Plant Response to Signals
Plant
Response to
Signals
Ch 39
Plant Response
Stimuli & a Stationary Life
animals respond to stimuli by changing behavior
move toward positive stimuli move away from negative stimuli
plants respond to stimuli by adjusting growth & development
Signal Transduction Pathway model
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 example
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
Signal Transduction Pathway example
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
Plants do not have brains
Or nervous systems for that matter,
So how do they communicate with itself and coordinate beneficial responses?
Plant hormones
Chemical signals that coordinate different parts of an organism
only tiny 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 abscisic acid ethylene
2005-2006
Hormones in review:
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
Response to light:
Phototropism
Growth towards light
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
Apical dominance
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
H
H
H
H
H
H
H
H
2
O
Plasma membrane
Cell wall
ATP
H
Plasma membrane
CYTOPLASM
Nucleus Cytoplasm
Vacuole
Gibberellins
Family of hormones
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
Abscisic acid (ABA)
Effects
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
Ethylene
Ethylene is a hormone gas released by plant cells
Multiple effects
response to mechanical stress
triple response
slow stem elongation thickening of stem curvature to stem growth leaf drop (like in Fall)
apoptosis fruit ripening
Apoptosis & Leaf drop: combination of hormones
Ethylene & auxin
many events in plants involve apoptosis
(pre-programmed cell death)
death of annual plant after flowering
differentiation of xylem vessels
loss of cytosol shedding of autumn leaves
What is the evolutionary advantage of loss of leaves in autumn?
Fruit ripening
Adaptation
hard, tart fruit protects developing seed from herbivores
ripe, sweet, soft fruit attracts animals to disperse seed
Ethylene
triggers ripening process
breakdown of cell wall
softening conversion of starch to sugar
sweetening positive feedback system
ethylene triggers ripening ripening stimulates more ethylene production
2005-2006
Applications
Truth in folk wisdom…..
one bad apple spoils the whole bunch
ripening apple releases ethylene to speed ripening of fruit nearby
Ripen green bananas by bagging them with an apple
Climate control storage of apples
high CO
2 storage = reduces ethylene production
Plant stimuli
Flowering Response
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
Circadian rhythms
Internal (endogenous) 24-hour cycles
Noon
4 O’clock
Midnight
Morning glory
2005-2006
Response 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
2005-2006
Response to touch
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
Plant defenses
Defenses against herbivores
2005-2006
Plant defenses
Defenses against herbivores
coevolution
Parasitoid wasp larvae emerging from a caterpillar
