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.
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
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)
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
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
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.
Notes de l'éditeur
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.
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.
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.