Phytohormones contain a variety of very different compounds
Phytohormones (Fig. 19.6) have very diverse structures and functions. Only a few examples of these functions will be summarized here. Indole acetic acid, an auxin derived from indole, stimulates cell elongation. Gibberellins, derivatives of gibberellane, induce elongation growth of internodes. Zeatin, a cytokinin, is a prenylated adenine and stimulates cell division. Abscisic acid, which is formed from carotenoids, regulates the water balance. Ethylene and jasmonic acid enhance senescence; methyl jasmonate plays a role in pathogene defense. Brassinosteroids have a key function in the regulation of cell devel-opment. Peptide hormones regulate plant development, and, in addition to salicylic acid and jasmonic acid, play a role in pathogen defense. In many cases, phytohormone function is caused by a pair of antagonistic phyto-hormones. Thus abscisic acid induces seed dormancy, and gibberellic acid terminates it.