Plant Hormones

1. Plant Hormones AP Biology – LAHS

2. What are Hormones? • Chemical signals that coordinate the various parts of an organism • Chemicals are made in one region and are target for some other region of the organism

3. The Discovery of Plant Hormones • Plant hormones were discovered as scientists were studying how it is that plants grow towards light • Phototropism – growth of a shoot towards light

4. Darwin’s Experiments with Phototropism • Coleoptile – term for the sheath that encloses a grass seedling • Studied growth of the coleoptile in different conditions • Darkness – grew straight • Illuminated uniformly from all sides – grew straight • Illuminated from one side only – grew towards the lighted side

5. Question: What Causes the Coleoptile to Bend towards Light? • Hypothesis: cells on the darker side of the coleoptile elongate faster than those on the lighted side. This causes the coleoptile to bend toward light. • How does this happen?

6. Darwin’s Ideas • The part of the coleoptile responsible for sensing light is the TIP. • Growth response that caused curvature of the coleoptile was BELOW the tip. • Hypothesis: some signal was transmitted downward from the tip

7. Diagrams of Experiments

8. Testing Darwin’s Hypothesis • Peter Boysen-Jensen • Tip was separated from the coleoptile • Control treatment: A gelatin block separated the tip form the lower parts of the plant • The gelatin block allowed the plant to be cut as it would be in the experimental treatment, but still allowed the chemicals from the tip to pass down • Resulted in curvature as normal

9. Testing Darwin’s Hypothesis • Experimental treatment: • An impermeable barrier was placed between the coleoptile tips and the lower parts of the plants • Prevented the chemicals made at the tip from moving down the plant • Result: Curvature growth did not occur

10. Diagrams of Experiments

11. Went’s Experiments • Extracted the chemical messenger from the coleoptile tip • Removed the tip and allowed it to diffuse onto a piece of agar • Removed and discarded growing tip from other coleoptile seedlings • Placed the agar block evenly centered onto the “decapitated” seedlings • They grew straight • Placed the agar block Uncentered onto other decapitated seedlings • Their growth caused them to curve AWAY from the side with the agar block

12. Went’s Experiments

13. Went’s Conclusions • The chemical in the tip stimulated growth as it passed down the coleoptile • The coleoptile curved toward light because of a HIGHER concentration of the growth-promoting chemical on the DARKER side of the coleoptile • Went named the chemical messenger that he studied AUXIN

14. Tropisms • Growth responses that result in curvatures of whole plant organs toward or away from some stimulus. • Mechanism • Elongation of cells on the OPPOSITE side of the organ region that is receiving the stimulus • Stimulii • Gravity • Light • Touch

15. How Does Auxin Stimulate Growth? • Causes cell walls to become “looser” and more malleable. Then they can be expanded/elongated

16. Plant Hormones - Auxin • IAA (indoleacetic acid) • Found: • Meristems of apical buds • Major Functions: • stimulation of stem elongation • Root growth, differentiation, branching • Apical dominance • Growth of a stem occurs only at the tip unless the tip is cut off • Absence of auxin from tip will allow lateral buds to emerge • This is why we prune • Actively transorted from cell to cell in a specific direction

17. Plant Hormones - Auxin • (IAA) cont. • Found • Embryos within seeds • Major Functions • Stimulate growth of fruit from ovary • Influences responses to light & gravity

18. Plant Hormones - Cytokinins Found • In actively growing tissues • Produced in roots, transported elsewhere • Major function: • Stimulate cytokinesis (cell division) • Work with auxins to control plant growth • Plant tissue treated with auxin w/o cytokiinin – cells will grow but not divide • Control of apical dominance – supports lateral buds (weakens apical dominance) • Anti-aging hormones • Delays senescence (aging) of leaves • Slow deterioration of leaves – used by florists

19. Plant Hormones - Gibberellins • Found in: • Apical meristems; young leaves/embryos • Major function: • Simulates growth in leaf and stem • Stem bolting – rapid elongation • Fruit growth • Grapes are sprayed with gib to cause them to grow larger and further apart • Germination of seeds • After water is imbibed, gibberellins are released in embryo to break from dormancy • Inhibition of aging leaves

20. Plant Hormones – Abscisic Acid • Found in: • Leaves, stems, roots • Seeds, green fruit • Major function: • Slow down growth • Dormancy for overwintering • Suspends primary and secondary growth • Promotes abscision of leaves (falling off) • In seeds – inhibits growth until ABA can be overcome or diminished by favorable conditions • Heavy rain may wash out ABA • Light may degrade • Increased gib to ABA ratio may determine germination • growth • Stress hormone • When a plant wilts, ABA accumulates causing stomata to close

21. Plant Hormones - Ethylene • Found in • Tissues of ripening fruit • Nodes of stems • Ageing leaves and flowers • Major functions • Changes of ovary to become fruit • Degradation of cell walls; softening • Dropping from plant • Leaf abscission • Loss of leaves to prevent water loss • Tissue at base of petiole dies • Senescence (aging) • Autumn leaves; withering flowers

22. Tropisms • Growth responses that result in curvatures of whole plant organs toward or away from some stimulus. • Mechanism • Elongation of cells on the OPPOSITE side of the organ region that is receiving the stimulus • Stimulii • Gravity • Light • Touch

23. Tropisms - Phototropsm • Phototropism: response to light • Achieved through auxin • When all sides equally lit, straight growth • When stem unequally lit, differential growth

24. Tropisms - Gravitropism • Also geotropism • Response to gravity be stems & roots • Gibberellins & Auxin involved (relative concentrations)

25. Tropisms - Gravitropism • If stem is horizontal: auxin at apical meristem moves down and concentrates on lower side – stem bends upwards • If root is horizontal, auxin produced at apical meristem moves up roots and concentrates on lower side – inhibits growth in roots • Special starch-storing plastics (staloliths) settle at lower ends of cells to influence auxin movement

26. Tropisms - Thigmotropism • Response to touch • Seen in climbing vines, venus fly trap, etc.

27. Photoperiodism • Response of plants to changes in the photoperiod (relative length of day/night) • A plant maintains circadian rhythm: internal clock that measures length of day/night

28. Phytochromes • Chemicals that function as photoreceptors in plants and allow plants to “measure” photoperiod

29. Phytochromes • The name given to the photoreceptor that is responsible for the reversible effects of red and far-red light is phytochrome • Phytochrome = a light absorbing protein • 2 forms • Pr = absorbs red light • Pfr = absorbs far red light • The two forms are photoreversible • When Pr is exposed to red, it becomes Pfr • When Pfr is exposed to far red, it becomes Pr

30. Phytochrome

31. Phytochrome • Pr is form of photochrome synthesized in plant cells. Pr synthesized in leaves. • Pr and Pfr in equilibrium during daylight. Pr -> Pfr since red light present in sunlight. • Pr accumulates at night. No sunlight for Pr -> Pfr. Pfr breaks down faster. Cells continue to make Pr at night. • Daybreak, light rapidly converts to accumulated Pr into Pfr. Equilibrium again.

32. Phytochromes • The Pr <-> Pfrinterconversion acts as a switching mechanism that controls various events in the life of a plant.

33. Phytochromes • Red light - 660nm • Wavelength of light that is most effective at interrupting the critical night length of a short day (long night) plant. • Exposure at night will cause the plant NOT to flower • HOWEVER, if this light briefly interrupts the night of a long day (short night) plant, the plant will flower • The red flash will shorten the plants perception of night length

34. Phytochromes • The shortening of night length can be negated by providing a flash of light at 730nm wavelength. • This is called the far-red part of the spectrum

36. Phytochrome • Night length is responsible for resetting the circadian rhythm clock • If daylight is interrupted with dark there is no effect • If dark is interrupted with flashes of red or far-red the clock can be affected • Red-light shortens night length • Because it converts Pr to Pfr – which would not normally accumulate at night • Far-red light restores – as though night was not broken • Because far red light flashes convert Pfr back to Pr

37. Phytochrome • Plants synthesize phytochrome as Pr • If left in the dark, nothing happens to this pigment • If the pigment is illuminated with sunlight, Pr changes to Pfr • Thus the plant can detect the presence of sunlight

38. Phytochrome • Pr = Pfr during daylight • If shade of larger trees were to block sunlight from a smaller tree, the radiation most blocked by canopy is red (not far red) • Pigments in the plant would be converted to Pr • This cue would stimulate the plant to grow taller.

39. Phytochrome • If ample sunlight were available, the reverse would happen – • Pfr proportions would increase and the plant would “sense” that it was in sun. • It would be cued to branch and vertical growth would be inhibited

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