plant responses to internal and external...
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![Page 1: Plant Responses to Internal and External Signalsstaff.katyisd.org/sites/thsbiologyapgt/Documents/Unit 09...• These are morphological adaptations for growing in darkness, ... •](https://reader031.vdocument.in/reader031/viewer/2022030416/5aa1dedf7f8b9a80378c3b65/html5/thumbnails/1.jpg)
LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures by
Erin Barley
Kathleen Fitzpatrick
Plant Responses to Internal and
External Signals
Chapter 39
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Concept 39.1: Signal
transduction pathways link
signal reception to response • A potato left growing in darkness
produces shoots that look unhealthy, and it lacks elongated roots.
• These are morphological adaptations for growing in darkness, collectively called etiolation
• After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally
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(a) Before exposure to light
(b) After a week’s exposure to natural daylight
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• A potato’s response to light is an example of cell-signal processing
• The stages are reception, transduction, and response
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Reception
CELL WALL
2 3 1 Transduction
CYTOPLASM
Response
Relay proteins and
second messengers
Activation of cellular responses
Receptor
Hormone or environmental stimulus Plasma membrane
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Figure 39.3
Reception
CELL WALL
2 3 1 Transduction
CYTOPLASM
Response
Relay proteins and
second messengers
Activation of cellular responses
Receptor
Hormone or environmental stimulus
Plasma membrane
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Figure 39.4-1
Reception 1
CYTOPLASM
Plasma membrane
Phytochrome
Cell wall
Light
- capable of detecting light
- responds to light by: Opens Ca2+
channels, which increases Ca2+ levels in
the cytosol.
- Activates an enzyme that produces cGMP
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Figure 39.4-2
Reception 2 1 Transduction
CYTOPLASM Plasma membrane
Phytochrome
Cell wall
Light
cGMP
Second messenger
Ca2
Ca2 channel
Protein kinase 1
Protein kinase 2
- Opens Ca2+ channels,
which increases Ca2+
levels in the cytosol.
Responds to light by:
Activating an enzyme that
produces cGMP
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Figure 39.4-3
Reception 2 3 1 Transduction Response
CYTOPLASM
Plasma membrane
Phytochrome
Cell wall
Light
cGMP
Second messenger
Ca2
Ca2 channel
Protein kinase 1
Protein kinase 2
Transcription factor 1
Transcription factor 2
NUCLEUS
Transcription
Translation
De-etiolation (greening)
response proteins
P
P Stimulation involve
increased activity of enzymes.
This can occur by
transcriptional regulation
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Concept 39.2: Plant hormones help
coordinate growth, development, and
responses to stimuli
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Tropism -
• Any response resulting in curvature of organs toward or away from a stimulus.
Phototropism-
• A plant bending toward light only if the tip of the coleoptile was present.
• That means a signal must transmitted from
the tip to the elongating region of the plant.
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Figure 39.5
Control
Light
Shaded side
Illuminated side
Boysen-Jensen
Light
Light
Darwin and Darwin
Gelatin (permeable)
Mica (impermeable)
Tip removed
Opaque cap
Trans- parent cap
Opaque shield over curvature
RESULTS
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• In 1913, Peter Boysen-
Jensen demonstrated
that the signal was a
mobile chemical
substance.
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• In 1926, Frits Went
extracted the chemical
messenger for
phototropism, auxin, by
modifying earlier
experiments
Control
RESULTS
Excised tip on agar cube
Growth-promoting chemical diffuses into agar cube
Control (agar cube lacking chemical)
Offset cubes
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Auxin
The Role of Auxin in Cell Elongation
• - Acid Growth Hypothesis - stimulates
proton pumps in the plasma membrane
• The proton pumps lower the pH in the
cell wall, activating expansins, enzymes
that loosen the wall’s fabric
• With the cellulose loosened, the cell can elongate
Plasma membrane
Cell wall
Nucleus Cytoplasm
Vacuole
H2O
• Refers to any chemical that promotes elongation of coleoptiles. Indoleacetic acid (IAA) is a common auxin in plants;
• Auxin is produced in shoot tips
and is transported down the stem
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Auxin’s Role in Plant Development
• Pattern formation of the developing plant
• Reduced auxin flow from the shoot of a branch stimulates growth in lower branches
• Plays a role in phyllotaxy, the arrangement of leaves on the stem
• Directs leaf venation pattern
• Activity of the vascular cambium is under control of auxin transport
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An overdose of synthetic auxins can kill plants For example 2,4-D is used as an herbicide on eudicots
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Cytokinins • stimulate cytokinesis (cell division)
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• Cytokinins work together with auxin to
control cell division and differentiation
Control of Apical Dominance
• Terminal bud suppresses development of axillary
buds
• If the terminal bud is removed, plants become
bushier
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(a) Apical bud intact (not shown in photo)
(b) Apical bud removed
(c) Auxin added to decapitated stem
Axillary buds
Lateral branches
“Stump” after removal of apical bud
Figure 39.9
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• Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination
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•Produced in young roots and leaves
•Stimulate growth of leaves and stems
•Stimulate cell elongation and
cell division
(a) Rosette form (left) and gibberellin-induced Bolting (right)
(b) Grapes from control vine (left) and gibberellin-
treated vine (right)
Fruit Growth Gibberellins are used in
spraying of Thompson
seedless grapes
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Germination
• After water is imbibed, release of gibberellins
from the embryo signals seeds to germinate
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Aleurone Endosperm
Water
Scutellum (cotyledon)
Radicle
-amylase Sugar GA
GA
1 2 3
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Abscisic Acid
Abscisic acid (ABA) slows growth
Two of the many effects of ABA
• Seed dormancy
• Drought tolerance
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Seed dormancy
• Ensures that the seed
will germinate only in
optimal conditions
• Dormancy is broken
when ABA is removed by
heavy rain, light, or
prolonged cold
Drought Tolerance
• ABA accumulation
causes stomata to close
rapidly
• Primary internal signal
that enables plants to
withstand drought
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Ethylene
Plants produce ethylene in response to stresses such
as drought, flooding, mechanical pressure, injury,
and infection
Effects of ethylene include
• response to mechanical stress
• Senescence
• leaf abscission
• fruit ripening
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• Senescence is the
programmed death of cells
(apoptosis) or organs
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0.5 mm
Stem Petiole
Protective layer Abscission layer
• Leaf Abscission
A change in the balance
of auxin and ethylene
controls- abcission layer
grows and cuts off
nutrients to leaf.
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Fruit Ripening
• Ethylene triggers ripening, and ripening triggers
release of more ethylene
• Fruit producers can control ripening by picking
green fruit and controlling ethylene levels
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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 called
photomorphogenesis
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• Plants detect light’s direction, intensity, and wavelength (color)
• An action spectrum depicts relative response of a process to different wavelengths
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(a) Phototropism action spectrum
(b) Coleoptiles before and after light exposures
1.0
0.8
0.6
0.4
0.2
0
436 nm
400 450 500 550 600 650 700
Wavelength (nm)
Ph
oto
tro
pic
eff
ecti
ven
ess
Light
Time 0 min
Time 90 min
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• There are two major classes of light receptors:
• blue-light photoreceptors - control
hypocotyl elongation, stomatal opening, and
phototropism
• phytochromes - responses include seed
germination and shade avoidance
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Figure 39.17
RESULTS
Red Red
Red Red Red Red
Far-red
Far-red Far-red Far-red
Dark (control)
Dark Dark
Dark
• Red light increased germination, while far-red light inhibited germination
• The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome
germination
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Biological Clocks and Circadian Rhythms
• Many plant processes oscillate during the day
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Noon Midnight
• Circadian rhythms
cycles that are about 24 hours
long and are governed by an
internal “clock”
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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
• the relative lengths of night and day
• the environmental stimulus plants use most often to
detect the time of year
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Photoperiodism and Control of Flowering
• Short-day plants
flower when a light period is shorter than a critical length
• Long-day plants
flower when a light period is longer than a certain number of hours
• Day-neutral plants
Flowering controlled by plant maturity, not photoperiod
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Needs a minimum number
of hours of darkness
responses to photoperiod are actually
controlled by night length
Critical Night Length
Needs a maximum number
of hours of darkness
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Figure 39.21
24 hours
Light Flash of light
Darkness
Critical dark period
Flash of light
(b) Long-day (short-night) plant – spring time bloomers
(a) Short day (long-night) plant – Cool season bloomers
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A Flowering Hormone? • Photoperiod is detected by leaves, which cue buds
to develop as flowers
• The flowering signal is called florigen
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24 hours
Graft
Short-day plant
Long-day plant grafted to
short-day plant
Long-day plant
24 hours 24 hours
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Gravity
• Gravitropism - response to gravity
• Roots show positive gravitropism;
• Shoots show negative gravitropism
• Plants may detect gravity by the
settling of statoliths, dense
cytoplasmic components
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Primary root of maize bending gravitropically (LMs)
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Mechanical Stimuli
• Thigmomorphogenesis -changes in growth that result from “touch”- results in “clingy/wrapping” growth
• occurs in vines and other climbing plants (Cudzu, Ivy)
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How do you grow Cudzu! It’s easy. You throw the seeds to the left and you run to the right!
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• Plants counter excessive herbivory with:
• physical defenses - thorns and trichomes
• chemical defenses - distasteful or toxic compounds
• Some plants even “recruit” predatory animals that help defend against specific herbivores
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Defenses Against Herbivores
Wounding
Signal transduction pathway
Chemical in saliva
Synthesis and release of volatile attractants
Recruitment of parasitoid wasps that lay their eggs within caterpillars
2
1 1
3
4
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Figure 39.28
Wounding
Signal transduction pathway
Chemical in saliva
Synthesis and release of volatile attractants
Recruitment of parasitoid wasps that lay their eggs within caterpillars
2
1 1
3
4
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Figure 39.UN03
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Figure 39.UN05