Isolation of a Bradyrhizobium japonicum serogroup 123 mutant which has an extended host range for nodulation-restricting soybean genotypes

We have isolated a Bradyrhizobium japonicum USDA 438 (serogroup 123) mutant which has the ability to form nodules on serogroup 123 nodulation‐restricting plant introduction genotypes and soybeans containing the Rj4 allele. The identity of the mutant was confirmed by using a serocluster 123‐specific DNA probe, restriction fragment length polymorphism analysis, and serogroup‐specific fluorescent antibodies. While the mutant contains Tn5 inserted into a cryptic, non nod gene‐containing locus, site‐directed mutagenesis and complementation studies indicated that the transposon is not responsible for host‐range extension. The mutant and the wild‐type parent had the same chromatographic profiles of [14C]acetate‐labelled extracellular B. japonicum nod factors.