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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.
Overview: Stimuli and a Stationary Life
• Linnaeus noted that flowers of different species
opened at different times of day and could be
used as a horologium florae, or floral clock
• Plants, being rooted to the ground, must
respond to environmental changes that come
their way
– For example, the bending of a seedling toward
light begins with sensing the direction, quantity,
and color of the light
© 2011 Pearson Education, Inc.
Figure 39.1
Figure 39.3
(1)
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
Figure 11.16
Reception
Binding of epinephrine to G protein-coupled receptor (1 molecule)
Transduction
(41)
Inactive G protein
Active G protein (102 molecules)
Inactive adenylyl cyclase
Active adenylyl cyclase (102)
ATP
Cyclic AMP (104)
Inactive protein kinase A
Active protein kinase A (104)
Inactive phosphorylase kinase
Active phosphorylase kinase (105)
Inactive glycogen phosphorylase
Active glycogen phosphorylase (106)
Response
Glycogen
Glucose 1-phosphate
(108 molecules)
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 (2-3)
© 2011 Pearson Education, Inc.
Figure 39.2
(a) Before exposure to light
(b) After a week’s exposure
to natural daylight
Figure 39.3
•
•
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
Plasma membrane
3 Response
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 39.4-3
(4-6)
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.
Post-Translational Modification of
Preexisting Proteins
• Post-translational modification involves
modification of existing proteins in the signal
response
• Modification often involves the phosphorylation
of specific amino acids
• The second messengers cGMP and Ca2+
activate protein kinases directly
© 2011 Pearson Education, Inc.
Figure 11.10
Signaling molecule
Receptor
Activated relay
molecule
Inactive
protein kinase
1
Active
protein
kinase
1
Inactive
protein kinase
2
ATP
ADP
P
Active
protein
kinase
2
PP
Pi
Inactive
protein kinase
3
ATP
ADP
Pi
Active
protein
kinase
3
PP
Inactive
protein
P
ATP
P
ADP
PP
Pi
Active
protein
Cellular
response
Transcriptional Regulation
• Specific transcription factors bind directly to
specific regions of DNA and control
transcription of genes
• Some transcription factors are activators that
increase the transcription of specific genes
• Other transcription factors are repressors that
decrease the transcription of specific genes
© 2011 Pearson Education, Inc.
De-Etiolation (“Greening”) Proteins
• De-etiolation activates enzymes that
– Function in photosynthesis directly
– Supply the chemical precursors for chlorophyll
production
– Affect the levels of plant hormones that regulate
growth
© 2011 Pearson Education, Inc.
Concept 39.2: Plant hormones help
coordinate growth, development, and
responses to stimuli
• Hormones were first discovered and animals
and defined with the following criteria: 1.
signal produced that acts elsewhere in the
body 2. Binds to a specific receptor and
triggers responses 3. Travels via circulatory
system
• Plant hormones (aka plant growth
regulators) are chemical signals that modify
or control one or more specific physiological
processes within a plant (7-8)
© 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 (9)
© 2011 Pearson Education, Inc.
Figure 39.5
RESULTS
(10-11)
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)
Figure 39.6
RESULTS
Excised tip on
agar cube
Growth-promoting
chemical diffuses
into agar cube
Control
(agar cube
lacking
Control chemical)
Offset
cubes
Went
Experiment
He gave this
substance the
name auxin
(greek for
increase). Its
structure was
later found to be
IAA (indoleacetic
acid) (12-13)
Auxin
• The term auxin refers to any chemical that
promotes elongation of coleoptiles
• Indoleacetic acid (IAA) is a common auxin in
plants; in this lecture the term auxin refers
specifically to IAA
• Auxin is produced in shoot tips and is
transported down the stem
• Auxin transporter proteins move the hormone
from the basal end of one cell into the apical end
of the neighboring cell
© 2011 Pearson Education, Inc.
Auxin Roles
• Stimulates stem elongation (low
concentrations)
• Promotes formation of lateral and
adventitius roots
• Regulates development of fruit
• Enhances apical dominance
• Fuctions in phototropism and gravitropism
• Promotes vascular differentiation
• Retards leaf abscission (14)
© 2011 Pearson Education, Inc.
Practical Uses for Auxins
• The auxin indolbutyric acid (IBA) stimulates
adventitious roots and is used in vegetative
propagation of plants by cuttings
• Monocots have inactivating enzymes, dicots do
not
• An overdose of synthetic auxins can kill plants
– For example 2,4-D is used as an herbicide on
eudicots (15)
© 2011 Pearson Education, Inc.
Cytokinins
• Cytokinins are so named because they
stimulate cytokinesis (cell division)
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 in shoots and
roots. (16-17partial next two on following slides)
© 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
• Cytokinin promotes lateral bud growth.
© 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 1) stem elongation, 2) fruit growth, 3)
pollen development and 4)seed germination
• (18)
© 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)
(a) Rosette form (left) and
gibberellin-induced bolting
(right)
Abscisic Acid
• Abscisic acid (ABA) slows growth
• Unlike previous discussed hormones it inhibits
growth
• When first discovered it was thought to play a
primary role in leaf abscission (no longer thought)
• Two of the many effects of ABA
– Seed dormancy
– Drought tolerance (Closes stomata)
– Inhibited growth (19-20)
© 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.
Ethylene
• Plants produce ethylene in response to stresses
such as drought, flooding, mechanical pressure,
injury, and infection
• Auxin can also stimulate ethylene production.
• The effects of ethylene include response to
mechanical stress, senescence, leaf abscission,
and fruit ripening (21-22)
© 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.
Table 39.1
Fruit Ripening
• A burst of ethylene production in a fruit triggers
the ripening process
• 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
• 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
© 2011 Pearson Education, Inc.
• Different plant responses can be mediated by
the same or different photoreceptors
• There are two major classes of light receptors:
blue-light photoreceptors and
phytochromes
• Phytochromes absorb red wavelengths of light.
• Red light (660nm) increased germination, while
far-red light (730nm) inhibited germination
• The photoreceptor responsible for the opposing
effects of red and far-red light is a phytochrome
(24-25 & 27)
© 2011 Pearson Education, Inc.
Figure 39.17
RESULTS
Red
Dark
Red Far-red
Dark
Dark (control)
Red Far-red Red
Dark
Red Far-red Red Far-red
Blue-Light Photoreceptors
• Various blue-light photoreceptors control
hypocotyl elongation, stomatal opening, and
phototropism (26)
© 2011 Pearson Education, Inc.
Figure 39.19
Pr
Pfr
Red light
Synthesis
Responses:
seed
germination,
control of
flowering, etc.
Far-red
light
Slow conversion
in darkness
(some plants)
Enzymatic
destruction
(28-30)
Cross out 31
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 (32 Skip
examples)
© 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 (33)
© 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 (34)
© 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
• Short-day plants are governed by whether the
critical night length sets a minimum number of
hours of darkness
• Long-day plants are governed by whether the
critical night length sets a maximum number of
hours of darkness
© 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
(35)
Flash
of light
• Red light can interrupt the nighttime portion of
the photoperiod
• A flash of red light followed by a flash of far-red
light does not disrupt night length
• Action spectra and photoreversibility
experiments show that phytochrome is the
pigment that receives red light
© 2011 Pearson Education, Inc.
Figure 39.22
24 hours
R
R FR
R FR R
R FR R FR
Critical dark period
Long-day
Short-day
(long-night) (short-night)
plant
plant
Figure 39.23
24 hours
24 hours
Long-day plant
grafted to
short-day plant
Long-day
plant
24 hours
Graft
Short-day
plant
A Flowering Hormone?
• Photoperiod is detected by leaves, which cue
buds to develop as flowers
• The flowering signal is called florigen
• Florigen may be a macromolecule governed
by the FLOWERING LOCUS T (FT) gene
(36)
© 2011 Pearson Education, Inc.
Figure 39.UN03