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Chapter 39

Chapter 39. Plant Responses to Internal and External Signals. LE 39-2.

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Chapter 39

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  1. Chapter 39 Plant Responses to Internal and External Signals

  2. LE 39-2 Before exposure to light. A dark-grown potato has tall, spindly stems and nonexpanded leaves—morphological adaptations that enable the shoots to penetrate the soil. The roots are short, but there is little need for water absorption because little water is lost by the shoots. After a week’s exposure to natural daylight. The potato plant begins to resemble a typical plant with broad green leaves, short sturdy stems, and long roots. This transformation begins with the reception of light by a specific pigment, phytochrome.

  3. LE 39-3 CYTOPLASM CELL WALL Reception Transduction Response Activation of cellular responses Relay molecules Receptor Hormone or environmental stimulus Plasma membrane

  4. LE 39-4_3 Reception Response Transduction Transcription factor 1 CYTOPLASM NUCLEUS Specific protein kinase 1 activated Plasma membrane cGMP Second messenger produced Transcription factor 2 Phytochrome activated by light Cell wall Specific protein kinase 2 activated Transcription Light Translation De-etiolation (greening) response proteins Ca2+ channel opened Ca2+

  5. Response • A signal transduction pathway leads to regulation of one or more cellular activities • In most cases, these responses to stimulation involve increased activity of enzymes

  6. The Discovery of Plant Hormones • Any response resulting in curvature of organs toward or away from a stimulus is called a tropism • Tropisms are often caused by hormones

  7. In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light • They observed that a seedling could bend toward light only if the tip of the coleoptile was present • They postulated that a signal was transmitted from the tip to the elongating region Video: Phototropism

  8. LE 39-5a Shaded side of coleoptile Control Light Illuminated side of coleoptile

  9. LE 39-5b Darwin and Darwin (1880) Light Base covered by opaque shield Tip removed Tip covered by trans- parent cap Tip covered by opaque cap

  10. In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance

  11. LE 39-5c Boysen-Jensen (1913) Light Tip separated by mica Tip separated by gelatin block

  12. In 1926, Frits Went extracted the chemical messenger for phototropism, auxin, by modifying earlier experiments

  13. LE 39-6 Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Control (agar block lacking chemical) has no effect Offset blocks cause curvature Control

  14. The Role of Auxin in Cell Elongation • The term auxin refers to any chemical that promotes cell elongation in target tissues • According to the acid growth hypothesis, auxin stimulates proton pumps in the plasma membrane • The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric • With the cellulose loosened, the cell can elongate

  15. LE 39-8a Cell wall enzymes Cross-linking cell wall polysaccharides Expansin CELL WALL Microfibril ATP Plasma membrane CYTOPLASM

  16. LE 39-8b H2O Cell wall Plasma membrane Nucleus Cytoplasm Vacuole

  17. Auxins as Herbicides • An overdose of auxins can kill eudicots but not monocots because eudicots can’t clear the hormone quickly. Herbicide 2,4-D for broadleafs.

  18. Other Effects of Auxin • Auxin affects secondary growth. • Developing seeds synthesize auxin, which promotes growth of the fruit. • Greenhouse tomatoes have fewer seeds, because of fewer pollinators. This would result in reduced fruits. • Synthetic auxins are sprayed on greenhouse tomatoes. This produces full fruits that can even be seedless!

  19. Control of Cell Division and Differentiation • Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits • Cytokinins work together with auxin

  20. Control of Apical Dominance • Cytokinins, auxin, and other factors interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds

  21. LE 39-9 Apical Dominance “Stump” after removal of apical bud Axillary buds Lateral branches Intact plant Plant with apical bud removed

  22. Gibberellins • Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination

  23. Stem Elongation • Gibberellins stimulate growth of leaves and stems • In stems, they stimulate cell elongation and cell division

  24. Fruit Growth • In many plants, both auxin and gibberellins must be present for fruit to set • Gibberellins are used in spraying of Thompson seedless grapes. This increases the internodal distance, allowing for larger grapes.

  25. Germination • After water is imbibed, release of gibberellins from the embryo signals seeds to germinate

  26. LE 39-11 Aleurone Endosperm a-amylase Sugar GA GA Water Radicle Scutellum (cotyledon)

  27. Brassinosteroids • Brassinosteroids are similar to the sex hormones of animals • They induce cell elongation and division

  28. Abscisic Acid • Two of the many effects of abscisic acid (ABA): • Seed dormancy • Drought tolerance

  29. Drought Tolerance • ABA is the primary internal signal that enables plants to withstand drought

  30. Ethylene • Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection

  31. The Triple Response to Mechanical Stress • Ethylene induces the triple response, which allows a growing shoot to avoid obstacles • The triple response consists of a slowing of stem elongation, a thickening of the stem, and horizontal growth

  32. LE 39-13 0.80 0.10 0.20 0.40 0.00 Ethylene concentration (parts per million)

  33. LE 39-15 Ethylene synthesis inhibitor Ethylene added Control Wild-type Ethylene insensitive (ein) Ethylene overproducing (eto) Constitutive triple response (ctr)

  34. Apoptosis: Programmed Cell Death • A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants

  35. Leaf Abscission • A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls

  36. LE 39-16 0.5 mm Protective layer Abscission layer Stem Petiole

  37. Fruit Ripening • A burst of ethylene production in a fruit triggers the ripening process

  38. Concept 39.3: Responses to light are critical for plant success • Plants detect not only presence of light but also its direction, intensity, and wavelength (color) • A graph called an action spectrum depicts relative response of a process to different wavelengths • Action spectra are useful in studying any process that depends on light

  39. LE 39-17 1.0 0.8 0.6 Phototropic effectiveness relative to 436 nm 0.4 0.2 0 450 650 400 500 550 600 700 Wavelength (nm) Light Time = 0 min. Time = 90 min.

  40. There are two major classes of light receptors: blue-light photoreceptors and phytochromes

  41. Blue-Light Photoreceptors • Various blue-light photoreceptors control hypocotyl elongation, stomatal opening, and phototropism

  42. LE 39-18 Dark (control) Red Dark Dark Red Far-red Dark Red Red Red Red Far-red Far-red Far-red

  43. Biological Clocks and Circadian Rhythms • Many plant processes oscillate during the day • Many legumes lower their leaves in the evening and raise them in the morning

  44. LE 39-21 Midnight Noon

  45. Cyclical responses to environmental stimuli are called circadian rhythms and are about 24 hours long • Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle

  46. Photoperiodism and Control of Flowering • Some processes, including flowering in many species, require a certain photoperiod • Plants that flower when a light period is shorter than a critical length are called short-day plants • Plants that flower when a light period is longer than a certain number of hours are called long-day plants • In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length

  47. LE 39-22 Darkness Flash of light 24 hours Critical dark period Light “Short-day” plants “Long-day” plants

  48. Gravity • Response to gravity is known as gravitropism • Roots show positive gravitropism • Stems show negative gravitropism

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