Molecular physiology of zinc transport in the Zn hyperaccumulator Thlaspi caerulescens

In this manuscript, recent research from this laboratory into physiological and molecular aspects of heavy metal (Zn) transport in the hyperaccumulating plant species, Thlaspi caerulescens is reviewed. This research is aimed at elucidating the processes that underlie the accumulation of extraordinarily high levels of Zn in the T. caerulescens shoot (up to 3% Zn dry wt.) without any associated toxicity symptom. Physiological studies focused on the use of radiotracer flux techniques (65Zn2+) to characterize zinc transport and compartmentation in the root, and translocation and accumulation in the shoot of T. caerulescens in comparison with a related non-accumulator, T. arvense. These studies indicated that Zn transport was stimulated at a number of sites in T. caerulescens, contributing to the hyperaccumulation trait. The transport processes that were stimulated included Zn influx into both root and leaf cells, and Zn loading into the xylem. The 4- to 5-fold stimulation of Zn influx into the root was hypothesized to be due to an increased abundance of Zn transporters in T. caerulescens root cells. Additionally, compartmental analysis (radiotracer wash out or efflux techniques) was used to show that Zn was sequestered in the vacuoles of T. arvense root cells which retarded Zn translocation to the shoot in this non-accumulator species. Molecular studies have 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 resulted in the recovery of a cDNA, ZNT1, that encodes a Zn transporter. Sequence analysis of ZNT1 indicated it is a member of a recently discovered micronutrient transport gene family which includes the Arabidopsis Fe transporter, IRT1, and the ZIP Zn transporters. Expression of ZNT1 in yeast allowed for a physiological characterization of this transporter. It 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 the ZNT1 gene in roots and shoots. In T. arvense, ZNT1 is expressed at far lower levels and this expression is stimulated by imposition of Zn deficiency.