Background: Recent results raised doubts regarding the earlier proposed C-23 hydroxylase function of CYP90A1/CPD in brassinosteroid biosynthesis. Results: The enzymatic role of CYP90A1/CPD is determined by analytical, genetic, and direct biochemical approaches. Conclusion: CYP90A1 catalyzes C-3 oxidation of early brassinosteroid intermediates. Significance: Our results highlight the primary role of the campestanol-independent pathway in brassinosteroid biosynthesis.

Abstract:Brassinosteroids (BRs) participate in the regulation of important physiological processes, such as germination, photomorphogenesis, elongation, and the development of reproductive organs. Unlike other phytohormones, BRs are not subject to active transport within the plant, therefore, their levels are determined by the balance between local biosynthetic and inactivation reactions. BR accumulation shows good correlation with the induction of biosynthetic genes, which are stringently regulated, primarily at the transcriptional level. These genes function under developmental, organ-specific and diurnal control, and respond to homeostatic feedback adjustment. Excess amounts of bioactive BRs are removed by reversible or irreversible inactivation mechanisms. The regulation of BR-inactivating genes is also fairly complex and often opposite to those of the biosynthetic functions. Though many details are yet to be clarified, the known regulatory mechanisms of BR metabolism reveal a finely orchestrated system capable of ensuring optimal BR supply from germination to seed production.

the wild phenotype, expression from the photosynthesisassociated CAB3 and the vasculature-specific SUC2 and ATHB8 promoters resulted in plants with varying morphogenic defects. Our results reveal complex differential regulation of BRI1 expression, and suggest that by influencing the distribution and abundance of the receptor this regulation can enhance or attenuate BR signalling.