Chapter 39: Plant responses to internal & external signals

1. Chapter 39: Plant responses to internal & external signals Plants= sessile must respond to environmental changes/cues by adjusting patterns of growth & development Signal transduction pathways in plants (ex. De- etiolation/greening) Environmental signal conformational change in protein Ex. Phytochrome (photoreceptor) in cytoplasm detects light Second messengers amplify signal Ex. Phytochrome activation increases levels of cGMP & Ca2+ changes in ion channels activating protein kinasesactivate other enzymes Regulation of one or more cell activities usually by activating enzymes or stimulating gene transcription Ex. Secondary messengers activate transcription factors stimulate photosynthesis enzymes, chlorophyll production, or affect hormones that regulate growth

3. Plant hormones & their responses • Hormones=chemical signals that coordinate different parts of an organism • Types: • Auxin (AA) • Cytokinins • Gibberellins • Brassinosteroids • Abscisic acid (ABA) • Ethylene

4. Auxin (AA)/Indoleacetic acid (IAA) • Functions: • Stimulate elongation of cells in young shoot • Stimulates proton pumps action potential activates expansin enzymesallow cell membrane to expandmore H2O enters • Increases gene expression of growth proteins • Induces cell division in vascular cambium & differentiation of secondary xylem • Regulates fruit development • Produced in: • Embryo of seed • Apical meristems

5. Cytokinins • Functions: • Stimulates cytokinesis • Influences differentiation • Stimulates germination • keeps plant green longer • Works in conjunction with auxin • Most common cytokinin=zeatin • Produced in: • Actively growing tissues, particularly roots, embryos, & fruit

6. Gibberellins • Functions: • Elongate stems (stimulates expansin enzymes) • Stimulate leaf growth • Stimulate fruit growth • Stimulate seed germination (break dormancy) • Produced in: • Roots • Young leaves • Embryos of seed

7. Brassinosteroids • Functions: • Induce cell elongation & division in stem segments & seedlings • Slow leaf abscission • Promote xylem differentiation • Inhibit root growth • Chemically similar to cholesterol & animal sex hormones • Produced in: • Seeds • Fruit • Shoots • Leaves • Floral buds

8. Abscisic Acid (ABA) • Functions: • Inhibits growth • Closes stomata during H2O stress (alters Ca2+ levels which effects K+) • Promotes seed dormancy • Produced in: • Leaves • Stems • Roots • Green fruit • Slows growth • Inactivated by large amounts of H2O, light or prolonged cold exposure • ABA to giberellin ratio often determines whether seed germinates or remains dormant

9. Ethylene • Functions: • Promote fruit ripening (attracts animals to spread seeds) • Causes enzymes to break down cell walls to soften fruit • Causes enzymes to convert starches to sugars for taste • Activates scent & color to display ripeness • May promote or inhibit development of roots, leaves, & flowers (dependant on species) • Opposes auxin affects • Activates enzymes to break down chemical components of cells leading to leaf abscission or plant death after flowering • positive feedback mechanism • Produced in: • Tissues or ripening fruit • Nodes of stems • Aging leaves & flowers

10. Photomorphogenesis • effects of light on plant morphology • Light effects plant growth & development • Plants detect light presence, direction, intensity, & wavelength (color) • Blue-light photoreceptors • Initiates responses in plants including • Phototropism (phototropin blue pigment) • Light induced stomata opening (carotenoid based zaxanthin) • Light induced retardation of hypocotyl elongation upon emerging from soil (cryptochrome pigment) • Phytochromes photoreceptors • Photoreceptor switches between Pr & Pfr forms • When Pr active- growth inhibited; Pfr active- growth stimulated • Responses include: • Stimulating seed germination • Aid in tracking seasons • Shade avoidance

11. Biological clocks & circadian rhythms • Physiological processes of plant including production of photosynthesis enzymes, stomata opening/closing, raising/lowering of leaves follow a 24 hour pattern even under controlled conditions • Circadian rhythms • Cycle in 24 hour frequency • Not necessarily connected to an environmental variable • Without environmental signal rhythm occurs in a 21-27 hour frequency • Light places rhythms on a 24 hour cycle • In plants a result of blue light photoreceptors & phytochrome • Light causes phytochrome to switch between Pr & Pfr • Allow plants to adjust to seasonal differences in day & night

12. Photoperiodism • Physiological response of plant to photoperiod (relative length of day or night) • Seasonal responses of plant • Short day plants (long night) • Flower only if light period is shorter than critical length • Flower in late summer, fall, or winter • Long day plants (short night) • Flower only if light period is longer than critical length • Flower in late spring or summer • Day neutral plants • Flowering unaffected by photoperiod • ***Plants really respond to night length not day length as names suggest*** • Leaves detect photoperiod • signal buds to develop flowers through signal florigen • Florigen activates organ identity genes

14. Other environmental influences on plant growth: abiotic • Gravity • Shoot grows upward (negative gravitropism) • Roots grow downward (positive gravitropism) • Statoliths (starch dense plastids) settle at lower portions of cell triggering redistribution of Ca2+ & thereby affecting auxin • Help determine up vs. down in plant • Mechanical stimuli • Thigmomorphogenesis- plant changes from due to mechanical perturbances • Viny plants grasp & grow around supports it encounters to stabalize itself • Touch sensitive plants can send action potentials similar to animal nerve impulses to neighboring organs

15. Other environmental influences on plant growth: abiotic • Drought • Low H2O in leaf- guard cells lose turgor- stomata close • H2O deficit increases abscisic acid in leaf to keep stomata closed • Cell expansion inhibited by lack of H2O; young leaves do not grow- decreases surface area for H2O loss • Conserves H2O but minimizes photosynthesis thereby decreasing crop yield • Flooding • Too much H2O suffocates roots- no O2 for respiration • Excess H2O- increased ethylene – root cortex undergoes apoptosis creating air tubes for O2 to reach submerged roots • Salt stress • Too much salt lowers H2O potential of soil causing roots to lose H2O to soil • Plant responds by producing organic compound solutes that keep Y of cells lower than soil

16. Other environmental influences on plant growth: abiotic • Heat stress • High heat denatures enzymes of plant, damaging plant metabolism • Transpiration lowers leaf temps through evaporative cooling • Heat shock proteins produced which protect enzymes from denaturing • Cold stress • Cold causes problems with membrane fluidity & transport • Plants increase unsaturated fatty acids & membrane solutes to prevent dehydration & keep membrane fluid

17. Other environmental influences on plant growth: biotic • Defenses against herbivores • Thorns • Chemical defenses • Distasteful or toxic compound production • Chemicals that attract defensive predators • Defenses against pathogens • First line: epidermis & periderm • Second line: chemicals released to kill pathogen & prevent its spread from infection site • R genes produce proteins that bind to pathogen protein • Elicitors produced • Oligosaccharins- released from damaged cell wall to stimulate production of antimicrobial phytoalexins • PR proteins activated that directly attack pathogen • Stimulate strengthening of cell wall to prevent spread