Three subfamilies of exocyst EXO70 family subunits in land plants: early divergence and ongoing functional specialization

Localized delivery of plasma membrane and cell wall components is an essential process in all plant cells. The vesicle-tethering complex, the exocyst, an ancient eukaryotic hetero-octameric protein cellular module, assists in targeted delivery of exocytosis vesicles to specific plasma membrane domains. Analyses of Arabidopsis and later other land plant genomes led to the surprising prediction of multiple putative EXO70 exocyst subunit paralogues. All land plant EXO70 exocyst subunits (including those of Bryophytes) form three distinct subfamilies-EXO70.1, EXO70.2, and EXO70.3. Interestingly, while the basal well-conserved EXO70.1 subfamily consists of multiexon genes, the remaining two subfamilies contain mostly single exon genes. Published analyses as well as public transcriptomic and proteomic data clearly indicate that most cell types in plants express and also use several different EXO70 isoforms. Here we sum up recent advances in the characterization of the members of the family of plant EXO70 exocyst subunits and present evidence that members of the EXO70.2 subfamily are often recruited to non-canonical functions in plant membrane trafficking pathways. Engagement of the most evolutionarily dynamic EXO70.2 subfamily of EXO70s in biotic interactions and defence correlates well with massive proliferation and conservation of new protein variants in this subfamily.