Rhizobium leguminosarum

Rhizobium leguminosarum is a Gram-negative, motile, rod-shaped, aerobic bacterium.

The symbiosis between R. leguminosarum and its host plants involves an exchange of chemical signals. Compounds released by legume roots, particularly flavonoids and other phenolic compounds, are detected by rhizobial regulatory proteins such as NodD, which activate bacterial nod genes. These genes direct the production of Nod factors, signalling molecules whose precise chemical structure is an important determinant of host specificity. In susceptible legumes, Nod-factor signalling causes changes including root hair deformation and curling. The bacteria enter through structures known as infection threads and are delivered into developing root nodules, where they differentiate into bacteroids capable of nitrogen fixation. Within the nodule, the plant supplies the bacteroids with carbon compounds, while the bacteria reduce atmospheric nitrogen to forms that can be assimilated by the plant. Nitrogenase is sensitive to oxygen, although the bacteroids require oxygen for respiration; leghemoglobin produced in the nodule helps maintain a low free-oxygen concentration while delivering oxygen to the bacteroids.

R. leguminosarum is widely used in the inoculation of legume seeds. The sv. trifolii strain U204 is commercially used to inoculate white and red clover in particular, but better strains for this purpose are being developed. Research has been carried out into the role that R. leguminosarum could play in promoting growth of canola and lettuce.