UC San Diego

Stem cell differentiation is key to plant development, which utilizes asymmetric cell divisions (ACD) to maintain stem cells, promote growth, and encourage cell diversity. Previous work in Arabidopsis thaliana elucidated some pathways controlling stem cell dynamics during morphogenesis. In the leaf epidermis, pre-mitotic, oriented nuclear migrations in meristemoids, a transient stem cell in Arabidopsis thaliana, allow creation of daughter cells with differing sizes, fates, and placements. Pre-division, the nucleus uses microtubules to migrate away from a polarity crescent that controls division orientation. Post-division, it is known that the nucleus orients towards the polarity crescent using F-actin, but precise mechanisms, including relevant nucleators, are unknown. Additionally, while behavior of the nucleus during ACDs in meristemoids has been characterized, cytoskeletal regulators linking the nucleus to F-actin remain unknown. Time-lapse imaging of Arabidopsis seedlings treated with an F-actin Arp2/3 complex inhibitor (CK-666) and a control compound (CK-689), revealed that Arp2/3 complex-mediated F-actin controls nuclear migration post-division. Novel regulators of nuclear migration were found by phenotypic characterization of stomatal formation in loss-of-function mutants in protein components linking the cytoskeleton to the nuclear envelope. We found three candidate proteins (CRWN1, CRWN3, and SINE1) decreased stomatal density, while one (SUN1) increased stomatal density. This suggests ACD frequency is altered in these mutants and identifies new factors to probe in future studies. These candidate proteins, SUN1, CRWN1, CRWN3, and SINE1, may control nuclear migration in Arabidopsis thaliana, alongside Arp2/3 complex-mediated F-actin. Determining mechanisms behind nuclear migration gives insight into how organelle positioning controls ACDs and stomatal development.