Location: Plant, Soil and Nutrition ResearchTitle: Elevated expression of TcHMA3 plays a key role in the extreme Cd tolerance exhibited by a Cd-hyperaccumulating ecotype of Thlaspi caerulescens) Author
|Zambrano, M. Glemencia|
|Jian, Feng Ma|
Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2011
Publication Date: 4/4/2011
Citation: Milner, M., Oeno, D., Naoki, Y., Yokosho, K., Zambrano, M., Kaskie, M., Ebbs, S., Jian, F., Kochian, L.V. 2011. Elevated expression of TcHMA3 plays a key role in the extreme Cd tolerance exhibited by a Cd-hyperaccumulating ecotype of Thlaspi caerulescens. Plant Journal. 66(5):852-862. Interpretive Summary: Heavy metal contamination of soils poses serious problems worldwide, and the current technologies used to remediate soils are costly and disruptive. There is considerable interest in the use of terrestrial plants to clean up heavy metals from the soil. Several metal hyperaccumulating plant species have been identified that tolerate highly contaminated soils and accumulate these metals to high concentrations. We have been studying the mechanisms for metal hyperaccumulation in Thlaspi caerulescens, a zinc/cadmium (Zn/Cd)hyperaccumulator. Because Cd is a highly toxic environmental contaminant, we are particularly interested in determining the molecular basis for Cd hyperaccumulation in Thlaspi. Thus we used gene chips to study global gene expression in two varieties of Thlaspi that differ in their ability to accumulate Cd. From this analysis, we found that a particular metal transport gene from the ATPase family of transporters was much more highly expressed in the Thlaspi variety that is a much better Cd accumulator (Ganges). Further analysis of this gene and the protein it encodes determined that it is a unique transporter that specifically accumulates Cd in the central vacuole of leaf cells in the Ganges variety of Thlaspi. The vacuole is a large storage compartment in plant cells where many waste products and environmental contaminants can be stored, protecting the plant cell from damage. These findings are significant, as we have identified the first gene that plays a key role in Cd hyperaccumulation in the leaves of Thlaspi caerulescens. This research is enhancing our understanding of toxic metal hyperaccumulation in plants, which ultimately will be useful in developing plants better suited for the phytoremediation of soils contaminated with Cd and other toxic metals.
Technical Abstract: Cadmium (Cd) is a highly toxic heavy metal for plants, but several unique Cd hyperaccumulating plant species are able to accumulate this metal to extraordinary concentrations in the above-ground tissues without showing any toxic symptoms. However, the molecular mechanisms underlying this hyper-tolerance to Cd are poorly understood. Here we have isolated and functionally characterized an allelic gene, TcHMA3 (Heavy Metal ATPase 3) from two ecotypes (Ganges and Prayon) of Thlaspi caerulescens contrasting in Cd accumulation and tolerance. The TcHMA3 alleles from the higher (Ganges) and lower Cd accumulating ecotype (Prayon) share 97.8% identity and encode a P1B-type ATPase. There were no differences in the expression pattern, cell-specificity of protein localization and transport substrate specificity of TcHMA3 between the two ecotypes. Both alleles which were characterized by constitutive expression in the shoot and root, a tonoplast-localization of the protein in all leaf cells, and specific transport activity for Cd. The only difference between the two ecotypes was the expression level of TcHMA3; Ganges showed 7-fold higher expression than Prayon due, in part, to a higher copy number. Over-expression of TcHMA3 in Arabidopsis resulted in enhanced tolerance to Cd, but not to Co or Pb. These results indicate that TcHMA3 is a tonoplast-localized transporter specific for Cd, which is responsible for sequestration of Cd into the leaf vacuoles and that a higher expression of this gene is required for Cd hyper-tolerance in the Cd-hyperaccumulating ecotype of Thlaspi caerulescens.