Chapter 30
Communication
Strategies in Plants
Albia Dugger • Miami Dade College
30.1 Prescription: Chocolate
• Cocoa is made from cacao beans, which are the seeds of the
Theobroma cacao tree
• Cocoa seeds have a high content of flavonoids such as
epicatechin, which functions in plant immunity
• In humans, epicatechin has a protective effect against
oxidative tissue damage that occurs after a stroke or heart
attack, enhances memory, and kills cancer cells
Cacao Tree
Cacao Fruit
30.2 Introduction to Plant Hormones
• Plant development depends on cell-to-cell communication –
mediated by plant hormones
• Plant hormones are extracellular signaling molecules that
exerts an effect at very low concentrations
• Hormones affect development and growth of plant parts;
defensive responses; circadian rhythms; flowering; fruit and
seed formation; aging; and dormancy
Chemical Signaling
• A hormone released by cells in a localized area usually alters
the activity of cells in a different area
• A cell’s response depends on the cell and the receptor, and
varies with the concentration of the hormone
• Typically, the response involves modification of nuclear or
mitochondrial DNA that causes a change in gene expression
• In some cases, cell function is affected with no change in
underlying gene expression patterns
Hormone Interactions
• Different hormones can have synergistic or opposing effects;
a cell’s response depends on integration of hormonal signals
• Plant hormones interact with one another mainly at the
transcriptional level
• Hormone expression may be controlled by negative or
positive feedback loops
Plant versus Animal Development
Table 30-1 p507
Table 30-1 p507
Table 30-1 p507
Take-Home Message: What regulates growth
and development in plants?
• Plant hormones are signaling molecules that coordinate
activities among cells in different parts of the plant body.
• Cells that bear receptors for a hormone—and thus can
respond to it—may be in the same tissue as the hormonereleasing cell, or in another region of the plant body.
• Plant hormones are involved in all aspects of growth,
development, and function in plants. They often work
together, with synergistic or opposing effects on cells.
30.3 Auxin: The Master Growth Hormone
• Auxin (IAA) coordinates the effects of other plant hormones
plays a critical role in all aspects of plant development
• First division of the zygote
• Polarity and tissue pattern in the embryo
• Formation of plant parts
• Differentiation of vascular tissues
• Formation of lateral roots
• Responses to environmental stimuli
Auxin and Plant Growth
• Auxins promote or inhibit cell division and elongation,
depending on the target tissue
• Auxin increases the activity of transport proteins that pump
hydrogen ions from cytoplasm into the cell wall
• Increased acidity softens the wall, and turgor stretches the
cell
Experiment: Response to Auxin
time
A A coleoptile
stops growing
after its auxinproducing tip
has been
removed.
time
B A block of agar that
absorbs auxin from a
cut tip can stimulate
a de-tipped coleoptile
to resume growth
time
C If an auxin-containing
agar block is placed to
one side of a cut tip,
the coleoptile will continue to grow, but it will
bend as it lengthens.
ANIMATED FIGURE: Auxin's effects
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Polar Transport
• Auxin made in shoot apical must be transported to parts of
the body where it is needed
• Auxin from shoots is loaded into phloem, travels to roots, and
is unloaded into root cells
• Auxin diffuses into cells or is actively transported through
plasma membrane proteins called influx carriers
• Once auxin has entered cytoplasm, it can only leave through
active transport proteins called efflux carriers
auxin
auxin
auxin
auxin
Figure 30-4a p509
Importance of Polar Transport
• This mechanism auxin flow is unique among plant hormones,
and it is important because it establishes auxin concentration
gradients across tissues, organs, and the entire plant
• Auxin coordinates the actions of other hormones, many of
which are expressed in different localized patterns
Apical Dominance
• Efflux carriers help balance the growth of a plant’s apical and
lateral buds
• Auxin travels through efflux carriers in a growing shoot tip and
prevents growth of lateral buds (apical dominance)
• Cell membranes in dormant lateral buds have few efflux
carriers; auxin produced by apical meristem is not traveling
• If a shoot’s tip breaks off, strigolactone level declines, lateral
buds acquire efflux carriers, and lateral buds begin to grow
Loss of a Shoot Tip
Ends Dormancy in Lateral Buds
Take-Home Message: What are the main
effects of auxin in plants?
• Auxin is a plant hormone that coordinates other hormones
during growth and development at all stages of the plant life
cycle.
• A polar distribution system sets up auxin concentration
gradients across a plant’s tissues and organs in response to
internal and external conditions.
30.4 Cytokinin
• A cytokinin is one of a group of plant hormones derived from
the nucleotide adenine
• Cytokinin stimulates cell divisions in shoot apical meristem,
and cell differentiation in root apical meristem
• Cytokinin and auxin work together, often antagonistically, and
they influence one another’s expression
Cytokinin and Auxin
• Cytokinin opposes auxin’s effect on lateral root formation
• In root apical meristem, cytokinin opposes auxin to maintain
the balance of differentiating and undifferentiated cells
• Cytokinin stimulates lateral bud growth by releasing lateral
buds from apical dominance
Interaction of Auxin and Cytokinin
in Release of Apical Dominance
auxin
auxin
A Auxin flowing
through a shoot
keeps the level
of cytokinin low
in the stem.
cytokinin
B Removing the tip
ends auxin flow in the
stem. As the auxin
level declines, the
cytokinin level rises.
auxin
C The cytokinin stimulates cell division in
apical meristem of
lateral buds. The cells
begin to produce auxin.
D Auxin gradients
form and direct the
development of the
growing lateral buds.
Take-Home Message: What are the main
effects of cytokinin in plants?
• Cytokinin stimulates cell divisions in shoot apical meristem,
and cell differentiation in root apical meristem.
• Cyokinin and auxin act together and often antagonistically.
The cytokinin–auxin balance controls cell division and
differentiation in shoot and root apical meristem.
30.5 Gibberellin
• A gibberellin is a hormone that promotes growth by inducing
cell division and elongation between nodes in stem tissue
• Gibberellin is also involved in slowing the aging of leaves and
fruits, breaking dormancy in seeds, germination of seeds,
and, in some plants, flowering
Effect of
Gibberellins
• Gibberellin works by
inhibiting inhibitors –
removing the brakes on
some cellular processes
Gibberellin and Germination
• Gibberellin and barley seed germination
• Barley seed absorbs water
• Embryo releases gibberellin
• Gibberellin induces transcription of amylase gene
• Amylase breaks stored starches into sugars used by
embryo for aerobic respiration
A Absorbed water causes cells of a barley
embryo to release gibberellin, which diffuses
through the seed into the aleurone layer of
the endosperm.
aleurone endosperm embryo
B Gibberellin triggers cells of the aleurone
layer to express the gene for amylase. This
enzyme diffuses into the starch-packed
middle of the endosperm.
C The amylase hydrolyzes starch into sugar
monomers, which diffuse into the embryo
and are used in aerobic respiration. Energy
released by the reactions of aerobic respiration
fuels meristem cell divisions in the embryo.
gibberellin
amylase
sugars
Figure 30-8 p511
Take-Home Message: What are the main
effects of gibberellin in plants?
• Gibberellin stimulates cell division and elongation in stems,
which causes stems to lengthen between nodes.
• Gibberellin affects the expression of genes for nutrient
utilization during seed germination.
30.6 Abscisic Acid
• Abscisic acid (ABA) mediates germination, inhibits growth,
and is part of protective responses to stress caused by living
and nonliving factors in the environment
• ABA also has an important role in embryo maturation,
stomata closure, seed and pollen germination, and fruit
ripening; and it suppresses lateral root formation
ABA Activity
• ABA synthesis begins in chloroplasts – its concentration is
highest in leaves and other photosynthetic parts
• ABA receptors occur on the plasma membrane, in cytoplasm,
and in the nucleus – ABA activates transcription factors that
govern the expression of thousands of genes
• Example: ABA enhances transcription of genes that encode
NADPH oxidase – resulting reactions produce H2O2 and NO
Hydrogen Peroxide and Nitric Oxide
hydrogen peroxide
H—O=O—H
nitric oxide
N≡O
ABA and Germination
• ABA accumulates in a seed as it forms and prevents the seed
from germinating too early by inhibiting expression of genes
involved in cell wall expansion and gibberellin synthesis
• A seed cannot germinate until its ABA level declines
• Hydrogen peroxide (from NADPH oxidase) enhances
expression of gibberellin synthesis genes
Premature Germination Without ABA
Take-Home Message: What
are the main effects
of abscisic acid in plants?
• Abscisic acid inhibits germination and growth.
• ABA also stimulates metabolism, stress responses, and
embryonic development.
30.7 Ethylene
• Ethylene is a gaseous hormone produced in all parts of a
plant from methionine and ATP
• Ethylene helps regulate many metabolic and developmental
processes, including germination, growth, abscission, fruit
ripening, and stress responses
• Expression of some genes is inhibited by ethylene (negative
feedback loop); expression of others is enhanced by ethylene
(positive feedback loop)
Ethylene and Fruit Ripening
• Ripening of fleshy fruits such as strawberries occurs after a
peak of cellular respiration followed by a burst of ethylene
produced in a positive feedback loop
• Chloroplasts are converted to chromoplasts; cell walls break
down; starch and organic acids are converted to sugars
• Synthetic ethylene is widely used to artificially ripen fruit
Ethylene production
Ethylene Production
During Strawberry Ripening
petals drop
fruit forms
green fruit enlarges
Days After Flower Opening
fruit ripens
fruit is mature
Take-Home Message: What are the main
functions of ethylene in plants?
• Ethylene produced in negative feedback loops participates in
ongoing metabolic and developmental processes.
• Ethylene produced in positive feedback loops is involved in
intermittent processes such as abscission, fruit ripening, and
defense responses.
30.8 Tropisms
• Tropisms
• Plants adjust the direction and rate of growth in response
to environmental stimuli such as gravity, light, contact, and
mechanical stress
• Hormones are typically part of this effect
Gravitropism
• Gravitropism
• A growth response to gravity which causes roots to grow
downward and shoots to grow upward
• Statoliths
• Amyloplasts containing heavy starch grains that sink to the
bottom of the cell
• A change in position results in movement of cell’s auxin
efflux carriers
Gravitropism
statoliths
A This micrograph shows heavy, starch-packed
statoliths settled on the bottom of gravity-sensing
cells in a corn root cap.
Figure 30-13a p514
statoliths
B This micrograph was taken ten minutes after the
root in A was rotated 90°. The statoliths are
already settling to the new “bottom” of the cells.
Figure 30-13b p514
Phototropism
• Phototropism
• Orientation of certain plant parts toward light
• Nonphotosynthetic pigments (phototropins) respond to
blue light, initiating signal cascades
• Auxin is redistributed to shady side of plant
• Heliotropism
• In some plants, leaves or flowers change position in
response to changing angle of the sun through the day
A Sunlight strikes only
one side of a coleoptile.
B Auxin flow is directed toward
the shaded side, so cells on
that side lengthen more.
Figure 30-14 p514
Movement of Chloroplasts
in Response to Light
• On the interior of a cell, chloroplasts are dragged from one
position to another on actin filament tracks of the cytoskeleton
• Chloroplasts move away from high-intensity light, which
minimizes damage from excess electrons accumulating in
electron transfer chains of the light reactions
• Chloroplasts move toward low-intensity light, maximizing
exposure to light for photosynthesis
Movement of Chloroplasts
in Response to Light
Thigmotropism
• Thigmotropism
• Contact with a solid object changes the direction of plant
growth
• Involves several gene products and calcium ions
• Results in unequal growth rates on opposite sides of the
shoot
• Mechanical stress (such as wind) inhibits stem lengthening
in a similar touch response
Take-Home Message: How do plants respond
to environmental cues?
• Via hormones, plants adjust the direction and rate of growth in
response to gravity, light, contact, mechanical stress, and
other environmental stimuli.
30.9 Sensing Recurring
Environmental Changes
• Shifts in biological activity that recur in 24-hour cycles are
mediated by cyclic shifts in gene expression
• Seasonal shifts in night length trigger seasonal shifts in
development in many plants
Circadian Cycles
• A circadian rhythm is a cycle of biological activity that recurs
every 24 hours
• Example: A bean plant holds its leaves horizontally during the
day but folds them close to its stem at night
• Circadian rhythms are driven by feedback loops involving
transcription factors that regulate their own expression
Rhythmic Leaf Movements of a Bean Plant
1 A.M.
6 A.M.
Noon
3 P.M.
10 P.M.
Midnight
ANIMATED FIGURE: Rhythmic leaf
movements
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Adjusting Circadian Clocks
• Different wavelengths of sunlight provide input to circadian
clocks by activating and inactivating photoreceptor pigments
such as phytochromes and cryptochromes
• Active phytochromes cause gene transcription for
components of rubisco, photosystem II, ATP synthase, and
other molecules involved in photosynthesis
• Some gene products are produced during the day and
degraded at night; others produced at night are degraded
during the day
Circadian Cycles of Gene Expression
Seasonal Changes
• Plants respond to seasonal changes in light availability with
seasonally appropriate behaviors such as entering or
breaking dormancy
• Photoperiodism is an organism’s response to changes in
the length of day relative to night
Photoperiodism and Flowering
• Inputs from phytochrome and cryptochrome converge on the
CO gene, which encodes a transcription factor
• The transcription factor induces expression of the FT gene
(flowering locus T) in companion cells
• During short-day seasons, CO protein never accumulates to a
high enough level to promote flowering in long-day plants
• Short-day plants have the same CO gene, but its product
inhibits FT gene expression in these plants
Time (hours)
0
4
8
12
16
20
24
long-day short-day
plant
plant
critical night length
A A flash of red light interrupting a long night causes plants to respond as
if the night were short. Long-day plants flower; short-day plants do not.
Time (hours)
0
4
8
12
critical night length
16
20
24
long-day short-day
plant
plant
B A flash of far-red light cancels the effect of a red light flash. Short-day
plants flower; long-day plants do not.
Figure 30-18 p517
ANIMATED FIGURE: Flowering response
experiments
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Vernalization
• Some biennials and perennials flower in the spring only after
exposure to cold winter temperatures (vernalization)
• Plants may perceive temperature via their plasma membrane,
which varies in lipid composition and calcium ion permeability
• A “cold” signal influences expression of the FT gene, and of
the VRN1 gene, which encodes a transcription factor that
promotes flowering when warm temperatures return
Take-Home Message: How do plants respond
to recurring environmental change?
• Plants respond to recurring cues from the environment with
recurring cycles of activity such as rhythmic leaf movements.
• Photoreceptors that detect daylight provide input into
circadian cycles.
• The main environmental cue for flowering is the length of
night relative to the length of day, which varies by the season
in most places.
• In some species, prolonged exposure to low temperature
stimulates flowering in spring.
ANIMATION: Phytochrome conversions
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30.10 Responses to Stress
• Living and nonliving stressors in the environment provoke
short-term and long-term defense responses in plants
• Defense responses in plants are mediated by hormones
Abiotic Stressors
• Abscisic acid synthesis is triggered by temperature extremes,
lack of water, and other abiotic stressors
• Example: ABA is part of a response that causes a plant’s
stomata to close when water is scarce
Biotic Stressors
• Cell surface receptors recognize molecules specific to
bacterial pathogens, triggering a burst of ethylene synthesis
• When ethylene is present, it binds to its receptors and locks
them in an inactive form that marks them for destruction
• Binding of additional bacteria triggers an ABA-mediated nitric
oxide burst that immediately closes stomata
• Beneficial bacteria and fungi avoid triggering defense
responses by engaging in a complex cross-talk
Hypersensitive Response
• A pathogen that penetrates a plant’s epidermis triggers a
large surge of hydrogen peroxide and nitric oxide that causes
cells in the infected region to commit suicide
• This “hypersensitive” response can prevent a pathogen from
spreading to other parts of the plant, because it often kills the
pathogen along with the infected tissue
Hypersensitive Response
Systemic Acquired Resistance
• Pathogen-triggered systemic acquired resistance increases
the plant’s resilience to both biotic and abiotic stress
• An infected tissue releases a signal that triggers cells to
produce salicylic acid, which increases transcription of
hundreds of genes involved in pathogen resistance
• The chemicals produced differ by species, but all confer
general hardiness to the plant
Interspecific Plant Defenses
• Wounding of a leaf by insects triggers production of ABA,
hydrogen peroxide, ethylene, and jasmonic acid
• Jasmonic acid increases transcription of genes resulting in
release of certain volatile chemicals into the air
• These secondary metabolites are detected by wasps that
parasitize insect herbivores
• These chemicals are also detected by neighboring plants,
which increase production of ethylene and jasmonic acid
Interspecific Plant Defenses
Take-Home Message:
How do plants respond to stress?
• Abscisic acid is involved in responses to nonliving
environmental stresses.
• Detection of plant pathogens can trigger stomatal closure or
cell death.
• Systemic acquired resistance triggered by pathogen attacks
increases a plant’s ability to withstand biotic and abiotic
stresses.