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SYSTEMS BIOLOGY, MOLECULAR BIOLOGY, AND GENE REGULATION

Diurnal Regulation of the Brassinosteroid-Biosynthetic CPD Gene in Arabidopsis^1,[W]

Institute of Plant Biology, Biological Research Center of the Hungarian Academy of Sciences, H–6701 Szeged, Hungary (S.B., A.-M.S., L.K.-B., F.N., M.S.); Institute of Biological Sciences, University of Wales, Aberystwyth SY23 3DD, United Kingdom (J.C., G.J.B.); Division of Biology, Imperial College London, Wye Campus, Wye TN25 5AH, United Kingdom (G.J.B.); and Department of Biosciences, Teikyo University, Utsunomiya 320–8551, Japan (K.S., T.Y.)

Plant steroid hormones, brassinosteroids (BRs), are essential for normal photomorphogenesis. However, the mechanism by which light controls physiological functions via BRs is not well understood. Using transgenic plants carrying promoter-luciferase reporter gene fusions, we show that in Arabidopsis (Arabidopsis thaliana) the BR-biosynthetic CPD and CYP85A2 genes are under diurnal regulation. The complex diurnal expression profile of CPD is determined by dual, light-dependent, and circadian control. The severely decreased expression level of CPD in phytochrome-deficient background and the red light-specific induction in wild-type plants suggest that light regulation of CPD is primarily mediated by phytochrome signaling. The diurnal rhythmicity of CPD expression is maintained in brassinosteroid insensitive 1 transgenic seedlings, indicating that its transcriptional control is independent of hormonal feedback regulation. Diurnal changes in the expression of CPD and CYP85A2 are accompanied by changes of the endogenous BR content during the day, leading to brassinolide accumulation at the middle of the light phase. We also show that CPD expression is repressed in extended darkness in a BR feedback-dependent manner. In the dark the level of the bioactive hormone did not increase; therefore, our data strongly suggest that light also influences the sensitivity of plants to BRs.

² These authors contributed equally to the paper.

³ Present address: David H. Smith Center for Vaccine Biology and Immunology, Aab Institute for Biomedical Sciences, University of Rochester Medical Center, 601 Elmwood Ave., Rochester, NY 14642.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Miklós Szekeres (szekeres{at}nucleus.szbk.u-szeged.hu).

^[W] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.106.079145.

^* Corresponding author; e-mail szekeres{at}nucleus.szbk.u-szeged.hu; fax 36–62–433434.

Received February 13, 2006; returned for revision February 13, 2006; accepted February 28, 2006.

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