Experimental evidences of pSym transfer in a native peanut-associated rhizobia

SummaryIn previous works we have characterized the native bacterial diversity associated with root nodules of peanut grown in Córdoba province, Argentina. Studies performed in the isolate Rhizobium sp. NET30 revealed a phylogenetic incongruence between housekeeping and nodulation genes. This discrepancy could be explained by the horizontal transfer of nodulation genes from the native peanut isolate Rhizobium sp. NCHA22, a bacterium that showed high identity percentages with Rhizobium tropici IIB strains in the basic and symbiotic genes analyzed. In this work, we demonstrate that, in R. sp. NCHA22, genes required for nodule formation in common bean (Phaseolus vulgaris L.) are plasmid-borne. A symbiotic plasmid capable of conjugal transfer to different genetic backgrounds was identified in this isolate. The mechanism involved in the plasmidic transfer differs from that described for R. tropici CIAT899 (R. tropici IIB type strain). The transfer of a symbiotic plasmid, and the subsequent homologous recombination of nodulation genes in R. sp. NET30 genetic background could account for the phylogenetic incongruence determined in this isolate. Results are also indicating that the transfer of the R. tropici NCHA22 pSym may be a frequent event in native conditions.