- Plants develop a vast array of cell shapes and sizes by selectively modifying their surrounding cell walls to expand in some regions and not in others. This process of morphogenesis requires the delivery of secretory vesicles to specific locations at the cell periphery, where exocytosis adds new membrane and proteins to the plasma membrane, and secretes materials to build and modify the cell wall. In yeast and mammals, the process of localized exocytosis has been found to involve an eight protein complex, the exocyst. Prior to my work, genes encoding all eight components of the exocyst had been identified in sequenced plant genomes, but no function had yet been determined for any of them. Utilizing T-DNA insertion mutants of putative exocyst components in Arabidopsis thaliana, I demonstrated that one component, SEC8, is required for pollen germination and competitive pollen tube growth. Subsequently, through genetic analyses, phenotypic observation, and cell biological experiments, I provided evidence that, combined with biochemical studies by my collaborators, demonstrated that the putative exocyst components function together as a complex (or complexes) in plants. Multiple components of the exocyst all localize to the tips of growing pollen tubes, the site of localized exocytosis, and strong mutations in exocyst components result in the formation of pollen tubes that are shorter and wider than wild-type. Exocyst mutants also have both an etiolated hypocotyl elongation defect and a root growth defect in the vegetative plant. Unexpectedly, characterization of these mutant plants implicated the exocyst for a role in phytohormone signaling, specifically, in the efficient reception of brassinosteroid signals. This work firmly establishes that the exocyst complex functions
in plants to affect plant morphogenesis in at least three developmental processes: polarized growth in pollen tubes, cell elongation in etiolated hypocotyls, and both cell division and cell elongation in growing roots. It also links the exocyst with at least two brassinosteroid -mediated developmental pathways. Finally, it strongly suggests that the exocyst also has a role in defining the cell elongation zone in roots, with exocyst mutants showing that the establishment of this zone can be disconnected from establishment of the root meristem.