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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 whatever environmental
change comes their way
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• For example, the bending of a grass seedling
toward light
– Begins with the plant sensing the direction,
quantity, and color of the light
Figure 39.1
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• Concept 39.1: Signal transduction pathways
link signal reception to response
• Plants have cellular receptors
– That they use to detect important 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
– Will produce shoots that do not appear healthy,
and will lack elongated roots
• These are morphological adaptations for
growing in darkness
– Collectively referred to as etiolation
(a) 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.
Figure 39.2a
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• After the potato is exposed to light
– The plant undergoes profound changes called deetiolation, in which shoots and roots grow normally
(b) 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.
Figure 39.2b
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• The potato’s response to light
– Is an example of cell-signal processing
CYTOPLASM
CELL
WALL
1 Reception
2
Transduction
Relay molecules
Receptor
Hormone or
environmental
stimulus
Plasma membrane
Figure 39.3
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3 Response
Activation
of cellular
responses
Transduction
• Second messengers
– Transfer and amplify signals from receptors to
proteins that cause specific responses
– CA++, cAMP, cGMP, Protein Kinases, etc.
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• An example of signal transduction in plants
2 Transduction
1 Reception
3 Response
Transcription
factor 1
NUCLEUS
CYTOPLASM
cGMP
Plasma
membrane
Second messenger
produced
Phytochrome
activated
by light
Cell
wall
2 One pathway uses cGMP as a
second messenger that activates
a specific protein kinase.The other
pathway involves an increase in
cytoplasmic Ca2+ that activates
another specific protein kinase.
Specific
protein
kinase 1
activated
P
Transcription
factor 2
P
Specific
protein
kinase 2
activated
Transcription
Light
Translation
1 The light signal is
detected by the
phytochrome receptor,
which then activates
at least two signal
transduction pathways.
Ca2+
channel
opened
Ca2+
Figure 39.4
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3 Both pathways
lead to expression
of genes for proteins
that function in the
de-etiolation
(greening) response.
De-etiolation
(greening)
response
proteins
Response
• Ultimately, a signal transduction pathway
– Leads to a regulation of one or more cellular
activities
• In most cases
– These responses to stimulation involve the
increased activity of certain enzymes
– Through transcription factors or posttranslational modifications
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Transcriptional/Post-Translational Regulation
• Transcription factors bind directly to specific
regions of DNA
– And control the transcription of specific genes
• Post-translational modification
– Involves the activation of existing proteins
involved in the signal response
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De-Etioloation (“Greening”) Proteins
• Many enzymes that function in certain signal
responses are involved in photosynthesis
directly
– While others are involved in supplying the
chemical precursors necessary 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 the
different parts of an organism
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The Discovery of Plant Hormones
• Any growth response
– That results in curvatures of whole plant
organs toward or away from a stimulus is
called a tropism
– Is often caused by hormones
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• Charles Darwin and his son Francis
– Conducted some of the earliest experiments
on phototropism, a plant’s response to light, in
the late 19th century
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EXPERIMENT In 1880, Charles Darwin and his son Francis designed an experiment to determine
what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to
determine how the signal for phototropism is transmitted.
RESULTS
Control
Boysen-Jensen (1913)
Darwin and Darwin (1880)
Shaded
side of
coleoptile
Light
Light
Light
Illuminated
side of
coleoptile
Tip
removed
Tip covered
by opaque
cap
Tip
covered
by transparent
cap
Base covered
by opaque
shield
Tip separated
by gelatin
block
Tip separated
by mica
CONCLUSION In the Darwins’ experiment, a phototropic response occurred only when light could
reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen
observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin)
but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested
that the signal is a light-activated mobile chemical.
Figure 39.5
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• In 1926, Frits Went
– Extracted the
chemical
messenger for
phototropism,
auxin, by
modifying earlier
experiments
EXPERIMENT
In 1926, Frits Went’s experiment identified how a growth-promoting
chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and
removed their tips, putting some tips on agar blocks that he predicted would absorb the
chemical. On a control coleoptile, he placed a block that lacked the chemical. On others,
he placed blocks containing the chemical, either centered on top of the coleoptile to
distribute the chemical evenly or offset to increase the concentration on one side.
RESULTS
The coleoptile grew straight if the chemical was distributed evenly.
If the chemical was distributed unevenly, the coleoptile curved away from the side with
the block, as if growing toward light, even though it was grown in the dark.
Excised tip placed
on agar block
Growth-promoting
chemical diffuses
into agar block
Control
Figure 39.6
Control
(agar block
lacking
chemical)
has no
effect
Agar block
with chemical
stimulates growth
Offset blocks
cause curvature
CONCLUSION
Went concluded that a coleoptile curved toward light because its dark
side had a higher concentration of the growth-promoting chemical, which he named auxin.
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A Survey of Plant Hormones
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• In general, hormones control plant growth and
development
– By affecting the division, elongation, and
differentiation of cells
• Plant hormones are produced in very low
concentrations
– But a minute amount can have a profound
effect on the growth and development of a
plant organ
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Auxin
• The term auxin
– Is used for any chemical substance that
promotes cell elongation in different target
tissues
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The Role of Auxin in Cell Elongation
• According to a model called the acid growth
hypothesis
– Proton pumps play a major role in the growth
response of cells to auxin
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• Cell elongation in response to auxin
3 Wedge-shaped expansins, activated
by low pH, separate cellulose microfibrils from
cross-linking polysaccharides. The exposed cross-linking
polysaccharides are now more accessible to cell wall enzymes.
Expansin
4 The enzymatic cleaving
of the cross-linking
CELL WALL
polysaccharides allows
the microfibrils to slide.
The extensibility of the
cell wall is increased. Turgor
causes the cell to expand.
Cell wall
enzymes
Cross-linking
cell wall
polysaccharides
Microfibril
H2O
Plasma
membrane
H+
H+
2 The cell wall
becomes more
acidic.
Cell
wall
H+
H+
H+
H+
H+
H+
1 Auxin
increases the
activity of
proton pumps.
Cytoplasm
Nucleus
Vacuole
ATP
H+
Figure 39.8
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Plasma membrane
Cytoplasm
5 With the cellulose loosened,
the cell can elongate.
Lateral and Adventitious Root Formation
• Auxin
– Is involved in the formation and branching of
roots
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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 (think cytokinesis)
– Stimulate cell division
– Are produced in actively growing tissues such
as roots, embryos, and fruits
– Work together with auxin
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Control of Apical Dominance
• Cytokinins, auxin, and other factors interact in
the control of apical dominance
– The ability of a terminal bud to suppress
development of axillary buds
Axillary buds
Figure 39.9a
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• If the terminal bud is removed
– Plants become bushier
“Stump” after
removal of
apical bud
Figure 39.9b
Lateral branches
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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 both leaves
and stems
• In stems
– Gibberellins 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
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• Gibberellins are used commercially
– In the spraying of Thompson seedless grapes
Figure 39.10
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Germination
• After water is imbibed, the release of gibberellins from
the embryo
– Signals the seeds to break dormancy and germinate
2 The aleurone responds by
synthesizing and secreting
digestive enzymes that
hydrolyze stored nutrients in
the endosperm. One example
is -amylase, which hydrolyzes
starch. (A similar enzyme in
our saliva helps in digesting
bread and other starchy foods.)
1 After a seed
imbibes water, the
embryo releases
gibberellin (GA)
as a signal to the
aleurone, the thin
outer layer of the
endosperm.
3 Sugars and other
nutrients absorbed
from the endosperm
by the scutellum
(cotyledon) are consumed
during growth of the
embryo into a seedling.
Aleurone
Endosperm
-amylase
GA
GA
Water
Radicle
Scutellum
(cotyledon)
Figure 39.11
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Sugar
Brassinosteroids
• Brassinosteroids
– Are similar to the sex hormones of animals
– Induce cell elongation and division
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Abscisic Acid
• Two of the many effects of abscisic acid (ABA)
are
– Seed dormancy
– Drought tolerance
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• Precocious germination is observed in maize
mutants
– That lack a functional transcription factor
required for ABA to induce expression of
certain genes
• ABA is the primary internal signal
– enables plants to
withstand drought
Figure 39.12
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Coleoptile
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 of etiolated
seedlings
EXPERIMENT Germinating pea seedlings were placed in the
dark and exposed to varying ethylene concentrations. Their growth
was compared with a control seedling not treated with ethylene.
– inhibition of elongation and
stem thickening
RESULTS
All the treated seedlings exhibited the triple
response. Response was greater with increased concentration.
– enhanced apical hook
curvature
– horizontal growth (90
degrees turned relative to
gravity)
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)
– this may be a response
associated with the seedling
growing into an obstacle
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CONCLUSION Ethylene induces the triple response in pea seedli
with increased ethylene concentration causing increased response.
Figure 39.13
• Ethylene-insensitive mutants
– Fail to undergo the triple response after
exposure to ethylene
ein mutant
Figure 39.14a
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Apoptosis: Programmed Cell Death
• A burst of ethylene
– Is associated with 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 (relative ratio)
– The process that occurs in autumn when a
leaf falls
0.5 mm
Protective layer
Abscission layer
Stem
Petiole
Figure 39.16
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Fruit Ripening
• A burst of ethylene production in the fruit
– Triggers the ripening process
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Systems Biology and Hormone Interactions
• Interactions between hormones and their signal
transduction pathways
– Make it difficult to predict what effect a genetic
manipulation will have on a plant
• Systems biology seeks a comprehensive
understanding of plants
– That will permit successful 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 what plant biologists call
photomorphogenesis
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• Plants not only detect the presence of light
– But also its direction, intensity, and wavelength
(color)
• A graph called an action spectrum
– Depicts the relative response of a process to
different wavelengths of light
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• Action spectra
– Are useful in the study of any process that depends
on light
EXPERIMENT Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow,
orange, and red light to test which wavelengths stimulate the phototropic bending toward light.
Phototropic effectiveness relative to 436 nm
RESULTS
The graph below shows phototropic effectiveness (curvature per photon) relative
to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and
after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only
with wavelengths below 500 nm and was greatest with blue light.
1.0
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
Wavelength (nm)
Light
Time = 0 min.
Time = 90 min.
Figure 39.17
CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is
sensitive to blue and violet light, particularly blue light.
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• Research on action spectra and absorption
spectra of pigments
– Led to the identification of 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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• In the 1930s, scientists at the U.S. Department
of Agriculture
– Determined the action spectrum for lightinduced germination of lettuce seeds
Dark (control)
Dark
Dark
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EXPERIMENT
During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red
light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in
the dark, and the results were compared with control seeds that were not exposed to light.
RESULTS
The bar below each photo indicates the sequence of red-light exposure, far-red light
exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed
to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right).
Dark (control)
Red
Dark
Red Far-red Red
Figure 39.18
Red Far-red
Dark
Dark
Red Far-red Red Far-red
CONCLUSION
Red light stimulated germination, and far-red light inhibited germination.
The final exposure was the determining factor. The effects of red and far-red light were reversible.
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• A phytochrome
– Is the photoreceptor responsible for the
opposing effects of red and far-red light
A phytochrome consists of two identical proteins joined to form
one functional molecule. Each of these proteins has two domains.
Chromophore
Photoreceptor activity. One domain,
which functions as the photoreceptor,
is covalently bonded to a nonprotein
pigment, or chromophore.
Kinase activity. The other domain
has protein kinase activity. The
photoreceptor domains interact with the
kinase domains to link light reception to
cellular responses triggered by the
kinase.
Figure 39.19
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• Phytochromes exist in two photoreversible
states
– With conversion of Pr to Pfr triggering many
developmental responses
Pr
Pfr
Red light
Responses:
seed germination,
control of
flowering, etc.
Synthesis
Far-red
light
Figure 39.20
Slow conversion
in darkness
(some plants)
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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 of a tree
– The phytochrome ratio shifts in favor of Pr
when a tree is shaded
– apical dominance increases, flowering altered,
etc.
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Biological Clocks and Circadian Rhythms
• Many plant processes
– Oscillate during the day
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• Many legumes
– Lower their leaves in the evening and raise
them in the morning
Figure 39.21
Noon
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Midnight
• Cyclical responses to environmental stimuli are
called circadian rhythms
– And are approximately 24 hours long
– 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
– Ratio of Pr to Pfr states of phytochrome protein
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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 developmental processes, including
flowering in many species
– Requires a certain 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
EXPERIMENT
During the 1940s, researchers conducted experiments in which periods of darkness were
interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering
in “short-day” and “long-day” plants.
RESULTS
Darkness
Flash of
light
Critical
dark
period
Light
(a) “Short-day” plants
flowered only if a period of
continuous darkness was
longer than a critical dark
period for that particular
species (13 hours in this
example). A period of
darkness can be ended by a
brief exposure to light.
Figure 39.22
(b) “Long-day” plants
flowered only if a
period of continuous
darkness was shorter
than a critical dark
period for that
particular species (13
hours in this example).
CONCLUSION
The experiments indicated that flowering of each species was determined by a critical period of
darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are
more properly called “long-night” plants, and “long-day” plants are really “short-night” plants.
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• Action spectra and photoreversibility experiments
– Show that phytochrome is the pigment that receives
red light, which can interrupt the nighttime portion of
the photoperiod
EXPERIMENT
A unique characteristic of phytochrome is reversibility in response to red and
far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods,
researchers observed how flashes of red light and far-red light affected flowering in “short-day”
and “long-day” plants.
RESULTS
24
20
R
FR
R
R
FR
R
FR
R
FR
R
16
12
8
4
0
Short-day (long-night) plant
Long-day (short-night) plant
CONCLUSION
Figure 39.23
A flash of red light shortened the dark period. A subsequent flash of far-red
light canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red
light was canceled. This reversibility indicated that it is phytochrome that measures the interruption
of dark periods.
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A Flowering Hormone?
• The flowering signal, not yet chemically
identified
– Is called florigen, and it may be a hormone or a
change in relative concentrations of multiple
hormones
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EXPERIMENT
To test whether there is a flowering hormone, researchers conducted an
experiment in which a plant that had been induced to flower by photoperiod was grafted to
a plant that had not been induced.
RESULTS
Plant subjected to photoperiod
that does not induce flowering
Plant subjected to photoperiod
that induces flowering
Graft
Time
(several
weeks)
Figure 39.24
CONCLUSION Both plants flowered, indicating the transmission of a flower-inducing
substance. In some cases, the transmission worked even if one was a short-day plant
and the other was a long-day plant.
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Meristem Transition and Flowering
• Whatever combination of environmental cues
and internal signals is necessary for flowering
to occur
– The outcome is the transition of a bud’s
meristem from a vegetative to a flowering state
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• Concept 39.4: Plants respond to a wide variety
of stimuli other than light
• Because of their immobility
– Plants must adjust to a wide 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
Statoliths
Figure 39.25a, b
(a)
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(b)
20 m
Mechanical Stimuli
• The term thigmomorphogenesis
– Refers to the changes in form that result from
mechanical perturbation
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• Rubbing the stems of young plants a couple of
times daily
– Results in plants that are shorter than controls
Figure 39.26
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• Growth in response to touch
– Is called thigmotropism
– Occurs in vines and other climbing plants
• Rapid leaf movements
in response to
mechanical
stimulation
(a) Unstimulated
– Are examples of
transmission of
electrical
impulses called
action potentials
(b) Stimulated
Side of pulvinus with
flaccid cells
Leaflets
after
stimulation
Side of pulvinus with
turgid cells
Pulvinus
(motor
organ)
(c) Motor organs
Vein
0.5 m
Figure 39.27a–c
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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
Vascular
cylinder
Air tubes
Epidermis
Figure 39.28a, b
100 m
(a) Control root (aerated)
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(b) Experimental root (nonaerated)
100 m
• Plants respond to salt stress by producing
solutes tolerated at high concentrations
– Keeping the water potential of cells more
negative than that of the soil solution
• Cold stress
– Altering lipid composition of membranes
• Heat stress
– Heat-shock proteins
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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
• Herbivory, animals eating plants
–
Is a stress that plants face in any ecosystem
• Plants counter excessive herbivory
–
With physical defenses such as thorns
–
With chemical defenses such as distasteful or toxic
compounds
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• Some plants even “recruit” predatory animals
– That help defend the plant against specific
herbivores
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
Figure 39.29
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Defenses Against Pathogens
• A plant’s first line of defense against infection
– Is the physical barrier of the plant’s “skin,” the
epidermis and the periderm
• Once a pathogen invades a plant
– The plant mounts a chemical attack as a
second line of defense that kills the pathogen
and prevents its spread
• The second defense system
– Is enhanced by the plant’s inherited ability to
recognize certain pathogens
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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
4 Before they die,
infected cells
release a chemical
signal, probably
salicylic acid.
3 In a hypersensitive
response (HR), plant
cells produce antimicrobial molecules,
seal off infected
areas by modifying
their walls, and
then destroy
themselves. This
localized response
produces lesions
and protects other
parts of an infected
leaf.
2 This identification
step triggers a
signal transduction
pathway.
1 Specific resistance is
based on the
binding of ligands
from the pathogen
to receptors in
plant cells.
Figure 39.31
4
3
2
Signal
5 The signal is
distributed to the
rest of the plant.
5
Hypersensitive
response
Signal transduction
pathway
Signal
transduction
pathway
6
Acquired
7 resistance
1
Avirulent
pathogen
R-Avr recognition and
hypersensitive response
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Systemic acquired
resistance
6 In cells remote from
the infection site,
the chemical
initiates a signal
transduction
pathway.
7 Systemic acquired
resistance is
activated: the
production of
molecules that help
protect the cell
against a diversity
of pathogens for
several days.
Systemic Acquired Resistance
• Systemic acquired resistance (SAR)
– Is a set of generalized defense responses in
organs distant from the original site of infection
– Protects against a diversity of pathogens,
briefly
– Is triggered by the signal molecule salicylic
acid
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings