BAS1 and SOB7 act redundantly to modulate Arabidopsis photomorphogenesis via unique brassinosteroid inactivation mechanisms

Active brassinosteroids (BRs), such as brassinolide (BL) and castasterone (CS), are growth-promoting plant hormones. An Arabidopsis cytochrome P450 monooxygenase (CYP734A1, formerly CYP72B1), encoded by the BAS1 gene, inactivates BRs and modulates photomorphogenesis. BAS1 was identified as the overexpressed gene responsible for a dominant, BR-deficient mutant, bas1-D. This mutant was isolated in an activation-tagged screen designed to identify redundant genes that might not be identified in classic loss-of-function screens. Here we report the isolation of a second activation-tagged mutant with a BR-deficient phenotype. The mutant phenotype is caused by the overexpression of SOB7 (CYP72C1), a homolog of BAS1. We generated single and double null-mutants of BAS1 and SOB7 to test the hypothesis that these two genes act redundantly to modulate photomorphogenesis. BAS1 and SOB7 act redundantly with respect to light promotion of cotyledon expansion, repression of hypocotyl elongation and flowering time in addition to other phenotypes not regulated by light. We also provide biochemical evidence to suggest that BAS1 and SOB7 act redundantly to reduce the level of active BRs, but have unique mechanisms. Overexpression of SOB7 results in a dramatic reduction in endogenous CS levels, and although single null-mutants of BAS1 and SOB7 have the same level of CS as the wild type, the double null-mutant has twice the amount. Application of BL to overexpression lines of BAS1 or SOB7 results in enhanced metabolism of BL, though only BAS1 overexpression lines confer enhanced conversion to 26-OHBL, suggesting that SOB7 and BAS1 convert BL and CS into unique products.

CYP72B1 inactivates brassinosteroid hormones: an intersection between photomorphogenesis and plant steroid signal transduction

Active brassinosteroids, such as brassinolide (BL) and castasterone, are growth promoting plant hormones. An Arabidopsis cytochrome p450 monooxygenase encoded by CYP72B1 has been implicated in brassinosteroid catabolism as well as photomorphogenesis. We expressed CYP72B1 in yeast, coupled with brassinosteroid feeding, and established the biochemical function to be the hydroxylation of BL and castasterone, to give 26-hydroxybrassinolide and 26-hydroxycastasterone, respectively. Brassinosteroid feeding experiments with wild-type Arabidopsis, a CYP72B1 null mutant, and a CYP72B1 overexpression line demonstrated that carbon 26 hydroxylation of active brassinosteroids is an endogenous function of CYP72B1. Seedling growth assays demonstrated that 26-hydroxybrassinolide is an inactive brassinosteroid. Genetic and physiological analysis of the hypocotyl response to exogenous BL and varying intensities of white and monochromatic light suggested that CYP72B1 modulates photomorphogenesis primarily through far-red light and to a lesser extent through blue- and red-light pathways. CYP72B1 transcript accumulation in dark-grown seedlings was organ specific and down-regulated after 1 h of illumination in dim white, red, and blue light, but not far-red light. CYP72B1 translational fusions with the beta-glucuronidase reporter gene demonstrated that protein levels increased in the hypocotyl elongation zone when shifted from the dark to far-red light, but not blue or red light. We propose a model in which Arabidopsis seedling development switches from dark-grown development (skotomorphogenesis) to light-grown development (photomorphogenesis) in part by rapid modulation of brassinosteroid sensitivity and levels. CYP72B1 provides an intersection between the light and brassinosteroid pathways mainly by far-red-light-dependent modulation of brassinosteroid levels.