The spatiotemporal control of cell divisions in Physcomitrella development

Cell division is fundamental in the development of all living organisms. In plants, cells are caged in rigid cell walls, thus plant cell shape is solely determined by cell expansion and the orientation of the cell division plane. Additionally, a newly divided cell can acquire a different cell fate through an asymmetric cell division, which is essential for tissue innovations. During my PhD, I used Physcomitrella patens, a model moss system with powerful genetic toolkits and single-cell-layer tissues, to investigate cell division control in plant development. During cytokinesis, the last step of cell division when cells are physically separated, two antiparallel sets of microtubules overlap at the midline where cell plate formation takes place. I found that subunit Sec6 of the exocyst complex is positioned to the microtubule overlaps before membranous vesicles arrive, suggesting that Sec6 bridges the cytoskeletal network and membranous compartments in cytokinesis. During moss development, cells grow as branching filaments to form a 2D filamentous network. Filaments then start to generate initial cells with a new fate, becoming gametophore initial cells. These commence growth of a 3D leafy gametophore. I identified markers that can predict the fate of initial cells, providing a new tool to investigate the early stages of the 2D-to-3D growth transition. Next, I investigated the contribution of Rho-GTPase proteins, Rho-of-Plant (ROP), to gametophore formation. I demonstrated that deletion of ROP2 results in defects in filamentous cell type transition, gametophore initiation, and leaf development. Additive deletion of ROP’s effector, RIC, in rop2 mutants only rescued phenotypes in gametophore development, suggesting that the ROP2-RIC module is functional in a tissue-specific manner. Finally, I developed a controllable wounding system to study cell reprogramming in excised moss leaves as a start to elucidate the interplay of cell reprogramming and cell division control in the early land plant lineage.