Exocytosis for endosymbiosis: membrane trafficking pathways for development of symbiotic membrane compartments

•SNARES and EXOCYST components conserved for AM symbiosis.•An exocytotic pathway shared by AM symbiosis and rhizobium-legume symbiosis.•EXO70i is essential for polarized exocytosis of membrane during AM symbiosis.•Multiple exocytosis pathways operate during endosymbiosis. During endosymbiosis with arbuscular mycorrhizal fungi or rhizobial bacteria, the microbial symbionts are housed within membrane-bound compartments in root cortex or nodule cells respectively. Their development involves polarized deposition of membrane around the symbionts as they enter the cells and the membranes show functional specialization, including transporters that mediate nutrient transfer between host and symbiont. The cellular changes associated with development of these compartments point to membrane deposition via exocytosis and over the past few years, researchers have uncovered several proteins within the exocytotic pathway that are required for development of endosymbiotic membrane compartments. The emerging theme is that unique membrane trafficking homologs or splice variants have evolved to enable exocytosis during endosymbiosis.