OsCYCD7;1 rigidly determines guard mother cell division in rice

Functional stomata depend on paired guard cells that arise from a single symmetric division of the guard mother cell (GMC). However, the mechanism governing this division in grasses with unique dumbbell-shaped guard cells remains poorly understood. Here, we identify OsCYCD7;1 as an essential component for GMC division in rice. OsCYCD7;1 was strictly confined to pre-divisional GMCs, and its knockout prevented GMC division. OsSMR9, a plant CDK (cyclin-dependent kinase) inhibitor, physically interacted with OsCYCD7;1 and suppressed GMC division. Yeast one-hybrid (Y1H) and EMSA assays revealed OsMUTE directly bound to OsCYCD7;1 promoter. Consistently, OsCYCD7;1 expression was repeatedly detected in ectopically dividing GMCs of osmute mutant. This ectopic division was markedly suppressed in oscycd7;1 osmute but enhanced by OsCYCD7;1 overexpression in osmute. Moreover, a similar enhancement was observed in osflp osmute. Furthermore, Y1H and dual-luciferase assays indicated OsMUTE also bound to and activated OsSMR9. We demonstrate that OsCYCD7;1 is essential for GMC division. Specifically, its activity is inhibited through OsMUTE-activated OsSMR9, and its expression is negatively influenced by OsMUTE and OsFLP, which likely function together to ensure a single symmetric GMC division in rice. Our data expand grass-specific GMC division of stomatal development, fundamentally diverging from Arabidopsis.