Zinc Phytoextraction in Thlaspi caerulescens

The existence of metal hyperaccumulator species demonstrates that plants have the genetic potential to remove toxic metals from contaminated soil. Possibly, one of the best-known hyperaccumulators is Thlaspi caerulescens. This species has been shown to accumulate very high Zn concentrations without manifesting any sign of toxicity. Thus, T. caerulescens represents an excellent experimental system for studying metal hyperaccumulation in plants as it relates to phytoremediation. In this article, we review the results of an investigation into the physiology, biochemistry, and molecular regulation of Zn transport and accumulation in T. caerulescens compared with a nonaccumulator relative T. arvense. Physiological studies focused on the use of ⁶⁵Zn radiotracer flux techniques to characterize zinc transport and compartmentation in the root, and translocation to the shoot. Transport studies 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 greater in T. caerulescens compared with the nonaccumulator T. arvense. The 4.5-fold stimulation of Zn influx into the roots of T. caerulescens was hypothesized to be due to an overexpression of Zn transporters in this species. Additionally, compartmental analysis (radiotracer wash out or efflux techniques) was used to show that Zn was sequestered in the root vacuole of T. arvense inhibiting Zn translocation to the shoot in this nonaccumulator species. Molecular studies focused on the cloning and characterization of Zn transport genes in T. caerulescens. Functional 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. Expression of ZNT1 in yeast allowed for a physiological characterization of this transporter. ZNT1 was shown to encode a high-affinity Zn transporter that can also mediate low-affinity Cd transport. Biochemical 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 may be linked to an alteration of the Zn tolerance mechanism in this species.