Major plant hormones include auxins, cytokinins, gibberellins, ethylene, and abscisic acid. Auxins promote cell elongation and root formation, inhibit lateral bud growth, and allow differential growth responses through areas of faster cell elongation. Cytokinins promote cell division and lateral bud growth. Gibberellins promote stem elongation and seed germination. Ethylene inhibits cell expansion and accelerates senescence and fruit ripening. Abscisic acid promotes stomatal closure and inhibits seed germination.

1. Signaling
PRACHEE RAJPUT
zoologist

2. Signaling (hormones, light, etc…)
Signal
Receptor
No response
Receptor
Signal
Relay proteins
Reception Transduction Response
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
Hypocotyl
(embryonic stem)
Cotyledons
(embryonic leaves)
Root
Turn seedling 90o
Root shows a
positive gravitropic
response
Hypocotyl shows a
negative gravitropic
response
Areas of differential growth (one side
grows faster than the other)

8. Differential growth
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.
a
b

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. Fig. 15-12, p. 246
Example: Auxin promotes adventitious root formation
from Ilex opaca (Holly) shoots.
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. Fig. 15-13, p. 246
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.

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

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
Survival hormones (tend to inhibit
growth)
Hormones that promote/control
growth (direction)

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.