Differentiation of stomata follows a series of stereotypical cell divisions and cell-state transitional steps specified by the master-regulatory transcription factors. The density and numbers of stomata are regulated by cell-cell signaling and flexibly modulated by environmental and physiological inputs. This review focuses on the latest breakthroughs in Arabidopsis elucidating the mechanisms behind the initiation of stomatal precursors, asymmetric cell division and stem cell behavior, and terminal differentiation of guard cells. I discuss new insights emerging from these studies: (i) competitive actions of signals and regulatory circuits initiating stomatal precursor pattern; (ii) a subcellular partitioning of signaling components determining the stomatal lineage stem-cell divisions; and (iii) epigenetic regulation maintaining the differentiated guard cell state.
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