Several transposon Tn5-induced mutants of the broad-host-range Rhizobium sp. strain NGR234 produce little or no detectable acidic exopolysaccharide (EPS) and are unable to induce nitrogen-fixing nodules on Leucaena kucocephala var. Peru or siratro plants. The ability of these Exo-mutants to induce functioning nodules on Leucaena plants was restored by coinoculation with a Sym plasmid-cured (Nod-Exo+) derivative of parent strain NGR234, purified EPS from the parent strain, or the oligosaccharide from the EPS. Coinoculation with EPS or related oligosaccharide also resulted in formation of nitrogen-fixing nodules on siratro plants. In addition, an Exo-mutant (ANU437) of Rhizobium trifolii ANU794 was able to form nitrogen-fixing nodules on white clover in the presence of added EPS or related oligosaccharide from R. trifolii ANU843. These results demonstrate that the absence of Rhizobium EPSs can result in failure of effective symbiosis with both temperate and subtropical legumes.A complex multistep interaction between the soil bacterium Rhizobium and specific leguminous plants results in the induction of nitrogen-fixing nodules on legume roots (19 and references therein). The early steps of the interaction are characterized by the distortion or curling of the root hair cells. The cell walls of the root hairs are penetrated after 24 h by a compatible Rhizobium strain, and an infection thread is synthesized by the plant after the nucleus of this cell has migrated to the infection site (4, 17). The bacteria are carried toward the root cortex inside the infection thread, where they actively divide. Shortly before or concurrent with initiation of infection thread synthesis, cortical cell division is thought to be induced by the Rhizobium strain, presumably by diffusible substances released by the bacterium (1, 2).Another feature of the Rhizobium-legume interaction is the host specificity displayed. Fast-growing ("temperate") Rhizobium strains, for example, usually nodulate only one plant species effectively, whereas slow-growing Bradyrhizobium strains typically have a broad host range. In contrast, the fast-growing Rhizobium sp. strain NGR234 (28) possesses an unusually extensive host range, which includes a variety of tropical and temperature legumes as well as the nonlegume tropical tree Parasponia andersonii (29).Since the initial interaction between the symbionts occurs at the surface of the two organisms, cell surface molecules may be important in determining the outcome of the infection. Rhizobia characteristically produce large amounts of exopolysaccharides (EPSs) on various laboratory media, and the colonies formed are mucoid (Muc+) Rhizobium polysaccharides, particularly EPSs and lipopolysaccharides, have been postulated to be involved in the infection and nodulation of legumes (24, 25), including specific adhesion to the root hair surfaces (9) and the determination of host specificity (13). Exo-mutants of Rhizobium meliloti are apparently affected at an early stage of infection (15). In other species, EPS m...