Exocyst functions in plants: secretion and autophagy.

Tethering complexes mediate vesicle-target compartment contact. Octameric complex exocyst initiate vesicle exocytosis at specific cytoplasmic membrane domains. Plant exocyst is possibly stabilized at the membrane by a direct interaction between SEC3 and EXO70A. Land plants evolved three basic membrane-targeting EXO70 subfamilies, the evolution of which resulted in several types of exocyst with distinct functions within the same cell. Surprisingly, some of these EXO70-exocyst versions are implicated in autophagy, as is animal exocyst, and are involved in host defense, cell wall fortification and transport of secondary metabolites. Interestingly, EXO70Ds act as selective autophagy receptors in the regulation of cytokinin signaling pathway. Secretion of double membrane autophagy-related structures formed with the contribution of EXO70s to the apoplast suggests the possibility of secretory autophagy in plants.