the complex. Using the
energy of another ATP, the complex dissociates completely and the previously misfolded
protein is released in a different folding state. If the repair is not completed by such a
cycle (i.e. there are denatured sequence regions left) the cycle will start anew. It is
estimated that the complete and correct folding of a medium sized protein requires up to
100 ATP molecules (Qiu et al. 2006, Young et al. 2004).
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Figure 4: Heat shock proteins (HSPs) act as molecular chaperones and serve to attain a proper
folding of misfolded or aggregated protein and also prevent thermal denaturation of proteins.
Isoforms of HSP60, HSP100, HSP70 and HSP90 are localized in different cell compartments,
including cytosol, endoplasmatic reticulum, chloroplasts and/or mitochondria.
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Figure 5: Short lived or misfolded proteins are recognized by the cell and labeled with Ubiquitin
(UB). The ubiquitin labeled protein is recognized by the proteasome, a large protein complex
consisting of several proteolytic subunits and regulatory factors, and degraded. This requires
ATP.
While the discussed heat shock proteins act within the cytosol or other cell components,
the unfolded protein response (UPR) is a reaction on heat shock which happens in the
endoplasmatic reticulum. UPR is a quality control mechanism, ensuring that only
correctly folded proteins exit the endoplasmic reticulum (ER) towards the cytoplasm. We
have discussed the mechanisms of UPR in Topic 1 and Figure 3b.
Binding luminal protein (BIP) is the most important protein which is expressed within the
unfolded protein response. It functions as a molecular chaperone in the ER and helps
refolding denatured proteins in reactions similar to those of the other heat shock proteins.
Cold Shock Response:
Interestingly, the protein set involved in the cold shock response is almost completely
different from the one involved in the heat shock response and comprises predominantly
proteins such as helicases, nucleases, and ribosome-associated components that
directly or indirectly interact with the biological information molecules DNA and RNA,
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acting as transcription or translation factors. They enhance or suppress the gene
expression.
Examples for this group of proteins are nucleases, which degrade RNAs and prevent
their translation. These enzymes often work in concert with helicases that unwind double
stranded regions (of RNA) and make them single stranded, a process that is often
required to make nucleic acids accessible for degradation. (Often, secondary structures
and double stranded regions protect nucleic acids from degradation by single strand
specific nucleases.) Alternatively, cold shock proteins like ribosome-associated
components could bind to mRNA and prevent their translation. One example how cold
shock proteins interact with their molecular targets is presented in Figure 6.
The best-characterized cold shock response proteins are those from prokaryotes like E.
coli. After storing the transcripts in the E. coli cell in a non-translatable form, the
bacterium slows down its growth under unfavorable conditions until it completely arrests
to prevent cell damages (no translation means no further growth until conditions get
more favorable again, El-Sharoud & Graumann (2007).
Drought and Osmotic Stress
By strict definition, these types of stress differ but we will discuss them together since
they have several aspects in common and result in similar physiological responses.
Although many proteins are induced by osmotic stress, their molecular function is not
well characterized: one example is osmotin, a protein that is induced upon different
types of stresses. Osmotin accumulates in salt adapted cells: the overexpression of
osmotin induces proline accumulation (a compatible solute, see lesson 4), but the
mechanism of this accumulation is still unclear.
Osmotin is also an example of a protein expressed after several stresss: it is also
expressed after drought stress, and it was demonstrated that osmotin has antifungal
activity against a variety of fungi and is therefore part of the response to pathogens.
After drought stress, several proteins are up-regulated:
•
Aquaporins: integral membrane proteins that form channels in the membrane of
biological cells. They are passive transporters of water but plants can regulate these
aquaporins by controlling their activity. The activity of these water channels increases
the permittivity of cell membranes to water. It has been shown that aquaporins of the
class SunTIP7 are up-regulated in response to water stress in sunflowers. SunTIP7
aquaporins are located in the guard cells of the stomatas of sunflowers are
responsible for the rapid water efflux from the guard cells necessary for stomatal
closure (Sarda et al. 1997; details in Topic 1c).
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Figure 6: Examples of cold shock proteins. (A) Helicases are enzymes which unwind
double-stranded substrates (RNA). This needs energy gained from ATP. (B) Afterwards,
degradation of RNA by a 3’ → 5’ exonuclease follows.
•
Enzymes for catalyzing the synthesis of compatible solutes: these are neutral
solutes that react minimally with the contents of a cell while protecting it from drying
out (Details in Topic 2, Lesson 4).
•
Proteases: They help degrading irreparable denatured proteins (we learnt about this
group in heat shock response; however, drought is often accompanied by
temperature stress and will also generate misfolded proteins).
•
ROS scavengers: After drought the stomates close and CO2 absence accompanied
by high light intensities can induce generations of reactive oxygen species, ROS,
which may damage cells. Therefore, scavengers deactivate these ROS (see Lesson
2 for details on ROS scavenger).
•
Dehydrins (also called LEA proteins or RAB proteins): They are proteins that
prevent the cell membrane proteins from drought damage. They prevent that the cell
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membranes become decomposed into lipids. They are induced after drought stress
but also after osmotic stress (the above discussed osmotin is also an example of a
dehydrin) and some of them by illumination. The group of these proteins was first
described to be active in seed maturation (seeds can desiccate completely during
their maturation), protecting the cell membranes within the seed from damage during
this process. Therefore, they were called late-embryogenesis-abundant (LEA)
proteins. However, later it was discovered that they are active in all plant
compartments in response to drought and osmotic stress, they were then called
dehydrins. As they are induced through ABA (the promoter of the dehydrin
expressing genes comprehend several ABREs, see Topic 1), they are also
sometimes referred to as RAB (responsive to ABA) proteins. Their molecular function
is not well characterized but recent data suggested that at least one dehydrin (ERD
10) not only has a high hydration capacity (can bind water) but it can also bind a
large amount of charged solute ions. In accord, dehydration stress function of this
protein probably results from its simultaneous action of retaining water by binding it in
the drying cells and preventing in parallel the increase in ionic strength of the
cytoplasm by binding salt ions, thus countering deleterious effects, such as protein
denaturation (Tompa, et al. 2006).
•
Expansins are plant cell wall proteins. They have unique "loosening" effects on plant
cell walls. Local expression of expansins induces the entire process of leaf
development and modifies leaf shape (as described in lesson 4). They are also
induced by ABA (Cosgrove 2005).
Drought stress is an excellent example to show that the molecular response of the plant
cell includes a complex network of enzymes, which modulate a network of different
metabolic pathways. As several stresses may interact (e.g. drought may be
accompanied by temperature stress and by oxidative stress), proteins of several
molecular stress responses may be involved in the response network.
Oxidative Stress
The response of a cell to oxidative stress is detailed in Lesson 2. In general, proteins that
are involved in the response to oxidative stress are oxygen scavengers. The typical
scavengers you have learnt about in the last lesson are the major cellular redox buffers
ascorbate and glutathione, as well as enzymatic scavengers like superoxide dismutase
(SOD), ascorbate peroxidase (APX), glutathione peroxidase (GPX) and catalase (CAT).
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Topic 3: The Molecular Responses of the Plant Cell: Physiological
Regulation and Acclimations?
In Lesson 1, you have learnt that limitation is the wider concept, stress a specific part of
it. We have specified that: Only extreme and continuous deviations from the optimal
environmental conditions to which the plant cannot respond maintaining its normal
metabolism are stress – otherwise, we are looking at a limitation. The transition from
limitation to stress is blurred. Let’s look at the example of heat shock to illustrate this
phenomenon:
In this lesson, we have learnt about different molecular mechanisms used by plants to
react to a one-time heat shock. So far, we have been mainly looking at physiological
regulations at the molecular level: The plant cell can react with a very rapid response
within minutes or hours after the heat shock signal.
A repeated, severe heat shock (e.g. during summer) lies somewhere on the blurred
border between limitation and stress: The heat shock response of the plant cell happens
at the cost of its normal metabolism and may result in a reduced growth rate.
Therefore, it is important for the plant to react effectively when facing this sort of
temperature stress:
•
After repeated occurrences of heat shock, many plant species recover and reduce
their damages faster than after the first heat shock. Although molecular and cellular
mechanisms causing this resistance are not known in detail yet, it was described that
the level of heat shock proteins was increased for several days after the appearance
of the first heat shock – reducing the time and duration of response to a second heat
shock. This results in a shorter time period for the plant cell to return to its normal
metabolism.
•
The development of such resistances, e.g. during summer, makes sense for the
plant, since, this way, the response of the cell is adapted to the particular season
(repeated occurrence of heat and drought).
•
Note that this type of response is an acclimation: The plant cell has reversibly
adjusted its processes and states to the continued exposure to heat.
There are many more examples of such acclimations at the molecular and cellular level:
•
We discussed the genetically induced growth limitation after oxidative stress in
Lesson 2. This was an example of an acclimation.
•
Additionally, resistance after repeated drought stress was observed in different plant
species. Presumably, this resistance is also caused by acclimations at the molecular
and cellular level.
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Topic 4: From RNA to Proteins to Metabolites: The Central Role of
Proteomics in Systems Biology
Understanding the response of a complex cellular system to e.g. environmental signals
like stress or limitation require a global analysis of all elements of the system. These
“systems elements” are grouped by their physico-chemical characteristics and their
position in the flow of genetic information (Figure 7). Presently, four basic groups of
systems elements and analytical procedures for their qualitative and quantitative
assessment are being distinguished, the latter carrying the suffix “-omics”.
•
Genomics studies the genes and their biochemical function in an organism:
Scientists in stress research characterize stress-related genes and their function.
This is usually done by determining the nucleotide sequence of the DNA. Since the
genome is static and its components are present in more or less stoichiometric
relation, genome DNA sequencing is a simple task with the technologies available
today.
•
Transcriptomics is the analysis of all mRNAs (the transcriptome) in a cell or
organism under certain environmental conditions: Scientists in stress research
characterize the mRNA actually transcribed after a stress or limitation signal. The
analysis by transcriptomics has to reveal dynamic and differing abundances of the
mRNAs. Transcriptomics uses the techniques of PCR and microarray, (see Lesson 8,
Topic “Phytoextraction”).
•
Proteomics is the determination of all proteins of a cell: Scientists in stress research
characterize the proteins actually translated after a stress or limitation signal.
Proteomics has a central role in the systems biology workflow and complements the
analysis of the transcriptome. One reason for the importance of proteomics is the
impossibility to predict cellular protein concentration from the abundance of the
respective mRNA. Although many analyses found positive correlations between
transcript and protein abundances, this applies only to a subset of all proteins. A
positive correlation indicates that transcripts translate linearly into proteins. For many
stress responses however, the level of a protein is adjusted by additional
mechanisms independent from transcription. These regulatory processes are the
control of the translation rate of an mRNA and post-translational modifications of the
protein (Figure 7). Remember the example on how translation of the binding luminal
protein is up-regulated in stressed cells (see Topic 1b). The regulatory network of this
stress response would have been inaccessible without data on the involved proteins.
This makes proteomics an indispensable tool in the analysis of stress responses.
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•
Stress Responses at the Cellular and Molecular Level:
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Metabolomics: The study of the metabolic profile (the metabolome) of a cell under
certain environmental conditions: Scientists in stress research characterize the
metabolites actually catalyzed by the proteins after a stress or limitation signal.
Figure 7: Schematic depiction of systems elements and the regulatory processes that determine
their concentration in the cell
In the next topic, we will focus on proteomics. You will learn about the two main
approaches used in proteomics. You can learn more about the techniques themselves in
Lesson 8, Topic “Proteomics”.
Topic 5: Proteomics Concepts
Proteomics defines an approach for the systematic analysis of all proteins expressed in a
cell. The progress of proteomics and proteomics-related technologies over the last
decade is mainly based on two parallel developments. First, the abundance of genome
information paved the way for the large-scale analysis of proteins, for which amino acid
sequences were deposited into databases (e.g., the Arabidopsis Genome initiative,
2000). Second, technological improvements in mass spectrometry, especially the
development of soft ionization techniques for peptide analysis (for techniques see
Lesson 8, Topic “Proteomics”), allowed the high-throughput identification of proteins from
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small amounts of samples (reviewed in Aebersold and Mann 2003). Although it is
accepted that the proteome is dynamic and difficult to define, scientists aim at the most
complete analysis of the protein complement of a cell or a tissue type under certain, well
defined conditions.
In principle, two basic proteomics approaches can be distinguished:
Protein profiling (also called expression proteomics) attempts the identification of all
proteins that are present in a sample and results in a list of proteins. Combined with
sophisticated protein or peptide fractionation strategies, protein profiling is a technically
relatively simple approach for high-throughput analyses of the proteome of an organelle
or a cell type and provides a snapshot of the major protein constituents (Washburn et al.
2001). The biological benefit of the acquired data could be twofold. First, proteins in
databases that are assigned as hypothetical, putative or unknown proteins, once
identified in a proteomics study, are no longer hypothetical and their annotation can be
changed to “expressed protein”. Second, protein profiling with a sub-cellular organelle
allows the definition of the organelle proteome and thus provides novel insights into
intracellular protein trafficking and sorting.
Functional proteomics concentrates on the identification of specific proteins related to
specific biological processes. Although the identification concentrates on the proteins
that are altered upon a stimulus or signal, the original analysis takes place at the level of
the complete proteome. This approach can best be illustrated by a 2-dimensional gel
electrophoretic analysis (more details on this approach in Lesson 8, Topic “Proteomics”).
All proteins of a treated and an untreated cell are displayed on the gel, and proteins that
are present or absent from one of the samples are then specifically analyzed and
identified.
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Stress Responses at the Cellular and Molecular Level:
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Figure 8: The functional proteomics approach illustrated by a 2-dimensional gel electrophoretic
analysis. Experimental design of a 2D-PAGE based differential display analysis of proteins
obtained from cells after two different treatments. Proteins that differ in abundance between the
two treatments or are shifted can be excised from the gel and identified by mass spectrometry.
Summary
You have learnt that regulation of gene expression can happen at several points.
•
Transcription is controlled by transcription factors that can interact with regulatory
DNA elements (=transcriptional regulation)
•
Translation is controlled by translation factors that initiate the translation
(=translational regulation also called post-transcriptional control)
•
Protein function is controlled by post-translational modification
We discussed the example of the unfolded protein response (UPR) to show you that
regulation of gene expression often involves a regulatory network.
You learnt about the function of several proteins that are involved in the molecular or
cellular response of a plant to a stress or limitation signal: heat shock response and heat
shock proteins, cold shock response and cold shock proteins, drought stress and drought
stress proteins, osmotic stress and osmotin.
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You learnt how the techniques of genomics, transcriptomics, proteomics and
metabolomics help to investigate the molecular response of a plant cell.
Literature Cited
•
AEBERSOLD R. & M. MANN (2003). Mass spectrometry-based proteomics. Nature 422: 198 –
207.
•
ARABIDOPSIS GENOME INITIATIVE (2000). Analysis of the genome sequence of the flowering
plant Arabidopsis thaliana. Nature 408, 796 – 815.
•
BARTELS, D. & SALAMINI, F. (2001). Dessiccation tolerance in the ressurection plant
Craterostigma plantagineum. A contribution to the study of drought tolerance at the molecular
level. Plant Physiology 127: 1346 – 1353.
•
BARTELS, D. & SUNKAR, R. (2005). Drought and Salt Tolerance in Plants. Critical Reviews in
Plant Sciences 24: 23 – 58.
•
BERNALES S, PAPA FR, WALTER P. (2006). Intracellular signaling by the unfolded protein
response. Annu. Rev. Cell Dev. Biol. 22: 487 – 508.
•
COSGROVE DJ. (2005). Growth of the plant cell wall. Nat Rev Mol Cell Biol 6: 850-861.
•
EL-SHAROUD WM, GRAUMANN PL. (2007). Cold shock proteins aid coupling of transcription and
translation in bacteria. Sci Prog. 90: 15 – 27.
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JAHANSSON, I., KARLSSON, M., SHUKLA, V.K., CRISPEELS, M.J., LARSSON, C. & KJELLBOM, P.
(1998). Water transport activity of the plasma membrane aquaporin PM28A is regulated by
phosphorylation. Plant Cell 10: 451 – 459.
•
MORIMOTO RI. (1998). Regulation of the heat shock transcriptional response: cross talk
between a family of heat shock factors, molecular chaperones, and negative regulators.
Genes Dev. 12: 3788 – 3796.
•
NOVER & HÖHFELD (1996) in: SCHULZE, E.-D. (2002), Pflanzenökologie. Heidelberg, Berlin:
Spektrum Akademischer Verlag, page 65.
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PRAK, S. HEM, J. BOUDET, G. VIENNOIS, N. SOMMERER, M. ROSSIGNOL, C. MAUREL, C & SANTONI,
V (2008) Multiple phosphorylations in the C-terminal tail of plant plasma membrane
aquaporins. Role in sub-cellular trafficking of AtPIP2;1 in response to salt stress. Mol Cell
Proteomics 7: 1019 – 1030
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SARDA, X., TOUSCH, D., FERARRE, K., LEGRAND, E., DUPUIS, J.M., CASSE-DALBERT, F. &LAMAZE,
T. (1997). Two TIP-like genes encoding aquaporins are expressed in sunflower guard cells.
Plant Journal 12: 1103 – 1111.
•
SHEN & HO (1995) in: Schulze, E.-D. (2002). Pflanzenökologie. Heidelberg, Berlin: Spektrum
Akademischer Verlag, page 146.
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Stress Responses at the Cellular and Molecular Level:
Gene Regulation and Gene Expression after Drought and Temperature Stress
SHEN, X., K. ZHANG & KAUFMAN R. (2004). The unfolded protein response – a stress signaling
pathway of the endoplasmic reticulum. J Chem Neuroanat. 28: 79 – 92.
•
SHINOZAKI K., YAMAGUCHI-SHINOZAKI K. & SEKI M. (2003). Regulatory network of gene
expression in the drought and cold stress responses. Curr. Opin. Plant Biol. 6: 410 – 417.
•
TOMPA P, BANKI P, BOKOR M, KAMASA P, KOVACS D, LASANDA G & TOMPA K. (2006). Proteinwater and protein-buffer interactions in the aqueous solution of an intrinsically unstructured
plant dehydrin: NMR intensity and DSC aspects. Biophys J. 91: 2243 – 2249.
•
QIU XB, SHAO YM, MIAO S & WANG L. (2006). The diversity of the DnaJ/Hsp40 family, the
crucial partners for Hsp70 chaperones. Cell Mol Life Sci. 63: 2560 – 2570.
•
WASHBURN, M., A. KOLLER, G. OSHIRO, R. ULASZEK, D. PLOUFFE, C. DECIU, E. WINZELER & J.
YATES 3
RD
(2003). Protein pathway and complex clustering of correlated mRNA and protein
expression analyses in Saccharomyces cerevisiae. Proc Natl Acad Sci USA 100: 3107 –
3112.
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YOUNG JC, AGASHE VR, SIEGERS K & HARTL FU. (2004). Pathways of chaperone-mediated
protein folding in the cytosol. Nat Rev Mol Cell Biol.: 781 – 791.
•
VAN WILDER, V., MIECIELICA, U., DEGAND, H., DERUA, R., WAELKENS, E., & CHAUMONT, F (2008)
Maize plasma membrane aquaporins belonging to the PIP1 and PIP2 subgroups are in vivo
phosphorylated, Plant Cell Physiol. 49: 1364 – 1377
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Responses to Drought Stress from the Cellular to the Whole-Plant Level
Responses to Drought Stress from the Cellular to the Whole-Plant Level
(Nikolaus Amrhein, Felix Keller & Enrico Martinoia)
Concept Map
Don’t Miss these Online-Learning Activities!
•
•
•
•
•
•
Exercise 1: Steps in Stomatal Opening under Normal Conditions (Topic 4)
Exercise 2: Ion Fluxes during Stomatal Opening (Topic 4)
Exercise 3: Model for Stomatal Opening (Topic 4)
Exercise 4: ABA-inducedSignal Transduction Cascade (Topic 5)
Exercise 5: Oleander Leaves are adapted to Drought (Topic 6)
Exercise 6: Describe the Crassulacean Acid Metabolism (Topic 7)
Introduction
Drought is part of plants’ daily life. It is experienced as limitation (a moderate deviation
from optimal conditions) or as stress (a severe deviation from optimal conditions) with
several nuances in between. Plants constantly evaporate water into the atmosphere as a
consequence of the steep water potential gradient between the plant and the
surrounding air. When they are able to absorb water from the soil through the roots,
water supply is maintained and plants do not suffer from drought. When water supply is
not maintained, however, they start to wilt and suffer from drought.
Water limitation and stress in plants is not only caused by drought but also by salt.
Metabolic adaptations of the plant to these environmental conditions need to occur both
at the cellular and at the whole-plant level. In these processes, several aspects are of
interest:
-
How does water limitation and stress in plants come about? Which are the processes
leading to wilting? (Topic 1)
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Responses to Drought Stress from the Cellular to the Whole-Plant Level
At the cellular level: How does the plant sense drought? How does signal
transduction proceed? Which processes are triggered within the plant cell? (Topic 2)
-
At the cellular level, plants synthesize high concentrations of compatible solutes ,
such as proline, mannitol, pinitol, glycinebetaine etc., which help the plant cell to
stabilize its turgor even under water deficit and, furthermore, to protect proteins
against excessive salt concentrations. This process is called osmotic adjustment.
(Topic 3)
-
From the cellular to the whole-plant level: Stomata close to reduce water loss
through transpiration. We will look at the regulation of this process by the plant
hormone abscisic acid (=ABA) in Topic 4 and 5 of this lesson.
-
At the whole-plant level, plants can respond to repeated or long-lasting drought
through several morphological and ontogenetic adaptations (= modifications). We will
discuss these modifications in Topic 6 of this lesson.
-
As an example of an evolutionary adaptation of photosynthesis to hot and dry
climates, we will discuss CAM plants in Topic 7 of this lesson.
-
The morphological and ontogenetic adaptations are controlled by mechanisms
regulated mainly through ABA. These mechanisms will be discussed in Topic 8 of
this lesson.
-
In Topic 9 of this lesson, we will discuss the connection between salinity and water
deficit.
Topic 1: Principles of Transpiration
At ambient relative humidity below 98 to 99%, plants constantly evaporate water into the
atmosphere. The intercellular space of the mesophyll of leaves is saturated with water
vapor, resulting in a large water potential difference between the interior of the leaf and
the surrounding atmosphere. This leads to water loss from the leaf. We can distinguish
two ways of water vapor loss:
•
Stomatal transpiration, which can be regulated through opening and closure of
stomatas (Topic 2)
•
Cuticular transpiration
Several environmental factors can affect transpiration as
•
Light: It is the signal for the opening of the stomata and, therefore, stimulates plant
transpiration. Additionally, through light the surrounding air of the leaves and the
leaves themselves heats up, which also increases transpiration.
•
Temperature: Plants transpire more rapidly at higher temperatures because
differences in water potential become larger.
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Responses to Drought Stress from the Cellular to the Whole-Plant Level
Wind and humidity: the layer of air above the vegetation becomes increasingly
humid through transpiration without wind. This cools the surrounding air of the leaves
and in turn decreases the transpiration rate. However, a breeze carries the humid air
away and in turn increases the transpiration rate.
Transpiration is an important physiological process in plants, necessary to cool down the
leaves. It is important for plants to lower the temperature in the leaves during strong solar
radiation. In this way, they are able to prevent heat shock (see Lesson 3: ‘Stress
Responses at the Cellular and Molecular Level’).
As long as plant roots maintain a water potential more negative than that of the
surrounding soil, they will be able to absorb water and pass it on to the transpiring
leaves. In this process, transpiration is the driving force causing a negative pressure
potential in leaves, which pulls water from the roots through the vessels of the xylem to
the leaves and thus water supply is maintained. When the soil becomes dry, i.e. its water
potential becomes more negative, and water supply is limited, plants reduce transpiration
by stomatal closure. Eventually, the mesophyll cells start to lose water. Under these
conditions, the water lost by transpiration is mainly taken from the vacuole. It diffuses
through the plasma membrane into the apoplast and evaporates into the intracellular
spaces. As a consequence, the water potential of the cell becomes more negative. At the
same time, the turgor decreases and the cell becomes increasingly flaccid. In this state,
the plant shows clear signs of wilting.
Topic 2: The Cellular Level – Signal Perception of the Drought Signal,
Endogenous Formation of Abscisic Acid, and ABA-Mediated Expression of
Drought-Responsive Genes
Plant cells must be able to perceive drought at an early stage, so that their metabolism
can quickly react to a drought signal. This perception usually occurs long before an
observer notices any signs of acute drought stress, e.g. wilting. Plants have multiple
sensors for drought. Some of them are not known in detail yet. However, it has been
shown that drought induces Ca2+ influx into the cell cytoplasm. Therefore, channels
responsible for this Ca2+ influx may represent one type of sensor for these stress signals.
Ca2+ acts as a second messenger in the subsequent signal transduction.
As a response to such a drought signal, the level of the plant hormone abscisic acid
(=ABA) in the plant cell rises up to 10-fold (through de novo synthesis). ABA is a key
signal in the cellular response, since it activates the expression of different droughtresponsive genes.
Additionally, ABA is responsible for the transformation of the cellular drought response
into a systemic response at the whole-plant level: For instance, ABA that is produced in
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the roots in response to drought can be directed to the shoot through the xylem flow and,
thus, trigger responses in the leaves, i.e. the closure of stomata (see Topic 5).
Accordingly, plant cells are thought to have ABA receptors at both the inside and the
outside of their plasma membranes, so that they can perceive an ABA signal generated
within or arriving outside. Such a signal is then passed on to the inside of the cell in a
specific signal transduction chain. Only little is known about this signal transduction – we
only know that, again, Ca2+ is involved in this process as a second messenger, but
others, such as cyclic ADP-ribose, inositol 1,4,5 trisphosphate (InsP3), inositol
hexakisphosphate (InsP6), diacylglycero phosphate or H2O2 have also been implied as
second messengers.
De novo Synthesis of ABA in the Cell in Response to a Drought Signal:
A drop in turgor pressure is considered to be the signal that causes ABA accumulation.
The ABA de novo synthesis is achieved by the induction of genes coding for enzymes
that catalyze ABA biosynthetic reactions. ABA is a sesquiterpene Its biosynthesis occur
in the chloroplast via the oxidative cleavage of C40 epoxycarotenoids, such as 9’- cisneoxanthin and 9’-cis-violaxanthin, by the key enzyme of ABA synthesis, 9’-cisepoxycarotenoid dioxygenase (NCED). The C15 cleavage product, xanthoxin, is
then,after export into the cytoplasm,
converted to ABA via ABA aldehyde by the
consecutive action of a dehydrogenase and an aldehyde oxidase (Figure 1).
ABA Changes Gene Expression Patterns in Cells
A rise in the cellular ABA level causes changes in the gene expression pattern. Different
drought-responsive genes are now expressed and several proteins are produced to help
plant cells adjust their metabolism to drought. We have looked at the synthesis of these
proteins in Lesson 3 (‘Stress Responses at the Cellular and Molecular Level: Gene
Regulation and Gene Expression after Drought and Temperature Stress’).
However, the expression of drought-responsive genes is not only triggered by ABA.
There are also other activators, but only very little is known about them.
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Figure 1: Pathway of ABA biosynthesis in Arabidopsis. Upregulation of genes coding for enzymes
(ABA1, VDE, NCED, ABA2, AAO) that catalyze the modification of the carotenoids of the
xantophyll cycle (zeaxanthin, antheraxanthin, and all-trans-violaxanthin are the carotenoids of the
xanthophyll cycle). Figure modified after Christmann et al. (2006).
Topic 3: Compatible Solutes – Osmotic Adjustment of Plant Cells
Compatible solutes are important molecules that help the plant cell protect itself against
drought. Compatible solutes are the end products of a pathway that starts with an ABAindependent activation of gene expression.
Compatible solutes (also known as counteracting solutes, osmoprotectants, or
compatible osmolytes) are small organic molecules which:
•
are highly water-soluble
•
tend to have no overall charge at physiological pH
•
accumulate in cells without perturbing their normal metabolic processes, even at high
concentrations
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The term was originally coined by A.D. Brown of the University of New South Wales in
the 1970s and used for a solute "which, at high concentration, allows an enzyme to
function effectively".
Compatible solutes occur in all living organisms from archaea to higher plants and
animals. Chemically speaking, they are quite diverse, comprising:
•
certain amino acids (e.g. proline, alanine)
•
quaternary ammonium compounds (e.g. glycine betaine, proline betaine)
•
tertiary sulfonium compounds (e.g. dimethylsulfoniopropionate)
•
sugars (e.g. trehalose, sucrose, fructans, raffinose oligosaccharides)
•
polyols (e.g. mannitol, sorbitol, pinitol, ononitol) (Figure 2)
Figure 2: Types of Compatible Solutes
In functional terms, they are less diverse, serving mainly as stress protectants involved in
osmotic adjustment and protection of cellular structures. The topic of compatible
solutes is, therefore, intimately associated with that of abiotic stress. We will focus here
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on water deficit stress which may be caused by different direct or indirect
environmental factors such as drought, high or low temperature, salinity and frost. The
danger of water deficit for plant life is obvious: Plants are aqueous organisms depending
on water as solvent, transport and reaction medium, substrate, evaporative coolant, and
turgor supplier. Turgor is vital for plant function and growth and must be
maintained at all costs. To do so, water deficit-stressed (and stress-tolerant)
plants react by lowering their water potential by actively increasing their net cell
solute concentrations (Figure 3). This process, called osmotic adjustment, may
theoretically be achieved by a number of mechanisms and solutes of which plants seem
to use only relatively few. As a rule of thumb, we can state that osmotic adjustment
requires the de novo synthesis of compatible solutes (sometimes in concert with a
reduced rate of their degradation). Compatible solute concentrations may reach values
as high as several hundred mM in response to stress. Correlative data shown in Table 1
illustrate this point.
Figure 3: Osmotic Adjustment in Plant Cells under Water Deficit. Compatible Solutes Stabilize the
Water Content in the Cell.
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Table 1: Selected Examples of Compatible Solute Accumulation in Plants under
Different Water Deficit Situations
Compound
Cause of
Water Deficit
Plant
Species
Leaf Tissue Concentration
after Stress (mM)
-fold
Increase
Glycine
betaine
Proline
Salt
15
6
115
414
Proline
Drought
32
150
Proline
Salt
294
10
Mannitol
Salt
170
2
Mannitol
Salt
79
3
Pinitol
Drought
Spinacia
oleracea1
Cajanus
cajan2
Solanum
tuberosum3
Arabidopsis
thaliana4
Lagunculari
a
racemosa5
Apium
graveolens6
Cajanus
cajan2
365
24
1
Drought
2
3
4
Robinson & Jones (1996), Keller & Ludlow (1993), Büssis & Heineke (1998), Chiang &
5
6
Dandekar (1995), Popp & Smirnoff (1995), Stoop & Pharr (1994)
Only little information on the cellular and subcellular compartmentation of compatible
solutes is available (Table 2). In storage parenchyma cells of unstressed celery petioles
(= stalk of leaves), mannitol, synthesized in the cytosol, was found both in the vacuoles
and the cytosol at considerable concentrations (Keller & Matile 1989). In leaves of the
natural mannitol producers, Antirrhinum (snapdragon; Moore et al. 1997), mannitol was
additionally found in chloroplasts. Conversely, glycine betaine, synthesized in the
chloroplast, was hardly present in the vacuoles but occurred at high concentrations in the
cytoplasm (including the chloroplasts) in salt-stressed spinach mesophyll cells (Robinson
& Jones1986). Finally, proline, synthesized in the cytosol, ended up in the chloroplast,
cytosol, and vacuole in water deficit-stressed potato leaves (Büssis & Heineke 1998).
This differential intracellular distribution of compatible solutes leads us to their second
function in plant cells, i.e. the protection of cellular structures, in the broadest sense,
from macromolecules to membranes. Enzymes are denatured upon extreme removal of
water and enzyme activities may be severely inhibited by high salt concentrations,
typically above 100 mM (more about salinity stress will be related in Topic 9). To protect
theses enzymes from denaturation, compatible solutes appear to have the capacity to
prevent the direct contact and interaction between enzymes and their environment
(which has a more negative osmotic potential) and allow the maintenance of an
undisturbed aqueous layer around the enzymes thus keeping them in their native (i.e.
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folded) conformation. In extremis, when the hydration shell is finally removed upon total
withdrawal of water, compatible solutes can replace it by hydrogen bonding resulting in a
stabilizing, glassy state (Figure 4).
Similar mechanisms have been proposed for processes in which compatible solutes
protect membrane functionality against stress.
Table 2: Concentrations of Compatible Solutes in Subcellular Compartments of
Leaves
Compatible Solute
Glycine betaine
Plant Species
Spinacia
In Vacuole
In Cytosol
In Chloroplast
(mM)
(mM)
(mM)
<1
300
300
20
91
160
5
69
76
103
280
nd
oleracea
Proline
Solanum
tuberosum
Mannitol
Antirrhinum
majus
Mannitol
Apium
graveolens
Topic 4: Physiological Regulation of Stomatal Opening and Closure under
Normal Conditions
Responses of plants to drought occur not only at the cellular level but also at the wholeplant level. For instance, the plant must close the stomata of the leaves to prevent an
interruption of the vital continuous water threads in the xylem. In this topic, we look at
stomatal regulation under normal conditions.
How is the mechanism of stomatal opening and closure regulated under normal
environmental conditions?
Stomata are regulated by light – they are open in the light to allow uptake of CO2
into the leaf.
Opening and closure of stomatais the result of interactions between metabolism,
transport, and regulatory processes. Guard cells are the sole cells in the epidermal layer
containing chloroplasts. In the light, they synthesize ATP, which is only partially used to
energize production of starch and soluble carbohydrates since these cannot be exported.
The ATP is rather used to energize the plasma membrane proton ATPase. This
eventually results in stomatal opening during illumination.
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Figure 4: Mechanisms of Protein Structure Stabilization at Different Stages of Water Loss. In fully
hydrated cells (a), the native (folded) form of a protein is thermodynamically favored. Molecular
crowding during water loss increases the probability of the cytoplasmic solutes to interact with the
protein surface. In stress-sensitive cells (b), the lack of compatible solutes, such as proline and
sugar results in binding of the destabilizing agents, which leads to protein unfolding and
denaturation. In tolerant cells (c), preferential exclusion from the protein surface dominates over
preferential binding, which maintains proteins in their native conformation at intermediate water
concentrations. Compatible solutes allow a preferential hydration of the protein surface (indicated
as the ring around the protein). With the disappearance of the water shell from the proteins, sugar
molecules that were previously excluded from the protein surface replace water via hydrogen
bonding, thus stabilizing the native protein structure in the water-deficient (glassy) cytoplasm in
tolerant cells (d). Compatible solutes other than sugars fail to stabilize proteins in the state of
water deprivation. In water-deprived, sensitive cells (e), the previously formed unfolded
conformation is fixed in a cytoplasmic glass. The reversibility of the processes occurring during
dehydration and rehydration is indicated by arrows.
Figure modified after: Hoekstra FA, Golovina EA, Buitink J (2001) Mechanisms of plant desiccation tolerance.
Trends in Plant Science 6: 431-438
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The Biochemical Mechanism of Stomatal Opening in the Light:
1. What is the signal for stomatal opening?
We know now that stomatal opening is caused by light, but light has many qualities. If
plants are exposed to red light only, which is sufficient to drive photosynthesis, only
incomplete stomatal opening can be observed. It is blue light that is required for full
stomatal opening. Also, low CO2 concentrations induce stomatal opening.
2. Which molecules are responsible for light perception?
Blue light is sensed by the flavoproteins phototropin 1 and phototropin 2 which are
found in the plasma membrane and play a major role in light perception. Deletion
mutants in only one of these genes are only slightly affected in their blue light response.
Double mutants, however, fail to exhibit the blue-light specific stomatal opening. This
result indicates that phototropins play an important role in the blue-light response of
stomata. However, it is still a matter of debate, whether phototropins are the sole bluelight photoreceptor in guard cells or play a role in later steps of the sensory transduction
cascade. Other factors of the transduction pathway for stomatal opening include Gproteins, phospholipase A which is likely to cause the release of linolenic acid from
membrane lipids in plants, protein phosphatases PP1 and PP2 and protein kinases.
However, the signal transduction cascade is not known in detail yet.
3. How does the signal transduction cascade mediate the activation of the plasma
membrane H+ ATPase?
Increased ATP levels alone are not sufficient to stimulate proton pumping by the plasma
membrane H+ ATPase. The signal transduction cascade results in the activation of
plasma membrane H+ ATPase, which is the initial event in stomatal opening:
•
The H+ ATPase is phosphorylated at its C-terminus
•
The phosphorylated H+ ATPase domain interacts with 14-3-3 proteins.
The activated guard cell plasma membrane H+ ATPase creates a proton gradient
between the cytosol and the apoplast. This steep H+ gradient means that the electrical
potential becomes more negative (hyperpolarization) in the cytosol and this has several
consequences:
•
The membrane potential becomes sufficiently negative to induce K+ channel opening.
•
Acidification in the apoplast activates the K+ inward rectifying (K+in) channels (= the
channels responsible for K+ influx from the apoplast into the cytoplasm).
•
As a result of the first two events, potassium (K+) moves through the K+in channels
along the electrical gradient into the guard cell (Figure 5).
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•
Responses to Drought Stress from the Cellular to the Whole-Plant Level
To maintain charge balance, anions are required in the cytoplasm. Three main
anions have been described to act as counterions: chloride, nitrate, and malate.
Depending on the plant species, chloride and nitrate (Allium cepa, Zea may) or
malate (Vicia faba) are the main counterions. Chloride and nitrate enter the guard cell
through a symport with protons, with a stoichiometry of 2 to 3 protons per anion.
Despite the strong hyperpolarization, the anions cannot leave the cell, since the
anion channel is blocked at hyperpolarizing membrane potentials. Malate is formed
during guard cell swelling by degradation of starch, followed by glycolysis and
carboxylation of phosphoenolpyruvate
•
The described ion fluxes and production of malate increase the osmolarity in the
cytoplasm, resulting in osmotic water influx which causes a higher turgor and, in
consequence, to swelling of the guard cells and their bowing apart with the
concomitant increase of the stomatal aperture. Within the cell, solutes are
transported into the vacuole, which swells, either by fusion of smaller vacuolar
vesicles or by swelling of a vacuolar network.
•
To maintain stomatal opening, many plants synthesize sucrose at considerable
concentrations in the guard cells. Sucrose production is a slow process and is not
directly related to the fast process of stomatal opening. Nevertheless, sucrose is
important to maintain high osmolarity within the cytoplasm when stomata are open.
Figure 5: Typical potassium staining in guard cells of leaf segments (A) and isolated epidermal
strips (B) after 3 hours of illumination at high levels of potassium. Staining immediately prior to
illumination is shown in C (leaf segments) and D (epidermal strips). By courtesy of Fischer (1971).
The Biochemical Mechanism of Stomatal Closure in the Dark
Similar to stomatal opening, stomatal closure is also a well-regulated process:
•
It starts with the down regulation of the plasma membrane H+ ATPase activity.
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•
Responses to Drought Stress from the Cellular to the Whole-Plant Level
Depolarization of the guard cell plasma membrane potential results in the opening of
the R (rapid)-type and S (slow)-type anion channels of the plasma membrane.
•
This effect increases the driving force for K+ efflux and activates the voltage gated K+
efflux channel.
•
It is still a matter of debate, whether malate is reincorporated into starch or excreted.
•
The more positive osmotic potential in the cytoplasm results in water efflux, decrease
in turgor, shrinking of guard cells, and, hence, stomatal closure.
Topic 5: Regulation of Stomatal Opening and Closure under Drought
Conditions
Under drought conditions, plants must close their stomata to reduce water loss
and avoid wilting. How is this process regulated?
Stomatal closure occurs in the dark but can also be induced by the plant hormone
abscisic acid (=ABA) which overrides the effect of light.
Physiological Effects of ABA
The response to ABA depends both on the hormone’s concentration within the tissue
and on the sensitivity of the tissue to the hormone. ABA biosynthesis and concentrations
can fluctuate in response to changing environmental conditions. Under conditions of
drought, ABA in the leaves will increase. Upon rewatering, the ABA level returns to
normal.
Cytosolic ABA increases during water deficit stress as a result of its
•
synthesis in the leaf,
•
redistribution within the mesophyll cell,
•
import from the roots (via the xylem), and
•
recirculation from other leaves.
The concentration of ABA declines after rewatering because of its
•
degradation and
•
export from leaf, as well as a
•
decrease in its rate of synthesis.
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With the onset of water deficit, ABA synthesis will increase in roots that are in direct
contact with the drying soil. From the roots, ABA is transported via the xylem stream
to the leaves and hence to the guard cells where it induces
stomatal closure.
Because this transport can occur before the increasingly negative water potential of the
soil causes any measurable change in the water status of the leaves, ABA can act as a
root signal that helps reduce the transpiration rate by closing stomata in leaves.
Additional hydraulic signals are likely to be involved.
Mutants that lack the ability to produce ABA exhibit permanent wilting because they are
unable to close their stomata. Application of exogenous ABA to such mutants causes
stomatal closure and a restoration of turgor pressure.
ABA Action Changes Ion Fluxes
Rapid physiological responses like stomatal closure induced by ABA involve alterations
in the fluxes of ions across membranes and are described in this topic (Figure 6).
Figure 6: Short-Term Effects of ABA: Via Alteration of Ion Fluxes to Stomatal Closure
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Responses to Drought Stress from the Cellular to the Whole-Plant Level
ABA Signaling in Stomatal Guard Cells
Signal transduction pathways, which amplify the primary signal generated when the
hormone binds to its receptor, are required for both the short-term and the long-term
effects of ABA. Two pathways have been proposed for the signal transduction:
Pathway 1:
•
Reactive oxygen species regulate the so called calcium inward rectifier channel
which leads to an elevated Ca2+ level in the cytoplasm. Increased cytoplasmic
calcium concentrations induce the further release of calcium from internal pools such
as the vacuole.
•
These increased calcium concentrations inhibit the plasma membrane H+ ATPase
and the so-called K+in channel and activate the R (rapid)-type and S-type anion
channels of the plasma membrane.
•
The decrease in anion concentration in the cytoplasm and the subsequent
depolarization lead to the activation of the so-called K+ out channel. The osmolarity of
the cytoplasm decreases and stomata close.
Pathway 2:
•
The distinction from pathway 1 (Figure 6) is that abscisic acid increases cyclic
ADPribose and InsP3 (also called IP3), two second messengers which activate
vacuolar calcium channels, leading to an elevated Ca2+ level in the cytoplasm. After
this process, the same reactions as described in pathway 1 occur.
•
In both cases, the signal transduction pathways include positive and negative
regulators such as phosphatases, protein kinases, phospholipases, and protein
farnesylation
•
Stomatal closure can also occur without the involvement of abscisic acid, e.g. under
oxidative stress conditions, when reactive oxygen species (ROS) bypass abscisic
acid).
It is interesting to note that in hot arid regions, stomatal closure occurs at noon. Whether
this effect is due to abscisic acid redistribution to prevent excessive water loss (as a
response to drought stress) or due to oxidative stress (as a response to high irradiation)
is not known yet. However, it shows once again that drought stress and oxidative stress
are normally combined.
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Responses to Drought Stress from the Cellular to the Whole-Plant Level
Topic 6: Modifications and Evolutionary Adaptations in Response to
Drought Stress at the Whole-Plant Level
When a plant is exposed to severe or repeated drought, it will protect itself against longlasting drought stress through morphological and ontogenetic adaptations.
While some of these are reversible (i.e. the paraheliotropic reactions), most of these are
modifications. The term ‘modification’ denotes an irreversible change in plant
architecture or structures that will help a plant to resist against repeated and future
drought events. The term “irreversible” normally means “not reversible within a season
(Lesson 1).
Examples for morphological and ontogenetic adaptations are:
•
Roots and shoots: ABA promotes root growth and inhibits shoot growth at lower
water potentials (Topic 8). The overall effect is a dramatic increase in the root/shoot
ratio during dehydrating conditions. Larger roots allow the plant to reach deeper
water layers. This helps the plant, along with the effect of ABA on stomatal closure, to
cope with water deficit stress.
•
Inhibition of leaf growth in response to the onset of drought: The plant reduces
its canopy leaf area to save water which would otherwise be lost through
evapotranspiration.
•
Changes in leaf position: Plants are able to change the position of their leaves. For
instance, they orient their leaves away from the sun (paraheliotropic reaction) when
subjected to water deficit stress, thus limiting the interception of radiation energy
which would raise temperature and hence evaporation.
•
Epidermis: Water loss can occur through the epidermis. Although cuticles consist of
waxes, n-alkanes, and other hydrophobic material, they are not highly efficient water
barriers and cannot prevent transpiration if they are thin. In contrast, thick cuticles are
quite efficient barriers. Leaves with thick cuticles are found in many plants growing in
arid environments. A plant frequently exposed to water deficit stress will
develop a thicker cuticle in comparison to a well watered plant.
•
Hairs: As mentioned previously, wind strongly affects transpiration by reducing the
thickness of the unstirred water vapor layer. To prevent water loss during windy
periods, plants have evolved several mechanisms to reduce turbulence in the vicinity
of stomata. The simplest one is to produce many hairs (trichomes) on the leaf
surface. They limit turbulence and, hence, increase the thickness of the boundary
layer. Plants can produce higher numbers of trichomes at the surface of newly grown
leaves when frequently exposed to water deficit.
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•
Responses to Drought Stress from the Cellular to the Whole-Plant Level
Cavities: Plants can also form cavities in the leaf surface in which the stomata are
hidden. In this case, stomata are also protected from wind. Some plants, such as
Nerium oleander, even have hairs in these invaginations to further increase the
unstirred layer.
•
Reduction of leaf surface: Yet another reaction to drought stress is the reduction of
the leaf surface. If a plant has developed a substantial leaf area, drought stress will
induce premature senescence of older leaves, which eventually abscise. Newly
grown leaves will then have a reduced leaf area.
•
Increase in stomata density but decrease in stomata size: in newly formed
drought-resistant leaves plants can form smaller stomata, but with a higher density in
the lower epidermis. This will help to improve water-use efficiency under prolonged
drought conditions.
•
The shape of a leaf also influences water loss. Thick leaves exhibit an altered
surface to volume ratio and, thus, limit transpiration. Plants can increase the
thickness of newly grown leaves when frequently exposed to drought.
Evolutionary Adaptation
Any of the modifications described above can ultimately result in an evolutionary
adaptation that will be passed on to the next generation. This phenomenon has occurred
in many plant species which have adapted to a hot and arid climate:
•
It can be observed in cacti, where the stem has developed into an often ball shaped
photosynthetically active organ, while the leaves have become photosynthetically
inactive thorns.
•
In Topic 7, we will discuss the example of CAM plants, in which the metabolism is
evolutionarily adapted to the conditions of hot and arid climatic conditions and
drought.
Topic 7: CAM Plants – Evolutionary Adaptation of Photosynthesis to Hot
and Arid Climatic Conditions
CAM plants: Adaptation to water deficit stress occurs not only at the morphological and
anatomical, but also at the biochemical level. For instance, plants have evolved a special
form of photosynthesis, which is better adapted to dry climates. In arid zones, some
plants belonging to a number of phylogenetically unrelated families, such as
Crassulaceae, Cactaceae and Euphorbiaceae, independently evolved a special form of
photosynthesis, the so-called CAM (crassulacean acid metabolism, Figure 7). CAM
enables the plants to improve water use efficiency. Typically, a CAM plant loses 50 to
100g of water per g of CO2 fixed. This is 5 to 10 times less compared to C3-plants.
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Lesson 4
•
Responses to Drought Stress from the Cellular to the Whole-Plant Level
CAM plants temporally separate primary CO2 fixation and the Calvin cycle. During
the night, when the conditions are favorable for reduced transpiration, CAM plants
open the stomata: CO2 can enter the plant and is fixed, as the bicarbonate ion, by
phosphoenolpyruvate
carboxylase
(PEPcase)
which
carboxylates
phosphoenolpyruvate (PEP) generated by the degradation of starch-derived glucose
in glycolysis.
•
In contrast to ribulose-1,5-bisphosphate carboxylase, PEP carboxylase has a very
high affinity to HCO3- and O2 does not compete. Consequently, photorespiration is
strongly reduced.
•
The product formed is oxaloacetate, which is reduced by a NADH-dependent malate
dehydrogenase to malate. Malate is transported to the large, central vacuole, where
it accumulates as free malic acid (see below) and, hence, the pH drops by two
orders of magnitude.
•
All CAM plants have succulent organs with very large cells. The vacuole occupies
more than 90% of the cellular volume and can thus function as a storage site for
water and malate.
•
Vacuolar malate uptake is energized by both the vacuolar H+ ATPase and H+PPase.
These proton pumps generate a membrane potential, which is more positive within
the vacuole compared to the cytosol. However, this membrane potential (20 to 40
mV) would not be sufficient to drive the strong accumulation of malate. Probably, the
driving force for the accumulation is the acidification of the vacuole during the dark
period from pH 5 down to pH 3. Malate is transported into the vacuole as malate2-. In
the acidic vacuole, dianionic malate (=malate2-) is protonated to monionic malate (=H
malic acid-), and further to undissociated malic acid (= H2 malic acid). These are
forms that cannot pass the malate transporter/channel.
•
During daytime, stomata are closed: The vacuole releases malic acid (by an
unknown
mechanism)
which
is
converted
to
pyruvate
and
CO2
by
a
decarboxylating malate dehydrogenase (malic enzyme). Pyruvate is used
directly for gluconeogenesis and starch synthesis, while CO2 is used for the
carboxylation reaction in the Calvin cycle.
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Figure 7: Overview of CAM Metabolism.
•
To prevent an unproductive cycle during daytime, the PEPcase activity is reduced via
the enzyme’s inhibition by malate. During nighttime, the enzyme is stimulated by
glucose-6-phosphate. Phosphorylation of a single serine residue results in this
differential sensitivity of PEPcase to the two metabolites. The phosphorylated night
form is less sensitive to malate inhibition and more sensitive to glucose-6-phosphate
activation, while the reverse is true for the non-phosphorylated day form. PEPcase
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kinase, the enzyme that catalyzes the phosphorylation of PEPcase, is stimulated by
the efflux of calcium from the vacuole to the cytosol.
•
In most CAM plants, crassulacean acid metabolism is constitutively expressed.
However, there are some exceptions in which plants can switch their metabolism
from C3 to CAM, when the environmental conditions become unfavorable for C3
photosynthesis. The best described plant performing such a modificatory and
acclimatory metabolic change is Mesembryanthemum crystallinum (also called “ice
plant”). Young, well watered Mesembryanthemum plants exhibit C3 photosynthesis.
When watering is stopped or when the plants are irrigated with saline water, leaves
become thicker, excretory glands are formed, and enzymes required for CAM are
synthesized (Bohnert et al 1995). High light intensities or exogenously applied ABA
can also induce this metabolic switch. These changes allow the plant to survive
under environmental conditions unfavorable to C3 photosynthesis.
Topic 8: From Physiological Regulations and Acclimations to Modifications
How can plants sense that long-term adaptations to drought are necessary and that the
investment is likely to pay off? How are modifications in the plant’s physiology and
architecture triggered? In these processes, the plant hormone ABA plays a crucial role,
too. ABA acts as a mediator for several complex signaling cascades and triggers the
expression of several enzymes responsible for changes in plant architecture (review in
Chaves et al. 2003). There are also some reaction cascades that are not triggered by
ABA, however, we will only focus on ABA-mediated responses here.
Roots:
•
ABA-mediated long-term responses to water stress normally start in the roots:
Water stress causes an increase in ABA concentrations in roots. Higher ABA
concentrations in roots cause increased root growth. In this process, ABA inhibits
ethylene production. Ethylene reduces root growth at normal conditions.
Shoots:
•
Mild drought reduces ABA concentrations in shoots. This shortage allows
ethylene to inhibit shoot growth.
Leaves:
•
The first reaction of the plant to drought is a reduction of leaf growth to decrease
the canopy leaf area (and avoid water loss through transpiration). Leaf growth
inhibition is regulated by changes in pH, resulting in a rapid decrease in
extensibility of expanding cell walls.
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Lesson 4
•
Responses to Drought Stress from the Cellular to the Whole-Plant Level
With the onset of drought, ABA from the roots (transported via the xylem stream)
acts as a signal that triggers changes in leaf shape and the development of
drought-resistant leaves.
•
ABA, therefore, induces signaling cascades both from within and from the outside
the leaf cells.
•
ABA affects transcription factors and induces the expression of several genes
through activation of ABA-responsive elements (ABREs in the promoter region of
drought-sensitive genes; see Lesson 3).
One of the consequences of enhanced transcription is the production of so-called
expansins. Expansins are plant cell wall proteins. They have unique "loosening" effects
on plant cell walls. Local expression of expansins induces the entire process of leaf
development and modifies leaf shape. In Lesson 3, you find an overview of other ABAinduced proteins involved in the long-term drought response of the plant.
Topic 9: Salinity Causes Water Deficits in Plants
In many arid zones, irrigation is a prerequisite for crop production. However, irrigation,
e.g. with river water, is problematic, because such water always contains a certain
amount of salts (calcium, magnesium and sodium). Salt concentration may be quite low
at the source of a river, but it increases downstream due to solubilization of minerals in
the river bed. Therefore, water for irrigation can contain 100 to 1000g of salt per cubic
meter. On average, annual irrigation is often in the range of 10’000 cubic meters per
hectare. This means that 1000 to 10’000 kilograms of salt can be deposited per hectare
of soil.
In arid zones, evaporation of water occurs fast and plants absorb only part of the water
and even less salts. As the water evaporates, Mg2+ and Ca2+ which are usually present in
low amounts can partially be used as nutrients by the plant. They usually do not have
harmful effects at the concentration deposited in the soil. In contrast, Na+ which is in
most cases the predominant cation of irrigation water is not a nutrient and accumulates
in the soil. As a result, in arid zones, especially in well irrigated regions with poor
drainage, soils become saline and are no longer suitable for agriculture. It is believed
that actually more than 30% of the presently irrigated soils will become saline within the
next ten to thirty years. This is a world-wide problem, but it is most acute in North and
Central Asia.
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What is the effect of soil salinity on plants?
For plants, both osmotic stress and ion toxicity are the two main problems arising from
salinity.
Osmotic stress causes water deficits in the plant cells and plants:
High salinity in the soil results in a more negative water potential. For the roots of plants,
it becomes increasingly difficult to absorb sufficient amounts of water. Water supply from
the roots may cease and the plants suffer from drought. Plant cells lose their turgor since
the water potential of the cell becomes less negative than the outside and, thus, they
have to release water. The plants start wilting.
Ion toxicity:
Finally, when plants take up high concentrations of sodium from the saline soil the
problem of ion toxicity arises:
•
High sodium concentrations can inhibit enzymes by disrupting the water shell
surrounding the enzymes which is required for their activity and structure.
•
Excessive uptake of Na+ and Cl- reduces uptake of other mineral nutrients, such as
K+, Ca2+, and Mn2+.
Plant Responses to Salinity Stress
Plants have found ways to cope with the negative effects of salinity:
Compatible solutes help to maintain a negative water potential difference between
the root and the environment even when the environment becomes saline and
hence the osmotic potential of the soil solution decreases. As already discussed in
Topic 2, compatible solutes help plants to cope with salinity by osmotic adjustment:
Plants produce large amounts of compatible solutes inside their cells. Through synthesis
of compatible solutes in the cell, the water potential of the cell decreases. Thus, the net
water flow out of the cell is inhibited.
Compatible solutes help to protect cellular structures against the toxicity of high
ion concentrations. Enzyme activities may, for instance, be severely inhibited by high
salt concentrations, typically above 100 mM. Compatible solutes appear to have the
capacity to prevent any direct interaction between the proteins and the salt, thus allowing
the maintenance of an undisturbed aqueous layer around the proteins and keeping them
in their native (i.e. folded) conformation.
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The SOS pathway helps plants to cope with the toxicity of high concentrations of
sodium within its cells and helps to maintain an intracellular homeostasis of
sodium:
•
This mechanism involves a plasma membrane localized Na+/H+ antiporter which
exports sodium in exchange for protons from the cell.
•
A second important transporter is located in the vacuolar membrane and allows the
accumulation of sodium within the vacuole by exchanging Na+ (from the cell into the
vacuole) against H+ (from the vacuole into the cell).
Salt Tolerance in Crop Plants?
At high salt concentrations in the soil, plant yields are drastically reduced. This leads to
poor harvests and famines in the affected regions. Therefore, it is important to change
irrigation strategies and find ways to administer low salt water. Thus, further salinization
of valuable agricultural soils could be reduced.
Another strategy to secure worldwide food supply in the future may be the development
of salt tolerant crops. There is a wide spectrum of salinity tolerance among higher plants:
Especially halophytes are known for their ability to live in saline environments. By
transferring the characteristics of these halophytes to our crop plants, we could possibly
develop agricultural plants more tolerant to salt. In this course, we look at the methods
used to create such transgenic plants. However, in this course we will not focus on
engineered salt tolerance, but rather on heavy metal tolerance – a closely related
subject. Learn about these methods in Lesson 8 (Phytoextraction).
Summary
You have learnt about short-term and long-term plant responses to drought stress at the
whole-plant level.
Responses at the cellular level:
•
Plants synthesize compatible solutes to maintain a lower water potential and to
protect enzymes.
•
Plants can regulate stomatal apertures to control the rate of transpiration.
•
Under normal environmental conditions, this process is regulated through red and
blue light and subsequent alterations in ion fluxes in and out of the guard cells.
•
Under drought conditions, this process is regulated through abscisic acid (= ABA)
and subsequent alterations of ion fluxes in and out of the guard cells.
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Responses at the whole-plant level:
•
You have learnt about irreversible changes in plant architecture (=modifications).
•
You have learnt about CAM Plants and the evolutionary adaptation of their
metabolism to hot arid climates.
•
You have learnt about the role of ABA in long-term responses to drought stress.
We discussed the example of salinity in plants. Salinity in soils causes stress:
•
Water deficit stress in plants: plants use compatible solutes to prevent water deficit
stress.
•
Toxicity of the absorbed ions, in particular sodium, in the cell: Plants use SOS
mechanisms to discharge sodium from their cells.
Now, you should be able to solve the following tasks:
•
Define water deficit stress!
•
Draw a mind map of the light-induced signal transduction resulting in stomatal
opening.
•
Draw a mind-map of the ABA-induced signal transduction resulting in stomatal
closure.
•
What does acclimation mean? Give a definition and examples of drought adjustments
of plants.
•
What does modification mean? Give a definition and examples of drought
adjustments of plants.
•
Describe the crassulacean acid metabolism!
•
Explain strategies employed by plants to prevent water deficit stress.
Literature Cited
•
BOHNERT H.J., NELSON, D.E., JENSEN, R.G. (1995). Adaptation to Environmental Stresses. The
Plant Cell 7: 1099-1111.
•
BROWN A.D. (1978). Compatible solutes and extreme water stress in eukaryotic microorganisms. Advances in Microbial Physiology 17: 181-242.
•
BÜSSIS D. & D. HEINEKE (1998). Acclimation of potato plants to polyethylene glycol-induced
water deficit II. Contents and subcellular distribution of organic solutes. Journal of
Experimental Botany 49: 1361-1370.
•
CHAVES, M.M., MAROCO, J.P & PEREIRA, J.S. (2003). Understanding plant responses to
drought – from genes to whole plant. Functional Plant Biology 30: 239-264.
•
CHIANG, H. & DANDEKAR A. (1995). Regulation of proline accumulation in Arabidopsis thaliana
(L.) Heynh during development and in response to desiccation. Plant Cell and Environment
18: 1280-1290.
•
CHRISTMANN, A., MOES, D., HIMMELBACH, A., YANG, Y., TANG, Y. & GRILL, E. (2006). Integration
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of Abscisic Acid Signalling into Plant Responses. Plant Biol. 8: 314 – 325. Online:
http://www.thieme-connect.com/ejournals/html_fg/plantbiology/doi/10.1055/s-2006924120/fg/F679-1).
•
FISCHER, R.A. (1971). Role of Potassium in Stomatal Opening in the Leaf of Vicia faba. Plant
Physiol. 47: 555–558.
•
HOEKSTRA F.A., GOLOVINA E.A., BUITINK J. (2001). Mechanisms of plant desiccation tolerance.
Trends in Plant Science 6: 431-438.
•
KELLER, F. & MATILE, P. (1989). Storage of sugars and mannitol in petioles of celery leaves.
New Phytologist 113, 291-299.
•
KELLER F. & M. LUDLOW (1993). Carbohydrate metabolism in drought-stressed leaves of
pigeonpea (Cajanus cajan). Journal of Experimental Botany 44: 1351-1359.
•
MOORE B., D. PALMQUIST & J. SEEMANN (1997). Influence of plant growth at high CO2
concentrations on leaf content of ribulose-1,5-bisphosphate carboxylase/oxygenase and
intracellular distribution of soluble carbohydrates in tobacco, snapdragon, and parsley. Plant
Physiology 115: 241-248.
•
POPP, M. & SMIRNOFF N. (1995). Polyol accumulation and metabolism during water deficit. In:
N. Smirnoff (ed.) Environment and Plant Metabolism: Flexibility and Acclimation, ed. N
Smirnoff, pp. 199– 215. Oxford: Bios Sci.
•
ROBINSON, S. & G. JONES (1986). Accumulation of glycinebetaine in chloroplasts provides
osmotic adjustment during salt stress. Australian Journal of Plant Physiology 13: 659-668.
•
STOOP, J., D. PHARR (1994). Mannitol metabolism in celery stressed by excess
macronutrients. Plant Physiology 106: 503-511
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Lesson 5
Stress at the Population Level: Responses to Drought and ‘Density Stress’
Stress at the Population Level: Responses to Drought and ‘Density Stress’
(Melanie Paschke & Bernhard Schmid)
Concept Map
Don’t Miss these Online-Learning Activities!
Exercise 1: Resource or Condition?
Exercise 2: Phenotypic, Genotypic or Genetic Variation?
Exercise 3: What Type of Plasticity Are You Looking at?
Topic 1: How Can an Environment Become Stressful for Plants in a
Population?
A plant population is made up by individuals of a single plant species co-occurring at a
certain site. Plants of a population share the environment at their site.
So far, we have looked at individual plants suffering from limitation or stress. Is it
possible, however, that an environment becomes limiting or stressful for an entire
population?
All organisms are in constant interaction with their environment and, at the same time,
have to maintain their internal functioning and to reproduce. For a plant, the environment
represents conditions and resources:
•
Conditions, e.g. temperature, affect the life of plants and can mean stress for a plant
(e.g. through extreme temperature as discussed in the previous lessons). Conditions
are not consumed by plants. Plants do not directly change conditions by their
presence.
•
Resources, e.g. water or soil nutrients, are consumed by the plants and thereby
reduced in the environment. They are limited and have to be constantly refilled (e.g.
water through rainfall, soil nutrients through mineralization).
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The distinction between environmental conditions and resources is important when we
look at plants in a population context.
•
Conditions can affect plant individuals independent of their number in a population.
The interactions between conditions and plants are to a large extent unilateral: high
temperatures affect plant individuals, but temperature itself is barely influenced by
the plants. In this case, the stress response of the population is the same as the
mean response of the individuals.
•
Resources, however, may become more limiting for plant individuals when they are
many rather than few in a population. In this case, stress results from the competition
between individual plants for a limited resource. Thus, the plant population generates
a potentially stressful environment by its own growth and by the consumption of
resources.
Topic 2: ‘Density Stress’ in Populations
When we look at plants as members of populations, we add a new aspect to the picture
which we did not consider in the previous lessons: in natural populations, most plant
individuals soon start to interfere with their neighbors. This interaction frequently occurs
in the form of competition: the plants may compete for light, water or other resources.
(Note: theoretically, light is an unlimited condition. In populations with high densities,
however, it can occur that, for example, the sunbeams cannot reach the young plants
since adult plants are in their way. Thus, light can be limited for part of the population.)
Figure 1: Limited Resources and Environmental Conditions Affect Plant Populations. (Note: Light
has also a resource aspect for plants, which is important in light competition within canopies.)
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The “limitedness” of the factors that the plants compete for depends largely on the
population density. The more individuals of a population consume a certain resource, the
more limited it becomes and the more elevated is the competition. Therefore, density
effects are one of the most prevalent features in plant populations. In fact, most plants in
populations are more suppressed by competition with neighbors than by the harshness
of the physical environment. Often one therefore loosely refers to “density stress”,
although we do not know if at physiological and molecular levels the plants experience
density more as limitation or as stress.
This example shows that the terminology in the field of “plant stress” is not set in stone
and that, depending on the research area, certain terms are used differently.
Topic 3: How Do Plant Populations Respond to ‘Density Stress’?
Plant individuals show the following growth responses to increasing population density:
•
reduction of branching frequency
•
increase of internode length
•
reduction of branch and leaf size, or loss of old branches and leaves
Therefore, plant individuals show an adjustment of the plant architecture that might be
interpreted as modifications. However, it is still not understood if plants “feel” the density
as stress at the level of organs, tissues, and cells or if they simply are exhausted of
resources. If the second is the case, plants would simply react with modifications to the
underlying stressor, e.g. to the limitation of water released by the competition of many
plants for this resource.
These responses at the individual plant level lead to an interesting effect at the
population level: the response to increasing density in a plant population leads to an
exact balancing between density and individual size, so that total biomass and,
sometimes, even total branch or leaf number per unit area remain constant across large
ranges of planting densities. This is called the law of constant yield. The constant yield
is an “emergent” population phenomenon, meaning it can not be assessed on single
individuals.
The constant yield can cover up subtle differences in density responses of individuals.
Thus, the reduced individual size with increased density only reflects the mean response
at the level of plant individuals. In fact, it has been found that in many plant species,
individuals vary tremendously in size especially in dense populations. This means that a
few large individuals may produce a large proportion of the individuals for the next
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Stress at the Population Level: Responses to Drought and ‘Density Stress’
generation (plant size is often strongly correlated with the ability to produce flowers and
fruits).
Obviously, not every beech seedling observed in spring on a forest floor could grow up to
the canopy — there would not even be enough space to put the tree trunks directly side
by side. Rather than all plants remaining small and thus producing a lower yield than
they would if they were allowed to grow taller, mortality of individuals leads to reduced
density and increased total biomass of the population. This process of mortality 
reduced density  increased total biomass continues until the level of constant yield for
the particular species investigated is reached. The phenomenon described here is called
self-thinning and is one of the very few general rules in plant population biology. The
mechanisms behind the phenomenon, e.g. how some plants “know” when to die are still
not understood.
Figure 2: The Laws of Constant Yield (a) and Self-Thinning (b).
Topic 4: Variation between Individuals as the Basis for Population Stress
Responses
We have seen how unfavorable environmental conditions or limited resources can affect
an entire plant population. However, some individual plants within this population may be
less affected by unfavorable environmental conditions or limited resources because they
are in a different physiological or developmental stage than others or because of
adaptive traits.
We can look at the effect of several weeks of summer drought on a plant population.
Plants individually react with several modifications to this stress, as discussed in Lesson
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Stress at the Population Level: Responses to Drought and ‘Density Stress’
4: they can build new leaves of reduced size and shape or increase leaf thickness and,
thereby, become more resistant to drought. They can delay or advance the onset and the
duration of blooming to hit a time frame when conditions are more favorable (e.g. a short
rainy period during a dry summer).
Although each plant of this population can make use of such adjustments, there is still
variation between individuals. Examples would be: even smaller, thicker, more droughtresistant leaves in one specific individual or blooming at the ideal point in time. In this
case, the drought event leads to differential damage and growth between individuals in
the population. The population response to the drought event differs from that of a
particular individual but can still be predicted from the mean individual response.
But what has changed at the population level after the drought period? Even though
some individuals may not have survived the event, the population as a whole has. There
may be fewer individuals or the ranking of individuals within the population with regard to
growth and reproduction may have changed. In the short term, this has little
consequences for the population. But due to the selective force of the stress event,
which eliminated or reduced the population contribution of individuals sensitive to
drought, the population as a whole may have increased its resistance to future similar
stress events. If the variation in stress resistance between individuals has a genetic
basis, then the progeny of these drought-resistant individuals will inherit the greater
resistance, which will increase their survival and, therefore, spread in the next
generation. The population has shifted towards an evolutionary adaptation to future
similar stress events. Eventually, evolutionary change may lead to a situation where the
event does not represent a stress anymore for the population!
In summary, evolutionary adaptation as the most long-term stress response
(beside physiological regulations, acclimations and modifications; Lesson 1) is a
population response.
There is one logical problem with the evolutionary stress response of a population being
based on its genetic variation among individuals. Each selective event reduces the
variation and, thus, the potential for further evolutionary response. Why can we normally
still find genetic variation for individual stress resistance within a population even after
selection over many generations? First of all, there are continued mutations that increase
genetic variation. However, mutation rates are usually much smaller than rates of their
elimination by selection. A second source of variation is genetic exchange between
individuals with different mutations. This so-called genetic recombination between
genetically different individuals is a strong force in maintaining variation.
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Stress at the Population Level: Responses to Drought and ‘Density Stress’
However, even with mutation and recombination, genetic variation may not be
maintained unless there is a third influence, and this is environmental heterogeneity and
trade-offs between different responses of plants to particular stresses. The individuals
that best resist a frost may not be the same as those that best resist a flooding event. If
these events and others occur from time to time, the population represents a mosaic of
subpopulations adapted to the different stresses that occurred in its evolutionary history.
The genetic variation within the population thus is a memory of the past and insurance
for the future at the same time.
Topic 5: Phenotypic Variation and Genotypic Variation
Populations of Cercis canadensis (redbud trees) at sites in Kansas (prairie: dry and
sunny) had smaller and thicker leaves than populations at sites shadowed with oak trees
(wet and shadow).
These differences did not disappear when seeds collected from the populations at the
dry sites and seeds collected from populations at the wet sites were grown in the
greenhouse under optimal water supply. Their leaf morphology was similar to that of their
ancestors at the original sites. From this result, we can conclude that we are really
looking at an evolutionary adaptation: It has led to more drought resistant genotypes in
populations at dry sites and to genotypes not resistant against drought at humid sites
(Figure 3).
However, even after this evolutionary adaptation, trees within the different populations
are able to respond by modifications to a new dry or wet situation: When we transplant
seeds from the drought resistant populations to the more humid sites, the resulting trees
increase leaf size and thickness; however, they do not reach the leaf size and thickness
of the original trees at the wet site. When we transplant seeds from the populations at
wet sites to the dry sites, the resulting trees decrease their leaf sizes, but they do not
reach the small leaf size of the drought resistant trees, either. The modifications in the
architecture of individual plants are part of a more general phenomenon called
phenotypic plasticity.
Now, we have to get familiar with some more terms:
Phenotypic variation (VP) refers to all variation among plant individuals (in e.g. leaf
size, plant size, number of flowers). If phenotypic variation between individuals has a
genetic basis, we call it genotypic variation (VG); if it has no genetic basis, we call it
environmentally-induced variation (VE). All environmentally-induced variation must
result from phenotypic plasticity: Plant individuals are able to respond to a new dry or wet
situation, for example, by adjusting the size and thickness of their leaves through
modifications.
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Stress at the Population Level: Responses to Drought and ‘Density Stress’
Figure 3: Result of an experiment with Cercis Canadensis – Genotypic differences
between populations and sites do not disappear when plants of both populations are
raised in the greenhouse. This reveals genotypic variation (=VGB) between populations.
(Data from: Abrams, M.C. (1994). Genotypic and phenotypic variation as stress
adaptation in temperate tree species: a review of several case studies. Tree Physiology
14, 833 – 842.)
Environmentally-induced variation may occur within populations (VEw) or between
populations (VEb). Similarly, genotypic variation may occur within populations (VGw) or
between populations (VGb). Within-environment phenotypic variation (VPw) is the
sum of genotypic and environmentally-induced variation within populations: VPw = VGw +
VEw. Between-environment phenotypic variation (VPb) is the sum of genotypic and
environmentally-induced variation between populations: VPb = VGb + VEb.
It is important to choose the appropriate experimental setting to assess phenotypic
plasticity, genotypic variation and environmentally-induced variation. For instance, if we
measure the leaf sizes of trees in several populations growing under different
environmental conditions, we normally find that leaves are small in dry environments and
large in wet environments. From the field observation, we do not know, however, to
which extent this between-environment phenotypic variation (VPb) is genotypic (VGb) or
environmentally induced (VEb).
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Stress at the Population Level: Responses to Drought and ‘Density Stress’
If the difference persists when plants from the different populations are raised in a
common environment, we know that there is genotypic variation between the populations
(VGb > 0). This genetic variation is most likely the result of a divergent past evolutionary
adaptation of populations, with which they adapted to environmental conditions at their
original sites. However, as we have seen in the example of Cercis canadensis, to a
certain degree plants from dry and wet environments can also react with the modification
of their leaves to changed environmental conditions: on average, plants from a wet site
build smaller leaves when raised under dry conditions and vice versa. Such phenotypic
plasticity of individuals is reflected in a positive estimate of environmentally-induced
variation between populations (VEb > 0).
Phenotypic plasticity can itself vary between genotypes and be passed on to the next
generation as an evolutionary adaptation. For example, individual plants within a
population may vary in their propensity to reduce their leaf size in response to drought
and may pass on their specific propensity to their offspring. Genotypic variation in
phenotypic plasticity leads to a new form of phenotypic variation which we have not
considered so far: variation due to genotype x environment interactions. In our formula
this is reflected as:
VP = VG + VE + VG x E
Figure 4: Environmentally-induced variation and genotypic variation within populations.
Genotypic variation (VGW) within a population is reflected as mean differences in plant
sizes between different genotypes (indicated by the colored bars in the insert figure) in a
common environment. Environmentally-caused variation within populations (VEW) is
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Stress at the Population Level: Responses to Drought and ‘Density Stress’
reflected as differences in plant sizes between the individuals of one genotype (indicated
by the black squares for each genotype).
How do scientists study phenotypic variation in stress responses?
To study variation in phenotypic stress response, we can look at the growth, survival and
reproduction of plants in the field. Scientists can use several traits, which can be
measured for single plant individuals and which show phenotypic variation (Figure 5).
Figure 5: Scientists can measure several traits in plants.
Of course, the chosen trait does not need to be measured in all plants of a population —
it is appropriate to measure it in a sufficiently large and representative sample of
individuals.
The problem with genotypic variation (as part of phenotypic variation) is that we cannot
measure it directly in the field, unless we know the genes themselves and have an easy
way to assess their presence in an individual. In the case of the sedge Carex flava var.
alpina, we could, for example, expect to find genes for increased frost resistance.
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Stress at the Population Level: Responses to Drought and ‘Density Stress’
However, nobody has looked for them, which is not surprising considering the low usevalue assigned to this species. Without knowing the genes for the phenotypic
differences, we need measurements of individual responses in differently stressful
environments to reveal amounts of environmentally-induced variation and genotypic
variation. Therefore, it is important to choose the appropriate experimental setting to
distinguish different causal components of total phenotypic variation:
•
Total phenotypic variation (VP) is the variation in e.g. leaf size (or other traits)
measured between individual plants regardless of their genotype.
•
To measure genotypic variation between populations (VGb), we need to grow
offspring of genotypes from different populations in a common environment.
For example, individuals from different altitudes often differ in plant height. In the
sedge Carex flava, this has even led to the distinction of a variety alpina. It was found
that the plants from high-altitude populations were generally still smaller than plants
from low-altitude populations after growing for a sufficiently long time at low altitudes
to acclimate and respond to the new environment by phenotypic plasticity. Thus,
some of the size differences between the populations must have had a genetic basis.
•
For the population to show an evolutionary response to stress, there should be
genotypic variation within populations (VGw). To measure this component of
within-population phenotypic variation, we have to grow offspring of different
genotypes from one population in a common environment. Replicate offspring of
each genotype need to be grown to differentiate between non-genetic (i.e.
environmentally-induced) variation that can already exist within genotypes and
genetic variation between genotypes (i.e. genotypic variation): regarding the
measured trait, two offspring of two different genotypes should on average differ
more from each other than two offspring of a single genotype.
•
To measure phenotypic plasticity, we need to grow offspring of single
genotypes from one population across a range. Several researchers have
transplanted plants from a single population to different drought conditions and
generally found reduced individual growth and size with increasing drought, which
could be interpreted as phenotypic plasticity in response to drought stress. Such
gradient experiments usually carried out in the experimental garden or the
glasshouse, allow us to measure phenotypic variation of individual offspring of single
genotypes as a function of known environmental variables. This function is called the
reaction norm of a single genotype. The reaction norm is synonymous with its
phenotypic plasticity.
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Stress at the Population Level: Responses to Drought and ‘Density Stress’
The reaction norm of a population is the sum of reaction norms of single genotypes
within the population.
•
Now, the amount of environmentally-caused variation between (VEb) or within (VEw)
populations is determined by subtracting the genotypic variation between or within
populations from the total phenotypic variation (VP is the sum of all reaction norms of
genotypes within or between populations): VE = VP – VG
Topic 6: Variation Is a Population Phenomenon
The response of a plant population to stress occurs gradually:
•
Variation between individuals is the basis for the stress response of a population.
Depending on the nature of this variation in a population, different stress responses
at the population level are possible.
•
Low levels of stress may be absorbed by phenotypic plasticity of individuals. If
stress levels get higher and reach beyond the reaction norm of some individuals,
these individuals will die.
•
However, genotypic variation between individuals means that their reaction norms
differ and thus progenies of some genotypes will still survive within a population
under higher levels of stress, leading to genetic change in the population and to
evolution.
•
With even higher stress levels, populations may be replaced by others of the same
species with a genotypic variation better adapted to a certain stress level and,
eventually, if the stress is so strong that it surpasses the potential of the species to
adjust, the species may be replaced by another one. This results in a community
change (Lesson 6).
Note: high density leads to a gradual response as well:
•
At moderately high densities, populations respond with the ‘law of constant yield’, i.e.
density “stress” is absorbed by size plasticity of plant individuals.
•
At very high densities, populations respond with self-thinning, i.e. density “stress”
becomes so severe that some individuals die while others benefit from the
subsequent mortality-caused density reduction and grow bigger. We do not
understand the mechanisms behind the phenomenon, e.g. how some plants “know”
when to die and if they experience density as limitation or stress at the level of
organs, tissues and cells. We also do not know how self-thinning affects genotypic
variation in plant populations.
Phenotypic, genotypic and environmentally-induced variation between individuals
are true population phenomena which can only be expressed if there is more than one
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individual. More importantly, higher levels of variation rather occur with many than with
few individuals. Thus, in addition to the average difference between individuals, their
number is a very important factor for maintaining variation, especially genotypic variation.
This means that the conservation of variation and genetic diversity within populations
and species requires a sufficiently large number of individuals, in plants as a very crude
rule probably > 1000
Genetic variation between populations is a phenomenon of a group of populations
because it is preceded by differential responses of entire plant populations to differential
selection pressures (e.g. small leaves in one population as an evolutionary adaptation to
drought and large ones in another population as an evolutionary adaptation to more
humid conditions) and because it can only be measured if there is more than one
population.
For future research, it is important to understand that we cannot predict the integrated
stress response of a population by looking at isolated or average individual responses.
In conclusion, it can be said that genotypic variation within and between populations is
vital for the persistence of plant species, in particular because it works as an insurance
against environmental stress. Conservation of plant species in general requires the
protection of entire populations with numerous different individuals rather than of a few
representative individuals.
Summary
Variation between individuals is the basis for population responses to stress and other
factors. Depending on the nature of this variation in a population, different responses at
the population level are possible. Low levels of stress may be absorbed by phenotypic
plasticity of individuals whereas higher levels of stress may be absorbed by genotypic
variation between individuals, leading to genetic change and evolutionary adaptation of
the population. In summary, evolutionary adaptation as the most long-term stress
response is a population response.
With even higher stress levels, populations may be replaced by others of the same
species that are better evolutionary adapted and, eventually, if the stress is so strong
that it surpasses the potential of the species to adapt, the species may be replaced by
another one.
Variation between individuals is a true population phenomenon which can only be
expressed if there is more than one individual. More importantly, higher levels of
variation rather occur with many than with few individuals. Thus, in addition to the
average difference between individuals, their number is a very important factor for
maintaining variation, especially genotypic variation. This means that the conservation of
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variation and genetic diversity within populations and species requires a sufficient
number of individuals, in plants as a very crude rule probably > 1000.
Now, you should be able to answer the following questions:
•
What are typical density effects in plant populations?
•
How can a plant population respond to stress?
•
What is plasticity and how can it be measured?
•
What is the difference between environmentally-induced variation between and within
populations?
Literature Cited
•
ABRAMS, M. (1994). Genotypic and phenotypic variation as stress adaptation in temperate tree
species: a review of several case studies. Tree Physiology 14, 833 – 842.)
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Plant Stress at the Community Level: Responses to Biotic Factors Causing Plant Stress
Plant Stress at the Community Level: Responses to Biotic Factors Causing
Plant Stress (Christine B. Müller & Melanie Paschke)
Concept Map
Don’t Miss these Online-Learning Activities!
•
Exercise 1: What Type of Response Are You Looking at? (Topic 2)
•
Exercise 2: At Which Level Does the Response Happen? (Topic 2)
•
Exercise 3: Is It a Positive or Negative Interaction? (Topic 5)
•
Exercise 4: What Type of Indirect Interaction Are You Looking at? (Topic 5)
•
Exercise 5: What is the Color of the Aquatic Microcosm? (Topic 5)
Topic 1: Stress and the Community Background
What can you do if you are rooted in the ground and fixed to one locality and you
experience adverse conditions or stress? You may think not much can be done because
you cannot run away. However, plants are equipped with an amazing array of possible
reactions to stress and limitation that allow them to withstand these perturbations.
Plants can tolerate a wide range of adverse conditions without actually being much
affected (more on tolerance, Lesson 1):
•
They can simply shut down their metabolism and withstand the adverse conditions
(as temperate plants do during winter).
•
Grazers (like cows or sheep or even lawn mowers) may even induce compensatory
growth of above ground tissue.
Plants can ‘escape’ in time (more on escaping, Lesson 1):
•
They can re-allocate resources in response to stress, such as perennial plants do.
They are able to re-allocate their resources into flowering in the subsequent year
when heavy pest outbreaks occur that destroy all above ground tissue (Senecio
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jacobaea attacked by caterpillars of the cinnabar moth Tyria jacobaeae; Islam &
Crawley 1983).
•
They can produce resting stages (seeds or bulbs) and await better conditions,
•
They can do so for a very long time (some seeds remain in the seed bank for many
years).
Plants can resist stress (more on resisting, Lesson 1):
Similar to animals they can mobilize defenses against biotic adversaries
•
by building new morphologies in response to stressors (e.g. trichomes, specialized
epidermal cells to protect the plant mechanically against pathogens; details in Topic
2)
•
by synthesis of their own secondary toxic compounds
•
by entering coalitions with animals (for example ants) or microbial symbionts that
help them in their defense against stressors
You already learned that there are two general kinds or stressors for plants, abiotic and
biotic ones. At the community level, stressful conditions act on individuals, populations,
plant species, and the community as a whole. The individual will respond through
physiological regulations or acclimations and modifications to the stressful condition on
spatial scales that include cellular responses as well as responses that function at the
whole-plant level (Lesson 1-4), but also through biotic interactions with other species
(plant-plant and plant-animal interactions, see Topic 2-8 of this lesson) including the
community as a whole into the response.
Especially the biotic interactions can be strong determinants of the structure of a natural
community and they can determine whether a particular plant species is able to be part
of this community. Why is that so?
A general principle in ecology is that organisms have to pass through certain filters and
fulfill certain criteria to get established in a given community. This will define their
fundamental (i.e. parameter space of abiotic factors in which organism can grow and
reproduce) and realized niches (i.e. parameter space of biotic interactions in which the
organism can persist and population growth occurs).
The filters function hierarchically:
(1) The plant species has to fulfill certain criteria to be part of the regional pool of
species. These are evolutionary and historical experiences and physiological constraints
that enable the organism to live in a certain geographic area.
(2) The plant species has to have the ability of dispersal to get from the regional pool
into a local community (e.g. via seeds).
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(3) The abiotic filter (specific environmental condition at a locality) and biotic
interactions with other plant or animal species will now determine whether the arrival of
the plant species at a locality results in the establishment of a population of the
species within or the exclusion from the local community. The community
background and the resulting interactions the plant experiences with the other members
determine the realized niche of this plant species and, ultimately, where it can live, grow,
reproduce, and expand its population.
Let’s start by looking at one plant species, perhaps an Oxeye-Daisy (Leucathemum
vulgare) that lives in calcareous grassland. Of course, no plant individual of a population
lives isolated but is surrounded by other organisms (other plants, animals, and
microbes). They will have negative, positive, or no effects on Daisy.
Can you imagine components of this community that could have stressful effects
on Daisy?
Daisy shares the ground with many other species and is well adapted to compete with
those for the essential resources, such as different nutrients, light, water and so one.
However, apart from competing with other plants for resources, Daisy will also
experience effects from herbivores (from the top of the food chain, or top-down) and soil
organisms, such as decomposers and pathogens (from the bottom, or bottom-up). Even
though Daisy is well adapted to its environment as its population has evolved over long
periods of time to live at nutrient-poor sites, there may still be changes in the community
that could stressfully affect Daisy’s life. For example, the identity of the neighboring plant
may matter. It could be a clover that, with the help of nitrogen-fixing bacteria, enriches
the soil with nitrogen. In contrast, Daisy may be unlucky because it grows in the
neighborhood of another daisy, with which it has to compete for the exact same
nutrients. Daisy may be attacked by a caterpillar, against which there is no natural
enemy available to call for help (Topic 7). Daisy may be neglected by the right pollinator
because the neighboring plants are more attractive, offering more and sweeter nectar to
the animal visitors; as a consequence, Daisy’s seed set may be poor. The above
description simplifies diverse effects for individuals, but many of these effects will
manifest at the population (Lesson 5) and species level and thus ultimately determine
which species are components of a given community and which are not. If we want to
understand how communities are structured, we need to understand how populations or
species of plants react to biotic stressors.
It is important to acknowledge that not all of these effects of biotic interactions are
equally strong; some might even be unimportant but some may cause stress to Daisy
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plants. Once we picture the full community context around our target plant, we will be
able to understand the mechanisms at play.
Topic 2: Plants React to Biotic Factors Causing Plant Stress - The
Response of the Plant Individual
•
Abiotic factors causing plant stress by water shortage, droughts, salinity, flooding,
temperature, CO2, nutrients, light (radiation and shade) will elicit responses that are
ultimately expressed as effects on growth, survival, reproduction, and, in some
cases, reallocation of resources in the individual plant. We have discussed these
responses in the previous lessons.
•
Here, we will concentrate on biotic factors causing stress or disturbance. In Lesson 1,
you have learnt that biotic factors, e.g. herbivory or pathogens, can result in stress or
disturbance or in both. A plant can react to stress but not to disturbance because it
involves the physical destruction of the organism. For instance, a caterpillar will at
first only destroy some leaves and thus presents a stress to the plant. The plant can
react to this stressor. When many caterpillars devour the plant, however, the stress
has become a disturbance and the plant is not able to respond to it anymore. In this
chapter, we will look at biotic factors at the stress and not at the disturbance level.
•
Biotic factors causing plant stress, like pathogen or herbivore attack, are normally
lasting stressors – to fight them, the plants can use several types of responses
(acclimations, irreversible modifications, and evolutionary adaptations passed on to
the next generation) at all levels (cellular, whole-plant, population and community
level).
•
To respond to the biotic factors causing plant stress, the plants have to adjust their
metabolism and reduce other “normal” metabolic pathways even at the cost of
growth. Therefore, biotic factors causing plant stress like herbivores and pathogens
will be expressed as effects on growth, survival, reproduction, and reallocation of
resources in the individual.
•
Let us first discuss some of the possible types of responses after biotic factors
causing plant stress:

Examples of acclimations: Synthesis and location of secondary toxic
components in response to pathogens or herbivores or the release of odors
(=volatiles) to call for the help of the natural enemy of the herbivore – this
response starts minutes after the attack and induces a response even in
neighboring leaves which were not attacked. The effect of the response
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outlasts the attack so that the plant is protected against future attacks of the
herbivore (=resistance, see Topic 7)

Examples of modifications: Plants build new morphologies like trichomes in
response to stressors. Trichomes are specialized epidermal cells which
develop early during ontogeny of a leaf – their development is irreversible. It
has been shown for several plant species that, under continuous pathogen or
herbivore attack, newly built leaves may show a higher trichome density – an
induced response to the biotic factors causing plant stress (Traw & Bergelson
and references therein, 2003). Trichomes act as an additional physical barrier
of the cell wall against attack of small pathogens or herbivores (e.g. making
attachment difficult). They can also improve the defense of plants against
fungal infection. Enhanced trapping of fungal spores, thanks to the trichomes,
might be the cause for the improved defense.

Examples of evolutionary adaptations: Plants evolve countermeasures –
evolutionary arms-races can be expected for interaction between plants and
herbivores or pathogens, while co-evolution is expected for interactions
between plants and symbionts or mutualists. Such processes are dynamic and
we cannot expect to find strict categories of responses but a whole array of
responses and counter-responses. (This is the essence of biotic interactions
and life in general, creating endless variation.)

The response of the plant to biotic factors causing plant stress, e.g. to
herbivory, starts at the cellular level (e.g. through the release of secondary
toxic components). The cellular response can induce a response at the wholeplant level (even not attacked parts of the plant become resistant).
Furthermore, biotic factors causing plant stress can function as a so-called
systemic signal: Neighboring plants and interacting organisms (e.g. enemies of
the herbivore) – thus, the entire community – are included in the response by
the attacked plant.
Let us now categorize the types of defenses:
Direct defenses are any plant traits (e.g., thorns, silica, trichomes, leathery leaves,
primary and secondary metabolites) that, by themselves, affect the susceptibility to
and/or the performance of attacking enemies and thus increase plant fitness in
environments with herbivores. These defenses can be constitutional (=always ready) or
induced (=mobilized when the stressor is present). Such direct defenses are:
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Physical barriers consist of thorns, spines, hairs, trichomes or tough leaves that
contain high amounts of lignin. Hairs and trichomes are found in combination with
toxic substances which they release when they break.
•
Toxins and feeding deterrents are secondary compounds synthesized by plants
that interfere with the insects’ physiology or behavior. There are endless
combinations of organic molecules that are built from carbon, oxygen, nitrogen and
sulfur and there can be synergistic effects when different toxins are mixed. The
delivery of the toxin is mostly passive with insects exposing themselves when
ingesting the plant tissue. Such chemical defenses can be induced responses after
damage of plant parts (either physical or by insect feeding).
•
Abscission defense (dropping of attacked leaves) is costly for the plant but very
effective against immobile, stationary feeders such as leafminers.
Indirect defenses are plant traits that attract predators and parasitoids of herbivores.
These types of defenses are normally induced after an attack of the predator – they
represent acquired defenses. This means that a mutualistic symbiont helps the plant to
defend itself:
•
Feeding and even oviposition by insects can induce the release of volatiles that are
used by parasitoids or predators to find their host or prey (details in Topic 7).
•
Plants can attract mutualistic symbionts, such as aggressive ants, that will keep
their host plant free of herbivores. Mutualistic symbionts get food (extrafloral
nectaries) and nesting sites (domatia) from the plants in exchange for their services
in herbivore defense. Plants also use microbial associates that live as
endosymbionts within the plant tissue and synthesize the toxic defense compounds
for the plant. For instance, temperate grasses do not synthesize secondary plant
compounds but rely on fungal endophytes for the production of defensive alkaloids.
Like the microbial symbionts in our intestines, such endophytes have also been
shown to protect the host plant against pathogen and herbivore attack (Figure 1).
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Figure 1: Plants use fungal endophytes to synthesize defense against pathogens: (first row) Natural colonisation of leaves by a diverse endophyte community (second row) - In leaves
inoculated with endophytes, pathogen attack resulted in little occupation of leaf cells. (third row) Pathogens became well established in endophyte-free control leaves. Modified after Clay (2004).
The following examples of plant responses and possible counter-responses of the
stressors illustrate the wide range of responses and the fantastic repertoire of defenses
that plants have evolved.
Plant – Herbivore
Insects are the most innovative herbivores with an enormous evolutionary potential to
exploit all kinds of plant species and to invent countermeasures against plant defenses.
Herbivorous insects exploit all parts of plants: leaves, stems, roots, seeds, fruits,
meristematic tissue. They occur in the form of chewers, suckers, gallers and miners.
They are skilled engineers and have the potential to reprogram genetic processes of the
plant. A spectacular example of this is the induction of gall formation by galling insects
(Figure 2). There is an estimated number of eight millions insect species, a major part of
which are herbivorous insects. Clearly, against such an army of attackers, there cannot
be only one defense. Plants can make use of several defenses against herbivores, some
of which are always present (constitutional defenses) and others are induced after
herbivore attack (induced defenses). It is important to remember that any kind of
defense will be costly to the plant as it requires resources that cannot be used otherwise.
Therefore, defensive plant traits are only useful if they (a) affect the degree by which a
plant is attacked and (b) increase the plant’s fitness in comparison with plants lacking the
defense or having lower degrees of defense.
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Plant – Pathogen
Plant pathogens are microbes (viruses, bacteria, and fungi) that live parasitically on plant
tissue and depend on the nutrients and shelter provided by the host plant. As parasites,
they do not return anything in exchange for shelter and food, but may cause
considerable damage to the host plant. Most pathogens are highly virulent and destroy
their host plant quickly. Countermeasures against pathogens are leaf abscission, local
ring-fencing of infected sites on leaves, toxins, and microbial or fungal endophytes. Many
pathogens are dispersed by wind but some need an insect vector (often aphids) to infect
new plants.
Reproduced with permission from: Stone and Schönrogge (2003). Photographs reproduced with
permission from: G. Csoka, J. Cook, J.P Koplke, U. Koruso, L. Mound.
Figure 2: The differentially shaped galls that are induced by gall insects are reflections of insects
taking control over the plant’s genes. The structures are created by the plants but each gall insect
species induces very specific shapes.
(a) Cross-section of leaf roll gall; (b,c) Leaf roll galls induced by sawfly (Euura weiffenbachii) on
willow and gall midge (Contarinia subulifex) on oak.
(d) Cross-section of pouch gall; Pouch galls induced by aphids: (e) Pemphigus borealis (note the
gall aperture, A); (f) P. spirothecae, both on poplar; (g) Astegopteryx styracophila and
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(h) Tuberaphis sumatrana, both on Styrax in Sumatra;
(i) and by thrips (Oncothrips rodwayi) on Acacia melanoxylon ; (j) opened woody stem gall
induced by thrips (Iotatubothrips sp.) on Acacia cunninghamiana, showing colony members;
(k) Cross-section of gall induced by acacia thrips (Oncothrips sterni), showing internal lamellae to
increase surface area for feeding; (l) section of wholly enclosed gall.
(m) Enclosed galls induced by gall midges on hickory; (n) (Mikiola fagi) on beech.
(o) Cryptic enclosed galls (arrows) induced inside developing fig fruit (Ficus rubiginosa).
(p) Enclosed sawfly gall (Pontania hastatae) on willow.
(q) Enclosed cynipid gallwasp gall (Diplolepis rosarum) on Rosa.
Topic 3: In a Community Context, Abiotic Stress Can Have Secondary
Effects on Biotic Interactions
You have learnt about the responses of individual plants or populations to abiotic
limitation or stress (e.g. oxidative stress, temperature, drought) in the previous lessons.
In the community context, we are interested in another question: How can abiotic factors
causing plant stress influence the biotic interactions between species within a
community? The abiotic stress of a plant can have several secondary effects on its
interactions with other organisms. Therefore, in a community context, a beforehand
abiotic stress is often transformed into biotic factors causing plant stress (a disconnected
or altered interaction between stressed plant and interacting organisms). The following
examples illustrate this phenomenon:
•
Abiotic factors causing plant stress, such as exposure of a plant to enhanced UV-B
radiation, can become stressful for organisms at the next trophic level, where the
consequences represent biotic factors causing plant stress. In particular, such
increased UV-B radiation can trigger the production of secondary metabolites, such
as phenolics, which are precursors for defensive chemicals. This has been shown for
black cottonwood (Populus trichocarpa) grown for three months under twice the
ambient UV-B radiation. Although the effects of this increased radiation were minimal
for photosynthesis, growth, and leave biomass of the plants, the concentration of
foliar nitrogen, chlorophylls, and several foliar phenolics increased under enhanced
UV-B radiation. Adults cottonwood leaf beetles (Chrysomela scripta) – a specialized
herbivore on cottonweed preferred leaf tissue exposed to enhanced radiation. We do
not know in detail why the beetles prefer leaves grown under enhanced UV-B,
however, other observations have shown that these specialized beetles seem to
prefer high concentrations of phenolics in the leaves of their host plant. In contrast to
the positive effect of UV-B radiation on the adult beetles, the growth of larvae feeding
on such exposed leaves was lower than the growth of larvae fed with non UV-B
leaves. For the larvae, the enhanced phenolic concentrations seem to have a
negative effect (Figure 3a, Warren, Bassmann & Eigenbrode 2002).
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Figure 3a: Example of how abiotic enhanced UV-B can influence the interactions between
Cottonwood and herbivore feeding on the tree: Growth of beetle larvae feeding on cottonwood
leaves exposed beforehand to enhanced UV-B is reduced compared to larvae feeding on
leaves resulting from normal radiation. However, adult beetles prefer the leaves under
enhanced UV-B levels.
Note, we discuss enhanced UV-B in this example as a stressor for plants, although
the experiment showed only minimal negative effects of enhanced UV-B on the plant,
therefore giving rise to the question if enhanced UV-B is really a stressor and not only
a limitation (Lesson 1).
•
Physical clipping of grasses that harbor endophytic symbionts can increase alkaloid
levels in the plant tissue (although a disturbance of the plant, see Lesson 1, the
clipping is interpreted by the plants and their endophytes as an herbivore attack, i.e.
an attack of a stressor – resulting in an enhanced production of alkaloids through the
endophytes). In turn, a higher defense capacity of the plant against herbivores
results. In long-term experiments, it has been shown that plants with a higher
defense capacity against herbivores have an advantage over other plant species
(because they are no longer attacked by herbivores) and are out-competing other
plant species in this indirect way as a consequence. A change in the community
structure results (Müller & Kraus 2005; Figure 3b, 3c).
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Figure 3b: Example of How Disturbance Can Effect Biotic Interactions: (I) Clipping enhances
resistance of grasses against herbivore (resistant grass shown in red). (II) Giving the species the
possibility to outcompete other species results in community changes.
Figure 3c: After experimental clipping, the resistant grass has a negative indirect effect on the
other species (right part of figure: red, broken line). The grass had no such effect before clipping
(left part of figure).
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Topic 4: Neighbors Can Help to Mitigate Stress
Plants not only pass on abiotic factors or biotic factors causing plant stress to their
neighbors, but interacting species within a community may help to mitigate stress.
Symbionts help to meliorate the effects of stressful abiotic factors and biotic hazards:
•
Mycorrhiza is perhaps the best known fungal root symbiont of plants that help
mitigate the effects of stressful conditions. Many woody plants experience increased
drought resistance when their roots are colonised by mycorrhiza.
•
In similar ways, endophyte infected grasses can tolerate higher drought levels and
show increased re-growth after drought compared to uninfected plants. Here, the
mechanisms are less clear since the endosymbiont does not colonize the roots.
Nevertheless, it has been observed that drought exposed plants produce more root
biomass. Endophyte-infected plants have lower leaf conductance, changes in
hormonal signals, higher water-use efficiency, greater capacity for osmotic
adjustment and turgor maintenance in leaves, and accumulation of polyhydroxyl
alcohols or the amino acid praline. It is hypothesized that a combination of these
various physiological adjustments causes increased drought resistance (Example
after Davies et al., 1996).
•
An interesting effect of fungal symbionts against a biotic factor causing plant stress
(plant pathogens) is found in cacao trees (Theobroma cacao). The diversity of fungi
in cacao leaves is enormous. These different fungi take almost all the space in the
leave tissue of older leaves. As a consequence, fungal pathogens have a hard time
to infect such leaves (Arnold et al. 2003). The provision of such protection can be
disrupted by the use of fungicides, which paradoxically makes trees more susceptible
to infections by pathogenic fungis.
Topic 5: Indirect Interactions
Biotic interactions among species within a web can act directly or indirectly.
What is an indirect effect? Think of the clipping experiment in Topic 3, where herbivore
attack is simulated. There, the herbivore-resistant grass outcompeted several other
species via shared herbivores: Herbivores did not attack the grass but other plant
species in the community. We can say that the herbivore-resistant grass has a negative
indirect effect on the other plant species through interactions with herbivores.
Indirect interactions are biotic interactions that involve at least three species,
while direct effects occur between two species. Indirect effects can only be observed
in a community context. Indirect interactions are intriguing because they often result in
unpredictable effects.
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In Figure 4, you see the direct and indirect interactions, which can occur among different
species in a community. They can be responsible for the structure of communities as we
observe them:
•
Competition (Figure 4a) occurs when two species affect each other’s densities
directly or indirectly. It may involve interference (for example aggressive interception
of space or light) or it can be mediated through the exploitation of limiting resources.
In the latter case of competition, the indirect effects occur because resources are
finite and are used up by neighboring consumers (Figure 4f).
•
Predation (Figure 4b) depicts the direct effect of one predator species on a prey
species: The predator gains a meal (positive effect) while the prey loses its life or is
damaged (negative effect).
•
Mutualism (Figure 4c) occurs when two species interact and both have a net gain
from the interaction. For example, a plant gets pollen transferred and can produce
fruits and seed, while the bee gets a meal in the form of nectar or pollen.
•
Amensalism (Figure 4d) describes two species interacting, whereby one species
suffers a negative effect while the other experiences no effect. For example, a
seedling has landed in the shade of a mature tree. The tree does not get an effect by
this but the seedling will perish in the shade.
•
Commensalism (Figure 4e) is the direct interaction between two species with one
species experiencing a positive effect and the other no net effect. For example, the
same situation as above but now the seedling needs shade to survive because it
would perish in full sun. Thus, it can grow while the shade tree is probably not
affected by the presence of a small seedling (this might change as the seedling
becomes a tree).
•
Apparent Competition (Figure 4g) depicts a situation where two prey species share
a predator and therefore interact indirectly. This occurs because the predator may
aggregate on the larger prey density when two species are on offer simultaneously or
because the predator increases its population size on two alternative prey species.
While the predator gains from prey (see predation), the two prey species have not
only the negative effect of the predator but also an additional negative effect from the
presence of an alternative prey. They may suffer doubly: directly and indirectly.
Apparent competition is a dynamic process that could end up with the exclusion of a
species from a community. Note that apparent competition is the mirror image of
resource competition. For example, invasive and native plants may interact through
shared generalist herbivores.
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Resource Competition (Figure 4f) depicts a situation where two plant species
compete for the same resource.
Figure 4: Direct and Indirect Interactions among Species. Negative effects are depicted in red,
positive effects in blue, and neutral effects in black. Direct effects are always solid lines, while
indirect effects are broken lines. R stands for resource species, C for consumer species, P for
primary producers, and H for herbivores. Note that resource species do not necessarily need to
be (but can be) plants as we move up the trophic chain. The direct effects (a-e) can occur at any
trophic level. Modified after Morin (1999).
Trophic Cascades (Figure 4h) depict indirect effects along food chains. Alternative
trophic levels experience positive indirect effects from the direct negative effects of
predation between adjacent trophic levels. The transmission of these indirect effects can
occur from the top down or from the bottom up. For example, blinking microcosms or
kelp forests.
•
Facilitation or Indirect Mutualism (Figure 4i) are positive indirect effects resulting
from complex interactions among several species whereby the ones on the receiving
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end experience positive net effects from the presence of other species with which
they do not directly interact. (Figure 5).
Figure 5: The most important pollinators of Trochetia are day geckos. The plant is also visited by
wasps, bees and birds. Pandanus and Trochetia interact indirectly via shared pollinators: The
reproductive success of Trochetia is increased when Pandanus grows closeby. This is caused by
a positive indirect effect of Pandanus on Trochetia. The indirect interaction is mediated by a
pollinating gecko that uses Pandanus as home and feeds on the nectar of Trochetia. As a
consequence gecko densities are increased around Pandanus. The graph from an exclusion
experiment (access of geckos to Trochetia plants prevented) demonstrate that fruit set of
Trochetia is increased near Pandanus and the increase is caused by the presence of the gecko
(Hansen et al. 2007).
Whether such biotic interactions are strong enough to cause stress for one of the
involved plant species and how they influence the organization of the community is
always subject to experiments. It is also important to understand that indirect effects are
not necessarily resource-mediated but could also be trait-mediated. The difference is that
in the first case, resources are changed as a consequence of changes in densities of
consumers, whilst in the second case, no changes in resources are required as the
effects are mediated by behavioral changes of consumers.
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Topic 6: Plants Live Embedded in Food Webs
We best describe the network of direct and indirect interactions between the species of a
community graphically in form of a web in which nodes are the species and edges are
the interactions between species. In this topic, we will discuss some more theory behind
ecological webs and will see why we need webs to investigate questions on stress
responses at a community level.
The simplest web is a food web that divides the community into separate trophic levels,
depicting who eats whom and what is the trophic position of each species. The food web
can be a:
•
Community Web (Figure 6a)
•
Source Web (Figure 6b),
•
Sink Web (Figure 6c)
Furthermore, there can be different degrees of quantification:
•
Connectance webs are qualitative, depicting all links between species with edges,
but without considering species abundance (Figure 7a).
•
Semi-quantitative webs depict the relative importance among species assemblages
feeding on particular components (Figure 7b).
•
Fully quantitative webs are fully quantified with all abundances and the strengths of
all interactions given (Figure 7c). The fully quantified web results in a very precise
cartoon of interactions where we distinguish between strong and weak links (edges)
among all species (nodes), and we get an idea, which species are rare and which are
common.
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Figure 6: Three Categories of Webs: (a) Community Web, (b) Source Web, (c) Sink Web
Figure 7: Differentially Quantified Food Webs: The species are depicted by dots or, when the
abundance is quantified, by boxes; the interaction or link between species is depicted by lines or,
when the abundance is quantified, by wedges, whereby the size of the wedge indicates the
strength of the interaction (a and b after Memmott and Godfray 1994, c after Müller et al 1999).
Think of the example of the herbivore-resistant grass in a community of several plant
species (Topic 3). What type of web do we need to take into account to be able to
answer the relevant questions?
For example, if we want to know which plant species are outcompeted by the indirect
effect of the herbivore-resistant grass, we can use a connectance web, showing all links
between herbivores and the different plant species of the community. We can assume
that there may be a negative indirect interaction between the herbivore-resistant grass
and all plants that share the same herbivores.
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However, with this type of web, we cannot decide how strong the negative indirect effect
will be. Perhaps, the herbivore-resistant grass may have a more negative effect on plant
species 1 than on plant species 2. To investigate this question, we need a fully
quantitative web.
Topic 7: Plants Can ‘Call’ for Help
Chemical cues are generally important for the location, evaluation, and utilization of
plants by invertebrate herbivores. Furthermore, herbivory often induces a variety of
chemical changes in plants, and it is reasonable that herbivores and their natural
enemies use this information to make foraging decisions. If we think in a food web
context, it becomes clear that indirect interactions may occur across trophic levels (i.e.
trophic cascades; see Figure 4h). This means that plants at the basis of a food chain
may communicate with natural enemies of their own stressors, attract them, and benefit
from their help in herbivore defense. Physiological changes in plants after damage or
stress are examples of induced responses, meaning that the synthesis, and thus the
costs of producing compounds, is not always mobilized but only when required.
Communication between plants and animals (predatory and parasitic insects) is achieved
by the release of a blend of odors (=volatiles), some components of which have
attracting effects equal to pheromones released by males to attract females. This blend
of volatiles attracts “friends” (natural enemies of the attackers) that help the plants to get
rid of the herbivorous enemies, effectively increasing plant fitness through increased
herbivore mortality. Interestingly, not only the attacked leaf releases the volatile, but also
unattacked leaves of the attacked plant. This indicates that the response is systemically
induced and expressed by the whole plant, even if only one leaf is damaged. The signals
start minutes after the herbivore attack and the response outlast it – so the plant is
protected against further herbivore attack. The plant has become resistant. Pinus
sylvestris, for example, emits such a signal in response to oviposition of pine sawfly
(Diprion pini). The signal attracts the parasitoid Chrysonotomyia ruforum and is emitted
by both attacked and not attacked needles. This preventive defense strategy protects the
plant against further damage by sawflies. (Hilker et al. 2002)
It appears that releases of volatiles are not single ‘freak’ cases but occur in a variety of
systems involving invertebrate herbivores and their natural enemies. This is perhaps not
surprising because much of insect biology is governed by a world of odor. There are
various plant-herbivore systems where such attraction of natural enemies by plant
volatiles after herbivore damage has been studied – we discuss only one system in
detail:
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Aphidius ervi, an aphid parasitoid, orientates towards the aphid-plant complex in a
wind tunnel and can further distinguish between bean plants (Vicia faba) damaged by
the pea aphid (Acyrthosiphon pisum), its host, and the black-bean aphid (Aphid
fabae), its non-host (Du et al. 1998).
•
Such signals between plants and natural enemies can be exploited by other
herbivores and by other plants – giving rise to a lot of opportunities for co-evolution
within the system.
•
For example, neighboring Lima beans beside a plant infected with spider mites
(Tetranychus urticae) will also emit volatiles that attract predatory mites (Phytoseiulus
persimilis). The take-up of this signal by a neighboring uninfested plant makes sense
as this plant has a high risk of becoming infested (Dicke & Dijkman 2001).
•
Another example is that the aphid Rhopalosiphon maidis which attacks corn (Zea
mays) is repulsed by volatile emissions of caterpillar infected plants. This may help
the aphid to find new undamaged plants (Müller & Goodfray 1999).
In conclusion, all these examples show a higher degree of sophistication than we might
have expected from plants. To fully appreciate what plants can do when stressed by
herbivores, we need to dive deeply into the olfactory world of chemical ecology.
Topic 8: Invasions: Plants in Foreign Terrain
Up to now, we have discussed stress in the community context from the point of view of
plant species: Within a community, plant species can experience or exert stress through
the biotic interactions with other species, but interaction with other species can also
avoid stress or limitation. However, can the community as a whole also get
stressed?
In Lesson 1, you already learnt that communities can never be stressed. A plant
species may suffer from stress exposure, which may result in its exclusion from the
community (e.g. through apparent competition or resource competition, Topic 5).
However, loss of species in a community will allow other species to fill in the free niches,
and a new community, at the loss of the old one, will establish.
We will now discuss the example of exotic invasions to show how introduction of a new
and invasive species into a community may result in the replacement of the old
community. Invasions are one of the largest threats to natural communities because they
alter the “natural balance” of existing communities. They occur at an ever increasing rate
across the globe and cause strong disturbance to co-evolved natural systems, often with
subsequent species extinctions, and possibly loss of many endemic species of a
community.
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It is important to remember that not all organisms arriving in new locations are actually
successful and invade the community. A rough rule is the ‘ten percent rule’ (Williamson
1996): 10% of translocated species survive and establish in a new environment; of these,
10% build up growing populations and spread; and of these spreading species, 10%
become pests and cause stress for the resident communities. Although it is only a small
fraction of introduced organisms that become pest species, these can cause immense
stress for resident plant species. There are the following exceptions from the ‘ten percent
rule’: Edible crop plants, biological control insects, and mammals on islands. They have
often reached 100% establishment. Can you think why this is so? Obviously, all these
organisms receive human assistance, either voluntarily (in the case of crop and pest
controls) or by association with human settlements (in the case of rats, cats, and dogs).
There are several reasons for the incredible success of some invaders and they
generally have to do with release from stresses that occur in their place of origin:
Invaders may find astonishingly nice environmental conditions to prosper.
Invaders may be superior competitors against native species and exploit the resources
better and grow faster (apparent competition or resource competition may play a role
here, Topic 5).
Invaders may leave their natural enemies (herbivores and pathogens) behind (“enemy
release”). After establishment and spread of an alien invader, we can thus expect all
kinds of negative direct and indirect effects for the resident plant and animal species.
Examples of Exotic Invading Plants Leaving Their Enemies Behind
In general, successful invasive plants grow larger and more vigorous in the new
environment. This is possibly due to the absence of competitors and natural enemies. A
good example is gorse (Ulex europeus) a shrubby plant in Europe that grows to huge
size and spreads quickly in New Zealand. Some extremely toxic plants, such as Senecio
jacobea, become aggressive invaders in foreign territory after leaving their specialist
herbivores behind. Equally, some European cool-season grasses, notably Lolium and
Festuca, which often associate with symbiotic fungal endophytes (see Topic 2), have
been translocated to other continents to improve pastures. As they are resistant to
herbivore attack they were able to spread from pastures into natural communities,
outcompeting and replacing the resident grass species.
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Examples of Invading Plants Attracting the Pollinators of the Established Plant
Community
A possible stress for native plants by an invader occurs via attraction of pollinators. It has
been experimentally demonstrated that invasive Himalayan balm (Impatiens glandulifera)
competes with the native Marsh Woundwort (Stachys palustris) for pollinating insects.
The invasive I. glandulifera produces much more and sweeter nectar than S. pallustris
and therefore monopolizes the services of pollinating bumblebees. When S. pallustris
grows close to I. glandulifera it receives significantly fewer visits by bees and this is
subsequently reflected in a decreased seed set. Thus, the native plant has lower
reproductive success when invasive plants are present and the experienced stress is
caused indirectly by the behavior of a shared pollinator (Chittka & Schürkens 2001).
Introduced Animals Can Cause Immense Stress to Native Plants
There are, of course, a variety of alien invasive animals that cause immense stress to
native plants. The most prominent are vertebrate grazers (goats and rabbits) and rats
that have been shown to destroy whole uniquely evolved plant communities in many
parts of the world. In general, effects of invasive species are far-reaching and, as the
‘global experiment’ of translocations goes on, we have a chance to observe, learn, and
study the complexity of community effects.
To finish this lesson, three complex case studies involving invaders and resulting in
stressful effects for native plant species are illustrated:
1. In Hawaii, where evolution of plant and animal communities occurred in strict
isolation, intricate co-evolution between plants and particularly birds has led to highly
specialized and unique mutualistic interactions. These relationships have been
destroyed in several ways after European colonization: pigs, European plants, and
birds were introduced. With the birds, bird malaria arrived and transferred onto
endemic bird species once the mosquitoes were also introduced. The endemic birds
could not withstand the malaria as they had no previous contact with the disease
and, unlike European bird species, had not evolved resistance. Introduced pigs
facilitated the spread of the disease as their actions created temporary water pools in
which the mosquitoes, the vector for malaria, could breed. In this case, it is clear that
the introduced malaria is responsible for the vanishing of many endemic bird species
because some are extinct now and the ones that survived are restricted to high
altitudes where malaria cannot exist due to missing mosquitoes, which are unable to
breed in these areas. As a consequence of the loss of birds, it is expected that
several species of plants that need the specialist pollination service by endemic birds
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will be lost, too. For some endemic plants, the stress is the lost pollinator, and, in this
special situation, there is no way out in the long run – plants still exist but are unable
to reproduce (Figure 8a).
Figure 8a: Disruption of Mutualistic Relationships between Plants and Birds on Hawaii. Direct
effects are solid arrows and indirect effects are broken arrows; positive effects are blue and
negative ones red. All introduced animals disrupt the mutualism between endemic plants and
birds (Benning et al. 2002).
2. In New Zealand, many alien European species that cause considerable damage to
native communities have been introduced. One of the most notorious and perhaps
impossible to control is the European wasp (Vespula germanica), which has farreaching impacts on native systems. As a consequence of their omnivory and
sociality, wasps are highly successful invaders. The wasps have invaded the South
Island’s beech forests that contain common native scale insects (Ultracoelostoma
sp.) feeding on endemic beech trees. Scale insects produce abundant carbohydrates
in the form of honeydew. The honeydew has been an important natural food resource
for nectar-feeding birds until the wasp arrived. Vespula can reach fantastically high
densities in the beech forests because of this superabundant honeydew and exploits
it now almost exclusively. The constant drain of the honeydew by harvesting wasps
deprives native birds of an important food resource and damages the trees. Wasps
also hunt invertebrate prey, and this, together with the honeydew exploitation, could
affect nutrient cycling. Peak wasp abundance needs to be reduced by 80 to 90% to
conserve the vulnerable native invertebrates and give some protection to the
sensitive endemic bird fauna. Here, the stress for native plants is direct drain of
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nutrients and water by wasps, and possibly the reduced cycling and the decrease of
soil nutrients (Figure 8b).
Figure 8b: Introduced Social Wasps Affect Native Trees through Several Indirect Pathways.
3. In the western United States, two species of gall flies (Urophora affinis and U.
quadrifasciata) have been introduced to biologically control knapweed (Centaurea
maculosa), a noxious invasive plant. The Urophora species successfully established,
remained host plant specific but for some reason failed to control the plant. In
Centaurea-invaded grasslands, the gall flies became super-abundant with densities
of 3000 larvae per m2. The abundant food resource attracted many native
consumers, one of which is the deer mouse (Peromyscus maniculatus) whose
populations are supported to a large degree by the gall fly larvae overwintering in
knapweed (85% of winter diet). This has led to an increased winter survival of the
deer mouse and elevated deer mouse populations in knapweed-invaded grassland.
Deer mice are aggressive predators of seeds and invertebrates, compete with other
small mammals, and are the primary vector of the deadly Sin Nombre Hantavirus.
Therefore, introduced gall flies subsidise deer mice populations by which established
food webs are disrupted and the prevalence of a deadly disease is elevated. Further
indirect effects are possible: Seed predation by deer mice can reduce recruitment of
native plant populations in an already knapweed-dominated plant community (Figure
8c).
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Figure 8c: Knock-on Effects of the Introduction of a Biocontrol Agent
(Pearson & Callaway 2003)
The complexity of the effects in the above examples and the various different pathways
should illustrate that there are intricate ways that cause stress to plants. The stress
effects on the plants are visible but the pathway remains hidden if we avoid looking at
several components in a community.
Summary
Biotic interactions occur in a community context of several species and can lead to direct
and indirect effects. We have learnt that biotic interactions can be strong determinants of
the structure of a natural community and they can determine whether a particular plant
species is able to establish a population within a community.
We have learnt that, in a community context,
•
plants can experience biotic factors causing plant stress through the biotic
interactions with their neighbors
•
neighbors can help to mitigate stress
•
each stress or limitation a plant experiences can be passed on to the neighbors via
changed interactions
Stress in a community context can therefore
•
disrupt or change the direct interactions between two species.
•
disrupt or establish new indirect interactions between three or more species – even
between species that never met.
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Not every change in the interactions is negative but we have discussed several
examples of negative interactions. Such effects can ripple through whole communities,
changing the composition of species involved.
A community can never be stressed. A plant species may suffer from stress exposure,
which may result in its exclusion from the community. However, loss of species in a
community will allow other species to fill in the free niches, and a new community, at the
loss of the old one, will establish. As an example, we discussed the invasion of alien
species to new areas. The unpredictable resulting effects of such invasions make us
aware of the importance of complex community interactions.
Literature Cited
•
ARNOLD, A.L., MEJÍA, D. KYLLO, E. ROJAS, Z. MAYNARD, N. ROBBINS & E. HERRE (2003).
Fungal endophytes limit pathogen damage in a tropical tree. Proceedings of the National
Academy of Sciences 2003, 15649-15654.
•
BENNING, T.L., LAPOINTE, D., ATKINSON C.T. & P.M. VITOUSEK (2002). Interactions of climate
change with biological invasions and land use in the Hawaiian Islands: Modeling the fate of
endemic birds using a geographic information system. PNAS 99: 14246 – 14249.
•
CHITTKA, L. & S. SCHÜRKENS (2001). Successful invasion of a floral market. Nature: 411: 653.
•
CLAY, K. (2004). Fungi and the food of the gods. Nature 427:401-402.
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DAVIES, F. S. SVENSON, J. COLE, L. PHAVAPHUTANON, S. DURAY, V. OLALDE-PORTUGAL, C.
MEIER & S. BO (1996). Non-nutritional stress acclimation of mycorrhizal woody plants
exposed to drought. Tree Physiology 16, 985-993.
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DICKE M. & H. DIJKMAN (2001). Within-plant circulation of systemic elicitor of induced defence
and release from roots of elicitor that affects neighbouring plants. Biochemical Systematics
and Ecology 29: 1075-1087.
•
DU, Y., POPPY, G., POWELL, W., PICKETT, J., WADHAMS, L. & C. WOODCOCK (1998).
Identification of semiochemicals released during aphid feeding that attract parasitoid Aphidius
ervi. Journal of Chemical Ecology 24:1355-1368.
•
HANSEN, D., HEINE, C., KIESBÜY, C. & C. MÜLLER (2007). Positive Indirect Interactions
between Neighboring Plant Species via a Lizard Pollinator. The American Naturalist 169:
534–542.
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HILKER, M.C. KOBS, M. VARAMA & K. SCHRANK. (2002). Insect egg deposition induces Pinus
sylvestris to attract egg parasitoids. The Journal of Experimental Biology 205, 455-461.
•
ISLAM, Z. & M.J. CRAWLEY (1983). Compensation and regrowth in ragwort (Senecio jacobaea)
attacked by cinnabar moth (Tyria jacobaeae). The Journal of Ecology 71, 829-843.
•
MEMMOTT, J. & H. GODFRAY (1994). The use and construction of parasitoid webs. In: Hawkins,
B. & Sheehan W (eds). Parasitoid Community Ecology. Oxford University Press, Oxford.
•
MORIN, P. (1999). Community Ecology. Blackwell Science.
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MÜLLER C.B.& J. KRAUSS (2005). Symbiosis between grasses and asexual fungal endophytes.
Current Opinion in Plant Biology 8: 450–456.
•
MÜLLER, C., ADRIAANSE, I., BELSHAW, R. & H. GODFRAY (1999). The structure of an aphidparasitoid community. Journal of Animal Ecology 68, 346-370.
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•
MÜLLER, C & H. GOODFRAY (1999). Indirect interactions in aphid-parasitoid communities.
Researches on Population Ecology 41: 93-106.
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PEARSON, D. & R. CALLAWAY (2003). Indirect effects of host-specific biological control agents.
Trends in Ecology and Evolution 18: 456-461.
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RICHARDS, R. (1926). Studies on the ecology of British heaths. III. Animal communities of the
felling and burn succession at Oxshott Heath, Surrey. Journal of Ecology 14, 244-281.
•
STONE, G. & K. SCHÖNROGGE (2003). Morphological diversity in insect induced galls. TREE 18,
p. 514.
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TRAW, M.B. & J. BERGELSON (2003). Interactive effects of jasmonic acid, salicylic acid, and
gibberellin on induction of Trichomes in Arabidopsis. Plant Physiology 133,1367-1375.
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WARREN, J., BASSMAN, J. & S. EIGENBRODE (2002). Leaf chemical changes induced in Populus
trichocarpa by enhanced UV-B radiation and concomitant effects on herbivory by Chrysomela
scripta (Coleoptera: Chrysomelidae). Tree Physiology 22, 1137-1146.
•
WILLIAMSON, M. (1996). Biological Invasions. Chapman & Hall, London.
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Plant Stress Implications at the Ecosystem Level
Plant Stress Implications at the Ecosystem Level (Nina Buchmann, Lutz
Merbold & Nina Buchmann)
Concept Map
Don’t Miss these Online-Learning Activities!
•
Exercise 1: Name the different compartments of an ecosystem (Topic 1)
•
Exercise 2: The stepwise process of ecosystem change (Topic 2)
•
Exercise 3: Name the compartments of the water cycle (Topic 3)
•
Exercise 4: Water and energy budget in several ecosystem types (Topic 3)
•
Exercise 5: Make a mind map of the consequences of global change on ecosystems
(Topic 4)
Topic 1: What Are Ecosystems?
Ecology/Ecosystem ecology has developed over the last hundred years, from its roots in
plant geography and plant physiology into an own discipline addressing issues of energy
and nutrient flow through organisms and the abiotic environment, often referred to as
ecosystem biogeochemistry. The most prominent representatives of plant geography,
Willdenow, von Humboldt, de Candolle and Grisebach as well as Darwin and Haeckel
primarily focused on describing vegetation composition and distribution around the world.
Empirical approaches were added by Stahl, Kerner von Marilaun, Warming and
Schimper, which included physiology, functional morphology and climatology (reviewed
in Buchmann 2002).
Early in the 20th century, plant ecophysiologists carried out field experiments on plant
water relations, on photosynthesis and plant adaptations to the environment (e.g.,
Clements, Wiesner, Fitting, Richards, Shreve, Stocker, Walter). Similarly, zoologists (e.g.
Elton, Hutchinson and Lindeman) developed the field of functional animal ecology,
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addressing questions such as lake productivity or energy flow through terrestrial
systems. When Tansley (1935), a British terrestrial plant ecologist, had introduced the
term „ecosystem“, the fields of prime interest were environmental (largely climatic)
influences on different organismic groups, i.e., animals, plants and microorganisms.
Later, a more holistic ecosystem approach was adopted, pioneered by Odum (1983),
Ellenberg (1988) or by Likens (1970). Major methodological advances during the 1980s
opened new research fields, in essence permitting unprecedented precision in the study
of ecosystem processes and in mathematical modeling. Global approaches became
possible by studying biosphere-atmosphere interactions, the global climate system and
its human perturbation.
An ecosystem is comprised of different compartments, which describe its environment.
Examples are:
•
bedrock (lithosphere),
•
soil (pedosphere),
•
water (hydrosphere),
•
vegetation, microorganisms, and animals (biosphere),
•
air (atmosphere)
The term “ecosystem” thus includes much more than the “plant community” , which is
defined mainly by the organisms and their biotic interactions – and actually only deals
with the biosphere.
Ecosystem processes describe the transfer of energy and matter from one
compartment to another. Some fundamental ecosystem processes:
•
the water cycle (details in Topic 3):
the circuit of water movement from the
atmosphere through the biosphere, pedosphere and lithosphere and the return to the
atmosphere.
•
the energy cycle, also called solar energy flow (details in Topic 3): How much
sunlight gets turned into biomass via photosynthesis? But how much energy in form
of heat can be stored in the pedosphere/biosphere?
•
the carbon cycle: carbon from atmospheric CO2 (atmosphere) via photosynthetic
fixation into plant tissues (biosphere), senescence of plant tissues and decomposition
by soil fauna and microorganisms (biosphere) to soil organic matter in the soil profile
(pedosphere). If dissolved organic carbon is leached out of deeper soil horizons into
an adjacent river (hydrosphere), this carbon might be deposited together with
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anorganic sediments in the ocean or enter the anorganic carbon cycle, creating new
bedrock over geological time scales (lithosphere).
•
nutrient cycle: the passage of nutrients through an ecosystem. Nutrients such as
nitrogen and phosphorous are assimilated or released by the organisms of the
ecosystem. Similarily nutrient are stored and leached from the pedosphere.
•
community dynamics: The concept of ecology suggests that communities are not
stable and that they are constantly undergoing a process of modification, e.g.
succession and disturbances.
Nowadays, the term “ecosystem” is used at very different spatial scales, from local to
regional or even global scales, depending on the research questions asked. When global
change issues are addressed, the “global ecosystem” might be the center of interest, but
also the response of biomes, such as the boreal forest zone, the arctic tundra, the
tropical forest or savannas.
Often, watersheds or catchments are used to study ecological processes at a regional
scale, e.g. the famous Hubbard Brook Experiment in the northeastern United States,
where the effect of clearcutting a temperate forest was examined on amount and quality
of water yield (Likens et al. 1970).
At the next smaller spatial scale, forest stands, wetlands or grasslands might be studied
using micrometeorological techniques or a combination of methods from the soil
sciences, plant ecophysiology, and micrometeorology (Figure 1). In general, ecosystem
ecology seeks to understand the relationships and interactions between organisms and
their environment.
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Plant Stress Implications at the Ecosystem Level
Figure 1: The ecosystem integrates the levels of tissue and leaves, plant, population and
community within their environment. Different disciplines, such as soil science (represented as
soil chemistry and soil physics), plant sciences (represented as ecophysiology and population
biology) or micrometeorology, are involved in studies of ecosystem ecology. Using the most
abundant definition, different ecosystems, such as forests, grasslands, arable lands or wetlands,
are embedded in a landscape (here in green). This landscape adds further aspects to ecosystem
ecology such as biogeochemical cycling or the distribution of biodiversity.
Topic 2: Stress Factors at the Ecosystem Level
We use global climate change and nutrient limitations as examples how the
pattern of stress factors at the ecosystem level may change.
Global changes due to climate warming are not only modeled scenarios for the future,
but have been measured already over the last 100 years, also in Switzerland. Among
higher carbon concentrations in the atmosphere, also air temperature and precipitation
patterns and amounts are affected. Current scenarios for Switzerland until 2050 predict:
•
an increase in winter precipitation, but a significant decrease in summer precipitation,
particularly between May and September, reduces the length of the growing season
for many crop and forest species. Periods of summer drought are likely to increase.
•
higher variability in precipitation (more frequent high-intensity rainfall events).
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Lesson 7
•
Plant Stress Implications at the Ecosystem Level
an increase in main annual air temperatures and thus evaporation demand, but also
a reduction of risk of freezing damage in e.g. exotic species.
Secondly ecosystems, particularly managed ecosystems, are often limited in nutrients
such as nitrogen or phosphorous. When biomass is harvested from a field several
components are regularly taken away from the system (biosphere/pedosphere) contrary
to natural ecosystems where the nutrients are mostly returned to the soil via
decomposition of dead biomass. The limitations in managed ecosystems may be
overcome by fertilization, however it is of crucial importance to study the magnitude and
usage of the several nutrient pools, to avoid nutrient starvation or nutrient excess. Either
limitation or excess may lead to negative feedbacks, e.g. decreasing yields or
eutrophication.
•
Increasing nitrogen levels within the groundwater and/or rivers (water cycle (Topic 3)
•
Decreasing phosphorous and nitrogen pools in the soil -> smaller yields if not
fertilized
•
Carbon losses due to land-use and land-use change (e.g. deforestation in the
tropics)
Since the ecosystem integrates over all compartments, an understanding of the
effects of global climate change/nutrient limitation needs monitoring of all
compartments considering the potential linkages among the compartments.
We will focus here on atmosphere, soil and biosphere (e.g. a forest, agricultural field).
We will first discuss how climate change/nutrient limitation will impact these
compartments and then show some potential linkages and feedbacks between the
atmosphere, the biosphere and the pedosphere:
Atmosphere:
•
Anticipated increases in summer temperatures will cause a decrease in air humidity.
Some feedback between atmosphere and biosphere:
•
Increase in summer temperatures intensifies evapotranspiration from vegetation (for
details see Topic 3, 4). The vapor pressure deficit (=difference between actual and
maximum vapor pressure, VPD) will increase. This is an important way in which
climate warming will increase the risk of drought stress in vegetation.
•
At the same time, higher winter and spring temperatures will reduce freezing stress.
Soil:
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Lesson 7
•
Plant Stress Implications at the Ecosystem Level
Where the number of high-intensity rainfalls increases, erosion might increase due to
high surface runoff because only small amounts of the rainfall water infiltrate the soil.
The higher the amount of rain per rain event, the less water infiltrates the soil due to
limited capacity to retain this water (Colombo et al 1998).
•
Regular harvests without fertilization will cause a decrease in soil carbon and
nitrogen pools and therefore a decrease in harvest yields and therefore a decrease in
aboveground biomass and further lead to erosion as pointed out before.
Some feedback between soil and biosphere:
•
Where soil water storage is poor (e.g., high conductivity in sandy soils or low capacity
to retain water), an increase in vapor pressure deficit (=VPD) will reduce soil water
availability and affect the productivity and survival of plants.
•
Soil water retention is improved by high amounts of organic material in the soil
because coarse-grained organic material and water holding capacity increases. The
following examples illustrate how much organic material might be deposited or
decomposed under increased water scarcity:

In some vegetation types, we expect less leaf litter due to reduced leaf growth,
when annual air temperature and periods of summer drought increase (see Topic
4).

However, drought may increase the fine root production (see lesson 4), which will
in turn increase root litter input into soils.

We expect higher decomposition rates of organic material and higher nutrient
release when air temperatures rise (rise by up to one third of present rate if
annual temperatures rise by 3°C, Schimel 1995).

However, under conditions of water scarcity in summer decomposition of organic
material may be reduced when microbiological activities become reduced.
•
decreases in soil carbon and nitrogen, will lead to less biomass and increased
erosion and leaching of the remaining nutrients over time but may also lead to a
vegetation shift towards legumes if not forced otherwise
•
However decreasing soil carbon and nitrogen pools may also lead to increased fine
root production and therefore higher litter inputs in the soil
Impacts of global climate change on biosphere:
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Lesson 7
•
Plant Stress Implications at the Ecosystem Level
Climate change will impact all levels, from cells to plants, populations, and
communities and we can expect all types of responses within an ecosystem –
physiological adaptations, acclimations, and modifications (at the cell and wholeplant level; see Lessons 2-4) as well as long-term evolutionary adaptations (at the
population and community level; see Lessons 5, 6):

Responses at the whole-plant level: Plant physiological processes and plant
architecture can change in response to climate change as well as due to nutrient
limitation

Responses at the population level: The genotypic variation in the plant
populations is the basis for evolutionary adaptation to climate conditions (see
Lesson 5).

Responses at the community level: Climate change alters the community
structure: Increased summer drought frequencies could also increase the
frequency of major insect and disease outbreaks while stressed plants are
already more sensitive to attacks. Similarly plants become more vulnerable to
attacks when starving nutrients but also species shifts within the community are
likely (legumes to bind nitrogen from the atmosphere)

Responses at the ecosystem level - Can an ecosystem become stressed by
global climate change? As outlined in Lesson 1, defining stressful habitats for
plants is not easy. Organisms native to a “stressful” environment, such as an arid
desert ecosystem or a boreal forest, are adapted to or evolved under these
conditions. Thus, they are not “stressed” per se (just because the human
observer feels hot or cold). However “stress” due to overexploitation of the
resources, e.g. nutrients in the soil, may lead to stress at the ecosystem level.

An ecosystem reacts in a long-term context. The life inventory is evolutionarily
adapted to the environmental conditions prevailing in this ecosystem – these
tolerated environmental conditions include the whole spectrum of situations ever
experienced by the ecosystem – from optimal to less optimal or even stressful
living conditions.

However, environmental conditions can become so different for the life inventory
of an ecosystem, either through increased stress or more disturbances, so that
productivity, reproduction, or regeneration are limited to such an extent that the
life inventory of the ecosystem will change.

Under global climate change, the evolutionary adaptations of the plants to the
142
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Plant Stress Implications at the Ecosystem Level
existing environmental conditions may become void and a new selection
process begins.

New evolutionary adaptations need a certain time to establish. If a plant species
at a certain site is not able to adapt to changing climatic conditions it will become
less competitive or even get extinct. Better adapted species, may invade the site
that is affected by the new climatic conditions and will establish new, locally better
adapted populations.
Ecosystem change caused by i.e. climate change/nutrient limitation comprises
several steps:
•
First, ecosystem rates change: i.e. as water and energy budgets changes due to
drought, the process rates of the ecosystem (e.g., its productivity, litter inputs etc.)
change (for details: Topic 3, 4).
•
Second, the ecosystem structure, the species composition (its life inventory),
and therefore, the ecosystem type change (details in Topic 5).
•
Each of these changes may result in a change in ecosystem services: If the
productivity of a vegetation stand decreases, the ecosystem may release more
carbon (through respiration) than it is fixing (through photosynthesis) – therefore, it
would not fulfill the requirements of a carbon storage any more (details on these
ecosystem services in Topic 5).
Topic 3: The Water and Energy Budget of Terrestrial Ecosystems
The water budget of terrestrial ecosystems is controlled by the balance between water
inputs into the system (precipitation) and water losses from the system (evaporation,
transpiration and runoff both below and above ground; Figure 2).
However, the net water budget is also determined by the type of vegetation present and
changes if the vegetation is removed by e.g. harvest
•
Interception is a function of the leaf area, the leaf size and rainfall pattern (= intervals
and magnitudes).
•
The evapotranspiration rate (i.e., evaporation from plant and non-plant surfaces and
transpiration by vegetation) from a canopy depends primarily on the climatic
conditions driving vapor pressure deficit and the coupling of the canopy to the
atmosphere (mainly determined by canopy and surface roughness – for a rough
surface, evapotranspiration will be higher).
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Lesson 7
•
Plant Stress Implications at the Ecosystem Level
Since forests are typically better coupled to the atmosphere (due to their greater
roughness and deeper rooting systems) than grasslands or scrublands, forest
evapotranspiration rates are higher than those of shorter vegetation over longer
periods, while surface runoff in forests is much smaller (the dense vegetation of a
forest acts like a sponge).
•
With smaller surface runoff and thus erosion, forest soils are less prone to nutrient
leaching than scrublands’, which in turn affects regeneration and productivity of the
vegetation.
Figure 2: Water Cycle of an Ecosystem.
Water balance is closely coupled to the energy balance of ecosystems. The net
solar energy absorbed by an ecosystem is generally balanced by heat released into the
atmosphere and by the energy dissipated by evapotranspiration. Heat storage in the
ground and biochemical energy stored in biomass play a minor role. The sensible heat
flux also dissipates energy from ecosystems and is driven by temperature differences
between the surface and the overlying air. It warms the air above vegetation.
Evapotranspiration (i.e., evaporation from soils, and evaporation of water intercepted by
the canopy as well as transpiration by the vegetation) dissipates energy from an
ecosystem by cooling its surfaces while transferring water back into the atmosphere.
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Plant Stress Implications at the Ecosystem Level
This is called the latent heat flux. Both pathways for energy dissipation dominate the
water and energy budget of an ecosystem and are closely linked:
•
Evaporation of water (through latent heat flux) cools the surface, thereby reducing the
temperature difference between the surface and the overlying air that drives the
sensible heat fluxes.
•
On the other hand, warming the surface air by sensible heat flux increases the
amount of water that the air can hold and causes convective movement of moist air
away from the evaporating surfaces. This, in turn, drives evaporation.
These interactions can be represented in the ratio of sensible (S) to latent heat fluxes
(L), the so-called Bowen ratio (β):
β = S/L.
In general, ecosystems with abundant moisture have higher evapotranspiration rates
(=higher latent heat flux) and, therefore, lower Bowen ratios (β<1) than ecosystems in
dry areas (β>1).
Topic 4: The Response of European Ecosystems Water Budget to the
Drought in 2003
Plants and microorganisms under field conditions are typically subjected to combinations
of stress factors, such as oxidative stress (lesson 3) and drought or cold and drought.
The anticipated stress factors of global climate change (e.g. increasing temperatures
globally, decreasing precipitation during summer in Europe) will affect ecosystem water
fluxes as discussed in Topic 3. Swiss ecosystems will not be exceptions in this respect
(see Topic 2).
What happens to our European ecosystems under such drought and heat
conditions? During summer 2003, Europe experienced a particularly extreme climate
event, with temperatures up to 6°C above long-term means and precipitation deficits up
to 300 mm per annum. During this time, various forest ecosystems in Europe were
studied intensively, with continuous CO2 and water flux measurements using the eddy
covariance method (for a description of this method see Lesson 8, Topic “Carbon
Fluxes and Drought Effects”). Forest sites all over Europe experienced water stress
during this period (Figure 3).
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Lesson 7
Plant Stress Implications at the Ecosystem Level
Figure 3: Intensities of Water Stress in 2003. Figure modified after Granier et al. unpublished.
The comparison of two consecutive years (2002 and 2003) demonstrated the short-term
effects of water scarcity on CO2 and water fluxes, i.e. on annual carbon budget and
evapotranspiration of the monitored ecosystems (Ciais et al. 2005):
•
Nearly all forest sites experienced a strong reduction in gross primary production
(GPP) in 2003 in comparison to 2002. The largest decrease in GPP occurred at
Hesse, a temperate deciduous beech forest in France where canopy conductance
during this time reached only 15% of the corresponding value in 2002. Southern
conifer forest sites, e.g. San Rossore in Italy, also experienced a reduction in GPP
during the heat wave (Figure 4). Moreover, GPP did not entirely recover from the
summer drought during the remainder of the growing season.
•
Also, total ecosystem respiration (TER) decreased at both sites in 2003 compared to
2002, parallel to the larger GPP reduction.
•
Therefore, a lower net ecosystem exchange (NEE) resulted.
•
The GPP drop coincided with reduced evapotranspiration due to stomatal closure
and soil drying due to the rainfall deficit (not shown in a figure) for many forest sites
across Europe. This nicely demonstrates that, at the ecosystem level, the
physiological control of foliar gas exchange (also see Lesson 4) generally leads to a
strong coupling of carbon and water fluxes.
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Lesson 7
Plant Stress Implications at the Ecosystem Level
Figure 4: Variations of temperature, monthly precipitation, and CO2 fluxes of daytime NEE (=net
primary exchange), GPP (=gross primary production), and TER (total ecosystem respiration) at
two forest sites: Hesse (France) & SanRossore (Italy). A 5-day running average was applied to
the original half-hourly flux and temperature data to remove daily variations. Data for the year
2002 are in black and for the year 2003 in color. The July to August heat wave period is shaded in
grey (from Ciais et al. 2005).
In a next step, models were used to extrapolate from the intensive measurement sites to
continental Europe (Figure 5). Similar to GPP, also the net primary productivity (NPP)
reduction in 2003 followed the pattern of the drought caused by rainfall deficit in Central
and Eastern Europe and of the extreme summer heat in temperate Western Europe.
Accordingly, the largest NPP reductions were found in Ukraine and Romania, France
and Italy, whereas NPP increased in southern Sweden in response to moderate warming
and no marked water deficits. Overall, the simulated NPP anomalies at the continental
scale in 2003 correlated better with annual rainfall changes than with summer air
147
Lesson 7
Plant Stress Implications at the Ecosystem Level
temperature changes, indicating the dominant role of water limitations on the ecosystem
carbon budgets.
Figure 5: Upper left: Changes in summer air temperature (average July to September) between
2003 and the average of 1998 - 2002. Upper right: Changes in precipitation over Europe
between 2003 and the average of 1998-2002 (average April to October). Lower left: Simulated
changes in annual mean NPP in response to climate. Lower right: Simulated changes in summer
NPP (from Ciais et al. 2005).
Topic 5: How will Ecosystems and their Services Change due to Climate
Change?
What will happen to ecosystems if the scenarios of climate change end of this century
become reality? Which ecosystem types are especially sensitive to the main implications
of climate change (Table 1)? Remember, the main implications of climate change in
Switzerland are: Increases in winter precipitation, but a large decrease in summer
precipitation, particularly between May and September, and increasing air temperatures.
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Lesson 7
Plant Stress Implications at the Ecosystem Level
If ecosystem types change, they will not be able to fulfill their most important services
and will not provide goods and services to humans anymore:

climate regulation through influence of land cover and of biologically mediated
processes (e.g. evapotranspiration) on climate and microclimate

disturbance prevention through influence of ecosystem structure on protection
against environmental disturbances (e.g. flood prevention by wetlands)

water regulation through land cover regulating runoff and river discharge

soil retention and formation through vegetation root matrix and soil biota

nutrient cycling through role of biota in storage and recycling of nutrients.
Table 1: Ecosystem Types Sensitive to Global Climate Change. Table modified after ‘Biodiversity
and
Climate
Change
Program,
UNEP
World
Conservation
Centre. Document
URL:
http:/www.wcms.org.uk /climate/impacts.html. Revised: January 11, 2006. Last visited: February
16, 2006.
Ecosystem Type
Wetlands
Tropical Montane Forests
Boreal Forests
Arctic Habitats
Implications/ Threads for the Ecosystem
- Inland wetlands may dry out.
- Warming of 3-4°C could eliminate 85% of all
remaining wetlands.
- Drying out and invasion or replacement of
montane species by lower montane or nonmontane species.
- Significant losses in some areas, mainly through
fires.
- Expansion of boreal forest into arctic areas.
- Vegetation changes with losses of tundra but
forest expansion.
- Thawing of permafrost leads to an additional
release of CO2 and CH4 in a positive feedback
loop.
Alpine Habitats
-
Arid and Semi-Arid Areas
-
Altitudinal migration of single species, with
invasion of alpine meadows by trees. Highest
altitude habitats may be unable to migrate and
disappear (extinction).
With a few exceptions, deserts are expected to
become hotter and drier.
Desertification extends even further in subSaharan Africa and Central Asian Steppes.
Salinization.
Loss of grassland.
Loss of arable land.
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Lesson 7
Plant Stress Implications at the Ecosystem Level
Ecosystem Type
Implications/ Threads for the Ecosystem
Wetlands
Inland wetlands may dry out.
Warming of 3-4°C could eliminate 85% of all remaining
wetlands.
Tropical Montane Forests
Drying out and invasion or replacement of montane
species by lower montane or non-montane species.
Boreal Forests
Significant losses in some areas, mainly through fires.
Expansion of boreal forest into arctic areas.
Arctic Habitats
Vegetation changes with losses of tundra but forest
expansion.
Thawing of permafrost leads to an additional release of
CO2 and CH4 in a positive feedback loop.
Alpine Habitats
Altitudinal migration of single species, with invasion of
alpine meadows by trees. Highest altitude habitats may
be unable to migrate and disappear (extinction).
Arid and Semi-Arid Areas
With a few exceptions, deserts are expected to become
hotter and drier.
Desertification extends even further in sub-Saharan
Africa and Central Asian Steppes.
Salinization.
Loss of grassland.
Loss of arable land.
Summary
This lesson on ‘Stress Implications at the Ecosystem Level’ introduced the concept of
ecosystem ecology and emphasized the fact that stress to entire ecosystems is relatively
rare. However, global climate change may be an example how on a global scale the
pattern of established stress factors in ecosystems can change. What will happen due to
this change in the ecosystems? What will happen primarily to managed ecosystems
when experiencing nutrient limitation?
•
Environmental conditions can become so different for the live inventory of an
ecosystem it undergoes changes due to stress imposed on plants.
•
Responses of ecosystems to the changing patterns of stress factors will be longterm.
•
A complete change in species composition will result in changes of ecosystem type
and ecosystem services.
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Lesson 7
•
Plant Stress Implications at the Ecosystem Level
Responses at the ecosystem level may be changes in the plant community (longterm) and reduced yields, which further result in similar outcomes as climate change
are possible (see above).
You should now be able to answer the following questions:
•
What is global climate change? What is nutrient limitation?
•
To which stress factors will ecosystems be exposed more frequently due to global
climate change? Due to nutrient limitation?
•
To what extent is global climate change a stress for an ecosystem?
•
In what stepwise process does an ecosystem react to global climate change?
•
Does global climate change/nutrient limitation influence the water, energy, and
carbon dioxide budgets? Explain your answer.
•
How will the composition of ecosystems change due to global climate change?
•
Will ecosystem services change with climate change/under nutrient limitations?
Explain the reactions.
Cited Literature
•
BUCHMANN N. (2002). Plant ecophysiology and forest response to global change. Tree
Physiology 22: 1177-1184.
•
CIAIS P.H., REICHSTEIN M., VIOVY N., GRANIER A.,. OGE ́E J, ALLARD V., AUBINET M., BUCHMANN
N., BERNHOFER CHR., CARRARA A., CHEVALLIER F., DE NOBLET N., FRIEND A. D., FRIEDLINGSTEIN
P., GRÜNWALD T., HEINESCH B., KERONEN P., KNOHL A., KRINNER G., LOUSTAU D., MANCA G.,
MATTEUCCI G., MIGLIETTA F., OURCIVAL J.M., PAPALE D., PILEGAARD K., RAMBAL S., SEUFERT G.,
SOUSSANA J.F., SANZ M.J., SCHULZE E. D., VESALA T. & VALENTINI R. (2005). Europe-wide
reduction in primary productivity caused by the heat and drought in 2003. Nature 437: 529 –
533.
•
ELLENBERG H. (1988). Vegetation ecology of Central Europe. Cambridge University Press,
New York.
•
IPCC - Intergovernmental Panel for Climate change (2001).
•
LIKENS G., BORMANN, F.H., JOHNSON N.M., FISHER D.W., PIERCE R.S. (1970) Effects of forest
cutting and herbicide treatment on nutrient budgets in the Hubbard Brook watershedecosystem. Ecological Monographs 40: 23-49.
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Lesson 7
Plant Stress Implications at the Ecosystem Level
•
ODUM, E.P. (1983). Basic ecology. Saunders College, London.
•
REICHSTEIN M., CIAIS P., PAPALE D., VALENTINI R., RUNNING S., VIOVY N., CRAMER W., GRANIER
A., OGÉE J., ALLARD V., AUBINET M., BERNHOFER C., BUCHMANN N., CARRARA A., GRÜNWALD T.,
HEIMANN M., HEINESCH B., KNOHL A., KUTSCH W., LOUSTAU D., MANCA G., MATTEUCCI G.,
MIGLIETTA F., OURCIVAL J.M., PILEGAARD K., PUMPANEN J., RAMBAL S., SCHAPHOFF S., SEUFERT
G., SOUSSANA J.F., SANZ M.J., VESALA T., ZHAO M. (in press). Reduction of ecosystem
productivity and respiration during the European summer 2003 climate anomaly: a joint flux
tower, remote sensing and modeling analysis. Global Change Biology.
•
SCHIMEL D.S. (1995) Terrestrial ecosystems and the carbon cycle. Global Change Biol. 1: 7791.
•
UNEP World Conservation Centre. Biodiversity and Climate Change Program, Document
URL: www.unep-wcms.org/climate/impacts.html. Revised: January 11, 2006. Last visited:
February 16, 2006
152
Index
Index
ABA ....................................................15, 52, 53, 57, 74, 75, 76, 84, 85, 86, 92, 93
Abiotic factors causing plant stress .................................................................... 114
abiotic factors causing stress ............................................................................. 119
abscisic acid ....................................................................................... 52, 74, 84, 87
acclimation ..................................................................................... 15, 65, 114, 135
acquired defense ................................................................................................ 116
adaptation...........................................................................15, 16, 18, 89, 102, 109
avoiding............................................................................................................... 18
biotic factors causing stress ............................................................... 114, 115, 119
Bowen ratio ........................................................................................................ 146
CAM ..................................................................................................................... 89
carbon cycle ....................................................................................................... 138
cis-regulatory elements ............................................................................ 48, 50, 51
co-evolution ........................................................................................ 115, 129, 131
cold shock proteins......................................................................................... 62, 63
community ..................................................14, 15, 20, 22, 112, 114, 115, 138, 143
community dynamics....................................................................................... 139
compatible solutes.................................................................. 63, 77, 79, 80, 81, 94
Compatible solutes ............................................................................. 76, 77, 81, 94
constitutional defense................................................................................. 115, 117
density stress ............................................................................................. 100, 108
direct interactions ...............................................................115, 122, 123, 124, 126
disturbance ............................................................................................. 12, 17, 120
153
Index
ecological optimum............................................................................................... 17
ecosystem .......................................................................................................... 137
ecosystem process rates ................................................................................... 144
ecosystem processes ................................................................................. 138, 139
ecosystem services ............................................................................ 131, 144, 150
ecosystem structure ................................................................................... 144, 150
ecosystem types ................................................................................................ 150
energy cycle ....................................................................................................... 138
escaping ....................................................................................................... 18, 111
evapotranspiration .............................................................................................. 144
expression proteomics ......................................................................................... 68
food web............................................................................................................ 126
functional proteomics ..................................................................................... 68, 69
Genomics ............................................................................................................ 66
genotypic variation ............................................................................................. 143
guard cell .................................................................................................. 82, 83, 84
guard cells ............................................................................................................ 80
H2O2/Singlet Oxygen Regulation .......................................................................... 41
Heat Shock ............................................................................................... 49, 50, 54
heat shock proteins ............................................................................ 50, 59, 61, 65
herbivore ....................................................................114, 115, 116, 117, 119, 120
indirect interactions ............................................122, 123, 124, 126, 128, 132, 133
induced defence ......................................................................... 115, 116, 117, 128
latent heat flux .................................................................................................... 146
law of constant yield ................................................................................... 100, 108
limitation ................................................................................................. 12, 17, 120
154
Index
Mehler Reaction.................................................................................................. 28
Metabolomics ..................................................................................................... 67
modification .......................................................................................... 15, 115, 139
net ecosystem exchange ................................................................................... 148
non-photochemical quenching ........................................................... 25, 26, 28, 30
nutrient cycle .................................................................................................... 139
osmotic stress .................................................................................................... 62
oxidative stress.........................................................24, 37, 42, 51, 64, 65, 87, 146
pathogen ........................................................................14, 15, 114, 115, 116, 135
photochemical quenching................................................................... 25, 26, 30, 42
photoinhibition ...................................................................................................... 30
Photoinhibition ...................................................................................................... 26
physiological optimum .......................................................................................... 17
physiological regulation .................................................................................. 11, 15
physiological regulations .................................................................................. 65
promoter ......................................................................................................... 49, 93
proteomics ................................................................................................ 66, 67, 68
reactive oxygen species ...................................................24, 25, 33, 37, 51, 63, 87
regulation of transcription ............................................................................... 47, 48
regulation of translation ........................................................................................ 54
resisting ........................................................................................................ 18, 112
resource ............................................................................................................... 99
ROS................24, 25, 27, 28, 32, 33, 34, 35, 36, 37, 39, 40, 41, 42, 51, 57, 63, 87
Rubisco ................................................................................................... 15, 26, 38
salinity .................................................................................................................. 94
scavengers ............................................................................. 37, 39, 41, 42, 63, 64
155
Index
second messengers ................................................................................. 40, 75, 87
self-thinning ................................................................................................ 101, 108
sensible heat flux........................................................................................ 145, 146
signal transduction .....................................................39, 40, 52, 74, 75, 82, 86, 87
singlet oxygen ..................................25, 26, 28, 30, 32, 33, 34, 35, 37, 41, 42, 43
SOS pathway ...................................................................................................... 95
stomata.........................................13, 22, 26, 73, 75, 80, 82, 83, 84, 85, 87, 89, 90
tolerating....................................................................................................... 18, 111
total ecosystem respiration................................................................................. 147
transcription .................................................................................................... 50, 93
transcription factor binding sites ......................................................... 48, 51, 52
transcription factors ..............................................40, 47, 48, 49, 51, 53, 57, 62, 93
transcriptomics ..................................................................................................... 66
Transcriptomics ................................................................................................. 66
translation factors ............................................................................ 47, 54, 57, 62
transpiration................................................................................ 13, 73, 85, 92, 144
unfolded protein response ............................................................ 54, 55, 56, 57, 61
water cycle ......................................................................................................... 138
156