Legume-Rhizobium-Symbiosis: Host’s Point of View

The legume-Rhizobium endosymbiosis may be the most highly evolved and perhaps ultimate association between a microbe and a plant in which the two partners can still grow independently. Undoubtedly the strong selective pressure on this association is the resulting nutritional complementation: the plant can be considerd a carbon-rich, nitrogen-deficient phototroph and the Rhizobium a carbon-deficient, nitrogen-fixing heterotroph. The resulting symbiosis which occurs in a specialized organ, the root nodule, makes the plant autotrophic with respect to the availability of reduced nitrogen, a limiting factor in plant nutrition. This unique intracellular association contributes significantly towards the yield of the agriculturally important legume crops.