[초청강연] To divide or differentiate: Transcription factor-orchestrated cell cycle switch from proliferative- to terminal division in stomatal development

Differentiation of specialized cell types from self-renewing progenitors requires precise cell
cycle control. Plant stomata are generated through asymmetric divisions of a stem-cell-like
precursor meristemoid followed by the single symmetric division that creates an adjustable
pore surrounded by paired guard cells. The stomatal-lineage-specific transcription factor
MUTE terminates the asymmetric divisions and triggers differentiation. However, the role of
cell cycle machinery in this transition remains unknown. Through time-lapse imaging, we
discover that the symmetric division is slower than the asymmetric division. We identify a
plant-specific cyclin-dependent kinase inhibitor, SIAMESE-RELATED4 (SMR4), as a
molecular brake that decelerates cell cycle during this transition. SMR4 is directly induced by
MUTE and transiently accumulates in differentiating meristemoids. SMR4 physically and
functionally associates with CYCD3;1 and extends 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 the stomatal fate specification.