Alleviation of Mercury Toxicity in Wheat by the Interaction of Mercury-Tolerant Plant Growth-Promoting Rhizobacteria

Heavy metal contamination of agricultural soils has increased along with industrialization. Mercury is a toxic heavy metal and a widespread pollutant in the ecosystem. Mercury-tolerant and plant growth-promoting rhizobacteria (PGPR) HG 1, HG 2, and HG 3 were isolated from the rhizosphere of plants growing in a mercury-contaminated site. These isolates were able to grow in the presence of mercury ranging from 10 to 200 µM in minimal medium and 25 to 500 µM in LB medium. The strains were characterized by morphological, biochemical, and plant growth-promoting traits. In the present study, these PGPR strains were analyzed for their involvement in metal stress tolerance in Triticum aestivum (wheat). Two bacterial strains, namely, Enterobacter ludwigii (HG 2) and Klebsiella pneumoniae (HG 3), showed better growth promotion of T. aestivum seedlings under metal stress. Different growth parameters like, water content and biochemical properties were analyzed in the PGPR-inoculated wheat plants under 75 µM HgCl₂. Shoot length, root length, shoot dry weight, root dry weight and relative water content (RWC) were significantly higher in inoculated plants compared to uninoculated plants under stress condition. Proline content, electrolyte leakage, and malondialdehyde content (shoots and roots) were significantly lower in inoculated plants with respect to uninoculated plants under mercury stress. Therefore, it could be assumed that all these parameters collectively improve plant growth under mercury stress conditions in the presence of PGPR. Hence, these PGPRs can serve as promising candidates for increasing plant growth and also have immense potential for bioremediation of mercury-contaminated soils.