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

• Environmental signals:
- Light
- Gravity
- Temperature
- Humidity
etc…

• Internal signals: Plant Hormones
- AUXIN
- CYTOKININ
- ETHYLENE
- ABSCISIC ACID
- GIBBERELLIC ACID

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

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

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

Differential growth b a
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.

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)

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.

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

Zeatin is one of many natural cytokinins found in plants
Zeatin

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

auxin cytokinin

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

The effect of cytokinin on senescence.
Cytokinin applied to the righthand primary leaf of this bean seedling inhibited its senescence. The left-hand did not get cytokinin.
Fig. 15-13, p. 246

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

Gibberellin effects
- promotes stem elongation growth
- promotes seed germination

Pea seedlings treated with GA
3
Pea seedlings

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

• The major plant hormones:
- Auxins
- Cytokinins
Hormones that promote/control growth (direction)
- Gibberellins
- Abscisic acid
- Ethylene
Survival hormones (tend to inhibit growth)

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

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

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

– 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

Abscisic acid effects
- promotes stomatal closure
- inhibits seed germination

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 H
2
O, thus rapidly reducing turgor pressure and closing the stomata

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.