Plant Physiology Preview
Published on July 21, 2006; 10.1104/pp.106.085811
Received June 26, 2006
Accepted July 13, 2006
Ectopic Expression of KNOX Homeodomain Protein Induces Expression of Cytokinin Biosynthesis Gene in Rice
Tomoaki Sakamoto , Hitoshi Sakakibara , Mikiko Kojima , Yuko Yamamoto , Hiroshi Nagasaki , Yoshiaki Inukai , Yutaka Sato , and Makoto Matsuoka *
Field Production Science Center, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Nishi-Tokyo, Tokyo 188-0002, Japan
RIKEN Plant Science Center, Yokohama, Kanagawa 230-0045, Japan
Bioscience and Biotechnology Center, Nagoya University, Nagoya, Aichi 464-8601, Japan
Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Aichi 464-8601, Japan
* Corresponding author; email: makoto{at}nuagr1.agr.nagoya-u.ac.jp.
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 L.) genes for adenosine phosphate isopentenyltransferase (IPT), which catalyzes the rate-limiting step of CK biosynthesis. Molecular and biochemical studies revealed that there are 8 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 overexpressors resembled those of KNOX-overproducing transgenic rice, although OsIPT overexpressors 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 by 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.
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