Plant hormones - Auxin

Chemical communication in plants

Plant hormones or plant growth regulators are:
- produced in a variety of tissues
- move from cell to cell (by diffusion or active transport) or carried in xylem sap and phloem sap

The two types we're going to look at auxin and gibberellin.

Auxin

Synthesized in meristems (the growing tips of shoots and tips of roots, where cell division occurs). It gets actively transported away from the meristems, from cell to cell and via phloem sap.

Function: controlling growth

1. cell division: mitosis
2. cell elongation (by absorption of water; auxin is involved)
3. cell differentiation

Mechanism of auxin

1. Auxin binds to receptor

2. Stimulates ATPase to pump H⁺ across the cell surface membrane (from cytoplasm to cell wall, therefore lowering the pH of the cell wall) 

3. Potassium ion channels stimulated to open; K⁺ move into the cytoplasm, therefore lowering the water potential inside the cytoplasm --> water moves into the cytoplasm via osmosis

4. Expansins (proteins) are activated by the decrease in pH. Expansins loosen linkages between cellulose microfibrils.

5. Disruption occurs briefly --> cells expands without losing overall wall strength

15.2  Control and co-ordination in plants Plant co-ordination systems involve rapid responses as in the case of the Venus fly trap, but also complex interactions between plant growth regulators, such as auxin and gibberellin. Plants respond quite differently to different concentrations of plant growth regulators. a) describe the rapid response of the Venus fly trap to stimulation of hairs on the lobes of modified leaves and explain how the closure of the trap is achieved b) explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls c) describe the role of gibberellin in the germination of wheat or barley d) explain the role of gibberellin in stem elongation including the role of the dominant allele, Le, that codes for a functioning enzyme in the gibberellin synthesis pathway, and the recessive allele, le, that codes for a non-functional enzyme