Allelic Variation of NtNramp5 Associated with Cultivar Variation in Cadmium Accumulation in Tobacco

Comprehensive transcriptome, physiological and biochemical analyses reveal that key role of transcription factor WRKY and plant hormone in responding cadmium stress

Cadmium (Cd) is readily absorbed by tobacco and accumulates in the human body through smoke inhalation, posing threat to human health. While there have been many studies on the negative impact of cadmium in tobacco on human health, the specific adaptive mechanism of tobacco roots to cadmium stress is not well understood. In order to comprehensively investigate the effects of Cd stress on the root system of tobacco, the combination of transcriptomic, biochemical, and physiological methods was utilized. In this study, tobacco growth was significantly inhibited by 50 μM of Cd, which was mainly attributed to the destruction of root cellular structure. By comparing the transcriptome between CK and Cd treatment, there were 3232 up-regulated deferentially expressed genes (DEGs) and 3278 down-regulated DEGs. The obvious differential expression of genes related to the nitrogen metabolism, metal transporters and the transcription factors families. In order to mitigate the harmful effects of Cd, the root system enhances Cd accumulation in the cell wall, thereby reducing the Cd content in the cytoplasm. This result may be mediated by plant hormones and transcription factor (TF). Correlational statistical analysis revealed significant negative correlations between IAA and GA with cadmium accumulation, indicated by correlation coefficients of -0.91 and -0.93, respectively. Conversely, ABA exhibited a positive correlation with a coefficient of 0.96. In addition, it was anticipated that 3 WRKY TFs would lead to a reduction in Cd accumulation. Our research provides a theoretical basis for the systematic study of the specific physiological processes of plant roots under Cd stress.

Variety-dependent responses of common tobacco with differential cadmium resistance: Cadmium uptake and distribution, antioxidative activity, and gene expression

Cadmium (Cd), which accumulates in tobacco leaves, enters the human body through inhalation of smoke, causing harmful effects on health. Therefore, identifying the pivotal factors that govern the absorption and resistance of Cd in tobacco is crucial for mitigating the harmful impact of Cd. In the present study, four different Cd-sensitive varieties, namely, ZhongChuan208 (ZC) with resistance, ZhongYan100 (ZY), K326 with moderate resistance, and YunYan87 (YY) with sensitivity, were cultivated in hydroponic with different Cd concentrations (20 µM, 40 µM, 60 µM and 80 µM). The results indicated that plant growth was significantly decreased by Cd. Irrespective of the Cd concentration, ZC exhibited the highest biomass, while YY had the lowest biomass; ZY and K326 showed intermediate levels. Enzymatic (APX, CAT, POD) and nonenzymatic antioxidant (Pro, GSH) systems showed notable variations among varieties. The multifactor analysis suggested that the ZC and ZY varieties, with higher levels of Pro and GSH content, contribute to a decrease in the levels of MDA and ROS. Among all the Cd concentrations, ZC exhibited the lowest Cd accumulation, while YY showed the highest. Additionally, there were significant differences observed in Cd distribution and translocation factors among the four different varieties. In terms of Cd distribution, cell wall Cd accounted for the highest proportion of total Cd, and organelles had the lowest proportion. Among the varieties, ZC showed lower Cd levels in the cell wall, soluble fraction, and organelles. Conversely, YY exhibited the highest Cd accumulation in all tissues; K326 and ZY had intermediate levels. Translocation factors (TF) varied among the varieties under Cd stress, with ZC and ZY showing lower TF compared to YY and K326. This phenomenon mainly attributed to regulation of the NtNramp3 and NtNramp5 genes, which are responsible for the absorption and transport of Cd. This study provides a theoretical foundation for the selection and breeding of tobacco varieties that are resistant to or accumulate less Cd.

NtARF11 positively regulates cadmium tolerance in tobacco by inhibiting expression of the nitrate transporter NtNRT1.1

Heavy metal cadmium (Cd) is widespread in contaminated soil and an important factor limiting plant growth. NO3- (nitrate) affects Cd uptake and thus changes Cd tolerance in plants; however, the underlying molecular regulatory mechanisms have not yet been elucidated. Here, we analyzed a novel gene, NtARF11 (auxin response factor), which regulates Cd tolerance in tobacco via the NO3- uptake pathway, through experiments with NtARF11-knockout and NtARF11-overexpression transgenic tobacco lines. NtARF11 was highly expressed under Cd stress in tobacco plants. Under Cd stress, overexpression of NtARF11 enhanced Cd tolerance in tobacco compared to that in wild-type tobacco, as shown by the low Cd concentration, high chlorophyll concentration, and low accumulation of reactive oxygen species in NtARF11-overexpressing tobacco. Moreover, low NO3- concentrations were observed in NtARF11-overexpressing tobacco plants. Further analyses revealed direct binding of NtARF11 to the promoter of the nitrate transporter NtNRT1.1, thereby negatively regulating its expression in tobacco. Notably, NtNRT1.1 knockout reduced NO3- uptake, which resulted in low Cd concentrations in tobacco. Altogether, these results demonstrate that the NtARF11-NtNRT1.1 module functions as a positive regulator of Cd tolerance by reducing the Cd uptake in tobacco, providing new insights for improving Cd tolerance of plants through genetic engineering.

The Uptake, Transfer, and Detoxification of Cadmium in Plants and Its Exogenous Effects

Cadmium (Cd) exerts a toxic influence on numerous crucial growth and development processes in plants, notably affecting seed germination rate, transpiration rate, chlorophyll content, and biomass. While considerable advances in Cd uptake and detoxification of plants have been made, the mechanisms by which plants adapt to and tolerate Cd toxicity remain elusive. This review focuses on the relationship between Cd and plants and the prospects for phytoremediation of Cd pollution. We highlight the following issues: (1) the present state of Cd pollution and its associated hazards, encompassing the sources and distribution of Cd and the risks posed to human health; (2) the mechanisms underlying the uptake and transport of Cd, including the physiological processes associated with the uptake, translocation, and detoxification of Cd, as well as the pertinent gene families implicated in these processes; (3) the detrimental effects of Cd on plants and the mechanisms of detoxification, such as the activation of resistance genes, root chelation, vacuolar compartmentalization, the activation of antioxidant systems and the generation of non-enzymatic antioxidants; (4) the practical application of phytoremediation and the impact of incorporating exogenous substances on the Cd tolerance of plants.

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.

NRAMPs and manganese: Magic keys to reduce cadmium toxicity and accumulation in plants

Cadmium (Cd), a toxic heavy metal, poses significant threats to both crop production and human health worldwide. Manganese (Mn), an essential micronutrient, plays a crucial role in plant growth and development. NRAMPs (Natural Resistance-Associated Macrophage Proteins) function as common transporters for both Cd and Mn. Deep understanding of the regulatory mechanisms governing NRAMP-mediated Cd and Mn transport is imperative for developing the crop varieties with high tolerance and low accumulation of Cd. This review reported the advance in studies on the fundamental properties and classification of NRAMPs in plants, and structural characteristics, expression patterns, and diverse functions of NRAMP genes across different plant species. We highlighted the pivotal role of NRAMPs in Cd/Mn uptake and transport in plants as a common transporter. Finally, we also comprehensively discussed over the strategies for reducing Cd uptake and accumulation in plants through using antagonism of Mn over Cd and altering the expression of NRAMP genes.

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.

NRAMP6c plays a key role in plant cadmium accumulation and resistance in tobacco (Nicotiana tabacum L.)

Tobacco plants (Nicotiana tabacum L.) exhibit considerable potential for phytoremediation of soil cadmium (Cd) pollutants, owing to their substantial biomass and efficient metal accumulation capabilities. The reduction of Cd accumulation in tobacco holds promise for minimizing Cd intake in individuals exposed to cigar smoking. NRAMP transporters are pivotal in the processes of Cd accumulation and resistance in plants; however, limited research has explored the functions of NRAMPs in tobacco plants. In this investigation, we focused on NtNRAMP6c, one of the three homologs of NRAMP6 in tobacco. We observed a robust induction of NtNRAMP6c expression in response to both Cd toxicity and iron (Fe) deficiency, with the highest expression levels detected in the roots. Subsequent subcellular localization and heterologous expression analyses disclosed that NtNRAMP6c functions as a plasma membrane-localized Cd transporter. Moreover, its overexpression significantly heightened the sensitivity of yeast cells to Cd toxicity. Through CRISPR-Cas9-mediated knockout of NtNRAMP6c, we achieved a reduction in Cd accumulation and an enhancement in Cd resistance in tobacco plants. Comparative transcriptomic analysis unveiled substantial alterations in the transcriptional profiles of genes associated with metal ion transport, photosynthesis, and macromolecule catabolism upon NtNRAMP6c knockout. Furthermore, our study employed plant metabolomics and rhizosphere metagenomics to demonstrate that NtNRAMP6c knockout led to changes in phytohormone homeostasis, as well as shifts in the composition and abundance of microbial communities. These findings bear significant biological implications for the utilization of tobacco in phytoremediation strategies targeting Cd pollutants in contaminated soils, and concurrently, in mitigating Cd accumulation in tobacco production destined for cigar consumption.

IlNRAMP5 is required for cadmium accumulation and the growth in Iris lactea under cadmium exposures

Iris lactea is potentially applied for remediating Cd-contaminated soils due to the strong ability of Cd uptake and accumulation. However, its molecular mechanism underlying Cd uptake pathway remains unknown. Here, we report a member of NRAMP (Natural Resistance-Associated Macrophage Protein) family, IlNRAMP5, is involved in Cd/Mn uptake and the growth in I. lactea response to Cd. IlNRAMP5 was localized onto the plasma membrane, and was induced by Cd. It was expressed in the root cortex rather than the central vasculature, and in leaf vascular bundle and mesophyll cells. Heterologous expression in yeast showed that IlNRAMP5 could transport Cd and Mn, but not Fe. Knockdown of IlNRAMP5 triggered a significant reduction in Cd uptake, further diminishing the accumulation of Cd. In addition, silencing IlNRAMP5 disrupted Mn homeostasis by lowering Mn uptake and Mn allocation, accompanied by remarkably inhibiting photosynthesis under Cd conditions. Overall, the findings suggest that IlNRAMP5 plays versatile roles in Cd accumulation by mediating Cd uptake, and contributes to maintain the growth via modulating Mn homeostasis in I. lactea under Cd exposures. This would provide a mechanistic understanding Cd phytoremediation efficiency in planta.

A comprehensive review on environmental pollutants and osteoporosis: Insights into molecular pathways

A significant problem that has an impact on community wellbeing is environmental pollution. Environmental pollution due to air, water, or soil pollutants might pose a severe risk to global health, necessitating intense scientific effort. Osteoporosis is a common chronic condition with substantial clinical implications on mortality, morbidity, and quality of life. It is closely linked to bone fractures. Worldwide, osteoporosis affects around 200 million people, and every year, there are almost 9 million fractures. There is evidence that certain environmental factors may increase the risk of osteoporosis in addition to traditional risk factors. It is crucial to understand the molecular mechanisms at play because there is a connection between osteoporosis and exposure to environmental pollutants such as heavy metals, air pollutants, endocrine disruptors, metal ions and trace elements. Hence, in this scoping review, we explore potential explanations for the link between pollutants and bone deterioration through deep insights into molecular pathways. Understanding and recognizing these pollutants as modifiable risk factors for osteoporosis would possibly help to enhance environmental policy thereby aiding in the improvement of bone health and improving patient quality of life.

Hydrogel-potassium humate composite alleviates cadmium toxicity of tobacco by regulating Cd bioavailability

Cadmium (Cd) removal from soil to reduce Cd accumulation in plants is essential for agroecology, food safety, and human health. Cd enters plants from soil and affects plant growth and development. Hydrogels can easily combine with Cd, thereby altering its bioavailability in soil. However, few studies have evaluated the effects of hydrogel on the complex phytotoxicity caused by Cd uptake in plants and the microbial community structure. Herein, a new poly (acrylic acid)-grafted starch and potassium humate composite (S/K/AA) hydrogel was added to soil to evaluate its impact on tobacco growth and the soil microenvironment. The results indicate that the addition of S/K/AA hydrogel can significantly improve the biomass, chlorophyll (Chl) content, and photosynthetic capacity of tobacco plants during Cd stress conditions, and decrease Cd concentration, probably by affecting Cd absorption through the expression of Cd absorption transporters (e.g., NRAMP5, NRAMP3, and IRT1). Moreover, the application of S/K/AA hydrogel not only reduced the accumulation of reactive oxygen species (ROS), but also reduced the antioxidant activities of peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT), suggesting that S/K/AA hydrogel alleviates Cd toxicity via a non-antioxidant pathway. Notably, we further analyzed the effectiveness of the hydrogel on microbial communities in Cd-contaminated soil and found that it increased the Cd-tolerant microbial community (Arthrobacter, Massilia, Streptomyces), enhancing the remediation ability of Cd-contaminated soil and helping tobacco plants to alleviate Cd toxicity. Overall, our study provides primary insights into how S/K/AA hydrogel affects Cd bioavailability and alleviates Cd toxicity in plants.

Two novel transporters NtNRAMP6a and NtNRAMP6b are involved in cadmium transport in tobacco (Nicotiana tabacum L.)

Plant natural resistance-associated macrophage protein (NRAMP) plays important roles in metal transport and tolerance. Tobacco is a typical cadmium (Cd) accumulator, while research on NRAMP in tobacco has been limited. In the current study, two novel NRAMP genes (NtNRAMP6a and NtNRAMP6b) were identified from the allotetraploid plant Nicotiana tabacum L. Real time‒PCR and GUS (β-glucuronidase) staining results showed that the two genes were expressed in roots, stems, leaves and flowers and induced by Cd stress. Subcellular localization revealed that they were located in the plasma membrane. Heterologously expressed NtNRAMP6a and NtNRAMP6b significantly increased the Cd sensitivity of the Δycf1 mutant, indicating that NtNRAMP6a and NtNRAMP6b had Cd transport functions in yeast. The difference in the manganese (Mn) transport activity of the two genes was demonstrated by point mutation, which was caused by the difference in the 18th amino acid. NRAMP6-N18K is a new key active site for manganese transport. After 50 μM Cd treatment for 7 days, the contents of Cd and Mn of the ntnramp6a/6b mutants was significantly lower than those of wild type in shoots, while the contents in roots were higher. Additionally, the mutant lines showed higher chorphyll contentration and lighter leaf damage. Knockout of NtNRAMP6a and NtNRAMP6b reduced Cd and Mn accumulation in tobacco shoots by influence root-to-shoot translocation. This provides new idea for cultivating tobacco varieties with low cadmium accumulation and high cadmium tolerance.

Application of oyster shell powder reduces cadmium accumulation by inhibiting the expression of genes responsible for cadmium uptake and translocation in rice

The application of waste oyster shell in agriculture is of extensive concern due to its benefits on improving yields and inhibiting cadmium (Cd) accumulation in edible parts of crops. However, the underlying mechanisms responsible for oyster shell powder (OSP) that decreases Cd accumulation in crops remain poorly understood. This study explored the effects of OSP on growth and Cd accumulation in rice via pot experiments and hydroponics. Pot experiments showed that the application of 1 g·kg-1 OSP improved rice yields and decreased Cd concentrations in all tissues of rice, especially in grains, which was reduced by 43.5%. The pH was increased and the phytoavailability of Cd in soil was reduced by OSP supplementation. In addition, OSP also exhibited high dissolution of Ca, Fe, Zn, and Se. In hydroponics, OSP supply also suppressed Cd accumulation in rice and increased plant growth. Pretreatment with OSP inhibited the accumulation of Cd in the roots and shoots. Simultaneously, OSP reduced the content of Cd in the root cell sap, cell wall, and xylem sap, and downregulated the expression of OsNramp5, OsNramp1, OsIRT1, and OsHMA2. These findings suggested that the application of OSP could reduce Cd accumulation by inhibiting the expression of genes responsible for Cd absorption and xylem loading in rice.

Effects of cadmium on transcription, physiology, and ultrastructure of two tobacco cultivars

Cadmium (Cd) is one of the most toxic heavy metal pollutants worldwide. Tobacco is an important cash crop; however, the accumulation of Cd in its biomass is very high. Cadmium may enter the body of smokers with contaminated tobacco and the surrounding environment via smoke. Therefore, it is important to understand the mechanisms of Cd accumulation and tolerance in tobacco plants, especially in the leaves. In this study, the effects of Cd on the growth, accumulation, and biochemical indices of two tobacco varieties, K326 (Cd resistant) and NC55 (Cd sensitive), were studied through transcriptomic and physiological experiments. Transcriptome and physiological analyses showed differences in the expression of Cd transport and Cd resistance related genes between NC55 and K326 under Cd stress. The root meristem cells of NC55 were more severely damaged. The antioxidant enzyme activity, ABA and ZT content, chlorophyll content, photosynthetic rate, and nitrogen metabolism enzyme activity in K326 leaves were higher than in NC55. These data elucidate the mechanisms of low Cd accumulation and high Cd tolerance in K326 leaves and provide a theoretical basis for cultivating tobacco varieties with low Cd accumulation and high Cd resistance.

Comparing cadmium uptake kinetics, xylem translocation, chemical forms, and subcellular distribution of two tobacco (Nicotiana tabacum L.) cultivars

Tobacco (Nicotiana tabacum L.) is a potential phytoremediator that can reduce soil cadmium (Cd) contamination. Pot and hydroponic experiments were conducted to investigate the difference in absorption kinetics, translocation patterns, accumulation capacity, and extraction amounts between two leading tobacco cultivars in China. We studied the chemical forms and subcellular distribution of Cd in the plants to understand the diversity of the detoxification mechanism of the cultivars. The concentration-dependent kinetics of Cd accumulation in leaves, stems, roots, and xylem sap for cultivars Zhongyan 100 (ZY100) and K326, fitted well with the Michaelis-Menten equation. K326 exhibited high biomass, Cd tolerance, Cd translocation, and phytoextraction abilities. The acetic acid, sodium chloride, and water-extractable fractions accounted for > 90% of Cd in all ZY100 tissues but only in K326 roots and stems. Moreover, the acetic acid and NaCl fractions were the predominant storage forms, while the water fraction was the transport form. The ethanol fraction also contributed significantly to Cd storage in K326 leaves. As the Cd treatment increased, more NaCl and water fractions were found in K326 leaves, while only NaCl fractions increased in ZY100 leaves. For subcellular distribution, > 93% Cd proportions were primarily stored in both cultivars' soluble or cell wall fraction. The proportion of Cd in the cell wall fraction of ZY100 roots was less than that of K326, while that proportion in the soluble fraction in ZY100 leaves was higher than in K326 leaves. These findings demonstrate that Cd accumulation patterns, detoxification, and storage strategies differ between the cultivars, providing a deeper understanding of Cd tolerance and accumulation mechanism in tobacco plants. It also guides the screening of germplasm resources or gene modification to improve the Cd phytoextraction efficiency of tobacco.

Tobacco as an efficient metal accumulator

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

Mutation of NtNRAMP3 improves cadmium tolerance and its accumulation in tobacco leaves by regulating the subcellular distribution of cadmium

Cadmium (Cd) is a harmful element that affects plant growth and development. Genetic improvements could be applied for enhancing Cd tolerance and accumulation in plants. Here, a novel Cd stress-induced gene, NtNRAMP3, was identified in tobacco. We constructed two NtNRAMP3-knockout (KO) tobacco lines using the CRISPR/Cas9 system, which enhanced Cd tolerance and Cd accumulation in tobacco leaves compared with those in the wildtype (WT). Subcellular localization analysis suggested that NtNRAMP3 is a tonoplast protein and GUS (β-glucuronidase) histochemical analysis showed that NtNRAMP3 is highly expressed in the conductive tissue of leaves. NtNRAMP3-KO tobacco showed reduced Cd translation from vacuole to cytosol in leaves compared with the WT, and its vacuolar Cd concentration was significantly higher (20.78-22.81%) than that in the WT; in contrast, Cd concentration in the cytosol was reduced by 13.72-20.15%, preventing chlorophyll degradation and reducing reactive oxygen species accumulation in the leaves. Our findings demonstrate that NtNRAMP3 is involved in regulating Cd subcellular distribution (controlling Cd transport from vacuoles to the cytosol) and affects Cd tolerance and its accumulation in tobacco. This provides a key candidate gene to improve the phytoremediation efficiency of plants via genetic engineering.

The NtNRAMP1 transporter is involved in cadmium and iron transport in tobacco (Nicotiana tabacum)

Plant natural resistance-associated macrophage protein (NRAMP) plays an important role in maintaining intracellular metal homeostasis and coping with environmental heavy metal stress. Until now, studies on NRAMP in tobacco have been limited. In this study, NtNRAMP1 was cloned from tobacco cultivar TN90, and the highest expression level was observed in the roots, which was strongly induced by Fe deficiency. Heterologously expressed NtNRAMP1 significantly increased the Cd sensitivity of the yeast Δycf1 mutant. Three overexpressed NtNRAMP1 lines were generated to reveal the biofunction of NtNRAMP1. In the soil pot experiments under natural conditions, the contents of Fe and total chlorophyll were increased in the leaves of transgenic tobacco compared with the WT. To reveal the characteristics of NtNRAMP1 in metal transport, transgenic plants were cultured in hydroponic solution with 50 μM Cd and 200 μM Fe. Compared with the WT, the Cd concentrations in transgenic plants increased by 1.26-2.02-fold in the roots. Interestingly, the Cd content in the shoots of transgenic plants was slightly reduced compared with that of the WT. Overexpression of NtNRAMP1 did not promote Fe uptake from the external environment into the roots but enhanced the transfer of Fe from the roots to shoots. Additionally, Fe overload in the leaves of transgenic tobacco resulted in increased levels of MDA and H2O2 while Fe toxicity may be relieved by POD. In conclusion, overexpression of NtNRAMP1 in tobacco could promote Cd uptake and Fe transport from the roots to shoots while disturbing Fe homeostasis in the leaves of transgenic tobacco.

Evaluation of the cadmium phytoextraction potential of tobacco (Nicotiana tabacum) and rhizosphere micro-characteristics under different cadmium levels

In this study, a field-scale and pot experiment were performed to evaluate the remedial efficiency of Cd contaminated soil by tobacco and explore rhizosphere micro-characteristics under different cadmium levels, respectively. The results indicated that tobacco could remove 12.9 % of Cd from soil within a short growing period of 80 d. The pot experiment revealed that tobacco could tolerate soil Cd concentrations up to 5.8 mg kg-1 and bioaccumulate 68.1 and 40.8 mg kg-1 Cd in shoots and roots, respectively. The high Cd bioaccumulation in tobacco might be attributed to strong acidification in the rhizosphere soil and the increase in Cd bioavailability. Rhizobacteria did not appear to be involved in Cd mobilization. In contrast, tobacco tended to enrich sulfate-reducing bacteria (such as Desulfarculaceae) under high Cd treatment (5.8 mg kg-1) but enrich plant growth-promoting bacteria (such as Bacillus, Dyadobacter, Virgibacillus and Lysobacter) to improve growth under low Cd treatment (0.2 mg kg-1), suggesting that tobacco employed different microbes for responding to Cd stress. Our results demonstrate the advantages of using tobacco for bioremediating Cd contaminated soil and clarify the rhizosphere mechanisms underlying Cd mobilization and tolerance.

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.

Genetic system underlying responses of Cryptococcus neoformans to cadmium

Cryptococcus neoformans, basidiomycetous pathogenic yeast, is basically an environmental fungus and, therefore, challenged by ever changing environments. In this study, we focused on how C. neoformans responds to stress caused by cadmium that is one of high-risk pollutants. By tracking phenotypes of the resistance or sensitivity to cadmium, we undertook forward and reverse genetic studies to identify genes involved in cadmium metabolism in C. neoformans. We found that the main route of Cd²⁺ influx is through Mn²⁺ ion transporter, Smf1, which is an ortholog of Nramp (natural resistance-associated macrophage protein 1) of mouse. We found that serotype A strains are generally more resistant to cadmium than serotype D strains and that cadmium resistance of H99, a representative of serotype A strains, was found to be due to a partial defect in SMF1. We found that calcium channel has a subsidiary role for cadmium uptake. We also showed that Pca1 (P-type-ATPase) functions as an extrusion pump for cadmium. We examined the effects of some metals on cadmium toxicity and suggested (i) that Ca²⁺ and Zn²⁺ could exert their protective function against Cd²⁺ via restoring cadmium-inhibited cellular processes and (ii) that Mg²⁺ and Mn²⁺ could have antagonistic roles in an unknown Smf1-independent Cd²⁺ uptake system. We proposed a model for Cd2⁺-response of C. neoformans, which will serve as a platform for understanding how this organism copes with the toxic metal.

Comparative root transcriptome analysis of two soybean cultivars with different cadmium sensitivities reveals the underlying tolerance mechanisms

Soybean can provide rich protein and fat and has great economic value worldwide. Cadmium (Cd) is a toxic heavy metal to organisms. It can accumulate in plants and be transmitted to the human body via the food chain. Cd is a serious threat to soybean development, particularly root growth. Some soybean cultivars present tolerant symptoms under Cd stress; however, the potential mechanisms are not fully understood. Here, we optimized RNA-seq to identify the differentially expressed genes (DEGs) in Cd-sensitive (KUAI) and Cd-tolerant (KAIYU) soybean roots and compared the DEGs between KAIYU and KUAI. A total of 1506 and 1870 DEGs were identified in the roots of KUAI and KAIYU, respectively. Through Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, and gene function analyses, we found that genes related to antioxidants and sequestration were responsible for Cd tolerance in KAIYU. In addition, overexpression of Glyma11g02661, which encodes a heavy metal-transporting ATPase, significantly improved Cd tolerance in transgenic hairy roots. These results provide a preliminary understanding of the tolerance mechanisms in response to Cd stress in soybean root development and are of great importance in developing Cd-resistant soybean cultivars by using the identified DEGs through genetic modification.

Comparative transcriptome combined with biochemical and physiological analyses provide new insights toward cadmium accumulation with two contrasting Nicotiana species

Cadmium (Cd) is known as one of the most hazardous elements in the environment and a persistent soil constraint toxic to all flora and fauna. In this study, we conducted physiological, biochemical, and transcriptomic analyses of Nicotiana rustica (N. rustica) and Nicotiana tabacum (N. tabacum) treated with CdCl2 to know the underlying molecular mechanisms of Cd accumulation. As a result, N. rustica had more dry weight than N. tabacum. Additionally, N. rustica accumulated higher Cd concentration (69.65 times), Cd2+ influx (1.32-fold), glutathione S-transferases (GST) enzyme activity (2.54 times), GSH/GSSG (oxidized form of GSH) ratio, increase of superoxide dismutase and CAT and a lower H2 O2 and superoxide (O2 •- ) accumulation in their roots than N. tabacum. Cd mainly distributed in the cytoplasm of both species and N. rustica had a significant proportion in the cell wall. Furthermore, the transcriptomic analysis revealed 173 and 710 differentially expressed genes (DEGs) between control and Cd-stressed plants in the leaves and roots of N. rustica, while 576 and 1543 DEGs were found in the leaves and roots of N. tabacum, respectively. In N. rustica, phenylpropanoid biosynthesis and phenylalanine metabolism were the most enriched pathways, while GSH metabolism, ATP-binding cassette transporters and phenylpropanoid biosynthesis were the most enriched in N. tabacum. Finally, we found that DEGs related to metal influx, sequestration, remobilization, and chelation were responsible for Cd accumulation. These results indicated that N. rustica accumulated higher Cd content than N. tabacum, suggesting that each species utilized different response mechanism under the same Cd stress conditions. The DEGs identified in this study might lead to the identification of genes or pathways related to Cd regulation. This study identifies important regulators related to Cd accumulation.

Research on the Mechanisms of Plant Enrichment and Detoxification of Cadmium

The heavy metal cadmium (Cd), as one of the major environmentally toxic pollutants, has serious impacts on the growth, development, and physiological functions of plants and animals, leading to deterioration of environmental quality and threats to human health. Research on how plants absorb and transport Cd, as well as its enrichment and detoxification mechanisms, is of great significance to the development of phytoremediation technologies for ecological and environmental management. This article summarises the research progress on the enrichment of heavy metal cadmium in plants in recent years, including the uptake, transport, and accumulation of Cd in plants. The role of plant roots, compartmentalisation, chelation, antioxidation, stress, and osmotic adjustment in the process of plant Cd enrichment are discussed. Finally, problems are proposed to provide a more comprehensive theoretical basis for the further application of phytoremediation technology in the field of heavy metal pollution.

OsNRAMP1 transporter contributes to cadmium and manganese uptake in rice

Rice is a major dietary source of the toxic metal, cadmium (Cd). Previous studies reported that the rice transporter, OsNRAMP1, (Natural resistance-associated macrophage protein 1) could transport iron (Fe), Cd and arsenic (As) in heterologous yeast assays. However, the in planta function of OsNRAMP1 remains unknown. Here, we showed that OsNRAMP1 was able to transport Cd and manganese (Mn) when expressed in yeast, but did not transport Fe or As. OsNRAMP1 was mainly expressed in roots and leaves and encoded a plasma membrane-localized protein. OsNRAMP1 expression was induced by Cd treatment and Fe deficiency. Immunostaining showed that OsNRAMP1 was localized in all root cells, except the central vasculature, and in leaf mesophyll cells. The knockout of OsNRAMP1 resulted in significant decreases in root uptake of Cd and Mn and their accumulation in rice shoots and grains, and increased sensitivity to Mn deficiency. The knockout of OsNRAMP1 had smaller effects on Cd and Mn uptake than knockout of OsNRAMP5, while knockout of both genes resulted in large decreases in the uptake of the two metals. Taken together, OsNRAMP1 contributes significantly to the uptake of Mn and Cd in rice, and the functions of OsNRAMP1 and OsNRAMP5 are similar but not redundant.

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.

Safer food through plant science: reducing toxic element accumulation in crops

Natural processes and human activities have caused widespread background contamination with non-essential toxic elements. The uptake and accumulation of cadmium (Cd), arsenic (As), and lead (Pb) by crop plants results in chronic dietary exposure and is associated with various health risks. Current human intake levels are close to what is provisionally regarded as safe. This has recently triggered legislative actions to introduce or lower limits for toxic elements in food. Arguably, the most effective way to reduce the risk of slow poisoning is the breeding of crops with much lower accumulation of contaminants. The past years have seen tremendous progress in elucidating molecular mechanisms of toxic element transport. This was achieved in the model systems Arabidopsis thaliana and, most importantly, rice, the major source of exposure to As and Cd for a large fraction of the global population. Many components of entry and sequestration pathways have been identified. This knowledge can now be applied to engineer crops with reduced toxic element accumulation especially in edible organs. Most obvious in the case of Cd, it appears likely that subtle genetic intervention has the potential to reduce human exposure to non-essential toxic elements almost immediately. This review outlines the risks and discusses our current state of knowledge with emphasis on transgenic and gene editing approaches.

MhMAPK4 from Malus hupehensis Rehd. decreases cell death in tobacco roots by controlling Cd2+ uptake

Cadmium (Cd) induces cell death in plant roots. Mitogen-activated protein kinase (MAPK) plays a role in the regulation of cell death induced by Cd in plant roots. In this study, MhMAPK4 was isolated from the roots of Malus hupehensis. Subcellular localization showed that the MhMAPK4 protein was located in the cell membrane and cytoplasm and is a transmembrane protein that is characterized by hydrophily. The expression of MhMAPK4 in the roots of M. hupehensis was up-regulated by Cd sulfate and Cd chloride. Phenotypic comparison under Cd stress showed that the growth of wild-type (WT) tobacco was lower than the transgenic lines overexpressing MhMAPK4. The fresh weight and the root length of WT also was lower than that of the transgenic tobacco. The net Cd2+ influx in the tobacco roots was decreased by the overexpression of MhMAPK4, as was root Cd accumulation. The recovery time of the Cd2+ influx to stable state in the transgenic tobacco was also shorter than the WT. The expression of iron-regulated transporter 1 (NtIRT1) and natural resistance associated macrophage protein 5 (NtNRAMP5) was relatively low in the transgenic lines under Cd stress. Cell death and apoptosis in the tobacco roots was reduced following the overexpression of MhMAPK4. The activity of vacuolar processing enzyme (VPE) and the transcript level of VPE in the transgenic tobacco was lower than that of WT under Cd stress. In addition, the electrolyte leakage and malondialdehyde and hydrogen peroxide contents in the transgenic tobacco were lower than those of WT, whereas the antioxidant enzyme activity and expression were higher. These results suggest that MhMAPK4 regulates Cd accumulation by mediating Cd2+ uptake by the roots, and controls Cd-caused cell death by adjusting VPE activity.

Comparison of heavy metal accumulation and cadmium phytoextraction rates among ten leading tobacco (Nicotiana tabacum L.) cultivars in China

Cadmium (Cd) contamination is one of the most serious global environmental problems, and phytoremediation, which uses Cd-accumulator plants, is potentially one of the sustainable solutions. Pot experiments with natural and Cd-amended soils were conducted to investigate the accumulation of heavy metals in 10 leading cultivars of tobacco in China. The extraction ability and profiles of Cd accumulation among plant organs were also analyzed. The tobacco roots accumulated cobalt, nickel, and Cd, while the leaf highly bioaccumulated Cd and lowly accumulated zinc, selenium and mercury. The transport from the tobacco stem to the leaf plays a critical role in the accumulation of these elements. The ratios of Cd concentration in the leaves at lower, middle and upper positions were comparatively stable. The high Cd-extracting cultivars were "Hongda", "NC89" and "Zhongyan 100" when grown in normal soils, "CuiBi 1" and "Hongda" in moderately contaminated soils, and "YuYan 87", "LongJiang 851" and "K326" in severely contaminated soils. Tobacco leaves could accumulate about 80% of the total Cd extracted from the soil by the plant. Considering the Cd-extraction limitations exhibited by leading tobacco cultivars, screening of germplasm resources for high or low levels of Cd-accumulation is still an important target for the future.

Evolution, and functional analysis of Natural Resistance-Associated Macrophage Proteins (NRAMPs) from Theobroma cacao and their role in cadmium accumulation

The presence of the toxic metal cadmium (Cd²⁺) in certain foodstuffs is recognised as a global problem, and there is increasing legislative pressure to reduce the content of Cd in food. The present study was conducted on cacao (Theobroma cacao), the source of chocolate, and one of the crops known to accumulate Cd in certain conditions. There are a range of possible genetic and agronomic methods being tested as a route to such reduction. As part of a gene-based approach, we focused on the Natural Resistance-Associated Macrophage Proteins (NRAMPS), a family of proton/metal transporter proteins that are evolutionarily conserved across all species from bacteria to humans. The plant NRAMP gene family are of particular importance as they are responsible for uptake of the nutritionally vital divalent cations Fe²⁺, Mn²⁺, Zn²⁺, as well as Cd²⁺. We identified the five NRAMP genes in cacao, sequenced these genes and studied their expression in various organs. We then confirmed the expression patterns in response to variation in nutrient cation availability and addition of Cd²⁺. Functional analysis by expression in yeast provided evidence that NRAMP5 encoded a protein capable of Cd²⁺ transport, and suggested this gene as a target for genetic selection/modification.

Expression of TpNRAMP5, a metal transporter from Polish wheat (Triticum polonicum L.), enhances the accumulation of Cd, Co and Mn in transgenic Arabidopsis plants

TpRNAMP5 is mainly expressed in the plasma membrane of roots and basal stems. It functions as a metal transporter for Cd, Mn and Co accumulation. Numerous natural resistance-associated macrophage proteins (NRAMPs) have been functionally identified in various plant species, including Arabidopsis, rice, soybean and tobacco, but no information is available on NRAMP genes in wheat. In this study, we isolated a TpNRAMP5 from dwarf Polish wheat (DPW, Triticum polonicum L.), a species with high tolerance to Cd and Zn. Expression pattern analysis revealed that TpNRAMP5 is mainly expressed in roots and basal stems of DPW. TpNRAMP5 was localized at the plasma membrane of Arabidopsis leaf protoplast. Expression of TpNRAMP5 in yeast significantly increased yeast sensitivity to Cd and Co, but not Zn, and enhanced Cd and Co concentrations. Expression of TpNRAMP5 in Arabidopsis significantly increased Cd, Co and Mn concentrations in roots, shoots and whole plants, but had no effect on Fe and Zn concentrations. These results indicate that TpNRAMP5 is a metal transporter enhancing the accumulation of Cd, Co and Mn, but not Zn and Fe. Genetic manipulation of TpNRAMP5 can be applied in the future to limit the transfer of Cd from soil to wheat grains, thereby protecting human health.

Cloning and Characterization of TpNRAMP3, a Metal Transporter From Polish Wheat (Triticum polonicum L.)

Essential transition metals and non-essential metals often co-exist in arable soils. In plants, some transition metal transporters, such as the natural resistance-associated macrophage proteins (NRAMPs), poorly selectively transport metals with similar chemical properties whether they are essential or non-essential. In this study, a member of the NRAMP transporter family, TpNRAMP3, was identified from dwarf Polish wheat (Triticum polonicum L.). TpNRAMP3 encodes a plasma membrane-localized protein and was highly expressed in leaf blades and roots at the jointing and booting stages, and in the first nodes at the grain filling stage. Expression of TpNRAMP3 increased sensitivity to Cd and Co, but not Zn, and increased the Cd and Co concentrations in yeast. TpNRAMP3 expression in Arabidopsis increased concentrations of Cd, Co, and Mn, but not Fe or Zn, in roots, shoots, and whole plant. However, TpNRAMP3 did not affect translocation of Cd, Co, or Mn from roots to shoots. These results suggest that TpNRAMP3 is a transporter for Cd, Co, and Mn accumulation, but not for Fe or Zn. However, Cd and Co are non-essential toxic metals; selective genetic manipulation of TpNRAMP3 will help breed low Cd- and Co-accumulating cultivars.