Signaling

PRACHEE RAJPUTzoologist

Signaling (hormones, light, etc…)

Signal

Receptor No response

Receptor

Signal

Relay proteins

Reception Transduction Response

Differential gene expression

Major signals that control plant growth and development

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

Major signals that control plant growth and development

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

Auxin

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

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)

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

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.

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.

Cytokinin

Zeatin

Zeatin is one of many natural cytokinins found in plants

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

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.

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

Gibberellin effects

- promotes stem elongation growth- promotes seed germination

Gibberellins promote stem elongation in many plant

species

Pea seedlingsPea seedlings treated with GA3

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

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)

Ethylene

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

Ethylene and senescence

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

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

Abscisic acid

Abscisic acid effects

- promotes stomatal closure- inhibits seed germination

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

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.