plant responses to internal and external signals response and... · signal transduction 2 pathway 1...
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1
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PLANT RESPONSES TO
INTERNAL AND
EXTERNAL SIGNALS
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Concept 39.1: Signal transduction pathways link signal reception to response
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(a) Before exposure to light (b) After a week’s exposure to natural daylight
2
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• A potato left growing in darkness produces
shoots that look unhealthy and 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
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Fig. 39-2
(a) Before exposure to light (b) After a week’s exposure to natural daylight
Fig. 39-4-1
CYTOPLASM
Reception
Plasma membrane
Cell wall
Phytochrome
activated
by light
Light
Transduction
Second messenger produced
cGMP
NUCLEUS
1 2
Specific protein kinase 1 activated
Fig. 39-4-2
CYTOPLASM
Reception
Plasma membrane
Cell wall
Phytochrome
activated
by light
Light
Transduction
Second messenger produced
cGMP Specific protein kinase 1 activated
NUCLEUS
1 2
Specific protein kinase 2 activated
Ca2+ channel opened
Ca2+
3
Fig. 39-4-3
CYTOPLASM
Reception
Plasma membrane
Cell wall
Phytochrome
activated
by light
Light
Transduction
Second messenger produced
cGMP Specific protein kinase 1 activated
NUCLEUS
1 2
Specific protein kinase 2 activated
Ca2+ channel opened
Ca2+
Response 3
Transcription factor 1
Transcription factor 2
NUCLEUS
Transcription
Translation
De-etiolation (greening)
response
proteins
P
P
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De-Etiolation (“Greening”) Proteins
• Many enzymes that function in certain signal
responses are directly involved in
photosynthesis
• Other enzymes are involved in supplying
chemical precursors for chlorophyll production
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Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli
• Hormones are chemical signals that
coordinate different parts of an organism
• Research on how plants grow toward light
led to the discovery of plant hormones
Fig. 39-5a
RESULTS
Control
Light
Illuminated
side of
coleoptile
Shaded
side of
coleoptile
4
Fig. 39-5b
RESULTS
Light
Tip
removed
Darwin and Darwin: phototropic response only when tip is illuminated
Tip covered
by opaque
cap
Tip
covered
by trans-
parent
cap
Site of
curvature
covered by
opaque
shield
1800’s
Fig. 39-5c
RESULTS
Light
Boysen-Jensen: phototropic response when tip is separated by permeable barrier, but not with impermeable barrier
Tip separated
by gelatin
(permeable)
Tip separated
by mica
(impermeable)
1913
Fig. 39-6
Excised tip placed on agar cube
RESULTS
Growth-promoting chemical diffuses into agar cube
Agar cube with chemical stimulates growth
Offset cubes cause curvature
Control (agar cube lacking chemical) has no effect Control
1926
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A Survey of Plant Hormones
• In general, hormones control plant growth and
development by affecting the division,
elongation, and differentiation of cells
• Plant hormones are produced in very low
concentration, but a minute amount can greatly
affect growth and development of a plant organ
5
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Auxin
• refers to any chemical that promotes elongation of
apical meristems.
• involved in root formation and branching
• Indoleacetic acid (IAA) is a common auxin in plants; in
this lecture the term auxin refers specifically to IAA
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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
20
6
Fig. 39-8
Cross-linking polysaccharides
Cellulose microfibril
Cell wall becomes more acidic.
2
1 Auxin increases proton pump activity.
Cell wall–loosening enzymes
Expansin
Expansins separate microfibrils from cross- linking polysaccharides.
3
4
5
CELL WALL
Cleaving allows microfibrils to slide.
CYTOPLASM
Plasma membrane
H2O
Cell wall Plasma
membrane
Nucleus Cytoplasm
Vacuole
Cell can elongate.
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Cytokinins
• stimulate cytokinesis (cell division)
• produced in actively growing tissues such as roots,
embryos, and fruits
• work together with auxin to control cell division and
differentiation
• retard the aging of some plant organs by inhibiting
protein breakdown, stimulating RNA and protein
synthesis, and mobilizing nutrients from
surrounding tissues
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Gibberellins
• stimulate growth of leaves and stems
• In stems, stimulate cell elongation and cell division
• In many plants, both auxin and gibberellins must
be present for fruit to set/grow
• Gibberellins are used in spraying of Thompson
seedless grapes
• After water is absorbed, release of gibberellins
from the embryo signals seeds to germinate
(a) Gibberellin-induced stem growth
(b) Gibberellin-induced fruit growth
7
Fig. 39-11
Gibberellins (GA) send signal to aleurone.
Aleurone secretes -amylase and other enzymes.
Sugars and other nutrients are consumed.
Aleurone
Endosperm
Water
Scutellum (cotyledon)
Radicle
1 2 3
GA
GA
-amylase Sugar
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Abscisic Acid (ABA)
• slows growth – often called the “stress hormone”
• two of the many effects of ABA:
– Seed dormancy - ensures that the seed will
germinate only in optimal conditions
– Drought tolerance (aids in closing stomata)
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Abscisic Acid (ABA)
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Ethylene
• Plants produce ethylene in response to stresses
such as drought, flooding, mechanical pressure,
injury, and infection
• A change in the balance of auxin and ethylene
controls leaf abscission, the process that occurs in
autumn when a leaf falls
• Senescence is the programmed death of plant
cells or organs - a burst of ethylene is associated
with apoptosis, the programmed destruction of
cells, organs, or whole plants
8
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Fruit Ripening
• A burst of ethylene
production in a fruit
triggers the ripening
process
Ethylene
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• End of Part 1
• Beginning of Part 2 – next slide
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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)
Ph
oto
tro
pic
eff
ec
tiven
es
s
Fig. 39-16 436 nm 1.0
0.8
0.6
0.4
0.2
0 400 450 500 550 600 650 700
Wavelength (nm)
(a) Action spectrum for blue-light phototropism
Light
Time = 0 min
Time = 90 min
(b) Coleoptile response to light colors
9
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Blue-Light Photoreceptors and Phytochromes as Photoreceptors
• There are two major classes of light receptors:
blue-light photoreceptors and phytochromes
• Various blue-light photoreceptors control hypocotyl
elongation, stomatal opening, and phototropism
• Phytochromes are pigments that regulate many of
a plant’s responses to light throughout its life
• These responses include seed germination and
shade avoidance
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Phytochromes and Shade Avoidance
• The phytochrome system also provides the
plant with information about the quality of light
• 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
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Phytochromes
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Phytochromes
• Blue-green leaf pigment
• 2 subunits – one larger one and one kinase
• Red light prevalent in the daylight activates it into Pfr form
• Far-red light is prevalent in the evening and it changes Pfr
to Pr which is inactive
• The Pr to Pfr conversion cycle controls various plant
growth functions
• Presence of Pfr indicates sunlight is present – seed
germination, leaf growth, stem branching
10
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Phytochromes
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The Effect of Light on the Biological Clock
• Many plant processes oscillate during the day – called
a Biological Clock (Circadian Rhythm)
• Phytochrome conversion marks sunrise and sunset,
providing the biological clock with environmental cues
• 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
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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 long-day plants
• Flowering in day-neutral plants is controlled by plant maturity, not photoperiod
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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
11
24 hours
Light
Critical dark period
Flash of light
Darkness
(a) Short-day (long-night) plant
Flash of light
(b) Long-day (short-night) plant
Fig. 39-22
24 hours
R
RFR
RFRR
RFRRFR
Critical dark period
Short-day (long-night)
plant
Long-day (short-night)
plant Red lig
ht can inte
rrupt th
e n
ightt
ime
port
ion o
f th
e p
hoto
period
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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
• Response to gravity is known as gravitropism
• Roots show positive gravitropism; shoots show
negative gravitropism
• Thigmotropism is growth in response to touch. It
occurs in vines and other climbing plants
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Concept 39.5: Defense 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 chemical
defenses such as distasteful or toxic
compounds
• Some plants even “recruit” predatory animals
that help defend against specific herbivores
12
Fig. 39-28
Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
Synthesis and release of volatile attractants
Chemical in saliva
Wounding
Signal transduction pathway
1 1
2
3
4
• The proteinase inhibitors produced which limit insect feeding –
thought to interfere with the digestive ability of the herbivore’s
saliva
• Also, Salicylic acid (main ingredient in aspirin) is a ligand in
plants to activate anti-herbivore chemical genes leading to
systemic acquired resistance – herbivores then avoid the plant
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Defenses Against Pathogens
• A plant’s first line of defense against infection is
the epidermis and periderm
• If a pathogen penetrates the dermal tissue, the
second line of defense is a chemical attack that
kills the pathogen and prevents its spread
• This second defense system is enhanced by
the inherited ability to recognize certain
pathogens – this includes the hypersensitive
response and systemic acquired resistance
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The Hypersensitive Response
• The hypersensitive response
– Causes cell and tissue death near the infection site
– Induces production of proteins, which bind the
pathogen – complex is “recognized”
– This recognition triggers a transduction pathway to
stimulate changes in the cell wall that confine the
pathogen by sealing off the infected area.
– Also gives rise to systemic acquired resistance
13
Fig. 39-29
Signal
Hypersensitive response
Signal transduction pathway
Avirulent pathogen
Signal transduction
pathway
Acquired resistance
R-Avr recognition and hypersensitive response
Systemic acquired resistance