GENOMIC HETEROGENEITY AMONG FRENCH RHIZOBIUM STRAINS ISOLATED FROM PHASEOLUS-VULGARIS L

Levels of DNA relatedness between strains isolated from root nodules o f Phaseolus vulgaris and reference strains of different Rhizobium spec ies were determined by performing DNA-DNA hybridization experiments (S 1 nuclease method). The nine strains examined were members of three ge nomic groups previously delineated by a restriction fragment length po lymorphism analysis among strains isolated from P. vulgaris at differe nt sites in France. In agreement with the results of the restriction f ragment length polymorphism analysis, three genomic species were found . We confirmed that one of these species corresponded to Rhizobium leg uminosarum since the strain examined was 100% related to the type stra in of this species. The other two species were new genomic species whi ch were less than 21% related to reference strains belonging to other Rhizobium species, including Rhizobium etli and Rhizobium tropici, and were 18% related to each other. As determined by an analysis of parti al 16S ribosomal DNA sequences, each of the genomic species was found to belong to a lineage independent from the lineages of previously des cribed Rhizobium species. Nevertheless, they were included in the grou p formed by the fast-growing Rhizobium species. Both genomic species 1 and genomic species 2 contained a majority of strains which were capa ble of nodulating both P. vulgaris and Leucaena leucocephala, like R. tropici. However, they also contained strains with a nodulation phenot ype restricted to P. vulgaris, like R. leguminosarum by. phaseoli and R. etli by. phaseoli. Our data are the first evidence that in Europe s pecies other than R. leguminosarum nodulate P. vulgaris.