Genes in Rhizobium bacteria and leguminous plants work together to fix nitrogen in a process called symbiotic nitrogen fixation. Rhizobium genes like nodulation and nitrogen fixation genes, along with leguminous plant genes called nodulins, play key roles in this process. When nodulation genes in Rhizobium are active, they trigger the production of nodulation factors. These genes are expressed differently in the root nodules of the plant and bacteria, and are controlled by nodulation factors and plant hormones.