PHYSIOLOGICAL AND MOLECULAR MECHANISMS OF HEAVY METAL TOLERANCE AND TRANSPORT IN THE HYPERACCUMULATOR PLANT SPECIES, THLASPI CAERULESCENS
Heavy metal pollution of the environment is significant problem throughout the world. One possible avenue for heavy metal decontamination of the environment is phytoremediation, which is a technology based on the remarkable abilities of certain plant species to tolerate and accumulate extremely high concentrations of heavy metals. One of the best known heavy metal hyperaccumulator plant species is Thlaspi caerulescens, which is a Zn/Cd-hyperaccumulator that can accumulate and tolerate up to 30,000 ppm Zn and 10,000 ppm Cd in the shoots without exhibiting toxicity symptoms. The research described in this dissertation focuses on identifying gene(s) that may be responsible for the extreme heavy metal accumulation phenotype in Thlaspi caerulescens. In the research conducted here, it was demonstrated that xylem metal loading may play a key role in heavy metal hyperaccumulation. In initial studies, the influence of altered plant metal status on metal (Zn, Cd) accumulation in Thlaspi caerulescens showed that increased metal status stimulated subsequent heavy metal (Cd) accumulation in the shoots but not roots, suggesting that growth on high metal levels stimulates metal loading into the xylem. Subsequently, a heavy metal transporting P1B-type ATPase, TcHMA4, was cloned from Thlaspi caerulescens and shown to mediate cellular heavy metal efflux and tolerance when expressed in yeast. TcHMA4 is expressed primarily in the root vascular tissue and its expression is strongly upregulated upon exposure to high concentrations of heavy metals. These findings indicate that TcHMA4 may be responsible for metal xylem loading, and thus play a key role in the enhanced root to shoot metal translocation that is so important to hyperaccumulation. Furthermore, peptides derived from the C terminus of the TcHMA4 protein that harbor several heavy metal binding domains were shown to confer a significant increase in metal accumulation and tolerance when expressed in transgenic yeast (Saccharomyces cerevisiae) and plants (Arabidopsis thaliana). These findings indicate that the C terminus peptides have the capacity to serve as heavy metal binding ligands, and may be useful for enhancing the phytoremediation potential of plants via biotechnology.
THLASPI CAERULESCENS, HEAVY METALS, HMA4, HYPERACCUMULATOR