Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2004
Publication Date: 12/17/2004
Citation: Papoyan, A., Kochian, L.V. 2004. Identification of thlaspi caerulescens genes that may be involved in heavy metal hyperaccumulation and tolerance: characterization of a novel heavy metal transporting atpase. Plant Physiology. 136:3814-3823. 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 the use of terrestrial plants to clean up heavy metals from the soil, a process termed phytoremediation. A small number of metal hyperaccumulating plants have been identified that can grow in highly contaminated soils and accumulate these metals to high shoot concentrations. We have been studying the mechanisms for metal hyperaccumulation in Thlaspi caerulescens, a zinc/cadmium hyperaccumulator, in order to identify mechanisms and associated genes for this unique hyperaccumulation phenotype. One of the hallmarks of all hyperaccumulating plant species is their ability to very efficiently translocate the metal that was absorbed from the soil to the above-ground shoot biomass. In this study, we expressed Thlaspi genes in yeast to identify genes that confer tolerance to cadmium. One of these genes encoded a heavy metal ATPase, that we showed pumped toxic metals out of yeast cells. However in Thlaspi, based on where the gene is expressed (only in the center of the root) we propose it is the key transporter that pumps metals from the root to the shoot. This possibility is now being tested in transgenic plants. If this is the case, this may be a very useful gene for use via biotechnology to improve the phytoremediation potential of different plant species.
Technical Abstract: Thlaspi caerulescens is a heavy metal hyperaccumulator plant species that accumulates extremely high levels of Zn and Cd in its shoots. It has been the subject of intense research to gain a better understanding of the mechanisms of heavy metal hyperaccumulation, and as a source of genes for developing plant species better suited for the phytoremediation of metal contaminated soils. In this study we report on the results of a yeast screen that identified a number of Thlaspi genes that conferred Cd tolerance to yeast, including possible metal binding ligands from the metallothionein gene family, and a P-type ATPase that is a member of the P1B-subfamily of heavy metal-translocating ATPases. A detailed characterization of the heavy metal ATPase, TcHMA4, demonstrated that it mediates yeast metal tolerance via active efflux of a number of different heavy metals (Cd, Pb, Zn, Cu) out of the cell. However in Thlaspi caerulescens, based on differences in tissue-specific and metal-responsive expression of this transporter compared with its homolog in Arabidopsis thaliana, we hypothesize that it may play a role in xylem loading of metals and thus could be a key player in the hyperaccumulation phenotype expressed in Thlaspi caerulescens. Additionally, we show that the C terminus of the TcHMA4 protein, which contains numerous histidine and cysteine repeats, participates in heavy metal binding. When partial peptides from this C terminal domain were expressed in yeast, they conferred an extremely high level of Cd tolerance and Cd hyperaccumulation. The possibilities for enhancing the metal tolerance and phytoremediation potential of higher plants via expression of these metal-binding peptides are also discussed.