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Rhizobium gallicum sp. nov. and Rhizobium giardinii sp. nov., from Phaseolus vulgaris nodules

N Amarger, V Macheret, G Laguerre
International Journal of Systematic Bacteriology 1997, 47 (4): 996-1006
Thirty-one strains of two new genomic species (genomic species 1 and 2) of rhizobia isolated from root nodules of Phaseolus vulgaris and originating from various locations in France were compared with reference strains of rhizobia by performing a numerical analysis of 64 phenotypic features. Each genomic species formed a distinct phenon and was separated from the other rhizobial species. A comparison of the complete 16S rRNA gene sequences of a representative of genomic species 1 (strain R602spT) and a representative of genomic species 2 (strain H152T) with the sequences of other rhizobia and related bacteria revealed that each genomic species formed a lineage independent of the lineages formed by the previously recognized species of rhizobia. Genomic species 1 clustered with the species that include the bean-nodulating rhizobia, Rhizobium leguminosarum, Rhizobium etli, and Rhizobium tropici, and branched with unclassified rhizobial strain OK50, which was isolated from root nodules of Pterocarpus klemmei in Japan. Genomic species 2 was distantly related to all other Rhizobium species and related taxa, and the most closely related organisms were Rhizobium galegae and several Agrobacterium species. On the basis of the results of phenotypic and phylogenetic analyses and genotypic data previously published and reviewed in this paper, two new species of the genus Rhizobium, Rhizobium gallicum and Rhizobium giardinii, are proposed for genomic species 1 and 2, respectively. Each species could be divided in two subgroups on the basis of symbiotic characteristics, as shown by phenotypic (host range and nitrogen fixation effectiveness) and genotypic data. For each species, one subgroup had the same symbiotic characteristics as R. leguminosarum biovar phaseoli and R. etli biovar phaseoli. The other subgroup had a species-specific symbiotic phenotype and genotype. Therefore, we propose that each species should be subdivided into two biovars, as follows: R. gallicum biovar gallicum and R. gallicum biovar phaseoli; and R. giardinii biovar giardinii and R. giardinii biovar phaseoli.


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