Exocyst subunit Sec6 is positioned by microtubule overlaps in the moss phragmoplast prior to the arrival of cell plate membrane

During plant cytokinesis a radially expanding membrane-enclosed is formed from fusing vesicles that compartmentalizes the cell in two. How fusion is spatially restricted to the site of cell plate formation is unknown. Aggregation of cell-plate membrane starts near regions of microtubule overlap within the bipolar phragmoplast apparatus of the moss . Since vesicle fusion generally requires coordination of vesicle tethering and subsequent fusion activity we analysed the subcellular localization of several subunits of the exocyst, a tethering complex active during plant cytokinesis. We found that Sec6, but neither Sec3 or Sec5 subunits localized to microtubule overlap regions in advance of cell plate construction started in moss. Moreover, Sec6 exhibited a conserved physical interaction with an orthologue of the Sec1/Munc18 protein KEULE, an important regulator for cell-plate membrane vesicle fusion in Arabidopsis. Recruitment of PpKEULE and vesicles to the early cell plate was delayed upon Sec6 gene silencing. Our findings thus suggest that vesicle-vesicle fusion is in part enabled by a pool of exocyst subunits at microtubule overlaps that is recruited independent of the delivery of vesicles.