Plant Response to Signals

1. Plant Response to Signals Ch 39

2. 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

3. 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?

4. 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.

5. 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.

6. Plants do not have brains • Or nervous systems for that matter, • So how do they communicate with itself and coordinate beneficial responses?

7. 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

8. Plant hormones • auxins • cytokinins • gibberellins • abscisic acid • ethylene

9. 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__Promotesstomatal closure __ __ Promotes seed and bud dormancy _ • ETHYLENE __ Promotes ripening of fruit

10. Response to light: Phototropism • Growth towards light • Hormone: Auxin • asymmetrical distribution of auxin, moves away from sunny side of stem (-ve phototropism, -vegravitropism) • cells on darker side elongate faster than cells on brighter side

11. Apical dominance • Controls cell division & differentiation • axillary buds do no grow while apical bud exerts control shoot root

12. Cross-linkingpolysaccharides Cell wall–looseningenzymes Expansin Figure 39.8 CELL WALL Cellulosemicrofibril H2O Plasmamembrane H H H Cell wall H H H H H Nucleus Cytoplasm Plasma membrane ATP H Vacuole CYTOPLASM

13. 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

14. 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

15. 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

16. Apoptosis & Leaf drop:combination of hormones What is the evolutionary advantage of loss of leaves in autumn? • 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

17. 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

18. 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

19. Plant stimuli

20. 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? Synchronizes plant responses to season Short-day plants Long-day plants

21. Circadian rhythms • Internal (endogenous) 24-hour cycles 4 O’clock Noon Midnight Morning glory

22. Response to gravity • How does a sprouting shoot “know” to grow towards the surface from underground? • environmentalcues? • roots = positive gravitropism • shoots = negative gravitropism • settling of statoliths (dense starch grains) may detect gravity

23. 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

24. Plant defenses • Defenses against herbivores

25. coevolution Plant defenses • Defenses against herbivores Parasitoid wasp larvae emerging from a caterpillar

26. Any Questions??