A Review on Mechanisms of Plant Tolerance and Role of Transgenic Plants in Environmental Clean-up

This article deals with the advances and implication of phytoremediation technologies with emphasis on remediation of toxic heavy metals from contaminated soil. Most of the conventional remedial technologies are expensive and inhibit the soil fertility and cause negative impacts on various ecosystem services. However, phytoremediation is a cost effective and ecofriendly approach, which does not adversely affect soil properties and ecosystem services. In recent years, major progresses have been made in understanding the physiological mechanisms of metal uptake and transport in hyper accumulators. However, the molecular mechanisms of metal uptake, translocation, accumulation and detoxification in plants and their further implication in transgenic development for efficient phytoremediation are not well understood. In view of above, present review article brings together existing bits of information to create a new direction for future research, critical gap in knowledge and a new perspective on remediation of toxic pollutants.     

铅污染现状及其修复机理研究进展

介绍了铅污染的现状、修复机理及存在的问题。其中重点阐述了国内在寻找铅的超累积植物的研究进展,国外研究者对铅在植物体内吸收、运输和储存机理方面的研究现状,同时对已发现的与铅污染有关的基因以及转基因技术的应用情况进行了简单介绍,并探讨了提高铅污染修复效率的一些方法。     

Phytoremediation of Heavy Metals: Physiological and Molecular Mechanisms

Heavy metals (HM) are a unique class of toxicants since they cannot be broken down to non-toxic forms. Concentration of these heavy metals has increased drastically, posing problems to health and environment, since the onset of the industrial revolution. Once the heavy metals contaminate the ecosystem, they remain a potential threat for many years. Some technologies have long been in use to remove, destroy and sequester these hazardous elements. Even though effective techniques for cleaning the contaminated soils and waters are usually expensive, labour intensive, and often disturbing. Phytoremediation, a fast-emerging new technology for removal of toxic heavy metals, is cost-effective, non-intrusive and aesthetically pleasing. It exploits the ability of selected plants to remediate pollutants from contaminated sites. Plants have inter-linked physiological and molecular mechanisms of tolerance to heavy metals. High tolerance to HM toxicity is based on a reduced metal uptake or increased internal sequestration, which is manifested by interaction between a genotype and its environment. The growing interest in molecular genetics has increased our understanding of mechanisms of HM tolerance in plants and many transgenic plants have displayed increased HM tolerance. Improvement of plants by genetic engineering, i.e., by modifying characteristics like metal uptake, transport and accumulation and plant’s tolerance to metals, opens up new possibilities of phytoremediation. This paper presents an overview of the molecular and physiological mechanisms involved in the phytoremediation process, and discusses strategies for engineering plants genetically for this purpose.     

Advances in development of transgenic plants for remediation of xenobiotic pollutants

Phytoremediation-the use of plants for cleaning up of xenobiotic compounds-has received much attention in the last few years and development of transgenic plants tailored for remediation will further enhance their potential. Although plants have the inherent ability to detoxify some xenobiotic pollutants, they generally lack the catabolic pathway for complete degradation/mineralization of these compounds compared to microorganisms. Hence, transfer of genes involved in xenobiotic degradation from microbes/other eukaryotes to plants will further enhance their potential for remediation of these dangerous groups of compounds. Transgenic plants with enhanced potential for detoxification of xenobiotics such as trichloro ethylene, pentachlorophenol, trinitro toluene, glycerol trinitrate, atrazine, ethylene dibromide, metolachlor and hexahydro-1,3,5-trinitro-1,3,5-triazine are a few successful examples of utilization of transgenic technology. As more genes involved in xenobiotic metabolism in microorganisms/eukaryotes are discovered, it will lead to development of novel transgenic plants with improved potential for degradation of recalcitrant contaminants. Selection of suitable candidate plants, field testing and risk assessment are important considerations to be taken into account while developing transgenic plants for phytoremediation of this group of pollutants. Taking advantage of the advances in biotechnology and 'omic' technologies, development of novel transgenic plants for efficient phytoremediation of xenobiotic pollutants, field testing and commercialization will soon become a reality.     

Transgenic plants for phytoremediation

Phytoremediation is a green, sustainable and promising solution to problems of environmental contamination. It entails the use of plants for uptake, sequestration, detoxification or volatilization of inorganic and organic pollutants from soils, water, sediments and possibly air. Phytoremediation was born from the observation that plants possessed physiological properties useful for environmental remediation. This was shortly followed by the application of breeding techniques and artificial selection to genetically improve some of the more promising and interesting species. Now, after nearly 20 years of research, transgenic plants for phytoremediation have been produced, but none have reached commercial existence. Three main approaches have been developed: (1) transformation with genes from other organisms (mammals, bacteria, etc.); (2) transformation with genes from other plant species; and (3) overexpression of genes from the same plant species. Many encouraging results have been reported, even though in some instances results have been contrary to expectations. This review will illustrate the main examples with a critical discussion of what we have learnt from them.     

Molecular Mechanisms and Genetic Basis of Heavy Metal Tolerance/Hyperaccumulation in Plants

Phytoremediation has gained increased attention as a cost-effective method for the remediation of heavy metal-contaminated sites. Because some plants possess a range of potential mechanisms that may be involved in the detoxification of heavy metals, they manage to survive under metal stresses. High tolerance to heavy metal toxicity could rely either on reduced uptake or increased plant internal sequestration,which is manifested by an interaction between a genotype and its environment. The growing application of molecular genetic technologies has led to increased understanding of mechanisms of heavy metal tolerance/accumulation in plants and, subsequently, many transgenic plants with increased heavy metal resistance,as well as increased uptake of heavy metals, have been developed for the purpose of phytoremediation. In the present review, our major objective is to concisely evaluate the progress made so far in understanding the molecular/cellular mechanisms and genetic basis that control the uptake and detoxification of metals by plants.     

金属结合蛋白基因及其在清除重金属污染中的应用

一些微生物和植物由于对毒性金属具有独特的抗性机制,使得利用它们来清除日益严重的环境污染已发展成为一种十分有效的技术——生物修复。研究表明,不同的金属结合蛋白（如MT 和PC）,在生物忍耐和降解过量重金属毒性机制中起重要作用。愈来愈多的MT 和PC基因被克隆,并已成功地应用于生物遗传转化,这些转基因生物在清除重金属污染方面已显示出潜在的应用价值。 Abstract:Heavy metal pollution has become a global environmental hazard.The use of microorganisms and plants for the decontamination of heavy metals is recognized as a low lost and high efficiency method for cleaning up metal contamination.It shows that various metal-binding proteins such as metallothioneins (MTs) or phytochelatines (PCs) play an important role in defense systems and detoxification to heavy metals in organisms.Many genes of MTs and PCs have been cloned and utilized successfully in genetically modified bacteria and plants for increasing remediation capacity.These transgenic organisms have been displayed a great potential in bioremediation and phytoremediation of heavy metals.     

重金属污染的转基因植物修复——原理与应用

污染环境的植物修复技术具有成本低、不造成二次污染等优点。从自然界中寻找用于污染环境修复的超富积植物不仅难度大 ,而且受生物量、生长周期以及地理环境等因素的限制。近几年迅速发展起来的通过转基因植物进行污染环境的修复技术显示了广阔的应用前景。外源基因在植物的高效表达可以提高植物吸收、运输、降解污染物的能力以及修复的效率 ,并可以作为研究不同污染物修复机理的实验系统。以转基因植物修复几种主要的重金属污染为例 ,介绍了转基因植物修复的原理、现状及存在问题 ,并探讨了提高转基因植物修复效率的一些方法 。     

Utilization of Multi-Tasking Non-Edible Plants for Phytoremediation and Bioenergy Source-A Review

Heavy metal contamination of land and freshwater resources is a serious concern worldwide. It adversely affects the health of animals, plants and humans. Therefore, remediation of toxic heavy metals must be highly considered. Unlike other techniques, phytoremediation is a holistic technology and can be used in large scale for soil remediation as it is costless, novel, environmentally-safe and solar-driven technology. Utilization of non-edible plants in phytoremediation is an ingenious technique as they are used to generate new bioenergy resources along with the remediation of contaminated soils. Some nonfood bioenergy crops such as Salix species, Miscanthus species, Populus species, Eucalyptus species, and Ricinus communis exhibit high capability to accumulate various metals and to grow in contaminated lands. However, there are still sustainable challenges facing coupling phytoremediation with bioenergy production from polluted lands. Therefore, there has long been a need for developing different strategies to resolve such challenges. In this article review, we will discuss the phytoremediation mechanism, the technique of phytoremediation coupling with bioenergy production, sustainable problems facing linking phytoremediation with energy production as well as possible strategies to enhance the efficiency of bioenergy plants for soil decontamination by improving their characteristics such as metal uptake, transport, accumulation, and tolerance.     

Phytoremediation: an overview of metallic ion decontamination from soil

In recent years, phytoremediation has emerged as a promising ecoremediation technology, particularly for soil and water cleanup of large volumes of contaminated sites. The exploitation of plants to remediate soils contaminated with trace elements could provide a cheap and sustainable technology for bioremediation. Many modern tools and analytical devices have provided insight into the selection and optimization of the remediation process by plant species. This review describes certain factors for the phytoremediation of metal ion decontamination and various aspects of plant metabolism during metallic decontamination. Metal-hyperaccumulating plants, desirable for heavily polluted environments, can be developed by the introduction of novel traits into high biomass plants in a transgenic approach, which is a promising strategy for the development of effective phytoremediation technology. The genetic manipulation of a phytoremediator plant needs a number of optimization processes, including mobilization of trace elements/metal ions, their uptake into the root, stem and other viable parts of the plant and their detoxification and allocation within the plant. This upcoming science is expanding as technology continues to offer new, low-cost remediation options.     

Phytoremediation of Metals Using Transgenic Plants

An ideal plant for environmental cleanup can be envisioned as one with high biomass production, combined with superior capacity for pollutant tolerance, accumulation, and/or degradation, depending on the type of pollutant and the phytoremediation technology of choice. With the use of genetic engineering, it is feasible to manipulate a plant's capacity to tolerate, accumulate, and/or metabolize pollutants, and thus to create the ideal plant for environmental cleanup. In this review, we focus on the design and creation of transgenic plants for phytoremediation of metals. Plant properties important for metal phytoremediation are metal tolerance and accumulation, which are determined by metal uptake, root-shoot translocation, intracellular sequestration, chemical modification, and general stress resistance. If we know which molecular mechanisms are involved in these tolerance and accumulation processes, and which genes control these mechanisms, we can manipulate them to our advantage. This review aims to give a succinct overview of plant metal tolerance and accumulation mechanisms, and to identify possible strategies for genetic engineering of plants for metal phytoremediation. An overview is presented of what has been achieved so far regarding the manipulation of plant metal metabolism. In fact, both enhanced metal tolerance and accumulation have been achieved by overproducing metal chelating molecules (citrate, phytochelatins, metallothioneins, phytosiderophores, ferritin) or by the overexpression of metal transporter proteins. Mercury volatilization and tolerance was achieved by introduction of a bacterial pathway. The typical increase in metal accumulation as the result of these genetic engineering approaches is 2-to 3-fold more metal per plant, which could potentially enhance phytoremediation efficiency by the same factor. As for the applicability of these transgenics for environmental cleanup, results from lab and greenhouse studies look promising for several of these transgenics, but field studies will be the ultimate test to establish their phytoremediation potential, their competitiveness, and risks associated with their use.     

Transgenic plants for enhanced biodegradation and phytoremediation of organic xenobiotics

Phytoremediation — the use of plants to clean up polluted soil and water resources — has received much attention in the last few years. Although plants have the inherent ability to detoxify xenobiotics, they generally lack the catabolic pathway for the complete degradation of these compounds compared to microorganisms. There are also concerns over the potential for the introduction of contaminants into the food chain. The question of how to dispose of plants that accumulate xenobiotics is also a serious concern. Hence the feasibility of phytoremediation as an approach to remediate environmental contamination is still somewhat in question. For these reasons, researchers have endeavored to engineer plants with genes that can bestow superior degradation abilities. A direct method for enhancing the efficacy of phytoremediation is to overexpress in plants the genes involved in metabolism, uptake, or transport of specific pollutants. Furthermore, the expression of suitable genes in root system enhances the rhizodegradation of highly recalcitrant compounds like PAHs, PCBs etc. Hence, the idea to amplify plant biodegradation of xenobiotics by genetic manipulation was developed, following a strategy similar to that used to develop transgenic crops. Genes from human, microbes, plants, and animals are being used successfully for this venture. The introduction of these genes can be readily achieved for many plant species using Agrobacterium tumefaciens-mediated plant transformation or direct DNA methods of gene transfer. One of the promising developments in transgenic technology is the insertion of multiple genes (for phase 1 metabolism (cytochrome P450s) and phase 2 metabolism (GSH, GT etc.) for the complete degradation of the xenobiotics within the plant system. In addition to the use of transgenic plants overexpressed with P450 and GST genes, various transgenic plants expressing bacterial genes can be used for the enhanced degradation and remediation of herbicides, explosives, PCBs etc. Another approach to enhancing phytoremediation ability is the construction of plants that secrete chemical degrading enzymes into the rhizosphere. Recent studies revealed that accelerated ethylene production in response to stress induced by contaminants is known to inhibit root growth and is considered as major limitation in improving phytoremediation efficiency. However, this can be overcome by the selective expression of bacterial ACC deaminase (which regulates ethylene levels in plants) in plants together with multiple genes for the different phases of xenobiotic degradation. This review examines the recent developments in use of transgenic-plants for the enhanced metabolism, degradation and phytoremediation of organic xenobiotics and its future directions.     

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.     

Understanding molecular mechanisms for improving phytoremediation of heavy metal-contaminated soils

Heavy metal pollution of soil is a significant environmental problem with a negative potential impact on human health and agriculture. Rhizosphere, as an important interface of soil and plants, plays a significant role in phytoremediation of contaminated soil by heavy metals, in which, microbial populations are known to affect heavy metal mobility and availability to the plant through release of chelating agents, acidification, phosphate solubilization and redox changes, and therefore, have potential to enhance phytoremediation processes. Phytoremediation strategies with appropriate heavy metal-adapted rhizobacteria or mycorrhizas have received more and more attention. In addition, some plants possess a range of potential mechanisms that may be involved in the detoxification of heavy metals, and they manage to survive under metal stresses. High tolerance to heavy metal toxicity could rely either on reduced uptake or increased plant internal sequestration, which is manifested by an interaction between a genotype and its environment.A coordinated network of molecular processes provides plants with multiple metal-detoxifying mechanisms and repair capabilities. The growing application of molecular genetic technologies has led to an increased understanding of mechanisms of heavy metal tolerance/accumulation in plants and, subsequently, many transgenic plants with increased heavy metal resistance, as well as increased uptake of heavy metals, have been developed for the purpose of phytoremediation. This article reviews advantages, possible mechanisms, current status and future direction of phytoremediation for heavy-metal–contaminated soils.     

环境重金属污染的植物修复及基因工程在其中的应用

随着工业技术的发展,重金属在土壤和水体中的含量越来越高,重金属污染已日益成为威胁人类健康和人类生活质量的严重的社会问题和环境问题。植物修复可部分解决这一问题且正引起人们的普遍关注。但现在发现许多用于修复的超量积累植物生长缓慢、植株矮小、地上部生物量小,成了实际应用中的最大限制。利用基因工程手段改变植物对重金属吸收、转运、积累和忍耐的机制,从而提高植物对重金属的富集能力,将成为今后植物修复领域研究的一个重要方向。     

重金属污染土壤植物修复中的微生物功能研究进展

综述了国内外在重金属污染土壤植物-微生物联合修复领域的研究报道,总结了近5年的研究实例。植物-微生物联合修复体系具有生物固定与生物去除土壤重金属的两种功能,根际微生物可以菌根、内生菌等方式与根系形成联合体,通过增强植物抗性和优化根际环境,促进根系发展,增强植物吸收和向上转运重金属的能力。建立植物-微生物联合修复体系,可充分发挥植物与微生物作用功能的优势,提高污染土壤的修复效率。增强植物修复体系中微生物功能的重点是深入研究根际微生物、根系和介质载体三者之间复合功能,结合污染土壤类型与植物群落配置的特点筛选扩繁高效菌种与菌群。     

Phytoremediation--a novel and promising approach for environmental clean-up

Phytoremediation is an eco friendly approach for remediation of contaminated soil and water using plants. Phytoremediation is comprised of two components, one by the root colonizing microbes and the other by plants themselves, which degrade the toxic compounds to further non-toxic metabolites. Various compounds, viz. organic compounds, xenobiotics, pesticides and heavy metals, are among the contaminants that can be effectively remediated by plants. Plant cell cultures, hairy roots and algae have been studied for their ability to degrade a number of contaminants. They exhibit various enzymatic activities for degradation of xenobiotics, viz. dehalogenation, denitrification leading to breakdown of complex compounds to simple and non-toxic products. Plants and algae also have the ability to hyper accumulate various heavy metals by the action of phytochelatins and metallothioneins forming complexes with heavy metals and translocate them into vacuoles. Molecular cloning and expression of heavy metal accumulator genes and xenobiotic degrading enzyme coding genes resulted in enhanced remediation rates, which will be helpful in making the process for large-scale application to remediate vast areas of contaminated soils. A few companies worldwide are also working on this aspect of bioremediation, mainly by transgenic plants to replace expensive physical or chemical remediation techniques. Selection and testing multiple hyperaccumulator plants, protein engineering ofphytochelatin and membrane transporter genes and their expression would enhance the rate of phytoremediation, making this process a successful one for bioremediation of environmental contamination. Recent years have seen major investments in the R&D, which have also resulted in competition of filing patents by several companies for economic gains. The details of science & technology related to phytoremediation have been discussed with a focus on future trends and prospects of global relevance.     

The use of transgenic plants in the bioremediation of soils contaminated with trace elements

The use of plants to clean-up soils contaminated with trace elements could provide a cheap and sustainable technology for bioremediation. Field trials suggested that the rate of contaminant removal using conventional plants and growth conditions is insufficient. The introduction of novel traits into high biomass plants in a transgenic approach is a promising strategy for the development of effective phytoremediation technologies. This has been exemplified by generating plants able to convert organic and ionic forms of mercury into the less toxic, volatile, elemental mercury, a trait that occurs naturally only in some bacteria and not at all in plants. The engineering of a phytoremediator plant requires the optimization of a number of processes, including trace element mobilization in the soil, uptake into the root, detoxification and allocation within the plant. A number of transgenic plants have been generated in an attempt to modify the tolerance, uptake or homeostasis of trace elements. The phenotypes of these plants provide important insights for the improvement of engineering strategies. A better understanding, both of micronutrient acquisition and homeostasis, and of the genetic, biochemical and physiological basis of metal hyperaccumulation in plants, will be of key importance for the success of phytoremediation.     

Phytoremediation—A Novel and Promising Approach for Environmental Clean-up

ABSTRACT

Phytoremediation is an eco friendly approach for remediation of contaminated soil and water using plants. Phytoremediation is comprised of two components, one by the root colonizing microbes and the other by plants themselves, which degrade the toxic compounds to further non-toxic metabolites. Various compounds, viz. organic compounds, xenobiotics, pesticides and heavy metals, are among the contaminants that can be effectively remediated by plants. Plant cell cultures, hairy roots and algae have been studied for their ability to degrade a number of contaminants. They exhibit various enzymatic activities for degradation of xenobiotics, viz. dehalogenation, denitrification leading to breakdown of complex compounds to simple and non-toxic products. Plants and algae also have the ability to hyper accumulate various heavy metals by the action of phytochelatins and metallothioneins forming complexes with heavy metals and translocate them into vacuoles. Molecular cloning and expression of heavy metal accumulator genes and xenobiotic degrading enzyme coding genes resulted in enhanced remediation rates, which will be helpful in making the process for large-scale application to remediate vast areas of contaminated soils. A few companies worldwide are also working on this aspect of bioremediation, mainly by transgenic plants to replace expensive physical or chemical remediation techniques. Selection and testing multiple hyperaccumulator plants, protein engineering of phytochelatin and membrane transporter genes and their expression would enhance the rate of phytoremediation, making this process a successful one for bioremediation of environmental contamination. Recent years have seen major investments in the R&D, which have also resulted in competition of filing patents by several companies for economic gains. The details of science & technology related to phytoremediation have been discussed with a focus on future trends and prospects of global relevance.     

Plants as extreme environments? Ni-resistant bacteria and Ni-hyperaccumulators of serpentine flora

During recent years there has been an increasing interest in the bacterial communities occurring in unusual, often extreme, environments. On serpentine outcrops around the world, a high diversity of plant species showing the peculiar features of metal hyperaccumulation is present. These metal hyperaccumulators have received much attention for their potential biotechnological exploitation in phytoremediation processes, but also as unusual, extreme habitats for the associated bacterial flora, which could reveal novel details concerning bacterial adaptation. This paper will briefly focus on the research topics that have been addressed to date on bacteria associated with serpentine plants and aims to provide a state of the art and to present possible future directions for research which could lead to new insights on microbial adaptation and evolution, and potentially applied in technologies for sustainable use and remediation of contaminated land.