Plant hormones

Plant hormones have been studied for more than a century because they have been heavily implicated in the control of plant growth and development. For many years, they have been considered to fall into five classes: auxins, cytokinins, gibberellins, abscisic acid and ethylene. More recently, a variety of additional classes such as brassinosteroids have been added and it seems likely that more will follow. It is difficult to produce a watertight definition of a plant hormone, and in this context some find the use of the word ‘hormone’ confusing, as any analogy with animal hormones is superficial. Although plant hormones co-ordinate plant growth and development, they do not carry specific signals, from specific tissues, to specific tissues. Perhaps a better analogy would be to compare plant hormones to animal nervous systems, which collect information from a variety of souces, process it and direct appropriate responses. Because of the nature of their role, it is not helpful to produce a list of events in which plant hormones have been implicated. This would be like providing a list of the many physiological and behavioural responses to, for example, hunger in animals. Rather than provide such a list, this article attempts to provide a conceptual framework for plant hormone biology.