1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 39: Plant Responses to Internal and external Signals

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants, being rooted to the ground – Must respond to whatever environmental change comes their way

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings e.g., bending of a grass seedling toward light – Begins with the plant sensing the direction, quantity, and color of the light Figure 39.1

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Signal transduction pathways link signal reception to response Plants cellular receptors detect changes in their environment For a stimulus to elicit a response cells must have an appropriate receptor

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Potato growing in darkness  Unhealthy shoots, lack elongated roots Morphological adaptation for growing in darkness (etiolation) Figure 39.2a (a) 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.

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings After exposed to light  Profound changes called de-etiolation, shoots and roots grow normally Figure 39.2b (b) 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.

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The potato’s response to light – Is an example of cell-signal processing Figure 39.3 CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Receptor Relay molecules Activation of cellular responses Hormone or environmental stimulus Plasma membrane

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 39.4 1 Reception 2 Transduction 3 Response CYTOPLASM Plasma membrane Phytochrome activated by light Cell wall Light cGMP Second messenger produced Specific protein kinase 1 activated Transcription factor 1 NUCLEUS P P Transcription Translation De-etiolation (greening) response proteins Ca 2+ Ca 2+ channel opened Specific protein kinase 2 activated Transcription factor 2 Signal transduction in plants 1 The light signal is detected by the phytochrome receptor, which then activates at least two signal transduction pathways. 2 One pathway uses cGMP as a second messenger that activates a specific protein kinase.The other pathway involves an increase in cytoplasmic Ca 2+ that activates another specific protein kinase. 3 Both pathways lead to expression of genes for proteins that function in the de-etiolation (greening) response.

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Response Ultimately, a signal transduction pathway – Leads to a regulation of one or more cellular activities, usually involves enzymes

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plant Hormones Chemical signals that coordinate growth, development, and responses to stimuli

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tropism Growth response that results in curvatures of plant toward or away from a stimulus, caused by hormones

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tropism experiments Figure 39.5 In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted. EXPERIMENT In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical. CONCLUSION RESULTS ControlDarwin and Darwin (1880) Boysen-Jensen (1913) Light Shaded side of coleoptile Illuminated side of coleoptile Light Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Base covered by opaque shield Light Tip separated by gelatin block Tip separated by mica

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In 1926, Frits Went – Extracted the chemical messenger for phototropism  auxin Went concluded that a coleoptile curved toward light because its dark side had a higher concentration of the growth-promoting chemical, which he named auxin. The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark. 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 Control Offset blocks cause curvature RESULTS CONCLUSION In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others, he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side. EXPERIMENT Figure 39.6

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Survey of Plant Hormones

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hormones control plant growth and development by affecting division, elongation, and differentiation of cells Hormones produced in very low concentrations, but have a profound effect

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxin Any chemical substance that promotes cell elongation in different target tissues

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Role of Auxin in Cell Elongation Acid growth hypothesis – Proton pumps involved in response of cells to auxin

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Expansin CELL WALL Cell wall enzymes Cross-linking cell wall polysaccharides Microfibril H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ ATP Plasma membrane Plasma membrane Cell wall Nucleus Vacuole Cytoplasm H2OH2O Cell elongation in response to auxin Figure 39.8 1 Auxin increases the activity of proton pumps. 4 The enzymatic cleaving of the cross-linking polysaccharides allows the microfibrils to slide. The extensibility of the cell wall is increased. Turgor causes the cell to expand. 2 The cell wall becomes more acidic. 5 With the cellulose loosened, the cell can elongate. 3 Wedge-shaped expansins, activated by low pH, separate cellulose microfibrils from cross-linking polysaccharides. The exposed cross-linking polysaccharides are now more accessible to cell wall enzymes.

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxin Formation and branching of roots

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Auxins as Herbicides An overdose of auxins can kill eudicots

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytokinins Stimulate cell division Produced in actively growing tissues Work together with auxin

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cytokinins and auxin interact to control apical dominance (ability of a terminal bud to suppress development of axillary buds) Figure 39.9a Axillary buds

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Remove terminal bud  plants become bushier Figure 39.9b “Stump” after removal of apical bud Lateral branches

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Anti-Aging Effects Cytokinins retard the aging of some plant organs

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gibberellins Stem elongation, fruit growth, and seed germination

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Stem Elongation Gibberellins stimulate growth of both leaves and stems (stimulate cell elongation and cell division)

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fruit Growth Auxin and gibberellins must be present for fruit to set

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gibberellins used commercially f/ spraying of Thompson seedless grapes Figure 39.10

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings After water is imbibed, the release of gibberellins from the embryo – Signals the seeds to break dormancy and germinate Germination Figure 39.11 2 2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is  -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA  -amylase Radicle Sugar 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 The aleurone responds by synthesizing and secreting digestive enzymes that hydrolyze stored nutrients in the endosperm. One example is  -amylase, which hydrolyzes starch. (A similar enzyme in our saliva helps in digesting bread and other starchy foods.) Aleurone Endosperm Water Scutellum (cotyledon) GA  -amylase Radicle Sugar 2 1 After a seed imbibes water, the embryo releases gibberellin (GA) as a signal to the aleurone, the thin outer layer of the endosperm. 3 Sugars and other nutrients absorbed from the endosperm by the scutellum (cotyledon) are consumed during growth of the embryo into a seedling.

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Abscisic Acid (ABA) Seed dormancy Drought tolerance

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Seed Dormancy Great survival value, seed germinates only when there are optimal conditions

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ethylene Plants produce ethylene – In response to stresses such as drought, flooding, mechanical pressure, injury, and infection

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Apoptosis (Programmed Cell Death) A burst of ethylene associated with apoptosis of cells, organs, or whole plants

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Leaf Abscission Auxin and ethylene controls leaf abscission – Occurs in autumn when a leaf falls Figure 39.16 0.5 mm Protective layer Abscission layer Stem Petiole

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Fruit Ripening A burst of ethylene in fruit triggers the ripening process

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Response to light Light cues many key events in plant growth and development

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Plants not only detect the presence of light, but also its direction, intensity, and wavelength (color)

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Action spectra Figure 39.17 Wavelength (nm) 1.0 0.8 0.6 0.2 0 450500550600650700 Light Time = 0 min. Time = 90 min. 0.4 400 Phototropic effectiveness relative to 436 nm Researchers exposed maize (Zea mays) coleoptiles to violet, blue, green, yellow, orange, and red light to test which wavelengths stimulate the phototropic bending toward light. EXPERIMENT The graph below shows phototropic effectiveness (curvature per photon) relative to effectiveness of light with a wavelength of 436 nm. The photo collages show coleoptiles before and after 90-minute exposure to side lighting of the indicated colors. Pronounced curvature occurred only with wavelengths below 500 nm and was greatest with blue light. RESULTS CONCLUSION The phototropic bending toward light is caused by a photoreceptor that is sensitive to blue and violet light, particularly blue light.

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochromes as Photoreceptors Regulate responses to light throughout its life

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings USDA (1930’s)  light-induced germination Figure 39.18 Dark (control) Dark Red Far-red Red Far-red Red DarkRed Far-red Red Far-red CONCLUSION EXPERIMENT RESULTS During the 1930s, USDA scientists briefly exposed batches of lettuce seeds to red light or far-red light to test the effects on germination. After the light exposure, the seeds were placed in the dark, and the results were compared with control seeds that were not exposed to light. The bar below each photo indicates the sequence of red-light exposure, far-red light exposure, and darkness. The germination rate increased greatly in groups of seeds that were last exposed to red light (left). Germination was inhibited in groups of seeds that were last exposed to far-red light (right). Red light stimulated germination, and far-red light inhibited germination. The final exposure was the determining factor. The effects of red and far-red light were reversible.

42. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochrome – Photoreceptor responsible for the opposing effects of red and far-red light A phytochrome consists of two identical proteins joined to form one functional molecule. Each of these proteins has two domains. Chromophore Photoreceptor activity. One domain, which functions as the photoreceptor, is covalently bonded to a nonprotein pigment, or chromophore. Kinase activity. The other domain has protein kinase activity. The photoreceptor domains interact with the kinase domains to link light reception to cellular responses triggered by the kinase. Figure 39.19

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biological Clocks and Circadian Rhythms Many plant processes oscillate during the day

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Many legumes (e.g. beans) – Lower their leaves in the evening and raise them in the morning Figure 39.21 Noon Midnight

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cyclical responses to environmental stimuli are called circadian rhythms, ~ 24 hours long

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photoperiodism and Responses to Seasons Photoperiod, the relative lengths of night and day – environmental stimulus plants use to detect the time of year Photoperiodism – response to photoperiod

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photoperiodism and Control of Flowering Flowering requires a certain photoperiod

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Critical Night Length Flowering controlled by night length Figure 39.22 During the 1940s, researchers conducted experiments in which periods of darkness were interrupted with brief exposure to light to test how the light and dark portions of a photoperiod affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS CONCLUSION The experiments indicated that flowering of each species was determined by a critical period of darkness (“critical night length”) for that species, not by a specific period of light. Therefore, “short-day” plants are more properly called “long-night” plants, and “long-day” plants are really “short-night” plants. 24 hours Darkness Flash of light Critical dark period Light (a) “Short-day” plants flowered only if a period of continuous darkness was longer than a critical dark period for that particular species (13 hours in this example). A period of darkness can be ended by a brief exposure to light. (b) “Long-day” plants flowered only if a period of continuous darkness was shorter than a critical dark period for that particular species (13 hours in this example).

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phytochrome (receives red light) can interrupt the nighttime portion of the photoperiod Figure 39.23 A unique characteristic of phytochrome is reversibility in response to red and far-red light. To test whether phytochrome is the pigment measuring interruption of dark periods, researchers observed how flashes of red light and far-red light affected flowering in “short-day” and “long-day” plants. EXPERIMENT RESULTS CONCLUSION A flash of red light shortened the dark period. A subsequent flash of far-red light canceled the red light’s effect. If a red flash followed a far-red flash, the effect of the far-red light was canceled. This reversibility indicated that it is phytochrome that measures the interruption of dark periods. 24 20 16 12 8 4 0 Hours Short-day (long-night) plant Long-day (short-night) plant R R FR R RR R Critical dark period

50. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Because of their immobility – Plants must adjust to a wide range of environmental circumstances

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gravity Response to gravity –  gravitropism Roots show positive gravitropism Stems show negative gravitropism

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanical Stimuli thigmomorphogenesis – Changes in form that result from mechanical perturbation

53. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rubbing the stems of young plants a couple of times daily – Results in plants that are shorter than controls Figure 39.26

54. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Growth in response to touch is called thigmotropism – Occurs in vines and other climbing plants

55. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Rapid leaf movements in response to mechanical stimulation – transmission of electrical impulses called action potentials Figure 39.27a–c (a) Unstimulated (b) Stimulated Side of pulvinus with flaccid cells Side of pulvinus with turgid cells Vein 0.5  m (c) Motor organs Leaflets after stimulation Pulvinus (motor organ)

56. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Environmental Stresses Adverse effect on a plant

57. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Drought Water deficit  reduced transpiration

58. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Flooding  Enzymatic destruction of cells –  air tubes  plants survive oxygen deprivation Figure 39.28a, b Vascular cylinder Air tubes Epidermis 100  m (a) Control root (aerated) (b) Experimental root (nonaerated)

59. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Salt Stress Water potential changes

60. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heat Stress Heat-shock proteins produced

61. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cold Stress Altered lipid composition of membranes

62. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Defenses Against Herbivores Physical defenses, e.g. thorns Chemical defenses, e.g. toxic compounds

63. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recruitment of parasitoid wasps that lay their eggs within caterpillars 4 3 Synthesis and release of volatile attractants 1 Chemical in saliva 1 Wounding 2 Signal transduction pathway Some plants “recruit” predatory animals to defend against herbivores Figure 39.29