HMA4 expression in tobacco reduces Cd accumulation due to the induction of the apoplastic barrier

NtGCN2 confers cadmium tolerance in Nicotiana tabacum L. by regulating cadmium uptake, efflux, and subcellular distribution

General control non-derepressible-2 (GCN2) is widely expressed in eukaryotes and responds to biotic and abiotic stressors. However, the precise function and mechanism of action of GCN2 in response to cadmium (Cd) stress in Nicotiana tabacum L. (tobacco) remains unclear. We investigated the role of NtGCN2 in Cd tolerance and explored the mechanism by which NtGCN2 responds to Cd stress in tobacco by exposing NtGCN2 transgenic tobacco lines to different concentrations of CdCl2. NtGCN2 was activated under 50 μmol·L-1 CdCl2 stress and enhanced the Cd tolerance and photosynthetic capacities of tobacco by increasing chlorophyll content and antioxidant capacity by upregulating NtSOD, NtPOD, and NtCAT expression and corresponding enzyme activities and decreasing malondialdehyde and O2·- contents. NtGCN2 enhanced the osmoregulatory capacity of tobacco by elevating proline (Pro) and soluble sugar contents and maintaining low levels of relative conductivity. Finally, NtGCN2 enhanced Cd tolerance in tobacco by reducing Cd uptake and translocation, promoting Cd efflux, and regulating Cd subcellular distribution. In conclusion, NtGCN2 improves the tolerance of tobacco to Cd through a series of mechanisms, namely, increasing antioxidant, photosynthetic, and osmoregulation capacities and regulating Cd uptake, translocation, efflux, and subcellular distribution. This study provides a scientific basis for further exploration of the role of NtGCN2 in plant responses to Cd stress and enhancement of the Cd stress signaling network in tobacco.

Overexpression of SQUAMOSA promoter binding protein-like 4a (NtSPL4a) alleviates Cd toxicity in Nicotiana tabacum

Squamosa Promoter Binding Protein-Like (SPL) plays a crucial role in regulating plant development and combating stress, yet its mechanism in regulating resistance to Cd toxicity remains unclear. In this study, we cloned a nuclear-localized transcription factor, NtSPL4a, from the tobacco cultivar TN90. Transient co-expression results showed that miR156 significantly reduced the expression of NtSPL4a by binding to the 3'-UTR of its transcript. We obtained transgenic tobacco overexpressing NtSPL4a (including the 3'-UTR) and NtSPL4aΔ (lacking the 3'-UTR) through Agrobacterium-mediated genetic transformation. Compared to the wild type (WT), overexpression of NtSPL4a/NtSPL4aΔ shortened the flowering time and exhibited a more developed root system. The transgenic tobacco showed significantly reduced Cd content, being 85.1% (OE-NtSPL4a) and 46.7% (OE-NtSPL4aΔ) of WT, respectively. Moreover, the upregulation of NtSPL4a affected the mineral nutrient homeostasis in transgenic tobacco. Additionally, overexpression of NtSPL4a/NtSPL4aΔ effectively alleviated leaf chlorosis and oxidative stress induced by Cd toxicity. One possible reason is that the overexpression of NtSPL4a/NtSPL4aΔ can effectively promote the accumulation of non-enzymatic antioxidants. A comparative transcriptomic analysis was performed between transgenic tobacco and WT to further unravel the global impacts brought by NtSPL4a. The tobacco overexpressing NtSPL4a had 183 differentially expressed genes (77 upregulated, 106 downregulated), while the tobacco overexpressing NtSPL4aΔ had 594 differentially expressed genes (244 upregulated, 350 downregulated) compared to WT. These differentially expressed genes mainly included transcription factors, metal transport proteins, flavonoid biosynthesis pathway genes, and plant stress-related genes. Our study provides new insights into the role of the transcript factor SPL in regulating Cd tolerance.

Cadmium and zinc accumulation and tolerance in two Egyptian cultivars (S53 and V120) of Helianthus annuus L. as potential phytoremediator

One of the most important oil crops in the world, sunflower (Helianthus annuus L.), is recognized to help in soil phytoremediation. Heavy metal (HM) contamination is one of the most abiotic challenges that may affect the growth and productivity of such an important crop plant. We studied the influence of HM-contaminated soils on metal homeostasis and the potential hypertolerance mechanisms in two sunflower Egyptian cultivars (V120 and S53). Both cultivars accumulated significantly higher cadmium concentrations in their roots compared to their shoots during Cd and Zn/Cd treatments. Higher root concentrations of 121 mg g-1 dry weight (DW) and 125 mg g-1 DW were measured in V120 plants compared to relatively lower values of 111 mg g-1 DW and 105 mg g-1 DW in the roots of S53 plants, respectively. Cadmium contamination significantly upregulated the expression of heavy metal ATPases (HaHMA4) in the shoots of V120 plants. On the other hand, their roots displayed a notable expression of HaHMA3. This study indicates that V120 plants accumulated and sequestered Cd in their roots. Therefore, it is advised to cultivate the V120 cultivar in areas contaminated with heavy metals as it is a promising Cd phytoremediator.

How plants respond to heavy metal contamination: a narrative review of proteomic studies and phytoremediation applications

MAIN CONCLUSION

Heavy metal pollution caused by human activities is a serious threat to the environment and human health. Plants have evolved sophisticated defence systems to deal with heavy metal stress, with proteins and enzymes serving as critical intercepting agents for heavy metal toxicity reduction. Proteomics continues to be effective in identifying markers associated with stress response and metabolic processes. This review explores the complex interactions between heavy metal pollution and plant physiology, with an emphasis on proteomic and biotechnological perspectives. Over the last century, accelerated industrialization, agriculture activities, energy production, and urbanization have established a constant need for natural resources, resulting in environmental degradation. The widespread buildup of heavy metals in ecosystems as a result of human activity is especially concerning. Although some heavy metals are required by organisms in trace amounts, high concentrations pose serious risks to the ecosystem and human health. As immobile organisms, plants are directly exposed to heavy metal contamination, prompting the development of robust defence mechanisms. Proteomics has been used to understand how plants react to heavy metal stress. The development of proteomic techniques offers promising opportunities to improve plant tolerance to toxicity from heavy metals. Additionally, there is substantial scope for phytoremediation, a sustainable method that uses plants to extract, sequester, or eliminate contaminants in the context of changes in protein expression and total protein behaviour. Changes in proteins and enzymatic activities have been highlighted to illuminate the complex effects of heavy metal pollution on plant metabolism, and how proteomic research has revealed the plant's ability to mitigate heavy metal toxicity by intercepting vital nutrients, organic substances, and/or microorganisms.

miR397b-LAC2 module regulates cadmium stress response by coordinating root lignification and copper homeostasis in Arabidopsis thaliana

Non-essential heavy metal cadmium (Cd) is toxic to plants and animals. Cadmium affects plant photosynthesis, respiration, and causes water imbalance and may lead to plant death. Cadmium induces toxicity by interfering with the essential metal copper (Cu) homeostasis, which affects plant nutrition. Though root lignin biosynthesis is positively regulated by Cd stress, the underlying mechanisms promoting lignin accumulation and controlling Cd-induced Cu limitation responses are unclear. Here, we elucidated the role of Cu-responsive microRNA (miR397b) in Arabidopsis thaliana plants for Cd stress by targeting the LACCASE2 (LAC2) gene. This study demonstrated the fundamental mechanism of miR397b-mediated Cd stress response by enhancing the lignin content in root tissues. We developed miR397b over-expressing plants, which showed considerable Cd stress tolerance. Plants with knockdown function of LAC2 also showed significant tolerance to Cd stress. miR397b overexpressing and lac2 mutant plants showed root reduction, higher biomass and chlorophyll content, and significantly lower Reactive Oxygen Species (ROS). This study demonstrated the miR397b-mediated Cd stress response in Arabidopsis by enhancing the lignin content in root tissues. We conclude that modulation in miR397b can be potentially used for improving plants for Cd tolerance and Cu homeostasis.

Differential absorption of cadmium and zinc by tobacco plants: Role of apoplastic pathway

Cadmium (Cd) contamination presents a significant challenge in global agriculture. This study explores the efficacy of chemical induction, specifically using sodium chloride (NaCl), to limit Cd uptake in tobacco (Nicotiana tabacum) and assesses its impact on essential divalent metal ions (DMIs). We conducted a comprehensive analysis encompassing ion absorption, root histology, and biochemistry to understand the influence of this method. Our results revealed that NaCl induction led to a notable 30 % decrease in Cd absorption, while maintaining minimal impact on zinc (Zn) uptake. Intriguingly, the absence of essential DMIs, such as calcium (Ca), magnesium (Mg), and Zn, was found to diminish the plant's capacity to absorb Cd. Furthermore, moderate NaCl induction resulted in an increased diameter of the root stele and enhanced lignin content, indicating a restriction of Cd absorption through the apoplastic pathway. Conversely, a compensatory absorption mechanism via the symplastic pathway appeared to be activated in the absence of essential elements. These findings highlight the potential of chemical induction as a strategy to mitigate agricultural Cd risks, offering insights into the complex interplay between plant ion transport pathways and metal uptake regulation.

Cell number regulator 8 from Salix linearistipularis enhances cadmium tolerance in poplar by reducing cadmium uptake and accumulation

Trace metals have relatively high density and high toxicity at low concentrations. Willow (Salix genus) is an excellent phytoremediation species for soil contaminated by trace metal ions. This study identified a cell number regulator (CNR) gene family member in Salix linearistipularis exhibiting strong metal ion resistance: SlCNR8. SlCNR8 expression was affected by various metal ions, including cadmium (Cd), zinc (Zn), copper (Cu), iron (Fe), and manganese (Mn). SlCNR8 overexpression enhanced Cd, Zn, Cu, and Fe resistance in transgenic poplar seedlings (84K) compared with the wild-type (WT). Moreover, transgenic poplar seedlings showed lower root Cd uptake and less Cd accumulation than WT under Cd stress. SlCNR8 was primarily localized to the nucleus and the plasma membrane-like cell periphery. Furthermore, SlCNR8 had transcriptional activation activity in yeast. The transcript levels of multiple metal ion transporters were altered in the roots of transgenic poplar seedlings compared to WT roots under Cd stress. These results suggest that SlCNR8 may enhance Cd resistance in transgenic poplar by reducing Cd uptake and accumulation. This may be related to altered transcription levels of other transporters or to itself. Our study suggests that SlCNR8 can be used as a candidate gene for genetic improvement of phytostabilisation of trace metals by genetic engineering.

Molecular-Assisted Breeding of Cadmium Pollution-Safe Cultivars

Cadmium (Cd) contamination in edible agricultural products, especially in crops intended for consumption, has raised worldwide concerns regarding food safety. Breeding of Cd pollution-safe cultivars (Cd-PSCs) is an effective solution to preventing the entry of Cd into the food chain from contaminated agricultural soil. Molecular-assisted breeding methods, based on molecular mechanisms for cultivar-dependent Cd accumulation and bioinformatic tools, have been developed to accelerate and facilitate the breeding of Cd-PSCs. This review summarizes the recent progress in the research of the low Cd accumulation traits of Cd-PSCs in different crops. Furthermore, the application of molecular-assisted breeding methods, including transgenic approaches, genome editing, marker-assisted selection, whole genome-wide association analysis, and transcriptome, has been highlighted to outline the breeding of Cd-PSCs by identifying critical genes and molecular biomarkers. This review provides a comprehensive overview of the development of Cd-PSCs and the potential future for breeding Cd-PSC using modern molecular technologies.

Significance and genetic control of membrane transporters to improve phytoremediation and biofortification processes

Humans frequently consume plant-based foods in their daily life. Contamination of agricultural soils by heavy metals (HMs) is a major food and nutritional security issue. The crop plants grown in HM-contaminated agricultural soil may accumulate more HMs in their edible part, further transferring into the food chain. Consumption of HM-rich crops can cause severe health issues in humans. On the other hand, the low content of the essential HM in the edible part of the crop also causes health problems. Therefore, researchers must try to reduce the non-essential HM in the edible part of the crop plants and improve the essential HMs. Phytoremediation and biofortification are the two strategies for resolving this problem. The genetic component helps to improve the efficiency of phytoremediation and biofortification processes in plants. They help eliminate HMs from soil and improve essential HM content in crop plants. The membrane transporter genes (genetic components) are critical in these two strategies. Therefore, engineering membrane transporter genes may help reduce the non-essential HM content in the edible part of crop plants. Targeted gene editing by genome editing tools like CRISPR could help plants achieve efficient phytoremediation and biofortification. This article covers gene editing's scope, application, and implication to improve the phytoremediation and biofortification processes in non-crop and crop plants.

Duckweed: a potential phytosensor for heavy metals

Globally, heavy metal (HM) contamination is one of the primary causes of environmental pollution leading to decreased quality of life for those affected. In particular, HM contamination in groundwater poses a serious risk to human health and the potential for destabilization of aquatic ecosystems. At present, strategies to remove HM contamination from wastewater are inefficient, costly, laborious, and often the removal poses as much risk to the environment as the initial contamination. Phytoremediation, plant-based removal of contaminants from soil or water, has long been viewed as an economical and sustainable solution to remove toxic metals from the environment. However, to date, phytoremediation has demonstrated limited successes despite a large volume of literature supporting its potential. A key aspect for achieving robust and meaningful phytoremediation is the selection of a plant species that is well suited to the task. For the removal of pollutants from wastewater, hydrophytes, like duckweed, exhibit significant potential due to their rapid growth on nutrient-rich water, ease of collection, and ability to survive in various ecosystems. As a model for ecotoxicity studies, duckweed is an ideal candidate, as it is easy to cultivate under controlled and even sterile conditions, and the rapid growth enables multi-generational studies. Similarly, recent advances in the genetic engineering and genome-editing of duckweed will enable the transition from fundamental ecotoxicity studies to engineered solutions for phytoremediation of HMs. This review will provide insight into the suitability of duckweeds for phytoremediation of HMs and strategies for engineering next-generation duckweed to provide real-world environmental solutions.

The regulatory role of abscisic acid on cadmium uptake, accumulation and translocation in plants

To date, Cd contamination of cropland and crops is receiving more and more attention around the world. As a plant hormone, abscisic acid (ABA) plays an important role in Cd stress response, but its effect on plant Cd uptake and translocation varies among plant species. In some species, such as Arabidopsis thaliana, Oryza sativa, Brassica chinensis, Populus euphratica, Lactuca sativa, and Solanum lycopersicum, ABA inhibits Cd uptake and translocation, while in other species, such as Solanum photeinocarpum and Boehmeria nivea, ABA severs the opposite effect. Interestingly, differences in the methods and concentrations of ABA addition also triggered the opposite result of Cd uptake and translocation in Sedum alfredii. The regulatory mechanism of ABA involved in Cd uptake and accumulation in plants is still not well-established. Therefore, we summarized the latest studies on the ABA synthesis pathway and comparatively analyzed the physiological and molecular mechanisms related to ABA uptake, translocation, and detoxification of Cd in plants at different ABA concentrations or among different species. We believe that the control of Cd uptake and accumulation in plant tissues can be achieved by the appropriate ABA application methods and concentrations in plants.

Advances in Genes-Encoding Transporters for Cadmium Uptake, Translocation, and Accumulation in Plants

Cadmium (Cd) is a heavy metal that is highly toxic for plants, animals, and human beings. A better understanding of the mechanisms involved in Cd accumulation in plants is beneficial for developing strategies for either the remediation of Cd-polluted soils using hyperaccumulator plants or preventing excess Cd accumulation in the edible parts of crops and vegetables. As a ubiquitous heavy metal, the transport of Cd in plant cells is suggested to be mediated by transporters for essential elements such as Ca, Zn, K, and Mn. Identification of the genes encoding Cd transporters is important for understanding the mechanisms underlying Cd uptake, translocation, and accumulation in either crop or hyperaccumulator plants. Recent studies have shown that the transporters that mediate the uptake, transport, and accumulation of Cd in plants mainly include members of the natural resistance-associated macrophage protein (Nramp), heavy metal-transporting ATPase (HMA), zinc and iron regulated transporter protein (ZIP), ATP-binding cassette (ABC), and yellow stripe-like (YSL) families. Here, we review the latest advances in the research of these Cd transporters and lay the foundation for a systematic understanding underlying the molecular mechanisms of Cd uptake, transport, and accumulation in plants.

The potential role of plasma membrane proteins in response to Zn stress in rice roots based on iTRAQ and PRM under low Cd condition

Cd pollution had already caused serious threats to crop growth and development, food safety and human health, and become a potential agricultural and global environmental problem. Zn had been used to reduce Cd accumulation in soil and plants. Proteins located in plasma membrane (PM) played important roles in transferring stress signals in plants. To further elucidate how PM proteins modulated Zn/Cd transport under low-Cd condition, quantitative proteomics was employed to identify and verify the differentially expressed proteins (DEPs) and their biological functions at proteome level. A total of 4008 proteins were identified, and 332 DEPs (192 up and 140 down, fold >1.50 or <0.66, p < 0.01) were screened. Functional analysis showed that DEPs were mainly catalytic active and binding proteins, involved in glutathione metabolism, phenylpropanoid biosynthesis, etc. DEPs involved in ion transport played key roles in regulating transmembrane transport, resisting stress and alleviating toxicity of heavy metals to rice roots. DEPs were as the marker proteins in rice root responding to heavy metal stress. This study had important guiding significances for metal ions transport mechanism and screening of biomarkers responding to abiotic stress, and provided references for further researches underlying abiotic stress and detoxication in rice and other plants.

Cadmium Phytotoxicity, Tolerance, and Advanced Remediation Approaches in Agricultural Soils; A Comprehensive Review

Cadmium (Cd) is a major environmental contaminant due to its widespread industrial use. Cd contamination of soil and water is rather classical but has emerged as a recent problem. Cd toxicity causes a range of damages to plants ranging from germination to yield suppression. Plant physiological functions, i.e., water interactions, essential mineral uptake, and photosynthesis, are also harmed by Cd. Plants have also shown metabolic changes because of Cd exposure either as direct impact on enzymes or other metabolites, or because of its propensity to produce reactive oxygen species, which can induce oxidative stress. In recent years, there has been increased interest in the potential of plants with ability to accumulate or stabilize Cd compounds for bioremediation of Cd pollution. Here, we critically review the chemistry of Cd and its dynamics in soil and the rhizosphere, toxic effects on plant growth, and yield formation. To conserve the environment and resources, chemical/biological remediation processes for Cd and their efficacy have been summarized in this review. Modulation of plant growth regulators such as cytokinins, ethylene, gibberellins, auxins, abscisic acid, polyamines, jasmonic acid, brassinosteroids, and nitric oxide has been highlighted. Development of plant genotypes with restricted Cd uptake and reduced accumulation in edible portions by conventional and marker-assisted breeding are also presented. In this regard, use of molecular techniques including identification of QTLs, CRISPR/Cas9, and functional genomics to enhance the adverse impacts of Cd in plants may be quite helpful. The review's results should aid in the development of novel and suitable solutions for limiting Cd bioavailability and toxicity, as well as the long-term management of Cd-polluted soils, therefore reducing environmental and human health hazards.

Recent advances in physiological and molecular mechanisms of heavy metal accumulation in plants

Among abiotic stress, the toxicity of metals impacts negatively on plants' growth and productivity. This toxicity promotes various perturbations in plants at different levels. To withstand stress, plants involve efficient mechanisms through the implication of various signaling pathways. These pathways enhance the expression of many target genes among them gene coding for metal transporters. Various metal transporters which are localized at the plasma membrane and/or at the tonoplast are crucial in metal stress response. Furthermore, metal detoxification is provided by metal-binding proteins like phytochelatins and metallothioneins. The understanding of the molecular basis of metal toxicities signaling pathways and tolerance mechanisms is crucial for genetic engineering to produce transgenic plants that enhance phytoremediation. This review presents an overview of the recent advances in our understanding of metal stress response. Firstly, we described the effect of metal stress on plants. Then, we highlight the mechanisms involved in metal detoxification and the importance of the regulation in the response to heavy metal stress. Finally, we mentioned the importance of genetic engineering for enhancing the phytoremediation technique. In the end, the response to heavy metal stress is complex and implicates various components. Thus, further studies are needed to better understand the mechanisms involved in response to this abiotic stress.

Transcriptome analysis and physiological indicators reveal the role of sulfur in cadmium accumulation and transportation in water spinach (Ipomoea aquatica Forsk.)

Cadmium (Cd) contamination of croplands has become a threat to crop food safety and human health. In this study, we investigated the effect of sulfur on the growth of water spinach under Cd stress and the amount of Cd accumulation by increasing the soil sulfate content. We found that the biomass of water spinach significantly increased after the application of sulfur while the shoot Cd concentration was considerably reduced (by 31%). The results revealed that sulfur could promote the expression of PME and LAC genes, accompanied by an increase in PME activity and lignin content. Also, the cell wall Cd content of water spinach roots was significantly increased under sulfur treatment. This finding suggests that sulfur could enhance the adsorption capacity of Cd by promoting the generation of cell wall components, thereby inhibiting the transportation of Cd via the apoplastic pathway. In addition, the higher expression of Nramp5 under the Cd1S0 (concentration of Cd and sulfur are 2.58 and 101.31 mg/kg respectively) treatment led to increased Cd uptake. The CAX3 and ABC transporters and GST were expressed at higher levels along with a higher cysteine content and GSH/GSSR value under Cd1S1 (concentration of Cd and sulfur are 2.60 and 198.36 mg/kg respectively) treatment, which contribute to the Cd detoxification and promotion of Cd compartmentalization in root vacuoles, thereby reducing the translocation of Cd to the shoot via the symplastic pathway.

Long-term effects of phytoextraction by a poplar clone on the concentration, fractionation, and transportation of heavy metals in mine tailings

Mine tailings are one of main causes of diffused heavy metal pollution since the heavy metals in there may acquire mobility. The current knowledge of the processes at work in long-term phytoremediation by woody species remains insufficient. Through a 4-year field study, we evaluated the phytoextraction efficiency of Populus deltoides CL. 'Xianglin 90' grown on a mine tailing co-polluted by Cd, Cu, Cr, Ni, Pb, and Zn. The concentrations of Cd, Cu, Ni, Pb, and Zn in the rhizospheric soil were reduced by amounts ranging from 12.86 to 42.19% during the study period. Bioconcentration factors and translocation factors showed that the accumulation of Cd and Zn occurring in the shoots was the most effective. Combined with the considerable biomass produced by poplar, the extracted amounts of Cd and Zn could reach 0.61 g and 10.66 g plant^-1, respectively, in which the shoots account for 77.3% (Cd) and 89.0% (Zn) of the overall extraction amounts. Acid-soluble Cd and Zn increased by 5.49% and 4.29%, respectively, in the rhizosphere compared to the bulk soil, indicating that poplar enhanced the mobility of Cd and Zn in the rhizosphere, which explained its ability for bioaccumulation and root-shoot translocation. Moreover, calculated time required to address the issue of Cd and Zn pollution was theoretically shortened by more than half from 2015 to 2019. This study brings new insights into the long-term effects of phytoextraction on the concentration, fractionation, and transportation of heavy metals and confirms the potential of poplar as a Cd and Zn remediation species.

Bio-remediation approaches for alleviation of cadmium contamination in natural resources

Cadmium (Cd) is a harmful heavy metal that can cause potent environmental and health hazards at different trophic levels through food chain. Cd is relatively non-biodegradable and persists for a long time in the environment. Considering the potential toxicity and non-biodegradability of Cd in the environment as well as its health hazards, this is an urgent issue of international concern that needs to be addressed by implicating suitable remedial approaches. The current article specifically attempts to review the different biological approaches for remediation of Cd contamination in natural resources. Further, bioremediation mechanisms of Cd by microbes such as bacteria, fungi, algae are comprehensively discussed. Studies indicate that heavy metal resistant microbes can be used as suitable biosorbents for the removal of Cd (up to 90%) in the natural resources. Soil-to-plant transfer coefficient (TC) of Cd ranges from 3.9 to 3340 depending on the availability of metal to plants and also on the type of plant species. The potential phytoremediation strategies for Cd removal and the key factors influencing bioremediation process are also emphasized. Studies on molecular mechanisms of transgenic plants for Cd bioremediation show immense potential for enhancing Cd phytoremediation efficiency. Thus, it is suggested that nano-technological based integrated bioremediation approaches could be a potential futuristic path for Cd decontamination in natural resources. This review would be highly useful for the biologists, chemists, biotechnologists and environmentalists to understand the long-term impacts of Cd on ecology and human health so that potential remedial measures could be taken in advance.

Distribution of cadmium in subcellular fraction and expression difference of its transport genes among three cultivars of pepper

Cadmium (Cd) tolerance mechanisms in plant are mainly divided into two categories: evasion mechanism and tolerance mechanism. However, due to the complexity of the mechanism of Cd absorption and accumulation in crops, there are still disputes and controversies about Cd toxicity to plants and the mechanism of Cd tolerance in plants. The Cd absorption and accumulation mechanism in edible parts of pepper remains unknown. The present study characterized three pepper cultivars with different cadmium tolerance under cadmium stress. One high-Cd-accumulation type (X55), a medium-Cd-accumulation type (Daguo 99) and a low-Cd-accumulation type (Luojiao 318) were selected to study distribution characteristics of Cd in subcellular fractions of the three pepper varieties as well as expression difference of key Cd accumulation and tolerance genes under different cadmium levels. The results showed that under Cd stress, X55 and Daguo 99 mainly migrated Cd from root to stems and leaves, while Luojiao318 migrated it to the fruit. The Cd concentration in the subcellular fractions of pepper roots, stems, leaves and fruits was as follow: cell wall (F1) > organelle (F2) > cell soluble fraction (F3). The roots, stems and leaf cells of X55 have strong Cd compartmentalization capacity. The fruit cells of Daguo 99 have strong Cd compartmentalization capacity, while the roots of Luojiao318 have strong ability to inhibit Cd absorption. Under Cd stress, HMA1, HMA2 and NRAMP1-6 were up-regulated in roots, stems and fruits of the three varieties. FTP1-2 and FTP1-3 genes were significantly up-regulated in different materials, except the roots of Daguo 99. Under Cd treatment, PCS gene expression of pepper showed an order of that of X55 > Luojiao 318 >Daguo 99. The present study revealed that the cell wall of pepper played an important role in Cd separation and resistance. The difference in Cd accumulation ability of the pepper varieties may be related to differences in main expression sites and expression levels of HMA, NRAMP, FTP and PCS genes.

Differential cadmium translocation and accumulation between Nicotiana tabacum L. and Nicotiana rustica L. by transcriptome combined with chemical form analyses

Cadmium (Cd) is a severely toxic and carcinogenic heavy metal. Cigarette smoking is one of the major source of Cd exposure in humans. Nicotiana tabacum is primarily a leaf Cd accumulator, while Nicotiana rustica is a root Cd accumulator among Nicotiana species. However, little is known about the mechanisms of differential Cd translocation and accumulation in Nicotiana. To find the key factors, Cd concentration, Cd chemical forms, and transcriptome analysis were comparatively studied between N. tabacum and N. rustica under control or 10 μM Cd stress. The leaf/root Cd concentration ratio of N. tabacum was 2.26 and that of N. rustica was 0.14. The Cd concentration in xylem sap of N. tabacum was significantly higher than that of N. rustica. The root of N. tabacum had obviously higher proportion of ethanol extractable Cd (40%) and water extractable Cd (16%) than those of N. rustica (16% and 6%). Meanwhile the proportion of sodium chloride extracted Cd in N. rustica (71%) was significantly higher than that in N. tabacum (30%). A total of 30710 genes expressed differentially between the two species at control, while this value was 30,294 under Cd stress, among which 27,018 were collective genes, manifesting the two species existed enormous genetic differences. KEGG pathway analysis showed the phenylpropanoid biosynthesis pathway was overrepresented between the two species under Cd stress. Several genes associated with pectin methylesterase, suberin and lignin synthesis, and heavy metal transport were discovered to be differential expressed genes between two species. The results suggested that the higher accumulation of Cd in the leaf of N. tabacum depends on a comprehensive coordination of Cd transport, including less cell wall binding, weaker impediment by the Casparian strip, and efficient xylem loading.

In situ phytoremediation of heavy metal-contaminated soil and groundwater: a green inventive approach

The heavy metal contamination of soil and groundwater is a serious threat to environment worldwide. The survival of human being primarily relies upon soil and groundwater sources. Therefore, the remediation of heavy metal-contaminated soil and groundwater is a matter of utmost concern. Heavy metals are non-degradable and persist in the environment and subsequently contaminate the food chain. Heavy metal pollution puts a serious impact on human health and it adversely affects our physical body. Although, numerous in situ conventional technologies have been utilized for the treatment purpose, but most of the techniques have some limitations such as high cost, deterioration of soil properties, disturbances to soil native flora and fauna and intensive labour. Despite that, in situ phytoremediation is a cost-effective, eco-friendly, solar-driven and novel approach with significant public acceptance. The past research reflects rare discussion addressing both (heavy metal in situ phytoremediation of soil and groundwater) in one platform. The present review article covers both the concepts of in situ phytoremediation of soil and groundwater with major emphasis on health risks of heavy metals, enhanced integrated approaches of in situ phytoremediation, mechanisms of in situ phytoremediation along with effective hyperaccumulator plants for heavy metals remediation, challenges and future prospects.

The Role of Aquaporin Overexpression in the Modulation of Transcription of Heavy Metal Transporters under Cadmium Treatment in Poplar

Populus alba ‘Villafranca’ clone is well-known for its tolerance to cadmium (Cd). To determine the mechanisms of Cd tolerance of this species, wild-type (wt) plants were compared with transgenic plants over-expressing an aquaporin (aqua1, GenBank GQ918138). Plants were maintained in hydroponic conditions with Hoagland’s solution and treated with 10 µM of Cd, renewed every 5 d. The transcription levels of heavy metal transporter genes (PaHMA2, PaNRAMP1.3, PaNRAMP2, PaNRAMP3.1, PaNRAMP3.2, PaABCC9, and PaABCC13) were analyzed at 1, 7, and 60 d of treatment. Cd application did not induce visible toxicity symptoms in wt and aqua1 plants even after 2 months of treatment confirming the high tolerance of this poplar species to Cd. Most of the analyzed genes showed in wt plants a quick response in transcription at 1 d of treatment and an adaptation at 60 d. On the contrary, a lower transcriptional response was observed in aqua1 plants in concomitance with a higher Cd concentration in medial leaves. Moreover, PaHMA2 showed at 1 d an opposite trend within organs since it was up-regulated in root and stem of wt plants and in leaves of aqua1 plants. In summary, aqua1 overexpression in poplar improved Cd translocation suggesting a lower Cd sensitivity of aqua1 plants. This different response might be due to a different transcription of PaNRAMP3 genes that were more transcribed in wt line because of the importance of this gene in Cd compartmentalization.

The kinase CIPK11 functions as a positive regulator in cadmium stress response in Arabidopsis

Cadmium (Cd) pollution in agricultural soil has always been a knotty problem, which made it necessary to find the mechanism related to Cd transport in plant. In this study, we found a novel character of the CIPK11 modulating the transport of Cd in Arabidopsis thaliana. Over-expression of CIPK11 (CIPK11OE#1-7, CIPK11OE#8-5) resulted in the increased tolerance to Cd stress, which embodied in higher fresh weight, lower Cd enrichment and reactive oxygen species (ROS) than the wild-type (WT) plants. qRT-PCR results showed a collective down-regulation of the expression of IRT1 and transcription factor genes FIT, bHLH039 in the CIPK11-overexpression plants after Cd stress. Overexpression of CIPK11 significantly increased the expression of ABA marker genes in Arabidopsis after Cd stress. With different concentrations of ABA treatment, the root length differences caused by Cd stress could be recovered. However the transcription levels of FIT and bHLH039 decreased in WT and cipk11 mutant when treated with ABA which indicated that ABA can inhibit the transcription of IRT1 by repressing FIT and bHLH039 expression. Taken together, our results demonstrated that the kinase CIPK11 responses to Cd stress by ABA signaling pathway.

Reduced cadmium accumulation in tobacco by sodium chloride priming

Cadmium (Cd) pollution threatens agricultural security worldwide. This study tested the efficacy of priming chemicals to decrease Cd uptake by tobacco plants (Nicotiana tabacum). After initial screening from nine different chemicals (NaCl, Cd(CH3COO)2, Cd(NO3)2, CdCl2, KHNO3, polyethylene glycol 6000 (PEG-6000), indole-3-acetic acid (IAA), ß-aminobutyric acid (BABA), and glutathione (GSH)), NaCl and PEG-6000 were further investigated because of their low risks to plant growth and efficiency to Cd reduction. Priming procedures (concentrations) were optimized for both chemicals and the best one (100 mM NaCl) was used to test both soil and hydroponic media. The results showed 31.3% lower Cd concentrations in shoots after priming with 100 mM NaCl. Phenotype parameters of the plants were also measured and showed no significant impacts of the priming procedures on the shoot biomass and the uptakes of nitrogen (N), phosphorus (P), and potassium (K), nor the photosynthetic capacity (net photosynthesis rate (Pn) and chlorophyll concentration (SPAD)). Histological observations of the roots showed a significant increase of the stele diameter after NaCl priming and a subsequent negative correlation between shoot Cd concentration and stele diameter was found after NaCl priming at different levels. This study confirmed 100 mM NaCl as an efficient priming treatment to decrease Cd uptake and the coarsening of the root stele was identified as a potential explanation for the observed decrease of Cd in tobacco shoots.

Phytoremediation of Cadmium: Physiological, Biochemical, and Molecular Mechanisms

Cadmium (Cd) is one of the most toxic metals in the environment, and has noxious effects on plant growth and production. Cd-accumulating plants showed reduced growth and productivity. Therefore, remediation of this non-essential and toxic pollutant is a prerequisite. Plant-based phytoremediation methodology is considered as one a secure, environmentally friendly, and cost-effective approach for toxic metal remediation. Phytoremediating plants transport and accumulate Cd inside their roots, shoots, leaves, and vacuoles. Phytoremediation of Cd-contaminated sites through hyperaccumulator plants proves a ground-breaking and profitable choice to combat the contaminants. Moreover, the efficiency of Cd phytoremediation and Cd bioavailability can be improved by using plant growth-promoting bacteria (PGPB). Emerging modern molecular technologies have augmented our insight into the metabolic processes involved in Cd tolerance in regular cultivated crops and hyperaccumulator plants. Plants’ development via genetic engineering tools, like enhanced metal uptake, metal transport, Cd accumulation, and the overall Cd tolerance, unlocks new directions for phytoremediation. In this review, we outline the physiological, biochemical, and molecular mechanisms involved in Cd phytoremediation. Further, a focus on the potential of omics and genetic engineering strategies has been documented for the efficient remediation of a Cd-contaminated environment.

Increased metal tolerance and bioaccumulation of zinc and cadmium in Chlamydomonas reinhardtii expressing a AtHMA4 C-terminal domain protein

The use of microalgal biomass for metal pollutant bioremediation might be improved by genetic engineering to modify the selectivity or capacity of metal biosorption. A plant cadmium (Cd) and zinc (Zn) transporter (AtHMA4) was used as a transgene to increase the ability of Chlamydomonas reinhardtii to tolerate 0.2 mM Cd and 0.3 mM Zn exposure. The transgenic cells showed increased accumulation and internalization of both metals compared to wild-type. AtHMA4 was expressed either as the full-length (FL) protein or just the C-terminal (CT) tail, which is known to have metal-binding sites. Similar Cd and Zn tolerance and accumulation was observed with expression of either the FL protein or CT domain, suggesting that enhanced metal tolerance was mainly due to increased metal binding rather than metal transport. The effectiveness of the transgenic cells was further examined by immobilization in calcium alginate to generate microalgal beads that could be added to a metal contaminated solution. Immobilization maintained metal tolerance, while AtHMA4-expressing cells in alginate showed a concentration-dependent increase in metal biosorption that was significantly greater than alginate beads composed of wild-type cells. This demonstrates that expressing AtHMA4 FL or CT has great potential as a strategy for bioremediation using microalgal biomass.

Prospect of phytoremediation combined with other approaches for remediation of heavy metal-polluted soils

Accumulation of heavy metals in agricultural soils due to human production activities-mining, fossil fuel combustion, and application of chemical fertilizers/pesticides-results in severe environmental pollution. As the transmission of heavy metals through the food chain and their accumulation pose a serious risk to human health and safety, there has been increasing attention in the investigation of heavy metal pollution and search for effective soil remediation technologies. Here, we summarized and discussed the basic principles, strengths and weaknesses, and limitations of common standalone approaches such as those based on physics, chemistry, and biology, emphasizing their incompatibility with large-scale applications. Moreover, we explained the effects, advantages, and disadvantages of the combinations of common single repair approaches. We highlighted the latest research advances and prospects in phytoremediation-chemical, phytoremediation-microbe, and phytoremediation-genetic engineering combined with remediation approaches by changing metal availability, improving plant tolerance, promoting plant growth, improving phytoextraction and phytostabilization, etc. We then explained the improved safety and applicability of phytoremediation combined with other repair approaches compared to common standalone approaches. Finally, we established a prospective research direction of phytoremediation combined with multi-technology repair strategy.

Expression of Genes Involved in Heavy Metal Trafficking in Plants Exposed to Salinity Stress and Elevated Cd Concentrations

Many areas intended for crop production suffer from the concomitant occurrence of heavy metal pollution and elevated salinity; therefore, halophytes seem to represent a promising perspective for the bioremediation of contaminated soils. In this study, the influence of Cd treatment (0.01-10.0 mM) and salinity stress (0.4 M NaCl) on the expression of genes involved in heavy metal uptake (irt2-iron-regulated protein 2, zip4-zinc-induced protein 4), vacuolar sequestration (abcc2-ATP-binding cassette 2, cax4-cation exchanger 2 pcs1-phytochelatin synthase 1) and translocation into aerial organs (hma4-heavy metal ATPase 4) were analyzed in a soil-grown semi-halophyte Mesembryanthemum crystallinum. The upregulation of irt2 expression induced by salinity was additionally enhanced by Cd treatment. Such changes were not observed for zip4. Stressor-induced alterations in abcc2, cax4, hma4 and pcs1 expression were most pronounced in the root tissue, and the expression of cax4, hma4 and pcs1 was upregulated in response to salinity and Cd. However, the cumulative effect of both stressors, similar to the one described for irt2, was observed only in the case of pcs1. The importance of salt stress in the irt2 expression regulation mechanism is proposed. To the best of our knowledge, this study is the first to report the combined effect of salinity and heavy metal stress on genes involved in heavy metal trafficking.

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.

Oryza sativa class III peroxidase (OsPRX38) overexpression in Arabidopsis thaliana reduces arsenic accumulation due to apoplastic lignification

ClassIII peroxidases are multigene family of plant-specific peroxidase enzyme. They are involved in various physiological and developmental processes like auxin catabolism, cell metabolism, various biotic, abiotic stresses and cell elongation. In the present study, we identified a class III peroxidase (OsPRX38) from rice which is upregulated several folds in both arsenate (AsV) and arsenite (AsIII) stresses. The overexpression of OsPRX38 in Arabidopsis thaliana significantly enhances Arsenic (As) tolerance by increasing SOD, PRX GST activity and exhibited low H₂O_2, electrolyte leakage and malondialdehyde content. OsPRX38 overexpression also affect the plant growth by increasing total biomass and seeds production in transgenics than WT under As stress condition. Confocal microscopy revealed that the OsPRX38-YFP fusion protein was localized to the apoplast of the onion epidermal cells. In addition, lignification was positively correlated with an increase in cell-wall-associated peroxidase activities in transgenic plants. This study indicates the role of OsPRX38 in lignin biosynthesis, where lignin act as an apoplastic barrier for As entry in root cells leading to reduction of As accumulation in transgenic. Overall the study suggests that overexpression of OsPRX38 in Arabidopsis thaliana activates the signaling network of different antioxidant systems under As stress condition, enhancing the plant tolerance by reducing As accumulation due to high lignification.

Genotype Variation in Rice (Oryza sativa L.) Tolerance to Fe Toxicity Might Be Linked to Root Cell Wall Lignification

Iron (Fe) is an essential element to plants, but can be harmful if accumulated to toxic concentrations. Fe toxicity can be a major nutritional disorder in rice (Oryza sativa) when cultivated under waterlogged conditions, as a result of excessive Fe solubilization of in the soil. However, little is known about the basis of Fe toxicity and tolerance at both physiological and molecular level. To identify mechanisms and potential candidate genes for Fe tolerance in rice, we comparatively analyzed the effects of excess Fe on two cultivars with distinct tolerance to Fe toxicity, EPAGRI 108 (tolerant) and BR-IRGA 409 (susceptible). After excess Fe treatment, BR-IRGA 409 plants showed reduced biomass and photosynthetic parameters, compared to EPAGRI 108. EPAGRI 108 plants accumulated lower amounts of Fe in both shoots and roots compared to BR-IRGA 409. We conducted transcriptomic analyses of roots from susceptible and tolerant plants under control and excess Fe conditions. We found 423 up-regulated and 92 down-regulated genes in the susceptible cultivar, and 42 up-regulated and 305 down-regulated genes in the tolerant one. We observed striking differences in root gene expression profiles following exposure to excess Fe: the two cultivars showed no genes regulated in the same way (up or down in both), and 264 genes were oppositely regulated in both cultivars. Plants from the susceptible cultivar showed down-regulation of known Fe uptake-related genes, indicating that plants are actively decreasing Fe acquisition. On the other hand, plants from the tolerant cultivar showed up-regulation of genes involved in root cell wall biosynthesis and lignification. We confirmed that the tolerant cultivar has increased lignification in the outer layers of the cortex and in the vascular bundle compared to the susceptible cultivar, suggesting that the capacity to avoid excessive Fe uptake could rely in root cell wall remodeling. Moreover, we showed that increased lignin concentrations in roots might be linked to Fe tolerance in other rice cultivars, suggesting that a similar mechanism might operate in multiple genotypes. Our results indicate that changes in root cell wall and Fe permeability might be related to Fe toxicity tolerance in rice natural variation.

The potential of genetic engineering of plants for the remediation of soils contaminated with heavy metals

The genetic engineering of plants to facilitate the reclamation of soils and waters contaminated with inorganic pollutants is a relatively new and evolving field, benefiting from the heterologous expression of genes that increase the capacity of plants to mobilize, stabilize and/or accumulate metals. The efficiency of phytoremediation relies on the mechanisms underlying metal accumulation and tolerance, such as metal uptake, translocation and detoxification. The transfer of genes involved in any of these processes into fast-growing, high-biomass crops may improve their reclamation potential. The successful phytoextraction of metals/metalloids and their accumulation in aerial organs have been achieved by expressing metal ligands or transporters, enzymes involved in sulfur metabolism, enzymes that alter the chemical form or redox state of metals/metalloids and even the components of primary metabolism. This review article considers the potential of genetic engineering as a strategy to improve the phytoremediation capacity of plants in the context of heavy metals and metalloids, using recent case studies to demonstrate the practical application of this approach in the field.

Wheat expansin gene TaEXPA2 is involved in conferring plant tolerance to Cd toxicity

Cadmium (Cd) is a severe and toxic heavy metal pollutant that affects plant growth and development. In this study, we found that the expression of an expansin gene, TaEXPA2, was upregulated in wheat leaves under CdCl2 toxicity. We characterized the involvement of TaEXPA2 in conferring Cd tolerance. Tobacco plants overexpressing TaEXPA2 showed higher germination rate, root elongation, and biomass accumulation compared to the wild-type (WT) plants upon CdCl2 treatment. The improved photosynthetic parameters and lesser cellular damage in transgenic plants exposed to Cd compared to that in the WT plants suggest that TaEXPA2 overexpression improves Cd tolerance in plants. Furthermore, we noticed that Cd was efficiently effluxed out of the cytoplasm in the transgenic plants owing to the enhanced activities of H+-ATPase, V-ATPase, and PPase, which helped in conferring Cd tolerance. Moreover, Cd concentration and ROS accumulation were lower in the transgenic plants than in WT plants as a consequence of enhanced antioxidant enzyme activities in the former. In addition, atexpa2, an Arabidopsis mutant, exhibited lower biomass and shorter primary root compared to its WT under Cd toxicity; however, the phenotype was recovered upon expression of TaEXPA2 in these mutants. Our results demonstrate that TaEXPA2 confers tolerance to Cd toxicity. The changed absorption/transportation of Cd and the antioxidative capacity may be involved in the improved tolerance of the transgenic plants with overexpression of TaEXPA2 to CdCl2 toxicity.

Contrasting cadmium resistance strategies in two metallicolous populations of Arabidopsis halleri

While cadmium (Cd) tolerance is a constitutive trait in the Arabidopsis halleri species, Cd accumulation is highly variable. Recent adaptation to anthropogenic metal stress has occurred independently within the genetic units of A. halleri and the evolution of different mechanisms involved in Cd tolerance and accumulation has been suggested. To gain a better understanding of the mechanisms underlying Cd tolerance and accumulation in A. halleri, ionomic inductively coupled plasma mass spectrometry (ICP-MS), transcriptomic (RNA sequencing) and metabolomic (high-performance liquid chromatography-mass spectrometry) profiles were analysed in two A. halleri metallicolous populations from different genetic units (PL22 from Poland and I16 from Italy). The PL22 and I16 populations were both hypertolerant to Cd, but PL22 hyperaccumulated Cd while I16 behaved as an excluder both in situ and when grown hydroponically. The observed hyperaccumulator vs excluder behaviours were paralleled by large differences in the expression profiles of transporter genes. Flavonoid-related transcripts and metabolites were strikingly more abundant in PL22 than in I16 shoots. The role of novel A. halleri candidate genes possibly involved in Cd hyperaccumulation or exclusion was supported by the study of corresponding A. thaliana knockout mutants. Taken together, our results are suggestive of the evolution of divergent strategies for Cd uptake, transport and detoxification in different genetic units of A. halleri.

Oxidative Stress and Heavy Metals in Plants

Oxidative stress is a pathological process related to not only animal kingdom but also plants. Regarding oxidative stress in plants, heavy metals are frequently discussed as causative stimuli with relevance to ecology. Because heavy metals have broad technological importance, they can easily contaminate the environment. Much of previous effort regarding the harmful impact of the heavy metals was given to their toxicology in the animals and humans. Their implication in plant pathogeneses is less known and remains underestimated.The current paper summarizes basic facts about heavy metals, their distribution in soil, mobility, accumulation by plants, and initiation of oxidative stress including the decline in basal metabolism. The both actual and frontier studies in the field are summarized and discussed. The major pathophysiological pathways are introduced as well and link between heavy metals toxicity and their ability to initiate an oxidative damage is provided. Mobility and bioaccessibility of the metals is also considered as key factors in their impact on oxidative stress development in the plant. The metals like lead, mercury, copper, cadmium, iron, zinc, nickel, vanadium are depicted in the text.Heavy metals appear to be significant contributors to pathological processes in the plants and oxidative stress is probably an important contributor to the effect. The most sensitive plant species are enlisted and discussed in this review. The facts presented here outline next effort to investigate pathological processes in the plants.

Recent strategies of increasing metal tolerance and phytoremediation potential using genetic transformation of plants

Avoidance and reduction of soil contamination with heavy metals is one of the most serious global challenges. Nowadays, science offers us new opportunities of utilizing plants to extract toxic elements from the soil by means of phytoremediation. Plant abilities to uptake, translocate, and transform heavy metals, as well as to limit their toxicity, may be significantly enhanced via genetic engineering. This paper provides a comprehensive review of recent strategies aimed at the improvement of plant phytoremediation potential using plant transformation and employing current achievements in nuclear and cytoplasmic genome transformation. Strategies for obtaining plants suitable for effective soil clean-up and tolerant to excessive concentrations of heavy metals are critically assessed. Promising directions in genetic manipulations, such as gene silencing and cis- and intragenesis, are also discussed. Moreover, the ways of overcoming disadvantages of phytoremediation using genetic transformation approachare proposed. The knowledge gathered here could be useful for designing new research aimed at biotechnological improvement of phytoremediation efficiency.

Heterologous expression of the yeast Tpo1p or Pdr5p membrane transporters in Arabidopsis confers plant xenobiotic tolerance

Soil contamination is a major hindrance for plant growth and development. The lack of effective strategies to remove chemicals released into the environment has raised the need to increase plant resilience to soil pollutants. Here, we investigated the ability of two Saccharomyces cerevisiae plasma-membrane transporters, the Major Facilitator Superfamily (MFS) member Tpo1p and the ATP-Binding Cassette (ABC) protein Pdr5p, to confer Multiple Drug Resistance (MDR) in Arabidopsis thaliana. Transgenic plants expressing either of the yeast transporters were undistinguishable from the wild type under control conditions, but displayed tolerance when challenged with the herbicides 2,4-D and barban. Plants expressing ScTPO1 were also more resistant to the herbicides alachlor and metolachlor as well as to the fungicide mancozeb and the Co²⁺, Cu²⁺, Ni²⁺, Al³⁺ and Cd²⁺ cations, while ScPDR5-expressing plants exhibited tolerance to cycloheximide. Yeast mutants lacking Tpo1p or Pdr5p showed increased sensitivity to most of the agents tested in plants. Our results demonstrate that the S. cerevisiae Tpo1p and Pdr5p transporters are able to mediate resistance to a broad range of compounds of agricultural interest in yeast as well as in Arabidopsis, underscoring their potential in future biotechnological applications.

The ratio of Zn to Cd supply as a determinant of metal-homeostasis gene expression in tobacco and its modulation by overexpressing the metal exporter AtHMA4

This study links changes in the tobacco endogenous metal-homeostasis network caused by transgene expression with engineering of novel features. It also provides insight into the concentration-dependent mutual interactions between Zn and Cd, leading to differences in the metal partitioning between wild-type and transgenic plants. In tobacco, expression of the export protein AtHMA4 modified Zn/Cd root/shoot distribution, but the pattern depended on their concentrations in the medium. To address this phenomenon, the expression of genes identified by suppression subtractive hybridization and the Zn/Cd accumulation pattern were examined upon exposure to six variants of low/high Zn and Cd concentrations. Five tobacco metal-homeostasis genes were identified: NtZIP2, NtZIP4, NtIRT1-like, NtNAS, and NtVTL. In the wild type, their expression depended on combinations of low/high Zn and Cd concentrations; co-ordinated responses of NtZIP1, NtZIP2, and NtVTL were shown in medium containing 4 µM Cd, and at 0.5 µM versus 10 µM Zn. In transgenics, qualitative changes detected for NtZIP1, NtZIP4, NtIRT1-like, and NtVTL are considered crucial for modification of Zn/Cd supply-dependent Zn/Cd root/shoot distribution. Notwithstanding, NtVTL was the most responsive gene in wild-type and transgenic plants under all concentrations of Zn and Cd tested; thus it is a candidate gene for the regulation of metal cross-homeostasis processes involved in engineering new metal-related traits.

Quantitative aspects of inductively coupled plasma mass spectrometry

Accurate determination of elements in various kinds of samples is essential for many areas, including environmental science, medicine, as well as industry. Inductively coupled plasma mass spectrometry (ICP-MS) is a powerful tool enabling multi-elemental analysis of numerous matrices with high sensitivity and good precision. Various calibration approaches can be used to perform accurate quantitative measurements by ICP-MS. They include the use of pure standards, matrix-matched standards, or relevant certified reference materials, assuring traceability of the reported results. This review critically evaluates the advantages and limitations of different calibration approaches, which are used in quantitative analyses by ICP-MS. Examples of such analyses are provided.This article is part of the themed issue 'Quantitative mass spectrometry'.

Engineering high Zn in tomato shoots through expression of AtHMA4 involves tissue-specific modification of endogenous genes

Background

To increase the Zn level in shoots, AtHMA4 was ectopically expressed in tomato under the constitutive CaMV 35S promoter. However, the Zn concentration in the shoots of transgenic plants failed to increase at all tested Zn levels in the medium. Modification of Zn root/shoot distribution in tomato expressing 35S::AtHMA4 depended on the concentration of Zn in the medium, thus indicating involvement of unknown endogenous metal-homeostasis mechanisms. To determine these mechanisms, those metal-homeostasis genes that were expressed differently in transgenic and wild-type plants were identified by microarray and RT-qPCR analysis using laser-assisted microdissected RNA isolated from two root sectors: (epidermis + cortex and stele), and leaf sectors (upper epidermis + palisade parenchyma and lower epidermis + spongy parenchyma).

Results

Zn-supply-dependent modification of Zn root/shoot distribution in AtHMA4-tomato (increase at 5 μM Zn, no change at 0.5 μM Zn) involved tissue-specific, distinct from that in the wild type, expression of tomato endogenous genes. First, it is suggested that an ethylene-dependent pathway underlies the detected changes in Zn root/shoot partitioning, as it was induced in transgenic plants in a distinct way depending on Zn exposure. Upon exposure to 5 or 0.5 μM Zn, in the epidermis + cortex of the transgenics’ roots the expression of the Strategy I Fe-uptake system (ethylene-dependent LeIRT1 and LeFER) was respectively lower or higher than in the wild type and was accompanied by respectively lower or higher expression of the identified ethylene genes (LeNR, LeACO4, LeACO5) and of LeChln. Second, the contribution of LeNRAMP2 expression in the stele is shown to be distinct for wild-type and transgenic plants at both Zn exposures. Ethylene was also suggested as an important factor in a pathway induced in the leaves of transgenic plants by high Zn in the apoplast, which results in the initiation of loading of the excess Zn into the mesophyll of “Zn accumulating cells”.

Conclusions

In transgenic tomato plants, the export activity of ectopically expressed AtHMA4 changes the cellular Zn status, which induces coordinated tissue-specific responses of endogenous ethylene-related genes and metal transporters. These changes constitute an important mechanism involved in the generation of the metal-related phenotype of transgenic tomato expressing AtHMA4.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2990-x) contains supplementary material, which is available to authorized users.

Determination the Usefulness of AhHMA4p1::AhHMA4 Expression in Biofortification Strategies

AhHMA4 from Arabidopsis thaliana encodes Zn/Cd export protein that controls Zn/Cd translocation to shoots. The focus of this manuscript is the evaluation of AhHMA4 expression in tomato for mineral biofortification (more Zn and less Cd in shoots and fruits). Hydroponic and soil-based experiments were performed. Transgenic and wild-type plants were grown on two dilution levels of Knop's medium (1/10, 1/2) with or without Cd, to determine if mineral composition affects the pattern of root/shoot partitioning of both metals due to AhHMA4 expression. Facilitation of Zn translocation to shoots of 19-day-old transgenic tomato was noted only when plants were grown in the more diluted medium. Moreover, the expression pattern of Zn-Cd-Fe cross-homeostasis genes (LeIRT1, LeChln, LeNRAMP1) was changed in transgenics in a medium composition-dependent fashion. In plants grown in soil (with/without Cd) up to maturity, expression of AhHMA4 resulted in more efficient translocation of Zn to shoots and restriction of Cd. These results indicate the usefulness of AhHMA4 expression to improve the growth of tomato on low-Zn soil, also contaminated with Cd.

Genome-Wide Analysis and Heavy Metal-Induced Expression Profiling of the HMA Gene Family in Populus trichocarpa

The heavy metal ATPase (HMA) family plays an important role in transition metal transport in plants. However, this gene family has not been extensively studied in Populus trichocarpa. We identified 17 HMA genes in P. trichocarpa (PtHMAs), of which PtHMA1-PtHMA4 belonged to the zinc (Zn)/cobalt (Co)/cadmium (Cd)/lead (Pb) subgroup, and PtHMA5-PtHMA8 were members of the copper (Cu)/silver (Ag) subgroup. Most of the genes were localized to chromosomes I and III. Gene structure, gene chromosomal location, and synteny analyses of PtHMAs indicated that tandem and segmental duplications likely contributed to the expansion and evolution of the PtHMAs. Most of the HMA genes contained abiotic stress-related cis-elements. Tissue-specific expression of PtHMA genes showed that PtHMA1 and PtHMA4 had relatively high expression levels in the leaves, whereas Cu/Ag subgroup (PtHMA5.1- PtHMA8) genes were upregulated in the roots. High concentrations of Cu, Ag, Zn, Cd, Co, Pb, and Mn differentially regulated the expression of PtHMAs in various tissues. The preliminary results of the present study generated basic information on the HMA family of Populus that may serve as foundation for future functional studies.

Overexpression of the iron transporter NtPIC1 in tobacco mediates tolerance to cadmium

KEY MESSAGE

Overexpression of the iron transporter NtPIC1 increases iron concentration in shoots and reduces Cd uptake/accumulation in plants, mediating tolerance to cadmium. Cadmium (Cd) is toxic to plant cells and causes plants to display a typical iron (Fe) deficiency phenotype. NtPIC1 (Permease In Chloroplast1) is an Fe transporter protein in tobacco, required for Fe homeostasis. Based on preliminary results in transformed Saccharomyces cerevisiae BY4741 cells, which showed that NtPIC1 expression increased Cd tolerance, this study evaluated Cd tolerance in tobacco plants overexpressing NtPIC1 (NtPIC1-OE). We show that these plants have longer roots and higher fresh weights than wild-type (WT) plants after Cd exposure. Under Cd stress, WT plants display more chlorosis, stronger growth inhibition, and lower chlorophyll concentrations than NtPIC1-OE plants. Importantly, NtPIC1-OE plants had higher Fe concentrations in shoots and lower Fe concentrations in roots, and Cd concentrations in NtPIC1-OE plants were significantly lower compared to those in WT plants. Moreover, Fe transport-related genes (NtPIC1, NtNRAMP1, and NtFER1) were upregulated in NtPIC1-OE plants, while Fe deficiency-related genes (NtFRO1, NtIRT1, and NtZIP1) that mediate Cd uptake were downregulated. We also found that the activities of several antioxidative enzymes were significantly higher in NtPIC1-OE plants than in WT plants under Cd stress. Overall, our results demonstrate that overexpression of NtPIC1 is an efficient way to increase shoot Fe concentrations and reduce Cd uptake/accumulation in plants.

Cation transporters/channels in plants: Tools for nutrient biofortification

Cation transporters/channels are key players in a wide range of physiological functions in plants, including cell signaling, osmoregulation, plant nutrition and metal tolerance. The recent identification of genes encoding some of these transport systems has allowed new studies toward further understanding of their integrated roles in plant. This review summarizes recent discoveries regarding the function and regulation of the multiple systems involved in cation transport in plant cells. The role of membrane transport in the uptake, distribution and accumulation of cations in plant tissues, cell types and subcellular compartments is described. We also discuss how the knowledge of inter- and intra-species variation in cation uptake, transport and accumulation as well as the molecular mechanisms responsible for these processes can be used to increase nutrient phytoavailability and nutrients accumulation in the edible tissues of plants. The main trends for future research in the field of biofortification are proposed.

Target or barrier? The cell wall of early- and later-diverging plants vs cadmium toxicity: differences in the response mechanisms

Increasing industrialization and urbanization result in emission of pollutants in the environment including toxic heavy metals, as cadmium and lead. Among the different heavy metals contaminating the environment, cadmium raises great concern, as it is ecotoxic and as such can heavily impact ecosystems. The cell wall is the first structure of plant cells to come in contact with heavy metals. Its composition, characterized by proteins, polysaccharides and in some instances lignin and other phenolic compounds, confers the ability to bind non-covalently and/or covalently heavy metals via functional groups. A strong body of evidence in the literature has shown the role of the cell wall in heavy metal response: it sequesters heavy metals, but at the same time its synthesis and composition can be severely affected. The present review analyzes the dual property of plant cell walls, i.e., barrier and target of heavy metals, by taking Cd toxicity as example. Following a summary of the known physiological and biochemical responses of plants to Cd, the review compares the wall-related mechanisms in early- and later-diverging land plants, by considering the diversity in cell wall composition. By doing so, common as well as unique response mechanisms to metal/cadmium toxicity are identified among plant phyla and discussed. After discussing the role of hyperaccumulators' cell walls as a particular case, the review concludes by considering important aspects for plant engineering.