Soils and waters with high levels of toxic metals such as cadmium (Cd), arsenic (As), lead (Pb) and mercury (Hg) are detrimental to human and environmental health. These four metals are among the Superfund;'s top five priority hazardous substances. Studies suggest that uptake of heavy metals into plant via the root system could provide a potent and cost effective approach for toxic metal removal and remediation of soils and waters. In plants and fungi, phytochelatins are major heavy metal chelating and detoxifying thiolate peptides, that form complexes with and detoxify heavy metals, including Cd, Zn, Pb, Hg and based on recent research also As. The enzyme phytochelatin synthase (PCS) produces phytochelatin, thus functioning as a major catalytic metal detoxification mechanisms in plants. However genes encoding phytochelatin synthases, had not yet been identified. We have recently cloned a new gene family (PCS) encoding phytochelatin synthases in plants and fungi. Expression of PCS cDNAs in S. cerevisiae dramatically enhance resistance to cadmium. Disruption of the PCS genes in S. pombe and Arabidopsis thaliana produces increased heavy metal sensitivity. Recombinant PCS proteins synthesize phytochelatins in vitro. We will test the hypotheses that stress-signaling pathways contribute to PCS induction and detoxification and that transgenic expression of PCS genes can, together with other metal-interacting mechanisms, enhance heavy metal hyper-accumulation and removal by plants. To test these hypotheses we will: (I) Characterize signaling mechanisms that induce PCS expression. (II) Characterize PCS expression and localization in Brassica juncea, which is one of the major plant species being studied for heavy metal biomediation. (III) Pursue transgenic over-expression in plants of PCS together with associated metal detoxification mechanisms to test for enhanced heavy metal tolerance and accumulation and (IV) provide selected transgenic lines to Phytotech Inc to include in field trials on super fund sites. (V) Pursue novel genetic activation-tagging screens in Arabidopsis and Cd-induced microarray analyses to identify new genes and pathways involved in heavy metal accumulation in plants. Results from these studies could play a central role in the development of future phytoremediation strategies for heavy metal uptake and biological removal of heavy metals form contaminated soils and waters.
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