Submitted to: Journal of Experimental Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/23/2000
Publication Date: N/A
Citation: Interpretive Summary: Heavy metal contamination of soils poses serious problems to both human health and agriculture in the U.S. Current engineering-based technologies used to remediate soils 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 an integrated molecular and physiological characterization of Zn accumulation in the heavy metal hyperaccumulating plant, Thlaspi caerulescens. A Zn transport gene was cloned from T. caerulescens and found to be 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 remediating plant species and agronomic practices that enhance phytoremediation.
Technical Abstract: This manuscript details findings from an integrated molecular and physiological investigation of heavy metal (Zn) transport in the metal accumulating plant species, Thlaspi caerulescens which exhibits extraordinarily high levels of Zn accumulation in the shoot (up to 3% Zn dry wt. without any toxicity symptoms). Physiological studies of Zn transport showed that a number of Zn transport sites were stimulated or altered in T. caerulescens, contributing to the hyperaccumulation trait. These transport sites included Zn influx into both root and leaf cells, and Zn loading into the xylem. The 4-5-fold stimulation of Zn influx into the root was hypothesized to be due to an increased expression of Zn transporters in T. caerulescens root cells. Additionally, compartmental analysis was used to show that Zn was sequestered in the root vacuole of T. arvense, which retarded Zn translocation to the shoot in this non-accumulator species. Molecular studies focused on the cloning and characterization of Zn transport genes in T. caerulescens. Complementation of a yeast Zn transport-defective mutant with a T. caerulescens cDNA library constructed in a yeast expression vector resulted in the cloning of a Zn transport cDNA, ZNT1. Sequence analysis of ZNT1 indicated it is a member of a recently discovered micronutrient transport gene family. ZNT1 was shown to encode a high affinity Zn transporter which can also mediate low affinity Cd transport. Northern analysis of ZNT1 and its homologue in the two Thlaspi species indicated that enhanced Zn transport in T. caerulescens results from a constitutively high expression of ZNT1 in roots and shoots. In T. arvense, the ZNT1 homologue is expressed to much lower levels and this expression is stimulated by imposition of Zn deficiency.