Physiology and molecular mode of action of brassinosteroids

Brassinosteroids (BRs) comprise a group of polyhydroxysteroids, which show close structural similarity to steroid hormones from arthropods and mammals . BRs are now accepted as a new class of phytohormones due to their ubiquit ous occurrence in plants, their highly effective elicitation of various res ponses and the identification of mutants defective in BR-biosynthesis or -r esponse. Important steps of BR-biosynthesis were elucidated with precursor- feeding experiments and by the analysis of BR-biosynthesis-deficient mutant s. The altered phenotypes of these mutants, particularly in Arabidopsis, re vealed the essential nature of BRs for normal growth and development. A maj or role of BRs is the positive regulation of cell expansion. Furthermore, B Rs modulate plant responses to biotic and abiotic stresses and to other phy tohormones, and influence differentiation processes of cells and tissues. B R-insensitive mutants such as bri1 hold the potential for uncovering BR-sig nalling pathway(s) at the molecular level. The identification of BR-regulat ed genes demonstrates a genetic basis for BR mode of action with reference to their multiple effects. This review focuses on the relevance of BRs to t he control of various physiological processes, BR-signalling and underlying molecular mechanisms by considering known mutants. (C) Elsevier, Paris.