A Plant-Specific Polarity Module Establishes Cell Fate Asymmetry in the Arabidopsis Stomatal Lineage

SUMMARY Generating cell polarity in anticipation of asymmetric cell division is required in many developmental contexts across a diverse range of species. Physical and genetic diversity among major multicellular taxa, however, demand different molecular solutions to this problem. The Arabidopsis stomatal lineage displays asymmetric, stem cell-like and oriented cell divisions, which require the activity of the polarly localized protein, BASL. Here we identify the plant-specific BREVIS RADIX (BRX) family as localization and activity partners of BASL. We show that members of the BRX family are polarly localized to peripheral domains in stomatal lineage cells and that their collective activity is required for asymmetric cell divisions. We further demonstrate a mechanism for these behaviors by uncovering mutual, yet unequal dependencies of BASL and the BRX family for each other’s localization and segregation at the periphery of stomatal lineage cells.