The roles of molecular motors and microtubule dynamics in assembly and function of the higher plant mitotic spindle

The directional axes of cell division and expansion are fundamental determinants of plant cell morphogenesis. Microtubules (MTs) assume cell cycle-specific organizations that play a fundamental role in the control of these processes. Understanding how higher plants form and transition between different MT arrays is elemental in our understanding of plant development. This thesis focuses on the formation and function of the higher plant mitotic spindle, the MT array responsible for segregation of chromosomes during cell division. In chapter one, the pathways and molecular aspects of spindle structure and function are discussed.;In chapters two and three, the role of the kinesin ATK5 in mitotic spindle formation and function is investigated. ATK5 is a minus end-directed kinesin-14 family member that facilitates organization of the mitotic spindle via MT-MT sliding. ATK5 is located predominately in the spindle midzone, where it provides inward forces during spindle formation, and later contributes to spindle integrity, most likely via promoting lateral MT interactions. The combination of mutant analysis with high resolution imaging of ATK5 has also revealed the existence of multiple pathways of spindle formation, which act redundantly to facilitate accurate and efficient mitosis. Chapter four explores the contributions of these pathways of spindle formation. I show that the preprophase band of MTs influences spindle development, specifically via the MTs connecting it to the spindle. Secondly, I describe a previously undocumented process I term 'capture and coalignment', wherein MTs with discordant orientation to the spindle axis become properly aligned during spindle formation. The third pathway is apparent in the absence of prophase spindle bipolarity, where MTs become sorted during prometaphase, resulting in a bipolar spindle. Hence, several different pathways augment one another to varying degrees during spindle formation.