Root nodules occur on the roots of plants (primarily
Fabaceae) that associate with
symbiotic nitrogen-fixing bacteria. Under
nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as
rhizobia. This process has evolved multiple times within the Fabaceae, as well as in other species found within the
Rosid clade. The Fabaceae include
legume crops such as
beans and
peas.