The taxonomic status of the species Rhizobium leguminosarum, Rhizobium trifolii and Rhizobium phaseoli was analysed in this study on the basis of their molecular and phenotypic characteristics. According to the results, the type strain R. phaseoli ATCC 14482(T) does not belong to any of the already described species of the genus Rhizobium and it should therefore be considered as a different species. In contrast, the strains of R. trifolii examined belonged to R. leguminosarum and thus R. trifolii is a later synonym of R. leguminosarum. The results of the analysis of 16S-23S intergenic spacer region and rrs, recA and atpD gene sequences as well as those of DNA-DNA hybridization experiments and phenotypic characterizations showed that the type strains R. leguminosarum USDA 2370(T) and R. leguminosarum DSM 30132 do not belong to the same species. Taking into account that strain USDA 2370(T) corresponds to the original strain of this species, 3Hoq18(T), this strain should be considered as the true type strain of R. leguminosarum whereas strain DSM 30132 should be reclassified as a different species, for which the name Rhizobium pisi sp. nov. is proposed (type strain, DSM 30132(T)=NCIMB 11478(T)).

Fifty-five Chinese isolates from nodules of Amorpha fruticosa were characterized and compared with the type strains of the species and genera of bacteria which form nitrogen-fixing symbioses with leguminous host plants. A polyphasic approach, which included RFLP of PCR-amplified 16S rRNA genes, multilocus enzyme electrophoresis (MLEE), DNA-DNA hybridization, 16S rRNA gene sequencing, electrophoretic plasmid profiles, cross-nodulation and a phenotypic study, was used in the comparative analysis. The isolates originated from several different sites in China and they varied in their phenotypic and genetic characteristics. The majority of the isolates had moderate to slow growth rates, produced acid on YMA and harboured a 930 kb symbiotic plasmid (pSym). Five different RFLP patterns were identified among the 16S rRNA genes of all the isolates. Isolates grouped by PCR-RFLP of the 16S rRNA genes were also separated into groups by variation in MLEE profiles and by DNA-DNA hybridization. A representative isolate from each of these DNA homology groups had a separate position in a phylogenetic tree as determined from sequencing analysis of the 16S rRNA genes. A new species, Mesorhizobium amorphae, is proposed for the majority of the isolates, which belonged to a moderately slow- to slow-growing, acid-producing group based upon their distinct phylogenetic position, their unique electrophoretic type, their low DNA homology with reference strains representing the species within the genus Mesorhizobium and their distinct phenotypic features. Strain ACCC 19665 was chosen as the type strain for M. amorphae sp. nov.