The Role of SIAMESE in G2 Checkpoint Regulation in Arabidopsis thaliana

Cyclin-cyclin dependent kinase (Cyclin-CDK) pairs regulate progression through the checkpoints of the cell cycle. They bind with some specificity and must be bound to for their regulative properties to be active. The presence of cyclin-CDKs is conserved throughout eukaryotes, while differences in specifics exist between yeast, animals, and plants. In Arabidopsis, SIAMESE (SIM) is a CDK inhibitor that restricts progression through mitosis. Overexpression of SIM is required for endoreplication in Arabidopsis trichomes, and overexpression of SIM results in increased endoreplication in leaf epidermal cells. SIM is expressed at high levels in the root elongation zone, where it plays a role in endoreplication, as well as at low levels in the meristem, where its function is not yet understood. Mathematical modeling of the cell cycle suggests that SIM may modulate the length of G2 during mitotic cycles, potentially replacing the WEE1/CDC25 circuit that regulates the timing of mitosis in animals and fungi. I found that the root meristem is longer in sim mutant roots than in WT roots, consistent with a role for SIM in the transition from the mitotic cycle to the endocycle in roots. Data from two different cell cycle measurement techniques, long-term live-cell imaging using a cell cycle marker system and EdU pulse-chase imaging, show that G2 length in root meristem cortex cells is shorter in sim mutant plants compared to wild-type. Thus, in addition to its known role in inducing endoreplication, SIM appears to play a role in regulating the length of G2 during mitotic cycles.