TRANSCRIPTION OF THE ARABIDOPSIS CPD GENE, ENCODING A STEROIDOGENIC CYTOCHROME-P450, IS NEGATIVELY CONTROLLED BY BRASSINOSTEROIDS

The Arabidopsis CPD gene encodes a cytochrome P450 steroid side-chain hydroxylase (CYP90) that plays an essential role in the biosynthesis o f the plant hormone brassinolide. Expression of the CPD gene is confin ed to cotyledons and leaf primordia in etiolated seedlings and detecta ble in the adaxial parenchyma of expanding leaves in light-grown plant s. Transcription of the CPD gene is not affected by the plant growth f actors auxin, ethylene, gibberellin, cytokinin, jasmonic acid and sali cylic acid, but is specifically down-regulated by brassinolide in both dark and light. Steady-state mRNA levels of a CPD promoter-driven uid A reporter gene correlate with the expression of resident CPD gene in transgenic plants. Intermediates of the early and late C-6 oxidation p athways of brassinolide, carrying C-22 and C-23 side-chain hydroxyls, efficiently inhibit the activity of the CPD promoter. Repression of CP D transcription by brassinosteroids is sensitive to the protein synthe sis inhibitor cycloheximide, indicating a requirement for de novo synt hesis of a regulatory factor.