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. Overview: Stimuli and a Stationary Life Plants, being rooted to the ground, must respond to environmental changes that come their way –For example, the bending of a seedling toward light begins with sensing the direction, quantity, and color of the light –Floral clock example © 2011 Pearson Education, Inc.

3. Figure 39.1

4. Concept 39.1: Signal transduction pathways 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.

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

6. 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

7. Transduction Second messengers transfer and amplify signals from receptors to proteins that cause responses 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 Protein kinases: enzymes that transfer a phosphate from ATP to a protein (phosphorylates a protein) © 2011 Pearson Education, Inc.

8. 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

9. Post-Translational Modification of Preexisting Proteins Post-translational modification involves modification of existing proteins in the signal response Modification often involves the phosphorylation of specific amino acids The second messengers cGMP and Ca 2+ activate protein kinases directly © 2011 Pearson Education, Inc.

10. De-Etiolation (“Greening”) Proteins De-etiolation activates enzymes that –Function in photosynthesis directly –Supply the chemical precursors for chlorophyll production –Affect the levels of plant hormones that regulate growth © 2011 Pearson Education, Inc.

11. 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.

12. The Discovery of Plant Hormones Any response resulting in curvature of organs toward or away from a stimulus is called a tropism Charles & Francis Darwin 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.

13. They postulated that a signal was transmitted from the tip to the elongating region, eventually scientists learned that the signal was a chemical called auxin. © 2011 Pearson Education, Inc. Video: Phototropism

14. 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

15. Figure 39.6 Control RESULTS Excised tip on agar cube Growth-promoting chemical diffuses into agar cube Control (agar cube lacking chemical) Offset cubes

16. A Survey of Plant Hormones Plant hormones are produced in very low concentration, but a minute amount can greatly affect growth and development of a plant organ In general, hormones control plant growth and development by affecting the division, elongation, and differentiation of cells © 2011 Pearson Education, Inc.

17. Auxin Auxin = any chemical that promotes elongation of coleoptiles Indoleacetic acid (IAA) is a common auxin in plants; Auxin is produced in shoot tips and is transported down the stem © 2011 Pearson Education, Inc.

18. Practical Uses for Auxins The auxin indolbutyric acid (IBA) stimulates adventitious roots 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 eudicots © 2011 Pearson Education, Inc.

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

20. 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.

21. (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

22. Figure 39.9a (a) Apical bud intact (not shown in photo) Axillary buds

23. 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.

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

25. 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.

26. Figure 39.10 (a) Rosette form (left) and gibberellin-induced bolting (right) (b)Grapes from control vine (left) and gibberellin-treated vine (right)

27. Fruit Growth In many plants, both auxin and gibberellins must be present for fruit to develop Gibberellins are used in spraying of Thompson seedless grapes © 2011 Pearson Education, Inc.

28. Figure 39.10b (b)Grapes from control vine (left) and gibberellin-treated vine (right)

29. Germination After water is imbibed, release of gibberellins from the embryo signals seeds to germinate © 2011 Pearson Education, Inc.

30. Figure 39.11 Aleurone Endosperm Water Scutellum (cotyledon) Radicle  -amylase Sugar GA 1 2 3

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

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

33. 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.

34. Figure 39.12 Red mangrove (Rhizophora mangle) seeds Maize mutant Coleoptile

35. 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.

36. Strigolactones The hormones called strigolactones –Stimulate seed germination –Help establish mycorrhizal associations –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.

37. 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, leaf abscission, and fruit ripening © 2011 Pearson Education, Inc.

38. 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.

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

40. Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene Other mutants undergo the triple response in air but do not respond to inhibitors of ethylene synthesis © 2011 Pearson Education, Inc.

41. Figure 39.14 (a) ein mutant (b) ctr mutant ctr mutant ein mutant

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 Fruit producers can control ripening by picking green fruit and controlling ethylene levels © 2011 Pearson Education, Inc.

46. Systems Biology and Hormone Interactions Interactions between hormones and signal transduction pathways make it hard to predict how genetic manipulation will affect a plant Systems biology seeks a comprehensive understanding that permits modeling of plant functions © 2011 Pearson Education, Inc.

47. 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.

48. 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.

49. Figure 39.16 (a) Phototropism action spectrum (b) Coleoptiles before and after light exposures 1.0 0.8 0.6 0.4 0.2 0 436 nm 400450 500 550 600 650 700 Wavelength (nm) Phototropic effectiveness Light Time  0 min Time  90 min

50. Different plant responses can be mediated by the same or different photoreceptors There are two major classes of light receptors: blue-light photoreceptors and phytochromes © 2011 Pearson Education, Inc.

51. Phytochromes and Seed Germination Many seeds remain dormant until light conditions change In the 1930s, scientists at the U.S. Department of Agriculture determined the action spectrum for light-induced germination of lettuce seeds © 2011 Pearson Education, Inc.

52. 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.

53. Figure 39.20 Noon Midnight

54. 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.

55. The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues © 2011 Pearson Education, Inc.

56. A Flowering Hormone? Photoperiod is detected by leaves, which cue buds to develop as flowers The flowering signal is called florigen Florigen may be a macromolecule governed by the FLOWERING LOCUS T (FT) gene © 2011 Pearson Education, Inc.

57. Figure 39.23 24 hours Graft Short-day plant Long-day plant grafted to short-day plant Long-day plant 24 hours

58. 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.

59. 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. Video: Gravitropism

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

61. Some mutants that lack statoliths are still capable of gravitropism Dense organelles, in addition to starch granules, may contribute to gravity detection © 2011 Pearson Education, Inc.

62. Mechanical Stimuli The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls © 2011 Pearson Education, Inc.

63. Figure 39.25

64. 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. Video: Mimosa Leaf

65. (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

66. Environmental Stresses Environmental stresses have a potentially adverse effect on survival, growth, and reproduction Stresses can be abiotic (nonliving) or biotic (living) Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress Biotic stresses include herbivores and pathogens © 2011 Pearson Education, Inc.

67. Drought During drought, plants reduce transpiration by closing stomata, slowing leaf growth, and reducing exposed surface area Growth of shallow roots is inhibited, while deeper roots continue to grow © 2011 Pearson Education, Inc.