Ch 39

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Stimuli & a Stationary Life
 animals respond to stimuli by changing behavior
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 move toward positive stimuli move away from negative stimuli
 plants respond to stimuli by adjusting growth & development

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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?

1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways
3. Both pathways lead kinase.
The other pathway involves increases in cytoplasmic Ca response of plant.
2+ that

1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways
3. Both pathways lead kinase.
The other pathway involves increases in cytoplasmic Ca response of plant.
2+ that

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Or nervous systems for that matter,
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So how do they communicate with itself and coordinate beneficial responses?

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Chemical signals that coordinate different parts of an organism
 only tiny amounts are required
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 produced by 1 part of body transported to another part binds to specific receptor triggers response in target cells & tissues

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 auxins cytokinins gibberellins abscisic acid ethylene
2005-2006

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AUXINS __ Promote cellular elongation __ by softening of cell walls __ Involved in phototropism __ Involved in geotropism __ Involved in apical dominance
CYTOKININS __ Promotes lateral growth_ growth in size of leaf cells __ Stimulate cell division (hence name) __ Release buds from apical dominance
GIBERELLINS __ seasonal growth___ Stimulate cell elongation __ Produce bolting in biennials __ Stimulate production of starch digestion enzymes in some seeds
ABSCISIC ACID __ opposite of giberellins__causes slow down or “cut off” of growth__Promotes stomatal closure __
__ Promotes seed and bud dormancy _
ETHYLENE __ Promotes ripening of fruit

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Growth towards light
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Hormone: Auxin
 asymmetrical distribution of auxin, moves away from sunny side of stem (-ve phototropism, -ve gravitropism)
 cells on darker side elongate faster than cells on brighter side

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Controls cell division & differentiation
 axillary buds do no grow while apical bud exerts control shoot root
2005-2006

Figure 39.8
Cell elongation in response to auxin: the acid growth hypothesis
Cross-linking polysaccharides
Cell wall
–loosening enzymes
Expansin
CELL WALL
Cellulose microfibril
H
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H
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H

H
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H

H
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H
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H
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H
2
O
Plasma membrane
Cell wall
ATP
H
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Plasma membrane
CYTOPLASM
Nucleus Cytoplasm
Vacuole

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Family of hormones
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 over 100 different gibberellins identified
Effects
 fruit growth seed germination plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

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Effects
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 slows growth seed dormancy
 high concentrations of Abscisic acid
 germination only after ABA is inactivated down or leeched out
 survival value: seed will germinate only under optimal conditions
 light, temperature, moisture drought tolerance
 rapid stomate closing

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Ethylene is a hormone gas released by plant cells
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Multiple effects
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 response to mechanical stress
 triple response
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 slow stem elongation thickening of stem curvature to stem growth leaf drop (like in Fall)
 apoptosis fruit ripening

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Ethylene & auxin
 many events in plants involve apoptosis
(pre-programmed cell death)
 death of annual plant after flowering
 differentiation of xylem vessels
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 loss of cytosol shedding of autumn leaves
What is the evolutionary advantage of loss of leaves in autumn?

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Adaptation
 hard, tart fruit protects developing seed from herbivores
 ripe, sweet, soft fruit attracts animals to disperse seed
Ethylene
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 triggers ripening process
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 breakdown of cell wall
 softening conversion of starch to sugar
 sweetening positive feedback system
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 ethylene triggers ripening ripening stimulates more ethylene production

2005-2006
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Truth in folk wisdom…..
 one bad apple spoils the whole bunch
 ripening apple releases ethylene to speed ripening of fruit nearby
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Ripen green bananas by bagging them with an apple
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Climate control storage of apples
 high CO
2 storage = reduces ethylene production

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Triggered by photoperiod
 relative lengths of day & night
 night length—“critical period”— is trigger
Plant is sensitive to red light exposure
What is the evolutionary advantage of photoperiodism?
Short-day plants Long-day plants
Synchronizes plant responses to season

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Internal (endogenous) 24-hour cycles
Noon
4 O’clock
Midnight
Morning glory

2005-2006
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How does a sprouting shoot “know” to grow towards the surface from underground?
 environmental cues?
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 roots = positive gravitropism shoots = negative gravitropism settling of statoliths
(dense starch grains) may detect gravity

2005-2006
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Thigmotropism
Mimosa (Sensitive plant) closes leaves in response to touch
Caused by changes in osmotic pressure = rapid loss of K + = rapid loss of H
2
O = loss of turgor in cells

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Defenses against herbivores
2005-2006

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Defenses against herbivores
Parasitoid wasp larvae emerging from a caterpillar