Cyclin-dependent kinases (CDKs) are at the heart of eukaryotic cell-cycle control. The yeast Cdc2/CDC28 PSTAIRE kinase and its orthologs such as the mammalian Cdk1 have been found to be indispensable for cell-cycle progression in all eukaryotes investigated so far. CDKA;1 is the only PSTAIRE kinase in the flowering plant Arabidopsis and can rescue Cdc2/CDC28 mutants. Here, we show that cdka;1 null mutants are viable but display specific cell-cycle and developmental defects, e.g., in S phase entry and stem cell maintenance. We unravel that the crucial function of CDKA;1 is the control of the plant Retinoblastoma homolog RBR1 and that codepletion of RBR1 and CDKA;1 rescued most defects of cdka;1 mutants. Our work further revealed a basic cell-cycle control system relying on two plant-specific B1-type CDKs, and the triple cdk mutants displayed an early germline arrest. Taken together, our data indicate divergent functional differentiation of Cdc2-type kinases during eukaryote evolution.