Ineffective nodulation of Sesbania macrocarpa by Sinorhizobium meliloti strain RCR2011

Sinorhizobium meliloti is a well-known symbiont that fixes nitrogen with plants of three genera of the family Leguminosae, Tribe Trifolieae: Medicago, Melilotus, and Trigonella. We have found that S. meliloti strain RCR2011 forms ineffective nodules on a distantly related legume species, Sesbania macrocarpa. Nodules are indeterminate and lack leghemoglobin. Although S. meliloti deformed root-hairs of S. macrocarpa, infection threads were not detected in these cells. Instead, cells of S. meliloti were found in infection pockets between cortical cells and within infection threads that had penetrated into these cells. Bacteria eventually were released and gave rise to symbiosomes, which contained large quantities of an amorphous matrix with parallel arrays of electron-dense striations. Mutants of strain RCR2011 lacking nodA or nodH failed to nodulate S. macrocarpa, inactivation of nodG or nodE resulted in smaller nodules, while inactivation of region IIa, which contains nodIJ, had no apparent effect on nodulation.