Phytoremediation and phytosensing of chemical contaminant, toluene: identification of the required target genes

As an industrial chemical produced worldwide in high volumes, toluene is commonly detected in ambient air and water. It can combine with oxygen and form compounds that are harmful to humans. In recent years, phytoremediation has been increasingly applied to repair the environmental damage caused by pollutants. However, insufficient knowledge is available regarding the response of plants to toluene. To detect the potential genes in plants that are related to the sensing mechanism and metabolism of toluene, a microarray analysis has been conducted on Arabidopsis thaliana seedlings grown on toluene-containing media. Following the validation of data and the application of appropriate selection criteria, the results show a coordinated induction and suppression of 202 and 67 toluene-responsive genes, respectively. Within the functional class "metabolism", the genes encoding detoxification proteins represent the most strongly up-regulated group. These include genes encoding cytochrome P450s, glucosyl transferases, and transporters. Subsequently, the toluene-induced genes of Arabidopsis are analyzed in detail.     

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 。     

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.     

Mercury detoxification with transgenic plants and other biotechnological breakthroughs for phytoremediation

Summary Phytoremediation, or the use of plants for removal and detoxification of environmental pollutants, has garnered great attention in recent years. This heightened interest is both scientifically, due the fascinating processes utilized by plants for tolerance and removal of harmful compounds, and commercially, as plants represent a more environmentally compatible and less expensive method of site remediation compared to standard approaches. The majority of phytoremediation studies have been with naturally occurring plant species after empirical discovery of their exceptional abilities for such applications. This has led to a growing body of literature and wider acceptance for plants in many aspects of environmental rehabilitation. However, this has occurred with little understanding of their basic biological mechanisms of action or investigation of alternative strategies for enhancing the capabilities of these extraordinary plants. Better understanding of plant physiology, biochemistry and molecular biology in response to specific contaminants is critical for optimization and advancement of phytoremediation. By applying the tools of biotechnology, the potential for plants as an aggressive method of environmental decontamination may be realized. This paper will serve as an introduction to the first Symposium assembled exclusively to review the use of molecular genetic and biotechnological methods for improvement of plants for phytoremediation. After a brief review of the other invited speakers' works (with more extensive papers following), the pioneering work using bacterial genes expressed in plants for removal of mercurial compounds will be surveyed.     

Transgenic rice plants expressing human p450 genes involved in xenobiotic metabolism for phytoremediation

Phytoremediation is the use of plants to remove xenobiotic compounds from the environment. Plants have the inherent ability to detoxify xenobiotic pollutants, but they are generally poor at degrading them. The introduction of genes involved in xenobiotic degradation is aimed at enhancing plants' potential further. Rice (Oryza sativa) is a good candidate for this purpose and has been transformed with genes encoding cytochrome P450 monooxygenases CYP1A1, CYP2B6, and CYP2C19. The transgenic plants were more tolerant to various herbicides than nontransgenic Nipponbare rice plants, owing to enhanced metabolism by the introduced P450 enzymes. Transgenic plants were able to remove atrazine and metolachlor from soil. Field testing and risk assessment are very important for developing transgenic plants for phytoremediation. Transgenic rice plants should become useful as herbicide-tolerant crops and for phytoremediation of xenobiotic pollutants in future.     

Genetic Engineering of Plants to Enhance Selenium Phytoremediation

Referee: Dr. Dean A. Martens, USDAARS Southwest Watershed, Research Center, 200 E. Allen Road, Tucson, AZ 85719 Phytoremediation is the use of plants to remove, contain, or render harmless environmental pollutants. In recent years, much attention has been focused on the improvement of such technologies for this purpose. In this review, we introduce selenium phytoremediation and describe the attempts made to enhance it through genetic engineering. Initial efforts have taken advantage of the knowledge of the enzymatic pathways for selenium assimilation and volatilization, especially by overexpressing genes of rate-limiting enzymes in plants. Another possible approach is to introduce additional metabolic pathways from selenium hyperaccumulators or organisms other than plants that can help detoxify selenium compounds. In this way the capacity of plants to take up, accumulate, and volatilize compounds can be increased beyond that of any naturally occurring plant species. Here we report on the progress that has been made in overexpressing potentially important enzymes involved in the selenium/sulfur pathways and discuss possible future directions in the enhancement of phytoremediation through genetic engineering.     

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 organochlorine pesticides: Concept,method, and recent developments

Rapid increase in industrialization of world economy in the past century has resulted in significantly high emission of anthropogenic chemicals in the ecosystem. The organochlorine pesticides (OCPs) are a great risk to the global environment and endanger the human health due to their affinity for dispersion, transportation over long distances, and bioaccumulation in the food chain. Phytoremediation is a promising technology that aims to make use of plants and associated bacteria for the treatment of groundwater and soil polluted by these contaminants. Processes known to be involved in phytoremediation of OCPs include phytoaccumulation, rhizoremediation, and phytotransformation. Vegetation has been accounted to considerably amplify OCP elimination from soil, in contrast to non-planted soil, attributable to both, uptake within plant tissues and high microbial degradation of OCP within the root zone. Developing transgenic plants is a promising approach to enhance phytoremediation capabilities. Recent advances in the application of phytoremediation technique for OCPs, including uptake by plants and plant–microbe association in the rhizosphere for the enhanced degradation and mineralization of these pollutants, is presented in this review. Additionally, some attempts to improve this technique using transgenesis and role of certain enzymes are also discussed.     

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.     

Plants,selenium and human health

Selenium is an essential nutrient for animals, microorganisms and some other eukaryotes. Although selenium has not been demonstrated to be essential in vascular plants, the ability of some plants to accumulate and transform selenium into bioactive compounds has important implications for human nutrition and health, and for the environment. Selenium-accumulating plants provide unique tools to help us understand selenium metabolism. They are also a source of genetic material that can be used to alter selenium metabolism and tolerance to help develop food crops that have enhanced levels of anticarcinogenic selenium compounds, as well as plants that are ideally suited for the phytoremediation of selenium-contaminated soils.     

Endophytes and their potential to deal with co-contamination of organic contaminants (toluene) and toxic metals (nickel) during phytoremediation

The aim was to investigate if engineered endophytes that are capable of degrading organic contaminants, and deal with or ideally improve uptake and translocation of toxic metals, can improve phytoremediation of mixed organic-metal pollution. As a model system, yellow lupine was inoculated with the endophyte Burkholderia cepacia VM1468 possessing (a) the pTOM-Bu61 plasmid, coding for constitutive toluene/TCE degradation, and (b) the chromosomally inserted ncc-nre Ni resistance/sequestration system. As controls, plants were inoculated with B. vietnamiensis BU61 (pTOM-Bu61) and B. cepacia BU72 (containing the ncc-nre Ni resistance/sequestration system). Plants were exposed to mixes of toluene and Ni. Only inoculation with B. cepacia VM1468 resulted in decreased Ni and toluene phytotoxicity, as measured by a protective effect on plant growth and decreased activities of enzymes involved in antioxidative defence (catalase, guaiacol peroxidase, superoxide dismutase) in the roots. Besides, plants inoculated with B. cepacia VM1468 and B. vietnamiensis BU61 released less toluene through the leaves than non-inoculated plants and those inoculated with B. cepacia BU72. Ni-uptake in roots was slightly increased for B. cepacia BU72 inoculated plants. These results indicate that engineered endophytes have the potential to assist their host plant to deal with co-contamination of toxic metals and organic contaminants during phytoremediation.     

Phytoremediation of toxic aromatic pollutants from soil

The enormous growth of industrialization, and the use of numerous aromatic compounds in dyestuffs, explosives, pesticides and pharmaceuticals has resulted in serious environmental pollution and has attracted considerable attention continuously over the last two decades. Many aromatic hydrocarbons, nitroaromatic compounds, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, diauxins and their derivatives are highly toxic, mutagenic and/or carcinogenic to natural microflora as well as to higher systems including humans. The increasing costs and limited efficiency of traditional physicochemical treatments of soil have spurred the development of new remediation technologies. Phytoremediation is emerging as an efficient treatment technology that uses plants to bioremediate pollutants from soil environments. Various modern tools and analytical devices have provided insight into the selection and optimization of remediation processes by various plant species. Sites heavily polluted with organic contaminants require hyperaccumulators, which could be developed by genetic engineering approaches. However, efficient hyperaccumulation by naturally occurring plants is also feasible and can be made practical by improving their nutritional and environmental requirements. Thus, phytoremediation of organics appears a very promising technology for the removal of contaminants from polluted soil. In this review, certain aspects of plant metabolism associated with phytoremediation of organic contaminants and their relevant phytoremediation efforts are discussed.IMTECH Communication No. 013/2002     

植物修复的生物学机制

随着现代化工业的发展,全球向土壤和环境中排放的重金属逐年增加。重金属污染已日益成为威胁人类健康和影响人类生活质量的严重环境问题和社会问题。这一问题可部分通过植物修复技术得以解决。植物修复技术是依据植物从环境中积累重金属元素和化合物的能力及其将这些有毒物质在植物体内代谢成无毒生物小分子的能力而建立的新的生物技术。本篇综述主要论及利用植物修复技术解决重金属污染的生物学机制。     

Phytoremediation of Organic Contaminants: A Review of Phytoremediation Research at the University of Washington

As overwhelmingly positive results have become available regarding the ability of plants to degrade compounds such as trichloroethylene, phytoremediation studies are expanding. Studies to determine the potential for phytoremediation of fully chlorinated compounds, such as carbon tetrachloride and tetrachloroethylene, brominated compounds, such as ethylene dibromide and dibromochloropropane, and nonhalogenated compounds, such as methyl-t-butyl ether (MTBE), are underway. When using phytoremediation, it is important to select not only a plant that is capable of degrading the pollutant in question, but also one that will grow well in that specific environment. In ecologically sensitive areas, such as the Hawaiian Islands, only plants native to the area can be used. One way to supplement the arsenal of plants available for remedial actions is to utilize genetic engineering tools to insert into plants those genes that will enable the plant to metabolize a particular pollutant. Hybrid technologies, such as using plants in pumping and irrigation systems, also enable plants to be used as a remedial method when the source of the pollutant is beyond the reach of plant roots, or when planting space directly over the pollutant is unavailable or restricted. Thus, the potential uses of phytoremediation are expanding as the technology continues to offer new, low-cost remediation options.     

Biogas production from water hyacinth and channel grass used for phytoremediation of industrial effluents

The paper reports on the biogas production from water hyacinth (Eichhornia crassipes) and channel grass (Vallisneria spiralis) employed separately for phytoremediation of lignin and metal-rich pulp and paper mill and highly acidic distillery effluents. These plants eventually grow well in diluted effluent up to 40% (i.e., 2.5-times dilution with deionized water) and often take up metals and toxic materials from wastewater for their metabolic use. Slurry of the two plants used for phytoremediation produced significantly more biogas than that produced by the plants grown in deionized water; the effect being more marked with plants used for phytoremediation of 20% pulp and paper mill effluent. Biogas production from channel grass was relatively greater and quicker (maximum in 6-9 days) than that from water hyacinth (in 9-12 days). Such variation in biogas production by the two macrophytes has been correlated with the changes in C, N and C/N ratio of their slurry brought by phytoremediation.     

Biological degradation of cyanide compounds

Cyanide compounds are produced as waste products of a number of industrial processes and several routes for their removal from the environment are under investigation, including the use of biodegradation. The most recent developments in this area have come from studies of the hydrolytic and oxidative pathways for biodegradation and the conditions that affect their activity. The biodegradation of cyanide under anaerobic conditions has also recently demonstrated the feasibility for concomitant biogas generation, a possible economic benefit of the process. Significant advances have been reported in the use of plants for the phytoremediation of cyanide compounds and evidence for the biodegradation of thiocyanate and metal-cyanide complexes has become available. Despite these advances, however, physical and economic factors still limit the application of cyanide biodegradation, as do competing technologies.     

PAH Phytoremediation: Rhizodegradation or Rhizoattenuation?

Dealing with soil contaminated with persistent organic pollutants (POP) is an increasing concern amplified by both regulatory constraints and the dramatic impact of human activities on the soil resource. The most used management options are treatments which totally eradicate the toxic compounds targeted. When possible, environmental-friendly processes should be used, and recent years have seen the emergence of green technologies using biological energies involving microorganisms (bioremediation) and plants (phytoremediation). Research has focused on phytoremediation and many have presented this technology as the process ideally combining efficiency, low cost and environmental acceptance. However, the applicability of phytoremediation on soils contaminated by bio-recalcitrant organic compounds, such as polycyclic aromatic hydrocarbons (PAH), has not yet proved as successful as expected. We propose here a review and discussion of the overall question of PAH status in soil and their potential for treatment. The limits and applicability of bioremediation technologies are discussed, and the specific beneficial effect of plants is objectively evaluated with a special interest to processes which lead to rhizoattenuation. Given the PAH high affinity to soil organic matter, availability is the main limitation to phytoremediation. In this context, bioavailability quantification remains an issue as well as the characterization of the recalcitrant fraction.     

Removal of Cyanide Contaminants from Rhizosphere Soil

Cyanide and cyanide-containing compounds from anthropogenic sources can be an environmental threat because of their potential toxicity. A remediation option for cyanide-contaminated soil may be through the use of plants and associated rhizosphere microorganimsms that have the ability to degrade cyanide compounds. Cyanogenic plant species are known to produce cyanide, but they also have the ability to degrade these compounds. In addition, the presence of these plants in soil may result in an increase in cyanide degrading microorganisms in the rhizosphere. Two cyanogenic species (Sorghum bicolor and Linum usitassium) and a noncyanogenic species (Panicum virgatum) were selected for a 200-day phytoremediation study to assess their potential use for removal of cyanide from soil. For both cyanogenic species, approximately 85% of the iron cyanide in soil was removed, whereas very little iron cyanide was removed in the unvegetated control or in the presence of Panicum virgatum. In addition, the activity of microbial communities in the rhizosphere of cyanogenic plants was higher than in cyanide-contaminated soil from unvegetated soil.     

Screening of plants for phytoremediation of oil-contaminated soil

Several species of ornamental flowering plants were evaluated regarding their phytoremediation ability for the cleanup of oil-contaminated soil in Japanese environmental conditions. Thirty-three species of plants were grown in oil-contaminated soil, and Mimosa, Zinnia, Gazania, and cypress vine were selected for further assessment on the basis of their favorable initial growth. No significant difference was observed in the above-ground and under-ground dry matter weight of Gazania 180 days after sowing between contaminated and non-contaminated plots. However, the other 3 species of plants died by the 180^th day, indicating that Gazania has an especially strong tolerance for oil-contaminated soil. The total petroleum hydrocarbon concentration of the soils in which the 4 species of plants were grown decreased by 45–49% by the 180^th day. Compared to an irrigated plot, the dehydrogenase activity of the contaminated soil also increased significantly, indicating a phytoremediation effect by the 4 tested plants. Mimosa, Zinnia, and cypress vine all died by the 180^th day after seeding, but the roots themselves became a source of nutrients for the soil microorganisms, which led to a phytoremediation effect by increase in the oil degradation activity. It has been indicated that Gazania is most appropriate for phytoremediation of oil-contaminated soil.     

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.