The role and regulation of D-type cyclins in the plant cell cycle

The G(1) phase of the cell cycle represents a period of commitment to cell division, both for cells stimulated to resume division from a resting or quiescent state, and for cells involved in repeated cell cycles. During this period, various signals that affect the cells' ability to divide must be assessed and integrated. G(1) culminates in the entry of cells into S phase, when DNA replication occurs. In addition, it is likely that several types of differentiation decision may be taken by cells in the G(1) phase. In both animals and plants, it appears that D-type cyclins play an important role in the cell cycle responses to external signals, by forming the regulatory subunit of cyclin-dependent kinase complexes. The phosphorylation targets of D-cyclin kinases in mammalian cells are the retinoblastoma (Rb) protein and close relatives. Unphosphorylated Rb can associate with E2F transcription factors, preventing transcription of genes under E2F control until the G(1)/S boundary is reached. The conservation of Rb and E2F proteins in plants suggests that this pathway is therefore conserved in all higher eukaryotes, although it is absent in fungi and yeasts. Here we review the current understanding of the roles and regulations of D-type (CycD) cyclins in plants.