Abstract
In higher plants, cis-zeatin (cZ) has been considered as low- or in-active cytokinin, based on traditional bioassays of cytokinin and studies using Arabidopsis. However, cZ and cZ derivates are accumulated in some plant species including maize and rice. In contract to known cytokinin receptors of Arabidopsis, maize cytokinin receptors are sensitive to cZ. Thus, further investigations of cZ is required to understand generality and diversity of the cytokinin metabolism and function in higher plants. In rice, cZ-O-glucoside (cZOG) and cZ riboside-O-glucoside (cZROG) are accumulated in high levels. In vitro assay of O-glucosyltransferase activity using recombinant protein showed that UDP-glucosyltransferase proteins encoded by at least three genes can catalyze O-glucosylation of cZ and cZ riboside (cZR). In transgenic rice plants ectopically overexpressing the each gene, accumulation of cZOG or cZROG was likely to be up-regulated. Together, we conclud that these UDP-glucosyltransferase proteins should function as cZ- or cZR-O-glucosyltransferases in rice.
