Sulfur dioxide (SO2) is one of the most common and harmful air pollutants. High concentrations of SO2 can cause stress and limit growth in plants. Some of the plants can resist stress by bacterial symbiosis such as Rhizobium symbiosis. Rhizobium is a beneficial bacterium that enhances plant growth and yield. To study the effects of SO2 pollution on growth indexes, protein, proline and sulfur contents, 31 days old plants of Trifolium resupinatum (Persian clover), inoculated with native and standard Rhizobium were exposed to the different concentrations of SO2 (0 as control, 0.5, 1, 1.5 and 2 ppm) for 5 consecutive days. Results showed that inoculation increased leaf area, leaf number, shoot height, root length, shoot fresh and dry weight and protein content of Persian clover but didn’t show any significant effect on proline and sulfur contents. Different concentrations of SO2 had a significant effect on leaf number, shoot height, root length, shoot fresh and dry weight, protein, proline and sulfur contents but didn’t have effects on leaf area. 0.5 ppm concentration of SO2 increased growth indexes and protein content. Proline and sulfur contents didn’t change in 0.5 ppm. Increasing SO2 decreased growth indexes and protein, and increased proline and sulfur contents. Interaction between Rhizobium inoculation and SO2 treatment improved the stress effects of high concentrations of SO2 on growth indexes, protein, proline and sulfur contents. It was therefore concluded that Rhizobium can increase tolerance and resistance of this plant to the abiotic stresses such as SO2 pollution
