Establishment of the symbiotic interaction between legume plants and Rhizobia occurs when the soil nitrogen source is limiting and leads to the de-novo formation of symbiotic nodules, generally formed on roots of the plant hosts. The symbiotic association allows the plant to overcome nitrogen limitation taking advantage of the bacterial nitrogenase activity. In return, the plant furnishes carbon derivatives to its hosts. The first steps of this symbiotic association and the formation of the symbiotic organ have been described in detail (Ferguson et al., 2010, J. Int Plant Biol., 52, 61-76 ; Oldroyd et al., 2011, Annu. Rev. Genet., 45, 119–144) but organ identity and later steps of the interaction related to bacterial accommodation and immunity are less understood. Rhizobia often invade the plant root using specialized symbiotic structures called infection threads. In the mature nitrogen fixing nodule, the rhizobia reside in symbiotic nodule cells within organelle-like structures, called symbiosome. In contrast to what is generally observed during microbial invasion, legumes do not elicit defense reactions during symbiosis despite that the bacterial population reaches massive densities in nodules.