1. Plant Hormones: Control of Growth & Flowering
DP Biology 2009

2. Plant Hormones Many aspects of plant function are controlled
and regulated through the action of hormones.
What hormones have we already discussed?

3. Plant Hormones Abscisic acid: closes guard cells
Many aspects of plant function are controlled
and regulated through the action of hormones.
Abscisic acid: closes guard cells
Gibberellin: stimulates embryo growth and production of amylase.

4. Plant Hormones Abscisic acid: closes guard cells
Many aspects of plant function are controlled
and regulated through the action of hormones.
Abscisic acid: closes guard cells
Gibberellin: stimulates embryo growth and production of amylase.
Just one other one you need to know for IB …
Auxin: stimulates stem & fruit growth, root formation, and phototropism.

5. Other Plant Hormones (FYI)
Brassinosteroids: growth of stem & pollen tubule, vascular tissue differentiation
Cytokinins: work with auxin, promote growth & inhibit senescence
Ethylenes: promotes fruit ripening and leaf abscission
Jasmonates / Oligosaccharins / Salicylic Acid : stimulates protection against herbivores and/or pathogens

6. Auxin & Plant Growth Auxin promotes the elongation of cells.
(Not the formation of new cells … why?)

7. Auxin & Plant Growth Auxin promotes the elongation of cells.
Plant cell elongation is usually prevented by … ?

8. Auxin & Plant Growth Auxin promotes the elongation of cells.
Plant cell elongation is usually prevented by the cell wall.

9. Auxin & Plant Growth Auxin promotes the elongation of cells.
Plant cell elongation is usually prevented by the cell wall.
Auxin loosens the cell wall of plants, allowing the central vacuole to take up more water and stretch the cell by tugor pressure.

10. Auxin & Plant Growth Auxin promotes the elongation of cells.
Plant cell elongation is usually prevented by the cell wall.
Auxin loosens the cell wall of plants, allowing the central vacuole to take up more water and stretch the cell by tugor pressure.
The cell wall can be re-strengthened by the addition of more cellulose fibers.

11. Auxin & Plant Growth How does auxin ‘loosen’ cell walls?
Auxin increases the H+ concentration in cell wall (with what effect on pH?)

12. Auxin & Plant Growth How does auxin ‘loosen’ cell walls?
Auxin increases the H+ concentration in cell wall (lowering the pH)

13. Auxin & Plant Growth How does auxin ‘loosen’ cell walls?
Auxin increases the H+ concentration in cell wall (lowering the pH)
H+ triggers proteins called expansions to break up hydrogen bonds between cellulose fibers, allowing them to slide past each other.
See animation:

14. Auxin & Phototropism Tropisms are directional growth responses.
Auxin mediates phototropism -- the growth of the stem towards light.

15. Auxin & Phototropism Photoreceptors in shoot tip absorb light.
Detection of light causes proteins that redistribute auxin to be produced.
Auxin is moved from
the tip (where it is
produced) down
through the stem.
photoreceptors

16. Auxin & Phototropism
Auxin is concentrated in the shady side of the stem, promoting greater elongation there.
Uneven elongation
promotes curving of
stem towards light.
See animation:
biosci10v/bis10v/media/
ch19/auxin_phototropism.html
photoreceptors

17. Other Tropisms (FYI)
Can you think of any other plant tropisms?

18. Other Tropisms (FYI) Geotropism: Growth towards or away from gravity.
Thigmotropism: Growth towards touch.
Time lapse videos for fun …

19. Control of Flowering Flowering in many plants is seasonal, and
closely related to day-length.
“Long-day” plants flower in the summer,
whereas “Short-day” plants flower in the
winter.
Note: not all plants are fall into these categories. Some are day-neutral. Some are controlled by a combination of other factors.

20. Control of Flowering Flowering in many plants is seasonal, and
closely related to day-length.
“Long-day” plants flower in the summer,
whereas “Short-day” plants flower in the
autumn.
Experiments have shown that it is not the
length of the day that matter, but rather the
length of the night.

21. Control of Flowering Plants have a critical night length that
determines the timing of flowering.
What does this mean?

22. Control of Flowering Plants have a critical night length that
determines the timing of flowering.
Long-day plants flower if the night is shorter than critical value.
Short-day plants flower if night is longer than critical value.

23. Control of Flowering How can we tell that it is night length that
matters?

24. Control of Flowering Experimenters interrupted dark and light
periods with a flash of light or a brief period of
dark. What do their results suggest? D

25. Control of Flowering A flash of light during night affects flowering
Dark spell during day does not affect flowering. D

26. Measuring “dark” Pigments in leaves called phytochromes absorb light.
PR absorbs red light (660 nm) and converts to PFR
PFR absorbs far-red light (730nm) and converts to PR
PFR slowly degrades into PR

27. Measuring “dark”
Sunlight contains far more red light than far-red light, so
while exposed to sunlight, most of phytochrome is in PFR
state.
At the end of the
night, much of
the PFR has
converted back
to PR.

28. Measuring “dark” The amount of PFR in the leaves at the start of the
day determines whether plants flower.
How does the response differ in short- and long-
day plants?
Flowering is inhibited by PFR in short-day plants
Flowering is promoted by PFR in long-day plants

29. Measuring “dark” Experiments support the role of PFR in determining
timing of
flowering.

30. ‘Florigen’ discussion
Phytochromes are found in leaves, but their effects are seen elsewhere in a plant.
A single leaf is enough to control flowering in a large plant -- even several plants grafted together.

31. ‘Florigen’ discussion
Scientists have long suspected that a hormone is involved in controlling this response -- its even been given a name: florigen.
Yet, despite decades of intense research, no one has been able to isolate and identify this hormone.

32. ‘Florigen’ discussion
Scientists have long suspected that a hormone is involved in controlling this response -- its even been given a name: florigen.
Yet, despite decades of intense research, no one has been able to isolate and identify this hormone.

33. ‘Florigen’ discussion
Why would the theory persist, even though scientists have failed to identify the hormone?
Does a failure to identify the hormone indicate a lack of evidence for its existence?

34. ‘Florigen’ discussion
Most scientists agree that a theory is not ‘scientific’ unless it is falsifiable. What does this mean?
What do you think of the following quote from the 1970s:
“"Flowering is a religion based on the totally unfounded dogma of florigen."

35. ‘Florigen’ discussion
What would a scientist have to do to show that florigen doesn’t exist?
Can you think of any other non-falsifiable ‘theories’ in science?

36. Recent Florigen Developments
Recently a few molecules (proteins & RNAs)
have been discovered that either are the
elusive florigen -- or are otherwise involved in
generating the signal.