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      • The ABC transporter AtPDR8 is a cadmium extrusion pump conferring heavy metal resistance

        Kim, Do-Young,Bovet, Lucien,Maeshima, Masayoshi,Martinoia, Enrico,Lee, Youngsook Blackwell Publishing Ltd 2007 The Plant journal Vol.50 No.2

        <P>Summary</P><P>Cadmium (Cd) and lead (Pb) are widespread pollutants that are toxic to plant growth. The expression of <I>AtPDR8</I> was upregulated in cadmium- or lead-treated <I>Arabidopsis thaliana.</I> To test whether AtPDR8 is involved in heavy metal resistance, we examined transgenic Arabidopsis that over-expressed AtPDR8 and RNAi plants that exhibited a severely reduced <I>AtPDR8</I> transcript level, as well as T-DNA insertion mutants of this ABC transporter. AtPDR8-over-expressing plants were more resistant to Cd<SUP>2+</SUP> or Pb<SUP>2+</SUP> than the wild-type and had lower Cd contents. In contrast, AtPDR8 RNAi transgenic plants and T-DNA insertion lines were more sensitive to Cd<SUP>2+</SUP> or Pb<SUP>2+</SUP> compared to wild-type plants and had higher Cd contents. The GFP–AtPDR8 protein was targeted to the plasma membrane, and GUS activity was present in most cells but strongest in the root hair and epidermal cells. Cd extrusion was higher in the AtPDR8-over-expressing plants in a flux assay using isolated protoplasts and radioactive <SUP>109</SUP>Cd, and was lower in the RNAi transgenic plants than in the wild-type. Together, these data strongly support a role for AtPDR8 as an efflux pump of Cd<SUP>2+</SUP> or Cd conjugates at the plasma membrane of Arabidopsis cells. As AtPDR8 has been suggested to be involved in the pathogen response and in the transport of chemicals that mediate pathogen resistance, this ABC protein is likely to transport a very broad range of substrates.</P>

      • AtATM3 is involved in heavy metal resistance in Arabidopsis.

        Kim, Do-Young,Bovet, Lucien,Kushnir, Sergei,Noh, Eun Woon,Martinoia, Enrico,Lee, Youngsook American Society of Plant Physiologists 2006 PLANT PHYSIOLOGY - Vol.140 No.3

        <P>AtATM3, an ATP-binding cassette transporter of Arabidopsis (Arabidopsis thaliana), is a mitochondrial protein involved in the biogenesis of iron-sulfur clusters and iron homeostasis in plants. Our gene expression analysis showed that AtATM3 is up-regulated in roots of plants treated with cadmium [Cd(II)] or lead (II); hence, we investigated whether this gene is involved in heavy metal tolerance. We found that AtATM3-overexpressing plants were enhanced in resistance to Cd, whereas atatm3 mutant plants were more sensitive to Cd than their wild-type controls. Moreover, atatm3 mutant plants expressing 35S promoter-driven AtATM3 were more resistant to Cd than wild-type plants. Since previous reports often showed that the cytosolic glutathione level is positively correlated with heavy metal resistance, we measured nonprotein thiols (NPSH) in these mutant plants. Surprisingly, we found that atatm3 contained more NPSH than the wild type under normal conditions. AtATM3-overexpressing plants did not differ under normal conditions, but contained less NPSH than wild-type plants when exposed to Cd(II). These results suggest a role for AtATM3 in regulating cellular NPSH level, a hypothesis that was further supported by our gene expression study. Genetic or pharmacological inhibition of glutathione biosynthesis led to the elevated expression of AtATM3, whereas expression of the glutathione synthase gene GSH1 was increased under Cd(II) stress and in the atatm3 mutant. Because the closest homolog of AtATM3 in fission yeast (Schizosaccharomyces pombe), HMT1, is a vacuolar membrane-localized phytochelatin-Cd transporter, it is tempting to speculate that glutathione-Cd(II) complexes formed in the mitochondria are exported by AtATM3. In conclusion, our data show that AtATM3 contributes to Cd resistance and suggest that it may mediate transport of glutamine synthetase-conjugated Cd(II) across the mitochondrial membrane.</P>

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