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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
Chapter 39
Plant Responses to Internal and
External Signals
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
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.
(a) Before exposure to light
• 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
(b)
© 2011 Pearson Education, Inc.
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
CELL
WALL
1 Reception
CYTOPLASM
2 Transduction
Relay proteins and
second messengers
Receptor
Hormone or
environmental
stimulus
© 2011 Pearson Education, Inc.
Plasma membrane
3 Response
Activation
of cellular
responses
Figure 39.3
CYTOPLASM
CELL
WALL
1 Reception
2 Transduction
Relay proteins and
second messengers
Receptor
Hormone or
environmental
stimulus
Plasma membrane
3 Response
Activation
of cellular
responses
Figure 39.4-1
1 Reception
CYTOPLASM
Plasma
membrane
Phytochrome -
capable of detecting
light
Cell
wall
-
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
2
1 Reception
CYTOPLASM
Plasma
membrane
Light
cGMP
Second
messenger
Phytochrome
Cell
wall
Transduction
Protein
kinase 1
Responds to light by:
Activating an enzyme that
produces cGMP
Opens Ca2+ channels,
which increases Ca2+
levels in the cytosol.
Ca2 channel
Ca2
Protein
kinase 2
Figure 39.4-3
2 Transduction
1 Reception
3 Response
Transcription
factor 1 NUCLEUS
CYTOPLASM
Plasma
membrane
cGMP
Second
messenger
Phytochrome
Cell
wall
P
Protein
kinase 1
Transcription
factor 2
Stimulation involve
increased activity of enzymes.
This can occur by
transcriptional regulation
Protein
kinase 2
P
Transcription
Light
Translation
Ca2 channel
Ca2
De-etiolation
(greening)
response proteins
Concept 39.2: Plant hormones help
coordinate growth, development, and
responses to stimuli
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.
© 2011 Pearson Education, Inc.
Figure 39.5
RESULTS
Shaded
side
Control
Light
Illuminated
side
Boysen-Jensen
Light
Darwin and Darwin
Light
Gelatin
(permeable)
Tip
removed
Opaque
cap
Transparent
cap
Opaque
shield over
curvature
Mica
(impermeable)
RESULTS
• In 1913, Peter BoysenJensen demonstrated
that the signal was a
mobile chemical
substance.
• In 1926, Frits Went
extracted the chemical
messenger for
phototropism, auxin, by
modifying earlier
experiments
© 2011 Pearson Education, Inc.
Excised tip on
agar cube
Growth-promoting
chemical diffuses
into agar cube
Control
(agar cube
lacking
Control chemical)
Offset
cubes
Auxin
• 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
H2O
Plasma
membrane
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
Cell wall
Nucleus
Cytoplasm
Vacuole
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
An overdose of synthetic auxins can kill plants
For example 2,4-D is used as an herbicide on eudicots
© 2011 Pearson Education, Inc.
Cytokinins
• stimulate cytokinesis (cell division)
• 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
© 2011 Pearson Education, Inc.
Figure 39.9
Lateral branches
“Stump” after
removal of
apical bud
(b) Apical bud removed
Axillary buds
(a) Apical bud intact (not shown in photo)
(c) Auxin added to decapitated stem
• Gibberellins have a variety
of effects, such as stem
elongation, fruit growth,
and seed germination
•Produced in young roots and leaves
•Stimulate growth of leaves and stems
•Stimulate cell elongation and
cell division
Fruit Growth
Gibberellins are used in
spraying of Thompson
seedless grapes
(b) Grapes from control vine
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(left) and gibberellintreated vine (right)
(a) Rosette form (left) and
gibberellin-induced
Bolting (right)
Germination
• After water is imbibed, release of gibberellins
from the embryo signals seeds to germinate
Aleurone
2
Endosperm 1
3
-amylase
GA
Water
Scutellum
(cotyledon)
© 2011 Pearson Education, Inc.
GA
Radicle
Sugar
Abscisic Acid
 Abscisic acid (ABA) slows growth
 Two of the many effects of ABA
• Seed dormancy
• Drought tolerance
Seed dormancy
• Ensures that the seed
will germinate only in
optimal conditions
Drought Tolerance
• ABA accumulation
causes stomata to close
rapidly
• Dormancy is broken
when ABA is removed by
heavy rain, light, or
prolonged cold
• Primary internal signal
that enables plants to
withstand drought
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
• Senescence is the
programmed death of cells
(apoptosis) or organs
0.5 mm
• Leaf Abscission
A change in the balance
of auxin and ethylene
controls- abcission layer
grows and cuts off
nutrients to leaf.
© 2011 Pearson Education, Inc.
Protective layer
Abscission layer
Stem
Petiole
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
Phototropic effectiveness
1.0
436 nm
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
Wavelength (nm)
(a) Phototropism action spectrum
Light
Time  0 min
Time  90 min
(b) Coleoptiles before and after light exposures
© 2011 Pearson Education, Inc.
• 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.
• 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
RESULTS
Red
Dark (control)
Dark
Dark
germination
Red Far-red Red
Figure 39.17
Red Far-red
Dark
Red Far-red Red Far-red
Biological Clocks and Circadian Rhythms
• Many plant processes oscillate during the day
• Circadian rhythms
cycles that are about 24 hours
long and are governed by an
internal “clock”
Noon
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Midnight
The Effect of Light on the Biological Clock
• Phytochrome conversion marks sunrise
and sunset, providing the biological clock with
environmental cues
• Photoperiodism
• the relative lengths of night and day
• the environmental stimulus plants use most often to
detect the time of year
© 2011 Pearson Education, Inc.
Photoperiodism and Control of Flowering
Critical Night Length
• 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.
responses to photoperiod are actually
controlled by night length
Needs a minimum number
of hours of darkness
Needs a maximum number
of hours of darkness
Figure 39.21
24 hours
(a) Short day (long-night) plant –
Cool season bloomers
Flash Darkness
of
Critical
dark period light
Light
(b) Long-day (short-night) plant –
spring time bloomers
Flash
of light
A Flowering Hormone?
• Photoperiod is detected by leaves, which cue buds
to develop as flowers
• The flowering signal is called florigen
24 hours
24 hours
Long-day plant
grafted to
short-day plant
Long-day
plant
24 hours
Graft
Short-day
plant
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
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)
How do you grow Cudzu!
It’s easy. You throw the
seeds to the left and
you run to the right!
© 2011 Pearson Education, Inc.
Defenses Against Herbivores
• 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
4 Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
1 Wounding
1 Chemical
in saliva
2 Signal transduction
pathway
© 2011 Pearson Education, Inc.
3 Synthesis
and release
of volatile
attractants
Figure 39.28
4 Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
1 Wounding
1 Chemical
in saliva
2 Signal transduction
pathway
3 Synthesis
and release
of volatile
attractants
Figure 39.UN03
Figure 39.UN05