Download Ch. 39 Plant Response to Signals

LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 39
Plant Responses to Internal and
External Signals
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
Overview: Stimuli and a Stationary Life
• Linnaeus noted that flowers of different species
opened at different times of day and could be
used as a horologium florae, or floral clock
• Plants, being rooted to the ground, must
respond to environmental changes that come
their way
– For example, the bending of a seedling toward
light begins with sensing the direction, quantity,
and color of the light
© 2011 Pearson Education, Inc.
Figure 39.1
Concept 39.1: Signal transduction pathways
link signal reception to response
• A potato left growing in darkness produces
shoots that look unhealthy, and it lacks
elongated roots
• These are morphological adaptations for
growing in darkness, collectively called
etiolation
• After exposure to light, a potato undergoes
changes called de-etiolation, in which shoots
and roots grow normally
© 2011 Pearson Education, Inc.
Figure 39.2
(a) Before exposure to light
(b) After a week’s exposure
to natural daylight
Figure 39.2a
(a) Before exposure to light
Figure 39.2b
(b) After a week’s exposure
to natural daylight
• A potato’s response to light is an example of
cell-signal processing
• The stages are reception, transduction, and
response
© 2011 Pearson Education, Inc.
Figure 39.3
CELL
WALL
1 Reception
CYTOPLASM
2 Transduction
3 Response
Relay proteins and
second messengers
Receptor
Hormone or
environmental
stimulus
Plasma membrane
Activation
of cellular
responses
Reception
• Internal and external signals are detected by
receptors, proteins that change in response to
specific stimuli
• In de-etiolation, the receptor is a phytochrome
capable of detecting light
© 2011 Pearson Education, Inc.
Transduction
• Second messengers transfer and amplify
signals from receptors to proteins that cause
responses
• Two types of second messengers play an
important role in de-etiolation: Ca2+ ions and
cyclic GMP (cGMP)
• The phytochrome receptor responds to light by
– Opening Ca2+ channels, which increases Ca2+
levels in the cytosol
– Activating an enzyme that produces cGMP
© 2011 Pearson Education, Inc.
Figure 39.4-1
1 Reception
CYTOPLASM
Plasma
membrane
Phytochrome
Cell
wall
Light
Figure 39.4-2
2 Transduction
1 Reception
CYTOPLASM
Plasma
membrane
cGMP
Second
messenger
Phytochrome
Protein
kinase 1
Cell
wall
Protein
kinase 2
Light
Ca2 channel
Ca2
Figure 39.4-3
2 Transduction
1 Reception
3 Response
Transcription
factor 1 NUCLEUS
CYTOPLASM
Plasma
membrane
cGMP
Second
messenger
Phytochrome
P
Protein
kinase 1
Transcription
factor 2
P
Cell
wall
Protein
kinase 2
Transcription
Light
Translation
Ca2 channel
Ca2
De-etiolation
(greening)
response proteins
Response
• A 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 or
post-translational modification
© 2011 Pearson Education, Inc.
Post-Translational Modification of
Preexisting Proteins
• Post-translational modification involves
modification of existing proteins in the signal
response
• Modification often involves the phosphorylation
of specific amino acids
• The second messengers cGMP and Ca2+
activate protein kinases directly
© 2011 Pearson Education, Inc.
Transcriptional Regulation
• Specific transcription factors bind directly to
specific regions of DNA and control
transcription of genes
• Some transcription factors are activators that
increase the transcription of specific genes
• Other transcription factors are repressors that
decrease the transcription of specific genes
© 2011 Pearson Education, Inc.
De-Etiolation (“Greening”) Proteins
• De-etiolation activates enzymes that
– Function in photosynthesis directly
– Supply the chemical precursors for chlorophyll
production
– Affect the levels of plant hormones that regulate
growth
© 2011 Pearson Education, Inc.
Concept 39.2: Plant hormones help
coordinate growth, development, and
responses to stimuli
• Plant hormones are chemical signals that
modify or control one or more specific
physiological processes within a plant
© 2011 Pearson Education, Inc.
The Discovery of Plant Hormones
• Any response resulting in curvature of organs
toward or away from a stimulus is called a
tropism
• In the late 1800s, Charles Darwin and his son
Francis conducted experiments on
phototropism, a plant’s response to light
• They observed that a grass seedling could bend
toward light only if the tip of the coleoptile was
present
© 2011 Pearson Education, Inc.
• They postulated that a signal was transmitted
from the tip to the elongating region
© 2011 Pearson Education, Inc.
Video: Phototropism
© 2011 Pearson Education, Inc.
Figure 39.5
RESULTS
Shaded
side
Control
Light
Illuminated
side
Boysen-Jensen
Light
Darwin and Darwin
Light
Gelatin
(permeable)
Tip
removed
Opaque
cap
Transparent
cap
Opaque
shield over
curvature
Mica
(impermeable)
• In 1913, Peter Boysen-Jensen demonstrated
that the signal was a mobile chemical substance
© 2011 Pearson Education, Inc.
• In 1926, Frits Went extracted the chemical
messenger for phototropism, auxin, by modifying
earlier experiments
© 2011 Pearson Education, Inc.
Figure 39.6
RESULTS
Excised tip on
agar cube
Growth-promoting
chemical diffuses
into agar cube
Control
(agar cube
lacking
Control chemical)
Offset
cubes
A Survey of Plant Hormones
• Plant hormones are produced in very low
concentration, but a minute amount can greatly
affect growth and development of a plant organ
• In general, hormones control plant growth and
development by affecting the division,
elongation, and differentiation of cells
© 2011 Pearson Education, Inc.
Table 39.1
Auxin
• The term auxin refers to any chemical that
promotes elongation of coleoptiles
• Indoleacetic acid (IAA) is a common auxin in
plants; in this lecture the term auxin refers
specifically to IAA
• Auxin is produced in shoot tips and is
transported down the stem
• Auxin transporter proteins move the hormone
from the basal end of one cell into the apical end
of the neighboring cell
© 2011 Pearson Education, Inc.
Figure 39.7
RESULTS
Cell 1
100 m
Cell 2
Epidermis
Cortex
Phloem
Xylem
Pith
25 m
Basal end
of cell
Figure 39.7a
100 m
Epidermis
Cortex
Phloem
Xylem
Pith
Figure 39.7b
Cell 1
Cell 2
25 m
Basal end
of cell
The Role of Auxin in Cell Elongation
• According to the acid growth hypothesis, auxin
stimulates proton pumps in the plasma
membrane
• The proton pumps lower the pH in the cell wall,
activating expansins, enzymes that loosen the
wall’s fabric
• With the cellulose loosened, the cell can
elongate
© 2011 Pearson Education, Inc.
Figure 39.8
Cross-linking
polysaccharides
Cell wall–loosening
enzymes
Expansin
CELL WALL
Cellulose
microfibril
H2O
H
Plasma
membrane
H
H
H
ATP
H
H
H
Cell wall
H
H
Plasma membrane
CYTOPLASM
Nucleus Cytoplasm
Vacuole
Figure 39.8a
Cross-linking
polysaccharides
Cell wall–loosening
enzymes
Expansin
CELL WALL
Cellulose
microfibril
H
H
H
ATP
H
H
H
H
H
H
Plasma membrane
CYTOPLASM
Figure 39.8b
H2O
Plasma
membrane
Cell wall
Nucleus Cytoplasm
Vacuole
• Auxin also alters gene expression and
stimulates a sustained growth response
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Auxin’s Role in Plant Development
• Polar transport of auxin plays a role in pattern
formation of the developing plant
• Reduced auxin flow from the shoot of a branch
stimulates growth in lower branches
• Auxin transport plays a role in phyllotaxy, the
arrangement of leaves on the stem
• Polar transport of auxin from leaf margins directs
leaf venation pattern
• The activity of the vascular cambium is under
control of auxin transport
© 2011 Pearson Education, Inc.
Practical Uses for Auxins
• The auxin indolbutyric acid (IBA) stimulates
adventitious roots and is used in vegetative
propagation of plants by cuttings
• An overdose of synthetic auxins can kill plants
– For example 2,4-D is used as an herbicide on
eudicots
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Cytokinins
• Cytokinins are so named because they
stimulate cytokinesis (cell division)
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Control of Cell Division and Differentiation
• Cytokinins are produced in actively growing
tissues such as roots, embryos, and fruits
• Cytokinins work together with auxin to control
cell division and differentiation
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Control of Apical Dominance
• Cytokinins, auxin, and strigolactone interact in
the control of apical dominance, a terminal bud’s
ability to suppress development of axillary buds
• If the terminal bud is removed, plants become
bushier
© 2011 Pearson Education, Inc.
Figure 39.9
Lateral branches
“Stump” after
removal of
apical bud
(b) Apical bud removed
Axillary buds
(a) Apical bud intact (not shown in photo)
(c) Auxin added to decapitated stem
Figure 39.9a
Axillary buds
(a) Apical bud intact (not shown in photo)
Figure 39.9b
Lateral branches
“Stump” after
removal of
apical bud
(b) Apical bud removed
Figure 39.9c
(c) Auxin added to decapitated stem
Anti-Aging Effects
• Cytokinins slow the aging of some plant organs by
inhibiting protein breakdown, stimulating RNA and
protein synthesis, and mobilizing nutrients from
surrounding tissues
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Gibberellins
• Gibberellins have a variety of effects, such
as stem elongation, fruit growth, and seed
germination
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Stem Elongation
• Gibberellins are produced in young roots and
leaves
• Gibberellins stimulate growth of leaves and
stems
• In stems, they stimulate cell elongation and cell
division
© 2011 Pearson Education, Inc.
Figure 39.10
(b) Grapes from control vine
(left) and gibberellin-treated
vine (right)
(a) Rosette form (left) and
gibberellin-induced bolting
(right)
Figure 39.10a
(a) Rosette form (left) and
gibberellin-induced bolting
(right)
Fruit Growth
• In many plants, both auxin and gibberellins must
be present for fruit to develop
• Gibberellins are used in spraying of Thompson
seedless grapes
© 2011 Pearson Education, Inc.
Figure 39.10b
(b) Grapes from control vine
(left) and gibberellin-treated
vine (right)
Germination
• After water is imbibed, release of gibberellins
from the embryo signals seeds to germinate
© 2011 Pearson Education, Inc.
Figure 39.11
Aleurone
2
Endosperm 1
3
-amylase
GA
Water
Scutellum
(cotyledon)
GA
Radicle
Sugar
Brassinosteroids
• Brassinosteroids are chemically similar to the
sex hormones of animals
• They induce cell elongation and division in stem
segments
• They slow leaf abscission and promote xylem
differentiation
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Abscisic Acid
• Abscisic acid (ABA) slows growth
• Two of the many effects of ABA
– Seed dormancy
– Drought tolerance
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Seed Dormancy
• Seed dormancy ensures that the seed will
germinate only in optimal conditions
• In some seeds, dormancy is broken when ABA
is removed by heavy rain, light, or prolonged
cold
• Precocious (early) germination can be caused
by inactive or low levels of ABA
© 2011 Pearson Education, Inc.
Figure 39.12
Red mangrove
(Rhizophora mangle)
seeds
Coleoptile
Maize mutant
Figure 39.12a
Red mangrove
(Rhizophora mangle)
seeds
Figure 39.12b
Coleoptile
Maize mutant
Drought Tolerance
• ABA is the primary internal signal that enables
plants to withstand drought
• ABA accumulation causes stomata to close
rapidly
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Strigolactones
• The hormones called strigolactones
– Stimulate seed germination
– Help establish mycorrhizal associations
– Help control apical dominance
• Strigolactones are named for parasitic Striga
plants
• Striga seeds germinate when host plants
exude strigolactones through their roots
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Ethylene
• Plants produce ethylene in response to stresses
such as drought, flooding, mechanical pressure,
injury, and infection
• The effects of ethylene include response to
mechanical stress, senescence, leaf abscission,
and fruit ripening
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The Triple Response to Mechanical Stress
• Ethylene induces the triple response, which
allows a growing shoot to avoid obstacles
• The triple response consists of a slowing of stem
elongation, a thickening of the stem, and
horizontal growth
© 2011 Pearson Education, Inc.
Figure 39.13
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)
• Ethylene-insensitive mutants fail to undergo the
triple response after exposure to ethylene
• Other mutants undergo the triple response in air
but do not respond to inhibitors of ethylene
synthesis
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Figure 39.14
ein mutant
ctr mutant
(a) ein mutant
(b) ctr mutant
Figure 39.14a
ein mutant
(a) ein mutant
Figure 39.14b
ctr mutant
(b) ctr mutant
Senescence
• Senescence is the programmed death of cells
or organs
• A burst of ethylene is associated with
apoptosis, the programmed destruction of
cells, organs, or whole plants
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Leaf Abscission
• A change in the balance of auxin and ethylene
controls leaf abscission, the process that
occurs in autumn when a leaf falls
© 2011 Pearson Education, Inc.
Figure 39.15
0.5 mm
Protective layer
Abscission layer
Stem
Petiole
Figure 39.15a
0.5 mm
Protective layer
Abscission layer
Stem
Petiole
Fruit Ripening
• A burst of ethylene production in a fruit triggers
the ripening process
• Ethylene triggers ripening, and ripening triggers
release of more ethylene
• Fruit producers can control ripening by picking
green fruit and controlling ethylene levels
© 2011 Pearson Education, Inc.
Systems Biology and Hormone Interactions
• Interactions between hormones and signal
transduction pathways make it hard to predict
how genetic manipulation will affect a plant
• Systems biology seeks a 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
• Effects of light on plant morphology are called
photomorphogenesis
© 2011 Pearson Education, Inc.
• Plants detect not only presence of light but also
its direction, intensity, and wavelength (color)
• A graph called an action spectrum depicts
relative response of a process to different
wavelengths
• Action spectra are useful in studying any
process that depends on light
© 2011 Pearson Education, Inc.
Figure 39.16
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) Phototropism action spectrum
Light
Time  0 min
Time  90 min
(b) Coleoptiles before and after light exposures
Figure 39.16a
Phototropic effectiveness
1.0
436 nm
0.8
0.6
0.4
0.2
0
400
450
500
550
600
Wavelength (nm)
(a) Phototropism action spectrum
650
700
Figure 39.16b
Light
Time  0 min
Time  90 min
(b) Coleoptiles before and after light exposures
Figure 39.16c
Time  0 min
Figure 39.16d
Time  90 min
• Different plant responses can be mediated by
the same or different photoreceptors
• There are two major classes of light receptors:
blue-light photoreceptors and
phytochromes
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Blue-Light Photoreceptors
• Various blue-light photoreceptors control
hypocotyl elongation, stomatal opening, and
phototropism
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Phytochromes as Photoreceptors
• Phytochromes are pigments that regulate many
of a 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 the 1930s, scientists at the U.S. Department of
Agriculture determined the action spectrum for
light-induced germination of lettuce seeds
© 2011 Pearson Education, Inc.
Figure 39.17
RESULTS
Red
Dark
Red Far-red
Dark
Dark (control)
Red Far-red Red
Dark
Red Far-red Red Far-red
Figure 39.17a
Dark (control)
Figure 39.17b
Red
Dark
Figure 39.17c
Red Far-red
Dark
Figure 39.17d
Red Far-red Red
Dark
Figure 39.17e
Red Far-red Red Far-red
• Red light increased germination, while far-red
light inhibited germination
• The photoreceptor responsible for the opposing
effects of red and far-red light is a phytochrome
© 2011 Pearson Education, Inc.
Figure 39.18
Two identical subunits
Chromophore
Photoreceptor activity
Kinase activity
Figure 39.UN01
Red light
Pr
Pfr
Far-red light
• Phytochromes exist in two photoreversible
states, with conversion of Pr to Pfr triggering
many developmental responses
• Red light triggers the conversion of Pr to Pfr
• Far-red light triggers the conversion of Pfr to Pr
• The conversion to Pfr is faster than the
conversion to Pr
• Sunlight increases the ratio of Pfr to Pr, and
triggers germination
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Figure 39.19
Pr
Pfr
Red light
Synthesis
Responses:
seed
germination,
control of
flowering, etc.
Far-red
light
Slow conversion
in darkness
(some plants)
Enzymatic
destruction
Phytochromes and Shade Avoidance
• The phytochrome system also provides the
plant with information about the quality of light
• Leaves in the canopy absorb red light
• Shaded plants receive more far-red than red
light
• In the “shade avoidance” response, the
phytochrome ratio shifts in favor of Pr when a
tree is shaded
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Biological Clocks and Circadian Rhythms
• Many plant processes oscillate during the day
• Many legumes lower their leaves in the evening
and raise them in the morning, even when kept
under constant light or dark conditions
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Figure 39.20
Noon
Midnight
Figure 39.20a
Noon
Figure 39.20b
Midnight
• Circadian rhythms are cycles that are about
24 hours long and are governed by an internal
“clock”
• Circadian rhythms can be entrained to exactly
24 hours by the day/night cycle
• The clock may depend on synthesis of a
protein regulated through feedback control and
may be common to all eukaryotes
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The Effect of Light on the Biological Clock
• Phytochrome conversion marks sunrise and
sunset, providing the biological clock with
environmental cues
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Photoperiodism and Responses to Seasons
• Photoperiod, the relative lengths of night and
day, is the environmental stimulus plants use
most often to detect the time of year
• Photoperiodism is a physiological response to
photoperiod
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Photoperiodism and Control of Flowering
• Some processes, including flowering in many
species, require a certain photoperiod
• Plants that flower when a light period is shorter
than a critical length are called short-day plants
• Plants that flower when a light period is longer
than a certain number of hours are called longday plants
• Flowering in day-neutral plants is controlled by
plant maturity, not photoperiod
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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
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• Short-day plants are governed by whether the
critical night length sets a minimum number of
hours of darkness
• Long-day plants are governed by whether the
critical night length sets a maximum number of
hours of darkness
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Figure 39.21
24 hours
(a) Short day
(long-night) plant
Flash Darkness
of
Critical
dark period light
Light
(b) Long-day
(short-night) plant
Flash
of light
• Red light can interrupt the nighttime portion of
the photoperiod
• A flash of red light followed by a flash of far-red
light does not disrupt night length
• Action spectra and photoreversibility
experiments show that phytochrome is the
pigment that receives red light
© 2011 Pearson Education, Inc.
Figure 39.22
24 hours
R
R FR
R FR R
R FR R FR
Critical dark period
Long-day
Short-day
(long-night) (short-night)
plant
plant
• Some plants flower after only a single exposure
to the required photoperiod
• Other plants need several successive days of
the required photoperiod
• Still others need an environmental stimulus in
addition to the required photoperiod
– For example, vernalization is a pretreatment
with cold to induce flowering
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A Flowering Hormone?
• Photoperiod is detected by leaves, which cue
buds to develop as flowers
• The flowering signal is called florigen
• Florigen may be a macromolecule governed
by the FLOWERING LOCUS T (FT) gene
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Figure 39.23
24 hours
24 hours
Long-day plant
grafted to
short-day plant
Long-day
plant
24 hours
Graft
Short-day
plant
Concept 39.4: Plants respond to a wide
variety of stimuli other than light
• Because of immobility, plants must adjust to a
range of environmental circumstances through
developmental and physiological mechanisms
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Gravity
• Response to gravity is known as gravitropism
• Roots show positive gravitropism; shoots show
negative gravitropism
• Plants may detect gravity by the settling of
statoliths, dense cytoplasmic components
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Video: Gravitropism
© 2011 Pearson Education, Inc.
Figure 39.24
Statoliths
(a) Primary root of maize
bending gravitropically
(LMs)
20 m
(b) Statoliths settling to
the lowest sides of
root cap cells (LMs)
Figure 39.24a
Figure 39.24b
Figure 39.24c
Statoliths
20 m
Figure 39.24d
Statoliths
20 m
• Some mutants that lack statoliths are still
capable of gravitropism
• Dense organelles, in addition to starch granules,
may contribute to gravity detection
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Mechanical Stimuli
• The term thigmomorphogenesis refers to
changes in form that result from mechanical
disturbance
• Rubbing stems of young plants a couple of
times daily results in plants that are shorter than
controls
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Figure 39.25
• Thigmotropism is growth in response to touch
• It occurs in vines and other climbing plants
• Another example of a touch specialist is the
sensitive plant Mimosa pudica, which folds its
leaflets and collapses in response to touch
• Rapid leaf movements in response to
mechanical stimulation are examples of
transmission of electrical impulses called
action potentials
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Video: Mimosa Leaf
© 2011 Pearson Education, Inc.
Figure 39.26
(a) Unstimulated state
(b) Stimulated state
Side of pulvinus
with flaccid cells
Pulvinus
(motor
organ)
Side of pulvinus
with turgid cells
Vein
0.5 m
Leaflets
after
stimulation
(c) Cross section of a leaflet pair in the stimulated state (LM)
Figure 39.26a
(a) Unstimulated state
Figure 39.26b
(b) Stimulated state
Figure 39.26c
Leaflets
after
stimulation
Pulvinus
(motor
organ)
(c) Cross section of a leaflet pair
in the stimulated state (LM)
Figure 39.26d
Side of pulvinus
with flaccid cells
Side of pulvinus
with turgid cells
0.5 m
Vein
(c) Cross section of a leaflet pair in
the stimulated state (LM)
Environmental Stresses
• Environmental stresses have a potentially
adverse effect on survival, growth, and
reproduction
• Stresses can be abiotic (nonliving) or biotic
(living)
• Abiotic stresses include drought, flooding, salt
stress, heat stress, and cold stress
• Biotic stresses include herbivores and
pathogens
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Drought
• During drought, plants reduce transpiration by
closing stomata, slowing leaf growth, and
reducing exposed surface area
• Growth of shallow roots is inhibited, while
deeper roots continue to grow
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Flooding
• Enzymatic destruction of root cortex cells
creates air tubes that help plants survive
oxygen deprivation during flooding
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Figure 39.27
Vascular
cylinder
Air tubes
Epidermis
100 m
(a) Control root (aerated)
100 m
(b) Experimental root (nonaerated)
Figure 39.27a
Vascular
cylinder
Epidermis
100 m
(a) Control root (aerated)
Figure 39.27b
Vascular
cylinder
Air tubes
Epidermis
100 m
(b) Experimental root (nonaerated)
Salt Stress
• Salt can lower the water potential of the soil
solution and reduce water uptake
• Plants respond to salt stress by producing
solutes tolerated at high concentrations
• This process keeps the water potential of cells
more negative than that of the soil solution
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Heat Stress
• Excessive heat can denature a plant’s
enzymes
• Heat-shock proteins help protect other
proteins from heat stress
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Cold Stress
• Cold temperatures decrease membrane fluidity
• Altering lipid composition of membranes is a
response to cold stress
• Freezing causes ice to form in a plant’s cell walls
and intercellular spaces
• Many plants, as well as other organisms, have
antifreeze proteins that prevent ice crystals from
growing and damaging cells
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Concept 39.5: Plants respond to attacks by
herbivores and pathogens
• Plants use defense systems to deter herbivory,
prevent infection, and combat pathogens
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Defenses Against Herbivores
• Herbivory, animals eating plants, is a stress
that plants face in any ecosystem
• Plants counter excessive herbivory with
physical defenses, such as thorns and
trichomes, and chemical defenses, such as
distasteful or toxic compounds
• Some plants even “recruit” predatory animals
that help defend against specific herbivores
© 2011 Pearson Education, Inc.
Figure 39.28
4 Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
1 Wounding
1 Chemical
in saliva
2 Signal transduction
pathway
3 Synthesis
and release
of volatile
attractants
• Plants damaged by insects can release volatile
chemicals to warn other plants of the same
species
• Arabidopsis can be genetically engineered to
produce volatile components that attract
predatory mites
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Defenses Against Pathogens
• A plant’s first line of defense against infection is
the barrier presented by the epidermis and
periderm
• If a pathogen penetrates the dermal tissue, the
second line of defense is a chemical attack that
kills the pathogen and prevents its spread
• This second defense system is enhanced by the
plant’s ability to recognize certain pathogens
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Host-Pathogen Coevolution
• A virulent pathogen is one that a plant has little
specific defense against
• An avirulent pathogen is one that may harm but
does not kill the host plant
© 2011 Pearson Education, Inc.
• Gene-for-gene recognition involves
recognition of elicitor molecules by the protein
products of specific plant disease resistance
(R) genes
• An R protein recognizes a corresponding
molecule made by the pathogen’s Avr gene
• R proteins activate plant defenses by triggering
signal transduction pathways
• These defenses include the hypersensitive
response and systemic acquired resistance
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The Hypersensitive Response
• The hypersensitive response
– Causes cell and tissue death near the infection
site
– Induces production of phytoalexins and PR
proteins, which attack the pathogen
– Stimulates changes in the cell wall that confine
the pathogen
© 2011 Pearson Education, Inc.
Figure 39.29
Infected tobacco leaf with lesions
4
3
Signal
5
Hypersensitive
response
Signal
transduction
pathway
6
2 Signal transduction pathway
7
Acquired
resistance
1
R protein
Avirulent
pathogen
Avr effector protein
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance
Figure 39.29a
4
3
Signal
5
Hypersensitive
response
6
Signal
transduction
pathway
2 Signal transduction pathway
7
Acquired
resistance
1
R protein
Avirulent
pathogen
Avr effector protein
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance
Figure 39.29b
Infected tobacco leaf with lesions
Systemic Acquired Resistance
• Systemic acquired resistance causes
systemic expression of defense genes and is
a long-lasting response
• Salicylic acid is synthesized around the
infection site and is likely the signal that
triggers systemic acquired resistance
© 2011 Pearson Education, Inc.
Figure 39.UN02
CELL
WALL
1 Reception
Hormone or
environmental
stimulus
CYTOPLASM
Plasma membrane
2 Transduction
Relay proteins and
second messengers
Receptor
3 Response
Activation
of cellular
responses
Figure 39.UN03
Figure 39.UN04
Photoreversible states of phytochrome:
Pfr
Pr
Red light
Responses
Far-red
light
Figure 39.UN05
Figure 39.UN06
Control
Wild-type
Ethylene insensitive
(ein)
Ethylene
overproducing (eto)
Constitutive triple
response (ctr)
Ethylene
added
Ethylene
synthesis
inhibitor
Figure 39.UN07