Challenging the plant cell cycle. Analysis of key cell cycle regulators in Arabidopsis thaliana

Throughout plant development cell differentiation is closely linked with cell cycle control. A class of highly conserved Serine/Threonine kinases, CYCLIN DEPENDENT KINASEs (CDKs) controls progression through the cell cycle. One important mechanism to regulate CDK activity is the binding of CDK inhibitors (CKIs). Recently, CKIs were also identified in plants and in previous studies, Arabidopsis plants misexpressing CKIs were found to have reduced endoreplication levels and decreased numbers of cells consistent with a function of CKIs in blocking the G1/S cell-cycle transition. I found that at least one inhibitor from Arabidopsis, KRP1, can also block entry into mitosis but allows S-phase progression causing endoreplication. The data presented in this work suggest that KRP1 acts in a concentration-dependent manner and has an important function in cell proliferation as well as in cell-cycle exit and in turning from a mitotic to an endoreplicating cell-cycle mode. Endoreplication is usually associated with terminal differentiation. Strikingly, endoreplicated cells were found to be able to re-enter mitosis emphasizing the high degree of flexibility of plant cells during development. Moreover, it could be shown that in contrast to animal CKIs KRP1 can move between cells. In animals CDKs regulate entry into S-phase via activation of the E2F-DP transcription factor, by phosphorylating the E2F-DP inhibiting RETINOBLASTOMA protein. Orthologs of Rb, E2F and DP have been identified in the Arabidopsis genome. In this work I misexpressed the RETINOBLASTOMA RELATED1 (RBR1) and three genes encoding for ADENOVIRUS E2 PROMOTOR BINDING FACTOR s (E2Fa, E2Fb and E2Fc) in endoreplicating trichomes. The obtained data suggest that RBR1 negatively regulates endoreplication, whereas E2Fa, E2Fb and E2Fc act as positive regulators, indicating that the RBR-E2F regulatory pathway is conserved in higher eukaryotes