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First published online July 21, 2006; 10.1104/pp.106.085811 Plant Physiology 142:54-62 (2006) © 2006 American Society of Plant Biologists Ectopic Expression of KNOTTED1-Like Homeobox Protein Induces Expression of Cytokinin Biosynthesis Genes in Rice1,[W]Field Production Science Center, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Nishi-Tokyo, Tokyo 188–0002, Japan (T.S.); Institute of Physical and Chemical Research Plant Science Center, Yokohama, Kanagawa 230–0045, Japan (H.S., M.K.); and Graduate School of Bioagricultural Sciences (H.N., Y.I., Y.S.) and Bioscience and Biotechnology Center (Y.Y., M.M.), Nagoya University, Nagoya, Aichi 464–8601, Japan
Some phytohormones such as gibberellins (GAs) and cytokinins (CKs) are potential targets of the KNOTTED1-like homeobox (KNOX) protein. To enhance our understanding of KNOX protein function in plant development, we identified rice (Oryza sativa) genes for adenosine phosphate isopentenyltransferase (IPT), which catalyzes the rate-limiting step of CK biosynthesis. Molecular and biochemical studies revealed that there are eight IPT genes, OsIPT1 to OsIPT8, in the rice genome, including a pseudogene, OsIPT6. Overexpression of OsIPTs in transgenic rice inhibited root development and promoted axillary bud growth, indicating that OsIPTs are functional in vivo. Phenotypes of OsIPT overexpressers resembled those of KNOX-overproducing transgenic rice, although OsIPT overexpressers did not form roots or ectopic meristems, both of which are observed in KNOX overproducers. Expression of two OsIPT genes, OsIPT2 and OsIPT3, was up-regulated in response to the induction of KNOX protein function with similar kinetics to those of down-regulation of GA 20-oxidase genes, target genes of KNOX proteins in dicots. However, expression of these two OsIPT genes was not regulated in a feedback manner. These results suggest that OsIPT2 and OsIPT3 have unique roles in the developmental process, which is controlled by KNOX proteins, rather than in the maintenance of bioactive CK levels in rice. On the basis of these findings, we concluded that KNOX protein simultaneously decreases GA biosynthesis and increases de novo CK biosynthesis through the induction of OsIPT2 and OsIPT3 expression, and the resulting high-CK and low-GA condition is required for formation and maintenance of the meristem.
1 This work was supported by the Ministry of Agriculture, Forestry, and Fisheries of Japan (Rice Genome Project IP–1010 to T.S. and Rice Genome Project IP–3003 to H.S.), by the Ministry of Education, Culture, Sports, Science and Technology of Japan (to H.S.), and by a Grant-in-Aid for the Center of Excellence from the Ministry of Education, Culture, Sports, Science and Technology of Japan (to M.M.). 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: Makoto Matuoka (makoto{at}nuagr1.agr.nagoya-u.ac.jp). [W] The online version of this article contains Web-only data. www.plantphysiol.org/cgi/doi/10.1104/pp.106.085811 * Corresponding author; e-mail makoto{at}nuagr1.agr.nagoya-u.ac.jp; fax 81–52–789–5226. Received June 26, 2006; accepted July 13, 2006. Related articles in Plant Physiol.:
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