The role of SIAMESE in G2 checkpoint regulation in Arabidopsis thaliana

Although the cell cycle is conserved between plants and other eukaryotes, there are also significant differences, particularly in G2 regulation. In particular, the WEE1/CELL DIVISION CYCLE25 (CDC25) circuit that establishes G2 timing in animals and fungi is absent in plants. In Arabidopsis thaliana, SIAMESE (SIM), a well-known regulator of endoreplication with homologs throughout land plants, is a cyclin-dependent kinase (CDK) inhibitor that restricts progression through mitosis. Mathematical modeling indicated that SIM may modulate the length of G2 during mitotic cycles in addition to its role in endoreplication. This prediction was tested several ways. First, root growth of sim lagged slightly behind that of wild-type (WT) and the root meristem was longer in sim than in WT. Second, two independent methods of monitoring cell cycle phases, long-term live-cell imaging and 5-ethynyl-2-deoxyuridine (EdU) pulse-chase labeling, showed that G2 is shorter in sim root meristem cortex cells than in WT. Finally, fluorescence levels of a CYCB:GFP fusion that responds directly to G2 CDK activity were consistent with sim mutants having greater G2 CDK activity. These results suggest that, in addition to its role in endoreplication, SIM plays a role in determining the length of G2 during mitotic cycles, potentially substituting in part for the functions of WEE1/CDC25.