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Nodules -
The Rhizobium groups of bacteria are the best known examples of symbiotic nitrogen fixing bacteria. They form bodules on the roots of leguminous plants (clover, alfalfa, peas, beans, etc.). Rhizobia that are found inside the plant cells of nodules are called Bacteroides. The bacteria first invade the root hairs of leguminous plants.
They then aggregate as threads and penetrate the plant cells. The presence of the bacteria stimulates the multiplication of infected cells, resulting in the formation of nodules. The legume, the bacteria and the nodule constitute the system for symbiotic nitrogen fixation.
Both the plant and the bacteria benefit by this association. The bacteria obtain their nitrogen and source of energy from the plant. In turn, they fix atmospheric nitrogen into nitrate and make it available to the plant Legume systems thus provide a highly integrated association between the partners.
The nodulating ability of rhizobium appears to be encoded by plasmid borne genes. Legumes have played a major role in agriculture throughout history.
Most legumes have the ability to fix all the nitrogen they require. Nodule bacteria may fix as much as 25 kg of nitrogen per acre per year.
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