Phosphate solubilization potential and modeling of stress tolerance of rhizobacteria from rice paddy soil in northern Iran

The purposes of this study were to evaluate the phosphate solubilization activity of bacteria isolated from the rhizosphere of rice paddy soil in northern Iran, and to study the effect of temperature, NaCl and pH on the growth of these isolates by modeling. Three of the most effective strains from a total of 300 isolates were identified and a phylogenetic analysis was carried out by 16S rDNA sequencing. The isolates were identified as Pantoea ananatis (M36), Rahnella aquatilis (M100) and Enterobacter sp. (M183). These isolates showed multiple plant growth-promoting attributes such as phosphate solubilization activity and indole-3-acetic acid (IAA) production. The M36, M100 and M183 isolates were able to solubilize 172, 263 and 254 mu g ml(-1) of Ca-3(PO4)(2) after 5 days of growth at 28 degrees C and pH7.5, and to produce 8.0, 2.0 and 3.0 mu g ml(-1) of IAA when supplemented with L-tryptophan (1 mg ml(-1)) for 72 h, at 28 degrees C and pH 7.0, respectively. The solubilization of insoluble phosphate was associated with a drop in the pH of the culture medium and there was an inverse relationship between pH and solubilized P (r = -0.98, P < 0.0952). There were no significant differences among isolates in terms of acidity tolerance based on their confidence limits as assessed by segmented model analysis and all isolates were able to grow at pH 4.3-11 (with optimum at 7.0-7.5). Based on a sigmoidal trend of a threeparameter logistic model, the salt concentration required for 50 % inhibition was 8.15, 6.30 and 8.23 % NaCl for M36, M100 and M183 isolates, respectively. Moreover, the minimum and maximum growth temperatures estimated by the segmented model were 5.0 and 42.75 degrees C for M36, 12.76 and 40.32 degrees C for M100, and 10.63 and 43.66 degrees C for M183. The three selected isolates could be deployed as inoculants to promote plant growth in an agricultural environment.