Using a nitrogen-free media, 232 bacteria were isolated from 50 soil samples collected in the provinces East Azerbaijan, Gilan and Ardabil in North of Iran. Plant growth-promoting traits of selected isolates were assessed under in vitro conditions. Moreover, PCR–RFLP analysis of the 16S rDNA and nifH genes was applied to investigate their genetic diversity. Forty percent of the bacteria were able to produce siderophores and the isolate 36A-2 (Pseudomonas) was identified as the best siderophore producer. All bacteria showed a much better phosphate solubilizing capacity with tricalcium phosphate than with rock phosphate as the phosphate source. The bacterial isolate 23A-6 (Pseudomonas) was observed to be the best phosphate solubilizers in the presence of tricalcium phosphate and 34SPIII (Pseudomonas) in the presence of rock phosphate. A K releasing capacity was shown for the bacterial isolates 36A-2 (Pseudomonas), 44SP-2 (Azotobacter chroococcum), 14SPIII (Achromobacter), 14SP2-1 (Azotobacter chroococcum) and 14A-4 (Agrobacterium). However, for most of the bacteria no K releasing ability was observed. The best IAA producer in the presence of 100 mg L-1 L-tryptophan (12.26 mg L-1) was found to be the bacterial isolate 23A-6 (Pseudomonas). Molecular and biochemical identification revealed that the isolates included in the study belong to the genera Azotobacter, Beijerinckia, Klebsiella, Pseudomonas, Rhizobium, Agrobacterium, Achromobacter, Stenotrophomonas and Sphingomonas. The Azotobacteria included in the study were all identified as A. chroococcum. With the exception of the isolate 35A-2, all of them produced the same PCR–RFLP pattern with the 16S rDNA and nifH PCR products. The bacterial isolates 23A-6, 34SPIII and 36A-2 (Pseudomonas) were identified as promising candidates to be used as plant growth-promoting bacteria.
