meliloti

Sinorhizobium meliloti, formerly Rhizobium meliloti, is a Gram-negative, motile, rod-shaped bacterium in the Alphaproteobacteria. It forms a nitrogen-fixing symbiosis with legume plants of the genus Medicago, most notably Medicago sativa (alfalfa). In soil, S. meliloti colonizes roots and initiates nodulation through a flavonoid-mediated signaling exchange with the plant. The bacterium produces Nod factors—lipo-chitooligosaccharide signals—that trigger root hair curling, infection-thread formation, and inward growth of bacteria into root cortex cells, where they differentiate into bacteroids within nodules.

Genomics: S. meliloti has a multipartite genome, comprising a chromosome (~3.6–3.9 Mb) and two megaplasmids, pSymA and pSymB, which harbor most of the symbiosis and nitrogen-fixation genes (nod, nif, fix). The model strain Rm1021 has had its genome sequenced, providing a basis for dissecting nodulation and host interaction. Gene transfer between S. meliloti strains and the megaplasmids contributes to variation in host range.

Ecology and agricultural relevance: as a legume root nodule symbiont, it fixes atmospheric nitrogen into ammonia, supplying the plant with nitrogen while receiving carbon compounds. Alfalfa inoculants based on S. meliloti are used to improve establishment and yield. The interaction is a classic example of signal exchange between bacteria and plants and a cornerstone in the study of biological nitrogen fixation.

Taxonomy: the species is now classified as Sinorhizobium meliloti, with historical name Rhizobium meliloti still encountered in older literature.