Meristem construction and control of CDK activities in plants

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In this project, we aimed to reveal the regulatory :mechanisms underlying CDK activities that control cell proliferation in plant. meristems. For the full activation of CDKs, not only cyclin binding but also phosphorylation is required. This phosphorylation is catalyzed by CDK-activating kinase (CAK). In Arabidopsis, there exist four CAKs, CAKI to CAK4. Among them, CAK2 and CAK4 can be classified into the vertebrate-type CAK in terms of enzyme activity, whereas each shows different substrate specificies. Namely, CAK2 and CAK4 preferentially phosphorylated CDK and the carboxy-terminal domain (TD) of the largest subunit of RNA polymerase II, respectively. Moreover, we found that CAK 1 phosphorylates CAK2 and CAK4, and activate the CTD-kinase activity of CAK4. These results suggested that Arabidopsis has a CDK-and CTD-phosphorylation cascade mediated by three CAKs.We overexpressed the cDNA of a B2-type cyclin, CycB2;2, in rice plants by using a glucocorticoid induction system. Our analyses revealed that the overexpression resulted in acceleration of root growth without any change in cell elongation. This indicated that cell proliferation in the root meristem had been activated by CYCB2;2 overexpression. By in vitro pull-down experiments, we showed that CYCB2;2 specifically interacted with CDKB2, and this interaction enhanced the kinase activity of CDKB2. Based on these data, we concluded that CYCB2;2 overexpression up-regulated the CDKB2 activity, which in turn promoted the transition from G2-to-M phase to accelerate the cell cycle duration in the root meristem.