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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
• 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.
Figure 39.2
(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.
Figure 39.3
CELL
WALL
1 Reception
CYTOPLASM
2 Transduction
3 Response
Relay proteins and
second messengers
Receptor
Hormone or
environmental
stimulus
Plasma membrane
Activation
of cellular
responses
Reception
• Internal and external signals are detected by
receptors, proteins that change in response to
specific stimuli
• In de-etiolation, the receptor is a phytochrome
capable of detecting light
© 2011 Pearson Education, Inc.
Transduction
• Second messengers transfer and amplify
signals from receptors to proteins that cause
responses
• Two types of second messengers play an
important role in de-etiolation: Ca2+ ions and
cyclic GMP (cGMP)
• The phytochrome receptor responds to light by
– Opening Ca2+ channels, which increases Ca2+
levels in the cytosol
– Activating an enzyme that produces cGMP
© 2011 Pearson Education, Inc.
Figure 39.4-1
1 Reception
CYTOPLASM
Plasma
membrane
Phytochrome
Cell
wall
Light
Figure 39.4-2
2 Transduction
1 Reception
CYTOPLASM
Plasma
membrane
cGMP
Second
messenger
Phytochrome
Protein
kinase 1
Cell
wall
Protein
kinase 2
Light
Ca2 channel
Ca2
Figure 39.4-3
2 Transduction
1 Reception
3 Response
Transcription
factor 1 NUCLEUS
CYTOPLASM
Plasma
membrane
cGMP
Second
messenger
Phytochrome
P
Protein
kinase 1
Transcription
factor 2
P
Cell
wall
Protein
kinase 2
Transcription
Light
Translation
Ca2 channel
Ca2
De-etiolation
(greening)
response proteins
Response
• A signal transduction pathway leads to
regulation of one or more cellular activities
• In most cases, these responses to stimulation
involve increased activity of enzymes
• This can occur by transcriptional regulation or
post-translational modification
© 2011 Pearson Education, Inc.
Concept 39.2: Plant hormones help
coordinate growth, development, and
responses to stimuli
• Plant hormones are chemical signals that
modify or control one or more specific
physiological processes within a plant
© 2011 Pearson Education, Inc.
The Discovery of Plant Hormones
• Any response resulting in curvature of organs
toward or away from a stimulus is called a
tropism
• In the late 1800s, Charles Darwin and his son
Francis conducted experiments on
phototropism, a plant’s response to light
• They observed that a grass seedling could bend
toward light only if the tip of the coleoptile was
present
© 2011 Pearson Education, Inc.
• They postulated that a signal was transmitted
from the tip to the elongating region
© 2011 Pearson Education, Inc.
Video: Phototropism
© 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)
Table 39.1
Auxin
• The term auxin refers to any chemical that
promotes elongation of coleoptiles
• Auxin is produced in shoot tips and is
transported down the stem
• Auxin transporter proteins move the hormone
from the basal end (bottom) of one cell into the
apical end (top) of the neighboring cell
© 2011 Pearson Education, Inc.
The Role of Auxin in Cell Elongation
• According to the acid growth hypothesis, auxin
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
© 2011 Pearson Education, Inc.
Figure 39.8
Cross-linking
polysaccharides
Cell wall–loosening
enzymes
Expansin
CELL WALL
Cellulose
microfibril
H2O
H
Plasma
membrane
H
H
H
ATP
H
H
H
Cell wall
H
H
Plasma membrane
CYTOPLASM
Nucleus Cytoplasm
Vacuole
Auxin’s Role in Plant Development
• Polar transport (shoots to roots) of auxin plays a
role in pattern formation of the developing
plant
• Reduced auxin flow from the shoot of a branch
stimulates growth in lower branches
• Auxin transport plays a role in phyllotaxy, the
arrangement of leaves on the stem
• Polar transport of auxin from leaf margins(leaf
edge) directs leaf venation pattern
© 2011 Pearson Education, Inc.
Practical Uses for Auxins
• The auxin indolbutyric acid (IBA) stimulates
adventitious roots (roots that form from
leaves/stems) and is used in vegetative
propagation of plants by cuttings
• An overdose of synthetic auxins can kill plants
– For example 2,4-D is used as an herbicide on
certain flowing plants
© 2011 Pearson Education, Inc.
Cytokinins
• Cytokinins are so named because they
stimulate cytokinesis (cell division)
© 2011 Pearson Education, Inc.
Control of Cell Division and Differentiation
• Cytokinins are produced in actively growing
tissues such as roots, embryos, and fruits
• Cytokinins work together with auxin to control
cell division and differentiation
© 2011 Pearson Education, Inc.
Control of Apical Dominance
• Cytokinins, auxin, and strigolactone interact in
the control of apical dominance, a terminal
bud’s ability to suppress 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
Anti-Aging Effects
• Cytokinins slow the aging of some plant organs by
inhibiting protein breakdown, stimulating RNA and
protein synthesis, and mobilizing nutrients from
surrounding tissues
© 2011 Pearson Education, Inc.
Gibberellins
• Gibberellins have a variety of effects, such
as stem elongation, fruit growth, and
seed germination
© 2011 Pearson Education, Inc.
Stem Elongation
• Gibberellins are produced in young roots and
leaves
• Gibberellins stimulate growth of leaves and
stems
• In stems, they stimulate cell elongation and cell
division
© 2011 Pearson Education, Inc.
Figure 39.10
(b) Grapes from control vine
(left) and gibberellin-treated
vine (right- fruit growth)
(a) Rosette form (left) and
gibberellin-induced bolting
(right-stem elongation)
Germination
• After water is taken in by the seed, release of
gibberellins from the embryo signals seeds to
germinate
© 2011 Pearson Education, Inc.
Seed Germination
Aleurone
2
Endosperm 1
3
-amylase
GA
Water
Scutellum
(cotyledon)
GA
Radicle
Sugar
Brassinosteroids
• Brassinosteroids are chemically similar to the
sex hormones of animals
• They induce cell elongation and division in stem
segments
• They slow leaf abscission (dropping) and promote
xylem differentiation
© 2011 Pearson Education, Inc.
Abscisic Acid
• Abscisic acid (ABA) slows growth
• Two of the many effects of ABA
– Seed dormancy
– Drought tolerance
© 2011 Pearson Education, Inc.
Seed Dormancy
• Seed dormancy ensures that the seed will
germinate only in optimal conditions
• In some seeds, dormancy is broken when ABA
is removed by heavy rain, light, or prolonged
cold
• Precocious (early) germination can be caused
by inactive or low levels of ABA
© 2011 Pearson Education, Inc.
Drought Tolerance
• ABA is the primary internal signal that enables
plants to withstand drought
• ABA accumulation causes stomata to close
rapidly
© 2011 Pearson Education, Inc.
Strigolactones
• The hormones called strigolactones
– Stimulate seed germination
– Help establish mycorrhizal associations (roots
& fungus)
– Help control apical dominance
• Strigolactones are named for parasitic Striga
plants
• Striga seeds germinate when host plants
exude strigolactones through their roots
© 2011 Pearson Education, Inc.
Ethylene
• Plants produce ethylene in response to stresses
such as drought, flooding, mechanical pressure,
injury, and infection
• The effects of ethylene include response to
mechanical stress, senescence (plant aging), leaf
abscission, and fruit ripening
© 2011 Pearson Education, Inc.
The Triple Response to Mechanical Stress
• Ethylene induces the triple response, which
allows a growing shoot to avoid obstacles
• The triple response consists of a slowing of
stem elongation, a thickening of the stem,
and horizontal growth
© 2011 Pearson Education, Inc.
Figure 39.13
0.00
0.10
0.20
0.40
0.80
Ethylene concentration (parts per million)
Senescence
• Senescence is the programmed death of cells
or organs
• A burst of ethylene is associated with
apoptosis, the programmed destruction of
cells, organs, or whole plants
© 2011 Pearson Education, Inc.
Leaf Abscission
• A change in the balance of auxin and ethylene
controls leaf abscission, the process that
occurs in autumn when a leaf falls
© 2011 Pearson Education, Inc.
Figure 39.15
0.5 mm
Protective layer
Abscission layer
Stem
Petiole
Fruit Ripening
• A burst of ethylene production in a fruit triggers
the ripening process
• Ethylene triggers ripening, and ripening triggers
release of more ethylene (positive feedback)
• 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 not only presence of light but also
its direction, intensity, and wavelength (color)
• A graph called an action spectrum depicts
relative response of a process to different
wavelengths
• Action spectra are useful in studying any
process that depends on light
© 2011 Pearson Education, Inc.
Phytochromes and Shade Avoidance
• The phytochrome system also provides the
plant with information about the quality of light
• Leaves in the canopy absorb red light
• Shaded plants receive more far-red than red
light
• In the “shade avoidance” response, the
phytochrome ratio shifts in favor of Pr (red)
when a tree is shaded and as a result a plant
will increase apical dominance and elongate
© 2011 Pearson Education, Inc.
Biological Clocks and Circadian Rhythms
• Many plant processes oscillate during the day
• Many legumes lower their leaves in the evening
and raise them in the morning, even when kept
under constant light or dark conditions
© 2011 Pearson Education, Inc.
Figure 39.20
Noon
Midnight
• Circadian rhythms are cycles that are about
24 hours long and are governed by an internal
“clock”
• Circadian rhythms can be entrained to exactly
24 hours by the day/night cycle
• The clock may depend on synthesis of a
protein regulated through feedback control and
may be common to all eukaryotes
© 2011 Pearson Education, Inc.
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
© 2011 Pearson Education, Inc.
Photoperiodism and Control of Flowering
• Some processes, including flowering in many
species, require a certain photoperiod
• Plants that flower when a light period is shorter
than a critical length are called short-day plants
• Plants that flower when a light period is longer
than a certain number of hours are called longday plants
• Flowering in day-neutral plants is controlled by
plant maturity, not photoperiod
© 2011 Pearson Education, Inc.
Critical Night Length
• In the 1940s, researchers discovered that
flowering and other responses to photoperiod
are actually controlled by night length, not
day length
© 2011 Pearson Education, Inc.
• A plant that requires a long period of darkness,
is termed a "short day" (long night) plant.
Short-day plants form flowers only when day
length is less than about 12 hours (typically
spring and fall plants)
• Other plants require only a short night to
flower. These are termed "long day" plants.
These bloom only when they receive more
than 12 hours of light (typically summer plants)
© 2011 Pearson Education, Inc.
Figure 39.21
24 hours
(a) Short day
(long-night) plant
Flash Darkness
of
Critical
dark period light
Light
(b) Long-day
(short-night) plant
Flash
of light
A Flowering Hormone?
• Photoperiod is detected by leaves, which cue
buds to develop as flowers
• The flowering signal is called florigen
© 2011 Pearson Education, Inc.
Concept 39.4: Plants respond to a wide
variety of stimuli other than light
• Because of immobility, plants must adjust to a
range of environmental circumstances through
developmental and physiological mechanisms
© 2011 Pearson Education, Inc.
Gravity
• Response to gravity is known as gravitropism
• Roots show positive gravitropism; shoots
show negative gravitropism
• Plants may detect gravity by the settling of
statoliths, dense cytoplasmic components
© 2011 Pearson Education, Inc.
Figure 39.24
Statoliths
(a) Primary root of maize
bending gravitropically
20 m
(b) Statoliths settling to
the lowest sides of
root cap cells
Gravitropism
Video: Gravitropism
© 2011 Pearson Education, Inc.
Mechanical Stimuli
• The term thigmomorphogenesis refers to
changes in form that result from mechanical
disturbance
• Responses by plants can be positive or
negative
• In experiments rubbing stems of young plants a
couple of times daily results in plants that are
shorter than controls
© 2011 Pearson Education, Inc.
Figure 39.25
• Thigmotropism is growth in response to touch
• It occurs in vines and other climbing plants
• Another example of a touch specialist is the
sensitive plant Mimosa pudica, which folds its
leaflets and collapses in response to touch
• Rapid leaf movements in response to
mechanical stimulation are examples of
transmission of electrical impulses called
action potentials
© 2011 Pearson Education, Inc.
Figure 39.26
(a) Unstimulated state
(b) Stimulated state
Side of pulvinus
with flaccid cells
Pulvinus
(motor
organ)
Side of pulvinus
with turgid cells
Vein
0.5 m
Leaflets
after
stimulation
(c) Cross section of a leaflet pair in the stimulated state (LM)