Enhanced mercury processing by plants genetically engineered for mercury phytoremediation

Abstract

Industrial practices have released mercury into soils and sediments that now require remediation.Current remediation strategies are costly,site-destructive and may cause a temporary increase in mobilization of mercury from contaminated sites.We are developing an alternative mercury remediation strategy,phytoremediation. Phytoremediation is the use of plants to extract,then sequester or detoxify contaminants from polluted soil or water.We have engineered several plant species with modified versions of the bacterial mercuric ion reductase gene,merA .The MerA enzyme electrochemically reduces Hg(II)to the less toxic and volatile Hg(0).Tobacco (Tabacum nicotiana),rice (Oryza sativa )and cottonwood (Populus deltoides )plants engineered with the merA gene have a strong survival advantage over wildtype counterparts when grown on highly Hg(II)-contaminated liquid or soil media.Direct measurement of Hg(0) volatilization from merA rice plants confirms that resistance is a result of MerA-mediated electrochemical reduction of Hg(II)to Hg(0).A lower tissue retention of root-absorbed Hg(II)in merA tobacco,rice and cottonwood than in wild-type counterparts provides further evidence of efficient Hg(II)reduction followed by Hg(0)volatilization.Tobacco plants were used as a model to study changes in mercury/plant interactions caused by the introduction of the merA gene.merA tobacco roots growing on semisolid growth medium penetrated an insoluble HgS barrier more effectively than wild-type roots. MerA activity provided partial protection from Hg(II)-induced transpiration stress in transgenic tobacco on HgCl2 -spiked hydroponics medium.Much lower leaf retention of atmospherically absorbed Hg(0)in merA than wild-type plants incubated in a closed chamber confirmed endogenous plant Hg(0)oxidation was efficiently countered by MerA Hg(II)reduction.Wild-type tobacco shoots grafted to merA tobacco roots accumulated and/or retained more root-absorbed mercury in aboveground tissues than intact merA or wild-type plants.This introduces the feasibility of engineering a mercury phytosequestering plant with root-specific merA activity.Results from experiments with merA tobacco,rice and cottonwood suggest that merA effectively mobilizes mercury within plants,allowing vertical mercury transport and efficient Hg(0)volatilization. Although the merA gene is likely to significantly enhance the ability of plant roots to encounter and absorb soil-bound Hg(II),additional engineering may be required for efficient soil Hg(II)extraction.