pollen tube growth, leaf bending and epinasty, root inhibition, induction of ethylene biosynthesis, proton-Recent 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 hor-1998). 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 their corresponding genes and gene products, are 1991). Despite this extensive research, a definitive proof essential for a better understanding of brassinosteroid that these steroids are essential for normal plant growth biosynthesis and its regulation. Molecular and genetic has been lacking. approaches have been undertaken to dissect the A clear role for these steroids in plant growth and brassinosteroid signalling pathway, leading to the development came from an entirely different field of identification of a putative brassinosteroid receptor studies: genetic analysis of plant photomorphogenesis. In and a few brassinosteroid-response genes. Further plants, immediate post-germinative growth usually occurs studies should expand our knowledge on how brassi- beneath the soil surface and newly germinated seedlings nosteroids are perceived and transduced to regulate need to bring their leaves into full sunlight so that they plant development. can start photosynthesis to provide the essential raw materials needed for long-term growth. An etiolated Key words: Brassinosteroid, biosynthesis, signal transduc- seedling, with an apical hook, closed cotyledons, and a tion, dwarf, receptor kinase. rapidly elongating hypocotyl, can reach the soil surface quickly without damaging its young leaves. Once the
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J Li (1999) studied this question.