Plant Hormones

1. Plant Hormones

2. Plant tropisms • Growth in a particular direction in response to an external stimulus Response to gravity is called gravitropism To light is phototropism To touch is thigmotropism Responses may be positive or negative

3. Early experiments Canary grass coleoptiles

4. Classes Hormones • Five classes are identified • Auxins • Gibberellins • Cytokinins • Ethylene • Abscisic Acid

5. Auxin • Primary form is Indole acetic acid (IAA) • Photo-, Gravi-, and Thigmotropisms come about in large part due to auxin effects • Regulate growth primarily by promoting cell elongation with some differentiation.

6. Auxin production and transport • Produced in shoot apical tips, leaves, & seeds • Moves from tip to base • Moves primarily through parenchyma cells surrounding vascular tissue

7. Fritz Went Experiments

8. Phototropism mechanism

9. Auxin Mechanism • IAA stimulates H+ pumps in the cell membrane. • H+ pumps secrete H+ into the cell wall, decreasing its pH. • This acidifies the cell wall which activates pH-dependent enzymes and breaks bonds between cellulose microfibrils. • The wall "loosens" because of the broken bonds and the turgor pressure expands the cell.

10. Gravitropism • Root & shoot differential growth in response to gravity • Auxin in higher amounts on “lower” side of organ • Roots: negative response • Root more sensitive to auxin - inhibits elongation

11. Organ Response to Hormone

12. Gravity sensing mechanism • Root cap cells contain amyloplasts (statoliths) containing starch grains • Density causes movement through cytoplasm to lower part of cell Statoliths at bottom of cells of pea root

14. One week Two weeks Apical dominance Control • Auxin production & transport from tip inhibits lateral bud growth • Pinching the tip releases buds for growth • The actual mechanism is not simplistic: IAA may induce ethylene production which inhibits lateral bud growth. Cytokinins which move apically may actually be of greater importance.

15. Leaf senescence

16. Senescence • Shorter days of fall, drought, or the lack of nutrients cause lower auxin production • A "senescence factor" stimulates cells to form ethylene which produces cellulase (an enzyme that breaks down cellulose) and pectinase. • Middle lamella is digested causing cells to separate causing abscission. • Ratio of auxin to cytokinin may play a role.

17. Other effects of auxin • Stimulates development of fruit • Can stimulate lateral root formation • May stimulate adventitious root formation in stems

18. Gibberellins • Translocated in xylem & pholem • Formed in young leaves, apical tips, embryo • Effects • Bolting • Can overcome dwarfing in some plants • Stimulates flowering in some plants • Affects fruit development • Stimulates germination of seeds

19. Difference: Auxin & Gibberellin • Gibberellin controls elongation in the mature regions of trees and shrubs • auxin regulates elongation in grass seedlings and herbs. • Gibberellin stimulates cell division and elongation • auxin stimulates only cellular elongation. • Plants can tolerate high levels of gibberellin but not of auxin. • Gibberellin has little effect on roots • auxin has more of an effect on roots.

20. Gibberellin example • Effect on cabbage • Treated once/week for 2 months • Evidence that cabbage comes from a tall, spindly ancestor

21. Dwarf Pea Control Gibberillin added

22. Cytokinins • Formed in roots • Translocated upward in xylem • Effects • Stimulates cell division • Shoot & root differentiation • Stimulates growth of lateral buds & leaf expansion • Chloroplast development • Delays leaf senescence • Often the auxin/cytokinin ratio is important

23. Abscisic acid • Seed maturation & stomatal function • May aid onset of seed dormancy • Transported from leaves in phloem • “Stress hormone” - effects help protect plant from unfavorable conditions • Levels increase in response to cold, drought, and high salt levels

24. Ethylene • Gas - diffuses through tissues • Stimulates abscission and fruit ripening • Used in commercial ripening for bananas & green picked fruit • Involved in leaf abscission & flower senescence • Primarily synthesized in response to stress

25. Photoperiodism classes • Response of plants to length of day is called photoperiodism • Flowering is a photoperiodism response

26. Florigen - hypothetical hormone Proposed to regulate the initiation of flowering

27. Critical Periods • Long-day plants: flowers when night length is less than critical period • Flower in spring & early summer when days grow long • Short-day plants: flowers when uninterrupted darkness is longer than critical period • Chrysanthemum: more than 10-12 hrs. light keeps them from flowering

28. Long day plants

29. Short day plants

30. Radish Potato

32. Other “day” classes • Day-neutral plants • tend to flower independent of day length • Long-short or short-long • a proper sequence is needed for flowering to begin • Intermediate-day plants • two critical periods - day length must be between them

33. Timing Mechanism?

34. Phytochrome

35. Dark etiolation

36. Florigen - the trigger? • Florigen has been proposed as the hormone that triggers flowering • No hormone has ever been isolated • Critical period inductions given to one part of the plant can trigger flowering in another part. • illumination of a leaf on a plant for the proper photoperiod can start flowering

37. Grafting experiments • One plant given proper period (short day) • Flowering begins • Later flowering starts in other plant • See Fig. 35.14 Graft union Light tight barrier

38. Phytochrome system • Plants can sense light both in quantity and quality • Plants respond to changes in light quality with different types of growth • Much of the sensing seems to come in the red part of the spectrum • Red light (~660 nm) and far red light (>700 nm) can be differentiated • The pigment system responsible is called the phytochrome system

39. Summary • Critical lengths can vary from species to species