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DEVELOPMENT AND HORMONE ACTION

BRASSINOSTEROID UPREGULATED1, Encoding a Helix-Loop-Helix Protein, Is a Novel Gene Involved in Brassinosteroid Signaling and Controls Bending of the Lamina Joint in Rice^1,[W],[OA]

Disease Resistance Research Unit (A.T., H.N., C.T., M.O., C.-J.J., J.G.D., M.M.) and Plant Genome Research Unit (S.K.), National Institute of Agrobiological Sciences, Tsukuba 305–8602, Japan; Graduate School of Science and Technology, Tokyo University of Science, Noda 278–8510, Japan (A.T., T.K.); Institute of Society for Techno-Innovation of Agriculture, Forestry and Fisheries, Tsukuba 305–0854, Japan (Z.S.); Faculty of Agriculture, Utsunomiya University, Utsunomiya 321–8505, Japan (C.T., H.S.); Department of Bioscience, Teikyo University, Utsunomiya 320–8551, Japan (T.N., T.Y.); and Department of Applied Biological Chemistry, University of Tokyo, Tokyo 113–8657, Japan (T.A.)

Brassinosteroids (BRs) are involved in many developmental processes and regulate many subsets of downstream genes throughout the plant kingdom. However, little is known about the BR signal transduction and response network in monocots. To identify novel BR-related genes in rice (Oryza sativa), we monitored the transcriptomic response of the brassinosteroid deficient1 (brd1) mutant, with a defective BR biosynthetic gene, to brassinolide treatment. Here, we describe a novel BR-induced rice gene BRASSINOSTEROID UPREGULATED1 (BU1), encoding a helix-loop-helix protein. Rice plants overexpressing BU1 (BU1:OX) showed enhanced bending of the lamina joint, increased grain size, and resistance to brassinazole, an inhibitor of BR biosynthesis. In contrast to BU1:OX, RNAi plants designed to repress both BU1 and its homologs displayed erect leaves. In addition, compared to the wild type, the induction of BU1 by exogenous brassinolide did not require de novo protein synthesis and it was weaker in a BR receptor mutant OsbriI (Oryza sativa brassinosteroid insensitive1, d61) and a rice G protein alpha subunit (RGA1) mutant d1. These results indicate that BU1 protein is a positive regulator of BR response: it controls bending of the lamina joint in rice and it is a novel primary response gene that participates in two BR signaling pathways through OsBRI1 and RGA1. Furthermore, expression analyses showed that BU1 is expressed in several organs including lamina joint, phloem, and epithelial cells in embryos. These results indicate that BU1 may participate in some other unknown processes modulated by BR in rice.

² Present address: National Institute for Basic Biology, Okazaki 444–8585, Japan.

³ Present address: Weed Science Center, Utsunomiya University, Utsunomiya 321–8505, Japan.

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: Masaki Mori (morimasa{at}affrc.go.jp).

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

^[OA] Open access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.140806

^* Corresponding author; e-mail morimasa{at}affrc.go.jp.

Received May 3, 2009; accepted July 29, 2009; published July 31, 2009.