Even though the understanding of cell-cycle regulators in plants has tremendously increased over the last years, still little is known about cell-cycle regulation in response to environmental signals like DNA damage. A ubiquitous stress for any organism is DNA stress that can either be caused by exogenous sources or internal processes like chromatid separation or DNA strands separation during replication. The posttranslational regulation of Cdk1-type kinases through inhibitory phosphorylation through Wee1-type kinases in the so-called P-loop at the residue Tyr15 or the analogous positions has been found to be of pivotal importance for the arrest of the cell cycle after DNA damage in yeast and animals. But this mechanism is apparently not conserved in plants, as suggested by the hypersensitivity analysis of CDKA;1 dephospho-mutants. The first half of this study focus on possible regulation of CDKA;1 through T-loop phosphorylation upon replication stress in Arabidopsis. The positively acting phosphorylation on T161 and analogous residues in the so-called T-loop of the kinase that is required for full CDK activity and serves in substrate recognition. Remarkably, a T-loop phospho-mimicry mutant of CDKA;1, was almost 100% resistant to hydroxyurea (HU) and can partially rescue the hypersensitivity of wee1 to HU. T-loop phosphorylation is catalyzed by CDK activating kinases (CAKs) that are themselves CDKs with typical P- and T-loop regions. Evidence is obtained that WEE1 might inhibit CDKDs (Cdk-activating kinases) that would subsequently result in reduced CDKA;1 activity, and thus, cell-cycle arrest upon DNA damage. It is revealed that dephospho-mimicry mutants of CDKD;2 and 3, which can not be inhibited through WEE1 showed hypersensitivity to HU and not to bleomycin, suggesting their involvement in cell-cycle arrest specifically upon replication stress. Hypersensitivity of cdkd;2cdkd;3 to replication stress suggested possible activation of CDKA;1 through CDKD;1 independent of WEE1. An essential role of CDKDs in stabilizing CDKA;1 kinase activity during gamete development has been suggested. Defects observed in cdka;1VFcdkd mutants during meiosis but not in cdka;1VF mutants emphasize on importance of CDKA;1 T-loop phosphorylation for appropriate meiotic division.In second part of this study interaction between cell-cycle and circadian has been studied. A feedback loop in which the cell cycle could potentially regulate the circadian clock was suggested as a number of circadian genes were found to be deregulated in a microarray experiment with holomorphic CDKA;1 mutants. Thus the circadian gating of cell division of wildtype and cdka;1 mutants was studied under diurnal growth conditions. The altered time of division observed in cell-cycle mutants supported the idea of cell-cycle regulation in a time dependent manner. Expression profile of clock genes were analyzed in cdka;1 mutants through luciferase assay system. An altered period and intensity of expression observed in these mutants compared to wild type plants suggested a direct or indirect effect of CDKA;1 activity on clock gene expression.
Cross-talk between cell-cycle control and the environment