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.