Ensifer

Ensifer is a genus of Gram-negative, motile, rod-shaped bacteria in the family Rhizobiaceae, within the order Rhizobiales of the Alphaproteobacteria. Members of this genus are best known for their role as nitrogen-fixing endosymbionts of leguminous plants; they form root nodules in cooperation with their plant hosts and convert atmospheric nitrogen into ammonia that the plant can use.

The genus comprises several species associated with legumes, and they vary in host range and symbiotic efficiency. Like other rhizobia, Ensifer species initiate nodulation through signaling molecules called nod factors produced in response to flavonoids exuded by legume roots. Within nodules, the bacteria express nitrogenase enzymes (nif) to fix nitrogen under microaerobic conditions provided by the nodule structure.

Taxonomic history is noted; some species historically placed in Sinorhizobium or related groups have been assigned to Ensifer as part of broader reclassifications of rhizobia. The grouping is based on phylogenetic analyses of 16S rRNA and whole-genome data; species of Ensifer commonly studied include those nodulating alfalfa, clovers, and other pasture legumes in various regions.

Genomes of several Ensifer strains have been sequenced, revealing typical rhizobial symbiosis gene clusters (nod, nif) and adaptations for plant interaction, stress tolerance, and environmental persistence. Due to their symbiotic nitrogen fixation capabilities, Ensifer species, like other rhizobia, are used in legume-based biofertilizers to improve soil fertility and reduce the need for chemical nitrogen inputs.