There are five major classes of plant hormones (Figure 23.25). They include the following:

1. The diterpene gibberellins are derived from isopentenyl pyrophosphate. Some gibberellins are also growth-promoting hormones and may stimulate expression of certain genes. Particular messenger RNAs are present in increased amounts as a result of gibberellin administration.

2. The sesquiterpene abscisic acid is also derived from isopentenyl pyrophosphate. Abscisic acid counteracts the effects of most other plant hormones. That is, it inhibits germination, growth, budding, and leaf senescence. Physiological evidence suggests a role for abscisic acid in ion and water balance.

3. The cytokinins are purine bases with a terpenoid side chain. Cytokinins are produced in the roots and promote growth and differentiation in many tissues. Cytokinins and auxins work together. The ratio of cytokinin to auxin is often crucial in determining whether a plant will grow or differentiate.

4. Of the auxins, the tryptophan metabolite indole-3-acetic acid is the most active. Auxins are synthesized in apical buds (at the tip) of growing shoots. They stimulate growth of the main shoot and inhibit lateral shoot development. A class of auxin-binding membrane proteins may represent auxin receptors. Auxin action involves pumping protons out of the cell, possibly in conjunction with a membrane ATPase.

5. Ethylene is a hydrocarbon that comes from the methionyl moiety of S-adenosylmethionine. Ethylene stimulates fruit ripening and the aging of flowers, and it inhibits seedling growth. It also redirects auxin transport to promote transverse, rather than longitudinal, growth of plants.

Ethylene and abscisic acid have been shown to function via signalling pathways that involve protein phosphorylation. The abscisic acid pathway involves a signalling component called cyclic ADP-ribose (cADPR), which regulates calcium release in animal cells.

Another plant protein is highly homologous to mammalian steroid 5 reductases (see here), which are involved in androgen synthesis. Plants lacking this protein have a growth defect, which can be reversed by brassinolide. Thus, the reductase-like protein is likely involved in the synthesis of a compound very similar (or identical) to brassinolide.

INTERNET LINK: Plant Hormones