Differential mercury volatilization by tobacco organs expressing a modified bacterial merA gene

INTRODUCTIONEnvironmental pollution is an increasing prob-lem both fOr developing and developed countries.Mercury, both in organic and ionic fOrm, is one of themost hazardous pollutants among the heavy met-als[l]and its accumuIation in human body results ininactivation of metabolic enzymes and structuralproteins[2, 3] giving rise to serious health problems(Minamatasyndrome).Usually mercury pollution is caused by indus-trial and agricultural activities, releasing mercuryinto air, water an…     

Phytodetoxification of hazardous organomercurials by genetically engineered plants

Methylmercury is a highly toxic, organic derivative found in mercury-polluted wetlands and coastal sediments worldwide. Though commonly present at low concentrations in the substrate, methylmercury can biomagnify to concentrations that poison predatory animals and humans. In the interest of developing an in situ detoxification strategy, a model plant system was transformed with bacterial genes (merA for mercuric reductase and merB for organomercurial lyase) for an organic mercury detoxification pathway. Arabidopsis thaliana plants expressing both genes grow on 50-fold higher methylmercury concentrations than wild-type plants and up to 10-fold higher concentrations than plants that express merB alone. An in vivo assay demonstrated that both transgenes are required for plants to detoxify organic mercury by converting it to volatile and much less toxic elemental mercury.     

Phytoremediation of Mercury- and Methylmercury-Polluted Soils Using Genetically Engineered Plants

Inorganic mercury in contaminated soils and sediments is relatively immobile, though biological and chemical processes can transform it to more toxic and bioavailable methylmercury. Methylmercury is neurotoxic to vertebrates and is biomagnified in animal tissues as it is passed from prey to predator. Traditional remediation strategies for mercury contaminated soils are expensive and site-destructive. As an alternative we propose the use of transgenic aquatic, salt marsh, and upland plants to remove available inorganic mercury and methylmercury from contaminated soils and sediments. Plants engineered with a modified bacterial mercuric reductase gene, merA, are capable of converting Hg(II) taken up by roots to the much less toxic Hg(0), which is volatilized from the plant. Plants engineered to express the bacterial organo-mercurial lyase gene, merB, are capable of converting methylmercury taken up by plant roots into sulfhydryl-bound Hg(II). Plants expressing both genes are capable of converting ionic mercury and methylmercury to volatile Hg(0) which is released into an enormous global atmospheric Hg(0) pool. To assess the phytoremediation capability of plants containing the merA gene, a variety of assays were carried out with the model plants Arabidopsis thaliana, and tobacco (Nicotiana tabacum).     

The biochemistry of environmental heavy metal uptake by plants: Implications for the food chain

Plants absorb a number of elements from soil, some of which have no known biological function and some are known to be toxic at low concentrations. As plants constitute the foundation of the food chain, some concerns have been raised about the possibility of toxic concentrations of certain elements being transported from plants to higher strata of the food chain. Special attention has been given to the uptake and biotransformation mechanisms occurring in plants and its role in bioaccumulation and impact on consumers, especially human beings. While this review draws particular attention to metal accumulation in edible plants, researched studies of certain wild plants and their consumers are included. Furthermore, this review focuses on plant uptake of the toxic elements arsenic, cadmium, chromium, mercury, and lead and their possible transfer to the food chain. These elements were selected because they are well-established as being toxic for living systems and their effects in humans have been widely documented. Arsenic is known to promote cancer of the bladder, lung, and skin and can be acquired, for example, through the consumption of As-contaminated rice. Cadmium can attack kidney, liver, bone, and it also affects the female reproduction system. Cadmium also can be found in rice. Chromium can produce cancer, and humans can be exposed through smoking and eating Cr-laden vegetables. Lead and mercury are well known neurotoxins that can be consumed via seafood, vegetables and rice.     

Expression of organomercurial lyase in eastern cottonwood enhances organomercury resistance

Summary Release of inorganic mercury pollutants into shallow aquatic environments has resulted in the bacterial production of a more toxic organic mercury species, methylmercury. The bacterial organomercurial lyase (MerB) catalyses the protonolysis of the carbon-mercury bond and releases Hg(II), a less toxic, non-biomagnified form of mercury. Our objective was to engineer eastern cottonwood (Populus deltoides), a fast-growing tree adapted to growth in riparian environments, with the merB gene to explore its potential for phytoremediation of mercury. We produced multiple eastern cottonwood clones expressing a modified bacterial merB gene, confirmed that the gene was expressed in the transclones and tested the regenerated plants for their ability to tolerate exposure to an organic mercury source, phenylmercuric acetate (PMA), in vitro and in hydroponic culture, compared to wild-type control trees. Transgenic merB plants expressed high levels of MerB protein and showed some evidence of higher resistance to the organic mercury than wild-type plants, producing longer roots under exposure to PMA in vitro, although hydroponic culture results were inconclusive. Our results indicate that in order for merB to be useful in eastern cottonwood trees designed to degrade methylmercury at mercury-contaminated aquatic sites, it will probably need to be combined with other genes such as merA.     

Mercury toxicity in plants

Mercury poisoning has become a problem of current interest as a result of environmental pollution on a global scale. Natural emissions of mercury form two-thirds of the input; manmade releases form about one-third. Considerable amounts of mercury may be added to agricultural land with sludge, fertilizers, lime, and manures. The most important sources of contaminating agricultural soil have been the use of organic mercurials as a seed-coat dressing to prevent fungal diseases in seeds. In general, the effect of treatment on germination is favorable when recommended dosages are used. Injury to the seed increases in direct proportion to increasing rates of application. The availability of soil mercury to plants is low, and there is a tendency for mercury to accumulate in roots, indicating that the roots serve as a barrier to mercury uptake. Mercury concentration in aboveground parts of plants appears to depend largely on foliar uptake of Hg⁰ volatilized from the soil. Uptake of mercury has been found to be plant specific in bryophytes, lichens, wetland plants, woody plants, and crop plants. Factors affecting plant uptake include soil or sediment organic content, carbon exchange capacity, oxide and carbonate content, redox potential, formulation used, and total metal content. In general, mercury uptake in plants could be related to pollution level. With lower levels of mercury pollution, the amounts in crops are below the permissible levels. Aquatic plants have shown to be bioaccumulators of mercury. Mercury concentrations in the plants (stems and leaves) are always greater when the metal is introduced in organic form. In freshwater aquatic vascular plants, differences in uptake rate depend on the species of plant, seasonal growthrate changes, and the metal ion being absorbed. Some of the mercury emitted from the source into the atmosphere is absorbed by plant leaves and migrates to humus through fallen leaves. Mercury-vapor uptake by leaves of the C₃ speciesoats, barley, and wheat is five times greater than that by leaves of the C₄ species corn, sorghum, and crabgrass. Such differential uptake by C₃ and C₄ species is largely attributable to internal resistance to mercury-vapor binding. Airborne mercury thus seems to contribute significantly to the mercury content of crops and thereby to its intake by humans as food. Accumulation, toxicity response, and mercury distribution differ between plants exposed through shoots or through roots, even when internal mercury concentrations in the treated plants are similar. Throughfall and litterfall play a significant role in the cycling and deposition of mercury. The possible causal mechanisms of mercury toxicity are changes in the permeability of the cell membrane, reactions of sulphydryl (-SH) groups with cations, affinity for reacting with phosphate groups and active groups of ADP or ATP, and replacement of essential ions, mainly major cations. In general, inorganic forms are thought to be more available to plants than are organic ones. Plants can be exposed to mercurials either by direct administration as antifungal agents, mainly to crop plants through seed treatment or foliar spray, or by accident. The end points screened are seed germination, seedling growth, relative growth of roots and shoots, and, in some case, studies of leaf-area index, internode development, and other anatomical characters. Accidental exposures occur through soil, water, and air pollution. The level of toxicity is usually tested under laboratory conditions using a wide range of concentrations and different periods of exposure. Additional parameters include biochemical assays and genetical studies. The absorption of organic and inorganic mercury from soil by plants is low, and there is a barrier to mercury translocation from plant roots to tops. Thus, large increases in mercury levels in soil produce only modest increases in mercury levels in plants by direct uptake from soil. Injuries to cereal seeds caused by organic mercurials has been characterized by abnormal germination and hypertrophy of the roots and coleoptile. Mercury affects both light and dark reactions of photosynthesis. Substitution of the central atom of chlorophyll, magnesium, by mercury in vivo prevents photosynthetic light harvesting in the affected chlorophyll molecules, resulting in a breakdown of photosynthesis. The reaction varies with light intensity. A concentration and time-dependent protective effect of GSH seems to be mediated by the restricted uptake of the metal involving cytoplasmic protein synthesis. Plant cells contain aquaporins, proteins that facilitate the transport of water, in the vacuolar membrane (tonoplast) and the plasma membrane. Many aquaporins are mercury sensitive, and in AQP1 a mercury-sensitive cysteine residue (Cys-189) is present adjacent to a conserved Asn-Pro-Ala motif. At low concentrations mercury has a toxic effect on the degrading capabilities of microorganisms. Sensitivity to the metal can be enhanced by a reduction in pH, and tolerance of mercury by microorganisms has been found to be in the order: total population > nitrogen fixers > nitrifiers. Numerous experiments have been carried out to study the genetic effects of mercury compounds in experimental test systems using a variety of genetic endpoints. The most noticeable and consistent effect is the induction of c-mitosis through disturbance of the spindle activity, resulting in the formation of polyploid and aneuploid cells and c-tumors. Organomercurials have been reported to be 200 times more potent than inorganic mercury. Exposure to inorganic mercury reduces mitotic index in the root-tip cells and increases the frequency of chromosomal aberrations in degrees directly proportional to the concentrations used and to the duration of exposure. The period of recovery after removal of mercury is inversely related to the concentration and duration of exposure. Bacterial plasmids encode resistance systems for toxic metal ions, including Hg²⁺, functioning by energy-dependent efflux of toxic ions through ATPases and chemiosmotic cationproton antiporters. The inducible mercury resistance (mer) operon encodes both a mercuric ion uptake and detoxification enzymes. In gram-negative bacteria a periplasmic protein,MerP, an inner-membrane transport protein,MerT, and a cytoplasmic enzyme, mercuric reductase, theMerA protein, are responsible for the transport of mercuric ions into cells and their reduction to elemental mercury, Hg(II). InThiobacillus ferrooxidans, an acidophilic chemoautotrophic bacterium sensitive to mercury ions, a group of mercury-resistant strains, which volatilize mercury, has been isolated. The entire coding sequence of the mercury-ion resistance gene has been located in a 2.3 kb fragment of chromosomal DNA (encoding 56,000 and 16,000 molecular-weight proteins) from strain E-l 5 ofEscherichia coli. Higher plants andSchizosaccharomyces pombe respond to heavy-metal stress of mercury by synthesizing phytochelatins (PCs) that act as chelators. The strength of Hg(II) binding to glutathione and phytochelatins follows the order: γGlu-Cys-Gly(γGlu-Cys)₂Gly(γGlu-Cys)₃Gly(γGlu-Cys)₄Gly. Suspension cultures of haploid tobacco,Nicotiana tabacum, cells were subjected to ethyl methane sulfonate to raise mercury-tolerant plantlets. HgCl₂-tolerant variants were selected from nitrosoguanidine (NTG)-treated suspension cell cultures of cow pea,Vigna unguiculata, initiated from hypocotyl callus and incubated with 18 ⧎g/ml HgCl₂. Experiments have been carried out to develop mercury-tolerant plants ofHordeum vulgare through previous exposure to low doses of mercury and subsequent planting of the next generation in mercury-contaminated soil. Phytoremediation involves the use of plants to extract, detoxify, and/or sequester environmental pollutants from soil and water. Transgenic plants cleave mercury ions from methylmercury complexes, reduce mercury ions to the metallic form, take up metallic mercury through their roots, and evolve less toxic elemental mercury. Genetically engineered plants contain modified forms of bacterial genes that break down methyl mercury and reduce mercury ions. The first gene successfully inserted into plants wasmerA, which codes for a mercuric ion reductase enzyme, reducing ionic mercury to the less toxic elemental form.MerB codes for an organomercurial lyase protein that cleaves mercury ions from highly toxic methyl mercury compounds. Plants with themerB gene have been shown to detoxify methyl mercury in soil and water. Both genes have been successfully expressed inArabidopsis thaliana, Brassica (mustard),Nicotiana tabacum (tobacco), andLiriodendron tulipifera (tulip poplar). Plants currently being transformed include cattails, wild rice, andSpartina, another wetland plant. The problem of mercury contamination can be reduced appreciably by combining the standard methods of phytoremediation—removal of mercury from polluted areas through scavenger plants—with raising such plants both by routine mutagenesis and by genetic engineering. The different transgenics raised utilizing the two genesmerA andmerB are very hopeful prospects.     

Subcellular targeting of methylmercury lyase enhances its specific activity for organic mercury detoxification in plants

Methylmercury is an environmental pollutant that biomagnifies in the aquatic food chain with severe consequences for humans and other animals. In an effort to remove this toxin in situ, we have been engineering plants that express the bacterial mercury resistance enzymes organomercurial lyase MerB and mercuric ion reductase MerA. In vivo kinetics experiments suggest that the diffusion of hydrophobic organic mercury to MerB limits the rate of the coupled reaction with MerA (Bizily et al., 2000). To optimize reaction kinetics for organic mercury compounds, the merB gene was engineered to target MerB for accumulation in the endoplasmic reticulum and for secretion to the cell wall. Plants expressing the targeted MerB proteins and cytoplasmic MerA are highly resistant to organic mercury and degrade organic mercury at 10 to 70 times higher specific activity than plants with the cytoplasmically distributed wild-type MerB enzyme. MerB protein in endoplasmic reticulum-targeted plants appears to accumulate in large vesicular structures that can be visualized in immunolabeled plant cells. These results suggest that the toxic effects of organic mercury are focused in microenvironments of the secretory pathway, that these hydrophobic compartments provide more favorable reaction conditions for MerB activity, and that moderate increases in targeted MerB expression will lead to significant gains in detoxification. In summary, to maximize phytoremediation efficiency of hydrophobic pollutants in plants, it may be beneficial to target enzymes to specific subcellular environments.     

Expression of mercuric ion reductase in Eastern cottonwood (Populus deltoides) confers mercuric ion reduction and resistance

Mercury is one of the most hazardous heavy metals and is a particular problem in aquatic ecosystems, where organic mercury is biomagnified in the food chain. Previous studies demonstrated that transgenic model plants expressing a modified mercuric ion reductase gene from bacteria could detoxify mercury by converting the more toxic and reductive ionic form [Hg(II)] to less toxic elemental mercury [Hg(0)]. To further investigate if a genetic engineering approach for mercury phytoremediation can be effective in trees with a greater potential in riparian ecosystems, we generated transgenic Eastern cottonwood (Populus deltoides) trees expressing modified merA9 and merA18 genes. Leaf sections from transgenic plantlets produced adventitious shoots in the presence of 50 microm Hg(II) supplied as HgCl2, which inhibited shoot induction from leaf explants of wild-type plantlets. Transgenic shoots cultured in a medium containing 25 microm Hg(II) showed normal growth and rooted, while wild-type shoots were killed. When the transgenic cottonwood plantlets were exposed to Hg(II), they evolved 2-4-fold the amount of Hg(0) relative to wild-type plantlets. Transgenic merA9 and merA18 plants accumulated significantly higher biomass than control plants on a Georgia Piedmont soil contaminated with 40 p.p.m. Hg(II). Our results indicate that Eastern cottonwood plants expressing the bacterial mercuric ion reductase gene have potential as candidates for in situ remediation of mercury-contaminated soils or wastewater.     

Mercury toxicity in livers of northern pike (Esox lucius) from Isle Royale, USA

Many laboratory studies have documented that mercury can be toxic to fish, but it is largely unknown if mercury is toxic to fish in their natural environments. The objective of our study was to investigate the toxic effects of mercury on northern pike (Esox lucius) at Isle Royale, Michigan. In 124 northern pike from eight inland lakes, concentrations of total mercury in skin-on fillets ranged from 0.069 to 0.622 microg/g wet mass (wet wt). Concentrations of total mercury in livers increased exponentially compared with concentrations in fillets, to a maximum of 3.1 microg/g wet wt. Methylmercury constituted a majority of the mercury in livers with total mercury concentrations <0.5 microg/g wet wt, but declined to 28-51% of the mercury in livers with total mercury concentrations >0.5 microg/g wet wt. Liver color (absorbance at 400 nm) varied among northern pike and was positively related to liver total mercury concentration. The pigment causing variation in liver color was identified as lipofuscin, which results from lipid peroxidation of membranous organelles. An analysis of covariance revealed lipofuscin accumulation was primarily associated with mercury exposure, and this association obscured any normal accumulation from aging. We also documented decreased lipid reserves in livers and poor condition factors of northern pike with high liver total mercury concentrations. Our results suggest (i) northern pike at Isle Royale are experiencing toxicity at concentrations of total mercury common for northern pike and other piscivorous fish elsewhere in North America and (ii) liver color may be useful for indicating mercury exposure and effects in northern pike at Isle Royale and possibly other aquatic ecosystems and other fish species.     

Mercury toxicity to terrestrial biota

The heavy metal mercury is a non-essential hazardous element which concentrates up the food chain. It is necessary to assess the ecological risk of mercury to establish proper regulatory guideline levels. Most of the toxicological assessment of mercury has been focused on aquatic organisms, however in terrestrial bodies the information is limited. Hence this review critically discusses the toxicity of inorganic mercury to key terrestrial biota from recent literature and evaluate whether these information are adequate to establish safe regulatory limits or precautionary values which is invaluable for risk assessment of mercury in soil. Till date soil microorganisms, plants and invertebrates have been utilized for assessing mercury toxicity; among them, microorganisms have been observed to be the most sensitive indicators to mercury stress. Large inconsistency among the measured toxic concentrations indicates that measuring mercury toxicity in soil may be influenced by soil characteristics and ageing period of contamination. This review warrants more studies to obtain widely acceptable safe limit of soil mercury.     

Impacts of Nutrient Loading on Mercury Fractionation and Mobility in an Estuarine Wetland in Nansi Lake,China

Total mercury is closely related to nutrient status in ecosystems. However, studies specifying the effects of various nutrient forms on the mobility of mercury are rare. In this study, three fractions of mercury (mobile, semi-mobile, and non-mobile) were examined in the soil of an estuarine wetland with excessive nutrient inputs in northern China. The wetland is located in the east route of China's South-to-North Water Transfer Project. Correlations between the three fractions of mercury and four nutrient parameters were analyzed. Results showed that toxic mobile and semi-mobile mercury comprised a great portion of total soil mercury in the studied wetland. Among the four nutrient parameters, ammonium nitrogen showed a significant negative correlation with mobile toxic mercury and was the principal nutrient component that influenced the mobility and toxicity of mercury. Nitrate nitrogen showed a positive correlation with mobile toxic mercury. Moreover, carbon/nitrogen ratio and soil acid were positively correlated with mercury mobility. Given that lower nitrogen loading is required in the watershed to ensure the water quality during the water transfer project, our study suggested that changes in nitrogen inputs might promote the mobility and bioavailability of mercury in the watershed. Thus, more attention should be provided to the potential hazard of mercury in the food web during the water transfer.     

Tolerance to Various Toxicants by Marine Bacteria Highly Resistant to Mercury

Bacteria highly resistant to mercury isolated from seawater and sediment samples were tested for growth in the presence of different heavy metals, pesticides, phenol, formaldehyde, formic acid, and trichloroethane to investigate their potential for growth in the presence of a variety of toxic xenobiotics. We hypothesized that bacteria resistant to high concentrations of mercury would have potential capacities to tolerate or possibly degrade a variety of toxic materials and thus would be important in environmental pollution bioremediation. The mercury-resistant bacteria were found to belong to Pseudomonas, Proteus, Xanthomonas, Alteromonas, Aeromonas, and Enterobacteriaceae. All these environmental bacterial strains tolerant to mercury used in this study were capable of growth at a far higher concentration (50 ppm) of mercury than previously reported. Likewise, their ability to grow in the presence of toxic xenobiotics, either singly or in combination, was superior to that of bacteria incapable of growth in media containing 5 ppm mercury. Plasmid-curing assays done in this study ascertained that resistance to mercury antibiotics, and toxic xenobiotics is mediated by chromosomally borne genes and/or transposable elements rather than by plasmids.     

Engineering expression of bacterial polyphosphate kinase in tobacco for mercury remediation

To develop the potential of plants to sequester and accumulate mercurials from the contaminated sites, we engineered a tobacco (Nicotiana tabacum) plant to express a bacterial ppk gene, encoding polyphosphate kinase (PPK), under control of a plant promoter. The designated plant expression plasmid pPKT116 that contains the entire coding region of ppk was used for Agrobacterium-mediated gene transfer into tobacco plants. A large number of independent transgenic tobacco plants were obtained, in some of which the ppk gene was stably integrated in the plant genome and substantially translated to the expected PPK protein in the transgenic tobacco. The presence of Hg²⁺ did not cause considerable morphological abnormalities in the transgenic tobacco, which grew, flowered, and set seed similarly to the wild-type tobacco on the medium containing normally toxic levels of Hg²⁺. The ppk-transgenic tobacco showed more resistance to Hg²⁺ and accumulated more mercury than its wild-type progenitors. These results suggest that ppk-specified polyphosphate has abilities to reduce mercury toxicity, probably via chelation mechanism, and also to accumulate mercury in the transgenic tobacco. Based on the results obtained in the present study, the expression of ppk gene in transgenic tobacco plants might provide a means for phytoremediation of mercury pollution.     

A Rhizosphere-Associated Symbiont,Photobacterium spp. Strain MELD1, and Its Targeted Synergistic Activity for Phytoprotection against Mercury

Though heavy metal such as mercury is toxic to plants and microorganisms, the synergistic activity between them may offer benefit for surviving. In this study, a mercury-reducing bacterium, Photobacterium spp. strain MELD1, with an MIC of 33 mg . kg^-1 mercury was isolated from a severely mercury and dioxin contaminated rhizosphere soil of reed (Phragmites australis). While the whole genome sequencing of MELD1 confirmed the presence of a mer operon, the mercury reductase MerA gene showed 99% sequence identity to Vibrio shilloni AK1 and implicates its route resulted from the event of horizontal gene transfer. The efficiency of MELD1 to vaporize mercury (25 mg . kg^-1, 24 h) and its tolerance to toxic metals and xenobiotics such as lead, cadmium, pentachlorophenol, pentachloroethylene, 3-chlorobenzoic acid, 2,3,7,8-tetrachlorodibenzo-p-dioxin and 1,2,3,7,8,9-hexachlorodibenzo-p-dioxin is promising. Combination of a long yard bean (Vigna unguiculata ssp. Sesquipedalis) and strain MELD1 proved beneficial in the phytoprotection of mercury in vivo. The effect of mercury (Hg) on growth, distribution and tolerance was examined in root, shoot, leaves and pod of yard long bean with and without the inoculation of strain MELD1. The model plant inoculated with MELD1 had significant increases in biomass, root length, seed number, and increased mercury uptake limited to roots. Biolog plate assay were used to assess the sole-carbon source utilization pattern of the isolate and Indole-3-acetic acid (IAA) productivity was analyzed to examine if the strain could contribute to plant growth. The results of this study suggest that, as a rhizosphere-associated symbiont, the synergistic activity between the plant and MELD1 can improve the efficiency for phytoprotection, phytostabilization and phytoremediation of mercury.     

Alleviation of Mercury Toxicity in Wheat by the Interaction of Mercury-Tolerant Plant Growth-Promoting Rhizobacteria

Heavy metal contamination of agricultural soils has increased along with industrialization. Mercury is a toxic heavy metal and a widespread pollutant in the ecosystem. Mercury-tolerant and plant growth-promoting rhizobacteria (PGPR) HG 1, HG 2, and HG 3 were isolated from the rhizosphere of plants growing in a mercury-contaminated site. These isolates were able to grow in the presence of mercury ranging from 10 to 200 µM in minimal medium and 25 to 500 µM in LB medium. The strains were characterized by morphological, biochemical, and plant growth-promoting traits. In the present study, these PGPR strains were analyzed for their involvement in metal stress tolerance in Triticum aestivum (wheat). Two bacterial strains, namely, Enterobacter ludwigii (HG 2) and Klebsiella pneumoniae (HG 3), showed better growth promotion of T. aestivum seedlings under metal stress. Different growth parameters like, water content and biochemical properties were analyzed in the PGPR-inoculated wheat plants under 75 µM HgCl₂. Shoot length, root length, shoot dry weight, root dry weight and relative water content (RWC) were significantly higher in inoculated plants compared to uninoculated plants under stress condition. Proline content, electrolyte leakage, and malondialdehyde content (shoots and roots) were significantly lower in inoculated plants with respect to uninoculated plants under mercury stress. Therefore, it could be assumed that all these parameters collectively improve plant growth under mercury stress conditions in the presence of PGPR. Hence, these PGPRs can serve as promising candidates for increasing plant growth and also have immense potential for bioremediation of mercury-contaminated soils.     

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.     

天津汉沽农田土壤－植物系统中汞的分布、迁移和积累

本文研究了受汞污染的农田土壤—植物系统中汞的分布,迁移和积累的规律。土壤中的汞在离污染源3公里的范围内含量最高;主要集中在0一20厘米的土壤上层,几乎不往下迁移。植物可以从土壤和大气中吸收、积累汞。在汉沽区没有发现由于汞污染所造成的植物受害症状。植物中的汞含量与土壤中的汞含量成正相关。土壤汞含量与水稻茎叶汞含量的相关系数为0.836(N=7),与糙米汞含量的相关系数为0.898(N=7)。植物不同部位的汞含量根>叶>茎>种子。不同作物种子比较,糙米>高粱>小麦。在大气中汞含量高的地段,植物地上部分汞含量高于根。土壤、植物中的汞不断地向大气扩散,而大气中的汞随着降雨、降尘等又不断地沉降到土壤和植物的气生表面,并可被植物吸收。汞向其邻近地区扩散的能力较小。     

三江平原典型湿地植物中汞的分布与库存量

分析了三江平原典型植物中的总Hg浓度．结果表明,各种植物中总Hg浓度差别较大,苔藓>藻类>苔草>禾草>灌木;干湿环境是影响总Hg浓度的重要因素;湿地植物总Hg浓度高于水稻和玉米等农作物．较高的土壤总Hg浓度是近地面大气中Hg的重要来源,间接地影响到植物中Hg的浓度．植物各构件中总Hg浓度具有立枯>根>叶>茎的特点．在植物整个生长季总Hg浓度先增加后减少．估算了三江平原湿地植物Hg的库存量．小叶章湿地植物地上部分库存量为24．9μg·m^-2;毛果苔草湿地植物地上部分库存量为35．8μg·m^-2,高于加拿大实验湖泊湿地．