Phytoremediation of potentially toxic elements (PTEs) contaminated soils using alfalfa (Medicago sativa L.): A comprehensive review

Soil contamination with potentially toxic elements (PTEs) is an increasing environmental problem, posing serious threats to the living organisms. Phytoremediation is a sustainable and highly accepted technology for remediation of PTE-contaminated soils. Alfalfa has been widely adopted for the phytoremediation of PTE-contaminated soils due to its large biomass productivity, high PTE tolerance, and strong capacity to take up PTEs. However, there are still no literature reviews systematically summarized the potential of alfalfa in the phytoremediation. Therefore, we review the available literatures that present its PTE uptake, phytotoxicity, tolerance mechanisms, and aided techniques improving the phytoremediation efficiency. In this review, alfalfa shows high amounts of PTEs accumulation, especially in their root tissue. Meanwhile, the inner mechanisms of PTE tolerance and accumulation in alfalfa are discussed including: (i) the activation of antioxidant enzyme system, (ii) subcellular localization, (iii) production of glutathione, phytochelatins, and proline, and (iv) regulation of gene expression. Indeed, excessive PTE can overcome the defense system, which causes oxidative damage in alfalfa plants, thereby inhibiting growth and physiological processes and weakening the ability of PTE uptake. Till now, several approaches have been developed to improve the tolerance and/or accumulation of PTE in alfalfa plants as follows: (i) selection of PTE tolerant cultivars, (ii) applying plant growth regulators, (iii) addition of chelating agents, fertilizer, and biochar materials, and (iv) inoculation of soil microbes. Finally, we indicate that the selection of PTE-tolerant cultivars along with inoculation of soil microbes may be an efficient and eco-friendly strategy of the soil PTE phytoremediation.