R

hizobium leguminosarum. Like with Aa below, we'll simply refer to this bug here as Rl, because I'm not typing that out every single time. Rl is a soil organism which was first identified in 1889. These organisms are interesting because they pair up with legumes in a symbiotic relationship -- residing within root nodules -- and fix nitrogen. A portion of this nitrogen remains in the soil after the legumes are harvested, which cuts down on fertilizer costs. Because of this, you'll see legumes often used in crop rotations, rotating with other plants which do not fix nitrogen.

The nodulation process (formation of root nodules) is very intricate. The legumes release chemicals (flavinoids) which are sensed by the bacteria. In turn, the bacteria secrete chemicals (nod factors) which force the legume roots to undergo cell division and direct root hair growth. Eventually (with a few more steps) this forms the nodules in which the rhizobia reside.

Root Nodule.