Yeast metallothionein in transgenic tobacco promotes copper uptake from contaminated soils

Tobacco as an efficient metal accumulator

Tobacco (Nicotiana tabacum L.) is an important industrial crop plant. However, it efficiently accumulates metals, primarily cadmium (Cd) and also zinc (Zn), in its leaves. Therefore, it could be a source of cadmium intake by smokers. On the other hand, as a high leaf metal accumulator, it is widely used for phytoremediation of metal-contaminated soil. Both issues provide an important rationale for investigating the processes regulating metal homeostasis in tobacco. This work summarizes the results of research to date on the understanding of the molecular mechanisms determining the effective uptake of Zn and Cd, their translocation into shoots and accumulation in leaves. It also discusses the current state of research to improve the phytoremediation properties of tobacco through genetic modification and to limit leaf Cd content for the tobacco industry.

Molecular mechanisms in phytoremediation of environmental contaminants and prospects of engineered transgenic plants/microbes

Concerns about emerging environmental contaminants have been growing along with industrialization and urbanization around the globe. Among various options for remediating these contaminants, phytotechnology is suggested as a feasible option to maintain the environmental sustainability. The recent advances in phytoremediation, genetic/molecular/omics/metabolic engineering, and nanotechnology are opening new paths for efficient treatment of emerging organic/inorganic contaminants. In this respect, elucidation of molecular mechanisms and genetic engineering of hyperaccumulator plants is expected to enhance remediation of environmental contaminants. This review was organized to offer valuable insights into the molecular mechanisms of phytoremediation and the prospects of transgenic hyperaccumulators with enhanced stress tolerance to diverse contaminants such as heavy metals and metalloids, xenobiotics, explosives, poly aromatic hydrocarbons (PAHs), petroleum hydrocarbons, pesticides, and nanoparticles. The roles of genoremediation and nanoparticles in augmenting the phytoremediation technology are also described in an interrelated framework with biotechnological prospects (e.g., plant molecular nano-farming). Finally, political debate on the preferential use of crops versus non-crop hyperaccumulators in genoremediation, limitations of transgenics in phytotechnologies, and their public acceptance issues are discussed in the policy framework.

Phytoextraction of Heavy Metals: A Promising Tool for Clean-Up of Polluted Environment?

Pollution by heavy metals (HM) represents a serious threat for both the environment and human health. Due to their elemental character, HM cannot be chemically degraded, and their detoxification in the environment mostly resides either in stabilization in situ or in their removal from the matrix, e.g., soil. For this purpose, phytoremediation, i.e., the application of plants for the restoration of a polluted environment, has been proposed as a promising green alternative to traditional physical and chemical methods. Among the phytoremediation techniques, phytoextraction refers to the removal of HM from the matrix through their uptake by a plant. It possesses considerable advantages over traditional techniques, especially due to its cost effectiveness, potential treatment of multiple HM simultaneously, no need for the excavation of contaminated soil, good acceptance by the public, the possibility of follow-up processing of the biomass produced, etc. In this review, we focused on three basic HM phytoextraction strategies that differ in the type of plant species being employed: natural hyperaccumulators, fast-growing plant species with high-biomass production and, potentially, plants genetically engineered toward a phenotype that favors efficient HM uptake and boosted HM tolerance. Considerable knowledge on the applicability of plants for HM phytoextraction has been gathered to date from both lab-scale studies performed under controlled model conditions and field trials using real environmental conditions. Based on this knowledge, many specific applications of plants for the remediation of HM-polluted soils have been proposed. Such studies often also include suggestions for the further processing of HM-contaminated biomass, therefore providing an added economical value. Based on the examples presented here, we recommend that intensive research be performed on the selection of appropriate plant taxa for various sets of conditions, environmental risk assessment, the fate of HM-enriched biomass, economical aspects of the process, etc.

Prospecting for Microelement Function and Biosafety Assessment of Transgenic Cereal Plants

Microelement contents and metabolism are vitally important for cereal plant growth and development as well as end-use properties. While minerals phytotoxicity harms plants, microelement deficiency also affects human health. Genetic engineering provides a promising way to solve these problems. As plants vary in abilities to uptake, transport, and accumulate minerals, and the key enzymes acting on that process is primarily presented in this review. Subsequently, microelement function and biosafety assessment of transgenic cereal plants have become a key issue to be addressed. Progress in genetic engineering of cereal plants has been made with the introduction of quality, high-yield, and resistant genes since the first transgenic rice, corn, and wheat were born in 1988, 1990, and 1992, respectively. As the biosafety issue of transgenic cereal plants has now risen to be a top concern, many studies on transgenic biosafety have been carried out. Transgenic cereal biosafety issues mainly include two subjects, environmental friendliness and end-use safety. Different levels of gene confirmation, genomics, proteomics, metabolomics and nutritiomics, absorption, metabolism, and function have been investigated. Also, the different levels of microelement contents have been measured in transgenic plants. Based on the motivation of the requested biosafety, systematic designs, and analysis of transgenic cereal are also presented in this review paper.

The heterologous expression of the Iris lactea var. chinensis type 2 metallothionein IlMT2b gene enhances copper tolerance in Arabidopsis thaliana

Iris lactea var. chinensis (I. lactea var. chinensis) is a widely adapted perennial species with a high level of copper tolerance. To evaluate the role of metallothioneins (MTs) in copper tolerance in I. lactea var. chinensis, a full-length cDNA homologue of MT2, designated IlMT2b (GenBank accession No. AB907788), was cloned using the RACE-PCR method. The expression level of IlMT2b in the leaves and roots of I. lactea var. chinensis was induced in response to copper (Cu) treatment. Ectopic expression of IlMT2b in Arabidopsis thaliana increased the Cu concentration and reduced H2O2 production in the transgenic plants. After treatment with 50 and 100 μM Cu, the root length of two transgenic seedlings was respectively about 1.5- and 3-fold longer than that of the wild-type. Together, these results suggested that IlMT2b may represent a useful target gene for the phytoremediation of Cu-polluted soil.

Transgenic mice expressing yeast CUP1 exhibit increased copper utilization from feeds

Copper is required for structural and catalytic properties of a variety of enzymes participating in many vital biological processes for growth and development. Feeds provide most of the copper as an essential micronutrient consumed by animals, but inorganic copper could not be utilized effectively. In the present study, we aimed to develop transgenic mouse models to test if copper utilization will be increased by providing the animals with an exogenous gene for generation of copper chelatin in saliva. Considering that the S. cerevisiae CUP1 gene encodes a Cys-rich protein that can bind copper as specifically as copper chelatin in yeast, we therefore constructed a transgene plasmid containing the CUP1 gene regulated for specific expression in the salivary glands by a promoter of gene coding pig parotid secretory protein. Transgenic CUP1 was highly expressed in the parotid and submandibular salivary glands and secreted in saliva as a 9-kDa copper-chelating protein. Expression of salivary copper-chelating proteins reduced fecal copper contents by 21.61% and increased body-weight by 12.97%, suggesting that chelating proteins improve the utilization and absorbed efficacy of copper. No negative effects on the health of the transgenic mice were found by blood biochemistry and histology analysis. These results demonstrate that the introduction of the salivary CUP1 transgene into animals offers a possible approach to increase the utilization efficiency of copper and decrease the fecal copper contents.

Phenotypic and molecular consequences of overexpression of metal-homeostasis genes

Metal hyperaccumulating plants are able to store very large amounts of metals in their shoots. There are a number of reasons why it is important to be able to introduce metal hyperaccumulation traits into non-accumulating species (e.g., phytoremediation or biofortification in minerals) and to engineer a desired level of accumulation and distribution of metals. Metal homeostasis genes have therefore been used for these purposes. Engineered accumulation levels, however, have often been far from expected, and transgenic plants frequently display phenotypic features not related to the physiological function of the introduced gene. In this review, we focus on an aspect often neglected in research on plants expressing metal homeostasis genes: the specific regulation of endogenous metal homeostasis genes of the host plant in response to the transgene-induced imbalance of the metal status. These modifications constitute one of the major mechanisms involved in the generation of the plant's phenotype, including unexpected characteristics. Interestingly, activation of so-called "metal cross-homeostasis" has emerged as a factor of primary importance.

Approaches for enhanced phytoextraction of heavy metals

The contamination of the environment with toxic metals has become a worldwide problem. Metal toxicity affects crop yields, soil biomass and fertility. Soils polluted with heavy metals pose a serious health hazard to humans as well as plants and animals, and often requires soil remediation practices. Phytoextraction refers to the uptake of contaminants from soil or water by plant roots and their translocation to any harvestable plant part. Phytoextraction has the potential to remove contaminants and promote long-term cleanup of soil or wastewater. The success of phytoextraction as a potential environmental cleanup technology depends on factors like metal availability for uptake, as well as plants ability to absorb and accumulate metals in aerial parts. Efforts are ongoing to understand the genetics and biochemistry of metal uptake, transport and storage in hyperaccumulator plants so as to be able to develop transgenic plants with improved phytoremediation capability. Many plant species are being investigated to determine their usefulness for phytoextraction, especially high biomass crops. The present review aims to give an updated version of information available with respect to metal tolerance and accumulation mechanisms in plants, as well as on the environmental and genetic factors affecting heavy metal uptake. The genetic tools of classical breeding and genetic engineering have opened the door to creation of 'remediation' cultivars. An overview is presented on the possible strategies for developing novel genotypes with increased metal accumulation and tolerance to toxicity.

Transgenic Nicotiana tabacum plants expressing a fungal copper transporter gene show enhanced acquisition of copper

The diets of two-thirds of the world's population are deficient in one or more essential elements and one of the approaches to enhance the levels of mineral elements in food crops is by developing plants with ability to accumulate them in edible parts. Besides conventional methods, transgenic technology can be used for enhancing metal acquisition in plants. Copper is an essential element, which is often deficient in human diet. With the objective of developing plants with improved copper acquisition, a high-affinity copper transporter gene (tcu-1) was cloned from fungus Neurospora crassa and introduced into a model plant (Nicotiana tabacum). Integration of the transgene was confirmed by Southern blot hybridization. Transgenic tobacco plants (T(0) and T(1)) expressing tcu-1, when grown in hydroponic medium spiked with different concentrations of copper, showed higher acquisition of copper (up to 3.1 times) compared with control plants. Transgenic plants grown in soil spiked with copper could also take up more copper compared with wild-type plants. Supplementation of other divalent cations such as Cd(2+) and Zn(2+) did not alter uptake of Cu by transgenic plants. The present study has shown that expression of a heterologous copper transporter in tobacco could enhance acquisition of copper.

Copper in plants: acquisition, transport and interactions

Copper is an essential metal for plants. It plays key roles in photosynthetic and respiratory electron transport chains, in ethylene sensing, cell wall metabolism, oxidative stress protection and biogenesis of molybdenum cofactor. Thus, a deficiency in the copper supply can alter essential functions in plant metabolism. However, copper has traditionally been used in agriculture as an antifungal agent, and it is also extensively released into the environment by human activities that often cause environmental pollution. Accordingly, excess copper is present in certain regions and environments, and exposure to such can be potentially toxic to plants, causing phytotoxicity by the formation of reactive oxygen radicals that damage cells, or by the interaction with proteins impairing key cellular processes, inactivating enzymes and disturbing protein structure. Plants have a complex network of metal trafficking pathways in order to appropriately regulate copper homeostasis in response to environmental copper level variations. Such strategies must prevent accumulation of the metal in the freely reactive form (metal detoxification pathways) and ensure proper delivery of this element to target metalloproteins. The mechanisms involved in the acquisition and the distribution of copper have not been clearly defined, although emerging data in last decade, mainly obtained on copper uptake, and both intra- and intercellular distribution, as well as on long-distance transport, are contributing to the understanding of copper homeostasis in plants and the response to copper stress. This review gives an overview of the current understanding of main features concerning copper function, acquisition and trafficking network as well as interactions between copper and other elements.

Cloning and characterization of TsMT3, a type 3 metallothionein gene from salt cress (Thellungiella salsuginea)

A full-length type 3 plant metallothionein cDNA was isolated from 200 mM NaCl stressed shoots of the salt cress (Thellungiella salsuginea). The 447 bp TsMT3 cDNA sequence has a 207 bp open reading frame (ORF) and encodes a deduced 69 residue peptide of molecular weight 7.52 kDa. Southern blot analysis indicates that, there is only one copy of TsMT3 in the T. salsuginea genome. The accumulation of TsMT3 mRNA is enhanced by the stress imposed by PEG6000, 200 mM NaCl, 50 microM ABA, 4 degrees C, 40 microM CuSO(4) or 25 microM CdCl2. The expression vector pET28-TsMT3 was heterologously expressed in Escherichia coli to define the contribution of TsMT3 to heavy metal tolerance. In the presence of 2 mM CuSO4, 0.3 mM Pb(NO3)2 or 0.4 mM CdCl2, TsMT3 expressing cells exhibited enhanced metal tolerance and accumulated more metal than the controls. We believe that TsMT3 is probably involved in the processes of metal homeostasis, tolerance, and reactive oxygen species (ROS) scavenging.

Copper dynamics and impact on microbial communities in soils of variable organic status

The effect of soil organic status on copper impact was investigated by means of a microcosm study carried out on a vineyard soil that had been amended with varying types of organic matter during a previous long-term field experiment. Soil microcosms were contaminated at 250 mg Cu kg(-1) and incubated for 35 days. Copper distribution and dynamics were assessed in the solid matrix by a sequential extraction procedure and in the soil solution by measuring total and free exchangeable copper concentrations. Copper bioavailability was also measured with a whole-cell biosensor. Modifications of microbial communities were assessed by means of biomass-C measurements and characterization of genetic structure using ARISA (automated-ribosomal-intergenic-spacer-analysis). The results showed that copper distribution, speciation, and bioavailability are strongly different between organically amended and nonamended soils. Surprisingly, in solution, bioavailable copper correlated with total copper but not with free copper. Similarly the observed differential copper impact on micro-organisms suggested that organic matter controlled copper toxicity. Bacterial-ARISA modifications also correlated with the estimated metal bioavailability and corresponded to the enrichment of the Actinobacteria. Contrarily, biomass-C and fungal-ARISA measurements did not relate trivially to copper speciation and bioavailability, suggesting that the specific composition of the indigenous-soil communities controls its sensitivity to this metal.

Copper and iron homeostasis in Arabidopsis: responses to metal deficiencies, interactions and biotechnological applications

Plants have developed sophisticated mechanisms to tightly control the acquisition and distribution of copper and iron in response to environmental fluctuations. Recent studies with Arabidopsis thaliana are allowing the characterization of the diverse families and components involved in metal uptake, such as metal-chelate reductases and plasma membrane transporters. In parallel, emerging data on both intra- and intercellular metal distribution, as well as on long-distance transport, are contributing to the understanding of metal homeostatic networks in plants. Furthermore, gene expression analyses are deciphering coordinated mechanisms of regulation and response to copper and iron limitation. Prioritizing the use of metals in essential versus dispensable processes, and substituting specific metalloproteins by other metal counterparts, are examples of plant strategies to optimize copper and iron utilization. The metabolic links between copper and iron homeostasis are well documented in yeast, algae and mammals. In contrast, interactions between both metals in vascular plants remain controversial, mainly owing to the absence of copper-dependent iron acquisition. This review describes putative interactions between both metals at different levels in plants. The characterization of plant copper and iron homeostasis should lead to biotechnological applications aimed at the alleviation of iron deficiency and copper contamination and, thus, have a beneficial impact on agricultural and human health problems.

Transgenic plants in phytoremediation: recent advances and new possibilities

Phytoremediation, the use of plants and their associated microbes to remedy contaminated soils, sediments, and groundwater, is emerging as a cost-effective and environmentally friendly technology. Due in large part to its aesthetic appeal, this technology has gained increasing attention over the past 10 years. Phytoremediation uses different plant processes and mechanisms normally involved in the accumulation, complexation, volatilization, and degradation of organic and inorganic pollutants. Certain plants, called hyperaccumulators, are good candidates in phytoremediation, particularly for the removal of heavy metals. Phytoremediation efficiency of plants can be substantially improved using genetic engineering technologies. Recent research results, including overexpression of genes whose protein products are involved in metal uptake, transport, and sequestration, or act as enzymes involved in the degradation of hazardous organics, have opened up new possibilities in phytoremediation. This paper provides a critical review of the recent progress made toward the development of transgenic plants with improved phytoremediation capabilities and their potential use in environmental cleanup.

Prospects of genetic engineering of plants for phytoremediation of toxic metals

Bioremediation is gaining a lot of importance in recent times as an alternate technology for removal of elemental pollutants in soil and water, which require effective methods of decontamination. Phytoremediation--the use of green plants to remove, contain or render harmless environmental pollutants--may offer an effective, environmentally nondestructive and cheap remediation method. The use of genetic engineering to modify plants for metal uptake, transport and sequestration may open up new avenues for enhancing efficiency of phytoremediation. Metal chelator, metal transporter, metallothionein (MT), and phytochelatin (PC) genes have been transferred to plants for improved metal uptake and sequestration. Transgenic plants, which detoxify/accumulate cadmium, lead, mercury, arsenic and selenium have been developed. A better understanding of the mechanisms of rhizosphere interaction, uptake, transport and sequestration of metals in hyperaccumulator plants will lead to designing novel transgenic plants with improved remediation traits. As more genes related to metal metabolism are discovered, facilitated by the genome sequencing projects, new vistas will be opened up for development of efficient transgenic plants for phytoremediation.

Phytoremediation: green technology for the clean up of toxic metals in the environment

The contamination of the environment by toxic metals poses a threat for "Man and biosphere", reducing agricultural productivity and damaging the health of the ecosystem. In developed nations, this problem is being addressed and solved to some extent by using "green technology" involving metal tolerant plants, to clean up the polluted soils. The use of naturally occurring metal tolerant plants and the application of genetic manipulation, should hasten the process of transferring this technology from laboratory to field. Therefore, it is essential to investigate and understand how plants are able to tolerate toxic metals and to identify which metabolic pathways and genes are involved in such a process. Recent advances in knowledge derived from the "omics", have considerable potential in developing this green technology. However, strategies to produce genetically altered plants to remove, destroy or sequester toxic metals from the environment and the long-term implications, must be investigated carefully.

Copper in plants

Copper is an essential metal for normal plant growth and development, although it is also potentially toxic. Copper participates in numerous physiological processes and is an essential cofactor for many metalloproteins, however, problems arise when excess copper is present in cells. Excess copper inhibits plant growth and impairs important cellular processes (i.e., photosynthetic electron transport). Since copper is both an essential cofactor and a toxic element, involving a complex network of metal trafficking pathways, different strategies have evolved in plants to appropriately regulate its homeostasis as a function of the environmental copper level. Such strategies must prevent accumulation of the metal in the freely reactive form (metal detoxification pathways) and ensure proper delivery of this element to target metalloproteins. The mechanisms involved in the acquisition of this essential micronutrient have not been clearly defined although a number of genes have recently been identified which encode potential copper transporters. This review gives a briefly overview of the current understanding of the more important features concerning copper toxicity and tolerance in plants, and brings information of recent findings on copper trafficking including copper detoxification factors, copper transporters and copper chaperones.