Phenotypic and genotypic diversity of Sinorhizobium meliloti strains isolated from soils in Zadar County

Soil bacteria, Sinorhizobium meliloti is of great agricultural importance because of its ability to fix atmospheric nitrogen in symbiosis with alfalfa very valuable forage crop. The main aim of this study was to evaluate tolerance of indigenous S. meliloti strains to stress environmental conditions. Twenty rhizobial strains, isolated from different regions in Croatia, were genotipically characterized to assess diversity amongst natural population. Stress tolerance assays were performed in order to select indigenous rhizobia with tolerance to unfavorable soil conditions. The growth of the strains was studied at different pH values, temperatures, NaCl and heavy metals concentrations. The results of 16S rDNA genotyping using PCR/RFLP analysis showed that 17 isolates could be assigned to S. meliloti while RAPD and ERIC-PCR fingerprints revealed significant genetic diversity among indigenous rhizobial strains. In comparison to the reference strain, higher resistance to adverse growth conditions of indigenous S. meliloti strains was determined. Most of the strains grew at temperatures higher than optimal and tolerated both acidic and alkaline environment. It was found that indigenous strains tolerate extremely high NaCl concentrations. Better understanding of rhizobial response to adverse environmental conditions is of potential value for improving rhizobial inoculants and efficiency of symbiotic nitrogen fixation.