Introduction: The Rhizobium-legume interaction leads to biological nitrogen-fixation and increases nitrogen of soil. The aim of this study was to characterize the properties of Sinorhizobium isolates from the roots of alfalfa plants in Iran. Materials and methods: Bacteria were isolated in yeast extract mannitol Agar and confirmed by plant infection test. After evaluation from the point of morphological and biochemical properties, a fragment of 16S rDNA gene with a size of approximately 1500 base pair was amplified using fD1 and rD1 primers. PCR (polymerase chain reaction) products were analyzed for digestion pattern by Taq1 endonuclease. Results: 63 bacteria were isolated from homogenized nodules. 42 isolates generated nodules in three replicates in infection test. Of the 42 isolates 8, were resistant to salinity. Seven isolates had better growth than others at pH 4. All isolates were resistant to CuCl2 (0.5 mmol), CdCl2 (0.65 mmol), MnSO4 (0.75 and 1.5 mmol) and ZnSO4 (0.125 mmol). Isolates S3Q and S22K were more resistant to salinity, acidity, temperature and heavy metals stresses. PCR products of all bacteria had the same restricted profile after digestion by Taq1 nuclease. Discussion and conclusion: The results showed that among isolated bacteria, there were some differences in the resistance to salinity, acidity, temperature and heavy metals stresses. Identification of native strains of rhizobia, especially strains resistant to salinity, temperature, heavy metals and acidity could be valuable due to their potentiality for using biological fertilizers in harsh conditions.

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