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."
Boyce Thompson Institute for Plant Research at Cornell University, Tower Road, Ithaca, NY 14853.
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Szalay