Molecular mechanisms of heavy metal hyperaccumulation and phytoremediation

A relatively small group of hyperaccumulator plants is capable of sequestering heavy metals in their shoot tissues at high concentrations. In recent years, major scientific progress has been made in understanding the physiological mechanisms of metal uptake and transport in these plants. However, relatively little is known about the molecular bases of hyperaccumulation. In this paper, current progresses on understanding cellular/molecular mechanisms of metal tolerance/hyperaccumulation by plants are reviewed. The major processes involved in hyperaccumulation of trace metals from the soil to the shoots by hyperaccumulators include: (a) bioactivation of metals in the rhizosphere through root–microbe interaction; (b) enhanced uptake by metal transporters in the plasma membranes; (c) detoxification of metals by distributing to the apoplasts like binding to cell walls and chelation of metals in the cytoplasm with various ligands, such as phytochelatins, metallothioneins, metal-binding proteins; (d) sequestration of metals into the vacuole by tonoplast-located transporters. The growing application of molecular-genetic technologies led to the well understanding of mechanisms of heavy metal tolerance/accumulation in plants, and subsequently many transgenic plants with increased resistance and uptake of heavy metals were developed for the purpose of phytoremediation. Once the rate-limiting steps for uptake, translocation, and detoxification of metals in hyperaccumulating plants are identified, more informed construction of transgenic plants would result in improved applicability of the phytoremediation technology.     

Influence of the zinc hyperaccumulator Thlaspi caerulescens J. & C. Presl. and the nonmetal accumulator Trifolium pratense L. on soil microbial populations

Metal hyperaccumulator plants like Thlaspi caerulescens J. & C. Presl. are used for phytoremediation of contaminated soils. Since little is known about the rhizosphere of hyperaccumulators, the influence of T. caerulescens was compared with the effects of Trifolium pratense L. on soil microbes. High- and low-metal soils were collected near a zinc smelter in Palmerton, Penn. Soil pH was adjusted to 5.8 and 6.8 by the addition of Ca(OH)2. Liming increased bacterial populations and decreased metal toxicity to levels allowing growth of both plants. The effects of the plants on total (culturable) bacteria, total fungi, as well as cadmium- and zinc-resistant populations were assessed in nonrhizosphere and rhizosphere soil. Both plants increased microbial populations in rhizosphere soil compared with nonrhizosphere soil. Microbial populations were higher in soils planted with T. pratense, but higher ratios of metal-resistant bacteria were found in the presence of T. caerulescens. We hypothesize that T. caerutescens acidifies its rhizosphere. Soil acidification in the rhizosphere of T. caerulescens would affect metal uptake by increasing available metals around the roots and consequently, increase the selection for metal-resistant bacteria. Soil acidification may be part of the hyperaccumulation process enhancing metal uptake from soil.     

Perspectives of plant-associated microbes in heavy metal phytoremediation

"Phytoremediation" know-how to do-how is rapidly expanding and is being commercialized by harnessing the phyto-microbial diversity. This technology employs biodiversity to remove/contain pollutants from the air, soil and water. In recent years, there has been a considerable knowledge explosion in understanding plant-microbes-heavy metals interactions. Novel applications of plant-associated microbes have opened up promising areas of research in the field of phytoremediation technology. Various metabolites (e.g., 1-aminocyclopropane-1-carboxylic acid deaminase, indole-3-acetic acid, siderophores, organic acids, etc.) produced by plant-associated microbes (e.g., plant growth promoting bacteria, mycorrhizae) have been proposed to be involved in many biogeochemical processes operating in the rhizosphere. The salient functions include nutrient acquisition, cell elongation, metal detoxification and alleviation of biotic/abiotic stress in plants. Rhizosphere microbes accelerate metal mobility, or immobilization. Plants and associated microbes release inorganic and organic compounds possessing acidifying, chelating and/or reductive power. These functions are implicated to play an essential role in plant metal uptake. Overall the plant-associated beneficial microbes enhance the efficiency of phytoremediation process directly by altering the metal accumulation in plant tissues and indirectly by promoting the shoot and root biomass production. The present work aims to provide a comprehensive review of some of the promising processes mediated by plant-associated microbes and to illustrate how such processes influence heavy metal uptake through various biogeochemical processes including translocation, transformation, chelation, immobilization, solubilization, precipitation, volatilization and complexation of heavy metals ultimately facilitating phytoremediation.     

Advances in Research on Genetically Engineered Plants for Metal Resistance

The engineering application of natural hyperaccumulators In removing or inactivating metal pollutants from soil and surface water In field trials mostly presents the insurmountable shortcoming of low efficiency owing to their little biomass and slow growth. Based on further understanding of the molecular mechanism of metal uptake, translocation, and also the separation, identification, and cloning of some related functional genes, this article highlights and summarizes In detail the advances in research on transgenlc techniques, such as Agrobacterlurn tumefaciens-medlated transformation and particle bombardment, in breeding of plants for metal resistance and accumulation, and points out that deepening the development of transgenlc plants Is one of the efficient approaches to improving phytoremedlatlon efficiency of metalcontaminated environments. From the viewpoint of sustainable development, governments should strengthen support to the development of genetic engineering for metal resistance and accumulation In plants.     

Phytoextraction of metals and metalloids from contaminated soils

The removal of inorganic contaminants by plants is termed phytoextraction. Recent studies have looked at the feasibility of phytoextraction, and demonstrate that both good biomass yields and metal hyperaccumulation are required to make the process efficient. Adding chelating agents to soil to increase the bioavailability of contaminants can sometimes induce hyperaccumulation in normal plants, but may produce undesirable environmental risks. Thus, it is necessary to investigate the mechanisms responsible for hyperaccumulation, using natural hyperaccumulators as model plant species. Recent advances have been made in understanding the mechanisms responsible for hyperaccumulation of Zn, Cd, Ni and As by plants. Attempts to engineer metal tolerance and accumulation have so far been limited to Hg, As and Cd, and although promising results have been obtained they may be some way from practical application. More fundamental understanding of the traits and mechanisms involved in hyperaccumulation are needed so that phytoextraction can be optimised.     

镍污染土壤植物采矿技术关键过程及其研究进展

植物采矿是利用超积累植物高量吸收土壤中的重金属,并从中提取、冶炼金属产品,在修复污染土壤的同时实现金属的资源化。全世界广泛分布着自然风化的镍污染土壤,植物采矿因其重要的环境、生态及资源价值,被作为一种环境友好且具备经济效益的土壤修复技术,在此类地区具有广阔的应用前景。该植物采矿技术关键过程主要包括超积累植物镍高选择性根际环境响应、植物镍高效吸收转运以及生物质中镍高附加值资源化等过程。近30年,污染土壤中镍的植物采矿已经在美国、阿尔巴尼亚、马来西亚等多个国家进行了野外实践,取得了良好效果。然而,相关技术在我国的研究与应用仍然处于起步阶段。文中通过综述植物采矿技术的关键过程的研究进展,发现其中的瓶颈,为接下来植物采矿的科学研究和技术在全世界推广提供理论基础和技术指导。     

植物对重金属的吸收和分布

植物修复是利用植物来清除污染土壤中重金属的一项技术。该技术成功与否取决于植 物从土壤中吸取金属以及向地上部运输金属的能力。植物对金属的吸收主要取决于自由态离子活度。许多螯合剂能诱导植物对重金属的吸收。金属离子在液泡中的区域化分布是植物耐 重金属的主要原因。同时,细胞内的金属硫蛋白、植物螯合肽等蛋白质以及有机酸、氨基酸等在金属贮存和解毒方面也起重要作用。本文还论述了重金属在植物体内运输的生理及分子 方面的研究进展。     

植物对重金属的吸收和分布

植物修复是利用植物来清除污染土壤中重金属的一项技术。该技术成功与否取决于植物从土壤中吸取金属以及向地上部运输金属的能力。植物对金属的吸收主要取决于自由态离子活度。许多螯合剂能诱导植物对重金属的吸收。金属离子在液泡中的区域化分布是植物耐重金属的主要原因。同时,细胞内的金属硫蛋白、植物螯合脓等蛋白质以及有机酸、氨基酸等在金属贮存和解毒方面也起重要作用。本文还论述了重金属在植物体内运输的生理及分子方面的研究进展。     

Opportunities and feasibilities for biotechnological improvement of Zn, Cd or Ni tolerance and accumulation in plants

Metals contaminate the soil when present in high concentrations causing soil and ultimately environmental pollution. “Phytoremediation” is the use of plants to remove pollutants from contaminated environments. Plants tightly regulate their internal metal concentrations in a process called “metal homeostasis”. Some species have evolved extreme tolerance and accumulation of Zn, Cd and Ni as a way to adapt to exposure to these metals. Such traits are beneficial for phytoremediation, however, most natural metal hyperaccumulator species are not adapted to agriculture and have low yields. A wealth of knowledge has been generated regarding metal homeostasis in plants, including hyperaccumulators, which can be used in phytoremediation of Zn, Cd and Ni. In this review, we describe the current state of Zn, Cd and Ni physiology in plants and the underlying molecular mechanisms. The ways to efficiently utilize this information in designing high biomass metal accumulator plants are discussed. The potential and application of genetic modification has extended our understanding about the mechanisms in plants dealing with the metal environment and has paved the way to achieve the goal of understanding metal physiology and to apply the knowledge for the containment and clean up of metal contaminated soils.     

粤东铅锌尾矿三种优势植物对重金属的吸收和富集特性研究

为探讨铅锌矿废弃地优势植物在重金属污染土壤植物修复中的应用潜力,利用野外采样分析法,从粤东梅县丙村铅锌尾矿区采集其三种优势植物类芦、黄荆、盐肤木的根、茎、叶和土壤样品,测定和分析Pb、Zn、Cu、Cd四种重金属含量.结果表明:该矿区土壤污染严重,Pb、Zn、Cd含量远超土壤环境质量的三级标准,Cu超出二级标准;根际土壤和非根际土壤重金属含量均为Pb＞Zn＞Cu＞Cd,但根际土壤的重金属含量显著低于非根际土壤;这三种植物对Pb、Zn、Cu的转移系数大于1.0,对Cu的富集系数最高,Pb最小,但对四种重金属的富集系数均小于1.0,均未达到超富集植物临界含量标准.三种植物为该矿区的优势植物,说明它们对土壤的重金属污染有很强的耐性,虽然并非典型的超富集植物,但对污染土壤仍有较好的修复效果.