Studies on biosynthesis of brassinosteroids and its regulation using mutants

所有的油菜素类固醇（BR）矮化突变体分离到目前为止，表现出一个短的强大的身材，深绿色的叶子，和降低的生育能力。此外，大多数BR侏儒表现出长期的发展，并表现出不同程度的去黄化时，在黑暗中生长。为了更好地了解拟南芥BR的生物合成及其调控，人们对拟南芥BR突变体进行了研究，通过生化和分子生物学分析揭示了BR生物合成突变体的缺陷步骤和相关基因的功能。研究表明，在BRs的生物合成中，FACKEL、DWF 7、DWF 5、DIM（DWF 1）、DET 2和DWF 4等多种酶的编码基因分别参与C14还原、C5去饱和、C7还原、C24还原、C5α还原和C22α羟基化。此外，上述BR生物合成突变体的侏儒症被发现是由于缺乏生物活性BR，表明BR是正常生长和发育所必需的。BR-不敏感突变体，bril，也进行了表征。我们发现bril突变体积累了非常大量的BR，并且BR水平与等位基因和表型严重程度呈正相关，这表明BRI 1是内源性BR水平稳态所必需的。此外，分析了bas 1突变体（phyB激活标记的抑制子）。成人表型bas 1模仿BR生物合成和响应突变体。在bas 1矮秆被证明是由于通过羟基化的油菜素酶的失活。本研究还揭示了BAS 1基因可能是多个光感受器系统与BR信号转导之间的控制点之一，从而揭示了BR生物激活、催化和信号转导相关基因的功能。该研究还提供了令人信服的证据，表明BR是高等植物正常生长和发育所必需的植物激素。

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.