Multiple hormonal control in plants

Hormones are but one class of compounds to regulate developmental processes in plants; however, they are probably the most important mediators in signal transduction. The classic concept of a plant hormone defined by Went & Thimann (1937) as a substance being produced in one part of the organism and then transferred to another part to influence a specific physiological process is, as expected after more than 60 years of plant hormone research, inadequate to explain hormonal regulation of development in plants. The complexity of multiple hormonal regulation is illustrated by the fact that each of the hormones have been found to be able to affect nearly every phase of plant growth and development (Leopold & Nooden 1984). The seeming lack of exclusive control of any one step of development by any one of the known hormones as well as the lack of correlation between hormone concentrations and changes in developmental processes has led to doubts as to whether the plant hormones are indeed the regulators of development (Trewavas 1980, 1983). In this connection it was postulated that hormonal control in development may be due to changes in the sensitivity to the hormones (Kende & Lang 1964; Trewavas 1983). Recently, Bradford & Trewavas (1994) proposed a quantitative model based on sensitivity threshold distributions and proportional rate responses to regulatory factor levels, which can account for a wide range of phenomena in plant growth and development.