Plant Responses to Signals

1. Plant Responses to Signals Chapter 39

2. Overview • Examples of some hormones • Phototropism experiments • Auxin’s mode of action • Apical dominance • Other tropisms • Seed dormancy, germination • Fruit ripening • Photoperiodism

3. Plant Growth Hormones • Hormone: a chemical substance produced in one part of the body and has a physiological response in another part of the body at very low concentrations. • coordinates metabolic activities • active in small amounts

4. Tropisms: growth response to particular stimulus • Phototropism plants grow towards light • Gravitropism- roots grow down into soil • statoliths in root cap • Thigomotropism-tendrils curl around branch, etc. Responds to touch. • wind blowing

5. Phototropism Investigators • Darwin and Son, 1880 • light sensitivity in tip. • Boysen-Jensen, 1913 • signal moving down from tip • Went, 1926 • signal is a chemical substance • Thimman isolated the structure of Auxin

11. Phototropism • Differential cell elongation causes stem to bend. Cells on shaded side elongate. • How do cells elongate? • Auxinin higher concentration on shaded side of stem. • Only cells directly underneath stimulated cells elongate • Auxin moves basipetally down stem • How?

12. How do Cells elongate? • Cell wall has constitutive enzymes that loosen connections between cellulose when activated. • An increase in cell wall acidity (lower pH) activates the enzymes. • Auxin in cytoplasm causes cell wall to acidify. • Water moves in by osmosis, swelling cell. • Cell membrane lays down additional cell wall material keeping new size / shape

13. How do Cells Elongate ?

15. Cell Elongation • How cellulose microfibrils are laid down determine the direct of elongation. • Elongation takes place when a cell only has primary cell wall. • Once proper size and shape is attained, secondary cell wall material is added inside the primary cell wall.

18. Plane of Cell division • Determines the cell shape

19. Plane of symmetry

20. higher pH in cytoplasm activates Auxin • Can only leave cell via transport proteins at base of cell. • Diffuse across into next cell, in inactive form.

21. Entering next cell down, auxin is activated by pH, causing proton pumps to start • Cell wall acidifies, activating enzymes.

22. Auxin moves down to next cell. • Now more diluted by process • Signal fades out farther down

23. Fig. 39-7 RESULTS Cell 1 100 µm Cell 2 Epidermis Cortex Phloem 25 µm Xylem Basal end of cell Pith

24. Gravitropism • Statoliths fall to bottom of cell at root cap. • Inhibit auxin production in lower cells elongate bending tip downward.

25. Solar tracking • Flowers or leaves bend towards sun’s path in the sky during the day. • Motor cells at base of flower or leaf uptake K+ and other ions causing them to swell, bending the stem towards light. • Stem continues to respond to direction of light during the day, and different cells swell, or relax changing direction. • Similar process cause an opening / closing response to flowers (poppies) or Leaves (prayer plants). • Can be under circadian rhythm.

26. Apical Dominance • Shoot apical meristem produces auxin which moves down stem and inhibits auxiliary bud from growing. • Roots produce cytokinins that move up and stimulate buds to grow. • Pinching back tops makes plants bushier • Pruning sends a surge of cytokinins up to remaining buds- fast growth in spring. • Limiting root growth can stunt plants.

28. Seed germination • Many annual’s have abscisic acid (ABA) in seed coat. • ABA keeps embryo, seed dormant • Rains wash out ABA • Embryo swells produces gibberillins which cause seed to germinate • Ensures germination after soil is wet enough. • Other seeds respond to cold, light etc.

29. Fig. 39-12 Early germination in red mangrove Coleoptile Early germination in maize mutant

30. GA stimulates seed germination:“malting” of grain

31. Fruit Ripening • Ethylene is the only gaseous hormone. • May spread to other plants • Causes fruit to ripen • “one bad apple…” • Positive feedback loop • Organic acids convert to sugars, pectin in middle lamella breaks down • Ethylene sensitive fruit can be stored green under carbon dioxide for months • Apples, bananas • not strawberries, mangoes • Gassed before sending to market • Potential area for biotechnology

32. Ethylene triple response • Growing tips meets on object • Secrete ethylene • Causes stem to • 1) slow elongation • 2) thicken • 3) grow sideways • Until around object and resumes upward growth.

33. Fig. 39-3 CYTOPLASM CELL WALL Transduction Response 1 2 3 Reception Relay proteins and Activation of cellular responses second messengers Receptor Hormone or environmental stimulus Plasma membrane

34. Ethylene mutants

35. Gibberellins • Testing in lab • dwarfism in many plants • Bolting- & flowering • Fruit set • Stimulate cell division & elongation • Promotes seed germination

36. Fig. 39-10 (b) Gibberellin-induced fruit growth Gibberellin-induced stem growth

37. Leaf abscission cause by balance of ethylene and auxin • Apoptosis cell death- recycles many essential nutrients to plant, stimulated by burst of ethylene • How do plants detect when this should happen?

38. Not covered

39. Table 39-1 Not covered

40. Plant Hormone ReviewTable 39.1 • Auxins: growth, cell elongation in stem root, Apical Dominance, seedless fruit • Cytokinins: (roots) root growth, stimulates cell differentiation & growth retards senescence (fruit, flower life), stimulates germination

41. Plant Hormone Review Table 39.1 • Gibberellins; stimulate cell division & elongation, fruit set, bolting, promotes seed germination • Ethylene: fruit ripening, opposes some auxin affects • Absicisc acid; inhibits growth, closes stomata, dormancy in seeds

42. FYI Only don’t need to know: • Oligosaccharins- Trigger defense mechanisms • short sugar chains released from cell wall by enzymatic breakdown of cellulose and pectin. • Brassinosteroids- steroids required for normal growth and development. • Studied mostly by mutations lacking these compounds.

43. Fig. 39-1

44. Phototropism responds to blue light levels Many responses to light detected by phytochromesensitive to red light. Plants can sense & measure light

45. Phytochrome responses: • Seed germination • Need light to germinate • Shade avoidance • Higher PR ratio in shade • Plants grows taller to reach brighter light • Flower response – Florigen • Other photoreceptors sense blue light: phototropism

47. Conversion of phytochrome • In light Pr converts rapidly to Pfr • In dark,Pfr slowly reverts to Pr • Used to time amount of darkness, or dawn • Resets internal biological clock

48. PhotoperoidismShort day and Long day plants • Actually refer to length of darkness • Many plants are day neutral

49. Effects of far-red light • Far red light counteracts red light, • erasing “day” signal

50. Fig. 39-23 24 hours 24 hours 24 hours Graft Short-day plant Long-day plant Long-day plant grafted to short-day plant