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: Major Functions:
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
Auxin/IAA: (indoleacetic acid) promotes plant growth. Facilitates elongation of developing cells. Increases H+ conc. In 1’ cell walls = activates enzymes to loosen cell wall fibers = increase in plasticity. Turgor pressure causes cell wall to expand.
Produced at tips of shoots and roots and also influences plant responses to light (phototropism) and gravity (geotropism),
Active in leaves, fruits, and germinating seeds.
Actively transported (uses ATP) from cell to cell in a specific direction (polar transport)

17. Plant Hormones - Auxin (IAA) cont. Found Major Functions
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
Cytokinins: hormones that stimulate cytokinesis (cell division). Variations structurally of nitrogenous base adenine. Include zeatin (natural) and kinetin (artificial).
Produced in roots and transported throughout plant.
Effects depend on target tissue and also presence of auxin.
Influence direction of organ development (organogenesis) = whether roots and shoots will develop.
Growth of lateral buds (weakening apical dominance)
Found to delay senescence (aging) of leaves.

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
Gibberellins: promote cell growth. More than 60 variations, all abbreviated GA1, GA2, GA3… for gibberellic acid.
Occurs in young leaves, roots, and seeds.
Interact with auxins to stimulate shoot growth. Promotion of fruit development, seed germination, and inhibition of aging leaves.
High concentrations cause rapid elongation of stems (bolting)

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
Phototropism: response to light, achieved by action of auxin.
Auxin produced in apical meristem moves downward by active transport to zone of elongation and gnerates growth.
When all sides equally stimulated by light, growth is straight.
When stem is unequally stimulated by light, auxin moves to shady region for elongation. = 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
Phytochrome: protein modified with a light-absorbing chromophore. Pr (P660) and Pfr(P730). Absorbing red/far red light. When absorbs 660 nm light, converted to Pfr. When absorbs 730 nm light, converted back to Pr.

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
Phytochrome: protein modified with a light-absorbing chromophore. Pr (P660) and Pfr(P730). Absorbing red/far red light. When absorbes 660 nm light, converted to Pfr. When absorbs 730 nm light, converted back to Pr.

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 <-> Pfr interconversion acts as a switching mechanism that controls various events in the life of a plant.
Night length responsible for resetting circadian-rhythm clock.
Daylight interrupted by brief dark period = no effect on clock.
Flashes of red/far-red light during night period can reset the clock.
In a series of flashes, only last flash affects perception of night length. Red light shortens night length and far-red restores night length.
Pfr appears to reset the circadian-rhythm clock. Pfr is active form of phytochrome.
Endogenous: internal clock that will keep cues even if external cues are absent. External cues (dawn/dusk) resent clock so its accurate.

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

40. Animated Videos