Prevalence of the Rhizobium etli-like allele in genes coding for 16S rRNA among the indigenous rhizobial populations found associated with wild beans from the Southern Andes in Argentina

Abstract

A collection of rhizobial isolates from nodules of wild beans, Phaseolus vulgaris var. aborigineus, found growing in virgin lands in 17 geographically separate sites in northwest Argentina was characterized on the basis of host range, growth, hybridization to a nifH probe, analysis of genes coding for 16S rRNA (16S rDNA), DNA fingerprinting, and plasmid profiles. Nodules in field-collected wild bean plants were largely dominated by rhizobia carrying the 16S rDNA allele of Rhizobium etli. A similar prevalence of the R. etli allele was observed among rhizobia trapped from nearby soil. Intragroup diversity of wild bean isolates with either R. etli-like or Rhizobium leguminosarum bv. phaseoli-like alleles was generally found across northwest Argentina. The predominance of the R. etli allele suggests that in this center of origin of P. vulgaris the coevolution of Rhizobium spp. and primitive beans has resulted in this preferential symbiotic association.

FIG. 1

Dendrogram (UPGMA) of genetic relationships among 16S rDNA genotypes identified by sequence analysis of a 260-bp 16S rRNA gene fragment. Asterisks indicate isolates from wild beans.

FIG. 2

Genotypic relatedness of isolates from wild beans. The dendrogram is derived from analysis of ERIC-PCR by using the computer-assisted system of analysis GelCompar from Applied Maths, Kortrijk, Belgium. The degree of relative genetic relatedness is indicated above the dendrogram on a scale from 10 to 100%. Each branch is followed by the respective wild bean isolate (preceded by the letter P) or soil isolate (preceded by the letter T) and site of origin according to Table 1 (in parentheses). Asterisks indicate R. leguminosarum bv. phaseoli-like isolates; all the other isolates have the R. etli 16S rDNA gene.

FIG. 3

Representative plasmid profiles of isolates obtained from wild beans and from NWA soils. R. etli isolates: lane 1, P40N1 (B8) (geographic origins of isolates according to Table 1 are given in parentheses); lane 2, P41N1 (B8); lane 3, T9N16P (B8); lane 4, P65N1 (A2). R. leguminosarum bv. phaseoli isolates: lane 5, P45N1 (B7); lane 6, P8N3 (B4); lane 7, P8N1 (B4). Lane 8, reference strain, R. etli CFN42, with plasmids ranging in size from 150 to 600 bp (5).