The growth of most plants depends on the presence of sufficient amounts of nitrogen contained in the soil. However, a family of plants, the legumes, is partially free of this constraint thanks to its ability to live in association with soil bacteria of the Rhizobium, genus, capable of fixing nitrogen from the air. When these bacteria come into contact with their host plant, they trigger in the roots the formation and development of organs, termed nodules, where they continue to live. This close relationship is symbiosis, which benefits both organisms involved: the plant supplies nutritive elements to the bacteria which in return pass on the nitrogen they have stored up.
These interactions improve crop yields of leguminous plants that are crucial for human diet (soybean, peas, ground nuts and so on) and as animal feed (alfalfa, clover, sainfoin). In addition, cultivation of legumes living in symbiotic association with bacteria can contribute to vegetation regeneration schemes on soils depleted in nitrogen owing to overexploitation, erosion or desertification. The plant cover thus formed can help achieve ecological restoration, by enriching the soils in nitrogen. However, the symbiotic processes studied predominantly concern the leguminous plants of temperate zones, very little those of the tropics.