In contrast, the mechanisms controlling cell division are much better understood than those regulating cell growth in plants (see Inze and De Veylder (2006) for an excellent recent review). Components of the plant cell cycle machinery (cyclins, cyclin-dependent kinases), orthologs of the retinoblastoma (Rb) gene, and E2F/DP-type transcription factors were identified based on their sequence homology (Vandepoele et al. 2002). Largely based on gain-of-function studies with transgenic plants over- or ectopically expressing cell cycle regulators and expression analysis, the following view is emerging: In association with CDKA (A-type cyclin-dependent kinase), D-type cyclins are involved in controlling the entry into the cell cycle (Menges et al. 2006, Riou-Khamlichi et al. 1999), whereas A- and B-type cyclins, in association with CDKA and CDKB play a major role in S-phase and entry into M-phase, respectively (Doerner et al. 1996; Weingartner et al. 2003). As in animal systems, the E2F/DP and related genes, promote S-phase and DNA synthesis, but are also involved in controlling the switch between mitotic cell cycles and the en-doreplication cycle. Likewise, CDK inhibitors function in post-translational control of cyclin-CDK complex activity. Anaphase promoting complex (APC) proteolytic activity at the metaphase-to-anaphase transition insures the irreversible directionality of cell cycle progression, as in other model systems.
Cell growth is coupled to cell division progression by mechanisms that monitor cell size. For example, in yeast, coupling of growth to cell cycle entry converges on the regulation of G1-type CLN3 cyclin abundance (Morgan 2007), although this view may be too simplified (Jorgensen and Tyers 2004). CLN3 abundance is regulated at the transcriptional, translational, and post-translational level (MacKay et al. 2001; Morgan 2007; Polymenis and Schmidt 1997). In aggregate, these mechanisms result in a steep stimulus-response coupling (ultrasensitive response) of CLN3 protein levels, and hence of CLN3-CDK complex activity, to the rate of mRNA translation by ribo-somes, which reflects the activity of the TOR and other growth regulating pathways. Cell cycle entry in plants requires D-type cyclins. In Arabidopsis, cyclin D3;1 mediates the stimulatory effect of cytokinins on proliferation, while cyclin D2 abundance is responsive to sucrose levels (Riou-Khamlichi et al. 1999, 2000). Cyclin D3;1 is a labile protein (Planchais et al. 2004), as would be expected of a limiting regulator responsive to potentially rapidly changing environments. Moreover, cyclin D3;1 promotes the G1/S transition (Menges et al. 2006). Based on this limited information, it is therefore reasonable to predict that key aspects of the mechanisms that couple cell growth to cell division are conserved in all eukaryotes.
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