Plant Responses to Internal and External Signals

1. Plant Responses to Internal and External Signals

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

4. Signal transduction pathways link signal reception to response • Plants have cellular receptors that detect changes in their environment • For a stimulus to elicit a response, certain cells must have an appropriate receptor

5. A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots • These are morphological adaptations for growing in darkness, collectively called etiolation

6. 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.

7. De-Etioloation (“Greening”) Proteins • Many enzymes that function in certain signal responses are directly involved in photosynthesis • Other enzymes are involved in supplying chemical precursors for chlorophyll production

8. Plant hormones help coordinate growth, development, and responses to stimuli • Hormones are chemical signals that coordinate different parts of an organism

9. 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

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

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

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

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

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

16. Auxin • The term auxin refers to any chemical that promotes cell elongation in target tissues • Auxin transporters move the hormone from the basal end of one cell into the apical end of the neighboring cell

17. 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

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

19. Lateral and Adventitious Root Formation • Auxin is involved in root formation and branching • An overdose of auxins can kill dicots • Auxin affects secondary growth by inducing cell division in the vascular cambium and influencing differentiation of secondary xylem

20. Cytokinins • Cytokinins are so named because they stimulate cytokinesis (cell division) • Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits • Cytokinins work together with auxin

21. 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 • If the terminal bud is removed, plants become bushier

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

23. Anti-Aging Effects • Cytokinins retard the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from surrounding tissues

24. Gibberellins • Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed germination • Gibberellins stimulate growth of leaves and stems • In stems, they stimulate cell elongation and cell division

25. 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

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

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

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

30. Seed Dormancy • Seed dormancy ensures that the seed will germinate only in optimal conditions • Precocious germination is observed in maize mutants that lack a transcription factor required for ABA to induce expression of certain genes

31. Coleoptile LE 39-12

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

33. 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

34. ein mutant ctr mutant LE 39-14 ein mutant. An ethylene-insensitive (ein) mutant fails to undergo the triple response in the presence of ethylene. ctr mutant. A constitutive triple-response (ctr) mutant undergoes the triple response even in the absence of ethylene.

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

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

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

38. 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 • Plants detect not only presence of light but also its direction, intensity, and wavelength (color)

39. 1.0 0.8 0.6 Phototropic effectiveness relative to 436 nm LE 39-17 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. Phytochromes as Photoreceptors • Phytochromes regulate many of a plant’s responses to light throughout its life • Studies of seed germination led to the discovery of phytochromes

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

44. The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome • Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses

45. Pr Pfr Red light Responses: seed germination, control of flowering, etc. LE 39-20 Synthesis Far-red light Slow conversion in darkness (some plants) Enzymatic destruction

46. Phytochromes and Shade Avoidance • The phytochrome system also provides the plant with information about the quality of light • In the “shade avoidance” response, the phytochrome ratio shifts in favor of Pr when a tree is shaded

47. 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

48. LE 39-21 Midnight Noon

49. 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

50. The Effect of Light on the Biological Clock • Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues