Cytokinin–induced Transcriptional Activation of NADPH–protochlorophyllide Oxidoreductase Gene in Cucumber

), BA remarkably increased the level of mRNA of NADPH-protochlorophyllide oxidoreductase (POR, EC1.3.1.33), which catalyzes the photoreduction of protochlorophyllide a to chlorophyllide a. In this study, we examined the regulatory mechanism of the BA–induced expression of the POR gene in more detail. In etiolated cotyledons of cucumber, the levels of POR mRNA and its protein were remarkably increased by BA treatment dose- and time-dependently. The increase was more marked in the mRNA than the protein. The BA-induced increase in the mRNA was repressed by treatment with cycloheximide. A run–on assay showed that BA increases the POR mRNA at least at the transcriptional level. Moreover, a transient reporter assay of the 5'- upstream region of the POR gene reveled this region to contain a cis–acting element(s) for cytokinin response. These results indicated that the BA–induced increase in the mRNA is mainly dependent on transcriptional activation, which accompanies de novo protein synthesis in cytoplasm.