DISTRIBUTION OF RHIZOBIA IN LEGUMINOUS PLANTS SURVEYED BY PHYLOGENETIC IDENTIFICATION

The distribution of rhizobia, bacteria which nodulate roots of legumin ous plants, was surveyed for 91 species of the family Leguminoceae. On e hundred and twenty-one strains of rhizobia were isolated, and 29 str ains were obtained from culture collections. The total of 150 strains were classified based on their small subunit (ssu) ribosomal RNA (rRNA ) sequences. The analyses of the partial sequences (157 bases from pos itions 1220 to 1377 in Escherichia coli numbering system) revealed the presence of three major groups which corresponded with the genera Rhi zobium, Bradyrhizobium, and Azorhizobium and the presence of 17 variet ies (eight varieties for the genus Rhizobium, eight for the genus Brad yrhizobium, and one for the genus Azorhizobium). Several phenotypic ch aracteristics and DNA base compositions were determined for 27 represe ntative strains from 17 varieties. Also, genomic relatedness among the 27 strains was estimated by DNA-DNA hybridization. The strains which were different in their ssu rRNA partial sequences never showed close relationships in DNA-DNA hybridization. Further, sequences covering mo st of ssu rRNA were compared among 17 representative strains of the 17 varieties including type strains of seven Rhizobium species, Bradyrhi zobium japonicum and Azorhizobium caulinodans, and phylogenetic relati onships among rhizobia were discussed. From phylogenetic analyses it i s inferred that the nodulation genes transferred among rhizobia.