Rhizobium meliloti nodulation genes specify the production of an alfalfa-specific sulfated lipo-oligosaccharide signal

The infection of leguminous plants by Rhizobium and the subsequent co-differentiation of both organisms leads ultimately to the formation of the unique nitrogen-fixing plant organ known as the root nodule (16). Within the nodule, the Rhizobium are furnished with photosynthate-derived energy and, in exchange, the endosymbiotic bacterium reduces atmospheric nitrogen to a form which can be assimilated by the host plant. This remarkable symbiosis has been the subject of extensive research for many years, but it is only recently that it has become possible to study, at the molecular level, the role of diffusible factors in plant-bacterial signalling and recognition, with the exciting prospect that such molecules might also be responsible for triggering plant morphogenesis.