Bioremediation of Heavy Metals using the Interaction between Plants and Genetically Engineered Microbes

Abstract

Excessive levels of heavy metals (HMs) in agricultural soil is a critical concerns for crop production and food safety and pose potential hazards to human and animal health. Anthropogenic sources including agriculture, mining, smelting, electroplating, and other industrial activities have resulted in the deposition of undesirable concentration of metals, such as arsenic (As), cadmium (Cd), chromium (Cr), and lead (Pb) in the soil. Unlike many other pollutants, HMs are difficult to remove from the environment as they cannot be degraded by any method, and are ultimately indestructible. The use of microorganisms and plants for soil remediation of HMs are of great interest because of their high efficiency, ease of use, and cost-effective application. Microorganisms can be used to remediate contaminated soil by detoxification, sequestration, and solubilization of HMs to facilitate their extraction. These microbes may act on HMs by chelation, precipitation, transformation (oxidation-reduction, methylation), biosorption, and accumulation. However, high concentrations of HMs in soil lead to decreased number of soil microbes. These symbiotic rhizospheric microbes depend on plant root exudates for their nutrition, thus to improve the number of microbes, it is also essential to optimize microbial nutrition by optimizing plant-microbe interaction. Different approaches were adopted to address these problems, from enrichment with rhizosphere bacterial consortia resistant to HMs to genetic engineering of plant growth-promoting symbiotic microbes. These genetically engineered (GE) microbes show improved bioremediation potential by enhancing their metal metabolism efficiency as well as increasing soil fertility. In this review, we describe how GE microbes and their association with plants enhance metal tolerance, accumulation, and detoxification in microbes and plants. We also describe the potential of bioremediation using symbiosis between plants and GE microbes.