Microtubule Organization at the Cell Cortex is a Determinant of Cell Shape via Division Plane Maintenance and Directional Cell Expansion in Plants

The proper organization of the microtubules is a major determinant of cell shape. Microtubule-associated proteins have essential roles in cell shape determination by direct microtubule interaction. During interphase, microtubules populate the cell cortex and are organized perpendicular to the primary axis of growth. During mitosis, microtubules form conserved structures that are essential for the normal completion of cell division. The preprophase band and phragmoplast are plant specific mitotic microtubule structures. TANGLED1 is localized to the division site and guides the expanding phragmoplast during the later stages of mitosis. To understand more about the role of division site localized proteins and their interacting partners on cell shape determination I analyzed microtubule dynamics, cell shape, and localization of TUBULIN and TANGLED1. I characterized an array of microtubules at the cell cortex during telophase. These microtubules were distinct from the phragmoplast and organized perpendicular to the division plane as they interacted with the division site and TANGLED1. The density and organization of microtubules in this array was reduced in tangled1 cells. In wild type cells, we observed individual microtubule addition to the phragmoplast with a preference for the leading edge, however, this preference was lost in tangled1 mutant cells. The asymmetric bundling of telophase microtubules at the cell cortex with phragmoplast microtubules altered the direction of phragmoplast expansion. These findings reveal a novel mechanism for phragmoplast guidance by division site localized proteins. KINECTIN was previously identified as a TANGLED1 interacting protein. We used CRISPR/CAS9 mutagenesis of the kinectin gene region in maize to describe a role for KINECTIN as a potential regulator of microtubule associated proteins in plants. The functional protein domains of KINECTIN were revealed by defects in cell expansion and increased cell shape isotropy in kinectin-1 but not kinectin-2 mutants. Microtubule dynamicity and TANGLED1 accumulation in the spindle were significantly different in kinectin-1 mutant cells. Together, our observations provide additional insight into how proper microtubule organization at the cell cortex determines cell shapes in plants via division plane maintenance and directional cell expansion