Zinc solubilization characteristics of efficient siderophore-producing soil bacteria

Background and Objectives: Iron and zinc are two essential micro-nutrients for plant growth and development. Therefore, isolation of siderophores-producing and zinc-solubilizing rhizobacteria involved in bio-availability of these elements is of great interest. Materials and Methods: In this study, soil samples collected from slightly alkaline soil types were screened for high levels of siderophore secretion and zinc solubilization. Results: Among positive colonies, three isolates, named F21A, F37 and F38, were able to secrete siderophore at high levels, ranged between 200 and 300 μM/liter. A close association was observed between siderophore production capability and growth rate as an indicator of active metabolism. Siderophore production was closely correlated with the level of zinc ion released into the medium as well. All three siderophore producing isolates were able to withstand temperature as high as 37°C, high concentration of NaCl (up to 2.5%) and a wide range of initial pH from 6 to 9 while hydrolyzing Zn compounds actively. One of the isolates, F21A, tolerated the presence of 200 mgl^-1 of zinc. Biochemical and molecular characteristics are indicative that these isolates are Pseudomonas japonica. As experienced in a greenhouse experiment, inoculation with the F21A and F37 isolates significantly increase the plants height, fresh and dry weight of corn with compared to control. Conclusion: These findings demonstrated that the potential of P. japonica strains as plants growth promoting rhizobacteria (PGPR) in iron and zinc deficient soils.