Regulation of Cell Proliferation Is an Essential Process in the Establishment of Plant Architecture.
The major checkpoints of the cell cycle are the G1/S and G2/M transitions. CDK/cycD complexes act at the G1/S checkpoint by phosphorylating the RBR protein, causing the release of the E2F transcription factor and entry into S-phase. In the G2/M transition, active CDK/cycB complexes induce the entry into mitosis. Their APC-mediated degradation completes mitosis. The cell cycle machinery responds to external signals such as hormones, sucrose, and light, which are integrated with developmental, positional, and epigenetic signals. As a consequence, cells modulate their activity to maintain proliferation competence, become quiescent, expand, differentiate, endoreduplicate, or die. The arrows represent the complex interconnections not only between the core of the cell cycle machinery and the different stimuli but also between expansion and differentiation or expansion and development.