1. Signaling

2. Signaling (hormones, light, etc…)
Reception
Transduction
Response
Signal
No response
Receptor
Relay proteins
Signal
Receptor
Differential gene expression

3. Major signals that control plant growth and development
Environmental signals:
- Light
- Gravity
- Temperature
- Humidity
- etc…

4. Major signals that control plant growth and development
Internal signals: Plant Hormones
- AUXIN
- CYTOKININ
- ETHYLENE
- ABSCISIC ACID
- GIBBERELLIC ACID

5. Auxin
Note: several different auxins are known to date (natural as well as synthetic). IAA is the most common natural auxin found in plants.

6. Auxin effects - promotes cell elongation
- inhibits lateral meristem activity
- promotes root formation

7. Auxin and differential growth: Gravitropic growth responses of Arabidopsis seedlings
Cotyledons
(embryonic leaves)
Turn seedling 90o
Hypocotyl
(embryonic stem)
Root
Hypocotyl shows a negative gravitropic response
Root shows a positive gravitropic response
Areas of differential growth (one side grows faster than the other)

8. Differential growth a b
Rate of cell elongation is higher on the a-side of the coleoptile compared to the b-side. This leads to differential growth: increased growth rate on one side of plant organ, results in curvature of the organ.
Figure 15.9: The dependence of coleoptile growth on auxin. If the tip of the coleoptile is removed, growth ceases (a); thus, the tip produces some factor needed for growth. If the tip is placed on an agar block (b), and later the block is placed on the decapitated shaft, growth resumes (c). This shows that the growth factor is a diffusable chemical (auxin). If the agar block is placed on one side of the shaft (d), the shaft bends as it grows (e), suggesting that the light-induced curvature may be caused by a movement of auxin to the side away from the light.

9. Auxin and shoot apical dominance
Decapitation of the apical bud releases the lateral buds. In the absence of auxin coming from the shoot apex, lateral buds become active leading to branching (and a more bushy shoot development)

10. Example: Auxin and lateral root formation in Arabidopsis
The synthetic auxin 2,4-D promotes lateral root formation in Arabidopsis
Note: 2,4-D is also used as a herbicide because it completely inhibits growth at higher concentrations.

11. Example: Auxin promotes adventitious root formation from Ilex opaca (Holly) shoots.
Fig , p. 246
Figure 15.12: Holly (Ilex opaca) shoots form roots at their bases faster when the bases are treated with an auxin. The ends of these shoots were dipped for 5 seconds in solutions containing 50% ethanol and (from left to right) 0%, 0.1%, and 0.5% naphthalene acetic acid, a synthetic auxin. They were then rooted in moist vermiculite for 2 weeks.
Shoots form roots at their bases faster when the bases are treated with auxin. The ends of these shoots were dipped for 5 seconds in solutions containing (from left to right) 0%, 0.1% and 0.5% auxin. They were then rooted in moist vermiculite for 2 weeks.

12. Cytokinin
Zeatin
Zeatin is one of many natural cytokinins found in plants

13. Cytokinin effects - promotes cell division/shoot formation
- promotes lateral meristem activity
- controls sink/source identity of plant organs
- delays senescence

14. auxin
cytokinin

15. Cytokinin and shoot apical dominance
By increasing the cytokinin concentration in the shoot, lateral buds become active resulting in increased branching (and a more bushy shoot development)
Cytokinin

16. The effect of cytokinin on senescence.
Cytokinin applied to the right-hand primary leaf of this bean seedling inhibited its senescence. The left-hand did not get cytokinin.
Figure 15.13: The effect of cytokinin on senescence. Cytokinin applied to the right-hand primary leaf of this bean (Phaseolus vulgaris) seedling inhibited its senescence. The left-hand leaf did not get cytokinin.
Fig , p. 246

17. Gibberellin Gibberellic acid 3
Note: several different gibberellins are known to date (natural as well as synthetic). GA3 is the most common natural gibberellin found in plants.

18. Gibberellin effects - promotes stem elongation growth
- promotes seed germination

19. Gibberellins promote stem elongation in many plant species
Pea seedlings
Pea seedlings
treated with GA3

20. Gibberellins and world food production
Norman Borlaug
Nobel Peace Prize 1970
Developed high-yielding wheat strains
Disadvantages
Strains require high levels of fertilizer (containing N, see lecture on absorption and transport of minerals)
Expensive (requires fossil fuels)
Create pollution

21. Coordination of Development via Hormone action
The major plant hormones:
- Auxins
- Cytokinins
- Gibberellins
- Abscisic acid
- Ethylene
Hormones that promote/control growth (direction)
Survival hormones (tend to inhibit growth)

22. Ethylene

23. Ethylene effects - inhibits cell expansion - accelerates senescence
- accelerates fruit ripening

24. Ethylene effects on etiolated seedlings
Arabidopsis seedlings grown in the dark display an etiolated growth pattern:
1) unexpanded cotyledons
2) Apical hook
3) long thin hypocotyl
Exposure to ethylene during growth in the dark results in:
1) Exagerated apical hook
curvature
2) Much shorter and
thicker hypocotyl

25. Ethylene and senescence
Solution that contains STS, an inhibitor of ethylene action. STS delays floral senescence.

26. Ethylene and fruit ripening
Ripening of fruit stimulated by ethylene
Ethylene is THE most damaging hormone in agriculture (accelerates ripening and consequently rotting of fruits)
Involves
Conversion of starch or organic acids to sugars
Softening of cell walls to form a fleshy fruit
Rupturing of cell membrane with resulting loss of cell fluid to form dry fruit
Overripe fruit is potent source of ethylene
Promotes ripening of adjacent fruits

27. Abscisic acid

28. Abscisic acid effects - promotes stomatal closure
- inhibits seed germination

29. Abscisic Acid and drought stress
Abscisic acid is a signal of this emergency situation. Under drought conditions, wilted mesophyll cells of a leaf rapidly synthesize and excrete abscisic acid (ABA). This ABA diffuses to the guard cells, where an ABA receptor recognizes the presence of the hormone and acts to release K+, Cl-, and as a result H2O, thus rapidly reducing turgor pressure and closing the stomata

30. Abscisic Acid and germination
Wild type (normal)
Corn seeds attached . Majority of seeds are dormant: they contain ABA that prevents germination.
ABA insensitive corn. Majority of seeds are already germinating while still attached to the parent plant because of a defect in ABA sensitivity.