Synergy among exocyst and SNARE interactions identifies a functional hierarchy in secretion during vegetative growth

Vesicle exocytosis underpins signaling and development in plants and is vital for wall remodeling during cell expansion. Vesicle tethering and fusion are thought to occur sequentially, tethering mediated by the exocyst and fusion driven by assembly of SNARE protein complexes. Interactions between these two protein complexes are known, although insights into their functional consequences are largely unexplored. We now identify a clear hierarchy of interactions leading to secretion in Arabidopsis. Mating-based split-ubiquitin screens and in vivo FRET analyses showed that exocyst EXO70 subunits bind preferentially with cognate plasma membrane SNAREs, notably SYP121 and VAMP721. The exo70A1 mutant affected SNARE distributions and suppressed vesicle traffic like the dominant-negative SYP121deltaC, consistent with the epistasis of exo70A1 over the exo70A1syp121 double mutant. However, the exo70A1vamp721 mutant showed a strong synergistic suppression of growth. These data are best explained by a spatiotemporal hierarchy of exocyst recruitment with the R-SNARE to the plasma membrane, plausibly with the VAMP721 longin domain as a nexus for binding