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

2. One pathway uses cGMP as a 2nd messenger to activate a protein kinase.The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase.

3. Both pathways lead to expression of genes for proteins that function in greening response of plant.

Signal Transduction Pathway example

1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal transduction pathways

2. One pathway uses cGMP as a 2nd messenger to activate a protein kinase.The other pathway involves increases in cytoplasmic Ca2+ that activates a different protein kinase.

3. Both pathways lead to expression of genes for proteins that function in greening response of plant.

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

2005-2006

shoot

root

Figure 39.8

Cross-linkingpolysaccharides

Cell wall–looseningenzymes

Cellulosemicrofibril

Expansin

CELL WALL

Plasma membrane

CYTOPLASM

Plasmamembrane

Cell wall

Nucleus CytoplasmVacuole

H2O

H

H

H

H HH

H

H

HATP

Cell elongation in response to auxin: the acid growth hypothesis

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

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 CO2 storage = reduces ethylene production

2005-2006

Plant stimuli

Flowering Response Triggered by photoperiod

relative lengths of day & night night length—“critical period”— is trigger

Short-day plants Long-day plants

Plant is sensitive to red light exposure

What is the evolutionary advantage of photoperiodism?

Synchronizes plant responses to season

Circadian rhythms Internal (endogenous) 24-hour cycles

Morning glory

4 O’clock

Noon Midnight

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

2005-2006

Mimosa (Sensitive plant) closes leaves in response to touch

Caused by changes in osmotic pressure = rapid loss of K+ = rapid loss of H2O = loss of turgor in cells

Plant defenses Defenses against herbivores

2005-2006

Plant defenses Defenses against herbivores

Parasitoid wasp larvae emerging from a caterpillar

coevolution

Any Questions??