Differential mercury volatilization by tobacco organs expressing a modified bacterial merA gene

HE Yu Ke^{1, 2}, Jian Ge SUN¹, Xian Zhong FENG^{1, 2}, Mihaly CZAKÓ¹, Laszlo MÁRTON¹

¹Department of Biological Sciences, University of South Carolina, 700 Sumter Street, Columbia, SC29208, USA
²National Laboratory of Plant Molecular Genetics, Shanghai Institute of Plant Physiology and Ecology, Chinese Academy of Sciences, 300 Fenglin Road, Shanghai 200032, China
Communicated by L¨¢szl¨® M¨¢rton, Department of Biological Sciences, University of South Carolina, 700 Sumter Street, Columbia, SC29208, USA

Mercury pollution is a major environmental problem accompanying industrial activities. Most of the mercury released ends up and retained in the soil as complexes of the toxic ionic mercury (Hg²⁺), which then can be converted by microbes into the even more toxic methylmercury which tends to bioaccumulate. Mercury detoxification of the soil can also occur by microbes converting the ionic mercury into the least toxic metallic mercury (Hg⁰) form, which then evaporates. The remediation potential of transgenic plants carrying the MerA gene from E. coli encoding mercuric ion reductase could be evaluated.A modified version of the gene, optimized for plant codon preferences (merApe9, Rugh et al. 1996), was introduced into tobacco by Agrobacterium-mediated leaf disk transformation. Transgenic seeds were resistant to HgCl₂ at 50 mM, and some of them (10-20% ) could germinate on media containing as much as 350 mM HgCl₂, while the control plants were fully inhibited or died on 50 mM HgCl₂. The rate of elemental mercury evolution from Hg²⁺ (added as HgCl₂) was 5-8 times higher for transgenic plants than the control. Mercury volatilization by isolated organs standardized for fresh weight was higher (up to 5 times) in the roots than in shoots or the leaves. The data suggest that it is the root system of the transgenic plants that volatilizes most of the reduced mercury (Hg⁰). It also suggests that much of the mercury need not enter the vascular system to be transported to the leaves for volatilization. Transgenic plants with the merApe9 gene may be used to mercury detoxification for environmental improvement in mercury-contaminated regions more efficiently than it had been predicted based on data on volatilization of whole plants via the upper parts only (Rugh et al. 1996).