Studies on biosynthesis of brassinosteroids and its regulation using mutants

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All of the brassinosteroid (BR) dwarf mutants isolated so far exhibit a short robust stature, dark-green leaves, and reduced fertility. In addition, most of the BR dwarfs display prolonged development, and exhibit varying degrees of de-etiolation when grown in the dark. Studies on BR mutants of Arabidopsis have been carried out to better understand BR biosynthesis and its regulation.Biochemical and molecular analyses disclosed defective steps of BR biosynthetic mutants, and functions of the genes. The genes encoding many enzymes in the biosynthesis of BRs, FACKEL, DWF7, DWF5, DIM (DWF1), DET2, and DWF4 were shown to be involved in the C14-reduction, C5-desaturation, C7-reduction, C24-reduction, C5α-reduction, and C22α-hydroxylation, respectively. In addition, dwarfism in the above BR biosynthetic mutants was found to be due to a deficiency of bioactive BRs, indicating that BRs are required for proper growth and development. BR-insensitive mutants, bril, were also characterized. We showed that bril mutants accumulate very large amounts of BRs, and that the levels of BRs are positively correlated with allele and phenotypic severity, suggesting that BRI1 is required for the homeostasis of endogenous BR levels. Furthermore, bas1 mutant (phyB activation-tagged suppressor) was analyzed. The adult phenotype of bas1 mimics that of BR biosynthetic and response mutants. Dwarfism in bas1 was shown to be due to an inactivation of brassinolide by means of hydroxylation. The study also revealed that BAS1 represents one of the control points between multiple photoreceptor systems and BR signal transduction.In conclusion, the present study disclosed the functions of the genes involved in BR biosynthsis, catabolism and signal trasduction. The study also provided the compelling evidence that BRs are essential plant hormones for proper growth and development in higher plants.