Chloroplast targeting of phytochelatin synthase in Arabidopsis: effects on heavy metal tolerance and accumulation

Relocation of chloroplast proteins from cytosols into chloroplasts

The chloroplasts in terrestrial plants play a functional role as a major sensor for perceiving physiological changes under normal and stressful conditions. Despite the fact that the plant chloroplast genome encodes around 120 genes, which are mainly essential for photosynthesis and chloroplast biogenesis, the functional roles of the genes remain to be determined in plant's response to environmental stresses. Photosynthetic electron transfer D (PETD) is a key component of the chloroplast cytochrome b6f complex. Chloroplast ndhA (NADH dehydrogenase A) and ndhB (NADH dehydrogenase B) interact with photosystem I (PSI), forming NDH-PSI supercomplex. Notably, artificial targeting of chloroplasts-encoded proteins, PETD, NDHA, or NDHB, was successfully relocated from cytosols into chloroplasts. The result suggests that artificial targeting of proteins to chloroplasts is potentially open to the possibility of chloroplast biotechnology in engineering of plant tolerance against biotic and abiotic stresses.

Enterohemorrhagic Escherichia coli O157:H7 antigens produced in transgenic lettuce effective as an oral vaccine in mice

KEY MESSAGE

Transgene with recombination sites to address biosafety concerns engineered into lettuce to produce EspB and γ-intimin C280 for oral vaccination against EHEC O157:H7. Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a food-borne pathogen where ruminant farm animals, mainly bovine, serve as reservoirs. Bovine vaccination has been used to prevent disease outbreaks, and the current method relies on vaccines subcutaneously injected three times per year. Since EHEC O157:H7 colonizes mucosal surfaces, an oral vaccine that produces an IgA response could be more convenient. Here, we report on oral vaccination against EHEC O157:H7 in mice orally gavaged with transgenic lettuce that produces EHEC O157:H7 antigens EspB and γ-intimin C280. Younger leaves accumulated a higher concentration of antigens; and in unexpanded leaves of 30-day-old T2 plants, EspB and γ-intimin C280 were up to 32 and 51 μg/g fresh weight, respectively. Mice orally gavaged with lettuce powders containing < 3 µg antigens for 6 days showed a mucosal immune response with reduced colonization of EHEC O157:H7. This suggests that the transgenic lettuce has potential to be used for bovine vaccination. To promote the biosafety of crop plants producing medically relevant proteins, recombination sites were built into our transgenic lines that would permit optional marker removal by Cre-lox recombination, as well as transgene deletion in pollen by CinH-RS2 recombination. The ability to upgrade the transgenic lettuce by stacking additional antigen genes or replacing older genes with newer versions would also be possible through the combined use of Bxb-att and Cre-lox recombination systems.

Exogenous abscisic acid alleviates Cd toxicity in Arabidopsis thaliana by inhibiting Cd uptake, translocation and accumulation, and promoting Cd chelation and efflux

Exogenous abscisic acid (ABA) has been implicated in plant response to cadmium (Cd) stress, but the underlying mechanism remains unclear. In the present study, we found that exogenous ABA application decreased Cd fixation in wild type (WT) root cell wall through reducing the hemicelluloses content, in parallel with the decreased expression of IRT1, ZIP1, ZIP4, HMA2 and HMA4, which are related to Cd uptake and translocation, and the increased expression of PDF2.6, PDR8 and AIT1, which are related to Cd chelation, efflux, and accumulation inhibition. These changes might be associated with the reduced Cd accumulation in roots and shoots and the alleviated Cd toxicity. In contrast, the mutation of ABI4, a transcription factor in ABA signaling pathway, significantly increased the expression of IRT1, ZIP1, ZIP4, HMA2 and HMA4, while decreased the expression of AIT1, PDF2.6 and PDR8, enhancing Cd accumulation in roots and shoots of abi4. The enhanced Cd-sensitivity in abi4 mutant could not be rescued by exogenous ABA addition compared with WT. In a word, we conclude that exogenous ABA mitigates Cd toxicity in Arabidopsis thaliana via inhibiting Cd uptake, translocation and accumulation, promoting Cd chelation and efflux, a pathway that might be regulated by ABI4.

Consequences of Arsenic Contamination on Plants and Mycoremediation-Mediated Arsenic Stress Tolerance for Sustainable Agriculture

Arsenic contamination in water and soil is becoming a severe problem. It is toxic to the environment and human health. It is usually found in small quantities in rock, soil, air, and water which increase due to natural and anthropogenic activities. Arsenic exposure leads to several diseases such as vascular disease, including stroke, ischemic heart disease, and peripheral vascular disease, and also increases the risk of liver, lungs, kidneys, and bladder tumors. Arsenic leads to oxidative stress that causes an imbalance in the redox system. Mycoremediation approaches can potentially reduce the As level near the contaminated sites and are procuring popularity as being eco-friendly and cost-effective. Many fungi have specific metal-binding metallothionein proteins, which are used for immobilizing the As concentration from the soil, thereby removing the accumulated As in crops. Some fungi also have other mechanisms to reduce the As contamination, such as biosynthesis of glutathione, cell surface precipitation, bioaugmentation, biostimulation, biosorption, bioaccumulation, biovolatilization, methylation, and chelation of As. Arsenic-resistant fungi and recombinant yeast have a significant potential for better elimination of As from contaminated areas. This review discusses the relationship between As exposure, oxidative stress, and signaling pathways. We also explain how to overcome the detrimental effects of As contamination through mycoremediation, unraveling the mechanism of As-induced toxicity.

Is Genetic Engineering a Route to Enhance Microalgae-Mediated Bioremediation of Heavy Metal-Containing Effluents?

Contamination of the biosphere by heavy metals has been rising, due to accelerated anthropogenic activities, and is nowadays, a matter of serious global concern. Removal of such inorganic pollutants from aquatic environments via biological processes has earned great popularity, for its cost-effectiveness and high efficiency, compared to conventional physicochemical methods. Among candidate organisms, microalgae offer several competitive advantages; phycoremediation has even been claimed as the next generation of wastewater treatment technologies. Furthermore, integration of microalgae-mediated wastewater treatment and bioenergy production adds favorably to the economic feasibility of the former process—with energy security coming along with environmental sustainability. However, poor biomass productivity under abiotic stress conditions has hindered the large-scale deployment of microalgae. Recent advances encompassing molecular tools for genome editing, together with the advent of multiomics technologies and computational approaches, have permitted the design of tailor-made microalgal cell factories, which encompass multiple beneficial traits, while circumventing those associated with the bioaccumulation of unfavorable chemicals. Previous studies unfolded several routes through which genetic engineering-mediated improvements appear feasible (encompassing sequestration/uptake capacity and specificity for heavy metals); they can be categorized as metal transportation, chelation, or biotransformation, with regulation of metal- and oxidative stress response, as well as cell surface engineering playing a crucial role therein. This review covers the state-of-the-art metal stress mitigation mechanisms prevalent in microalgae, and discusses putative and tested metabolic engineering approaches, aimed at further improvement of those biological processes. Finally, current research gaps and future prospects arising from use of transgenic microalgae for heavy metal phycoremediation are reviewed.

Safe, efficient, and economically beneficial remediation of arsenic-contaminated soil: possible strategies for increasing arsenic tolerance and accumulation in non-edible economically important native plants

Anthropogenic activities, geological processes, and biogenic sources have led to the enhanced concentration of arsenic (As), a toxic metalloid in water and soil. Non-edible, economically important plants can be employed for safe As phytoremediation in addition to generating extra income. However, these plants may get affected by stressful local environmental conditions. Native plant species are adapted to local environmental conditions and hence overcome this problem. Native non-edible economic plant species which show high As tolerance and accumulation are promising candidate for safe, efficient, and economically beneficial phytoremediation of As-contaminated sites. The current review discusses the potential of native economic plant species that can be used in As phytoremediation programme. However, since their phytoremediation potential is moderate, possible strategies for increasing As  olerance and accumulation, especially genetic modification, have been discussed in detail. Knowledge gained from the review can be used for the development of As tolerance and accumulation in non-edible economic native plants.

Metal transporters in organelles and their roles in heavy metal transportation and sequestration mechanisms in plants

Heavy metal toxicity is one of the major concerns for agriculture and health. Accumulation of toxic heavy metals at high concentrations in edible parts of crop plants is the primary cause of disease in humans and cattle. A dramatic increase in industrialization, urbanization, and other high anthropogenic activities has led to the accumulation of heavy metals in agricultural soil, which has consequently disrupted soil conditions and affected crop yield. By now, plants have developed several mechanisms to cope with heavy metal stress. However, not all plants are equally effective in dealing with the toxicity of high heavy metal concentrations. Plants have modified their anatomy, morphophysiology, and molecular networks to survive under changing environmental conditions. Heavy metal sequestration is one of the essential processes evolved by some plants to deal with heavy metals' toxic concentration. Some plants even have the ability to accumulate metals in high quantities in the shoots/organelles without toxic effects. For intercellular and interorganeller metal transport, plants harbor spatially distributed various transporters which mainly help in uptake, translocation, and redistribution of metals. This review discusses different heavy metal transporters in different organelles and their roles in metal sequestration and redistribution to help plants cope with heavy metal stress. A good understanding of the processes at stake helps in developing more tolerant crops without affecting their productivity.

An amiRNA screen uncovers redundant CBF and ERF34/35 transcription factors that differentially regulate arsenite and cadmium responses

Arsenic stress causes rapid transcriptional responses in plants. However, transcriptional regulators of arsenic-induced gene expression in plants remain less well known. To date, forward genetic screens have proven limited for dissecting arsenic response mechanisms. We hypothesized that this may be due to the extensive genetic redundancy present in plant genomes. To overcome this limitation, we pursued a forward genetic screen for arsenite tolerance using a randomized library of plants expressing >2,000 artificial microRNAs (amiRNAs). This library was designed to knock-down diverse combinations of homologous gene family members within sub-clades of transcription factor and transporter gene families. We identified six transformant lines showing an altered response to arsenite in root growth assays. Further characterization of an amiRNA line targeting closely homologous CBF and ERF transcription factors show that the CBF1,2 and 3 transcription factors negatively regulate arsenite sensitivity. Furthermore, the ERF34 and ERF35 transcription factors are required for cadmium resistance. Generation of CRISPR lines, higher-order T-DNA mutants and gene expression analyses, further support our findings. These ERF transcription factors differentially regulate arsenite sensitivity and cadmium tolerance.

Glutathione Restores Hg-Induced Morpho-Physiological Retardations by Inducing Phytochelatin and Oxidative Defense in Alfalfa

Simple Summary

An ecofriendly approach to mitigate mercury (Hg) toxicity in alfalfa, one of the important forage crops, is highly desirable for environmental sustainability. In this study, the exogenous glutathione (GSH) substantially improved the morphological hindrance and photosynthesis inefficiency in Hg-exposed alfalfa plants. In addition, the Fe and S status of Cd-toxic alfalfa was restored due to GSH supplementation. Interestingly, GSH applied to Hg-exposed plants showed elevated Hg concentration in roots resulted in a substantial deposition of Hg in the root cell wall due to the upregulation of MsPCS1 and MsGSH1 genes in roots. It implies that GSH induces PC accumulation in roots enabling excess Hg bound to the cell wall, thereby limiting the transport of Hg to the aerial part of alfalfa. In silico analysis further suggests a conserved motif linked to the phytochelatin synthase domain (CL0125). In addition, GSH induced the GSH concentration and GR activity in protecting alfalfa plants from Hg-induced oxidative damage. These findings can be useful to formulate GSH-based fertilizer or to develop Hg-tolerant alfalfa plants.

Abstract

Mercury (Hg) is toxic to plants, but the effect of glutathione in Hg alleviation was never studied in alfalfa, an important forage crop. In this study, Hg toxicity showed morphological retardation, chlorophyll reduction, and PSII inefficiency, which was restored due to GSH supplementation in alfalfa plants treated with Hg. Results showed a significant increase of Hg, but Fe and S concentrations substantially decreased in root and shoot accompanied by the downregulation of Fe (MsIRT1) and S (MsSultr1;2 and MsSultr1;3) transporters in roots of Hg-toxic alfalfa. However, GSH caused a significant decrease of Hg in the shoot, while the root Hg level substantially increased, accompanied by the restoration of Fe and S status, relative to Hg-stressed alfalfa. The subcellular analysis showed a substantial deposition of Hg in the root cell wall accompanied by the increased GSH and PC and the upregulation of MsPCS1 and MsGSH1 genes in roots. It suggests the involvement of GSH in triggering PC accumulation, causing excess Hg bound to the cell wall of the root, thereby reducing Hg translocation in alfalfa. Bioinformatics analysis showed that the MsPCS1 protein demonstrated one common conserved motif linked to the phytochelatin synthase domain (CL0125) with MtPCS1 and AtMCS1 homologs. These in silico analysis further confirmed the detoxification role of MsPCS1 induced by GSH in Hg-toxic alfalfa. Additionally, GSH induces GSH and GR activity to counteract oxidative injuries provoked by Hg-induced H₂O₂ and lipid peroxidation. These findings may provide valuable knowledge to popularize GSH-derived fertilizer or to develop Hg-free alfalfa or other forage plants.

Anatomic features, tolerance index, secondary metabolites and protein content of chickpea (Cicer arietinum) seedlings under cadmium induction and identification of PCS and FC genes

ABSTRACT

Chickpea (Cicer arietinum) belonging to the Fabaceae family is a major legume crop and is a good source of protein and carbohydrates. Industrialization has resulted in soil contamination with heavy metals such as cadmium. Adsorption of cadmium by plants can lead to reduced yields and heavy metal toxicity. In the current study, changes in the anatomical, morphological features and biochemical properties of the chickpea plant were evaluated. Two indexes DWSTI and PHSTI were determined. Anatomically, there was a change in the number of xylem poles within the root structure which was most significant at treatments of 125 μg cadmium. There was also a noticeable change in leaf pigmentation, the total phenolics and soluble protein in the plant. Cadmium levels were elevated attaining concentrations of 0.21, 0.40 and 0.52 mg per gram dry weight in plants exposed to 62, 125 and 250 μg/g Perlit cadmium after a period of 30 days. A noticeable increase in the level of cadmium in the plant was observed. Two PCS genes, glutathione gamma-glutamylcysteinyltransferase 1 and glutathione gamma-glutamylcysteinyltransferase and four FC genes, 4 proteins and 4 mRNA were detected in chickpeas. Bioinformatics tools were utilized to predict enzyme structure and binding sites. Chickpea may be classified as a cadmium hyperaccumulator and may be considered for use in phytoremediation. This study provides a better understanding with regards to the response of chickpeas to cadmium and the genetic mechanism by which the plant regulates heavy metal toxicity.

Effects of Exogenous Organic Acids on Cd Tolerance Mechanism of Salix variegata Franch. Under Cd Stress

Chelate induction of organic acids has been recognized to enhance metal uptake and translocation in plants, but the underlying mechanism remains unclear. In this study, seedlings of Salix variegata were hydroponically exposed to the combinations of Cd (0 and 50 μM) and three exogenous organic acids (100 μM of citric, tartaric, or malic acid). Plant biomass, antioxidant enzymes, non-protein thiol compounds (NPT) content, and the expression of candidate genes associated with Cd accumulation and tolerance were determined. Results showed that Cd significantly inhibited plant biomass but stimulated the activity of antioxidant enzymes in the roots and leaves, while the lipid peroxidation increased as well. Respective addition of three organic acids greatly enhanced plant resistance to oxidative stress and reduced the lipid peroxidation induced by Cd, with the effect of malic acid showing greatest. The addition of organic acids also significantly increased the content of glutathione in the root, further improving the antioxidant capacity and potential of phytochelatin biosynthesis. Moreover, Cd induced the expression level of candidate genes in roots of S. variegata. The addition of three organic acids not only promoted the expression of candidate genes but also drastically increased Cd accumulation in S. variegata. In summary, application of citric, tartaric, or malic acid alleviated Cd-imposed toxicity through the boost of enzymatic and non-enzymatic antioxidants and candidate gene expression, while their effects on Cd tolerance and accumulation of S. variegata differed.

Mechanisms of arsenic assimilation by plants and countermeasures to attenuate its accumulation in crops other than rice

Arsenic is a ubiquitous metalloid in the biosphere, and its origin can be either geogenic or anthropic. Four oxidation states (-3, 0, +3 and + 5) characterize organic and inorganic As- compounds. Although arsenic is reportedly a toxicant, its harmful effects are closely related to its chemical form: inorganic compounds are most toxic, followed by organic ones and finally by arsine gas. Although drinking water is the primary source of arsenic exposure to humans, the metalloid enters the food chain through its uptake by crops, the extent of which is tightly dependent on its phytoavailability. Arsenate is taken up by roots via phosphate carriers, while arsenite is taken up by a subclass of aquaporins (NIP), some of which involved in silicon (Si) transport. NIP and Si transporters are also involved in the uptake of methylated forms of As. Once taken up, its distribution is regulated by the same type of transporters albeit with mobility efficiencies depending on As forms and its accumulation generally occurs in the following decreasing order: roots > stems > leaves > fruits (seeds). Besides providing a survey on the uptake and transport mechanisms in higher plants, this review reports on measures able to reducing plant uptake and the ensuing transfer into edible parts. On the one hand, these measures include a variety of plant-based approaches including breeding, genetic engineering of transport systems, graft/rootstock combinations, and mycorrhization. On the other hand, they include agronomic practices with a particular focus on the use of inorganic and organic amendments, treatment of irrigation water, and fertilization.

Prospects of genetic engineering utilizing potential genes for regulating arsenic accumulation in plants

The rapid pace of industrial, agricultural and anthropogenic activities in the 20^th century has resulted in contamination of heavy metals across the globe. Arsenic (As) is a ubiquitous, naturally occurring toxic metalloid, contaminating the soil and water and affecting human health in several countries. Several physicochemical methods exist for the cleanup of As contamination but these are expensive and disastrous to microbes and soil. Plant based remediation approaches are low cost and environmentally safe. Hence, extensive biochemical, molecular and genetic experiments have been conducted to understand plants' responses to As stress and have led to the identification of potential genes. The available knowledge needs to be utilized to either reduce As accumulation in crop plants (rice) or to enhance As levels in shoots of hyperaccumulators (Pteris vittata). Gene manipulation using biotechnological tools can be an effective approach to exploit the potential genes (plasmamembrane and vacuolar transporters, glutathione and phytochelatin biosynthetic enzymes, etc.) playing pivotal roles in uptake, translocation, transformation, complexation, and compartmentalization of As in plants. The transgenic plants with increased tolerance to As and altered (increased/decreased) As accumulation have been developed. The need, however, exists to design plants with altered expression of two or more genes for harmonizing various events (like arsenate reduction, arsenite complexation, sequestration and translocation) so as to achieve desirable reduction (crop plants) or increase (phytoremediator plants) in As content. This review sheds light on transgenic approaches adopted to modulate As levels in plants and proposes future directions to achieve desirable results.

The phytochelatin synthase gene in date palm (Phoenix dactylifera L.): Phylogeny, evolution and expression

We studied date palm phytochelatin synthase type I (PdPCS1), which catalyzes the cytosolic synthesis of phytochelatins (PCs), a heavy metal binding protein, in plant cells. The gene encoding PdPCS1 (Pdpcs) consists of 8 exons and 7 introns and encodes a protein of 528 amino acids. PCs gene history was studied using Notung phylogeny. During evolution, gene loss from several lineages was predicted including Proteobacteria, Bilateria and Brassicaceae. In addition, eleven gene duplication events appeared toward interior nodes of the reconciled tree and four gene duplication events appeared toward the external nodes. These latter sequences belong to species with a second copy of PCs suggesting that this gene evolved through subfunctionalization. Pdpcs1 gene expression was measured in seedling hypocotyls exposed to Cd, Cu and Cr using quantitative real-time polymerase chain reaction (qPCR). A Pdpcs1 overexpression was evidenced in P. dactylifera seedlings exposed to metals suggesting that 1-the Pdpcs1 gene is functional, 2-there is an implication of the enzyme in metal detoxification mechanisms. Additionally, the structure of PdPCS1 was predicted using its homologue from Nostoc (cyanobacterium, NsPCS) as a template in Discovery studio and PyMol software. These analyses allowed us to identify the phytochelatin synthase type I enzyme in date palm (PdPCS1) via recognition of key consensus amino acids involved in the catalytic mechanism, and to propose a hypothetical binding and catalytic site for an additional substrate binding cavity.

Evidence of prokaryote like protein associated with nickel resistance in higher plants: horizontal transfer of TonB-dependent receptor/protein in Betula genus or de novo mechanisms?

Mechanisms of metal resistance have been reported in many plants but knowledge in woody species is scarce. The TonB-dependent receptors family (TBDTs) is a large group of proteins that facilitate the transport of molecules across the membrane of Gram-negative bacteria. Some evidence exists that TBDTs are involved in metal stress. The existence of a TonB-like mechanism in non-prokaryotes has not been established. The recent development of the Betula papyrifera (white birch) transcriptome has allowed the discovery of genes involved in plant adaptation to stress. The main objective of the present study was to identify novel genes associated with nickel resistance in B. papyrifera. Our results from next generation sequencing and RT-qPCR analyses show that genes involved in transport activities are upregulated in nickel-resistant genotypes compared with susceptible forms. Detailed analysis of gene expression and genome analysis shows for the first time the existence of a TonB-dependent receptor and TonB-like family protein in non-prokaryotes. In addition, we have found that these proteins are associated with nickel resistance in B. papyrifera. Our experiments suggest that the TonB-dependent receptor may be exclusive to the Betula genus, suggesting that Betula species may have acquired the gene via horizontal gene transfer from prokaryotes or fungi.

Review of remediation techniques for arsenic (As) contamination: a novel approach utilizing bio-organisms

Arsenic (As) contamination has recently become a worldwide problem, as it is found to be widespread not only in drinking water but also in various foodstuffs. Because of the high toxicity, As contamination poses a serious risk to human health and ecological system. To cope with this problem, a great deal of effort have been made to account for the mechanisms of As mineral formation and accumulation by some plants and aquatic organisms exposed to the high level of As. Hence, bio-remediation is now considered an effective and potent approach to breakdown As contamination. In this review, we provide up-to-date knowledge on how biological tools (such as plants for phytoremediation and to some extent microorganisms) can be used to help resolve the effects of As problems on the Earth's environment.

Mechanisms of arsenic tolerance and detoxification in plants and their application in transgenic technology: a critical appraisal

Arsenic (As) contamination of the environment has emerged as a serious problem. Consequently, there is an urge to understand plants' responses to As. The analysis of various hypertolerant and hyperaccumulator plants and comparison of their responses with non-tolerant and nonaccumulators have provided valuable information about the mechanisms of As tolerance and detoxification. Therefore, we understand why most of the pteridophytes are able to hyperacumulate As, why it is difficult to find hyperaccumulators among angiosperms and why rice is able to translocate As to its grains more efficiently than any other cereal crop. This information can be employed to generate As hyperaccumulators in angiosperms and to develop safe cultivars of rice for human consumption through biotechnological approaches. Although measurable success, in terms of application in the field, has so far not been achieved, transgenic research has yielded promising results, which shed light on the approaches to be taken up in future endeavor. In this review, we discuss the mechanisms of As tolerance and detoxification in plants and transgenic research conducted.

Glutathione in plants: an integrated overview

Plants cannot survive without glutathione (γ-glutamylcysteinylglycine) or γ-glutamylcysteine-containing homologues. The reasons why this small molecule is indispensable are not fully understood, but it can be inferred that glutathione has functions in plant development that cannot be performed by other thiols or antioxidants. The known functions of glutathione include roles in biosynthetic pathways, detoxification, antioxidant biochemistry and redox homeostasis. Glutathione can interact in multiple ways with proteins through thiol-disulphide exchange and related processes. Its strategic position between oxidants such as reactive oxygen species and cellular reductants makes the glutathione system perfectly configured for signalling functions. Recent years have witnessed considerable progress in understanding glutathione synthesis, degradation and transport, particularly in relation to cellular redox homeostasis and related signalling under optimal and stress conditions. Here we outline the key recent advances and discuss how alterations in glutathione status, such as those observed during stress, may participate in signal transduction cascades. The discussion highlights some of the issues surrounding the regulation of glutathione contents, the control of glutathione redox potential, and how the functions of glutathione and other thiols are integrated to fine-tune photorespiratory and respiratory metabolism and to modulate phytohormone signalling pathways through appropriate modification of sensitive protein cysteine residues.

Glutathione

Glutathione is a simple sulfur compound composed of three amino acids and the major non-protein thiol in many organisms, including plants. The functions of glutathione are manifold but notably include redox-homeostatic buffering. Glutathione status is modulated by oxidants as well as by nutritional and other factors, and can influence protein structure and activity through changes in thiol-disulfide balance. For these reasons, glutathione is a transducer that integrates environmental information into the cellular network. While the mechanistic details of this function remain to be fully elucidated, accumulating evidence points to important roles for glutathione and glutathione-dependent proteins in phytohormone signaling and in defense against biotic stress. Work in Arabidopsis is beginning to identify the processes that govern glutathione status and that link it to signaling pathways. As well as providing an overview of the components that regulate glutathione homeostasis (synthesis, degradation, transport, and redox turnover), the present discussion considers the roles of this metabolite in physiological processes such as light signaling, cell death, and defense against microbial pathogen and herbivores.

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.

Dissection of glutathione conjugate turnover in yeast

Xenobiotics are widely used as pesticides. The detoxification of xenobiotics frequently involves conjugation to glutathione prior to compartmentalization and catabolism. In plants, degradation of glutathione-S-conjugates is initiated either by aminoterminal or carboxyterminal amino acid cleavage catalyzed by a gamma-glutamyl transpeptidase and phytochelatin synthase, respectively. In order to establish yeast as a model system for the analysis of the plant pathway, we used monochlorobimane as a model xenobiotic in Saccharomyces cerevisiae and mutants thereof. The catabolism of monochlorobimane is initiated by conjugation to form glutathione-S-bimane, which is then turned over into a gamma-GluCys-bimane conjugate by the vacuolar serine carboxypeptidases CPC and CPY. Alternatively, the glutathione-S-bimane conjugate is catabolized by the action of the gamma-glutamyl transpeptidase Cis2p to a CysGly-conjugate. The turnover of glutathione-S-bimane was impaired in yeast cells deficient in Cis2p and completely abolished by the additional inactivation of CPC and CPY in the corresponding triple knockout. Inducible expression of the Arabidopsis phytochelatin synthase AtPCS1 in the triple knockout resulted in the turnover of glutathione-S-bimane to the gamma-GluCys-bimane conjugate as observed in plants. Challenge of AtPCS1-expressing yeast cells with zinc, cadmium, and copper ions, which are known to activate AtPCS1, enhanced gamma-GluCys-bimane accumulation. Thus, initial catabolism of glutathione-S-conjugates is similar in plants and yeast, and yeast is a suitable system for a study of enzymes of the plant pathway.

Effect of cadmium on cellular viability in two species of microalgae (Scenedesmus sp. and Dunaliella viridis)

We determined the effect of several concentrations of cadmium (0, 5, 10, and 20 microg/l) on cellular viability in the microalgae Scenedesmus sp. and Dunaliella viridis, by measuring growth at 0, 24, 48, 72, and 96 h and pigment production at 10 days. Algae were obtained from the Nonvascular Plant Laboratory collection, in the Facultad Experimental de Ciencias, Universidad del Zulia, Venezuela. Growth was measured by cellular counting, while pigment content was evaluated using conventional spectrophotometric techniques. Growth of both species decreased in the exposed cultures comparing with the control, but its behavior was similar, because in both control and exposed cultures, its was observed an adaptive phase in the first hours, as well as a growth phase after 72 h. Cadmium concentrations above 10 microg/l produced an adverse effect on pigment production, depending on the concentration and/or exhibition time. However, even though cadmium inhibited growth and pigment production, levels of both parameters indicated cellular viability, demonstrating the adaptability of the algae cultures when they were exposed to the metal.

Arsenic uptake and metabolism in plants

Arsenic (As) is an element that is nonessential for and toxic to plants. Arsenic contamination in the environment occurs in many regions, and, depending on environmental factors, its accumulation in food crops may pose a health risk to humans.Recent progress in understanding the mechanisms of As uptake and metabolism in plants is reviewed here. Arsenate is taken up by phosphate transporters. A number of the aquaporin nodulin26-like intrinsic proteins (NIPs) are able to transport arsenite,the predominant form of As in reducing environments. In rice (Oryza sativa), arsenite uptake shares the highly efficient silicon (Si) pathway of entry to root cells and efflux towards the xylem. In root cells arsenate is rapidly reduced to arsenite, which is effluxed to the external medium, complexed by thiol peptides or translocated to shoots. One type of arsenate reductase has been identified, but its in planta functions remain to be investigated. Some fern species in the Pteridaceae family are able to hyperaccumulate As in above-ground tissues. Hyperaccumulation appears to involve enhanced arsenate uptake, decreased arsenite-thiol complexation and arsenite efflux to the external medium, greatly enhanced xylem translocation of arsenite, and vacuolar sequestration of arsenite in fronds. Current knowledge gaps and future research directions are also identified.

Phytochelatin synthesis is essential for the detoxification of excess zinc and contributes significantly to the accumulation of zinc

The synthesis of phytochelatins (PCs) is essential for the detoxification of nonessential metals and metalloids such as cadmium and arsenic in plants and a variety of other organisms. To our knowledge, no direct evidence for a role of PCs in essential metal homeostasis has been reported to date. Prompted by observations in Schizosaccharomyces pombe and Saccharomyces cerevisiae indicating a contribution of PC synthase expression to Zn2+ sequestration, we investigated a known PC-deficient Arabidopsis (Arabidopsis thaliana) mutant, cad1-3, and a newly isolated second strong allele, cad1-6, with respect to zinc (Zn) homeostasis. We found that in a medium with low cation content PC-deficient mutants show pronounced Zn2+ hypersensitivity. This phenotype is of comparable strength to the well-documented Cd2+ hypersensitivity of cad1 mutants. PC deficiency also results in significant reduction in root Zn accumulation. To be able to sensitively measure PC accumulation, we established an assay using capillary liquid chromatography coupled to electrospray ionization quadrupole time-of-flight mass spectrometry of derivatized extracts. Plants grown under control conditions consistently showed PC2 accumulation. Analysis of plants treated with same-effect concentrations revealed that Zn2+-elicited PC2 accumulation in roots reached about 30% of the level of Cd2+-elicited PC2 accumulation. We conclude from these data that PC formation is essential for Zn2+ tolerance and provides driving force for the accumulation of Zn. This function might also help explain the mysterious occurrence of PC synthase genes throughout the plant kingdom and in a wide range of other organisms.

Altered expression of cytosolic/nuclear HSC70-1 molecular chaperone affects development and abiotic stress tolerance in Arabidopsis thaliana

Molecular chaperones of the heat shock cognate 70 kDa (HSC70) family are highly conserved in all living organisms and assist nascent protein folding in normal physiological conditions as well as in biotic and abiotic stress conditions. In the absence of specific inhibitors or viable knockout mutants, cytosolic/nuclear HSC70-1 overexpression (OE) and mutants in the HSC70 co-chaperone SGT1 (suppressor of G(2)/M allele of skp1) were used as genetic tools to identify HSC70/SGT1 functions in Arabidopsis development and abiotic stress responses. HSC70-1 OE caused a reduction in root and shoot meristem activities, thus explaining the dwarfism of those plants. In addition, HSC70-1 OE did not impair auxin-dependent phenotypes, suggesting that SGT1 functions previously identified in auxin signalling are HSC70 independent. While responses to abiotic stimuli such as UV-C exposure, phosphate starvation, or seedling de-etiolation were not perturbed by HSC70-1 OE, it specifically conferred gamma-ray hypersensitivity and tolerance to salt, cadmium (Cd), and arsenic (As). Cd and As perception was not perturbed, but plants overexpressing HSC70-1 accumulated less Cd, thus providing a possible molecular explanation for their tolerance phenotype. In summary, genetic evidence is provided for HSC70-1 involvement in a limited set of physiological processes, illustrating the essential and yet specific functions of this chaperone in development and abiotic stress responses in Arabidopsis.

A large gene cluster encoding peptide synthetases and polyketide synthases is involved in production of siderophores and oxidative stress response in the cyanobacterium Anabaena sp. strain PCC 7120

Non-ribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs) are necessary for the production of a variety of secondary metabolites, such as siderophores involved in iron acquisition. In response to iron limitation, the cyanobacterium Anabaena sp. strain PCC 7120 synthesizes several siderophores. The chromosome of this organism contains a large gene cluster of 76 kb with 24 open-reading frames from all2658 to all2635, including those that encode seven NRPSs and two PKSs. The function of this gene cluster was unknown, and one possibility could be the synthesis of siderophores. These genes were indeed activated under conditions of iron limitation. One mutant, MDelta41-49, bearing a large deletion of 43.4 kb in this gene cluster, synthesized considerably less siderophores and contained less iron as compared with the wild type. Its growth rate was similar to the wild type in the presence of iron, but was reduced when iron became limiting. Two other mutants, MDelta44-45 and MDelta47-49, lacking either all2644 and all2645, or all2647, all2648 and all2649 respectively, produced more siderophores than MDelta41-49, but less than the wild type. These genes were also activated under oxidative stress conditions to which MDelta41-49 was highly sensitive, consistent with the importance of iron in oxidative stress response. We propose that this gene cluster is involved in the synthesis of siderophores in Anabaena sp. PCC 7120 and plays an important role in defence against oxidative stress.

Vitamin E is essential for the tolerance of Arabidopsis thaliana to metal-induced oxidative stress

Arabidopsis (Arabidopsis thaliana) plants were grown in a hydroponic culture system for 7 to 14 d in the absence or presence of 75 microM Cd or 75 microM Cu. The Cu treatment resulted in visual leaf symptoms, together with anthocyanin accumulation and loss of turgor. Pronounced lipid peroxidation, which was detected by autoluminescence imaging and malondialdehyde titration, was observed in Cu-treated leaves. The Cd treatment also resulted in loss of leaf pigments but lipid peroxidation and oxidative stress were less pronounced than in the leaves exposed to Cu. Analysis of low-molecular-weight chloroplast and cytosolic antioxidants (ascorbate, glutathione, tocopherols, carotenoids) and antioxidant enzymes (thiol-based reductases and peroxidases) revealed relatively few responses to metal exposure. However, there was a marked increase in vitamin E (alpha-tocopherol) in response to Cd and Cu treatments. Ascorbate increased significantly in Cu-exposed leaves. Other antioxidants either remained stable or decreased in response to metal stress. Transcripts encoding enzymes of the vitamin E biosynthetic pathway were increased in response to metal exposure. In particular, VTE2 mRNA was enhanced in Cu- and Cd-treated plants, while VTE5 and hydroxylpyruvate dioxygenase (HPPD) mRNAs were only up-regulated in Cd-treated plants. Consistent increases in HPPD transcripts and protein were observed. The vitamin E-deficient (vte1) mutant exhibited an enhanced sensitivity towards both metals relative to the wild-type (WT) control. Unlike the vte1 mutants, which showed enhanced lipid peroxidation and oxidative stress in the presence of Cu or Cd, the ascorbate-deficient (vtc2) mutant showed WT responses to metal exposure. Taken together, these results demonstrate that vitamin E plays a crucial role in the tolerance of Arabidopsis to oxidative stress induced by heavy metals such as Cu and Cd.

Interaction of heavy metals with the sulphur metabolism in angiosperms from an ecological point of view

The metabolism of sulphur in angiosperms is reviewed under the aspect of exposure to ecologically relevant concentrations of sulphur, heavy metals and metalloids. Because of the inconsistent use of the term 'metal tolerance', in this review the degree of tolerance to arsenic and heavy metals is divided into three categories: hypotolerance, basal tolerance and hypertolerance. The composition of nutrient solutions applied to physiological experiments let see that the well-known interactions of calcium, sulphate and zinc supply with uptake of heavy metals, especially cadmium are insufficiently considered. Expression of genes involved in reductive sulphate assimilation pathway and enzyme activities are stimulated by cadmium and partially by copper, but nearly not by other heavy metals. The synthesis of the sulphur-rich compounds glucosinolates, metallothioneins and phytochelatins is affected in a metal-specific way. Phytochelatin levels are low in all metal(loid)-hypertolerant plant species growing in the natural environment on metal(loid)-enriched soils. If laboratory experiments mimic the natural environments, especially high Zn/Cd ratios and good sulphur supply, and chemical analyses are extended to more mineral elements than the single metal(loid) under investigation, a better understanding of the impact of metal(loid)s on the sulphur metabolism can be achieved.

Arsenic hazards: strategies for tolerance and remediation by plants

Arsenic toxicity has become a global concern owing to the ever-increasing contamination of water, soil and crops in many regions of the world. To limit the detrimental impact of arsenic compounds, efficient strategies such as phytoremediation are required. Suitable plants include arsenic hyperaccumulating ferns and aquatic plants that are capable of completing their life cycle in the presence of high levels of arsenic through the concerted action of arsenate reduction to arsenite, arsenite complexation, and vacuolar compartmentalization of complexed or inorganic arsenic. Tolerance can also be conferred by lowering arsenic uptake by suppression of phosphate transport activity, a major pathway for arsenate entry. In many unicellular organisms, arsenic tolerance is based on the active removal of cytosolic arsenite while limiting the uptake of arsenate. Recent molecular studies have revealed many of the gene products involved in these processes, providing the tools to improve crop species and to optimize phytoremediation; however, so far only single genes have been manipulated, which has limited progress. We will discuss recent advances and their potential applications, particularly in the context of multigenic engineering approaches.

Phytochelatin-cadmium-sulfide high-molecular-mass complexes of Euglena gracilis

High-molecular-mass PC complexes (PC-HMWCs) constituted by phytochelatins (PCs), cadmium and sulfide are synthesized by several organisms after exposure to cadmium. In this study, PC-HMWCs were isolated from photoheterotrophic Euglena gracilis and purified to homogeneity, resulting in compounds of molecular mass 50-380 kDa depending on the CdCl2 and sulfate concentrations in the culture medium. In contrast with plants and some yeasts, PC-HMWCs from E. gracilis mainly comprise (57-75%) monothiol molecules (Cys, gamma-glutamylcysteine, GSH) and, to a lesser extent (25-43%), PCs. A similar acid-soluble thiol compound composition was found in whole cell extracts. The -SH/Cd2+ and S2-/Cd2+ ratios found in purified PC-HMWCs were 1.5 and 1.8, respectively; the (-SH + S2-)/Cd2+ ratio was 3.2. PC-HMWCs of molecular mass 60 and 100 kDa were also localized inside Percoll-purified chloroplasts, in which cadmium and PCs were mainly compartmentalized. Cadmium and sulfur-rich clusters with similar sulfur/cadmium stoichiometries to those of the purified PC-HMWCs were detected in the chloroplast and throughout the cell by energy dispersive microanalysis and atomic resolution electron microscopy. The presence of PC-HMWCs in primitive photosynthetic eukaryotes such as the protist, E. gracilis, suggests that their function as the final cadmium-storage-inactivation process is widespread. Their particular intracellular localization suggests that chloroplasts may play a major role in the cadmium-resistance mechanism in organisms lacking a plant-like vacuole.

Leaf-targeted phytochelatin synthase in Arabidopsis thaliana

One of the key steps in developing transgenic plants for the phytoremediation of metal containing soils is to develop plants that accumulate metals in the aerial tissues. With the goal of changing the distribution of phytochelatin (PC)-dependent cadmium accumulation from roots to the leaves, the phytochelatin synthase (PCS) deficient cad1-3 mutant and wild type (Col-0) Arabidopsis plants were transformed with an Arabidopsis phytochelatin synthase (AtPCS1) under the control of a leaf-specific promoter. Three independent transformant lines from each genetic background were chosen for further analysis and designated cad-PCS and WT-PCS. PCS activity in the cadPCS lines was restored in the leaves, but not in the roots. Additionally, when whole plants were treated with cadmium, PCs were found only in the leaves of cad-PCS plants. Although the inserted AtPCS1 gene was leaf-specific, cad-PCS lines showed an overall decrease in cadmium toxicity evidenced by a partial amelioration of the "brown-root" phenotype and root growth was restored to wild type levels when treated with cadmium and arsenate. WT-PCS lines showed an increase in leaf PCS activity but had only wild type PC levels. In addition, cadmium uptake studies indicated that there was no difference in cadmium accumulation among all types tested. So, while we were able to protect the plants against cadmium by expressing PC synthase only in the leaves, we were not able to limit cadmium accumulation to aerial tissues.