FIGURE 1.

Overview of stomatal development. The stomatal lineage initiates from meristemoid mother cells (MMCs). MMC undergoes asymmetric entry divisions and produces a meristemoid (M) as well as a larger sister cell (termed a stomatal lineage ground cell, SLGC). Meristemoids can undergo asymmetric amplifying divisions before differentiating into a guard mother cell (GMC). SLGCs can differentiate into a pavement cells or reacquire a MMC fate and initiate asymmetric spacing divisions that produce “satellite meristemoids.” Stomatal complexes form after at least one unequal division of a stem cell, and then by a single equal division of a GMC. SPCH, MUTE, FAMA, and FLP/MYB88 are transcription factors that regulate key fate transitions during stomatal development. SCRM and SCRM2 heterodimerize with SPCH, MUTE, and FAMA to promote stomatal–lineage transitions. AGO1 is involved in an AGL16-mediated microRNA post-transcriptional regulatory pathway that blocks asymmetric spacing divisions in SLGCs. ABA might be involved in stomatal initiation and differentiation by repressing SPCH and MUTE. RBR activity is predominantly regulated by CDKA;1. RBR participates in the regulation of asymmetric as well as symmetric divisions.