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Plant Responses to Internal and External Signals

This article explores the important plant hormones and signaling pathways that regulate various plant responses to internal and external factors such as light, gravity, touch, and stress. It covers topics such as cell elongation, fruit ripening, plant movement, photoperiodism, and plant defense mechanisms.

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Plant Responses to Internal and External Signals

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  1. Plant Responses to Internal and External Signals

  2. Important plant hormones: • Auxin – stimulate cell elongation  phototropism & gravitropism (high concentrations = herbicide) • Cytokinins – cell division (cytokinesis) & differentiation • Gibberellins – stem elongation, leaf growth, germination, flowering, fruit development • Abscisic Acid – slows growth; closes stomata during H2O stress; promote dormancy • Ethylene – promote fruit ripening (positive feedback!); involved in apoptosis (shed leaves, death of annuals)

  3. The effects of gibberellin on stem elongation and fruit growth

  4. Ethylene Gas: Fruit Ripening Canister of ethylene gas to ripen bananas in shipping container Untreated tomatoes vs. Ethylene treatment

  5. Plant Movement • Tropisms: growth responses  SLOW • Phototropism – light (auxin) • Gravitropism – gravity (auxin) • Thigmotropism – touch • Turgor movement: allow plant to make relatively rapid & reversible responses • Venus fly trap, mimosa leaves, “sleep” movement

  6. Phototropism Experiments with Light

  7. Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Further Experiments:Cells on darker side elongate faster than cells on brighter sideAUXIN = chemical messenger that stimulates cell elongation Control (agar block lacking chemical) has no effect Agar block with chemical stimulates growth Offset blocks cause curvature Control

  8. Gravitropism

  9. Positive gravitropism in roots: the statolith hypothesis.

  10. Thigmotropism: rapid turgor movements by Mimosa plant  action potentials

  11. Plant Responses to Light • Plants can detect direction, intensity, & wavelenth of light • Phytochromes: light receptors, absorbs mostly red light • Regulate seed germination, shade avoidance

  12. Biological Clocks Circadian rhythm: biological clocks • Persist w/o environmental cues • Frequency = 24 hours Phytochrome system + Biological clock = plant can determine time of year based on amount of light/darkness

  13. Photoperiodism: physiological response to the relative length of night & day (i.e. flowering)*more predictable than air temperature changes* • Short-day plants: flower when nights are long (mums, poinsettia) • Long-day plant: flower when nights are short (spinach, iris, veggies) • Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions)

  14. How does interrupting the dark period with a brief exposure to light affect flowering?

  15. Plant responses to stress

  16. Drought (H2O deficit): • close stoma • release abscisic acid to keep stoma closed • Inhibit growth • roll leaves  reduce SA & transpiration • deeper roots • Flooding (O2 deprivation): • release ethylene  root cell death  air tubes formed to provide O2 to submerged roots

  17. Excess Salt: • cell membrane – impede salt uptake • produce solutes to ↓ψ - retain H2O • Heat: • evap. cooling via transpiration • heat shock proteins – prevent denaturation • Cold: • alter lipid composition of membrane (↑unsat. fatty acids, ↑fluidity) • increase cytoplasmic solutes • antifreeze proteins

  18. Herbivores: • physical (thorns) • chemicals (garlic, mint) • recruit predatory animals (parasitoid wasps) • Pathogens: • 1st line of defense = epidermis • 2nd line = pathogen recognition, host-specific

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