Physcomitrella Cyclin-Dependent Kinase A Links Cell Cycle Reactivation to Other Cellular Changes during Reprogramming of Leaf Cells

During regeneration, differentiated plant cells can be reprogrammed to produce stem cells, a process that requires coordination of cell cycle reactivation with acquisition of other cellular characteristics. However, the factors that coordinate the two functions during reprogramming have not been determined. Here, we report a link between cell cycle reactivation and the acquisition of new cell-type characteristics through the activity of cyclin-dependent kinase A (CDKA) during reprogramming in the moss Physcomitrella patens. Excised gametophore leaf cells of P. patens are readily reprogrammed, initiate tip growth, and form chloronema apical cells with stem cell characteristics at their first cell division. We found that leaf cells facing the cut undergo CDK activation along with induction of a D-type cyclin, tip growth, and transcriptional activation of protonema-specific genes. A DNA synthesis inhibitor, aphidicolin, inhibited cell cycle progression but prevented neither tip growth nor protonemal gene expression, indicating that cell cycle progression is not required for acquisition of protonema cell-type characteristics. By contrast, treatment with a CDK inhibitor or induction of dominant-negative CDKA;1 protein inhibited not only cell cycle progression but also tip growth and protonemal gene expression. These findings indicate that cell cycle progression is coordinated with other cellular changes by the concomitant regulation through CDKA;1.