Download The Hypersensitive Response

Fig. 39-1
Chapter 39
Plant Responses to Internal and External
Signals
39.2 Regulation of Plant Growth
The Search for a Plant Hormone
1. Charles Darwin and his son Francis-late 1800s
a. Phototropism-a plant’s response to or from light
b. grass seedling could bend toward light only if the
tip of the coleoptile was present
c. Coleoptile senses light!
*But the growth response occurred some
distance from the tip, how was the
coleoptile sending the message?
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-5
RESULTS
Shaded
side of
coleoptile
Control
Light
Illuminated
side of
coleoptile
Darwin and Darwin: phototropic response
only when tip is illuminated
Light
Tip
removed
Tip covered
by opaque
cap
Tip
covered
by transparent
cap
Site of
curvature
covered by
opaque
shield
Boysen-Jensen: phototropic response when tip separated
by permeable barrier, but not with impermeable barrier
Light
Tip separated
by gelatin
(permeable)
Tip separated
by mica
(impermeable)
2. Frits Went-1926
a. Cut off coleptile and put an agar block
between the tip and the rest of the plant.
b. Hypothesized that the chemical would
diffuse into the agar.
c. Substituted coleptile tip with agar block
d. Agar block caused phototropism!
i. Hormone = Auxin.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-6
RESULTS
Excised tip placed
on agar cube
Growth-promoting
chemical diffuses
into agar cube
Control
Control
(agar cube
lacking
chemical)
has no
effect
Agar cube
with chemical
stimulates growth
Offset cubes
cause curvature
A Survey of Plant Hormones
a. Chemical produced in one tissue and targeted
for use in another.
*must be small molecules to pass through cell walls!*
b. Usually target growth and development
tissues; no specialized organs for hormone
production.
c. A [hormone] relative to other hormones and
development stage of the plant are important
to the result = gene expression!
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
1. Auxins
a. Indoleacetic acid (IAA)
1. Synthesized at the apical meristem (root formation and
branching)
2. Stimulates elongation by making cell wall more plastic
3. Supresses growth of lateral buds
4. Stimulates activity of vascular cambium and influencing
differentiation of secondary xylem
f. Synthetic auxins1. Can induce fruit formation without pollination seedless tomatoes.
2. Prevents abscission layer  fruits remain on tree longer
3. Cause broadleaf weeds (eudicots like dandelions and weeds) to grow to
death-herbicides!
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-8
3 Expansins separate
Cross-linking
polysaccharides
Cell wall–loosening
enzymes
microfibrils from crosslinking polysaccharides.
Expansin
CELL WALL
4 Cleaving allows
microfibrils to slide.
Cellulose
microfibril
H2O
2 Cell wall
Plasma
membrane
becomes
more acidic.
Cell
wall
1 Auxin
increases
proton pump
activity.
Plasma membrane
Nucleus
Cytoplasm
Vacuole
CYTOPLASM
5 Cell can elongate.
2. Cytokinins-stimulate cytokinesis
a. Produced in root
b. Works together with auxin to stimulate cell
division and differentiation
c. Stimulates growth of lateral branches
(antagonistic to auxin)
d. Stimulates RNA and protein synthesis and
prohibit protein breakdown (anti-aging effects)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
3. Gibberellins
a. Causes bolting (growth of floral stock)
b. Work with auxin to control stem elongation
c. Produced in roots and young leaves
d. Helps break down starch in germinating seeds
e. Discovered in rice attacked by a fungus
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-11
1 Gibberellins (GA)
2 Aleurone secretes
send signal to
aleurone.
-amylase and other enzymes.
3 Sugars and other
nutrients are consumed.
Aleurone
Endosperm
-amylase
GA
GA
Water
Scutellum
(cotyledon)
Radicle
Sugar
4. Abscisic Acid
a. Produced in the buds
b. Antagonistic to gibberelins
c. Induces formation of winter buds (Seed
dormancy)
d. Inhibits cell % in vascular cambiumannual
rings.
e. Causes stomata to close in low water conditions
(drought tolerance)
*Also found in mammalian brain-possible a gene repressor!*
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5. Ethylene
a. Gas that diffuses through air spaces in cells
b. Acts as an inhibitor to cell growth can trigger
apoptosis.
c. Causes defoliation (autumn)
d. Ripens fruit (positive feedback loop!)
e. Production increases in response to stress.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
39. 1 Signal transduction pathways in plant cells.
1. Reception-hormone binds to a specific receptor in
or on target cell
–
Receptor could be on membrane or in cytoplasm
2. Transduction-amplifies the stimulus and converts it to
a chemical capable of changing the cells activities
–
Second messenger (transfers and amplify) such as
calcium-calmodulin complex
3. Induction- amplified signal induces a specific
response by the cell.
–
Ex. bend toward light, stomata open or close
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-3
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
Fig. 39-4-1
1
Reception
2
Transduction
CYTOPLASM
Plasma
membrane
cGMP
Second messenger
produced
Phytochrome
activated
by light
Cell
wall
Light
Specific
protein
kinase 1
activated
NUCLEUS
Fig. 39-4-2
1
Reception
2
Transduction
CYTOPLASM
Plasma
membrane
cGMP
Second messenger
produced
Specific
protein
kinase 1
activated
Phytochrome
activated
by light
Cell
wall
Specific
protein
kinase 2
activated
Light
Ca2+ channel
opened
Ca2+
NUCLEUS
Fig. 39-4-3
1
Reception
2
Transduction
3
Response
Transcription
factor 1
CYTOPLASM
Plasma
membrane
cGMP
Second messenger
produced
Specific
protein
kinase 1
activated
NUCLEUS
P
Transcription
factor 2
Phytochrome
activated
by light
P
Cell
wall
Specific
protein
kinase 2
activated
Transcription
Light
Translation
Ca2+ channel
opened
Ca2+
De-etiolation
(greening)
response
proteins
39.3 Response-In most cases, these responses to stimulation involve
increased activity of enzymes
Tropisms-growth to or away from a stimuli
a. Phototropism-to light; photoreceptors in shoot tip
• Auxin-induced acidification of cell walls causes
cell elongation
b. Gravitropism: positive gravitropism-roots
negative gravitropism-shoots
i. statoliths-specialized plastids redistribute calcium
c. Thigmotropism is growth in response to touch
ex. It occurs in vines and other climbing plants
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Fig. 39-24
Statoliths
(a) Root gravitropic bending
20 µm
(b) Statoliths settling
Fig. 39-26ab
(a) Unstimulated state
(b) Stimulated state
Responses to light are critical for plant success
• Light cues many key events in plant growth
and development
• Plants detect not only presence of light but also
its direction, intensity, and wavelength (color)
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Circadian rhythms
are cycles that are about 24 hours long and are
governed by an internal “clock”
Noon
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Midnight
Photoperiodism-is a physiological response to day
length
a. Short-day plants-require a light period is
shorter than a critical length in order to flower
–
Flower in late summer, fall, winter
b. Long-day plants-require a light period is
longer than a critical period.
–
Flower in spring or early summer
*Night length is actually what is important!*
c. Day-neutral plants is controlled by plant
maturity, not photoperiod
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Some plants flower after only a single exposure
to the required photoperiod
• Other plants need several successive days of
the required photoperiod
• Still others need an environmental stimulus in
addition to the required photoperiod
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
d. Phytochromes
• Phytochromes are pigments that regulate
many of a plant’s responses to light throughout
its life
• Phytochrome conversion marks sunrise and
sunset, providing the biological clock with
environmental cues
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Long-night vs. Short-night Plants
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
39.5 Plants respond to a wide variety of other
stimuli
• Environmental stressors: Because of immobility, plants
must adjust to a range of environmental circumstances through
developmental and physiological mechanisms
Abiotic
•Drought/Flooding
•Heat/cold stress
•Salt stress
Biotic
•Herbivores
(thorns,Methyljasmonic
acid)
•Pathogens
(epidermis,
hypersensitive response)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-28
4 Recruitment of
parasitoid wasps
that lay their eggs
within caterpillars
3 Synthesis and
release of
volatile attractants
1 Wounding
1 Chemical
in saliva
2 Signal transduction
pathway
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Plants damaged by insects can release volatile
chemicals to warn other plants of the same
species
• Methyljasmonic acid can activate the
expression of genes involved in plant defenses
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• A virulent pathogen is one that a plant has
little specific defense against
• An avirulent pathogen is one that may harm
but does not kill the host plant
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Gene-for-gene recognition involves
recognition of pathogen-derived molecules by
protein products of specific plant disease
resistance (R) genes
• An R protein recognizes a corresponding
molecule made by the pathogen’s Avr gene
• R proteins activate plant defenses by triggering
signal transduction pathways
• These defenses include the hypersensitive
response and systemic acquired resistance
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Hypersensitive Response
• The hypersensitive response
– Causes cell and tissue death near the infection
site
– Induces production of phytoalexins and PR
proteins, which attack the pathogen
– Stimulates changes in the cell wall that confine
the pathogen
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 39-29
Signal
Hypersensitive
response
Signal transduction
pathway
Signal
transduction
pathway
Acquired
resistance
Avirulent
pathogen
R-Avr recognition and
hypersensitive response
Systemic acquired
resistance