Legumerhizobia

Legumerhizobia are nitrogen-fixing bacteria that form specialized nodules on the roots of leguminous plants. Through symbiosis, they convert atmospheric nitrogen into ammonia that the plant can use, while receiving carbon compounds in return. They inhabit soil and the rhizosphere and can establish durable associations under suitable environmental conditions.

Symbiosis begins when legume roots exude flavonoids that induce nodulation genes in compatible rhizobia. In response, bacteria produce Nod factors that are recognized by host receptors, triggering root hair curling, infection thread formation, and nodule primordia development. Inside nodules, bacteroids are maintained in a low-oxygen environment by leghemoglobin, enabling nitrogenase to fix N2.

Legumerhizobia are represented by several genera within Alphaproteobacteria, including Rhizobium, Bradyrhizobium, Sinorhizobium (Ensifer), Mesorhizobium and Azorhizobium. They vary in growth rate and host range; some nodulate broad groups of legumes, others are more specialized. Examples include Rhizobium leguminosarum with peas and beans, Sinorhizobium meliloti with alfalfa, and Bradyrhizobium japonicum with soybean.

In agriculture, inoculation with well-adapted legumerhizobia strains can increase nitrogen fixation, reduce synthetic fertilizer use, and raise legume yields. Inoculant performance depends on soil pH, temperature, salinity, competition from native microbes, and compatibility with the host. Strain diversity and regional adaptation influence inoculation success and sustainability.

Genetically, legumerhizobia carry nodulation (nod) and nitrogen-fixation (nif/ fix) gene clusters, often on plasmids or genomic islands. Horizontal gene transfer can spread these traits among populations, shaping host range and efficiency. Genome sequencing reveals features that enable signal perception, nodule development, and bacteroid persistence in nodules.