Transcript Plant Responses

Plant Responses
Chapter 39
Signal Transduction

Plants have cellular receptors that detect
important changes in their environment
Grown in Dark
Etiolation
1 week exposure to Light
Greening
Response
Signal Transduction Pathway Model
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signal triggers receptor
receptor triggers internal cellular
messengers & then cellular response
 receptor
 signal
pathway
(2° messengers)
 response
What kinds of
molecules are
the receptors?
Signal Transduction Pathway
Plant Hormones

Chemical signals that coordinate
different parts of an organism
 only
minute amounts are required
 produced by 1 part of body
 transported to another part
 binds to specific receptor
 triggers response in target cells & tissues
Plant Hormones
auxins
 cytokinins
 gibberellins
 brassinosteroids
 abscisic acid
 ethylene

The Discovery of Plant Hormones
Tropisms : Phototropism
Signal is a a mobile chemical
The Went Experiments
Went named the chemical
“auxin”
Later purified as
indolacetic acid
(IAA)
How Auxins Cause Cell Elongation

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H+ pump
Increase in H+ activates expansins
Weakened cell wall allows more water to enter
Auxins
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Herbicides2-4-D
Affects secondary growth Synthetic auxins
sprayed on green house plants induce fruit
development
Cytokinins
Stimulate Cytokinesis
 Are produced in actively growing tissues

Gibberellins
Stimulate cell elongation and division
 Bolting
 Fruit Growth
 Germination
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Effects
 stem
elongation
 fruit growth
 seed germination
Figure 39.11 The effect of gibberellin treatment on seedless grapes
Abscisic Acid (ABA)
Slows growth
 Maintains seed dormancy
 Drought tolerance

Ethylene A magic gas?
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Response to stress
Leaf abscission
Fruit ripening
Slowing, thickening,
curving
Ethylene induces the triple response in pea seedlings
Slowing, thickening,curving
ETHYLENE
A magic gas?
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Response to stress
Leaf abscission
Fruit ripening
Abscission of a maple leaf
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Apoptosis
Caused by
ethylene/auxin
shift
Responses to gravity

How does a sprouting shoot “know” to grow
towards the surface from underground?
environmental
cues?
 roots
= positive
gravitropism
 shoots = negative
gravitropism
 settling of statoliths
(dense starch
grains) may
detect gravity
Molecular Switch Mechanism of
Phytochromes
Phytochrome photoreceptors

Molecular switch reaction to red light
 conversion
of Pr  Pfr in sunlight stimulates
germination, flowering, branching…
 conversion of Pfr  Pr in dark inhibits response, &
stimulates other responses: growth in height
Light induced
Chromophore
Photorecptor
Kinase activity
Phytochrome
Response:
Vertical growth
Phytochrome
Circadian Rhythms

Internal (endogenous) 24-hour cycles
4 O’clock
Noon
Morning glory
Midnight
Figure 39.21 Sleep movements of a bean plant
Figure 39.x1 Biological clocks
Figure 39.22 Photoperiodic control of
flowering
Figure 39.23 Reversible effects of red and far-red
light on photoperiodic response
Is there a flowering hormone?

Plant on left is
induced to flower &
then grafted onto
plant on right
 plant
on right is
triggered to flower
What can you conclude?
Thigmomorphogenesis
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Change in form
resulting from
mechanical
perturbation
Altering gene expression by
touch in Arabidopsis
Figure 39.27 Rapid turgor movements by the sensitive
plant (Mimosa pudica)
Plant Defenses
Defense against herbivores
 Defense against pathogens
 Systematic acquired resistance

Figure 39.29 A corn leaf recruits a parasitoid wasp as a
defensive response to an herbivore, an army-worm
caterpillar
Figure 39.31 Defense responses against an avirulent
pathogen