A Novel Cytochrome P450 Is Implicated in Brassinosteroid Biosynthesis via the Characterization of a Rice Dwarf Mutant, <i>dwarf11</i>, with Reduced Seed Length

We have characterized a rice (Oryza sativa) dwarf mutant, dwarf11 (d11), that bears seeds of reduced length. To understand the mechanism by which seed length is regulated, the D11 gene was isolated by a map-based cloning method. The gene was found to encode a novel cytochrome P450 (CYP724B1), which showed homology to enzymes involved in brassinosteroid (BR) biosynthesis. The dwarf phenotype of d11 mutants was restored by the application of the brassinolide (BL). Compared with wild-type plants, the aberrant D11 mRNA accumulated at higher levels in d11 mutants and was dramatically reduced by treatment with BL, implying that the gene is feedback-regulated by BL. Precise determination of the defective step(s) in BR synthesis in d11 mutants proved intractable because of tissue specificity and the complex control of BR accumulation in plants. However, 6-deoxotyphasterol (6-DeoxoTY) and typhasterol (TY), but not any upstream intermediates before these compounds, effectively restored BR response in d11 mutants in a lamina joint bending assay. Multiple lines of evidence together suggest that the D11/CYP724B1 gene plays a role in BR synthesis and may be involved in the supply of 6-DeoxoTY and TY in the BR biosynthesis network in rice.