Overexpression of SmZIP plays important roles in Cd accumulation and translocation, subcellular distribution, and chemical forms in transgenic tobacco under Cd stress

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.

SpCTP3 from the hyperaccumulator Sedum plumbizincicola positively regulates cadmium tolerance by interacting with SpMDH1

Cadmium (Cd) is a heavy metal pollutant mainly originating from the discharge of industrial sewage, irrigation with contaminated water, and the use of fertilizers. The phytoremediation of Cd polluted soil depends on the identification of the associated genes in hyperaccumulators. Here, a novel Cd tolerance gene (SpCTP3) was identified in hyperaccumulator Sedum plumbizincicola. The results of Cd2+ binding and thermodynamic analyses, revealed the CXXC motif in SpCTP3 functions is a Cd2+ binding site. A mutated CXXC motif decreased binding to Cd by 59.93%. The subcellular localization analysis suggested that SpCTP3 is primarily a cytoplasmic protein. Additionally, the SpCTP3-overexpressing (OE) plants were more tolerant to Cd and accumulated more Cd than wild-type Sedum alfredii (NHE-WT). The Cd concentrations in the cytoplasm of root and leaf cells were significantly higher (53.75% and 71.87%, respectively) in SpCTP3-OE plants than in NHE-WT. Furthermore, malic acid levels increased and decreased in SpCTP3-OE and SpCTP3-RNAi plants, respectively. Moreover, SpCTP3 interacted with malate dehydrogenase 1 (MDH1). Thus, SpCTP3 helps regulate the subcellular distribution of Cd and increases Cd accumulation when it is overexpressed in plants, ultimately Cd tolerance through its interaction with SpMDH1. This study provides new insights relevant to improving the Cd uptake by Sedum plumbizincicola.

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.

Contribution of Cd passivating functional bacterium H27 to tobacco growth under Cd stress

The cadmium (Cd) embedded in tobacco not only affects yield and quality but also harms human health. Microbial remediation has attracted widespread attention due to its low cost and minimal risk of secondary pollution. Therefore, researching microbes capable of inhibiting crop absorption of heavy metals or removing heavy metals from the environment has significant practical implications. This study screened a strain named H27 with a Cd immobilization efficiency of up to 76.60%. Static cultivation experiments showed that immobilization of Cd by H27 is achieved through intracellular absorption, hydroxyl, carboxyl, and phosphate group reactions on the cell wall. The bacterium can also secrete extracellular substances to adsorb Cd and increase the environmental pH, reducing the bioavailability of Cd. H27 reduced the accumulation of Cd in the stems of hydroponically grown tobacco by 55.23% and decreased the expression of three Cd transport genes, HAM2, IRT1, and NRAMP1, in the roots. Additionally, H27 increased the mineralization rate of organic matter, increased the content of humic acid in the soil, promoted the formation of smaller soil particles, and enhanced the adsorption and fixation of Cd by soil components while simultaneously raising the pH of rhizosphere and non-rhizosphere soils in tobacco growth environments. Both hydroponic and potted experiments showed that H27 alleviated the inhibitory effect of Cd on tobacco growth, significantly reducing Cd accumulation in various parts of tobacco and lowering the transfer coefficient of Cd within the tobacco plant. This study aims to effectively reduce the Cd content in tobacco using microbes, mitigate the harm of heavy metals in cigarettes to human health, and provide theoretical and practical basis for the application of microbial techniques to control heavy metal absorption in tobacco.

CIP1, a CIPK23-interacting transporter, is implicated in Cd tolerance and phytoremediation

Environmental pollution from cadmium (Cd) presents a serious threat to plant growth and development. Therefore, it's crucial to find out how plants resist this toxic metal to develop strategies for remediating Cd-contaminated soils. In this study, we identified CIP1, a transporter protein, by screening interactors of the protein kinase CIPK23. CIP1 is located in vesicles membranes and can transport Cd2+ when expressed in yeast cells. Cd stress specifically induced the accumulation of CIP1 transcripts and functional proteins, particularly in the epidermal cells of the root tip. CIKP23 could interact directly with the central loop region of CIP1, phosphorylating it, which is essential for the efficient transport of Cd2+. A loss-of-function mutation of CIP1 in wild-type plants led to increased sensitivity to Cd stress. Conversely, tobacco plants overexpressing CIP1 exhibited improved Cd tolerance and increased Cd accumulation capacity. Interestingly, this Cd accumulation was restricted to roots but not shoots, suggesting that manipulating CIP1 does not risk Cd contamination of plants' edible parts. Overall, this study characterizes a novel Cd transporter, CIP1, with potential to enhance plant tolerance to Cd toxicity while effectively eliminating environmental contamination without economic losses.

Phytobial remediation advances and application of omics and artificial intelligence: a review

Industrialization and urbanization increased the use of chemicals in agriculture, vehicular emissions, etc., and spoiled all environmental sectors. It causes various problems among living beings at multiple levels and concentrations. Phytoremediation and microbial association are emerging as a potential method for removing heavy metals and other contaminants from soil. The treatment uses plant physiology and metabolism to remove or clean up various soil contaminants efficiently. In recent years, omics and artificial intelligence have been seen as powerful techniques for phytobial remediation. Recently, AI and modeling are used to analyze large data generated by omics technologies. Machine learning algorithms can be used to develop predictive models that can help guide the selection of the most appropriate plant and plant growth-promoting rhizobacteria combination that is most effective at remediation. In this review, emphasis is given to the phytoremediation techniques being explored worldwide in soil contamination.

Rice tonoplast intrinsic protein member OsTIP1;2 confers tolerance to arsenite stress

The International Agency for Research on Cancer categorizes arsenic (As) as a group I carcinogen. Arsenic exposure significantly reduces growth, development, metabolism, and crop yield. Tonoplast intrinsic proteins (TIPs) belong to the major intrinsic protein (MIP) superfamily and transport various substrates, including metals/metalloids. Our study aimed to characterize rice OsTIP1;2 in As[III] stress response. The gene expression analysis showed that the OsTIP1;2 expression was enhanced in roots on exposure to As[III] treatment. The heterologous expression of OsTIP1;2 in S. cerevisiae mutant lacking YCF1 (ycf1∆) complemented the As[III] transport function of the YCF1 transporter but not for boron (B) and arsenate As[V], indicating its substrate selective nature. The ycf1∆ mutant expressing OsTIP1;2 accumulated more As than the wild type (W303-1A) and ycf1∆ mutant strain carrying the pYES2.1 vector. OsTIP1;2 activity was partially inhibited in the presence of the aquaporin (AQP) inhibitors. The subcellular localization studies confirmed that OsTIP1;2 is localized to the tonoplast. The transient overexpression of OsTIP1;2 in Nicotiana benthamiana leaves resulted in increased activities of enzymatic and non-enzymatic antioxidants, suggesting a potential role in mitigating oxidative stress induced by As[III]. The transgenic N. tabacum overexpressing OsTIP1;2 displayed an As[III]-tolerant phenotype, with increased fresh weight and root length than the wild-type (WT) and empty vector (EV line). The As translocation factor (TF) for WT and EV was around 0.8, while that of OE lines was around 0.4. Moreover, the OE line bioconcentration factor (BCF) was more than 1. Notably, the reduced TF and increased BCF in the OE line imply the potential of OsTIP1;2 for phytostabilization.

Comparative transcriptome analysis and Arabidopsis thaliana overexpression reveal key genes associated with cadmium transport and distribution in root of two Capsicum annuum cultivars

The molecular mechanisms underlying high and low cadmium (Cd) accumulation in hot pepper cultivars remain unclear. In this study, comparative transcriptome analysis of root between high-Cd (J) and low-Cd (Z) cultivars was conducted under hydroponic cultivation with 0 and 0.4 mg/L Cd, respectively. The results showed that J enhanced the root uptake of Cd by elevating the expression of Nramp5 and counteracting Cd toxicity by increasing the expression of genes, such as NIR1, GLN1, and IAA9. Z reduced Cd accumulation by enhancing the cell wall lignin synthesis genes PAL, COMT, 4CL, LAC, and POD and the Cd transporters ABC, MTP1, and DTX1. Elevated expression of genes related to sulfur metabolism was observed in Z, potentially contributing to its ability to detoxify Cd. To investigate the function of CaCOMT1, an Arabidopsis thaliana overexpression line (OE-CaCOMT1) was constructed. The results revealed that OE-CaCOMT1 drastically increased the lignin content by 38-42% and reduced the translocation of Cd to the aboveground parts by 32%. This study provides comprehensive insights into the mechanisms underlying Cd accumulation in hot pepper cultivars using transcriptome analysis. Moreover, this study elucidates the critical function of CaCOMT1, providing a theoretical foundation for the production of low-Cd vegetables for food safety.

Physiological, transcriptome and gene functional analysis provide novel sights into cadmium accumulation and tolerance mechanisms in kenaf

Kenaf is considered to have great potential for remediation of heavy metals in ecosystems. However, studies on molecular mechanisms of root Cd accumulation and tolerance are still inadequate. In this study, two differently tolerant kenaf cultivars were selected as materials and the physiological and transcriptomic effects were evaluated under Cd stress. This study showed that 200 µmol/L CdCl2 treatment triggered the reactive oxygen species (ROS) explosion and membrane lipid peroxidation. Compared with the Cd-sensitive cultivar 'Z', the Cd-tolerant cultivar 'F' was able to resist oxidative stress in cells by producing higher antioxidant enzyme activities and increasing the contents of ascorbic acid (AsA) and glutathione (GSH). The root cell wall of 'F' exhibited higher polysaccharide contents under Cd treatment, providing more Cd-binding sites. There were 3,439 differentially expressed genes (DEGs) that were co-regulated by Cd treatment in two cultivars. Phenylpropanoid biosynthesis and plant hormone signal transduction pathways were significantly enriched by functional annotation analysis. DEGs associated with pectin, cellulose, and hemi-cellulose metabolism were involved in Cd chelation of root cell wall; V-ATPases, ABCC3 and Narmp3 could participated in vacuolar compartmentalization of Cd; PDR1 was responsible for Cd efflux; the organic acid transporters contributed to the absorption of Cd in soil. These genes might have played key roles in kenaf Cd tolerance and Cd accumulation. Moreover, HcZIP2 was identified to be involved in Cd uptake and transport in kenaf. Our findings provide a deeper understanding of the molecular pathways underlying Cd accumulation and detoxification mechanisms in kenaf.

Expression of Sailx suchowensis SsIRT9 enhances cadmium accumulation and alters metal homeostasis in tobacco

Cadmium (Cd) contamination in soils is of great concern for plant growth and human health. Willow (Salix spp.) is a promising phytoextractor because of its high biomass production. However, as a non-hyperaccumulator, willow has a low competitive ability in extraction of Cd. Thus, improving Cd concentrations in developing tissues is one of the primary tasks. Here, our study uncovers a novel SsIRT9 gene from Sailx suchowensis which manipulates plant Cd accumulation. SsIRT9 was more highly expressed in willow roots than other SsIRT genes. As a plasma membrane-localized protein, when expressed in yeast, SsIRT9 retarded cell growth more severely than other SsIRT proteins in the presence of Cd. Furthermore, SsIRT9 was cloned and expressed in tobacco and SsIRT9 did not affect plant growth. In hydroponic experiments, SsIRT9 lines displayed higher Cd in the shoots than the wild type. When grown in Cd-contaminated soils, Cd levels in transgenic tobacco increased by 152-364% in roots and by 135-444% in shoots, demonstrating significant superiority in Cd accumulation over other functional IRT/ZIP transporters. Moreover, expressing SsIRT9 in tobacco altered metal homeostasis, especially manganese and zinc. Taken together, we envision that SsIRT9 expression in plants is a promising strategy for upgrading extraction of Cd from soils.

The maize WRKY transcription factor ZmWRKY64 confers cadmium tolerance in Arabidopsis and maize (Zea mays L.)

KEY MESSAGE

ZmWRKY64 positively regulates Arabidopsis and maize Cd stress through modulating Cd uptake, translocation, and ROS scavenging genes expression. Cadmium (Cd) is a highly toxic heavy metal with severe impacts on crops growth and development. The WRKY transcription factor is a significant regulator influencing plant stress response. Nevertheless, the function of the WRKY protein in maize Cd stress response remains unclear. Here, we identified a maize WRKY gene, ZmWRKY64, the expression of which was enhanced in maize roots and leaves under Cd stress. ZmWRKY64 was localized in the nucleus and displayed transcriptional activity in yeast. Heterologous expression of ZmWRKY64 in Arabidopsis diminished Cd accumulation in plants by negatively regulating the expression of AtIRT1, AtZIP1, AtHMA2, AtNRAMP3, and AtNRAMP4, which are involved in Cd uptake and transport, resulting in Cd stress tolerance. Knockdown of ZmWRKY64 in maize led to excessive Cd accumulation in leaf cells and in the cytosol of the root cells, resulting in a Cd hypersensitive phenotype. Further analysis confirmed that ZmWRKY64 positively regulated ZmABCC4, ZmHMA3, ZmNRAMP5, ZmPIN2, ZmABCG51, ZmABCB13/32, and ZmABCB10, which may influence Cd translocation and auxin transport, thus mitigating Cd toxicity in maize. Moreover, ZmWRKY64 could directly enhance the transcription of ZmSRG7, a reported key gene regulating reactive oxygen species homeostasis under abiotic stress. Our results indicate that ZmWRKY64 is important in maize Cd stress response. This work provides new insights into the WRKY transcription factor regulatory mechanism under a Cd-polluted environment and may lead to the genetic improvement of Cd tolerance in maize.

Physiological and molecular mechanisms of boron in alleviating cadmium toxicity in Capsicum annuum

Soil cadmium (Cd) contamination threatens food safety and human health, particularly in developing countries. Previously, we have proposed that boron (B) could reduce Cd uptake and accumulation in hot peppers (Capsicum annuum) by regulating the expression of genes related to Cd transport in roots. However, only few studies have examined the role of B in plant leaves under Cd stress. It is unclear how B induces the expression of relevant genes and metabolites in hot pepper leaves and to what extent B is involved in leaf growth and Cd accumulation. The purpose of this study was to investigate the effects of B on growth and Cd accumulation in hot pepper leaves by determining physiological parameters and transcriptome sequencing. The results showed that B application significantly improved the concentration of chlorophyll a and intercellular CO2, stomatal conductance, and photosynthetic and transpiration rates by 18-41 % in Cd-stressed plants. Moreover, B enhanced Cd retention in the cell wall by upregulating the expression levels of pectin-, lignin-, and callose-related genes and improving the activity of pectin methylesterase by 30 %, resulting in an approximate 31 % increase in Cd retention in the cell wall. Furthermore, B application not only enhanced the expression levels of genes related to antioxidant enzymes (superoxide dismutase, catalase, and peroxidase) and their activities by 28-40 %, thereby counteracting Cd-induced oxidative stress, but also improved Cd chelation, sequestration, and exclusion by upregulating the expression levels of genes related to sulfur metabolism, heavy metal-associated isoprenylated plant protein (HIPP), and transporters such as vacuolar cation/proton exchanger (CAX3), metal-nicotianamine transporter (YSL), ATP-binding cassette (ABC), zinc/iron transporters (ZIP) and oxic-compound detoxification (DTX), ultimately reinforcing Cd tolerance. Together, our results suggest that B application reduces the negative effects of Cd on leaf growth, promotes photosynthesis, and decreases Cd transfer to fruits through its sequestration and retention.

Transcriptome analysis reveals the molecular mechanism of different forms of selenium in reducing cadmium uptake and accumulation in wheat seedlings

Selenium (Se) can counteract cadmium (Cd) toxicity in wheat, but the molecular mechanism of different Se forms reducing Cd uptake and accumulation in wheat seedlings remain unclear. Here, a hydroponic experiment was conducted to investigate the effects of three Se forms (selenite (Se(IV)), selenate (Se(VI)) and seleno-L-methionine (SeMet)) on Cd2+ influx, Cd subcellular distribution, and Cd accumulation in wheat seedlings, and the underlying molecular mechanisms were investigated through transcriptome analysis. Consequently, Se(IV) and Se(VI) addition significantly reduced root Cd concentration by 74.3% and 80.8%, respectively, and all Se treatments significantly decreased shoot Cd concentration by approximately 34.2%-74.9%, with Se(IV) addition having the most pronounced reducing effect. Transcriptome analysis showed the reduction of Cd accumulation after Se(IV) addition was mainly due to the downregulation of Cd uptake genes. The inhibition of Cd accumulation after Se(VI) addition was not only associated with the downregulation of Cd uptake genes, but also related to the sequestration of Cd in vacuole. For SeMet addition, the reduction of Cd accumulation was mainly related to the sequestration of Cd in vacuole as GSH-Cd. The above findings provide novel insights to understand the effects of different forms of Se on Cd uptake and accumulation and tolerance in wheat.

Inventory of cadmium-transporter genes in the root of mangrove plant Avicennia marina under cadmium stress

Mangrove Avicennia marina has the importantly potential for cadmium (Cd) pollution remediation in coastal wetlands. Unfortunately, the molecular mechanisms and transporter members for Cd uptake by the roots of A. marina are not well documented. In this study, photosynthetic and phenotypic analysis indicated that A. marina is particularly tolerant to Cd. The content and flux analysis indicated that Cd is mainly retained in the roots, with greater Cd influx in fine roots than that in coarse roots, and higher Cd influx in the root meristem zone as well. Using transcriptomic analysis, a total of 5238 differentially expressed genes were identified between the Cd treatment and control group. Moreover, we found that 54 genes were responsible for inorganic ion transport. Among these genes, AmHMA2, AmIRT1, and AmPCR2 were localized in the plasma membrane and AmZIP1 was localized in both plasma membrane and cytoplasm. All above gene encoding transporters showed significant Cd transport activities using function assay in yeast cells. In addition, the overexpression of AmZIP1 or AmPCR2 in Arabidopsis improved the Cd tolerance of transgenic plants. This is particularly significant as it provides insight into the molecular mechanism for Cd uptake by the roots of mangrove plants and a theoretical basis for coastal wetland phytoremediation.

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.

Expression of TaNCL2-A ameliorates cadmium toxicity by increasing calcium and enzymatic antioxidants activities in arabidopsis

Cadmium (Cd) is a heavy metal that occurs naturally in the environment and is toxic to both animals and plants. The impact of Cd toxicity is shown to be reduced by the exogenous application of calcium (Ca) in crop plants. The sodium/calcium exchanger-like (NCL) protein is involved in Ca enrichment in the cytoplasm by transporting it from the vacuole in the exchange of cytosolic sodium (Na). However, it has not been utilized to ameliorate the Cd toxicity, to date. An elevated expression of TaNCL2-A gene in the root and shoot tissues of bread wheat seedlings, and a higher growth rate of recombinant yeast cells, suggested its role in Cd stress response. The TaNCL2-A expressing transgenic Arabidopsis lines exhibited significant Cd tolerance with increased Ca (∼10-fold) accumulation. The proline content and antioxidant enzymes activities were increased while oxidative stress-related molecules such as H₂O₂ and MDA were reduced in the transgenic lines. In addition, the growth and yield parameters of transgenic lines such as seed germination rate, root length, leaf biomass, leaf area index, rosette diameter, leaf length and width, and silique count, along with various physiological indicators like chlorophyll, carotenoid, and relative water contents were also improved in comparison to the control plants. Further, the transgenic lines exhibited significant salinity and osmotic stress tolerance, as well. Taken together, these results suggested that the TaNCL2-A could mitigate Cd toxicity along with salinity and osmotic stress. This gene may also be utilized for phytoremediation and Cd sequestration in future studies.

Detoxification mechanisms of biochar on plants in chromium contaminated soil: Chromium chemical forms and subcellular distribution

The complete pathway of chromium (Cr) transfer from soil to plant tissues and subcellular components under biochar amendment remains to be quantified, as well as the involved diverse detoxification processes in roots and stems respectively. Pot experiments and quantitative analysis were conducted to investigate Cr fixation in soil amended with Enteromorpha prolifera-derived biochar and subsequent phytoprocesses (Cr uptake, transfer, and phytotoxicity) in cultivated Secale cereale L. (rye). The results indicated that adding 5-30 g kg^-1 of biochar increased the residual form of Cr (B4) in soil by 8-21% and decreased the bioavailable form of Cr (B1) by 9-29%. For Cr transferred to rye, Cr in the rye was mainly present in the low-toxicity bound state, with the acetic acid-extracted Cr (F4) (45-54%) in roots and the NaCl-extracted Cr (F3) (37-47%) in stems. The subcellular distribution of Cr in both roots and stems was predominantly in the cell wall and residues (T1), followed by the cytoplasm (T4). Partial least squares path model (PLS-PM) was used for quantifying the effect of biochar on the form changes and subcellular detoxification of Cr from soil to roots and stems to sub-cells. In soils, biochar reduced the bioavailability of Cr and decreased the transfer of Cr to rye. In plant roots, Cr was distributed mainly as low-toxicity phosphate complexes in cell walls and vacuoles in sub-cells (with the largest path coefficients of 0.90 and -0.91, respectively). In the stems, Cr was distributed mainly as proteins integrated into the cell walls and vacuoles. This was due to the difference in subcellular compartmentalization of detoxification in the roots and stems. These PLS-PM results provide new insights into the entire process of pollutant detoxification in complex environments.

A novel gene SpCTP3 from the hyperaccumulator Sedum plumbizincicola redistributes cadmium and increases its accumulation in transgenic Populus × canescens

A cadmium (Cd) tolerance protein (SpCTP3) involved in the Sedum plumbizincicola response to Cd stress was identified. However, the mechanism underlying the Cd detoxification and accumulation mediated by SpCTP3 in plants remains unclear. We compared wild-type (WT) and SpCTP3-overexpressing transgenic poplars in terms of Cd accumulation, physiological indices, and the expression profiles of transporter genes following with 100 μmol/L CdCl₂. Compared with the WT, significantly more Cd accumulated in the above-ground and below-ground parts of the SpCTP3-overexpressing lines after 100 μmol/L CdCl₂ treatment. The Cd flow rate was significantly higher in the transgenic roots than in the WT roots. The overexpression of SpCTP3 resulted in the subcellular redistribution of Cd, with decreased and increased Cd proportions in the cell wall and the soluble fraction, respectively, in the roots and leaves. Additionally, the accumulation of Cd increased the reactive oxygen species (ROS) content. The activities of three antioxidant enzymes (peroxidase, catalase, and superoxide dismutase) increased significantly in response to Cd stress. The observed increase in the titratable acid content in the cytoplasm might lead to the enhanced chelation of Cd. The genes encoding several transporters related to Cd²⁺ transport and detoxification were expressed at higher levels in the transgenic poplars than in the WT plants. Our results suggest that overexpressing SpCTP3 in transgenic poplar plants promotes Cd accumulation, modulates Cd distribution and ROS homeostasis, and decreases Cd toxicity via organic acids. In conclusion, genetically modifying plants to overexpress SpCTP3 may be a viable strategy for improving the phytoremediation of Cd-polluted soil.

Ca alleviated Cd-induced toxicity in Salix matsudana by affecting Cd absorption, translocation, subcellular distribution, and chemical forms

Cadmium (Cd), a ubiquitous and highly toxic heavy metal pollutant, is toxic to animals and plants. Calcium (Ca) is an essential component for plant growth and reduces plant Cd absorption by competing with Cd. To gain deeper insight into the effects of Ca on Cd absorption, translocation, subcellular distribution, and chemical forms in S. matsudana seedlings under Cd stress, an investigation was conducted on these properties. Adding Ca alleviated Cd physiological toxicity in S. matsudana, reduced Cd absorption, increased the translocation from roots to shoots, lead to subcellular redistribution of Cd by increasing the proportion of Cd in soluble fractions but decreasing Cd in the cell wall and changed the chemical forms of Cd from 0.6 mol/L HCl- and 2% HAc-extracted Cd to 1 mol/L NaCl-extracted Cd. The energy dispersive X-ray analyses (EDXA) results revealed that after adding Ca, Cd was transferred through the root epidermis, cortex, endodermis, and vascular cylinder, transported to the shoots, and was highly accumulated in leaf epidermal and mesophyll cells, but less in leaf vein and guard cells. The genes involved in Cd uptake and xylem loading included NRAMP1, ZIP8, HMA2, and HMA4, which were up-regulated significantly (P < 0.05) in the Cd and Cd + Ca treatments compared to the control. The findings of this study provide new insight into the mechanism that Ca alleviates Cd toxicity in woody tree species, as well as propose an important prospect of Ca addition for improving the phytoremediation of Cd contamination.

Cadmium distribution and transformation in leaf cells involved in detoxification and tolerance in barley

Barley is a diagnostic plant that often used in the research of soil pollution by heavy metals, our research explored the detoxification and tolerance mechanism of cadmium(Cd) in barley through pot experiment. We investigated subcellular distribution, chemical forms and oxidative damage of Cd in barley leaves, combing with the transmission electron microscopy and Fourier-transform infrared spectroscopy(FT-IR) to further understand the translocation, transformation characteristics and toxic effect of Cd in cells. The results showed that, the bioaccumulation factors in roots and shoots of barley were ranged of 4.03-7.48 and 0.51-1.30, respectively. Barley reduces the toxic effects by storing Cd in the roots and reducing its transport to the shoots. Compared to the control treatment (0 mg/kg), the percentage of Cd in the cell wall fractions of leaves in 300 mg/kg Cd treatment increased from 34.74 % to 38.41 %; the percentage of the organelle fractions increased from 24.47 % to 56.02 %; and the percentage of soluble fraction decreased from 40.80 % to 5.57 %. We found that 69.13 % of the highly toxic inorganic Cd and water-soluble Cd were converted to less toxic pectates and protein-integrated Cd (50.20 %) and undissolved Cd phosphates (18.93 %). This conversion of Cd was mainly due to its combination with -OH, -NH, -CN, -C-O-C, and -C-O-P groups. Excessive Cd induced a significant (P < 0.05) increase in the levels of peroxidase, malondialdehyde, and cell membrane permeability, which damaged the cell membrane and allowed Cd to enter the organelles. The chloroplasts and mitochondria were destroyed, and eventually the metabolism of intracellular substances was affected, resulting in symptoms of toxicity. Our research provides cellular-scale insight into the mechanisms of Cd tolerance in barley.

Spraying sorbitol-chelated calcium affected foliar calcium absorption and promoted the yield of peanut (Arachis hypogaea L.)

The prevalent use of foliar calcium fertilizers in peanut production is inorganic, but calcium absorbed from the foliar has poor availability. Sorbitol-chelated calcium is a novel organic foliar calcium fertilizer that has rarely been studied for application in peanut production. To explore whether calcium absorption and peanut yields can be affected by foliar application of sorbitol-chelated calcium, this study conducted two field experiments using Virginia peanut (Huayu-22) in 2020 and 2021. The five spray treatments included: deionized water (CK), sorbitol (Sor), calcium nitrate (CaN), a mixture of sorbitol and calcium nitrate (SN), and sorbitol-chelated calcium (SC). The yield of peanuts treated with sorbitol-chelated calcium was increased by 12.31-16.63%, 10.22-11.83%, 6.31-9.69%, and 4.18-6.99% compared to the CK, Sor, CaN, and SN treatments, respectively. Sorbitol-chelated calcium had the lowest contact angle due to the wetting effect of sorbitol, which promoted calcium absorption by leaves. Sorbitol-chelated calcium improved the leaf calcium concentration by 13.12-19.32% and kernel calcium concentration by 6.49-8.15% compared to the CK treatment. Foliar fertilization increased the calcium concentration of each subcellular fraction of leaves and changed the distribution of calcium in mesophyll cells. This change was directly observed by transmission electron microscopy. Additionally, spraying sorbitol alone obtained similar effects to spraying calcium nitrate alone, indicating that the benefits of sorbitol itself were not negligible. The results of the principal component and correlation analysis showed that the increase in calcium concentrations and the change in calcium distribution improved the pod traits of the peanut, thus affecting the peanut yield. The above results showed that from the perspective of calcium absorption and distribution, sorbitol-chelated calcium is a more effective foliar calcium fortifier for peanuts and effectively improves peanut yields.

Exogenous 5-aminolevulinic acid alleviates low-temperature damage by modulating the xanthophyll cycle and nutrient uptake in tomato seedlings

5-Aminolevulinic acid (ALA), an antioxidant existing in plants, has been widely reported to participate in the process of coping with cold stress of plants. In this study, exogenous ALA promoted the growth of tomato plants and alleviated the appearance of purple tomato leaves under low-temperature stress. At the same time, exogenous ALA improved antioxidant enzyme activities, SlSOD gene expression, Fv/Fm, and proline contents and reduced H₂O₂ contents, SlRBOH gene expression, relative electrical conductivity, and malondialdehyde contents to alleviate the damage caused by low temperature to tomato seedlings. Compared with low-temperature stress, spraying exogenous ALA before low-temperature stress could restore the indicators of photochemical quenching, actual photochemical efficiency, electron transport rate, and nonphotochemical quenching to normal. Exogenous ALA could increase the total contents of the xanthophyll cycle pool, the positive de-epoxidation rate of the xanthophyll cycle and improved the expression levels of key genes in the xanthophyll cycle under low-temperature stress. In addition, we found that exogenous ALA significantly enhanced the absorption of mineral nutrients, promoted the transfer and distribution of mineral nutrients to the leaves, and improved the expression levels of mineral nutrient absorption-related genes, which were all conducive to the improved adaptation of tomato seedlings under low-temperature stress. In summary, the application of exogenous ALA can increase tomato seedlings' tolerance to low-temperature stress by improving the xanthophyll cycle and the ability of the absorption of mineral nutrients in tomato seedlings.

The mechanism of arbuscular mycorrhizal enhancing cadmium uptake in Phragmites australis depends on the phosphorus concentration

Arbuscular mycorrhizal fungi (AMF) is a vital strategy to enhance the phytoremediation of cadmium (Cd) pollution. However, the function of AMF was influenced by phosphorus (P) concentration. To reveal the effect of AMF on the Cd accumulation of host plants under different P concentrations and how the AMF and P interact, this study comparatively analyzed the regulatory effects of AMF on the Cd response, extraction, and transportation processes of Phragmites australis (P. australis) under different P levels, and explored its physiological, biochemical and molecular biological mechanisms. The study showed that AMF could induce different growth allocation strategies in response to Cd stress. Moreover, AMF promoted plant Cd tolerance and detoxification by enhancing P uptake, Cd passivation, Cd retention in the cell wall, and functional group modulation. Under P starvation treatments, AMF promoted Cd uptake by inducing Cd to enter the iron pathway, increased the transport coefficient by 493.39%, and retained Cd in stems. However, these effects disappeared following the addition of P. Additionally, AMF up-regulated the expression of ZIP, ZIP, and NRAMP genes to promote cadmium uptake at low, medium, and high phosphorus levels, respectively. Thus, the Cd response mechanism of the AMF-P. australis symbiotic system was P dose-dependent.

Subcellular distribution and chemical forms of manganese in Daucus carota in relation to its tolerance

Daucus carota is a biennial herb of the Umbelliferae family, which is a candidate plant for the phytoremediation of Mn pollution. To reveal the mechanism of this plant to adapt to Mn stress, plant growth, anatomical structure, Mn accumulation characteristic, Mn subcellular distribution, and chemical forms of D. carota under six Mn²⁺ concentrations by pot culture experiments were studied. The results showed that with the rising Mn concentrations, the total dry weight and leaf area of D. carota increased firstly and then decreased, while the specific leaf area increased. The thickness of the main vein, upper epidermis, and lower epidermis; the thickness of the palisade tissue; and the thickness of the spongy tissue of the leaves increased firstly and then decreased. The Mn content in the aboveground and underground parts of D. carota increased, and the values of the bioconcentration factor (BCF) and translocation factor (TF) were higher than 1. The Mn existing in the cell wall and soluble components accounted for the largest proportion, and the proportion of Mn in the cell wall increased with increasing concentrations of Mn. In addition, Mn mainly existed in ethanol extraction state, deionized water extraction state, and sodium chloride extraction state. The results showed that D. carota could alleviate the damage caused by high manganese concentration by storing most of manganese in the cell wall and vacuole and existing in the form of low-activity state.

Comparison of two willow genotypes reveals potential roles of iron-regulated transporter 9 and heavy-metal ATPase 1 in cadmium accumulation and resistance in Salix suchowensis

Willows (Salix spp.) are promising extractors of cadmium (Cd), with fast growth, high biomass production, and high Cd accumulation capacity. However, the molecular mechanisms underlying Cd uptake and detoxification are currently poorly understood. Analysis of the Cd uptake among 30 willow genotypes in hydroponic systems showed that the S. suchowensis and S. integra hybrids, Jw8-26 and Jw9-6, exhibited distinct Cd accumulation and resistance characteristics. Jw8-26 was a high Cd-accumulating and tolerant willow, while Jw9-6 was a low Cd-accumulating and relatively Cd-intolerant willow. Therefore, these two genotypes were ideal specimens for determining the molecular mechanisms of Cd uptake and detoxification. To identify relevant genes in Cd handling, the parent S. suchowensis was treated with Cd and RNA-seq analysis was performed. SsIRT, SsHMA, and SsGST, in addition to the transcription factors SsERF, SsMYB, and SsZAT were identified as being associated with Cd uptake and resistance. Because membrane-localised heavy metal transporters mediate Cd transfer to plant tissues, a total of 17 SsIRT and 12 SsHMA family members in S. suchowensis were identified. Subsequently, a thorough bioinformatics analysis of the SsIRT and SsHMA families was conducted, and their transcript levels were analysed in the roots of the two hybrids. The transcript levels of SsIRT9 in roots were positively correlated with the observed differences in Cd accumulation in Jw8-26 versus Jw9-6. Jw8-26 displayed higher SsIRT9 expression levels and higher Cd accumulation than Jw9-6; therefore, SsIRT9 may be involved in Cd uptake. Gene expression analysis also revealed that SsHMA1 was a candidate gene associated with Cd resistance. These results lay the foundation for understanding the molecular mechanism of Cd transfer and detoxification in willows, and provide guidance for the screening and breeding of high Cd-accumulating and tolerant willow genotypes via genetic engineering.

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.

Cytological and physiological tolerance of transgenic tobacco to Cd stress is enhanced by the ectopic expression of SmZIP8

Zrt and Irt-like proteins (ZIPs) are responsible for transporting various divalent metal cations. However, information about the characteristics of the cellular and physiological tolerance of plant ZIPs to Cd stress is still limited. The expression levels of SmZIP8 in Salix matsudana Koidz were upregulated by Cd stress. The complete length of SmZIP8 from S. matsudana was cloned, and transgenic tobacco was obtained by Agrobacterium-mediated transformation. Then, the tolerance to Cd stress of wild-type (WT) and transgenic tobacco seedlings was analyzed and compared by studying the cytotoxicity of the root tip cells, photosynthetic parameters, histochemical staining of O2- and H2O2, the activities of antioxidant enzymes, and malondialdehyde content under Cd stress. In comparison with WT tobacco, the ectopic expression of SmZIP8 in tobacco promoted the cytological tolerance of the transgenic tobacco to Cd stress by reducing cell damage, raising the mitotic indexes, and reducing the rate of chromosome aberration of the root cells. Meanwhile, the results of increased photosynthetic capacity, decreased oxidative damage, and activated antioxidant enzymes showed that the physiological tolerance of transgenic tobacco to Cd was enhanced. The principal component analysis for the above physiological parameters explained 96.08% of the total variance (PC1, 77.77%; PC2, 18.31%), indicating a significant difference in Cd tolerance abilities between the tobacco expressing SmZIP8 and WT tobacco. Therefore, SmZIP8 may be considered as an important genetic resource for the phytoremediation of Cd or other heavy metal pollution via the use of transgenic plants obtained through genetic transformation.

Effect of nitrilotriacetic acid and tea saponin on the phytoremediation of Ni by Sudan grass (Sorghum sudanense (Piper) Stapf.) in Ni-pyrene contaminated soil

Phytoremediation is commonly used in the remediation of soils co-contaminated by heavy metals and polycyclic aromatic hydrocarbons (PAHs) because of its economy and effectiveness. Sudan grass (Sorghum sudanense (Piper) Stapf.) has well-developed roots and strong tolerance to heavy metals, so it has been widely concerned. In this study, nitrilotriacetic acid (NTA) and tea saponin (TS) were used as enhancers and combined with Sudan grass for improving the remediation efficiency of Ni-pyrene co-contaminated soil. The results of the pot experiment in soils showed that enhancers promoted the enrichment of Ni in plants. With the function of enhancers, more inorganic and water-soluble Ni were converted into low-toxic phosphate-bonded and residual Ni, so as to reinforce the tolerance of Sudan grass to Ni. In the pot experiment based on vermiculite, it was found that enhancers increased the accumulation of Ni in cell wall by 49.71-102.73%. Enhancers also had the positive effect on the relative abundance of Proteobacteria, Patescibacteria and Bacteroidetes that could tolerate heavy metals at phylum level. Simultaneously, the study found that pyrene reduced the exchangeable Ni in soils. More Ni entered the organelles and transfer to more high-toxic forms in Sudan grass when pynere coexisted. The study manifested that enhancers improved the phytoremediation effect of Ni significantly, yet the co-existence of pyrene weakened the process. Our results provided meaningful references for remediating actual co-contaminated soil of heavy metals and PAHs.

Utilization of Legume-Nodule Bacterial Symbiosis in Phytoremediation of Heavy Metal-Contaminated Soils

Simple Summary

The legume–rhizobium symbiosis is one of the most beneficial interactions with high importance in agriculture, as it delivers nitrogen to plants and soil, thereby enhancing plant growth. Currently, this symbiosis is increasingly being exploited in phytoremediation of metal contaminated soil to improve soil fertility and simultaneously metal extraction or stabilization. Rhizobia increase phytoremediation directly by nitrogen fixation, protection of plants from pathogens, and production of plant growth-promoting factors and phytohormones.

Abstract

With the increasing industrial activity of the growing human population, the accumulation of various contaminants in soil, including heavy metals, has increased rapidly. Heavy metals as non-biodegradable elements persist in the soil environment and may pollute crop plants, further accumulating in the human body causing serious conditions. Hence, phytoremediation of land contamination as an environmental restoration technology is desirable for both human health and broad-sense ecology. Legumes (Fabaceae), which play a special role in nitrogen cycling, are dominant plants in contaminated areas. Therefore, the use of legumes and associated nitrogen-fixing rhizobia to reduce the concentrations or toxic effects of contaminants in the soil is environmentally friendly and becomes a promising strategy for phytoremediation and phytostabilization. Rhizobia, which have such plant growth-promoting (PGP) features as phosphorus solubilization, phytohormone synthesis, siderophore release, production of beneficial compounds for plants, and most of all nitrogen fixation, may promote legume growth while diminishing metal toxicity. The aim of the present review is to provide a comprehensive description of the main effects of metal contaminants in nitrogen-fixing leguminous plants and the benefits of using the legume–rhizobium symbiosis with both wild-type and genetically modified plants and bacteria to enhance an efficient recovery of contaminated lands.

Increase in Phytoextraction Potential by Genome Editing and Transformation: A Review

Soil metal contamination associated with productive activities is a global issue. Metals are not biodegradable and tend to accumulate in soils, posing potential risks to surrounding ecosystems and human health. Plant-based techniques (phytotechnologies) for the in situ remediation of metal-polluted soils have been developed, but these have some limitations. Phytotechnologies are a group of technologies that take advantage of the ability of certain plants to remediate soil, water, and air resources to rehabilitate ecosystem services in managed landscapes. Regarding soil metal pollution, the main objectives are in situ stabilization (phytostabilization) and the removal of contaminants (phytoextraction). Genetic engineering strategies such as gene editing, stacking genes, and transformation, among others, may improve the phytoextraction potential of plants by enhancing their ability to accumulate and tolerate metals and metalloids. This review discusses proven strategies to enhance phytoextraction efficiency and future perspectives on phytotechnologies.

Antioxidant system response, mineral element uptake and safe utilization of Polygonatum sibiricum in cadmium-contaminated soil

Chinese herbal medicine is widely cultivated in Southwest China, where the soil cadmium (Cd) contamination of farmland is more serious than that in China as a whole. In this study, Polygonatum sibiricum was exposed to Cd at concentrations of e⁻¹, e⁰, e², and e⁴ mg/kg for 30, 60, and 90 days, and the physiological stress responses, Cd and mineral element uptake, antioxidant enzyme activities, and content changes of pharmaceutical ingredients (polysaccharides) were analyzed to decipher the feasibility of safe utilization in Cd-contaminated soil. The results show that the activity of antioxidant enzymes (SOD and CAT) in the aboveground part was always higher than that in the underground part. The underground part of Polygonatum sibiricum mobilizes nonenzymatic systems to facilitate the synthesis of polysaccharides (PCP1, PCP2) with antioxidant properties to cope with Cd stress. Mineral elements (P, K, Ca, Mg, Fe, Cu, and Zn) significantly (p < 0.05) changed after 90 d of cultivation. In particular, the changes in the iron and zinc content were significantly correlated (p < 0.05) with the activities of SOD and POD. Soil Cd at e⁰ mg/kg can guarantee the safe production and utilization of Polygonatum sibiricum, and the stimulation of Cd promotes polysaccharide synthesis and biomass growth.

The Endophytic Fungus Piriformospora Indica-Assisted Alleviation of Cadmium in Tobacco

Increasing evidence suggests that the endophytic fungus Piriformospora indica helps plants overcome various abiotic stresses, especially heavy metals. However, the mechanism of heavy metal tolerance has not yet been elucidated. Here, the role of P. indica in alleviating cadmium (Cd) toxicities in tobacco was investigated. It was found that P. indica improved Cd tolerance to tobacco, increasing Cd accumulation in roots but decreasing Cd accumulation in leaves. The colonization of P. indica altered the subcellular repartition of Cd, increasing the Cd proportion in cell walls while reducing the Cd proportion in membrane/organelle and soluble fractions. During Cd stress, P. indica significantly enhanced the peroxidase (POD) activity and glutathione (GSH) content in tobacco. The spatial distribution of GSH was further visualized by Raman spectroscopy, showing that GSH was distributed in the cortex of P. indica-inoculated roots while in the epidermis of the control roots. A LC-MS/MS-based label-free quantitative technique evaluated the differential proteomics of P. indica treatment vs. control plants under Cd stress. The expressions of peroxidase, glutathione synthase, and photosynthesis-related proteins were significantly upregulated. This study provided extensive evidence for how P. indica enhances Cd tolerance in tobacco at physiological, cytological, and protein levels.