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 2. One pathway uses cGMP as a 2nd transduction pathways messenger to activate 3. Both pathways leada protein kinase. to expression of genes The pathway forother proteins that involves increases Ca2+ that function in in cytoplasmic greening activates different responseaof plant. protein kinase. Signal Transduction Pathway example 1. Light signal is detected by the phytochrome receptor, which then activates at least 2 signal 2. One pathway uses cGMP as a 2nd transduction pathways messenger to activate 3. Both pathways leada protein kinase. to expression of genes The pathway forother proteins that involves increases Ca2+ that function in in cytoplasmic greening activates different responseaof plant. protein kinase. 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 2005-2006 Plant hormones auxins cytokinins gibberellins abscisic acid ethylene 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 2005-2006 Apical dominance Controls cell division & differentiation axillary buds do no grow while apical bud exerts control shoot root 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 H2O H Plasma membrane H H H ATP H H H Cell wall H 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 softening conversion of cell wall 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 CO2 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 4 O’clock Noon Morning glory Midnight 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 H2O = loss of turgor in cells 2005-2006 Plant defenses Defenses against herbivores Plant defenses Defenses against herbivores Parasitoid wasp larvae emerging from a caterpillar coevolution Any Questions??