Download Chapter 39 - Plant Responses to Internal and External Signals

Figure 39.1
CAMPBELL
BIOLOGY
Figure 39.1a
Stimuli and a Stationary Life
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
!  Plants receive signals from the environment and
respond by altering growth and development
39
!  For example, the bending of a dodder seedling
toward a host plant occurs in response to chemicals
released by the host
Plant Responses to Internal
and External Signals
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
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Figure 39.2
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Figure 39.2a
Figure 39.2b
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
(b) After a week’s exposure
to natural daylight
(a) Before exposure to light
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(b) After a week’s exposure
to natural daylight
(a) Before exposure to light
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Reception
Transduction
Figure 39.3
CELL
WALL
!  A potato’s response to light is an example of cell
signal processing
CYTOPLASM
1 Reception
!  The stages are reception, transduction, and
response
Receptor
2 Transduction
3 Response
Relay proteins and
Activation
of cellular
responses
second messengers
!  Internal and external signals are detected by
receptors, proteins that change in response to
specific stimuli
!  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)
!  In de-etiolation, the receptor is a phytochrome
capable of detecting light
!  The phytochrome receptor responds to light by
!  Opening Ca2+ channels, which increases Ca2+
levels in the cytosol
Hormone or
environmental
stimulus
Plasma membrane
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!  Activating an enzyme that produces cGMP
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Figure 39.4-1
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Figure 39.4-2
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Figure 39.4-3
Response
1
Reception
1
CYTOPLASM
Reception
2 Transduction
1
CYTOPLASM
Plasma
membrane
Plasma
membrane
Phytochrome
activated
by light
Cell
wall
cGMP
Cell
wall
Light
3
2 Transduction
Plasma
membrane
Protein
kinase 1
cGMP
Second
messenger
produced
Phytochrome
activated
by light
Light
Protein
kinase 1
P
Transcription
factor 2
Protein
kinase 2
Transcription
Light
Translation
Ca2+ channel
opened
Ca2+ channel
opened
Ca2+
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!  This can occur by transcriptional regulation or
post-translational modification
De-etiolation
(greening)
response
proteins
Ca2+
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!  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
P
Cell
wall
Protein
kinase 2
Response
Transcription
factor 1
NUCLEUS
CYTOPLASM
Second
messenger
produced
Phytochrome
activated
by light
Reception
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Post-Translational Modification of Preexisting
Proteins
Transcriptional Regulation
!  Post-translational modification involves
modification of existing proteins in the signal
response
!  Specific transcription factors bind directly to
specific regions of DNA and control transcription of
genes
!  Modification often involves the phosphorylation of
specific amino acids
!  Some transcription factors are activators that
increase the transcription of specific genes
!  The second messengers cGMP and Ca2+ activate
protein kinases directly
!  Other transcription factors are repressors that
decrease the transcription of specific genes
!  Protein phosphatases “switch off” the signal
transduction pathways by dephosphorylating
proteins
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De-Etiolation (“Greening”) Proteins
Concept 39.2: Plant hormones help coordinate
growth, development, and responses to stimuli
!  De-etiolation activates enzymes that
!  Plant hormones are chemical signals that modify
or control one or more specific physiological
processes within a plant
!  Function in photosynthesis directly
!  Supply the chemical precursors for chlorophyll
production
!  Plant hormones are produced in very low
concentrations, but can have profound effects on
growth and development
!  Affect the levels of plant hormones that regulate
growth
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Table 39.1
A Survey of Plant Hormones
!  Each hormone has multiple effects, but multiple
hormones can influence a single process
Auxin
!  The major plant hormones include
!  Any response resulting in curvature of organs
toward or away from a stimulus is called a tropism
!  Auxin
!  Plant responses depend on amount and
concentration of specific hormones and often on
the combination of hormones present
!  In the late 1800s, Charles Darwin and his son
Francis conducted experiments on phototropism,
a plant’s response to light
!  Cytokinins
!  Gibberellins
!  Abscisic acid
!  They observed that a grass seedling could bend
toward light only if the tip of the coleoptile was
present
!  Ethylene
!  Brassinosteroids
!  Jasmonates
!  Strigolactones
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Figure 39.5
Figure 39.5b
Shaded
side
Control
Light
Darwin and Darwin
Illuminated
side
Shaded
side
Control
!  In 1913, Peter Boysen-Jensen demonstrated that
the signal was a mobile chemical substance
Light
Darwin and Darwin
Boysen-Jensen
Light
Illuminated
side
Tip
removed
Light
Light
Tip
removed
Opaque
cap
Transparent
cap
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Figure 39.5a
Results
!  They postulated that a signal was transmitted from
the tip to the elongating region
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Opaque
shield
over
curvature
Gelatin
(permeable)
Opaque
cap
Transparent
cap
Opaque
shield
over
curvature
Mica
(impermeable)
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Figure 39.5c
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Figure 39.6
Video: Phototropism
!  The term auxin refers to any chemical that
promotes elongation of coleoptiles
Boysen-Jensen
Light
Gelatin
(permeable)
Mica
(impermeable)
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Results
Cell 1
100 µm
!  Indoleacetic acid (IAA) is a common auxin in
plants; in this lecture the term auxin refers
specifically to IAA
Cortex
!  Auxin is produced in shoot tips and is transported
down the stem
Xylem
!  Auxin transporter proteins move the hormone from
the basal end of one cell into the apical end of the
neighboring cell
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Cell 2
Epidermis
Phloem
Pith
25 µm
Basal end
of cell
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Figure 39.6a
Figure 39.6b
Figure 39.7
CELL WALL
The Role of Auxin in Cell Elongation
100 µm
Cell 2
Xylem
Pith
Acidity
increases.
Figure 39.7a
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4
Low pH
activates
expansins.
2
Acidity
increases.
pump
activity
increases.
Proton
pump
activity
increases.
1
H 2O
Plasma
membrane
!  Auxin also alters gene expression and stimulates a
sustained growth response
Cell
wall
H+
Nucleus
H+
H+
CYTOPLASM
Nucleus
Cytoplasm
Vacuole
5
Sliding cellulose
microfibrils allow
cell to elongate.
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Auxin’s Role in Plant Development
!  Polar transport of auxin plays a role in pattern formation of
the developing plant
!  Polar transport of auxin from leaf margins directs leaf
venation pattern
Sliding cellulose microfibrils
allow cell to elongate.
Plasma membrane
H+
Plasma membrane
H+
!  Auxin transport plays a role in phyllotaxy, the arrangement
of leaves on the stem
Cytoplasm
Vacuole
5
ATP
ATP
!  Reduced auxin flow from the shoot of a branch stimulates
growth in lower branches
H+
H+
H+
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H+
H+
H+
Figure 39.7b
Cell wall-loosening enzymes cleave cross-linking
polysaccharides.
H+
H+
H+
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CELL WALL
3
H+
Cell
wall
1 Proton
Basal end
of cell
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Plasma
membrane
H+
H+
!  With the cellulose loosened, the cell can elongate
25 µm
H 2O
H+
2
!  The proton pumps lower the pH in the cell wall,
activating expansins, enzymes that loosen the
wall’s fabric
Phloem
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enzymes cleave
cross-linking
polysaccharides.
activates
expansins.
!  According to the acid growth hypothesis, auxin
stimulates proton pumps in the plasma membrane
Epidermis
Cortex
4 Cell wall-loosening
3 Low pH
Cell 1
CYTOPLASM
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!  The activity of the vascular cambium is under control of
auxin transport
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Cytokinins
Practical Uses for Auxins
!  Cytokinins are so named because they stimulate
cytokinesis (cell division)
!  The auxin indolbutyric acid (IBA) stimulates
adventitious roots and is used in vegetative
propagation of plants by cuttings
Control of Cell Division and Differentiation
Control of Apical Dominance
!  Cytokinins are produced in actively growing
tissues such as roots, embryos, and fruits
!  Cytokinins, auxin, and strigolactone interact in the
control of apical dominance, a terminal bud’s
ability to suppress development of axillary buds
!  Cytokinins work together with auxin to control cell
division and differentiation
!  An overdose of synthetic auxins can kill plants
!  For example 2,4-D is used as an herbicide on
eudicots
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Figure 39.8
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Figure 39.8a
!  If the terminal bud is removed, plants become
bushier
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Figure 39.8b
Figure 39.8c
Lateral branches
Lateral branches
“Stump” after
removal of
apical bud
(b) Apical bud removed
“Stump” after
removal of
apical bud
(c) Auxin added to decapitated stem
Axillary buds
(a) Apical bud intact (not shown in photo)
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Axillary buds
(c) Auxin added to decapitated stem
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(b) Apical bud removed
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(a) Apical bud intact (not shown in photo)
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Figure 39.9
Gibberellins
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
!  Gibberellins have a variety of effects, such as
stem elongation, fruit growth, and seed
germination
Stem Elongation
!  Gibberellins are produced in young roots and
leaves
!  Gibberellins stimulate growth of leaves and stems
(b) Grapes from control vine
(left) and gibberellintreated vine (right)
!  In stems, they stimulate cell elongation and cell
division
(a) Rosette form (left) and
gibberellin-induced bolting
(right)
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Figure 39.9a
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Figure 39.9b
Fruit Growth
Germination
!  In many plants, both auxin and gibberellins must
be present for fruit to develop
!  After water is imbibed, release of gibberellins from
the embryo signals seeds to germinate
!  Gibberellins are used in spraying of Thompson
seedless grapes
(b) Grapes from control vine
(left) and gibberellintreated vine (right)
(a) Rosette form (left) and
gibberellin-induced bolting
(right)
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Figure 39.10
Figure 39.11
Abscisic Acid
Aleurone
Endosperm
1
2
3
α-amylase
α-amylase
GA
!  Abscisic acid (ABA) slows growth
Seed Dormancy
!  Two of the many effects of ABA
!  Seed dormancy ensures that the seed will
germinate only in optimal conditions
!  Seed dormancy
Sugar
Water
Scutellum
(cotyledon)
Coleoptile
!  In some seeds, dormancy is broken when ABA is
removed by heavy rain, light, or prolonged cold
!  Drought tolerance
GA
Red mangrove
(Rhizophora mangle)
seeds
!  Precocious (early) germination can be caused by
inactive or low levels of ABA
Radicle
Maize mutant
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Figure 39.11a
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Figure 39.11b
Red mangrove
(Rhizophora mangle)
seeds
Ethylene
Coleoptile
Drought Tolerance
!  Plants produce ethylene in response to stresses
such as drought, flooding, mechanical pressure,
injury, and infection
!  ABA is the primary internal signal that enables
plants to withstand drought
!  ABA accumulation causes stomata to close rapidly
!  The effects of ethylene include response to
mechanical stress, senescence, leaf abscission,
and fruit ripening
Maize mutant
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Figure 39.12
Figure 39.13
ein mutant
ctr mutant
The Triple Response to Mechanical Stress
!  Ethylene-insensitive mutants fail to undergo the
triple response after exposure to ethylene
!  Ethylene induces the triple response, which allows
a growing shoot to avoid obstacles
!  Other mutants undergo the triple response in air
but do not respond to inhibitors of ethylene
synthesis
!  The triple response consists of a slowing of stem
elongation, a thickening of the stem, and
horizontal growth
0.00
0.10
0.20
0.40
0.80
(a) ein mutant
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Ethylene concentration (parts per million)
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Figure 39.13a
(b) ctr mutant
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Figure 39.13b
ein mutant
ctr mutant
Senescence
Leaf Abscission
!  Senescence is the programmed death of cells or
organs
!  A change in the balance of auxin and ethylene
controls leaf abscission, the process that occurs in
autumn when a leaf falls
!  A burst of ethylene is associated with apoptosis,
the programmed destruction of cells, organs, or
whole plants
(b) ctr mutant
(a) ein mutant
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Figure 39.14
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Figure 39.14a
More Recently Discovered Plant Hormones
0.5 mm
0.5 mm
Fruit Ripening
!  A burst of ethylene production in a fruit triggers the
ripening process
!  Ethylene triggers ripening, and ripening triggers
release of more ethylene
Protective layer
Stem
Abscission layer
Protective layer
Petiole
Stem
Petiole
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!  They induce cell elongation and division in stem
segments
!  They slow leaf abscission and promote xylem
differentiation
!  Fruit producers can control ripening by picking
green fruit and controlling ethylene levels
Abscission layer
!  Brassinosteroids are chemically similar to the
sex hormones of animals
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Concept 39.3: Responses to light are critical for
plant success
!  Jasmonates, including jasmonate (JA) and methyl
jasmonate (MeJA) play important roles in plant
defense and development
!  They are produced in response to wounding and
involved in controlling plant defenses
!  Strigolactones are xylem-mobile chemicals that
!  Jasmonates also regulate many other physiological
processes, including
!  Fruit ripening
!  Suppress adventitious root formation
!  Pollen production
!  Help establish mycorrhizal associations
!  Flowering time
!  Strigolactones are named for parasitic Striga
plants
!  Root growth
!  Tuber formation
!  Mycorrhizal symbiosis
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!  Tendril coiling
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!  Effects of light on plant morphology are called
photomorphogenesis
!  Help control apical dominance
!  Seed germination
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!  Light cues many key events in plant growth and
development
!  Stimulate seed germination
!  Nectar secretion
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!  Striga seeds germinate when host plants exude
strigolactones through their roots
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Figure 39.15
Blue-Light Photoreceptors
!  A graph called an action spectrum depicts
relative response of a process to different
wavelengths
!  Different plant responses can be mediated by the
same or different photoreceptors
!  Various blue-light photoreceptors control hypocotyl
elongation, stomatal opening, and phototropism
!  There are two major classes of light receptors:
blue-light photoreceptors and phytochromes
!  Action spectra are useful in studying any process
that depends on light
1.0
Phototropic effectiveness
!  Plants detect not only presence of light but also its
direction, intensity, and wavelength (color)
436 nm
Refracting prism
0.8
0.6
0.4
White
light
0.2
0
400
450
500
550
600
650
700
Wavelength (nm)
(a) Light wavelengths below 500 nm induce
curvature.
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Figure 39.15a
(b) Blue light induces the most curvature of
coleoptiles.
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Phytochrome Photoreceptors
Phytochromes and Seed Germination
Figure 39.15b
Phototropic effectiveness
1.0
436 nm
0.8
Refracting prism
0.6
0.4
!  Many seeds remain dormant until light and other
conditions are near optimal
!  These responses include seed germination and
shade avoidance
!  In the 1930s, scientists at the U.S. Department of
Agriculture determined the action spectrum for
light-induced germination of lettuce seeds
White
light
0.2
0
!  Phytochromes are pigments that regulate many of
a plant’s responses to light throughout its life
400
450
500
550
600 650
700
Wavelength (nm)
(a) Light wavelengths below 500 nm induce
curvature.
(b) Blue light induces the most curvature of
coleoptiles.
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Figure 39.16
!  Red light increased germination, while far-red light
inhibited germination
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Figure 39.16a
Figure 39.16b
Results
!  The effects of red and far-red light are reversible;
the final light exposure determines the response
Red
!  The photoreceptors responsible for the opposing
effects of red and far-red light are phytochromes
Dark
Red
Far-red
Dark
Dark (control)
Far-red
Red
Dark
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Red
Far-red
Red
Dark
Far-red
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Figure 39.16c
Red
Dark (control)
Red
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Figure 39.16e
Figure 39.16d
!  Phytochromes exist in two photoreversible states,
with conversion of Pr to Pfr triggering many
developmental responses
!  Red light triggers the conversion of Pr to Pfr
!  Far-red light triggers the conversion of Pfr to Pr
Red
Far-red
Dark
Red
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Far-red
Red
Dark
Red
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Far-red
Red
Far-red
!  The conversion of Pr to Pfr is faster than the
reverse process
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!  Sunlight, containing both red and far-red light,
increases the ratio of Pfr to Pr and triggers
germination
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Figure 39.17
Figure 39.18
Phytochromes and Shade Avoidance
Red light
Pr
Synthesis
Pfr
Far-red
light
Slow conversion
in darkness
(some species)
Responses to Pfr:
• Seed germination
• Inhibition of vertical
growth and stimulation of branching
• Setting internal clocks
• Control of flowering
Biological Clocks and Circadian Rhythms
!  The phytochrome system also provides the plant
with information about the quality of light
!  Many plant processes oscillate during the day in
response to light and temperature changes
!  Leaves in the canopy absorb red light
!  Many legumes lower their leaves in the evening
and raise them in the morning, even when kept
under constant light or dark conditions
!  Shaded plants receive more far-red than red light
!  In the “shade avoidance” response, the
phytochrome ratio shifts in favor of Pr when a tree
is shaded
Enzymatic
destruction
Noon
10:00 PM
!  This shift induces vertical growth
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Figure 39.18a
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Figure 39.18b
The Effect of Light on the Biological Clock
!  Circadian rhythms are cycles that are about 24
hours long and are governed by an internal “clock”
!  Circadian rhythms can be set to exactly 24 hours
by the day/night cycle
Noon
!  The clock may depend on synthesis of a protein
regulated through feedback control and may be
common to all eukaryotes
10:00 PM
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Photoperiodism and Responses to Seasons
Photoperiodism and Control of Flowering
!  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
!  Phytochrome conversion marks sunrise and
sunset, providing the biological clock with
environmental cues
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!  Some processes, including flowering in many
species, require a certain photoperiod
Critical Night Length
!  In the 1940s, researchers discovered that
flowering and other responses to photoperiod are
actually controlled by night length, not day length
!  Plants that flower when a light period is shorter
than a critical length are called short-day plants
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!  Plants that flower when a light period is longer
than a certain number of hours are called longday plants
!  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
!  Flowering in day-neutral plants is controlled by
plant maturity, not photoperiod
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Figure 39.19
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Figure 39.20
24 hours
24 hours
(a) Short-day
(long-night) plant
Light
Critical
dark period
!  Red light can interrupt the nighttime portion of the
photoperiod
!  Some plants flower after only a single exposure to
the required photoperiod
!  A flash of red light followed by a flash of far-red
light does not disrupt night length
Darkness
Flash
of light
!  Action spectra and photoreversibility experiments
show that phytochrome is the pigment that detects
the red light
(b) Long-day
(short-night) plant
!  Other plants need several successive days of the
required photoperiod
R
!  Still others need an environmental stimulus in
addition to the required photoperiod
R FR
!  For example, vernalization is a pretreatment with
cold to induce flowering
R FR R
R FR R FR
Flash of light
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Critical dark period
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Short-day
(long-night)
plant
Long-day
(short-night)
plant
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Figure 39.21
A Flowering Hormone?
24 hours
24 hours
Concept 39.4: Plants respond to a wide variety
of stimuli other than light
24 hours
!  Photoperiod is detected by leaves, which cue buds
to develop as flowers
!  Because of immobility, plants must adjust to a
range of environmental circumstances through
developmental and physiological mechanisms
Graft
!  The flowering signal molecule is called florigen
!  Response to gravity is known as gravitropism
!  Roots show positive gravitropism; shoots show
negative gravitropism
!  Florigen may be a protein governed by the
FLOWERING LOCUS T (FT) gene
!  Plants may detect gravity by the settling of
statoliths, dense cytoplasmic components
Short-day
plant
Long-day plant
grafted to
short-day plant
Long-day
plant
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Figure 39.22
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Figure 39.22a
Statoliths
Gravity
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Figure 39.22b
Figure 39.22c
20 µm
Statoliths
(a) Primary root of maize
bending gravitropically
(LMs)
20 µm
(b) Statoliths settling to
the lowest sides of
root cap cells (LMs)
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Figure 39.22d
Video: Gravitropism
Mechanical Stimuli
!  Some mutants that lack statoliths are still capable
of gravitropism
Statoliths
20 µm
!  Dense organelles, in addition to starch granules,
may contribute to gravity detection
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!  Rubbing stems of young plants a couple of times
daily results in plants that are shorter than controls
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Figure 39.23
!  The term thigmomorphogenesis refers to
changes in form that result from mechanical
disturbance
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Figure 39.24
Figure 39.24a
!  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
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(a) Unstimulated state (leaflets spread
apart)
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(b) Stimulated state (leaflets folded)
(a) Unstimulated state (leaflets spread
apart)
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Figure 39.24b
Video: Mimosa Leaf
Environmental Stresses
Drought
!  Environmental stresses have a potentially adverse
effect on survival, growth, and reproduction
!  Stresses can be abiotic (nonliving) or biotic
(living)
!  During drought, plants reduce transpiration by
closing stomata, reducing exposed surface area,
and in some species, shedding leaves
!  Abiotic stresses include drought, flooding, salt
stress, heat stress, and cold stress
!  Biotic stresses include herbivores and pathogens
(b) Stimulated state (leaflets folded)
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Figure 39.25
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Figure 39.25a
Figure 39.25b
Flooding
Vascular
cylinder
Vascular
cylinder
!  Enzymatic destruction of root cortex cells creates
air tubes that help plants survive oxygen
deprivation during flooding
Vascular
cylinder
Air tubes
Air tubes
Epidermis
(a) Control root
(aerated)
100 µm
Epidermis
(b) Experimental root
(nonaerated)
(a) Control root
(aerated)
100 µm
133
Epidermis
(b) Experimental root
(nonaerated)
100 µm
134
100 µm
135
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© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Salt Stress
Heat Stress
Cold Stress
Concept 39.5: Plants respond to attacks by
pathogens and herbivores
!  Salt can lower the water potential of the soil
solution and reduce water uptake
!  Plants respond to salt stress by producing solutes
tolerated at high concentrations
!  This process keeps the water potential of cells
more negative than that of the soil solution
!  Excessive heat can denature a plant’s enzymes
!  Cold temperatures decrease membrane fluidity
!  Transpiration helps cool leaves by evaporative
cooling
!  Altering lipid composition of membranes is a
response to cold stress
!  Heat-shock proteins help protect other proteins
from heat stress
!  Freezing causes ice to form in a plant’s cell walls
and intercellular spaces
!  Many plants, as well as other organisms, have
antifreeze proteins that prevent ice crystals from
growing and damaging cells
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Defenses Against Pathogens
Immune Responses of Plants
!  The first line of immune defense depends on the
plant’s ability to recognize pathogen-associated
molecular patterns (PAMPs)
!  Pathogens can enter through wounds or natural
openings, such as stomata
!  These molecular sequences are specific to certain
pathogens
!  PAMP recognition starts a chain of signaling
events leading to the production of antimicrobial
chemicals and toughening of the cell wall
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!  A plant’s first line of defense against infection is
the barrier presented by the epidermis and
periderm
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!  Plants use defense systems to deter herbivory,
prevent infection, and combat pathogens
!  Plants and pathogens have engaged in an
evolutionary arms race
!  Effector-triggered immunity results from the action
of hundreds of disease resistance (R) genes
!  Pathogens that have evolved the ability to deliver
effectors into plant cells can suppress PAMPtriggered plant immunity
!  Each R protein is activated by a specific effector
!  Effectors are pathogen-encoded proteins that
cripple the host’s innate immune system
!  R proteins activate plant defenses by triggering
signal transduction pathways
!  These defenses include the hypersensitive
response and systemic acquired resistance
!  A second level of plant immune defense evolved in
response to these pathogens
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Figure 39.26
Figure 39.26a
Figure 39.26b
The Hypersensitive Response
!  The hypersensitive response
4
!  Causes cell and tissue death near the infection site
Infected tobacco leaf with lesions
3
!  Induces production of enzymes that attack the
pathogen
4
!  Stimulates changes in the cell wall that confine the
pathogen
3
Signal
5
Hypersensitive
response
2 Signal transduction pathway
Signal
5
Hypersensitive
response
Signal
transduction
pathway
2 Signal transduction pathway
Signal
transduction
pathway
7
6
7
6
Acquired
resistance
1
Acquired
resistance
R protein
1
Infected tobacco leaf with lesions
Pathogen
R protein
Effector protein
Pathogen
Effector protein
Hypersensitive
response
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Systemic acquired
resistance
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© 2014 Pearson Education, Inc.
Systemic Acquired Resistance
Defenses Against Herbivores
Hypersensitive
response
146
Systemic acquired
resistance
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Figure 39.27a
!  Systemic acquired resistance causes systemic
expression of defense genes and is a long-lasting
response
!  Methylsalicylic acid is synthesized around the
infection site and carried in the phloem to other
remote sites where it is converted to salicylic acid
MAKE CONNECTIONS:
Levels of Plant Defenses Against Herbivores
!  Herbivory, animals eating plants, is a stress that
plants face in any ecosystem
!  Plants counter excessive herbivory with physical
defenses, such as thorns and trichomes, and
chemical defenses, such as distasteful or toxic
compounds
Molecular-Level
Molecular-Level Defenses
Cellular-Level
Tissue-Level
!  Chemical compounds including terpenoids,
phenolics, and alkaloids can be produced to deter
attackers
Organ-Level
!  Salicylic acid triggers the defense system to
respond rapidly to another infection
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Figure 39.27aa
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Figure 39.27ab
Molecular-Level
Defenses
Opium poppy fruit
Cellular-Level
Defenses
Cellular-Level Defenses
Tissue-Level Defenses
!  Cells may be specialized to form trichomes, store
chemical deterrents, or produce irritants
!  Leaves may be toughened with sclerenchyma
tissue
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Raphide crystals from
taro plant
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Figure 39.27ac
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Figure 39.27ad
Figure 39.27ae
Organ-Level Defenses
Organ-Level Defenses
Tissue-Level Defenses
Organ-Level Defenses
!  Leaves can be modified into spines and bristles;
stems into thorns
!  Some species have leaves that appear partially
eaten, others have structures that mimic insect
eggs
Cross section through the major vein
of an olive leaf
Leaf of snowflake plant
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Bristles on cactus spines
158
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159
160
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Figure 39.27af
Figure 39.27b
Figure 39.27ba
MAKE CONNECTIONS:
Levels of Plant Defenses Against Herbivores
Organ-Level Defenses
Organismal-Level Defenses
Organismal-Level
Organismal-Level Defenses
!  Plants may respond to attack by altering flowering
time
Community-Level
Population-Level
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Egg mimicry on leaf of passion flower plant
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Hummingbird pollinating wild tobacco plant
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Figure 39.27bb
Figure 39.27bc
Community-Level
Defenses
Population-Level Defenses
Population-Level Defenses
Community-Level Defenses
!  Some plants release chemicals in response to
herbivore attack that trigger defense responses in
nearby members of the population
!  Some plants “recruit” predatory animals that help
defend against specific herbivores
!  Others populations synchronously produce
massive amounts of seeds after long intervals
Flowering bamboo plants
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Figure 39.27bd
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Figure 39.UN01a
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Figure 39.UN01b
Roots not connected
Stomatal opening (µm)
Community-Level Defenses
Roots connected
Osmotic shock
Parasitoid wasp cocoons on
caterpillar host
168
Figure 39.UN01c
14
Control
13
12
15 minutes after plant 6 osmotically shocked
11
10
9
8
7
6
5
4
3
2
1
0
Adult wasp emerging from a
cocoon
1
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Figure 39.UN02
1 Reception
Hormone or
environmental
stimulus
CYTOPLASM
Plasma membrane
2
Transduction
Relay proteins and
second messengers
3 Response
Activation
of cellular
responses
Receptor
Major Responses
Auxin
Stimulates cell elongation; regulates branching
and organ bending
Cytokinins
Stimulate plant cell division; promote later bud
growth; slow organ death
Gibberellins
Promote stem elongation; help seeds break
dormancy and use stored reserves
Abscisic acid
Promotes stomatal closure in response to drought;
promotes seed dormancy
Ethylene
Mediates fruit ripening and the triple response
Brassinosteroids
Chemically similar to the sex hormones of animals;
induce cell elongation and division
Jasmonates
Mediate plant defenses against insect herbivores;
regulate a wide range of physiological processes
Strigolactones
Regulate apical dominance, seed germination,
and mycorrhizal associations
5
6
7
Plant number
8
9
10
Pea plant (Pisum sativum)
11
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Figure 39.UN05
Red light
Pr
Pfr
Responses
Environmental Stress
Major Response
Drought
ABA production, reducing water loss by
closing stomata
Flooding
Formation of air tubes that help roots survive
oxygen deprivation
Salt
Avoiding osmotic water loss by producing
solutes tolerated at high concentrations
Heat
Synthesis of heat-shock proteins, which reduce
protein denaturation at high temperatures
Cold
Adjusting membrane fluidity; avoiding
osmotic water loss; producing antifreeze
proteins
Far-red light
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4
Figure 39.UN04
Plant Hormone
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3
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Figure 39.UN03
CELL
WALL
2
170
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Figure 39.UN06
Figure 39.UN07
Control
Ethylene
added
Ethylene
synthesis
inhibitor
Wild-type
Ethylene insensitive
(ein)
Ethylene
overproducing (eto)
Constitutive triple
response (ctr)
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