Trends in Legume-Rhizobia Symbiosis in Remediation of Mercury-Contaminated Agricultural Soils

The agricultural ecosystem is moderately contaminated with mercury due to industries, mining, and farming activities. Mercury concentrations in agrarian lands depend on pH, organic matter, soil texture, vegetation cover, and land usage. The legume-rhizobia symbiosis is an effective rhizoremediation tool to lessen mercury and recover soil nitrogen. Mercury-resistant rhizobia in the roots of legumes assist the host by alleviating the phytotoxicity of Hg by stabilizing Hg in the roots and producing plant growth-promoting substances. The mechanisms of Hg resistance in rhizobia include reducing mercury to a volatile form (Hg0) and sequestering in the cell wall. Legume roots can phytostabilize mercury due to their high biomass. Thus, the translocation of mercury to shoot is low, with less than one translocation and bioaccumulation factor. Sulfur compounds, enzymes, amino polycarboxylic acids, and low molecular weight organic acids can enhance mercury phytoremediation by improving plant biomass and mercury uptake. Mercury stress increases free radical scavenging in nodules. Therefore, legumes and the associated rhizobia are sustainable tools for the remediation of polluted soil. However, Hg toxicity affects nodulation by rhizobia, nodule number, morphology, weight, and micro-community structure. Contemplating the harmfulness of mercury on rhizobia, effective rhizoremediation strategies can be developed to reduce mercury in agroecosystems.