Ectopic expression of the Rhizobium etli amtB gene affects the symbiosome differentiation process and nodule development

Under conditions of nitrogen limitation, soil bacteria of the genus Rhizobi um are able to induce the development of symbiotic nodules on the roots of leguminous plants, During nodule organogenesis, bacteria are released endoc ytotically inside the invaded plant cells where they differentiate into the ir endosymbiotic form called bacteroids. Bacteroids surrounded by a plant-d erived peribacteroid membrane are nondividing, organelle-like structures, c alled symbiosomes, that use nitrogenase to reduce Nz to ammonia. Experiment s performed in vitro with isolated symbiosomes have previously led to the s uggestion that the NH3 produced by the bacteroids is released as NH4+ into the plant cytosol, Furthermore, it was observed that the bacterial amtB (am monium/methylammonium transport B) gene is switched off very early during s ymbiosis, just when bacteria are released into the host cells. We report he re that the ectopic expression of amtB in bacteroids alters the ability of bacteria to invade the host cells and the symbiosome differentiation proces s. Both the NtrC protein, which controls the expression of the bacterial ge nes involved in NH4+ assimilation, and the nitrogenase activity are essenti al to observe the amtB-mediated effect. Our results support the idea that i n vivo bacteroids do not take up NH4+ and demonstrate that the transcriptio nal downregulation of the amtB gene is essential for an effective symbiotic interaction.