Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 19, 2007
Publication Date: July 5, 2007
Citation: Papoyan, A., Pineros, M., Kochian, L.V. 2007. The effect of plant cadmium and zinc status on root and shoot heavy metal accumulation in the heavy metal hyperaccumulator, Thlaspi caerulescens. New Phytologist. 175:51-58. Interpretive Summary: Heavy metal contamination of soils poses serious problems worldwide, and the current technologies used to remediate soils are costly and disruptive. There is considerable interest in the use of terrestrial plants to clean up heavy metals from the soil. Several metal hyperaccumulating plant species have been identified that tolerate highly contaminated soils and accumulate these metals to high concentrations. We have been studying the mechanisms for metal hyperaccumulation in Thlaspi caerulescens, a zinc/cadmium (Zn/Cd)hyperaccumulator. In the current study, we investigated the influence of altering plant metal status (Zn and Cd) on the uptake of these metals, as well as uptake of the essential micronutrients nickel, copper and manganese (which also are heavy metals). It was found that growing plants on high levels of Zn or Cd conferred even greater metal tolerance in this already highly tolerant plant species. This increased tolerance was associated with stimulated uptake of heavy metals into the root and accumulation in the shoot. A model was developed based on these results suggesting that a key transport site for heavy metal hyperaccumulation involves root-to-shoot metal translocation. These findings are providing us a better understanding of metal hyperaccumulation, which ultimately will be useful in developing plants better suited for the phytoremediation of metal-contaminated soils.
Technical Abstract: Thlaspi caerulescens is a plant species capable of tolerating and accumulating extremely high concentrations of the heavy metals, Zn and Cd, in the shoot. In this study, we investigated the impact of changes in plant heavy metal status (i.e. Zn and Cd) on the accumulation of heavy metals, including essential micronutrients. Thlaspi caerulescens plants grown hydroponically on a high Zn regimen (500 µM) were found to be more Cd tolerant than plants grown on standard Zn media (1 µM Zn). Furthermore, shoot Cd concentrations were significantly higher in the high Zn-grown plants. A positive correlation was also found between shoot Zn accumulation and increasing plant Cd status, such that plants grown on increasing levels of Cd accumulated more Zn in the shoots. However, this response disappeared in plants grown on high levels of Zn. That is, increasing the concentration of Cd in the growth solution had no effect on shoot Zn accumulation in high Zn-grown plants. Root radiotracer 109Cd influx experiments indicated that plants grown on high (500 µM) Zn maintained significantly higher root Cd2+ influx values, when compared with plants grown on normal (1 µM) Zn. In addition, we also examined the effect of increasing plant Zn status on the shoot accumulation of other micronutrients that also are heavy metals (Cu, Ni, and Mn). It was found that both Ni and Cu shoot accumulation were also stimulated by high plant Zn status, while Mn accumulation was not affected. These findings have led us to speculate that xylem loading may be involved in these responses, and also may be a key transport sites involved in excessive Zn and Cd accumulation in hyperaccumulator plants.