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Ch. 39 Plant Responses to Internal and External Signals

Ch. 39 Plant Responses to Internal and External Signals. 39.1 Signal transduction pathways link signal reception to response. 39.2 Plant hormones help coordinate growth. Hormones control plant growth and development by affecting the division, elongation, and differentiation of cells.

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Ch. 39 Plant Responses to Internal and External Signals

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  1. Ch. 39 Plant Responses to Internal and External Signals

  2. 39.1 Signal transduction pathways link signal reception to response

  3. 39.2 Plant hormones help coordinate growth • Hormones control plant growth and development by affecting the division, elongation, and differentiation of cells. • Some hormones also mediate the responses of plants to environmental stimuli.

  4. 39.3 Responses to light are critical to plant success • Blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism. • Phytochromes act like molecular “on-off” switches. Red light turns phytochrome “on”, and far-red light turns it “off.” • Phytochromeregulates shade avoidance and the germination of many seed types.

  5. Phytochrome conversion also provides information about the relative lengths of day and night (photoperiod) and hence the time of year. Photoperiodism regulates the time of flowering in many species. • Short-day plants require a night longer than a critical length to flower. Long-day plants need a night length shorter than a critical period to flower.

  6. Many daily rhythms in plant behavior are controlled by an internal circadian clock. • Free-running circadian cycles are approximately 24 hours long but are entrained to exactly 24 hours by dawn and dusk effects on phytochrome form.

  7. 39.4 Plants respond to a wide variety of stimuli other than light • Gravitropism is the bending of an organ in response to gravity. Roots show positive gravitropism, and stems show negative gravitropism. • Statoliths, starch filled plastids, enable plant roots to detect gravity.

  8. Plants are highly sensitive to touch. Thigmotropism is a growth response to touch. Rapid leaf movements involve transmission of electrical impulses called action potentials.

  9. Plants are sensitive to environmental stresses, including drought, flooding, high salinity, and extremes of temperature.

  10. 39.5 Plants respond to attacks by herbivores and pathogens • In addition to physical defenses such as thorns and trichomes, plants produce distasteful or toxic chemicals, as well as attractants that recruit animals that destroy herbivores. • The hypersensitive response seals off an infection and destroys both pathogen and host cells in the region.

  11. Systemic acquired resistance is a generalized defense response in organs distant from the infection site.

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