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Chapter 39
Plant Responses to Internal
and External Signals
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Stimuli and a Stationary Life
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 39.1: Signal transduction pathways link
signal reception to response
• Plants have cellular receptors that detect changes
in their environment
• For a stimulus to elicit a response, certain cells
must have an appropriate receptor
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• A potato left growing in darkness produces shoots
that look unhealthy and lacks elongated roots
• These are morphological adaptations for growing
in darkness, collectively called etiolation
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LE 39-2
Before exposure to light. A
dark-grown potato has tall,
spindly stems and
nonexpanded leaves—
morphological adaptations
that enable the shoots to
penetrate the soil. The roots
are short, but there is little
need for water absorption
because little water is lost by
the shoots.
After a week’s exposure to
natural daylight. The potato
plant begins to resemble a
typical plant with broad
green leaves, short sturdy
stems, and long roots. This
transformation begins with
the reception of light by a
specific pigment,
phytochrome.
• After exposure to light, a potato undergoes
changes called de-etiolation, in which shoots and
roots grow normally
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• A potato’s response to light is an example of cellsignal processing
• The stages are reception, transduction, and
response
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 39-3
CELL
WALL
Reception
CYTOPLASM
Transduction
Relay molecules
Receptor
Hormone or
environmental
stimulus
Plasma membrane
Response
Activation
of cellular
responses
Reception
• Internal and external signals are detected by
receptors, proteins that change in response to
specific stimuli
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Transduction
• Second messengers transfer and amplify signals
from receptors to proteins that cause responses
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LE 39-4_3
Reception
Response
Transduction
Transcription
NUCLEUS
factor 1
CYTOPLASM
Plasma
membrane
cGMP
Second messenger
produced
Specific
protein
kinase 1
activated
Transcription
factor 2
Phytochrome
activated
by light
Cell
wall
Specific
protein
kinase 2
activated
Transcription
Light
Translation
Ca2+ channel
opened
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
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Transcriptional Regulation
• Transcription factors bind directly to specific
regions of DNA and control transcription of genes
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Post-Translational Modification of Proteins
• Post-translational modification involves activation
of existing proteins in the signal response
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De-Etioloation (“Greening”) Proteins
• Many enzymes that function in certain signal
responses are directly involved in photosynthesis
• Other enzymes are involved in supplying chemical
precursors for chlorophyll production
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Concept 39.2: Plant hormones help coordinate
growth, development, and responses to stimuli
• Hormones are chemical signals that coordinate
different parts of an organism
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The Discovery of Plant Hormones
• Any response resulting in curvature of organs
toward or away from a stimulus is called a tropism
• Tropisms are often caused by hormones
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• In the late 1800s, Charles Darwin and his son
Francis conducted experiments on phototropism,
a plant’s response to light
• They observed that a seedling could bend toward
light only if the tip of the coleoptile was present
• They postulated that a signal was transmitted from
the tip to the elongating region
Video: Phototropism
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LE 39-5a
Shaded
side of
coleoptile
Control
Light
Illuminated
side of
coleoptile
LE 39-5b
Darwin and Darwin (1880)
Light
Tip
Tip
removed covered
by opaque
cap
Base covered
Tip
covered by opaque
by trans- shield
parent
cap
• In 1913, Peter Boysen-Jensen demonstrated that
the signal was a mobile chemical substance
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LE 39-5c
Boysen-Jensen (1913)
Light
Tip separated
Tip
separated by by mica
gelatin block
• In 1926, Frits Went extracted the chemical
messenger for phototropism, auxin, by modifying
earlier experiments
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LE 39-6
Excised tip placed
on agar block
Growth-promoting
chemical diffuses
into agar block
Control
Control
(agar block
lacking
chemical)
has no
effect
Agar block
with chemical
stimulates growth
Offset blocks
cause curvature
A Survey of Plant Hormones
• In general, hormones control plant growth and
development by affecting the division, elongation,
and differentiation of cells
• Plant hormones are produced in very low
concentration, but a minute amount can greatly
affect growth and development of a plant organ
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Auxin
• The term auxin refers to any chemical that
promotes cell elongation in target tissues
• Auxin transporters move the hormone from the
basal end of one cell into the apical end of the
neighboring cell
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LE 39-7
Cell 1
100 µm
Cell 2
Epidermis
Cortex
Phloem
Xylem
Pith
Basal end
of cell
25 µm
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
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LE 39-8a
Cross-linking
cell wall
polysaccharides
Cell wall
enzymes
Expansin
CELL WALL
Microfibril
ATP
Plasma membrane
CYTOPLASM
LE 39-8b
H 2O
Plasma
membrane
Cell
wall
Nucleus Cytoplasm
Vacuole
Lateral and Adventitious Root Formation
• Auxin is involved in root formation and branching
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Auxins as Herbicides
• An overdose of auxins can kill eudicots
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Other Effects of Auxin
• Auxin affects secondary growth by inducing cell
division in the vascular cambium and influencing
differentiation of secondary xylem
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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
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Control of Apical Dominance
• Cytokinins, auxin, and other factors interact in the
control of apical dominance, a terminal bud’s
ability to suppress development of axillary buds
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LE 39-9
Axillary buds
“Stump” after
removal of
apical bud
Lateral branches
Intact plant
Plant with apical bud removed
• If the terminal bud is removed, plants become
bushier
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Anti-Aging Effects
• Cytokinins retard 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 stimulate growth of leaves and stems
• In stems, they stimulate cell elongation and cell
division
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Fruit Growth
• In many plants, both auxin and gibberellins must
be present for fruit to set
• Gibberellins are used in spraying of Thompson
seedless grapes
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Germination
• After water is imbibed, release of gibberellins from
the embryo signals seeds to germinate
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 39-11
Aleurone
Endosperm
a-amylase
GA
Water
Scutellum
(cotyledon)
GA
Radicle
Sugar
Brassinosteroids
• Brassinosteroids are similar to the sex hormones
of animals
• They induce cell elongation and division
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Abscisic Acid
• Two of the many effects of abscisic acid (ABA):
– Seed dormancy
– Drought tolerance
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Seed Dormancy
• Seed dormancy ensures that the seed will
germinate only in optimal conditions
• Precocious germination is observed in maize
mutants that lack a transcription factor required for
ABA to induce expression of certain genes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 39-12
Coleoptile
Drought Tolerance
• ABA is the primary internal signal that enables
plants to withstand drought
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Ethylene
• Plants produce ethylene in response to stresses
such as drought, flooding, mechanical pressure,
injury, and infection
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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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 39-13
0.00
0.10
0.20
0.40
Ethylene concentration (parts per million)
0.80
• Ethylene-insensitive mutants fail to undergo the
triple response after exposure to ethylene
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LE 39-14
ein mutant
ctr mutant
ein mutant. An ethylene-insensitive
(ein) mutant fails to undergo the triple
response in the presence of ethylene.
ctr mutant. A constitutive triple-response
(ctr) mutant undergoes the triple response
even in the absence of ethylene.
• Other mutants undergo the triple response in air
but do not respond to inhibitors of ethylene
synthesis
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LE 39-15
Control
Wild-type
Ethylene insensitive
(ein)
Ethylene
overproducing (eto)
Constitutive triple
response (ctr)
Ethylene
added
Ethylene
synthesis
inhibitor
Apoptosis: Programmed Cell Death
• 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
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LE 39-16
0.5 mm
Protective layer
Stem
Abscission layer
Petiole
Fruit Ripening
• A burst of ethylene production in a fruit triggers the
ripening process
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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
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• 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
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Phototropic effectiveness relative to 436 nm
LE 39-17
1.0
0.8
0.6
0.4
0.2
0
400
450
500
550
600
Wavelength (nm)
Light
Time = 0 min.
Time = 90 min.
650
700
• 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 regulate many of a plant’s
responses to light throughout its life
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Phytochromes and Seed Germination
• Studies of seed germination led to the discovery of
phytochromes
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• In the 1930s, scientists at the U.S. Department of
Agriculture determined the action spectrum for
light-induced germination of lettuce seeds
Dark (control)
Dark
Dark
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LE 39-18
Dark (control)
Red
Dark
Red Far-red Red
Red Far-red
Dark
Dark
Red Far-red Red Far-red
• The photoreceptor responsible for the opposing
effects of red and far-red light is a phytochrome
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LE 39-19
A phytochrome consists of two identical proteins joined to form
one functional molecule. Each of these proteins has two domains.
Chromophore
Photoreceptor activity
Kinase activity
• Phytochromes exist in two photoreversible states,
with conversion of Pr to Pfr triggering many
developmental responses
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LE 39-20
Pr
Pfr
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
• The phytochrome system also provides the plant
with information about the quality of 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
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LE 39-21
Noon
Midnight
• Cyclical responses to environmental stimuli are
called circadian rhythms and are about 24 hours
long
• Circadian rhythms can be entrained to exactly 24
hours by the day/night cycle
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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
• 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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LE 39-22
Darkness
Flash of
light
Critical
dark
period
Light
“Short-day” plants
“Long-day” plants
• Action spectra and photoreversibility experiments
show that phytochrome is the pigment that
receives red light
• Red light can interrupt the nighttime portion of the
photoperiod
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LE 39-23
24
20
R
FR
R
16
12
8
4
0
Short-day (long-night) plant
Long-day (short-night) plant
R
FR
R
FR
R
FR
R
A Flowering Hormone?
• The flowering signal, not yet chemically identified,
is called florigen
• Florigen may be a hormone or a change in relative
concentrations of multiple hormones
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LE 39-24
Graft
Time
(several
weeks)
Meristem Transition and Flowering
• For a bud to form a flower instead of a vegetative
shoot, meristem identity genes must first be
switched on
• Researchers seek to identify the signal
transduction pathways that link cues such as
photoperiod and hormonal changes to the gene
expression required for flowering
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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
• Stems show negative gravitropism
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• Plants may detect gravity by the settling of
statoliths, specialized plastids containing dense
starch grains
Video: Gravitropism
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 39-25
Statoliths
20 µm
Mechanical Stimuli
• The term thigmomorphogenesis refers to changes
in form that result from mechanical perturbation
• Rubbing stems of young plants a couple of times
daily results in plants that are shorter than controls
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Thigmotropism is growth in response to touch
• It occurs in vines and other climbing plants
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• Rapid leaf movements in response to mechanical
stimulation are examples of transmission of
electrical impulses called action potentials
Video: Mimosa Leaf
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LE 39-27
Unstimulated
Stimulated
Side of pulvinus with
flaccid cells
Leaflets
after
stimulation
Side of pulvinus with
turgid cells
Vein
Pulvinus
(motor
organ)
Motor organs
0.5 mm
Environmental Stresses
• Environmental stresses have a potentially adverse
effect on survival, growth, and reproduction
• They can have a devastating impact on crop
yields in agriculture
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Drought
• During drought, plants respond to water deficit by
reducing transpiration
• Deeper roots continue to grow
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Flooding
• Enzymatic destruction of cells creates air tubes
that help plants survive oxygen deprivation during
flooding
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LE 39-28
Vascular
cylinder
Air tubes
Epidermis
100 µm
Control root (aerated)
100 µm
Experimental root (nonaerated)
Salt Stress
• 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
• Heat-shock proteins help plants survive heat
stress
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Cold Stress
• Altering lipid composition of membranes is a
response to cold stress
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Concept 39.5: Plants defend themselves against
herbivores and pathogens
• Plants counter external threats with defense
systems that deter herbivory and prevent infection
or 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 chemical defenses
such as distasteful or toxic compounds
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• Some plants even “recruit” predatory animals that
help defend against specific herbivores
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LE 39-29
Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
Synthesis and
release of
volatile attractants
Wounding
Chemical
in saliva
Signal transduction
pathway
Defenses Against Pathogens
• A plant’s first line of defense against infection is its
“skin,” the epidermis or 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
inherited ability to recognize certain pathogens
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Gene-for-Gene Recognition
• A virulent pathogen is one that a plant has little
specific defense against
• An avirulent pathogen is one that may harm but
not kill the host plant
• Gene-for-gene recognition involves recognition of
pathogen-derived molecules by protein products
of specific plant disease resistance (R) genes
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• A pathogen is avirulent if it has a specific Avr gene
corresponding to an R allele in the host plant
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LE 39-30a
Signal molecule (ligand)
from Avr gene product
Receptor coded by R allele
R
Avr allele
Avirulent pathogen
Plant cell is resistant
If an Avr allele in the pathogen corresponds to an R
allele in the host plant, the host plant will have
resistance, making the pathogen avirulent.
• If the plant host lacks the R gene that counteracts
the pathogen’s Avr gene, then the pathogen can
invade and kill the plant
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LE 39-30b
R
No Avr allele;
virulent pathogen
R allele; plant cell
becomes diseased
Avr allele
Avr allele;
virulent pathogen
No R allele; plant cell
becomes diseased
No Avr allele;
virulent pathogen
No R allele; plant cell
becomes diseased
If there is no gene-for-gene recognition because of
one of the above three conditions, the pathogen
will be virulent, causing disease to develop.
Plant Responses to Pathogen Invasions
• A hypersensitive response against an avirulent
pathogen seals off the infection and kills both
pathogen and host cells in the region of the
infection
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LE 39-31
Signal
Hypersensitive
response
Signal transduction
pathway
Signal
transduction
pathway
Acquired
resistance
Avirulent
pathogen
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance
Systemic Acquired Resistance
• Systemic acquired resistance (SAR) is a set of
generalized defense responses in organs distant
from the original site of infection
• Salicylic acid is a good candidate for one of the
hormones that activates SAR
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