Phytoremediation of Mercury and Organomercurials in Chloroplast Transgenic Plants: Enhanced Root Uptake, Translocation to Shoots, and Volatilization

Abstract

Transgenic tobacco plants engineered with bacterial merA and merB genes via the chloroplast genome were investigated to study the uptake, translocation of different forms of mercury (Hg) from roots to shoots, and their volatilization. Untransformed plants, regardless of the form of Hg supplied, reached a saturation point at 200 microM of phenylmercuric acetate (PMA) or HgCl2, accumulating Hg concentrations up to 500 microg g(-1) with significant reduction in growth. In contrast, chloroplast transgenic lines continued to grow well with Hg concentrations in root tissues up to 2000 microg g(-1). Chloroplasttransgenic lines accumulated both the organic and inorganic Hg forms to levels surpassing the concentrations found in the soil. The organic-Hg form was absorbed and translocated more efficiently than the inorganic-Hg form in transgenic lines, whereas no such difference was observed in untransformed plants. Chloroplast-transgenic lines showed about 100-fold increase in the efficiency of Hg accumulation in shoots compared to untransformed plants. This is the first report of such high levels of Hg accumulation in green leaves or tissues. Transgenic plants attained a maximum rate of elemental-Hg volatilization in two days when supplied with PMA and in three days when supplied with inorganic-Hg, attaining complete volatilization within a week. The combined expression of merAB via the chloroplast genome enhanced conversion of Hg2+ into Hg,0 conferred tolerance by rapid volatilization and increased uptake of different forms of mercury, surpassing the concentrations found in the soil. These investigations provide novel insights for improvement of plant tolerance and detoxification of mercury.

Chloroplast Vector Systems for Biotechnology Applications

“…Chloroplast transgenic lines absorbed mercury exceeding the levels in soil and translocated 100-fold more to shoots than untransformed plants (Hussein et al, 2007). Tobacco is ideal for phytoremediation of contaminated soil because it is a non-food non-feed crop.…”

Complete plastid genome sequence of the chickpea (Cicer arietinum) and the phylogenetic distribution of rps12 and clpP intron losses among legumes (Leguminosae)

“…Plastid transgenic lines also lack gene silencing (DeCosa et al, 2001;Lee et al, 2003), position effect due to site specific transgene integration and pleiotropic effects due to subcellular compartmentalization of transgene products (Lee et al, 2003;Daniell et al, 2001;Leelavathi et al, 2003); concerns of transgene silencing, position effect and pleiotropic effects are often encountered in nuclear genetic engineering. Therefore, transgenes have been integrated into plastid genomes to confer valuable agronomic traits, including herbicide resistance (Daniell et al, 1998), insect resistance (McBride et al, 1995;DeCosa et al, 2001), disease resistance (DeGray et al, 2001), drought tolerance (Lee et al, 2003), salt tolerance (Kumar et al, 2004), phytoremediation (Ruiz et al, 2003;Hussein et al, 2007) or expression of various therapeutic proteins or biomaterials (Verma and Daniell, 2007;Kamarajugadda and Daniell, 2006;Daniell et al, 2005). However, soybean is the only legume that has been transformed via the plastid genome so far (Dufoumantel et al, 2004(Dufoumantel et al, , 2005 and more genome sequence information is needed to facilitate plastid genetic engineering in other economically important legumes.…”

Transplastomic plants for innovations in agriculture. A review

“…Therefore, the development of transplastomic plants resistant to such pollutants, e.g., organomercurials (like phenyl mercuric acetate) and mercury salts (HgCl 2 ), by successfully integrating a native operon containing the genes of bacterial mercuric ion reductase (merA) and organomercurial lyase (merB) represents an important example for phytoremediation by transplastomic plants (Ruiz et al 2003;Hussein et al 2007). Again, it should be emphasized that in this case, the transplastomic tobacco plants represent a useful tool for phytoremediation only and accumulate very high levels of Hg in their tissues even in the aerial part of the plants (Hussein et al 2007). Therefore, this method is suitable to clean polluted agronomic fields, but such genetic modifications are useful probably only in non-feed and nonfood crops, especially when they confer in planta tolerance and would result in the accumulation of these harmful compounds in edible plant parts.…”