Many bacteria in the family Rhizobiaceae (a clade of proteobacteria) are able to infect and establish a nitrogen-fixing symbiosis on the roots of leguminous plants. This symbiosis is of economic importance to humans in a variety of ways and decreases the need for nitrogen fertilizer for some key agriculturally important plants (e.g. soybean and alfalfa). The establishment of the symbiosis involves complex interactions between the plant host and the bacteria, resulting in the formation of a novel organ, the nodule, which the bacteria colonize as intracellular symbionts. Stacey et al. (2006) reviewed recent discoveries relating to how this symbiosis is established.
Rhizobacteria are legumes’ best friends. All plants need nitrogen. Most can only get it from soil. But legumes, such as beans, can get it from air. This is thanks to rhizobacteria. These bacteria live in legumes’ roots. They turn nitrogen from air into a form the plant can use. Plants return the favor by giving the bacteria food and shelter.
^Garrity, George M.; Brenner, Don J.; Krieg, Noel R.; Staley, James T. (eds.) (2005). Bergey's Manual of Systematic Bacteriology, Volume Two: The Proteobacteria, Part C: The Alpha-, Beta-, Delta-, and Epsilonproteobacteria. New York, New York: Springer. ISBN 978-0-387-24145-6.