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Plant and Cell Physiology Advance Access originally published online on July 15, 2009
Plant and Cell Physiology 2009 50(9):1627-1635; doi:10.1093/pcp/pcp106
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Involvement of C-22-Hydroxylated Brassinosteroids in Auxin-Induced Lamina Joint Bending in Rice

Ayako Nakamura1,7,*, Shozo Fujioka1, Suguru Takatsuto2, Masafumi Tsujimoto1, Hidemi Kitano3, Shigeo Yoshida4, Tadao Asami1,5 and Takeshi Nakano1,6

1RIKEN Adavanced Science Institute, Wako, Saitama, 351-0198 Japan
2Department of Chemistry, Joetsu University of Education, Joetsu, Niigata, 943-8512 Japan
3Bioscience and Biotechnology Center, Nagoya University, Chikusa, Nagoya, Aichi, 464-8601 Japan
4RIKEN Plant Science Center, Tsurumi, Yokohama, Kanagawa, 230-0045 Japan
5Graduate School of Agricultural and Life Science, Tokyo University, Yayoi, Bunkyo, Tokyo, 113-8657 Japan
6JST-PRESTO, Kawaguchi, Saitama, 332-0012 Japan

*Corresponding author: E-mail, iripple1103{at}gmail.com; Fax, +81-3-5841-8466.


  Abstract

The rice lamina joint is ideal material for investigating the activity of brassinosteroids (BRs) and auxin because of its high sensitivity to these compounds. Using a series of rice BR biosynthetic and receptor mutants, we conducted lamina joint tests to elucidate the mechanism of cross-talk between BR and auxin signaling in lamina joint bending. In BR biosynthetic mutants d2 and brd1, which are defective in C-23 hydroxylase and C-6 oxidase, respectively, the lamina joint response to auxin was significantly higher than that of wild-type plants. The other BR-biosynthetic mutants, brd2, osdwarf4 and d11, which are defective in C-22-hydroxylated BRs, showed less or no response to auxin. These results suggest that C-22-hydroxylated BRs are involved in auxin-induced lamina joint bending. The results were supported by the observation that inhibition of the hyper-response to auxin in d2 was reduced by treatment with brassinazole, which inhibits the function of DWARF4, the C-22 hydroxylase. In d61, which is defective in OsBRI1, a possible BR receptor in rice, the bending angle of the lamina joint in response to auxin and C-22-hydroxylated 6-deoxoBRs was nearly the same as that in wild-type plants. This implies that C-22-hydroxylated BRs function in auxin signaling independently of OsBRI1. From these observations, we propose that C-22-hydroxylated BRs participate in auxin signaling via a novel OsBRI1-independent signaling pathway.

Keywords: Auxin - Brassinosteroid - Lamina joint - Rice

Abbreviations: BL, brassinolide; BR, brassinosteroid; Brz, brassinazole; CS, castasterone; CT, cathasterone; 3DT, 3-dehydroteasterone; 22-OHCR, (22S)-22-hydroxycampes-terol; TE, teasterone; TY, typhasterol.

7Present address: Graduate School of Science and Engineering, Saitama University Shimo-okubo, Sakura Saitama, Saitama, 338-8570 Japan.

(Received June 8, 2009; Accepted July 10, 2009)
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