Molecular Genetics of Metal Detoxification: Prospects for Phytoremediation

Abstract

Inorganic elements that include toxic metals and radionuclides present an unusual pollution challenge to ecosystems. These elements persist so that even low rates of deposition over time leads to toxic accumulation. Engineering nonfood plants to mine inorganics from contaminated sites offers an economically feasible solution for reclamation. Plants, however, generally have a low tolerance for these toxins. Oxidative stress is induced by high accumulation of heavy metals and radionuclides. Through a first strategy of analyzing fission yeast mutants, we identified and characterized several genes involved in heavy metal tolerance in the fission yeast. Most extensive characterization was conducted on a gene encoding a mitochondrial sulfide dehydrogenase. This gene prevents metal-induced sulfide toxicity in the mitochondria.Interestingly, highly homologous genes of unknown functions are also present in higher eukaryotes including humans.Through another strategy involving the direct expression and selection of cDNA clones in fission yeast, we identified approximately 50 cDNA clones that confer hypertolerance to cadmium and/or oxidative stress in fission yeast. These cDNAs have been sequenced and many of them have been transferred to vectors for transformation in plant and animal cells. By the end of the funding period, we were ready to test the function of these cDNAs when overexpressed in animal cells and transgenic plants.Functional identification of genes that ameliorate toxic effects of heavy metals and radionuclides will provide vital tools for engineering bioremediating plants.Additionally, these genes provide valuable insights into cellular defense mechanisms against exposure to these harmful oxidants.DOE final report -page 4 Research Objectives Introduction Promises of phytoextractionThe global industrial revolution has led to an unprecedented dissemination of toxic substances into the environment. Exposure to these pollutants, including dietary intake of plant-derived food and beverages, can have long-term effects on human health. This is of particular concern with substances that have a long biological half-life. The metal cadmium, for instance, can have a half-life of up to 30 years in the human kidney. Chronic bioaccumulation, at low quantities that might otherwise be considered insignificant, could nonetheless contribute to renal failure. Most organic compounds can be inactivated eventually through biotic and abiotic mineralization processes. Inorganic compounds, however, such as most heavy metals and some radionuclides, are practically indestructible. Their persistence in the environment requires that they be removed from the contaminated source or be converted into a biologically inert form. A conversion process is possible for only a few of the toxic elements, such as the biovolatilization of Se, Hg, and 3 H (2,38,50) . For most other toxic metals and radionuclides, effective remediation requires the physical extraction from soil and water systems. Conventional engineering technologies, the "dig and t...