Genetic uniformity and symbiotic properties of acid-tolerant alfalfa-nodulating rhizobia isolated from dispersed locations throughout Argentina

Abstract

Alfalfa-nodulating rhizobia from acidic soils (pH 5.52-6.2) in Argentina comprise a distinct group of acid-tolerant Rhizobium isolates which were found to be related to Rhizobium sp. strain Or 191 (Del Papa et al., 1999). Lipopolysaccharide profiles and 16S rDNA sequences demonstrated that all twelve acid-tolerant Rhizobium isolates belonged to one species. Furthermore, the group of acid-tolerant Rhizobium isolates was formed by one strain type, since they gave the same plasmid profile as well as the same IS- and ERIC-fingerprints. All acid-tolerant Rhizobium isolates were genetically identical even though they were isolated at very different locations in Argentina. The isolates nodulated a range of Medicago, Melilotus and Trigonella species, and Phaseolus vulgaris, but were weak in nitrogen fixation. M. sativa nodules induced by one of the acid-tolerant rhizobial isolates contained several bacteroids within a vesicle-like structure, in contrast to nodules induced by the reference strain S. meliloti 2011. Reisolation of bacteria from root nodules resulted in approx. 40 times more colony forming bacteria in case of the acid-tolerant strains. These strains mark the borderline between a symbiotic and a parasitic plant-microbe interaction.