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Plant Response
Plant reactions
• Stimuli & a Stationary Life
– Animals respond to stimuli
by changing behavior
• Move toward positive
stimuli
• Move away from negative
stimuli
– Plants respond to stimuli
by adjusting growth &
development
Signal Transduction Pathways in Plants
• Plants have cellular receptors that
detect changes in their
environment
– For a stimulus to elicit a response,
certain cells must have an appropriate
receptor
• Signal triggers receptor
• Receptor triggers internal cellular
messengers which transfer and
amplify signals from receptors to
proteins that cause responses
• Cellular response
– may involve increased activity of
enzymes by:
• Stimulating transcription of mRNA
for an enzyme (Transcriptional
regulation)
• Activating an existing enzyme (Posttranslational modification)
Transcriptional Regulation
• Specific transcription factors bind directly to
specific regions of DNA and control transcription of
genes
• Positive transcription factors
– increase the transcription of specific genes
• Negative transcription factors
– decrease the transcription of specific genes
Post-Translational Modification of
Proteins
• Involves modification of existing proteins in the
signal response
– Ex. phosphorylation of enzymes for activation
Signal Transduction Pathway Example
Both pathways lead to
expression of genes
for proteins that
functions in greening
response of plants
Greening Response in Potato Plants
Grown in dark
1 week exposure to light
Discovery of Plant Hormones
• In the late 1800s, Charles Darwin and his son Francis
conducted experiments on phototropism, a plant’s response
to light
– Curvatures of whole plant organs toward or away from stimuli is
called a tropism
– Grass 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
Discovery of Plant Hormones
• In 1913, Peter Boysen-Jensen demonstrated
that the signal was a mobile chemical
substance
Discovery of Plant Hormones
• In 1926, Frits Went
extracted the
chemical messenger
for phototropism,
auxin, by modifying
earlier experiments
Plant hormones
• Chemical signals that coordinate different
parts of an organism
– Only minute amounts are required
– Produced in one part of the body and is
transported to another part
– Binds to specific receptor
– Triggers response in target cells & tissues
Plant hormones
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Auxins
Cytokinins
Gibberellins
Brassinosteroids
Abscisic acid
ethylene
Table 39-1
• Indoleacetic acid (IAA)
– Stimulates cell elongation
– Involved in lateral root
formation and branching
– Enhances apical dominance
– An overdose of synthetic auxins
can kill eudicots (used as
herbicide)
– Classical explanation of
phototropism
• Asymmetrical distribution of
auxin
• Cells on darker side elongate
faster than cells on brighter side
Auxin
• Produced in actively growing tissues:
roots, fruits & embryos
• Effects
– Control of cell division &
differentiation
– Enhances apical dominance
• Terminal bud suppresses
development of axillary buds
• If terminal bud is removed,
plants become bushier
– Retard aging of some plant organs
by inhibiting protein breakdown
and stimulating RNA and protein
synthesis
– auxin & cytokinins interact to
control cell division and
differentiation
Cytokinins
Gibberellins
• Over 100 different
gibberellins identified
• Effects
– Stem elongation
• Stimulate cell elongation and
cell division
– Fruit growth
• Auxin and gibberellins must be
present for fruit to set
– Seed germination
• After seed is imbibed, release of
gibberellins from embryo signals
seeds to germinate
Fig. 39-11
1 Gibberellins (GA)
2 Aleurone secretes
send signal to
aleurone.
3 Sugars and other
α-amylase and other enzymes.
nutrients are consumed.
Aleurone
Endosperm
-amylase
GA
GA
Water
Scutellum
(cotyledon)
Radicle
Sugar
Brassinosteroids
• Similar to sex hormones of animals
• Effects
– Similar to auxins
– Cell elongation & division in shoots & seedlings
Abscisic acid (ABA)
• Effects
– Slows growth
– High concentration of ABA promotes
seed dormancy
• Germination occurs only after ABA is inactivated or
leached out
• Ensures that seeds will germinate only under optimal
conditions
– Drought tolerance
• Rapid stomata closure to allow plant to withstand drought
Ethylene
• Ethylene is a gas released by plant cells
in response to stresses such as drought,
flooding, injury, infection
• Multiple effects
– Response to mechanical stress
• Such as a seedling growing around
a stone (an obstacle)
– Apoptosis
• Ex. shedding leaves in autumn
Ethylene Effects: Fruit Ripening
• Hard, tart fruit protects developing seeds from
herbivores
• Ripe, sweet, soft fruit attracts animals to
disperse seed
– Burst of ethylene triggers ripening process
• Breakdown of cell wall = softening
• Conversion of starch to sugar = sweetening
– Positive feedback system
• Ethylene triggers ripening
• Ripening stimulates more ethylene production
Applications
• “One bad apple DOES spoil the whole bunch”
– Ripening apple releases ethylene to speed ripening of fruit
nearby
• Ripen green bananas by bagging them with an apple
• Climate control storage of apples
– Air is circulated to prevent ethylene buildup
– Stored in high amounts of CO2 which inhibits the release of
ethylene
Responses to light
• Photomorphogenesis
– Effect of light on plant growth
• Plants can detect
– Presence of light
– Intensity of light
– Direction of light
– Wavelength (color)
• Blue-light receptors
• Phytochromes (red-light receptors)
• An action spectrum depicts responses of a plant
process to different wavelengths
Blue-Light Photoreceptors
• Various blue-light photoreceptors control hypocotyl
elongation, stomatal opening, and phototropism
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
• Photoreceptor activity
– In each subunit, one domain, which
functions as a photoreceptor, is
covalently bonded to a nonprotein
pigment, chromophore
• Kinase activity
– The other domain has protein kinase
activity. The two domains interact
linking light reception to cellular
responses triggered by the kinase
Phytochromes as Photoreceptors
• The chromophore of a phytochrome is
photoreversible, reverting back and forth
between two isomeric forms, depending on
the color of light
– Pr absorbs red (r) light maximally
– Pfr absorbs far-red (fr) light
• The conversion triggers many developmental
responses such as germination
Phytochrome photoreceptors
• Molecular switch reaction to red light
– Conversion of Pr
Pfr in sunlight stimulates germination,
flowering, branching…
– Conversion of Pfr
Pr in dark inhibits response &
stimulates other responses: growth in height
• “Shade avoidance” response
Circadian Rhythms
• Internal 24-hour cycles
Morning Glory
The Effect of Light on the Biological
Clock
• The clock may depend on synthesis of a protein
regulated through feedback control
• Phytochrome conversion marks sunrise and sunset,
providing the biological clock with environmental
cues
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
Photoperiodism and Control of
Flowering
• 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
– Chrysanthemums, soybeans
• Plants that flower when a light period is longer than
a certain number of hours are called long-day
plants
– Spinach, lettuce, iris
• Flowering in day-neutral plants is controlled by
plant maturity, not photoperiod
– Tomatoes, rice, dandelions
Flowering Response
• Controlled by night
length – “critical period”
– Short-day plants (longnight) flower when night
exceeds a minimum
number of hours of
darkness
– Long-day plants (shortnight) flower only if the
night is shorter than a
critical dark period
– Flash of light can interrupt
the nighttime portion of
the photoperiod
Flowering Response
• Red light can interrupt
the nighttime portion
of the photoperiod
• Action spectra and
photoreversibility
experiments show
that phytochrome is
the pigment that
detects red light
– If a flash of R light
during dark period is
followed by a flash of
FR light, the plant
detects no
interruption of the
night length
Flowering Response
• 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
Is there a flowering
hormone?
• A flowering signal, not
yet chemically
identified is called
florigen
Responses to stimuli: gravity
• How does a sprouting
shoot “know” to grow
towards the surface
from underground?
– Environmental cues
• Roots = positive
gravitropism
• Shoots = negative
gravitropism
• Settling of statoliths
(dense starch grains in
plastids) may detect
gravity
Responses to stimuli: touch
• Thigmotropism
– Growth in response to
touch
– Caused by changes in
osmotic pressure = rapid
loss of K+ = rapid loss of
H2O = loss of turgor in
cells
• Example
– Mimosa closes leaves in
response to touch
Responses to Stimuli: Touch
• Thigmomorphogenesis
– Changes in form resulting
from mechanical disturbance
– Ex. Rubbing stems of young
plants a couple of times daily
results in plants that are
shorter than controls
Responses to Stimuli
• Environmental Stresses
– have a potentially adverse effect on survival, growth,
and reproduction
– Stresses can be abiotic or biotic
• Abiotic stresses include drought, flooding, salt stress, heat
stress, and cold stress
• 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
• Flooding
– Enzymatic destruction of root cortex cells creates air tubes that
help plants survive oxygen deprivation during flooding
Responses to Stimuli
• 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
• water potential of cells becomes more negative than that of the
soil solution
• Heat Stress
– Excessive heat can denature a plant’s enzymes
• Heat-shock proteins help protect other proteins from heat stress
• Cold Stress
– Cold temperatures decrease membrane fluidity
• Plants can alter lipid composition of membranes
– Freezing causes ice to form in a plant’s cell walls and
intercellular spaces
Plant defenses
• Defenses against
herbivores
– Thorns
– Chemicals
– Recruitment of predatory
animal to defend against
specific herbivores
– Volatile chemicals to warn
other plants of same
species
• Methyljasmonic acid can
activate expression of
genes involved in plant
defenses
Plant Defenses
• Defenses against pathogens
– First line of defense
• Epidermis and periderm
– Second line of defense
• Chemical attack that kills pathogen
• Enhanced by ability to recognize pathogens
Gene-for-gene Recognition
• involves recognition of
pathogen-derived molecules
by protein products of specific
plant disease resistance (R)
genes
– An R protein recognizes a
corresponding molecule made
by the pathogen’s Avr
(avirulence) gene
– R proteins activate plant
defenses by triggering signal
transduction pathways
• hypersensitive response and
systemic acquired resistance
Signal Transduction Pathways
• The hypersensitive response
– Causes cell and tissue death near the infection site
– Induces production of phytoalexins and PR
(pathogenesis-related) proteins which attack the
pathogen
– Stimulates changes in the cell wall that confine the
pathogen
• 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
Plant defenses: Signal Transduction Pathways
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Specific resistance is based on
pathogen-receptor binding
Signal transduction pathway is
triggered
Antimicrobial molecules that
seal off infected areas are
produced
Infected cells release
methylsalicylic acid before they
die
The signaling molecule is
distributed to rest of the plant
In cells away from infection
site, methylsalicylic acid is
converted to salicylic acid
which initiates a signal
transduction pathway
Molecules that help protect
the cell against a diversity of
pathogens for several days are
produced