Rapid identification of bean Rhizobium isolates by a nifH gene-PCR assay

Two oligonucleotide primer pairs, based on the nifH gene sequence of Rhizobium etli, were used in the polymerase chain reaction to amplify genomic DNA from a collection of bean-nodulating Rhizobium strains that were isolated from the Americas. The primers that were derived from a poorly conserved region among the nifH sequences of other rhizobial species, generated a positive and specific reaction with all the R. etli strains assayed. By contrast, strains belonging to type II R. tropici and to unclassified bean strains did not yield an amplification product. Since the nifH primers appeared to be universal for a collection of R. etli strains representing a diversity of genotypes, the nifH-PCR method provides a tool for the rapid typing of bean nodule isolates.