COMPARISON OF GEOGRAPHICALLY DISTANT POPULATIONS OF RHIZOBIUM ISOLATED FROM ROOT-NODULES OF PHASEOLUS-VULGARIS

Seventy-two rhizobial strains were isolated from the root nodules of french beans (Phaseolus vulgaris). They were sampled from two geographically distant field populations and 18 additional different sites in France. They were characterized by a) plasmid profiles (b) RFLP analysis of total cellular DNA using various chromosomal and symbiotic gene probes (including nifH from Rhizobium etli bv. phaseoli) and c) their ability to nodulate a potential alternative host, L. leucocephala. Over half of the isolates were ascribed to Rhizobium leguminosarum bv. phaseoli on the basis of the hybridization analysis, the possession of multiple copies of nifH and their inability to nodulate L. leucocephala. The remaining isolates belonged to 2 groups which were shown to be genomically distinct from R. leguminosarum bv. phaseoli, R. etli bv. phaseoli and R. tropici. Most members of these two groups shared with R. tropici the ability to nodulate L. leucocephala and, for isolates of only one of these groups, the presence of one copy of nifH. Members of each of the 3 taxa were widely distributed in France and circumstantial evidence of pSym transfer between them was shown. R. leguminosarum bv. phaseoli and one of the two novel groups co-occurred within the two geographically distant populations. Individual genotypes were conserved between them. The finding of a third taxon at various other locations indicated additional diversity among rhizobia nodulating beans.