Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia in legumes and strains of actinobacteria of the genus Frankia in other nodulating plants. This process has evolved multiple times within the legumes, as well as in other species found within the Rosid clade. Legume crops include beans, peas, and soybeans.