Opposing, Polarity-Driven Nuclear Migrations Underpin Asymmetric Divisions to Pattern Arabidopsis Stomata.

Multicellular development depends on generating and precisely positioning distinct cell types within tissues. During leaf development, pores in the epidermis called stomata are spaced at least one cell apart for optimal gas exchange. This pattern is primarily driven by iterative asymmetric cell divisions (ACDs) in stomatal progenitors, which generate most of the cells in the tissue. A plasma membrane-associated polarity crescent defined by BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL) and BREVIS RADIX family (BRXf) proteins is required for asymmetric divisions and proper stomatal pattern, but the cellular mechanisms that orient ACDs remain unclear. Here, utilizing long-term, quantitative time-lapse microscopy, we identified two oppositely oriented nuclear migrations that precede and succeed ACD during epidermal patterning. The pre- and post-division migrations are dependent on microtubules and actin, respectively, and the polarity crescent is the unifying landmark that is both necessary and sufficient to orient both nuclear migrations. We identified a specific and essential role for MYOXI-I in controlling post-ACD nuclear migration. Loss of MYOXI-I decreases stomatal density, owing to an inability to accurately orient a specific subset of ACDs. Taken together, our analyses revealed successive and polarity-driven nuclear migrations that regulate ACD orientation in the Arabidopsis stomatal lineage.