Deceleration of the cell cycle underpins a switch from proliferative to terminal divisions in plant stomatal lineage.

Differentiation of specialized cell types requires precise cell-cycle control. Plant stomata are generated through asymmetric divisions of a stem-cell-like precursor followed by a single symmetric division that creates paired guard cells surrounding a pore. The stomatal-lineage-specific transcription factor MUTE terminates the asymmetric divisions and commits to differentiation. However, the role of cell-cycle machineries in this transition remains unknown. We discover that the symmetric division is slower than the asymmetric division in Arabidopsis. We identify a plant-specific cyclin-dependent kinase inhibitor, SIAMESE-RELATED4 (SMR4), as a MUTE-induced molecular brake that decelerates the cell cycle. SMR4 physically and functionally associates with CYCD3;1 and extends the G1 phase of asymmetric divisions. By contrast, SMR4 fails to interact with CYCD5;1, a MUTE-induced G1 cyclin, and permits the symmetric division. Our work unravels a molecular framework of the proliferation-to-differentiation switch within the stomatal lineage and suggests that a timely proliferative cell cycle is critical for stomatal-lineage identity.