Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes. Rhizobia require a plant host; they cannot independently fix nitrogen. The bacterium's enzyme system supplies a constant source of nitrogen for the plant while the plant furnishes nutrients and energy for the activities of the bacterium
Where Rhizobia Come From
Rhizobia bacteria are free living and motile in the soil, feeding on the remains of dead organisms. Free living rhizobia, however, cannot fix nitrogen, in part due to the higher oxygen content in the soil. There are multiple strains of rhizobia which occur naturally in soils. One rhizobium strain will be able to "infect" certain species of legumes but not others.
As a legume establishes itself in the soil it releases flavanoids that attract the rhizobia bacteria and encourage the growth of rhizobia around the roots of the plant. These flavanoids activitate the nod genes in the bacteria which then induce nodule formation. The rhizobia then excrete nod factors that cause the root hair on the plant to curl. Rhizobia then invade the root through the hair tip where they induce the formation of an infection thread. As the bacteria multiply, more and more threads are formed and the bacteria spread within the expanding network of tubes, continuing to produce nod factors which stimulate the root cells to proliferate, eventually forming a root nodule. Within a week of infection small nodules are visible to the naked eye. Each root nodule is packed with thousands of living Rhizobium bacteria. Portions of plant cell membrane surround the root nodule, forming the symbiosome where nitrogen fixing takes place.
How Rhizobia Fix Nitrogen
Rhizobia fix nitrogen through an enzyme called nitrogenase which catalyzes the conversion of nitrogen gas to ammonia. This reaction requires hydrogen as well as energy from ATP. The nitrogenase complex is sensitive to oxygen, becoming inactivated when exposed to it. Rhizobia control the oxygen levels in the nodule by excreting a red, iron-containing protein that bonds to oxygen. This leaves sufficient oxygen for the metabolic processes of the rhizobia but not so much that the nitrogenase complex is inactivated.
Implications for Agriculture and the Environment
As nitrogen is one the primary limiting nutrients to plant growth, increasing the amount of fixed nitrogen available to crops can stimulate increased agricultural production. In the past century this has been done mainly with fertilizers. Increased use of fertilizers has in fact been one of the main ingredients in the "green revolution". However, fertilizer typically deposits far more nitrogen in the soil than the plants' ability to assimilate it, which results in nitrogen runoff into aquatic environments with negative environmental consequences.
Coating legume seeds with rhizobium inoculum prior to planting is one method by which farmers have increased crop yields while decreasing fertilizer needs.
In addition to increased production of legumes, stimulation of rhizobia symbiosis in other (non-legume) food crops has potential to greatly increase crop yields without requiring nitrogen deposition through fertilizers, which leads to europhication of rivers and lakes, generation of acid rain, and other environmental issues. Scientists are currently looking into genetically engineering crops, particularly grains such as wheat, corn, and rice, to fix their own nitrogen through developing rhizobium symbiosis.