Maize Cyclin D2 Expression, Associated Kinase Activity and Effect of Phytohormones During Germination

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Abstract
A cDNA corresponding to 16 kDa of the maize cyclin D2 N-terminus was cloned and this polypeptide was overexpressed to produce homologous antibodies. This antibody recognized a 38 kDa protein in extracts from maize embryonic axes which corresponds to the predicted size for cyclin D2 protein. Expression of cyclin D2 was followed at the transcriptional and protein levels, and the effect of cytokinins and abscisic acid (ABA) was followed during maize germination. Cytokinins importantly stimulated cyclin D2 gene expression at late germination times and sucrose was necessary for stimulation, whereas the effect of ABA was not different from that in controls. However, cyclin D2 protein levels in control axes reached a peak at 6 h germination, declining thereafter, and neither cytokinins nor ABA modified this behavior. Two cyclic-dependent kinase A (Cdk-A)-type proteins and proliferating cell nuclear antigen (PCNA) were found co-immunoprecipitating with cyclin D2, and these immunoprecipitates were able to phosphorylate both histone H1 and the maize retinoblastoma-related protein (RBR). This protein kinase activity differed from the pattern of protein accumulation during germination, and the activity was not modified by either cytokinins or ABA. We discuss these findings in terms of the importance of the cell cycle for the germination process.