Submitted to: International Journal of Phytoremediation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2002
Publication Date: 7/1/2002
Citation: LASAT, M.M., PENCE, N.S., LETHAM, D.L., KOCHIAN, L.V. ZINC PHYTOEXTRACTION IN THLASPI CAERULESCENS. INTERNATIONAL JOURNAL OF PHYTOREMEDIATION. 2002. Interpretive Summary: Heavy metal contamination of soils poses serious problems to our society, and the current technologies used to remediate soils are quite costly and disruptive. There is considerable interest in a remediation technology using terrestrial plants to extract heavy metals from the soil. A small number of 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, we have been conducting an integrated molecular and physiological characterization of Zn accumulation in Thlaspi caerulescens, a heavy metal hyperaccumulator. Physiological investigations showed that root Zn influx, Zn transport to the shoot, and entry of Zn into leaf cells are all stimulated in this hyperaccumulator. A molecular characterization of a gene that encodes an important Zn transporter in this plant showed that this gene is expressed to very high levels in the root and shoot. In a related non-accumulator plant (T. arvense), the same gene was expressed to very low levels in Zn sufficient plants, and then expression of the gene increased with Zn deficiency. Thus, the mechanisms by which Zn transporters are regulated by Zn status are altered in the hyperaccumulator, such that Zn transporters are produced at very high levels throughout the plant. This information will help us gain a better understanding of metal hyperaccumulation, and this information will be used to develop more effective heavy metal-remediating plant species.
Technical Abstract: The existence of metal hyperaccumulator species demonstrates that plants have the genetic potential to remove toxic metals from contaminated soil. We have been studying the Zn/Cd hyperaccumulator, Thlaspi caerulescens, as a model system for understanding metal hyperaccumulation as it relates to phytoremediation. In this paper, we describe the physiology, biochemistry and molecular regulation of Zn transport and accumulation in T. caerulescens in comparison with a related nonaccumulator, T. arvense. These findings indicated that a number of Zn transport sites were stimulated in T. caerulescens, contributing to the hyperaccumulation trait. Thus, Zn influx into root and leaf cells, and Zn loading into the xylem was much greater in T. caerulescens compared with T. arvense. This enhanced Zn transport was hypothesized to be due to an overexpression of Zn transport proteins in the plant. Additionally, compartmental analysis (radiotracer wash out or efflux techniques) was used to show that Zn was sequestered in the root vacuole of T. arvense, reducing Zn translocation to the shoot in the nonaccumulator. Molecular studies focused on the cloning and characterization of Zn transport genes in T. caerulescens. Functional complementation of a yeast Zn transport-defective mutant resulted in the cloning of a Zn transport cDNA, ZNT1. Expression of ZNT1 in yeast allowed for a physiological characterization of this transporter. ZNT1 was shown to encode a high affinity Zn transporter which can also mediate low affinity Cd transport. Northern analyses indicated that enhanced Zn transport in T. caerulescens results from a constitutively high expression of ZNT1 in roots and shoots. These results suggest that overexpression of ZNT1 is linked to the metal hyperaccumulation trait in this species.