Deoxyribonucleic Acid Homologies Among Acid-Producing Strains of Rhizobium

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Abstract
Plant specificities and DNA homologies were studied among 122 strains of Rhizobium. Some strains were assigned to species on the basis of their source of isolation and present nodulation capabilities, but many did not fit into 1 of the 6 currently recognized species of the genus Rhizobium. Among those strains assigned to species were many which also nodulated plants outside their species-specific, cross-inoculation group. Isolates from a wide variety of plants could be designated R. phaseoli since they were capable of nodulating Phaseolus vulgaris. Acid production and growth rate on yeast-mannitol agar were tested for all strains. Some strains grew rapidly but did not produce and acid reaction; these were grouped with the fast growing acid producers. DNA homology was used to identify 4 genetic groups of fast-growing, acid-producing rhizobia. Group 1 included strains of R. trifolii (except strains obtained from Trifolium lupinaster), R. leguminosarum, R. phaseoli (obtained from P. vulgaris) and 2 strains obtained from Neptunia gracilis. Group 2 comprised 6 American strains obtained from crown vetch (Coronilla varia), sainfoin (Onobrychis vicifolia), Sophora spp. Species status for this group should remain tentative until further strains are studied. Group 3 corresponded with R. meliloti as presently defined. Group 4 included fast-growing Lotus rhizobia, 2 strains obtained from T. lupinaster and a wide variety of previously unclassified strains. Nine fast-growing strains could not be included in any of these groups. The 9 slow-growing, non-acid producing strains included in this study showed < 10% homology with DNAs from 7 fast-growing reference strains. The relationships between subgroups in group 1 are discussed and the genetic diversity of strains obtained from P. vulgaris is examined. Apparently fast-growing rhizobia comprise at least 4-species corresponding with the 4 genetic groups described.