Extracellular\nVesicle-Driven Crosstalk between Legume\nPlants and Rhizobia: The Peribacteroid Space of Symbiosomes as a Protein\nTrafficking Interface

Prokaryotes and eukaryotes secrete extracellular vesicles\n(EVs)\ninto the surrounding milieu to preserve and transport elevated concentrations\nof biomolecules across long distances. EVs encapsulate metabolites,\nDNA, RNA, and proteins, whose abundance and composition fluctuate\ndepending on environmental cues. EVs are involved in eukaryote-to-prokaryote\ncommunication owing to their ability to navigate different ecological\nniches and exchange molecular cargo between the two domains. Among\nthe different bacterium–host relationships, rhizobium–legume\nsymbiosis is one of the closest known to nature. A crucial developmental\nstage of symbiosis is the formation of N<sub>2</sub>-fixing root nodules\nby the plant. These nodules contain endocytosed rhizobiacalled\nbacteroidsconfined by plant-derived peribacteroid membranes.\nThe unrestricted interface between the bacterial external membrane\nand the peribacteroid membrane is the peribacteroid space. Many molecular\naspects of symbiosis have been studied, but the interbacterial and\ninterdomain molecule trafficking by EVs in the peribacteroid space\nhas not been questioned yet. Here, we unveil intensive EV trafficking\nwithin the symbiosome interface of several rhizobium–legume\ndual systems by developing a robust EV isolation procedure. We analyze\nthe EV-encased proteomes from the peribacteroid space of each bacterium–host\npartnership, uncovering both conserved and differential traits of\nevery symbiotic system. This study opens the gates for designing EV-based\nbiotechnological tools for sustainable agriculture.