Isolation, identification, and application of zinc-solubilizing bacteria exhibiting beneficial traits to promote plant growth and metal uptake

This study was conducted to isolate and characterize zinc (Zn)-solubilizing, plant growth-promoting (PGP) bacterial strains from mine soils and evaluate their inoculation effects on the growth and Zn uptake of corn and Indian mustard. A total of 171 Zn-tolerant bacterial strains were isolated from the mine soils, out of which six strains showed a significant increase (P < 0.01) in Zn solubility in a sucrose-minimal salt low-phosphate liquid medium. This increase in solubility was accompanied by a significant decrease (P < 0.01) in the pH values of the medium. The Zn-solubilizing bacteria (ZSB) were found to exhibit multiple PGP traits. All the strains were found to produce varying amounts of indole acetic acid and siderophore. Additionally, four of the strains displayed the ability to solubilize phosphate. These strains also demonstrated resistance to heavy metals, salinity, and diverse pH ranges. Based on the 16S rRNA gene sequence analysis, it was determined that the Zn-solubilizing strains belonged to four genera: Bacillus, Paenibacillus, Klebsiella, and Brevibacterium. Corn plants grown in Zn-contaminated sand exhibited enhanced shoot growth and Zn accumulation after inoculation with the selected Zn-solubilizing PGP strains, namely Bacillus tequilensis YSP3 and Paenibacillus pabuli YSP148. The efficiency of the YSP3, YSP148, and YSP3+YSP148 consortium bacterial inocula was also demonstrated in enhancing growth and metal (Zn, Pb, and Cd) uptake of corn and Indian mustard grown in a metal-contaminated soil. The study thus suggests the ZSB could play an important role in remediating metal-contaminated lands through microbe-assisted phytoremediation approaches.