Activation of the cytochrome P450 gene, CYP72C1, reduces the levels of active brassinosteroids in vivo.

A pool of Arabidopsis lines transformed with the activation vector was screened for short hypocotyl mutants under dim far-red light, and three mutant lines designated chibi1-3 (chi) were isolated. Among the chi mutants, chi2 was dominant. The chi2 seedlings were short, regardless of the light conditions. The chi2 mature plants exhibited phenotypic features such as dwarfism, reduced male fertility and dark green, rounded epinastic leaves, which are characteristics of brassinosteroid-deficient mutants such as det2, cpd, and dwf4. Furthermore, the hypocotyl phenotype was restored by the addition of brassinolide to the culture medium, suggesting that brassinosteroids had been affected in this mutant. The molecular analysis of the chi2 mutant revealed that the CYP72C1 gene was overexpressed by the enhancing activity of the inserted DNA. Wild-type plants that were transformed with a vector containing a chimeric gene between the 35S promoter and the CYP72C1 genomic DNA exhibited a similar phenotype. Consistent with the morphological and physiological phenotype, the levels of active brassinosteroids were reduced in the chi2 mutant. Hence, CYP72C1, together with BAS1/CYP72B1, is speculated to regulate active brassinosteroid levels in plants. Expression analysis suggested that wild-type CYP72C1 transcript levels increased after exposure to white light, although the physiological significance of such a response remains obscure.