Endocytic accommodation of microbes in plants

Plants host many different microbes within their cells. These endosymbiotic relationships are characterized by the formation of new specialized membrane compartments inside the plant cells in which the microbes live and where nutrients and signals are efficiently exchanged. Such symbiotic interfaces include arbuscules produced by arbuscular mycorrhiza (AM), organelle-like symbiosomes formed during the rhizobium-legume symbiosis, and haustoria produced by biotrophic fungi and oomycetes. The formation and maintenance of such new membrane compartments require a major reorganization of the host endomembrane system. In the last decade, much progress has been made in understanding how arbuscules, symbiosomes, and haustoria are formed. In this chapter, we will summarize the recent developments in each field, with a major focus on the AM and rhizobial endosymbiosis. It has become clear that rhizobia have co-opted a signalling pathway as well as a cellular mechanism to make the interface membrane compartment from the ancient and most successful AM symbiosis. Both AM symbiosis and rhizobium symbiosis depend on the secretion of lipo-chito-oligosaccharides that trigger a symbiotic signalling cascade, which is required for both arbuscule and symbiosome formation. In both interactions a shared specific exocytosis pathway is recruited to facilitate the formation of the symbiotic interface resulting in a membrane compartment with distinct protein composition. Given the structural similarity of haustoria to arbuscules, similar mechanisms are envisioned to be involved in the formation of a haustorium.