Plant Science and Molecular Biology

Plant Hormones

Plant hormones are essential regulators of plant development beginning with seed germination and culminating in whole-plant senescence. Until recently it was generally believed that the five classes of compounds comprising abscisic acid, auxin, cytokinins, ethylene, and gibberellins could account for most or all of the growth regulatory effects of plant hormones. During the past 5 years, however, application of molecular genetics and biochemical analysis to a number of dwarf mutants has revealed that another group of compounds, termed brassinosteroids (BRs), are as critical in the normal development of a plant as are the classical plant hormones. This article provides a brief history of BR research, summarizes the structure, natural occurrence, and biosynthesis of BRs, examines physiological responses to BRs along with practical agricultural applications, and surveys molecular approaches to understanding the mode of action of BRs in promoting elongation, division, and differentiation of cells in multiple developmental programs.