The plant cell cycle: G1/S regulation

G1/S controls appear central to the commitment to further cell division or differentiation in eukaryotic cells. Accumulating evidence implicates plant D-type cyclins as integrators of environmental and developmental signals within the cell cycle progression. The recent identification of plant homologs of retinoblastoma proteins (Rb) and E2F transcription factors strongly suggests that plants may deploy the E2F/Rb pathway in G1/S transition control in a similar manner as in mammalian cells. Over all, a picture of regulation is emerging: upon mitogenic signal stimulation, D-type cyclins are produced and associate with cyclin-dependent kinases (CDKs); the resulted CycD/CDK complexes phosphorylate subsequently the downstream Rb target proteins, leading to the release of free and transcriptionally active E2F factors from E2F/Rb complexes; the released E2F factors further promote transcription of genes whose products are required for G1/S transition as well as for S phase progression. Potential roles of ubiquitin-dependent proteolysis in G1/S controls are also discussed.