Isolation, Screening and Evaluation of Plant Growth Stimulating Traits of Cd and Pb Resistant Microorganisms

Success of the phytoremediation technique depends not only on plant species, but also largely on the interactions of plant roots with the rhizosphere microorganisms. These microorganisms, especially bacteria with plant growth promoting traits, can improve efficiency of phytoremediation by helping to proper plant establishment, increasing root system growth and, consequently, increasing plant growth and enhancing heavy metal uptake. Considering the important role of soil microbial community in increasing the remediation of polluted soil with plants, this research was conducted with the aim of isolating, screening, investigating the traits of cadmium and lead-resistant bacteria and introducing superior isolates. Soil samples were taken from Cd and Pb contaminated soils of the Shahid Tondguyan oil refinery and after measuring some physical and chemical properties, heavy metals resistant microorganisms were isolated from them. Resistance to cadmium and lead was determined in the isolates, and then the ability of the superior isolates to produce phytohormones of auxin, secretion of growth inhibitor metabolites and solubilization of insoluble inorganic phosphate were evaluated. In this study, thirty microorganisms were isolated from contaminated soils. After examining the appearance of the colony, its color and margin, as well as the growth rate, at the end, 20 different isolates were selected. 70% of the studied isolates showed a very good growth in culture medium up to a concentration of 8 mM l-1 of lead and cadmium. The results of evaluation of plant growth promoting traits in the top 10 isolates in terms of resistance to heavy metals of lead and cadmium showed that all of these isolates had the ability to produce auxins and dissolve insoluble inorganic phosphates. The highest (10.20 mg l-1) and the lowest (0.64 mg l-1) auxin production were observed for C4 and C2 isolates, respectively. The average solubility of tricalcium phosphate by isolates was 106.91 mg l-1. 80% of isolates had the same ability to produce siderophore. The highest rate of production of this metabolite was observed in the isolate C1 with a halo to colony ratio of 23.3. Among 10 studied isolates, three isolates, K2, K5 and C8, were able to produce hydrogen cyanide, protease and cellulase enzymes.