1. LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson © 2011 Pearson Education, Inc. Lectures by Erin Barley Kathleen Fitzpatrick Plant Responses to Internal and External Signals Chapter 39

2. Concept 39.1: Signal transduction pathways link signal reception to response A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots These are morphological adaptations for growing in darkness, collectively called etiolation After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally © 2011 Pearson Education, Inc.

3. Figure 39.2 (a) Before exposure to light(b) After a week’s exposure to natural daylight

4. A potato’s response to light is an example of cell-signal processing The stages are reception, transduction, and response © 2011 Pearson Education, Inc.

5. Figure 39.3 Reception CELL WALL 2 31 Transduction CYTOPLASM Response Relay proteins and second messengers Activation of cellular responses Receptor Hormone or environmental stimulus Plasma membrane

6. Reception Internal and external signals are detected by receptors, proteins that change in response to specific stimuli In de-etiolation, the receptor is a phytochrome capable of detecting light © 2011 Pearson Education, Inc.

7. Transduction Second messengers transfer and amplify signals from receptors to proteins that cause responses Two types of second messengers play an important role in de-etiolation: Ca 2+ ions and cyclic GMP (cGMP) The phytochrome receptor responds to light by –Opening Ca 2+ channels, which increases Ca 2+ levels in the cytosol –Activating an enzyme that produces cGMP © 2011 Pearson Education, Inc.

8. Figure 39.4-1 Reception 1 CYTOPLASM Plasma membrane Phytochrome Cell wall Light

9. Figure 39.4-2 Reception 2 1 Transduction CYTOPLASM Plasma membrane Phytochrome Cell wall Light cGMP Second messenger Ca 2  Ca 2  channel Protein kinase 1 Protein kinase 2

10. Figure 39.4-3 Reception 23 1 Transduction Response CYTOPLASM Plasma membrane Phytochrome Cell wall Light cGMP Second messenger Ca 2  Ca 2  channel Protein kinase 1 Protein kinase 2 Transcription factor 1 Transcription factor 2 NUCLEUS Transcription Translation De-etiolation (greening) response proteins P P

11. 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 This can occur by transcriptional regulation or post-translational modification © 2011 Pearson Education, Inc.

12. Concept 39.2: Plant hormones help coordinate growth, development, and responses to stimuli Plant hormones are chemical signals that modify or control one or more specific physiological processes within a plant © 2011 Pearson Education, Inc.

13. The Discovery of Plant Hormones Any response resulting in curvature of organs toward or away from a stimulus is called a tropism In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present © 2011 Pearson Education, Inc.

14. They postulated that a signal was transmitted from the tip to the elongating region © 2011 Pearson Education, Inc.

15. Video: Phototropism

16. Figure 39.5 Control Light Shaded side Illuminated side Boysen-Jensen Light Darwin and Darwin Gelatin (permeable) Mica (impermeable) Tip removed Opaque cap Trans- parent cap Opaque shield over curvature RESULTS

17. Table 39.1

18. Auxin The term auxin refers to any chemical that promotes elongation of coleoptiles Auxin is produced in shoot tips and is transported down the stem Auxin transporter proteins move the hormone from the basal end (bottom) of one cell into the apical end (top) of the neighboring cell © 2011 Pearson Education, Inc.

20. The Role of Auxin in Cell Elongation 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 © 2011 Pearson Education, Inc.

21. Figure 39.8 Cross-linking polysaccharides Cell wall–loosening enzymes Cellulose microfibril Expansin CELL WALL Plasma membrane CYTOPLASM Plasma membrane Cell wall Nucleus Cytoplasm Vacuole H2OH2O HH HH HH HH HH HH HH HH HH ATP

22. Auxin’s Role in Plant Development Polar transport (shoots to roots) of auxin plays a role in pattern formation of the developing plant Reduced auxin flow from the shoot of a branch stimulates growth in lower branches Auxin transport plays a role in phyllotaxy, the arrangement of leaves on the stem Polar transport of auxin from leaf margins(leaf edge) directs leaf venation pattern © 2011 Pearson Education, Inc.

23. Practical Uses for Auxins The auxin indolbutyric acid (IBA) stimulates adventitious roots (roots that form from leaves/stems) and is used in vegetative propagation of plants by cuttings An overdose of synthetic auxins can kill plants –For example 2,4-D is used as an herbicide on certain flowing plants © 2011 Pearson Education, Inc.

24. Cytokinins Cytokinins are so named because they stimulate cytokinesis (cell division) © 2011 Pearson Education, Inc.

25. Control of Cell Division and Differentiation Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits Cytokinins work together with auxin to control cell division and differentiation © 2011 Pearson Education, Inc.

26. Control of Apical Dominance Cytokinins, auxin, and strigolactone interact in the control of apical dominance, a terminal bud’s ability to suppress development of axillary buds If the terminal bud is removed, plants become bushier © 2011 Pearson Education, Inc.

27. (a) Apical bud intact (not shown in photo) (b) Apical bud removed (c) Auxin added to decapitated stem Axillary buds Lateral branches “Stump” after removal of apical bud Figure 39.9

28. Anti-Aging Effects Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues © 2011 Pearson Education, Inc.

29. Gibberellins Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination © 2011 Pearson Education, Inc.

30. Stem Elongation Gibberellins are produced in young roots and leaves Gibberellins stimulate growth of leaves and stems In stems, they stimulate cell elongation and cell division © 2011 Pearson Education, Inc.

31. Figure 39.10 (a) Rosette form (left) and gibberellin-induced bolting (right-stem elongation) (b)Grapes from control vine (left) and gibberellin-treated vine (right- fruit growth)

32. Germination After water is taken in by the seed, release of gibberellins from the embryo signals seeds to germinate © 2011 Pearson Education, Inc.

33. Seed Germination Aleurone Endosperm Water Scutellum (cotyledon) Radicle  -amylase Sugar GA 1 2 3

34. Brassinosteroids Brassinosteroids are chemically similar to the sex hormones of animals They induce cell elongation and division in stem segments They slow leaf abscission (dropping) and promote xylem differentiation © 2011 Pearson Education, Inc.

35. Abscisic Acid Abscisic acid (ABA) slows growth Two of the many effects of ABA –Seed dormancy –Drought tolerance © 2011 Pearson Education, Inc.

36. Seed Dormancy Seed dormancy ensures that the seed will germinate only in optimal conditions In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold Precocious (early) germination can be caused by inactive or low levels of ABA © 2011 Pearson Education, Inc.

37. Drought Tolerance ABA is the primary internal signal that enables plants to withstand drought ABA accumulation causes stomata to close rapidly © 2011 Pearson Education, Inc.

38. Strigolactones The hormones called strigolactones –Stimulate seed germination –Help establish mycorrhizal associations (roots & fungus) –Help control apical dominance Strigolactones are named for parasitic Striga plants Striga seeds germinate when host plants exude strigolactones through their roots © 2011 Pearson Education, Inc.

39. Ethylene Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection The effects of ethylene include response to mechanical stress, senescence (plant aging), leaf abscission, and fruit ripening © 2011 Pearson Education, Inc.

40. 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 © 2011 Pearson Education, Inc.

41. Ethylene concentration (parts per million) 0.00 0.10 0.200.40 0.80 Figure 39.13

42. Senescence Senescence is the programmed death of cells or organs A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants © 2011 Pearson Education, Inc.

43. Leaf Abscission A change in the balance of auxin and ethylene controls leaf abscission, the process that occurs in autumn when a leaf falls © 2011 Pearson Education, Inc.

44. Figure 39.15 0.5 mm Stem Petiole Protective layer Abscission layer

45. Fruit Ripening A burst of ethylene production in a fruit triggers the ripening process Ethylene triggers ripening, and ripening triggers release of more ethylene (positive feedback) Fruit producers can control ripening by picking green fruit and controlling ethylene levels © 2011 Pearson Education, Inc.

46. Concept 39.3: Responses to light are critical for plant success Light cues many key events in plant growth and development Effects of light on plant morphology are called photomorphogenesis © 2011 Pearson Education, Inc.

47. 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 © 2011 Pearson Education, Inc.

48. Phytochromes and Shade Avoidance The phytochrome system also provides the plant with information about the quality of light Leaves in the canopy absorb red light Shaded plants receive more far-red than red light In the “shade avoidance” response, the phytochrome ratio shifts in favor of P r (red) when a tree is shaded and as a result a plant will increase apical dominance and elongate © 2011 Pearson Education, Inc.

50. 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, even when kept under constant light or dark conditions © 2011 Pearson Education, Inc.

51. Figure 39.20 Noon Midnight

52. Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock” Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle The clock may depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes © 2011 Pearson Education, Inc.

53. Photoperiodism and Responses to Seasons Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year Photoperiodism is a physiological response to photoperiod © 2011 Pearson Education, Inc.

54. 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 Flowering in day-neutral plants is controlled by plant maturity, not photoperiod © 2011 Pearson Education, Inc.

55. Critical Night Length In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length © 2011 Pearson Education, Inc.

56. A plant that requires a long period of darkness, is termed a "short day" (long night) plant. Short-day plants form flowers only when day length is less than about 12 hours (typically spring and fall plants) Other plants require only a short night to flower. These are termed "long day" plants. These bloom only when they receive more than 12 hours of light (typically summer plants) © 2011 Pearson Education, Inc.

57. Figure 39.21 24 hours Light Flash of light Darkness Critical dark period Flash of light (b) Long-day (short-night) plant (a)Short day (long-night) plant

58. A Flowering Hormone? Photoperiod is detected by leaves, which cue buds to develop as flowers The flowering signal is called florigen © 2011 Pearson Education, Inc.

59. Concept 39.4: Plants respond to a wide variety of stimuli other than light Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms © 2011 Pearson Education, Inc.

60. Gravity Response to gravity is known as gravitropism Roots show positive gravitropism; shoots show negative gravitropism Plants may detect gravity by the settling of statoliths, dense cytoplasmic components © 2011 Pearson Education, Inc.

61. Figure 39.24 Statoliths 20  m (a) Primary root of maize bending gravitropically (b) Statoliths settling to the lowest sides of root cap cells

62. © 2011 Pearson Education, Inc. Video: Gravitropism Gravitropism

63. Mechanical Stimuli The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance Responses by plants can be positive or negative In experiments rubbing stems of young plants a couple of times daily results in plants that are shorter than controls © 2011 Pearson Education, Inc.

64. Figure 39.25

65. Thigmotropism is growth in response to touch It occurs in vines and other climbing plants Another example of a touch specialist is the sensitive plant Mimosa pudica, which folds its leaflets and collapses in response to touch Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials © 2011 Pearson Education, Inc.

66. (a) Unstimulated state (b) Stimulated state (c) Cross section of a leaflet pair in the stimulated state (LM) Leaflets after stimulation Pulvinus (motor organ) Side of pulvinus with flaccid cells Side of pulvinus with turgid cells Vein 0.5  m Figure 39.26