This study was conducted to investigate the role of 1-aminocyclopropane-1-carboxylate (ACC) deaminase in Rhizobium strains indigenous to Iranian soils and their ability to reduce stress ethylene and promote the elongation of the roots of wheat seedlings. One hundred different rhizobial strains isolated from some Iranian soils were cultured on Rhizobial Minimal Medium (RMM) with two different sources of nitrogen (ACC & ammonium chloride). RMM without any nitrogen was taken as the control. From the rhizobial strains tested, sixty five were positive for ACC deaminase and were then divided into three groups of strong, mild and weak based on ACC deaminase production. According to this classification, all of the strong group members and most of the mild ones belonged to Sinorhizobium meliloti. In addition, some strains were tested for their ability to promote the root elongation of wheat seedling under gnotobiotic conditions. ACC deaminase containing rhizobia could increase the length of root and also shoot of wheat seedlings significantly, in comparison with the negative control. Those strains belonged to strong group were more successful in elongation of wheat seedling roots.

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