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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
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
© 2011 Pearson Education, Inc.
(a) Before exposure to light
(b) After a week’s exposure to natural daylight
• A potato’s response to light is an example of cell-signal processing
• The stages are reception, transduction, and response
© 2011 Pearson Education, Inc.
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
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
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
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
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
Concept 39.2: Plant hormones help
coordinate growth, development, and
responses to stimuli
© 2011 Pearson Education, Inc.
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.
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
• In 1913, Peter Boysen-
Jensen demonstrated
that the signal was a
mobile chemical
substance.
© 2011 Pearson Education, Inc.
• 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
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
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
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
(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
• Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination
© 2011 Pearson Education, Inc.
•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
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
Abscisic Acid
Abscisic acid (ABA) slows growth
Two of the many effects of ABA
• Seed dormancy
• Drought tolerance
© 2011 Pearson Education, Inc.
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
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.
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
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
• 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
• 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
© 2011 Pearson Education, Inc.
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
Biological Clocks and Circadian Rhythms
• Many plant processes oscillate during the day
© 2011 Pearson Education, Inc.
Noon Midnight
• Circadian rhythms
cycles that are about 24 hours
long and are governed by an
internal “clock”
The Effect of Light on the Biological Clock
• Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues
© 2011 Pearson Education, Inc.
• Photoperiodism
• the relative lengths of night and day
• the environmental stimulus plants use most often to
detect the time of year
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
© 2011 Pearson Education, Inc.
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
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
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
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)
Mechanical Stimuli
• Thigmomorphogenesis -changes in growth that result from “touch”- results in “clingy/wrapping” growth
• occurs in vines and other climbing plants (Cudzu, Ivy)
© 2011 Pearson Education, Inc.
How do you grow Cudzu! It’s easy. You throw the seeds to the left and you run to the right!
• 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
© 2011 Pearson Education, Inc.
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
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
Figure 39.UN03
Figure 39.UN05