Brassinosteroid actions in plants

pollen tube growth, leaf bending and epinasty, root inhibition, induction of ethylene biosynthesis, protonRecent studies on dwarf mutants of Arabidopsis, pump activation, xylem differentiation, and regulation of tomato and pea have provided convincing evidence gene expression (Mandava, 1988; Clouse and Sasse, that brassinosteroids are a unique class of plant hor1998). In addition, useful agricultural applications have mones that are essential for normal plant growth. been found such as increasing yield and improving stress Detailed metabolic analyses of these mutants, coupled resistance of several major crop plants (Cutler et al., with thorough molecular and biochemical studies of 1991). Despite this extensive research, a definitive proof their corresponding genes and gene products, are that these steroids are essential for normal plant growth essential for a better understanding of brassinosteroid has been lacking. biosynthesis and its regulation. Molecular and genetic A clear role for these steroids in plant growth and approaches have been undertaken to dissect the development came from an entirely different field of brassinosteroid signalling pathway, leading to the studies: genetic analysis of plant photomorphogenesis. In identification of a putative brassinosteroid receptor plants, immediate post-germinative growth usually occurs and a few brassinosteroid-response genes. Further beneath the soil surface and newly germinated seedlings studies should expand our knowledge on how brassineed to bring their leaves into full sunlight so that they nosteroids are perceived and transduced to regulate can start photosynthesis to provide the essential raw plant development. materials needed for long-term growth. An etiolated seedling, with an apical hook, closed cotyledons, and a