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

• Published in: The Plant journal : for cell and molecular biology Vol. 42; no. 1; pp. 23 - 34
• Main Authors: Turk, E.M, Fujioka, S, Seto, H, Shimada, Y, Takatsuto, S, Yoshida, S, Wang, H, Torres, Q.I, Ward, J.M, Murthy, G
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01.04.2005; Blackwell Science; Blackwell Publishing Ltd
• Subjects: 24-methylenecholesterol; 3-dehydro-6-deoxoteasterone; 3-dehydroteasterone; 6-deoxocastasterone; 6-deoxocathasterone; 6-deoxotyphasterol; 6-oxocampestanol; activation‐tagging; Amino Acid Sequence; amino acid sequences; Arabidopsis; Arabidopsis - growth & development; Arabidopsis - metabolism; Arabidopsis - physiology; Arabidopsis Proteins - metabolism; Arabidopsis Proteins - physiology; Arabidopsis thaliana; Biological and medical sciences; Brassinosteroids; campesterol; castasterone; cathasterone; Chemical agents; Cholestanols; cytochrome P-450; Cytochrome P-450 Enzyme System - physiology; cytochrome P450; Fundamental and applied biological sciences. Psychology; Gene expression; gene expression regulation; Gene Expression Regulation, Developmental - physiology; Gene Expression Regulation, Plant - physiology; gene overexpression; insertional mutagenesis; Light; lighting; Molecular Sequence Data; Mutation; nucleotide sequences; oxygenases; Peroxidases - physiology; Peroxiredoxins; Phenotype; photomorphogenesis; phyb-4 activation-tagged suppressor 1 gene; Plant growth; Plant physiology and development; plant proteins; Sequence Alignment; Steroids; Steroids, Heterocyclic; suppressor of phyb-47 gene; teasterone; transfer DNA; typhasterol; Vegetative apparatus, growth and morphogenesis. Senescence; Light effect; brassinosteroids; Growth; cytochrome P450; Gene expression; Metabolism; Genetic determinism; Arabidopsis thaliana; Morphogenesis; Brassinosteroid; photomorphogenesis; Hypocotyl; Gene; Cruciferae; Dicotyledones; Angiospermae; Plant growth substance; Spermatophyta; activation-tagging; Mutation
• ISSN: 0960-7412; 1365-313X
• DOI: 10.1111/j.1365-313x.2005.02358.x

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.