Fig. 6.

Model of the developmental integration of CYCD7;1 to ensure lineage-specific cell cycle regulation. Cell cycle regulators are integrated with stomatal specific transcription factors to ensure that the last formative division of the lineage creates a pair of symmetric GCs. Initiation of CYCD7;1 expression in GMCs requires MUTE and additional unknown factors (question mark). CYCD7;1 together with its CDK partner executes the formative division of the GMC. Owing to the observation that this last division is not completely abolished in cycd7;1 mutants, other D-type cyclins likely promote the G1-S phase transition. CDK/CYCD complexes phosphorylate RBR1 in order to release its negative function on S phase-promoting factors. To ensure termination of the lineage, the transcription factor FAMA, itself slightly later expressed than CYCD7;1, binds to the CYCD7;1 promoter to temporally restrict CYCD7;1 expression to GMCs and to restrict the cell cycle right after the last division. Transcriptional regulation is marked by dashed lines. This regulatory network ensures high cell cycle activity for the last formative division in the stomatal lineage and terminates cell division activity to ‘one and only one’ division.