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Chapter 39
Plant Responses to Internal
and External Signals
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Stimuli and a Stationary Life
• Linnaeus noted flowers of different species
opened at different times of day/could be
used by horologium florae, or floral clock
–
Reflects time insect pollinators active, one of
environmental factors plant senses to compete successfully
–
Plant’s morphology/physiology tuned to surrounding by complex
interactions between environmental stimuli/internal signals
• Plants, being rooted to ground, must respond to environmental
cues by adjusting individual pattern of growth/development
–
Must detect changes first
–
Bending of seedling toward light begins with sensing direction,
quantity, and color of light
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 39.1: Signal transduction pathways link
signal reception to response
• All organisms received specific signals/respond to them in
ways that enhance survival/reproductive success
• Plants have cellular receptors that detect changes in their
environment (molecule affected by stimulus)
– For stimulus to elicit response, certain cells must have
appropriate receptor
– Stimulation of receptor initiates specific signal
transduction pathway
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-2
(a) Before exposure to light
Tall, spindly stem/nonexpanded leaves
(morphological adaptations called etiolation
enable shoots to penetrate soil, including
short roots due to little need for water
absorption from little water loss by shoots)
Expanded leaves hindrance as shoots
push through soil/chlorophyll waste of
energy (underground)
(b) After a week’s exposure to
natural daylight
Begins to resemble typical plant w/broad
green leaves, short sturdy stems, long
roots (transformation begins w/reception
of light by specific pigment, phytochrome)
by undergoing changes (de-etiolation) by
reception of signal (light) which is
transduced into responses (greening)
Fig. 39-3
Review of a general model for signal transduction pathways
CELL
WALL
1 Reception
CYTOPLASM
2 Transduction
Relay proteins and
second messengers
Receptor
Hormone or
environmental
stimulus
Plasma membrane
3 Response
Activation
of cellular
responses
Reception/Transduction
• Internal/external signals are detected by receptors,
proteins that change in response to specific stimuli
– Phytochrome functioning in de-etiolation located in
cytoplasm, not built into plasma membrane as
most receptors
• Second messengers (small molecules/ions in cell
transfer and amplify signals from receptors to proteins
that cause responses) activate hundreds of molecules
of specific enzyme from one activated phytochrome
molecule
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-4-3 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response
1
Reception
2
Transduction
3
Transcription
factor 1
CYTOPLASM
Plasma
membrane
cGMP
Second messenger
produced
Phytochrome
activated
by light
Cell
wall
Response
2. One pathway uses cGMP as 2nd
messenger that activates specific
protein kinase. Other pathway
involves increase in cytosolic level
of Ca2+, which activates different
protein kinase
Specific
protein
kinase 1
activated
P
Transcription
factor 2
P
Specific
protein
kinase 2
activated
3. Both pathways lead to
expression of genes for
proteins that function in
de-etiolation (greening)
response
1. Light signal detected
by phytochrome receptor;
phytochrome undergoes
change in shape, which
then activates at least
two signal transduction
pathways
Ca2+ channel
opened
Ca2+
NUCLEUS
Transcription
Translation
De-etiolation
(greening)
response
proteins
Response
• Signal transduction pathway leads to regulation of
one or more cellular activities
– In most cases, these responses to stimulation
involve increased activity of enzymes
– This can occur by transcriptional regulation
(increases/decreases synthesis of mRNA that
codes for enzyme) or post-translational
modification (activates existing enzyme
molecules)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Transcriptional Regulation
• Specific transcription factors bind directly to specific regions
of DNA and control transcription of genes
–
In phytochrome-induced de-etiolation, several transcription factors
activated by phosphorylation in response to appropriate light conditions
• Mechanism by which signal promotes new developmental
course may depend on
–
Positive transcription factors (activators): proteins that increase
transcription of specific genes
–
Negative transcription factors (repressors): proteins that decrease
transcription of specific genes
• Mutation in Arabidopsis allows for light-grown morphology
underground (except for pale color because no chlorophyll grown
because no light) because genes activated by light normally
blocked by repressor eliminated by mutation
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Post-Translational Modification of Proteins
• Post-translational modification involves modification of
existing proteins in signal response
• Modification often involves phosphorylation of specific
amino acids which alters protein’s hydrophobicity/
activity
– Many 2nd messengers (including some forms of
phytochrome) activate protein kinases directly,
often leading to kinase cascades
– Many signal pathways ultimately regulate
synthesis of new proteins, usually by turning
specific genes on or off
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
De-Etiolation (“Greening”) Proteins
•
Many enzymes that function in certain signal
responses
– Are directly involved in photosynthesis
– Are involved in supplying chemical precursors
for chlorophyll production
– Affect levels of plant hormones that regulate
growth
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 39.2: Plant hormones help coordinate
growth, development, and responses to stimuli
• Hormones: chemical signals that coordinate
different parts of organism
– Signaling molecule produced in tiny amounts
by one part of organism and transported to
other parts, where it binds to specific receptor
and triggers responses in target cells/tissues
– Many plant biologists prefer plant growth
regulator to describe organic compounds that
modify or control one or more specific
physiological processes within plant
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Discovery of Plant Hormones
http://bcs.whfreeman.com/thelifewire/content/chp38/3802001.html
• Tropism: any response resulting in curvature of organs toward
or away from stimulus (often caused by hormones)
• In late 1800s, Charles Darwin/son Francis conducted
experiments on phototropism (plant’s response to light)
–
Observed that grass seedling could bend toward light only if
tip of coleoptile was present (maximum exposure of leaves to
light for photosynthesis)
• Shoot of sprouting grass (enclosed in coleoptile) grows straight
upward if seedling kept in dark/illuminated for all sides uniformly
• If illuminated form one side, grows toward light (results from
differential growth of cells on opposite sides of coleoptile; cells
on darker side elongate faster than those on brighter side)
–
Postulated signal was transmitted from tip to elongating region
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Darwin/Darwin: phototropic response only when tip is illuminated:
Only tip of coleoptile senses light
Phototrophic bending occurred at distance from site of light perception (tip)
Fig. 39-5c
RESULTS
Boysen-Jensen: phototropic response when tip is separated
by permeable barrier, but not with impermeable barrier
Demonstrated that signal was mobile chemical substance
Light
Tip separated by gelatin
(permeable)
Phototrophic response
occurred
Tip separated by mica
(impermeable)
No phototrophic
response occurred
Fig. 39-6
http://bcs.whfreeman.com/thelifewire/content/chp38/3802002.html
RESULTS
Excised tip placed
on agar cube
In 1926, Frits Went
extracted chemical
messenger for
phototropism,
auxin, by modifying
earlier experiments
Auxin later purified/
chemical structure
found to be IAA
(indoleacetic acid)
by Kenneth
Thimann
Growth-promoting
chemical diffuses
into agar cube
Control
Control
(agar cube
lacking
chemical)
has no
effect
Agar cube
with chemical
stimulates growth
Offset cubes
cause curvature
Off center blocks caused
decapitated coleoptiles to
bend away from side with
agar block, as though
growing toward light
Concluded that agar block
contained chemical
produced in coleoptile tip
that stimulated growth as
it passed down coleoptile/
which curved toward light
because higher concentration of growth-promoting
chemical on darker side of
coleoptile
• These studies show asymmetrical distribution of auxin
moving down from coleoptile tip and causes cells on
darker side to elongate faster than cells on brighter
side
• Studies of organs other than grass coleoptiles provide
less support for this, but there is asymmetrical
distribution of certain substances that may act as
growth inhibitors, and these are more concentrated on
lighted side of stem
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A Survey of Plant Hormones
• Plant hormones are produced in very low concentration, but minute
amount can greatly affect growth/development of plant organ
–
Amplified in some way
–
Hormone may act by altering expression of genes, by affecting activity
of existing enzymes, by changing properties of membranes
–
Any of these actins could redirect metabolism and development of cell
responding to small number of hormone molecules
• In general, hormones control plant growth/development by affecting
division, elongation, and differentiation of cells
–
Some mediate shorter-term physiological responses to environmental
stimuli
–
Each hormone has multiple effects, depending on site of action,
concentration, developmental stage of plant
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Table 39-1
Auxin
• Auxin refers to any chemical that promotes elongation of
coleoptiles (multiple functions in flowering plants: cell
elongation/lateral root formation)
– Indoleacetic acid (IAA) is common natural auxin in plants
– Transported directly through parenchyma tissue, from one
cell to another
• Polar transport: unidirectional transport of auxin (in
shoot, moves only from tip to base)
– Polarity of auxin movement attributable to polar
distribution of auxin transport protein in cells that
move hormone from basal end of one cell into apical
end of neighboring cell
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The Role of Auxin in Cell Elongation
• One of chief functions of auxin is to stimulate elongation of cells
within young developing shoots
–
Apical meristem of shoot major site of auxin synthesis, moving
down to region of cell elongation
• According to acid growth hypothesis, auxin stimulates proton
pumps in plasma membrane 
–
Proton pumps lower pH in cell wall, activating expansins
(enzymes that loosen wall’s fabric by breaking hydrogen bonds
between cellulose microfibrils and other cell wall constituents)
–
With the cellulose loosened, cell can elongate
• Also rapidly alter gene expression, causing cells in region of
elongation to produce new proteins within minutes
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Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis
3 Expansins separate
Cross-linking
polysaccharides
Cell wall–loosening
enzymes
microfibrils from crosslinking polysaccharides.
Expansin
CELL WALL
4 Cleaving allows
microfibrils to slide.
Cellulose
microfibril
H2O
2 Cell wall
Plasma
membrane
becomes
more acidic.
Cell
wall
1 Auxin
increases
proton pump
activity.
Plasma membrane
Nucleus
Cytoplasm
Vacuole
CYTOPLASM
http://bcs.whfreeman.com/thelifewire/content/chp38/3802003.html
5 Cell can elongate.
Lateral and Adventitious Root Formation
Auxins as Herbicides/Other Effects of Auxin
• Auxin is involved in root formation and branching
– Used in vegetative propagation of plants by cuttings
• Overdose of synthetic auxins can kill eudicots
– Monocots rapidly inactivate these synthetic auxins
• Auxin affects secondary growth by inducing cell
division in vascular cambium and influencing
differentiation of secondary xylem
– Developing seeds produce auxin which promotes fruit
growth (can induce normal fruit development without
pollination for commercially grown seedless tomatoes)
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Cytokinins
Control of Cell Division and Differentiation
• Cytokinins are so named because they stimulate
cytokinesis (cell division)
– Produced in actively growing tissues (roots, embryos,
and fruits)
• Work together w/auxin to control cell division/differentiation
– When concentration of both at certain levels, mass of
cells continues to grow, but remains cluster of
undifferentiated cells (callus)
• If cytokinin levels increase, shoot buds develop
• If auxin level increase, roots form
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Control of Apical Dominance
• Cytokinins, auxin, and other factors interact in control of apical
dominance, terminal bud’s ability to suppress development of
axillary buds
– Direct inhibition hypothesis says auxin/cytokinins act
antagonistically in regulating axillary bud growth
• Auxin transported down shoot from apical bud directly
inhibits axillary buds from growing, causing stem to
elongate
• Cytokinins entering shoot system from roots counter
action by signaling axillary buds to grow
• Does not account for all experimental findings
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-9
Lateral branches
“Stump” after
removal of
apical bud
(b) Apical bud removed, enables lateral
branches to grow (removes inhibition)
Axillary buds
(a)
Apical bud intact (primary source of auxin)
Inhibition of growth of axillary buds, possibly influenced by
auxin from apical bud, favors elongation of shoot’s main axis
(c) Auxin added to decapitated stem
prevents lateral branches from growing
Anti-Aging Effects
• Cytokinins retard aging of some plant organs by
inhibiting protein breakdown, stimulating RNA and
protein synthesis, and mobilizing nutrients from
surrounding tissues
– If leaves removed from plant dipped in cytokinin
solution, stay greener much longer
– Also slows deterioration of leaves on intact
plants (used to spray on cut flowers to keep
fresh)
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Gibberellins
Stem Elongation/Fruit Growth
• Gibberellins have variety of effects, such as stem elongation,
fruit growth, and seed germination
• Major sites of gibberellin production are roots/young leaves
–
Stimulate growth of leaves/stems, little effect on roots
–
In stems, they stimulate cell elongation and cell division
• Both auxin/gibberellins must be present for some fruit to ripen
–
Used in spraying of Thompson
seedless grapes which makes
individual grapes grow larger/
make internodes elongate,
allowing more space for individual
grapes (reduces yeasts/other
microorganisms from infecting fruit)
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Fig. 39-11 Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley
1 After water is imbibed,
gibberellins (GA) from
embryo signals to
aleurone to germinate
2 Aleurone secretes
-amylase and other enzymes.
3 Sugars and other
nutrients are consumed.
Aleurone
Endosperm
-amylase
GA
GA
Water
Scutellum
(cotyledon)
Radicle
Germination
Sugar
Brassinosteroids
• Brassinosteroids are steroids, chemically similar
to sex hormones of animals
– Induce cell elongation/division in stem
segments and seedlings
– Reduce leaf abscission (leaf drop)
– Promote xylem differentiation
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Abscisic Acid
• Abscisic acid (ABA) slows growth
– Antagonistic to growth hormones and ratio
determines final physiological outcome
– Two of many effects of ABA
– Seed dormancy
– Drought tolerance
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Seed Dormancy
• Seed dormancy ensures that seed will
germinate only in optimal conditions
• In some seeds, dormancy is broken
when
–
ABA is removed by heavy rain,
light, or prolonged cold
–
Light/prolonged exposure to cold
to inactivate ABA
• Precocious (early) germination is
observed in maize mutants that lack
transcription factor required for ABA to
induce expression of certain genes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Drought Tolerance
• ABA is primary internal signal that enables plants
to withstand drought
– Plant begins to wilt  ABA accumulates in
leaves  rapid closing of stomata  reduces
transpiration/prevents further water loss
• ABA causes potassium channels in guard
cells to open  loss of potassium ions from
cells  osmotic loss of water  guard cell
turgor reduction  closing of stomatal pores
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Ethylene (H2C=CH2)
The Triple Response to Mechanical Stress
• Plants produce ethylene in response to stresses such as
drought, flooding, mechanical pressure, injury, and infection
–
Effects of ethylene include response to mechanical stress, senescence,
leaf abscission, fruit ripening, in response to high concentrations of
externally applied auxin
• Ethylene induces triple response,
which allows growing shoot to
avoid obstacles (rock in ground)
–
Consists of slowing of stem
elongation, thickening of stem
(making it stronger), and horizontal
growth (curvature)
–
Resumes vertical growth when
obstacle gone
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• Ethylene-insensitive mutants fail to undergo triple response
after exposure to ethylene because they lack functional
ethylene receptor (ein or ethylene-insensitive mutants)
• Others undergo triple response in air without physical obstacles
and have regulatory defect that causes them to produce
ethylene at rates 20x normal (eto or ethylene overproducing
mutants)
–
Can be restored to wild-type by treating seedlings with inhibitors of
ethylene synthesis
• Other mutants undergo triple response in air but do not respond
to inhibitors of ethylene synthesis (ctr, constitutive tripleresponse mutants)
–
Do not respond to inhibitors of ethylene synthesis
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Fig. 39-14
ein mutant
ctr mutant
(a) ein mutant fails to undergo
triple response in presence of
ethylene
(b) ctr mutant undergoes triple response
even in absence of ethylene
Senescence
• Senescence: programmed death of plant cells or
organs or entire plant
– Starts w/onset of apoptosis (programmed cell
death) where newly formed enzymes break
down many chemical components (chlorophyll,
DNA, RNA, proteins, membrane lipids) to be
salvaged by plant
– Burst of ethylene associated w/programmed
destruction of cells, organs, entire plant
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Leaf Abscission
• Change in balance of auxin
and ethylene controls leaf
abscission (process that
occurs in autumn when leaf
falls)
–
Essential elements
salvaged/stored in stem
parenchyma cells
–
Nutrients recycled back to
developing leaves next
spring
–
After leaf falls, protective
layer of cork becomes leaf
scar that prevents pathogens
from invading plant
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fruit Ripening
• Immature fleshy fruit (tart, hard, green) protect
them from herbivores
• Ripe fruit attracts animals to disperse seeds
• Burst of ethylene production (positive feedback) in
fruit triggers ripening process (enzymatic
breakdown of cell wall softens fruit, conversion of
starches/acids to sugars makes them sweet)
– Moving air prevents ethylene accumulation/
carbon dioxide prevents ethylene production 
slows ripening of stored fruits
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Systems Biology and Hormone Interactions
• Interactions between hormones and signal
transduction pathways make it hard to predict how
genetic manipulation will affect plant
• Systems biology seeks comprehensive
understanding that permits modeling of plant
functions
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Concept 39.3: Responses to light are critical for
plant success
• Light cues many key events in plant growth and development,
photosynthesis, measuring passage of days and seasons
–
Photomorphogenesis: effects of light on plant morphology
• Plants detect not only presence of light but also direction,
intensity, and wavelength (color)
–
Graph (action spectrum) depicts relative effectiveness of
different wavelengths of radiation in driving particular process
• Two peaks (red/blue light) for photosynthesis
–
Action spectra are useful in studying any process that depends
on light (phototropism)
–
Two major classes of light receptors: blue-light photoreceptors
and phytochromes
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Fig. 39-16 Action spectrum for blue-light-stimulated phototropism in maize coleoptiles
Phototropic effectiveness
1.0
436 nm
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
Wavelength (nm)
(a) Action spectrum for blue-light phototropism
Light
Time = 0 min
Time = 90 min
(b) Coleoptile response to light colors
Phototropic bending toward light controlled by phototropin (photoreceptor
sensitive to blue/violet light, particularly blue light
Blue-Light Photoreceptors
Phytochromes as Photoreceptors
• Various blue-light photoreceptors control hypocotyl
elongation, stomatal opening, and phototropism
• Phytochromes are pigments that regulate many
of plant’s responses to light throughout its life
– These responses include seed germination and
shade avoidance
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Phytochromes and Seed Germination
• Many seeds remain dormant
until light conditions change
• In 1930s, scientists at U.S.
Department of Agriculture
determined action spectrum
for light-induced germination
of lettuce seeds
–
Red light increased
germination, while far-red
light inhibited germination
–
Final light exposure was
determining factor
–
Effects of red/far-red light
reversible
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Fig. 39-18
Photoreceptor responsible for opposing effects of red/far-red light are phytochromes
Two identical subunits, each consisting of polypeptide
component covalently bonded to nonpolypeptide chromophore
Chromophore
Photoreceptor activity
Kinase activity
• Phytochromes exist in two photoreversible states
– Depend on color of light provided
– Converts Pr (inhibits germination) to Pfr, which
triggers many developmental responses (germination)
– Though light contains both
red and far red light,
conversion to Pfr faster than
conversion to Pr so ratio of
Pfr to Pr increases in light,
triggering germination
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Fig. 39-19
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter41/animation_-_phytochrome_signaling.html
Pfr
Pr
Red light
Responses:
seed germination,
control of
flowering, etc.
Synthesis
Far-red
light
Slow conversion
in darkness
(some plants)
Enzymatic
destruction
Phytochromes and Shade Avoidance
• Phytochrome system also provides plant with information about
quality of light
– Sunlight contains both red/far-red radiation
– During day, Pr ⇄ Pfr interconversion reaches dynamic
equilibrium, with ratio of two phytochrome forms indicating
relative amounts of red/far-red light
– Allows plants to adapt to changes in light conditions
• Shaded plants receive more far-red than red light
– In “shade avoidance” response, phytochrome ratio shifts in
favor of Pr when tree is shaded, inducing tree to allocate more
resources to growing taller
• Direct sunlight stimulates branching/inhibits vertical growth
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Biological Clocks and Circadian Rhythms
• Phytochrome helps plant keep track of passage of
days/seasons
• Many plant processes (transpiration, synthesis of certain
enzymes) oscillate during the day
– Response to changes in light levels, temperature, and
relative humidity accompanying 24-hour cycle of
day/night
– Many legumes lower their
leaves in evening and
raise them in morning,
even when kept under
constant light or dark conditions
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• Circadian rhythms: cycles that are about 24 hours long and
are governed by internal “clock” rather than daily responses to
environmental cycle
–
Can be entrained (set) to exactly 24 hours by day/night cycle
from environment
–
Deviations (free-running periods of 21-27 hours) occur when
organism kept in constant environment (not erratic because
still keep perfect time, just not synchronized with outside
world)
–
We can interfere with biological rhythm, but will reestablish
position for time of day when interference removed
–
Clock may depend on synthesis of protein regulated through
feedback control and may be common to all eukaryotes
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The Effect of Light on the Biological Clock
• Light is factor that entrains biological clock to precisely 24
hours each day
–
Desynchronization occurs when clock on wall not synchronized
w/internal clocks (jet lag)
• Phytochrome conversion marks sunrise/sunset, providing
biological clock w/environmental cues
–
Reduction of Pfr due to
• No sunlight to convert Pr to Pfr
• Pigment of phytochrome synthesized as Pr
• Decrease of Pfr in darkness when enzymes destroy more Pfr
than Pr
–
Increase of Pfr at dawn (light returns) resets biological clock
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Photoperiodism and Responses to Seasons
• Seasonal events critical in life cycles of most plants (seed
germination, flowering, onset/breaking of bud dormancy)
require photoperiod
– Photoperiod (relative lengths of night and day) is
environmental stimulus plants use most often to detect
time of year
– Photoperiodism is physiological response to
photoperiod, or response to change in length of night,
that results in flowering in low-day and short-day
plants/preparation for winter
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Photoperiodism and Control of Flowering
• Some processes, including flowering in many species, require
certain photoperiod
• Critical length: period of daylight, specific in length for any
given species, that appears to initiate flowering in long-day
plants or inhibit flowering in short-day plants
– Short-day plant: plants that flower when light period is
shorter than critical length/chrysanthemums, poinsettias,
some soybean
– Long-day plants: plants that flower when light period is
longer than certain number of hours/spinach, radishes,
lettuce, irises, many cereal varieties
– Day-neutral plants: flowering controlled by plant maturity,
not photoperiod/tomatoes, rice, dandelions
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Critical Night Length
• In the 1940s, researchers discovered that flowering and other
responses to photoperiod are actually controlled by night
length, not day length
•
Short-day plants (really long-night plants) governed by whether
critical night length sets minimum number of hours of darkness
• If daytime portion of photoperiod broken by brief exposure to
darkness, no effect on flowering
• If nighttime part interrupted by even few minutes of dim light,
flowering will not occur
•
Long-day plants (really short-night plants) governed by whether
critical night length sets maximum number of hours of darkness
• Brief flash of light interrupting long period of darkness will
induce flowering, even if not enough daylight hours 
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-21 Photoperiodic control of flowering
24 hours
(a) Short-day (long-night)
plant, Pfr inhibits flowering
Light
Critical
dark period
Flash
of
light
Darkness
(b) Long-day (short-night)
plant, Pfr induces flowering
Flash
of
light
Fig. 39-22 Reversible effects of red and far-red light on photoperiodic response
24 hours
Red light is most effective color in interrupting nighttime portion of photoperiod
Action spectra/photoreversibility experiments show that phytochrome is
pigment that receives red light
R
flash of red light shortens dark period (night length)
RFR subsequent flash of far-red light cancels red flash’s effect
(restores night length)
RFRR
RFRRFR
Critical dark period
http://bcs.whfreeman.com/thelifewire/content/chp39/3902002.html
Long-day
Short-day
(long-night) (short-night)
plant
plant
• Some plants flower after only single exposure to
required photoperiod
• Other plants need several successive days of
required photoperiod
• Still others need environmental stimulus in addition
to required photoperiod
– For example, vernalization is pretreatment with
cold to induce flowering
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A Flowering Hormone?
• Only one leaf needed to cue buds to develop flowers
• Florigen: flowering signal, not yet chemically identified
– May be macromolecule governed by CONSTANS
gene, which is expressed in leaves (as is florigen)
– CONSTANS might turn on gene called FLOWERING
LOCUS T (FT) in leaf/FT protein travels to shoot apical
meristem and initiates flowering
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-23 Experimental evidence for a flowering hormone
24 hours
24 hours
24 hours
Graft
Short-day plant
grown under short-day
conditions will flower
Long-day plant grafted
to short-day plant
Long-day plant
grown under short-day
induced to grow by transmission conditions doesn’t flower
of florigen across grafts
Meristem Transition and Flowering
• For bud to form flower instead of vegetative shoot,
meristem identity genes must first be switched on
– Organ identity genes that specify spatial
organization of floral organs (sepals, petals,
stamens, carpels) activated in appropriate
regions of meristem
• Researchers seek to identify signal transduction
pathways that link cues such as photoperiod and
hormonal changes to gene expression required for
flowering
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Concept 39.4: Plants respond to a wide variety of
stimuli other than light/Gravity
• Because of immobility, plants must
adjust to range of environmental
circumstances through
developmental and physiological
mechanisms
• Gravitropism: Response to
gravity/what causes roots to grow
down (positive gravitropism) and
shoots up (negative gravitropism)
– Auxin plays key role in
gravitropism
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Plants may detect gravity by settling of statoliths, specialized
plastids containing dense starch grains located in lower
portions of cells (in roots, certain root cap cells)
–
Aggregates at lower points trigger redistribution
calcium, which causes lateral transport of auxin
root
within
–
Calcium/auxin accumulate on lower side of
root’s zone of elongation
–
At high concentrations, auxin inhibits cell
elongation, slowing growth on root’s lower side
–
More rapid elongation on upper side causes
root to curve as it grows
–
Still occurs in plants w/no statoliths (dense organelles, in addition to
starch granules, may contribute to gravity detection)
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Mechanical Stimuli
• Thigmomorphogenesis: changes in form that
result from mechanical disturbance
–
Rubbing stems of young plants couple of times daily results in plants
that are shorter than controls/tree growing on windy hill shorter/stockier
than those in sheltered location
–
Mechanical stimulation  signal transduction pathway  increase in
cytosolic Ca2+  activation of specific genes of proteins  cell wall
properties
• Thigmotropism is growth in response to touch
–
Occurs in vines/other climbing plants where
coil rapidly around supports
tendrils that
• Grow straight until they touch something that stimulates coiling
response (caused by differential growth of cells on opposite sides
of tendril)
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• Rapid leaf movements in
response to mechanical
stimulation are examples of
transmission of electrical
impulses (action potentials)
– Rapid loss of turgor in cells
within pulvini (specialized
motor organs at joints of
leaf)  loss of potassium 
osmotic loss of water 
motor cells become flaccid
 regain turgor (~10
minutes)  open again
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Environmental Stresses
• Environmental stresses have potentially adverse
effect on survival, growth, and reproduction
• Stresses can be
– Abiotic (nonliving) include drought, flooding,
salt stress, heat stress, cold stress
– Biotic (living) include pathogens, herbivores
• Plants not tolerating environmental stress will
succumb/be outcompeted by other plants, and
become locally extinct
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Drought
• During drought, plants reduce transpiration by
– Closing stomata (stimulates increased synthesis/release of
abscisic acid in leaf which helps keep stomata closed by
acting on guard cell membranes)
– Slowing leaf growth (need turgor/accumulation of abscisic
acid)
– Reducing exposed surface area
– Also reduces photosynthesis (why drought diminishes crop
growth)
• Growth of shallow roots is inhibited (dries from surface down),
while deeper roots continue to grow (look for deeper water)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Flooding (Oxygen deprivation)
• Waterlogged soil lacks air
spaces that provide oxygen
for cellular respiration in roots
– Oxygen deprivation 
ethylene causes some
cells in root cortex to
undergo enzymatic
apoptosis  air tubes
provide oxygen to
submerged roots
– Aerial roots (mangroves)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Salt Stress
• Salt can lower water potential of soil solution
–
As water potential of soil solution becomes more negative, water
potential gradient from soil to roots lowered, reducing water uptake
• Na/certain other ions toxic to plants w/relatively high concentrations
–
Selectively permeable membranes of root cells impede uptake of most
harmful ions, which causes problem acquiring water from solute-rich
soils
• Plants respond to salt stress by producing solutes tolerated at high
concentrations
–
Keeps water potential of cells more negative than that of soil solution
without admitting toxic quantities of salt
• Halophytes: special anatomical/physiological adaptations to grow
best in salty soils (salt gland on leaves eliminate excess salt
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Heat Stress
• Excessive heat can denature plant’s enzymes/damages
metabolism
• Evaporative cooling associated w/transpiration keeps
temperature of leaf 3°-10°C lower than ambient air
temperature
– When stomata close, cooling is lost to conserve water
• Most plants will begin producing heat-shock proteins
(protect other proteins from heat stress) when exposed to
excessive temperatures (40°C or above for temperate zone
plants)
– Some are chaperone proteins (chaperonins) which help
other proteins fold into functional shapes
– Molecules might bind to other proteins/help prevent
denaturation
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Cold Stress
• Cold temperatures decrease membrane fluidity
–
Below critical point, lipids become locked into crystalline structure
which alters solute transport across membrane/ adversely affects
function of membrane proteins
–
Altering lipid composition (unsaturated fatty acids) of membranes is
response to cold stress
• Freezing causes ice to form in plant’s cell walls/intercellular spaces
–
Generally don’t freeze because solutes lower freezing point, but
water loss in cell wall can cause dehydration/high concentrations of
solutes can be toxic
–
Many frost-resistant species increase cytoplasmic levels of specific
solutes (sugars) that are well tolerated at high concentrations/help
reduce loss of water
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 39.5: Plants respond to attacks by herbivores
and pathogens/Defenses Against Herbivores
• Plants use defense systems to deter herbivory, prevent
infection, and combat pathogens
• Herbivory (animals eating plants) is stress that plants face in
any ecosystem
– Plants counter excessive herbivory with physical defenses
(thorns)/chemical defenses (distasteful/toxic compounds)
• Canavanine resembles arginine replaces it in insect’s
proteins  disrupts protein conformation  insect dies
– Some plants even “recruit” predatory animals that help
defend against specific herbivores 
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Figure 39.28 A maize leaf “recruiting” a parasitoid wasp as a defensive response to an armyworm caterpillar, an herbivore
4 Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
3 Synthesis and
release of
volatile attractants
1 Wounding
1 Chemical
in saliva
2 Signal transduction
pathway
• Plants damaged by insects can release volatile
chemicals to warn other plants of the same
species
– Methyljasmonic acid from spider mite-infested
lima beans signals noninfested nearby plants 
activate expression of genes  become less
susceptible to spider mites and more attractive to
another species of mites that preys on spider mites
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Defenses Against Pathogens
http://bcs.whfreeman.com/thelifewire/content/chp40/40020.html
• Plant’s first line of defense against infection is epidermis of
primary plant body and periderm of secondary plant body
• If pathogen penetrates dermal tissue, second line of
defense is chemical attack that kills pathogen and
prevents its spread
– Is enhanced by inherited ability to recognize certain
pathogens
• Virulent pathogen : one plant has little specific defense
against
• Avirulent pathogen: one that may harm but does not kill
host plant
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Gene-for-gene recognition involves recognition of
pathogen-derived molecules by protein products of
specific plant disease resistance (R) genes
– Pathogen is avirulent if it has specific Avr gene
corresponding to particular R allele in host plant
• R protein usually recognizes single corresponding
pathogen molecule encoded by one of pathogen’s
Avr gene
– R proteins activate plant defenses by triggering signal
transduction pathways
– These defenses include hypersensitive response and
systemic acquired resistance
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Hypersensitive Response
• Hypersensitivity response: genetically programmed death
of infected cells/tissues, as well as tissue reinforcement and
antibiotic production, at infection site (localized, specific,
containment response based on gene-for-gene recognition
between host and pathogen)
– Cells at infection site  mount chemical defense  seal off
area  destroy themselves
– Induces production of phytoalexins (toxic compounds
w/general fungicidal and bactericidal properties) and PR
proteins (pathogenesis-related proteins, many being
enzymes that hydrolyze components in cell walls of
pathogens)
– Stimulates changes in cell wall that confine pathogen
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Systemic Acquired Resistance
• Systemic acquired resistance: long-lasting systemic
response that primes plant for resisting broad spectrum of
pathogens in organs distant from infection site
(nonspecific, provide protection against diversity of
pathogens that lasts for days)
– Methylsalicylic acid most likely candidate for
signaling molecule that moves from infection site to
elicit systemic acquired resistance
– Converted to salicylic acid in areas remote from sites
of infection, which then activates signal transduction
pathway that induces production of PR proteins and
resistance to pathogen attack
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-UN4 Indicate the response to each condition by drawing a straight seedling or one with the triple response.
Control
Wild-type
Ethylene insensitive
(ein)
Ethylene
overproducing (eto)
Constitutive triple
response (ctr)
Ethylene
added
Ethylene
synthesis
inhibitor
Fig. 39-UN5
You should now be able to:
1.
Compare the growth of a plant in darkness (etiolation) to the characteristics of
greening (de-etiolation)
2.
List six classes of plant hormones and describe their major functions
3.
Describe the phenomenon of phytochrome photoreversibility and explain its
role in light-induced germination of lettuce seeds
4.
Explain how light entrains biological clocks
5.
Distinguish between short-day, long-day, and day-neutral plants; explain why
the names are misleading
6.
Describe how plants tell up from down
7.
Distinguish between thigmotropism and thigmomorphogenesis
8.
Describe the challenges posed by, and the responses of plants to, drought,
flooding, salt stress, heat stress, and cold stress
9.
Describe how the hypersensitive response helps a plant limit damage from a
pathogen attack
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings