Bradyrhizobium japonicum mutants deficient in exo- and capsular polysaccharides cause delayed infection and nodule initiation

Four strains of Bradyrhizobium japonicum 2143 that were unable to bind soybean lectin were isolated following Tn5-insertion mutagenesis. Each of the mutants had a dry colony morphology compared with the mucoid parent and were found to be deficient in the amount of exo- (EPS) and capsular (CPS) polysaccharides. Nodulation and nitrogen-fixing characteristics of four unique mutants were investigated and all four mutants exhibited delayed nodulation on soybean (Glycine max) cv, Williams 82 and significantly reduced acetylene reduction activities. The infection of one of the mutant strains was observed by light microscopy and was found to be delayed as early as 5 days post infection (dpi). The ultrastructure of nodules formed by these strains was examined at 31 dpi: two of the mutants appeared to be similar to nodules formed by the parent strain, but the other two mutants appeared to be defective in plant cell invasion and nodule initiation, The results presented here provide additional evidence that extracellular polysaccharides are required for efficient infection and nodulation of determinate nodules.