Figure 4.

(A) The plant circadian clock affects primary and secondary growth by regulating the cell cycle. This figure summarizes how the core circadian clock may interact with the cell cycle to regulates cell proliferation and growth in Populus spp.; further details are provided in the text. (B) The morning-phased and evening-phased clock components regulate the cell cycle. The morning-phased components LHY1 and LHY2 in Populus spp. repress CYCD3 expression, thus restricting CYCD3 to the early day. During the G1 phase, CYCD3 interacts with cyclin-dependent kinases (CDKs) to phosphorylate RETINOBLASTOMA RELATED 1 (RBR1), targeting it for degradation by the proteasome. The subsequent release from repression of replication factors in the heterodimer of the E2F–DP complex promotes expression of proliferating cellular nuclear antigen, mini-chromosome maintenance protein complex and CDC6, enabling their build-up prior to DNA replication in the S phase. TOC1 is an evening-phased component that may represses CDC6 (in grey, with question mark) transcription, restricting CDC6 production to the daytime. Several replication factors together with CDC6 act to enable cells to undergo DNA replication in the S phase. The E2F–DP complex also promotes expression of CDKs, which act with CYCA/B/D to control the transition between the S and G2 (G2 to M) phases; the kinase WEE1 may phosphorylate CDKAs at this stage. Note: (B) is adapted from Dante et al. (2014).