Rhizobiaceae

Rhizobiaceae is a family of Gram-negative, rod-shaped bacteria within the order Rhizobiales of the class Alphaproteobacteria. The family includes several genera, notably Rhizobium, Sinorhizobium, Bradyrhizobium, Mesorhizobium, Azorhizobium, and Agrobacterium. Taxonomic revisions in recent decades have moved some species into newly defined genera, but these groups remain linked by phylogeny and ecological roles at the plant–microbe interface.

Many Rhizobiaceae form nitrogen-fixing symbioses with leguminous plants. In root nodules, rhizobial bacteroids express nitrogenase to convert atmospheric nitrogen (N2) to ammonia, which the plant uses for growth. In exchange, the plant provides carbon compounds and maintains a low-oxygen environment in nodules with leghemoglobin to protect nitrogenase.

Some members are plant pathogens; Agrobacterium species, especially Agrobacterium tumefaciens and Agrobacterium rhizogenes, cause crown gall and hairy root diseases, respectively. Agrobacterium tumefaciens is known for its Ti plasmid, which enables DNA transfer to plant cells and has been harnessed as a tool for plant genetic engineering.

Beyond symbiosis and pathogenicity, Rhizobiaceae are widespread in soil and the rhizosphere, and some are free-living saprotrophs. They play a significant role in the nitrogen cycle and are used in agricultural inoculants to improve legume crop yields, as well as in biotechnology for plant transformation methods.