Plant Growth Regulators II: Cytokinins, their Analogues and Antagonists

Hormones in plants differ from most of those in animals by having pleiotropic effects; that is, they are involved in the control of a wide range of developmental processes. At the same time the effect of a hormone on any developmental process depends on the species. For example, ethylene inhibits growth in dicotyledons and most monocotyledons but is promotory in deepwater rice and other hydrophytes. Moreover, two or more hormones can interact synergistically or antagonistically in many circumstances. Equally, any given hormone may affect the biosynthesis or metabolism of another, thus affecting endogenous levels. The issue is further complicated by the fact that environmental factors e.g. light, water status, wounding, pathogens may modify responses and indeed hormone levels themselves. The reason for this appears to be that hormones (and growth regulators) and environmental factors share many components in their transduction chains (i.e. the very early events which occur after the signal abiotic or biotic is perceived by the plant tissue). These transduction chains interact to produce an integrated response.