Fig. 4.

Asymmetric cell division and division polarity of stomata in Arabidopsis. The asymmetric divisions of MMCs with and without influences from stomatal lineage neighbors are referred to as having intrinsic and oriented polarity, respectively. Polarity is seen in cell sizes and division orientations, but also in the localization of the cortical proteins BASL (blue) and POLAR (not shown, but following the same trajectory as BASL). Prior to MMC asymmetric division, BASL localizes at the cortex of the MMC in crescents on the side at which the future SLGC will form. Models predict that such localization could generate cell polarity if the polarized protein acts to repel the nucleus and subsequent division plane. During amplifying divisions (top pathway), self-renewing meristemoids reorient cortical crescents away from the most recently formed walls, leading to a stomatal progenitor surrounded by lineally related non-stomatal sisters (see also Fig. 1, top pathway). White dotted lines indicate the future division plane and blue dotted lines indicate the previous position of BASL. In SLGCs adopting MMC fates (lower pathway), BASL must redistribute within the same cell into a crescent abutting the pre-existing meristemoid or GMC. For simplicity, the nuclear localization of BASL is not indicated. Although divisions of MMCs are precisely oriented relative to immediate neighbors, the divisions of MMCs and GMCs do not appear to be oriented relative to any landmarks or axes of the leaf.