1. Chapter 39: Control Systems in Plants

2. Question  Do plants sense and respond to their environment ?  Yes - By adjusting their pattern of growth and development. In DarkIn Light

3. Comment  Plants can’t “move” away from a stimulus, but can change their growth response.  Result – plant bodies are more “flexible” in morphology than animals.

4. Classical Example  Phototropism - plant growth response to unilateral light.  Observation – plants bend or grow towards the light.

5. Phototropism Experimenters  Darwins: late 1800's.  Boysen & Jenson: early 1900's.  F.W. Went: 1926

7. Went Experiments

8. Mechanism of Phototropism  Cells on the dark side elongate faster than the cells on the light side.  The uneven growth rate causes the bending of the stem toward the light.

9. Question  What is the adaptive value of phototropism?  It tilts the leaves toward the light source for more efficient photosynthesis.

10. Cause of Phototropism  Chemical messenger from the tip caused the growth response in the stem.  The distribution of the chemical changes in the unequal light, resulting in unequal cell elongation.

12. Hormone  Chemical signal produced in one location, transported, has effect in another location.  Phototropism is caused by a plant hormone.

13. Plant Hormones  Are produced in small quantities.  Effects may reflect balance between several hormones.

14. Mechanism

15. Plant Hormones 1. Auxins 2. Cytokinins 3. Gibberellins 4. Abscisic Acid 5. Ethylene

17. Auxins  Named by Went in 1926.  First plant hormone described.  Ex: IAA (natural) 2,4-D (synthetic)

18. Major Functions  Stimulates cell elongation.  Fruit development.  Apical Dominance.  Tropism responses.

19. Apical Dominance

20. Where Produced  Apical Meristems.  Young leaves.  Embryos.

21. Cytokinins  Isolated from coconut "milk" (endosperm) in the 1940’s.  Named because they stimulate cell division.  Ex: Zeatin

22. Major Effects  Stimulates cell division.  Delays senescence.  Root growth and differentiation.  Where Produced - roots

23. Auxin/Cytokinin Ratios  Control shoot or root differentiation in tissue cultures.

24. Gibberellins  Found from the "Foolish Seedling" disease in rice.  Ex: GA 3 70 types known

25. Foolish Seedlings

26. Major Effects  Internode elongation.  Seed/Bud germination.  Flowering (some species).  Fruit development. Extra GA3 No GA3

27. Lack GA3 Have GA3

28. Where Produced  Apical Meristems.  Young leaves.  Embryos.

29. Abscisic Acid  Slows or inhibits plant growth.  "Stress" hormone produced under unfavorable conditions.

30. Major Effects  Inhibits growth  Seed/Bud dormancy.  Stomata closure.  Leaf drop – produces abscission layer.

31.  Abscission Layer

32. Where Produced  Leaves  Stems  Green fruit

33. Ethylene  Gaseous hormone (fast diffusion rates).  Often interacts with Auxin.

34. Major Effects  Fruit ripening.  Accelerates Senescence.  Stem/Root Elongation (+ or -).

35. Where Produced  Ripening fruits.  Senescent tissue.  Nodes.

36. New Hormones  Oligosaccharins – short chains of sugars released from the cell wall.  Function: Pathogen responses Cell differentiation Flowering

37. New Hormones  Brassinosteroids – steroid hormones similar to animal sex hormones.  Function: Needed for normal growth and development.

38. Commercial Applications of Plant Hormones  Weed killers  Seedless fruit  Rooting of cuttings  Tissue culture

39. Plant Movements 1. Tropisms 2. Circadian Rhythms

40. Tropisms  Growth responses in response to external stimuli.  + toward a stimulus  - away from stimulus

41. Examples 1. Phototropism 2. Gravitropism

42. Phototropism  Response to light (blue).

43. Movie

44. Gravitropism  Response to gravity.  Stems are – gravitropic and roots are + gravitropic.

45. Gravitropism - mechanism  Statolith movement may be the receptor for the stimulus.

46. Thigmotropism  Response to touch.  A series of 5 genes are involved.  Ex: Tendrils Climbing stems Wind direction response of stems.

47. Turgor Movements  Movement caused by turgor pressure differences in certain cells.

48. Types 1. Rapid Leaf Movement Ex: Mimosa 2. Sleep Movements Ex: Bean Leaves Prayer Plant Day Night Sleep Movements

49. Mimosa Rapid Leaf Movement

50. Circadian Rhythms  A physiological cycle about 24 hours long.  Ex: Stomata opening Sleep movements

51. Causes  Synthesis of a transcription factor protein that regulates is own manufacturing through feedback control.  Gene is believed to be common in most eukaryotic organisms.

52. Photoperiodism  A physiological response to changing day lengths.  Used to detect and direct growth responses to seasonal changes.

53. Advantages  Match growth responses to proper season.  Ex: Leaf drop in fall Flowering

54. Flowering Types 1. Short - Day Plants 2. Long - Day Plants 3. Day - Neutral Plants

55. Short-Day Plants  Flower when days are shorter than a critical period (long nights).  Ex: Mums Poinsettias

56. Long-Day Plants  Flower when days are longer than a critical period (short nights).  Ex: Spinach Iris Lettuce

57. Day-Neutral Plants  Flower whenever they have enough energy.  Ex: Roses African Violets

58. Night Length  Actually controls flowering response, not day length.  Proof – experiments show that if you interrupt the dark period, you reset the “clock”.

60. Comment  Length of night not absolute, but relative for the response to be triggered.

61. Question  What detects day/night length changes?  Phytochrome - plant pigment involved with photoperiodism.

62. Phytochrome Forms  P r - responds to 660nm (red light).  P fr - responds to 730nm (far red).

63. Phytochrome  Changes between the two forms.  Ratio or accumulation of enough P fr triggers the responses

64.  In Red light: P r  P fr  Far-red light or darkness: P fr  P r

65. Photoperiodism  Very sensitive (1 minute difference).  Sets clocks for plant responses.

66. Other Effects  Seed Germination  Stomatal Opening  Leaf Drop

67. Lettuce Germination

68. Responses to Stress  Stress – an environmental condition that can have an adverse effect on a plant’s growth, reproduction and survival.

69. Plant Response 1. Developmental changes 2. Physiological changes

70. Water Deficit  During high Ts, guard cells may close.  Young leaves may slow expansion.  Leaves may roll to reduce surface area.

71. Oxygen Deprivation  Common in roots in water-logged soils.  Air tubes in roots may bring oxygen to the cells.

73. Salt Stress  Damages the plant through unfavorable soil water potentials and toxic ions.  Some plants can concentrate and excrete salt through salt glands (ex. halophytes).

74. Heat and Cold Stress  Heat - use heat-shock proteins to protect other proteins from denaturing.  Cold – lipid shifts to keep lipid bilayers “liquid”.  Cold – solute changes to lower freezing point.

75. Herbivores  Plants have many physical and chemical defenses against herbivores.  Physical – thorns  Chemical – crystals, tannins and other toxic compounds.

76. Herbivores  Often trigger a plant to release chemicals to attract predators or to warn other plants to increase their production of toxins.

78. Pathogens  First Defense – epidermis  Second Defense – chemical events to restrict or kill the invader.

79. SAR  Systemic Acquired Resistance: chemicals that spread the “alarm” of an infection to other parts of the plant.  Possible Candidate: salicylic acid

81. Summary  Know the general plant hormones and their effects.  Know tropisms.  Know photoperiodism.  Know general ideas about how plants respond to stress.