Deoxyribonucleic Acid Homologies Among Acid-Producing Strains of Rhizobium

Plant specificities and deoxyribonucleic acid 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 one of the six 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. Conversely, isolates from a wide variety of plants could be designated Rhizobium phaseoli since they were capable of nodulating Phaseolus vulgaris. Acid production and growth rate on yeastmannitol agar were tested for all strains. Some strains grew rapidly but did not produce an acid reaction; these were grouped with the fast growing acid producers. Deoxyribonucleic acid homology was used to identify four genetic groups of fastgrowing, acid-producing rhizobia. Group 1 included strains of Rhizobium trifolii (except strains obtained from Trifolium lupinaster, Rhizobium leguminosarum, Rhizobium phaseoli (obtained from Phaseolus vulgaris), and two strains obtained from Neptunia gracilis. Group 2 comprised six American strains obtained from crown vetch (Coronilla varia), sainfoin (Onobrychis vicifolia), and Sophora spp. Species status for this group should remain tentative until further strains have been studied. Group 3 corresponded with Rhizobium meliloti as presently defined. Group 4 included fast-growing Lotus rhizobia, two strains obtained from T. lupinaster, and a wide variety of previously unclassified strains. Nine fastgrowing strains could not be included in any of these groups. The nine slowgrowing, non-acid producing strains included in this study showed < 10% homology with DNAs from seven fast-growing reference strains. The relationships between subgroups in group 1 are discussed, and the genetic diversity of strains obtained from Phaseolus vulgaris is examined. It is proposed that fast-growing rhizobia comprise at least four species corresponding with the four genetic groups described.