Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/1999
Publication Date: N/A
Interpretive Summary: Heavy metal (Pb, Zn, Cd, Cu, Cr and Ni) contamination of soils poses serious problems to both human health and agriculture in the U.S. Current engineering-based technologies used to remediate soils (e.g., removal of top soil for storage in landfills) are quite costly, and often dramatically disturb the landscape. Recently, there has been considerable interest focused on the use of terrestrial plants to absorb heavy metals from the soil and concentrate them in the easily harvestable shoot tissues as an alternative remediation technology. A small number of very interesting plant species have been identified that can grow in soils containing high levels of heavy metals, and will also accumulate these metals to high concentrations in the shoot. Despite the intense interest in these hyperaccumulator plants, very little is known about mechanisms of heavy metal transport, translocation and sequestration involved in heavy metal hyperaccumulation in plants. Therefore, in this study we conducted a physiological characterization of Zn accumulation in the heavy metal hyperaccumulating plant, Thlaspi caerulescens. It was found that a number of different Zn transport sites within the plant were altered in this plant species. The results indicate that metal hyperaccumulation is a complex trait involving a number of different transport sites within the plant. This fundamental information will help us gain a better understanding of metal hyperaccumulation, and this information will be used to develop more effective remediating plant species and agronomic practices that enhance phytoremediation.
Technical Abstract: We have previously shown that zinc hyperaccumulation in shoots of Thlaspi caerulescens involves a stimulated Zn2+ influx into the root symplasm. In the current study, we investigated the role of Zn compartmentation in the root, Zn transport into the xylem, and Zn absorption into leaf cells in Zn hyperaccumulation in T. caerulescens compared with a related nonaccumulator, Thlaspi arvense. 65Zn compartmental analysis showed that significant fraction of symplasmic zinc is stored in the root vacuole of T. arvense, and presumably is unavailable for loading into the xylem and subsequent translocation to the shoot. In T. caerulescens, however, a smaller fraction of the absorbed Zn was stored in the root vacuole and the vacuolar Zn was readily transported out of the vacuole back into the cytoplasm. We conclude that in T. caerulescens, Zn absorbed by roots is readily available for loading into xylem. This is supported by analysis of fxylem exudate collected from detopped Thlaspi seedlings. When seedlings o the two species were grown on either low (1 uM) or high (50 uM) Zn, xylem sap of T. caerulescens contained approximately 5-fold more Zn than T. arvense. This increased xylem Zn in T. caerulescens was not correlated with a stimulated production of any specific organic or amino acid. The capacity of Thlaspi leaf cells to absorb 65Zn was studied in leaf sections and leaf protoplasts. At high Zn levels typical of that found in xylem (1 mM, 2.2-fold more zinc was accumulated in leaf sections of T. caerulescens. These findings indicate that altered tonoplast Zn transport in root cells, as well as stimulated Zn uptake in leaf cells play a role in the dramatic Zn hyperaccumulation expressed in T. caerulescens.