XVI International Botanical Congess

Rhizobia and legume roots have evolved the ability to associate with one another symbiotically. This results in the production of a new organ, the root-nodule, which houses internalized nitrogen-fixing bacteria. These symbioses are specific to varying extents: In some cases, a bacterial speciesCor biovarCmatches up with a legume species or group of related species to form nodules. In other cases, either the legume or the bacterium can nodulate with a broad range of symbiotic partners. Two such promiscuous bacterial strains, Rhizobium fredii USDA257 and Rhizobium sp. NGR234, nodulate 452 species distributed among all three subfamilies of the Leguminosae. Although both microbes nodulate a wide variety of legumes that differ in geographical origin and growth habit, patterns of selectivity are evident, and the host range of USDA257 is almost a precise subset of that of NGR234. The observed patterns of host specificity cannot be readily explained by patterns of Nodulation factor substituents.