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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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• 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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How Plants Respond To Their Environment
• 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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• 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
– mainly IAA (indolacetic acid)
– Produced in shoot, distributed down to roots
– Too much causes INHIBITION of growth and
branching
– Needed for fruit development (as well as apical
growth and root development)
– Commercial uses: more fruits/plant, seedless
fruits, larger fruits, delayed fruit drop (riper)
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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 dicots
•2,4-D
•The defoliant Agent Orange
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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
Stimulate cell division
example: Zeatin
•Causes cell division; found in seed endosperms,
meristems, fruits, roots
•Increases rate of protein synthesis
•Can reverse auxin inhibition
•Prevents leaf aging
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Control of Cell Division and Differentiation
• Cytokinins
– 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
•(GA) > 70 kinds
•Affect cell elongation
•Can cause hyperelongation of
stems; "bolting" and
"flowering"
•Can also induce cellular
differentiation
•Immature seeds have high
[GA]
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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
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
• Two of the many effects of abscisic acid (ABA)
are
– Present in seeds to maintain dormancy
– Preserves buds (dormancy)
– Drought tolerance
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Seed Dormancy
• Seed dormancy has great survival value
– Because it ensures that the seed will
germinate only when there are optimal
conditions
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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
Coleoptile
Figure 39.12
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Ethylene
• Plants produce ethylene
– In response to stresses such as drought,
flooding, mechanical pressure, injury, and
infection
H2C=CH2
•Gaseous hormone, simple hydrocarbon
•Made in cell membranes of plants, bacteria,
fungi
•Causes- fruit ripening (improved color/flavor in
citrus)
•Regulates leaf drop (abscission)
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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
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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Light Responses
• Responses to light/day length/season are
critical for plant success
• Light cues many key events in plant growth
and development
• Light affects
– Plant morphology
– Flowering
– Seed germination
– Etc.
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Photoperiod
•
Plants not only detect the presence of light
–
•
But also its direction, intensity, and wavelength (color)
Seasonal changes = differences in growth rates, etc.
*not really a function of temperature, but more of DAY LENGTH
How do plants know? …A pigment called phytochrome
•
PHYTOCHROME exists in two forms: red-light sensitive (Pr) and farred-light sensitive (Pfr)
•
Pr absorbs red light (660nm) during sunny hours and is converted to
Pfr
•
At night, Pfr is slowly converted back to Pr
•
Scientists think that the ratio of Pr to Pfr is a chemical means of
measuring day length
•
The changing proportions of these two chemicals are what may trigger
hormonal release and flowering
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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
Circadian Rhythms
• 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
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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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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
– vernalization: some plants must be exposed to
cold temp. before they will flower (turnips,
beets, carrots)
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Response to Environmental Stimuli
• 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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Response to Environmental Stimuli
NASTIC MOVEMENTS- plants movements that
occur in response to environmental stimulus
BUT that are independent of the direction of
the stimulus
– ex: closing of Mimosa leaves as response to
touch; Venus flytrap
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Response to Environmental Stimuli
TROPISM: a growth response of a plant part
toward (+) or away (-) from an external
stimulus that determines the direction of
movement
– ex: plant bending toward light; roots growing
downward in response to gravity
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Phototropism
young seedlings will bend forward a unilateral
light source
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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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Bending toward light
Plant tip bending caused by:
• photoreceptive yellow pigment (absorbs blue gamma best)
• diffusion of "chemical messenger" (auxins) from tip from
darkened side of coleoptile
• high concentration of auxin (on darkened side) causes cell
elongation on that side => causes curvature away from dark,
toward light
• if pigment and chemical can’t get from light side to dark side =
no bending
• conclusion= a redistribution of auxin from light side to dark side
of coleoptile
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Response to gravity
– Is known as gravitropism or geotropism
• 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
Response to Mechanical Stimulus (touch)
• Growth in response to touch
– Is called thigmotropism
– Occurs in vines and other climbing plants
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Touch Response
• Rapid leaf movements in response to
mechanical stimulation
– Are examples of 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
Mechanical Stimuli
• The term thigmomorphogenesis
– Refers to the changes in form that result from
mechanical perturbation
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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Environmental Stresses
• Environmental stresses
– Have a potentially adverse effect on a plant’s
survival, growth, and reproduction
– 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
– Hydrotropism: growth of roots toward water
source
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