Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/5/2008
Publication Date: 8/1/2008
Citation: Klein, M., Sekimoto, H., Milner, M., Kochian, L.V. 2008. Investigation of Heavy Metal Hyperaccumulation at the Cellular Level: Development and Characterization of Thlaspi caerulescens Suspension Cell Lines. Plant Physiology. 147:2006-2016. 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. In this study, we investigated whether the extreme metal tolerance and accumulation exhibited by Thlaspi caerulescens plants is due to traits expressed at the cellular level. This was done by creating a suspension cell culture from Thlaspi caerulescens tissue. Suspension cells are single cells, or small groups of cells, that divide and grow in liquid culture. It was found that these suspension cell lines exhibited several of the traits associated with metal hyperaccumulation expressed by whole plants. These include increased Zn/Cd tolerance, altered metal transport, and increased expression of metal transport genes. Interestingly the Thlaspi suspension cells accumulated less, and not more Zn and Cd compared with suspension cell lines of the related non-accumulator, Arabidopsis thaliana. These findings are providing us a better understanding of metal hyperaccumulation, which ultimately will be useful in developing plants better suited for the phytoremediation of metal-contaminated soils.
Technical Abstract: The ability of metal hyperaccumulator plant species to accumulate high concentrations of toxic heavy metals requires the coordinated uptake, transport and sequestration of these metals to avoid damage to photosynthetic mechanisms. A number of previous studies have examined how hyperaccumulating plant species tolerate and accumulate metals using whole plant, organ and protoplast specific studies. In this study the construction of stable suspension cell lines from Thlaspi caerulescens, the subsequent physiological and molecular characterization, and stable Agrobacterium mediated transformation of T. caerulescens suspension cell lines is reported. Through both short and long term physiological and molecular experiments, the behavior of these undifferentiated cell lines were examined. It was found that the T. caerulescens suspension cell lines exhibit enhanced zinc and cadmium tolerance relative to Arabidopsis thaliana suspension cell lines, which is consistent with the increased metal tolerance seen in intact T. caerulescens plants. Interestingly, the T. caerulescens cells accumulated lesser amounts of Zn and Cd than did Arabidopsis thaliana cells. Like intact T. caerulescens plants that exhibit hyperexpression of a number of transporter genes, T. caerulescens cell lines also show increased expression of HMA4, ZNT1 and MTP1 relative to A. thaliana suspension cells. Finally, the stable transformation of T. caerulescens suspension cell lines suggest a potential mechanism for further dissecting the altered metal accumulation and homeostasis seen in this metal hyperaccumulator model plant species.