THE ROLE OF NOD SIGNAL STRUCTURES IN THE DETERMINATION OF HOST-SPECIFICITY IN THE RHIZOBIUM LEGUME SYMBIOSIS

During the past five years the structure of nodulation signals from mo re than a dozen different Rhizobium species has been elucidated. In ad dition, the role of numerous nod genes in the biosynthesis of the lipo oligosaccharides has been identified. This review discusses how Nod si gnal structure is determined by the specificity of the various biosynt hetic steps and how this influences variation in host specificity. Unt il recently, it appeared that the decorations of a common lipochitooli gosaccharide core determine the host-specific recognition of the signa ls, possibly via specific receptors in the host plant cell. A number o f recent publications, however, suggest that beyond the interaction of Nod signals with a putative receptor, certain structural features of Nod factors are involved in controlling the concentration of the signa ls during their uptake by the root tissue.