Altered exopolysaccharides of Bradyrhizobium japonicum mutants correlate with impaired soybean lectin binding, but not with effective nodule formation

The exact mechanism(s) of infection and symbiotic development between rhizo bia and legumes is not vet known, but changes in rhizobial exopolysaccharid es (EPSs) affect both infection and nodule development of the legume host. Early events in the symbiotic process between Bradyrhizobium japonicum and soybean (Glycine max [L.] Merr.) were studied using two mutants, defective in soybean lectin (SBL) binding, which had been generated from B. japonicum 2143 (USDA 3I-1b-143 derivative) by Tn5 mutagenesis. In addition to their SBL-binding deficiency, these mutants produced less EPS than the parental s train. The composition of EPS varied with the genotype and with the carbon source used for growth. When grown on arabinose, gluconate, or mannitol, th e wild-type parental strain, B. japonicum 2143, produced EPS typical of DNA homology group I Bradyrhizobium, designated EPS I. When grown on malate, s train 2143 produced a different EPS composed only of galactose and its acet ylated derivative and designated EPS II. Mutant 1252 produced EPS II when g rown on arabinose or malate, but when grown on gluconate or mannitol, mutan t 1252 produced a different EPS comprised of glucose, galactose, xylose and glucuronic acid (1:5:1:1) and designated EPS III. Mutant 1251, grown on an y of these carbon sources, produced EPS III. The EPS of strain 2143 and mut ant 1252 contained SBL-binding polysaccharide. The amount of the SBL-bindin g polysaccharide produced by mutant 1252 varied with the carbon source used for growth. The capsular polysaccharide (CPS) produced by strain 2143 duri ng growth on arabinose, gluconate or mannitol, showed a high level of SBL b inding, whereas CPS produced during growth of strain 2143 on malate showed a low level of SBL binding. However, the change in EPS composition and SBL binding of strain 2143 grown on malate did not affect the wild-type nodulat ion and nitrogen fixation phenotype of 2143. Mutant 1251, which produced EP S III, nodulated 2d later than parental strain 2143, but formed effective, nitrogen-fixing tap root nodules. Mutant 1252, which produced either EPS II or III, however nodulated 5-6 d later and formed few and ineffective tap r oot nodules. Restoration of EPS I production in mutant 1252 correlated with restored SBL binding, but not with wild-type nodulation and nitrogen fixat ion.