Molecular mechanisms controlled by cytokinin ofM phase progression in division cycle at tobacco BY-2 cells

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The aim of the present research is to identify processes in M phase of division cycle of tobacco BY-2 cells that might be regulated by cytokinin. In particular, we have sought for target molecular events of cytokinin during phase focusing on expression of Ntmyb genes that are required for transcription of genes during G2/M phase; activation and inactivation of CDK (cyclin-dependent protein kinase); formation &the NACK/MAPKKK complex that is required for activation of the MAP kinase cascade controlling cytokinesis; the activation of the MAP kinase. We have examined effects of depletion of cytokinin and those of expression of genes for cytokinin oxydase, which can degrade this phytohormone, on the M phase progression of BY-2 cells. Our results showed no obvious difference between patterns of M phase progression of BY2 cells incubated with and without cytokinin, respectively. When the cytokinin oxydase genes were introduced and expressed in transgenic Arabidopsis plants, levels of transcr … More ipts of G2/M-phase-specific genes, however, decreased. We also have compared the pattern of the progression of M phase of synchronized BY-2 cells incubated with hormone-free medium to that incubated with trans-zeatin-containing medium. We did not observe clear difference between two patterns obtained in these experiments. We found similar patterns of CDK activation and inactivation when we incubated BY-2 cells with or without cytokinin. Unexpectedly, we, however, found that the level of NACK1 protein increased by 2-fold in the hormone-free medium. Further investigations must be required for understanding of physiological relevance of the unexpected observations obtained by this research project. Although our results did not dearly show target processes of cytokinin during M phase progression, our researches related to cytokinesis have contributed to understanding of new aspects of regulatory mechanisms of cytokinesis in plant cells: (1) CDK phosphorylated both NACK1 and MAPKKK before metaphase, which resulted in interfering the formation of the NACK/MAPKKK complex and after metaphase, these proteins were dephosphorylated, which induced the interaction of these proteins that activated the MAP kinase cascade, leading to the progression of cytokinesis; (2) MYB factors that positively or negatively controlled G2/M-phase-specific transcription in Arabidopsis plants were identified and unidentified factors that may be also involved in the transcription have been predicted. Less