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

0129844Leon V. Kochian土壤有毒重金属污染是一个严重的世界性问题，对人类健康和农业都有影响。据估计，仅在美国，目前使用的基于工程的技术处理危险废物的成本至少为4000亿美元。近年来，人们对利用陆生植物作为一种替代的"绿色技术"来修复受有毒重金属污染的表层土壤产生了相当大的兴趣。植物修复背后的原则是在受污染的场地上种植植物，就像在农业土壤上种植作物一样。如果植物对土壤中的重金属具有亲和力，它们可以从土壤中提取重金属并将其积累在地上部生物量中。然后收获这些含重金属的枝条组织并将其灰化以减少其体积，然后储存在废物储存库中。重复这一植物生长和收获过程，直到土壤中的污染物水平降低到可接受的水平最近对金属污染土壤的植物修复感兴趣的一个主要因素是科学界越来越意识到存在许多植物物种，它们不仅可以耐受土壤中高水平的有毒重金属，但实际上可以在容易收获的地上茎生物量中将这些金属积累到非常高的水平。据报道，超过200种陆生植物生长在高金属土壤上，可以耐受并积累高水平的重金属，如Zn，Cd，Cu和Ni。这些植物已被创造hyperoxemia;这些有趣的金属超富集植物物种的存在本身表明，植物修复是成功的遗传潜力。然而，这些超富集植物大多数都很小，生长缓慢，而且由于它们产生的芽生物量有限，它们对污染土壤进行大规模净化的潜力有限。将表达超积累表型的基因转移到生产更高生物量的植物中被认为是提高植物修复作为一种可行的商业技术的潜力的一种途径。然而，由于对重金属超积累的分子、生化和生理机制缺乏了解，这一目标的进展受到阻碍，其中最著名的金属超积累植物之一是Thlaspi caerulescens，它是卷心菜家族的一员，可以在地上部积累极高水平的重金属镉（Cd）和锌（Zn）。此外，T.本研究的目的是明确植物体内重金属超积累的基本机制，为植物体内重金属超积累机理的研究提供理论基础，为植物体内重金属超积累的研究奠定基础。并分离和表征在Thlaspi caerulescens中这种超积累特性下的一组基因。Kochian博士的研究小组将利用植物分子生物学和基因组学的最新进展来鉴定参与金属积累和耐受性的金属转运基因，以及参与低分子量有机化合物产生的基因（例如，多肽、有机基因、氨基酸、金属硫蛋白、植物螯合素等）可以结合并解毒植物细胞中的锌和镉。基于最近对拟南芥基因组的测序和分析，现在已知高等植物采用最近在酵母、细菌和哺乳动物中鉴定和表征的相同家族的金属转运蛋白用于金属积累和稳态。Kochian博士的研究小组已经克隆了T. caerulescens从这些不同的金属转运蛋白基因家族，并将这些转运蛋白的特点，以确定其在金属超积累的作用。这种表征将包括确定在哪些植物组织和细胞类型中表达不同的基因，转运蛋白的膜定位以帮助分配每个转运蛋白的潜在作用，以及阐明单个金属转运蛋白的生理功能。他们也表达了T。这些方法将使研究人员能够鉴定出在T. caerulescens中引起重金属过度积累的基因组。caerulescens和阐明这一特点的分子机制。本研究的最终目标是利用这些超积累基因培育出既能吸收金属又能产生高地上部生物量的转基因植物，从而很好地适用于金属污染土壤的植物修复。本项目由美国环境保护署、美国国家科学基金会、美国海军研究办公室、战略环境研究与发展计划。

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.