Abstract
Main conclusionTransgenic Arabidopsis thaliana and Populus alba plants overexpressing the zinc transporter ScZRC1 in shoots exhibit Zn tolerance. Increased Zn concentrations were observed in shoots of P. alba, a species suitable for phytoremediation.Genetic engineering of plants for phytoremediation is worth to consider if genes leading to heavy metal accumulation and tolerance are expressed in high biomass producing plants. The Saccharomyces cerevisiae ZRC1 gene encodes a zinc transporter which is primarily involved in the uptake of Zn into the vacuole. The ZRC1 gene was expressed in the model species A. thaliana and P. alba (cv. Villafranca). Both species were transformed with constructs carrying ScZRC1 under the control of either the CaMV35S promoter for constitutive expression or the active promoter region of the tobacco Rubisco small subunit (pRbcS) to limit the expression to the above-ground tissues. In hydroponic cultures, A. thaliana and poplar ScZRC1-expressing plants accumulated more Zn in vegetative tissues and were more tolerant than untransformed plants. No differences were found between plants carrying the CaMV35::ScZRC1 or pRbcS::ScZRC1 constructs. The higher Zn accumulation in transgenic plants was accompanied by an increased superoxide dismutase (SOD) activity, indicating the activation of defense mechanisms to prevent cellular damage. In the presence of cadmium in addition to Zn, plants did not show symptoms of metal toxicity, neither in hydroponic cultures nor in soil. Zn accumulation increased in shoots, while no differences were observed for Cd accumulation, in comparison to control plants. These data suggest that ectopic expression of ScZRC1 can increase the potential of poplar for the remediation of Zn-polluted soils, although further tests are required to assay its application in remediating multimetal polluted soils.
Highlights
Environmental heavy metal pollution is a worldwide issue that arises mainly from industrial activities, mining and agricultural practices (Nriagu and Pacyna 1988)
A. thaliana was transformed with constructs carrying the ScZRC1 coding sequence under the control of the CaMV35S promoter conferring constitutive expression, or under the control of the light-inducible promoter of the tobacco small subunit of Rubisco, to test the expression of ScZRC1 only in the above-ground plant tissues
The promising results regarding Zn accumulation and tolerance in A. thaliana have brought us to investigate the effects of the expression of this transporter in the poplar clone Villafranca, a good candidate for metal phytoextraction due to its fast and consistent biomass production (Romeo et al 2014)
Summary
Environmental heavy metal pollution is a worldwide issue that arises mainly from industrial activities, mining and agricultural practices (Nriagu and Pacyna 1988). Biological remediation of metal-polluted soils can prove challenging, since contrary to organic compounds, biodegradation of heavy metals is impossible and they continuously accumulate in the environment (Shim et al 2013; He et al 2015a, b) To this respect, plants can be exploited to help detoxifying polluted soils, due to their ability in the uptake and root-to-shoot transport of heavy metals. Phytoremediation encompasses two main processes: (i) pollutant containment and stabilization, when the vegetation covers the polluted areas, preventing migration to ground waters and nearby areas, reducing the inlet of toxic metals into the food chain (Raskin and Ensley 2000) and (ii) extraction in which the plant species accumulate pollutants in harvestable tissues The latter has the advantage that the heavy metal is permanently removed, and in some cases, can be recovered from the plant tissues (Elekes 2014). At the end of the process the concentration of heavy metals in the shoot biomass should be higher than that present in the soil
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