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Chapter 39: Plant Responses to
Internal and external Signals
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• Plants, being rooted to the ground
–
Must respond to whatever
environmental change comes their
way
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• e.g., 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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• Signal transduction pathways link signal
reception to response
• Plants cellular receptors detect changes in their
environment
• For a stimulus to elicit a response cells must
have an appropriate receptor
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• Potato growing in darkness
Unhealthy shoots, lack elongated roots
• Morphological adaptation for growing in
darkness (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 exposed to light
Profound changes called de-etiolation, 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
• 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, usually involves enzymes
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Plant Hormones
• Chemical signals that coordinate growth,
development, and responses to stimuli
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Tropism
• Growth response that results in curvatures of
plant toward or away from a stimulus, caused
by hormones
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Tropism experiments
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
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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• Hormones control plant growth and
development by affecting division, elongation,
and differentiation of cells
• Hormones produced in very low
concentrations, but have a profound effect
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Auxin
• Any chemical substance that promotes cell
elongation in different target tissues
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The Role of Auxin in Cell Elongation
• Acid growth hypothesis
– Proton pumps involved in 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.
H2O
Cell wall
enzymes
Cross-linking
cell wall
polysaccharides
Microfibril
Plasma
membrane
H+
H+
2 The cell wall
becomes more
Cell
wall
H+
acidic.
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.
Auxin
• Formation and branching of roots
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Auxins as Herbicides
• An overdose of auxins can kill eudicots
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Cytokinins
• Stimulate cell division
• Produced in actively growing tissues
• Work together with auxin
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• Cytokinins and auxin interact to control apical
dominance (ability of a terminal bud to
suppress development of axillary buds)
Axillary buds
Figure 39.9a
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• Remove terminal bud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
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Gibberellins
• Stem elongation, fruit growth, and seed
germination
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Stem Elongation
• Gibberellins stimulate growth of both leaves
and stems (stimulate cell elongation and cell
division)
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Fruit Growth
• Auxin and gibberellins must be present for fruit
to set
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• Gibberellins used commercially f/ 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
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)
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Sugar
Abscisic Acid (ABA)
• Seed dormancy
• Drought tolerance
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Seed Dormancy
• Great survival value, seed germinates only
when there are optimal conditions
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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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Apoptosis (Programmed Cell Death)
• A burst of ethylene associated with apoptosis
of cells, organs, or whole plants
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Leaf Abscission
• Auxin and ethylene controls leaf abscission
– Occurs in autumn when a leaf falls
0.5 mm
Protective layer
Abscission layer
Figure Stem
39.16
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Petiole
Fruit Ripening
• A burst of ethylene in fruit triggers the ripening
process
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Response to light
• Light cues many key events in plant growth
and development
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• Plants not only detect the presence of light, but
also its direction, intensity, and wavelength
(color)
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• Action spectra
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
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and violet light, particularly blue light.
Phytochromes as Photoreceptors
• Regulate responses to light throughout its life
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USDA (1930’s) light-induced germination
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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• Phytochrome
– 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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Biological Clocks and Circadian Rhythms
• Many plant processes oscillate during the day
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• Many legumes (e.g. beans)
– 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, ~ 24 hours long
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Photoperiodism and Responses to Seasons
• Photoperiod, the relative lengths of night and
day
– environmental stimulus plants use to detect the
time of year
• Photoperiodism
–
response to photoperiod
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Photoperiodism and Control of Flowering
• Flowering requires a certain photoperiod
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Critical Night Length
• Flowering controlled by night 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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• Phytochrome (receives red light) 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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• Because of their immobility
– Plants must adjust to a wide range of
environmental circumstances
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Gravity
• Response to gravity
– gravitropism
• Roots show positive gravitropism
• Stems show negative gravitropism
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Mechanical Stimuli
• thigmomorphogenesis
– 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
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• Rapid leaf movements in response to
mechanical stimulation
– transmission of electrical impulses called
action potentials
(a) Unstimulated
(b) Stimulated
Side of pulvinus with
flaccid cells
Leaflets
after
stimulation
Side of pulvinus with
turgid cells
Pulvinus
(motor
organ)
Figure 39.27a–c
(c) Motor organs
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Vein
0.5 m
Environmental Stresses
• Adverse effect on a plant
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Drought
• Water deficit reduced transpiration
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Flooding
• Enzymatic destruction of cells
– air tubes plants survive oxygen deprivation
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
Salt Stress
• Water potential changes
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Heat Stress
• Heat-shock proteins produced
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Cold Stress
• Altered lipid composition of membranes
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Defenses Against Herbivores
• Physical defenses, e.g. thorns
• Chemical defenses, e.g. toxic compounds
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• Some plants “recruit” predatory animals to
defend against 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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