title: Ectopic expression of KNOTTED1-like homeobox protein induces expression of cytokinin biosynthesis genes in rice.
abstract: 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.
authors.affiliation: Field Production Science Center, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Nishi-Tokyo, Tokyo 188-0002, Japan.
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Sakamoto
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Sakakibara
authors.forename: Mikiko
authors.initials: M
authors.lastname: Kojima
authors.forename: Yuko
authors.initials: Y
authors.lastname: Yamamoto
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Nagasaki
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Inukai
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Sato
authors.forename: Makoto
authors.initials: M
authors.lastname: Matsuoka
mesh_terms: D019883:Alkyl and Aryl Transferases / Q000235:genetics / Q000378:metabolism; D000595:Amino Acid Sequence; D003583:Cytokinins / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D018398:Homeodomain Proteins / Q000502:physiology*; D008969:Molecular Sequence Data; D012275:Oryza / Q000235:genetics / Q000378:metabolism*; D010940:Plant Proteins / Q000502:physiology*; D030821:Plants, Genetically Modified / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism
chemical_list: D003583:Cytokinins; D018398:Homeodomain Proteins; C405985:Knox1 protein, plant; D010940:Plant Proteins; D011994:Recombinant Proteins; D019883:Alkyl and Aryl Transferases; C024274:adenylate isopentenyltransferase