The Molecular Basis for Heavy Metal Accumulation and Tolerance in the Hyperaccumulating Plant Species, Thlaspi Caerulescens

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0129844Leon V. KochianContamination of soils with toxic heavy metals is a serious worldwide problem both for human health and agriculture. Cleanup of hazardous wastes by the currently used engineering-based technologies has been estimated to cost at least $400 billion in the U.S. alone. Recently, there has been considerable interest in the use of terrestrial plants as an alternative, "green technology" for the remediation of surface soils contaminated with toxic heavy metals. The principal behind phytoremediation is to grow plants on a contaminated site in much the same way crop plants are grown on agricultural soils. If the plants have an affinity for the heavy metals in the soil, they can extract the metals from the soils and accumulate them in the above-ground shoot biomass. These heavy metal-containing shoot tissues are then harvested and ashed to reduce their volume prior to storage in a waste repository. This plant growth and harvesting process is repeated until the level of contaminant in the soil is reduced to acceptable levels (usually a number of years).A major factor behind the recent interest in phytoremediation of metal polluted soils has been the growing awareness by the scientific community of the existence of a number of plant species that not only can tolerate high levels of toxic heavy metals in the soil, but actually can accumulate these metals to very high levels in the easily harvested above-ground shoot biomass. Over 200 terrestrial species have been reported that grow on high metal soils and can tolerate and accumulate high levels of heavy metals such as Zn, Cd, Cu, and Ni in their shoots. These plants have been coined hyperaccumulators; the very existence of these interesting metal hyperaccumulator species suggests that the genetic potential exists for phytoremediation to be successful. Most of these hyperaccumulator species, however, are small and slow growing, and because they produce limited shoot biomass their potential for large-scale decontamination of polluted soils is limited. Transferring the genes expressing the hyperaccumulating phenotype to higher shoot biomass-producing plants has been suggested as an avenue for enhancing the potential of phytoremediation as a viable commercial technology. Progress towards this goal, however, has been hindered by a lack of understanding of the basic molecular, biochemical and physiological mechanisms involved in heavy metal hyperaccumulation.One of the best known metal hyperaccumulators is Thlaspi caerulescens, a member of the cabbage family that can accumulate the heavy metals cadmium (Cd) and zinc (Zn) to extremely high levels in the shoot. Additionally, certain ecotypes of T. caerulescens have been reported to accumulate high levels of other heavy metals, including Ni and Co. The unique physiology of heavy metal transport and tolerance in Thlaspi caerulescens makes it a very interesting experimental system for basic research aimed at elucidating plant mechanisms and the associated genes controlling heavy metal hyperaccumulation.The goals of this research are to identify the basic mechanisms of heavy metal hyperaccumulation in plants, and to isolate and characterize the suite of genes that underly this hyperaccumulation trait in Thlaspi caerulescens. Dr. Kochian's group will use recent advances in plant molecular biology and genomics to identify both metal transporter genes involved in metal accumulation and tolerance, as well as genes involved in the production of low molecular weight organic compounds (e.g., peptides, organic genes, amino acids, metallothioneins, phytochelatins) that can bind and detoxify Zn and Cd in plant cells. Based on the recent sequencing and analysis of the Arabidopsis genome, it is now known that higher plants employ the same families of metal transporters recently identified and characterized in yeast, bacteria and mammals for metal accumulation and homeostasis. Dr. Kochian's group has cloned genes in T. caerulescens from these different metal transporter gene families and will characterize these transporters to determine their role in metal hyperaccumulation. This characterization will include determining in which plant tissue and cell type different genes are expressed, the membrane localization of transport proteins to help assign a potential role for each transporter, and the elucidation of the physiological function of individual metal transporters. They also are expressing T. caerulescens genes in yeast to look for genes conferring metal tolerance through the production of metal binding organic ligands.These approaches should allow the investigators to identify the suite of genes that confer heavy metal hyperaccumulation in T. caerulescens and to elucidate the molecular mechanism(s) for this trait. The ultimate goal of this research is to use these hyperaccumulation genes to develop transgenic plants that both are metal hyperaccumulators and produce high shoot biomass , and thus will be well suited for the phytoremediation of metal contaminated soils.This project was funded through the Joint Program on Phytoremediation, co-sponsored by the Environmental Protection Agency, the National Science Foundation, the Office of Naval Research, and the Strategic Environmental Research and Development Program.