Isolation and characterization of nodulation genes from Bradyrhizobium sp. (Vigna) strain IRc 78

An 11.76-kilobase-pair (kb) segment of DNA from Bradyrhizobium sp. (Vigna) strain IRc 78 that hybridizes to nodulation genes of Rhizobium meliloti strain 41 was isolated. Hybridization of the 11.76-kb DNA fragment to DNA from other Bradyrhizobium species revealed a high degree of sequence conservation in this region. Transfer of the 11.76-kb segment to nodulation-defective (Nod^-) mutants of R. meliloti restored their ability to induce nodules on Medicago sativa (alfalfa). Mutants of strain IRc 78 generated by Tn5 mutagenesis of the 11.76-kb segment fell into three classes according to their symbiotic reaction with Vigna unguiculata (cowpea). Class I mutants of strain IRc 78 were unable to induce root-hair curling or to nodulate; class II induced small, ineffective nodules; and class III showed delayed and decreased nodulation with reduction in amount of nitrogen fixed. Furthermore, in contrast to the wild-type strain, class I mutants could not induce nodules on Glycine max (soybean), Cajanus cajan (pigeon pea), or Arachis hypogaea (peanut). This finding suggests a common function of the 11.76-kb region in the infection of host plants by Bradyrhizobium either through root hairs or by “crack entry.”