Nodulation Mutants of Rhizobium fredii USDA257. D. S. Heron. Department of Plant Pathology, University of Missouri, Columbia, 65211 U.S.A. T. Érsek, H. B. Krishnan, and S. G. Pueppke. Department of Plant Pathology, University of Missouri, Columbia, 65211 U.S.A. MPMI 2:4-10. Accepted 16 November 1988. Copyright 1989 The American Phytopathological Society.

Rhizobium Fredii strain USDA257 nodulates cowpea, siratro, primitive soybean genotypes, and several other legume species. It does not nodulate the improved soybean cultivar McCall. By using Tn5 mutagenesis, we have isolated two classes of symbiotically altered mutants of strain USDA257. Three mutants of one class failed to nodulate all tested legumes. Five mutants of the second class retained the nodulating ability of the parent strain, but acquired the capacity to nodulate McCall soybean. Each mutant contains a single copy of Tn5, but marker exchange experiments confirmed that the transposon is responsible for the altered phenotype of only two. In one, Tn5 is in the Sym plasmid. Nodules produced by this mutant on McCall soybean are morphologically normal and reduce acetylene. In the other, the transposon is in the chromosome. On McCall soybean, this mutant produces a small number of abnormal nodules that fail to reduce acetylene.