Comparative transcriptome analysis between two different cadmium-accumulating genotypes of soybean (Glycine max) in response to cadmium stress.

BACKGROUND: Cadmium (Cd) is extremely toxic and non-essential for plants. Different soybean varieties differ greatly in their Cd accumulation ability, but little is known about the underlying molecular mechanisms. RESULTS: Here, we performed transcriptomic analysis using Illumina pair-end sequencing on root tissues from two soybean varieties (su8, high-Cd-accumulating (HAS) and su7, low Cd-accumulating (LAS)) grown with 0 or 50 µM CdSO4. A total of 18.76 million clean reads from the soybean root samples were obtained after quality assessment and data filtering. After Cd treatment, 739 differentially expressed genes (DEGs; 265 up and 474 down) were found in HAS; however, only 259 DEGs (88 up and 171 down) were found in LAS, and 64 genes were same between the two varieties. Pathway enrichment analysis suggested that after cadmium treatment, the DEGs between LAS and HAS were mainly enriched in glutathione metabolism and plant-pathogen interaction pathways. KEGG analysis showed that phenylalanine metabolism responding to cadmium stress in LAS, while ABC transporters responding to cadmium stress in HAS. Besides we found more differential expressed heavy metal transporters such as ABC transporters and zinc transporters in HAS than LAS, and there were more transcription factors differently expressed in HAS than LAS after cadmium treatment in two soybean varieties, eg. bHLH transcription factor, WRKY transcription factor and ZIP transcription factor. CONCLUSIONS: Findings from this study will shed new insights on the underlying molecular mechanisms behind the Cd accumulation in soybean.

Photosynthetic variation and detoxification strategies based on cadmium uptake, non-protein thiols, and secondary metabolites in Miscanthus sacchariflorus under cadmium exposure.

Miscanthus sacchariflorus is previously demonstrated to be a potential candidate for remediation of cadmium (Cd) pollution. To explore its resistance strategy to Cd, a hydroponic experiment was conducted to determine the variations of photosynthetic activity in leaves and physiological response in roots of this plant. Results showed that the root of M. sacchariflorus was the primary location for Cd accumulation. The bioconcentration factor in the roots and rhizomes was >1, and the translocation factor from underground to aboveground was <1. Throughout the experimental period, treatment with 0.06 mM Cd2+ did not significantly alter the contents of chlorophyll a, chlorophyll b, or carotenoid. By contrast, treatment with 0.15 and 0.30 mM Cd2+ decreased the contents of chlorophyll a, chlorophyll b, and carotenoid; caused the deformation of the chlorophyll fluorescence transient curve; reduced the photochemical efficiency of photosystem II; and increased the contents of non-protein thiols, total flavone, and total phenol. These results indicate that M. sacchariflorus has good adaptability to 0.06 mM Cd2+. Moreover, the accumulation of the non-protein thiols, total flavone, and total phenol in roots may promote the chelation of Cd2+, thus alleviating Cd toxicity. This study provides theoretical support for using M. sacchariflorus to remediate Cd-polluted wetlands.

Responses of soil and rhizosphere microbial communities to Cd-hyperaccumulating willows and Cd contamination.

BACKGROUND: The pollution of soil by heavy metals, particularly Cd, is constitutes a critical international environmental concern. Willow species are renowned for their efficacy in the phytoremediation of heavy metals owing to their high Cd absorption rate and rapid growth. However, the mechanisms underlying microbial regulation for high- and low-accumulating willow species remain poorly understood. Therefore, we investigated the responses of soil and rhizosphere microbial communities to high- and low-Cd-accumulating willows and Cd contamination. We analyzed soil properties were analyzed in bulk soil (SM) and rhizosphere soil (RM) planted with high-accumulating (H) and low-accumulating (L) willow species. RESULTS: Rhizosphere soil for different willow species had more NH4+ than that of bulk soil, and RM-H soil had more than RM-L had. The available phosphorus content was greater in hyper-accumulated species than it was in lower-accumulated species, especially in RM-H. Genome sequencing of bacterial and fungal communities showed that RM-L exhibited the highest bacterial diversity, whereas RM-H displayed the greatest richness than the other groups. SM-L exhibited the highest diversity and richness of fungal communities. Ralstonia emerged as the predominant bacterium in RM-H, whereas Basidiomycota and Cercozoa were the most enriched fungi in SM-H. Annotation of the N and C metabolism pathways revealed differential patterns: expression levels of NRT2, NarB, nirA, nirD, nrfA, and nosZ were highest in RM-H, demonstrating the effects of NO3-and N on the high accumulation of Cd in RM-H. The annotated genes associated with C metabolism indicated a preference for the tricarboxylic pathway in RM-H, whereas the hydroxypropionate-hydroxybutyrate cycle was implicated in C sequestration in SM-L. CONCLUSIONS: These contribute to elucidation of the mechanism underlying high Cd accumulation in willows, particularly in respect of the roles of microbes and N and C utilization. This will provide valuable insights for repairing polluted soil using N and employing organic acids to improve heavy metal remediation efficiency.

Physiological and transcriptomic analyses reveal the cadmium tolerance mechanism of Miscanthus lutarioriparia.

Miscanthus lutarioriparia is a promising energy crop that is used for abandoned mine soil phytoremediation because of its high biomass yield and strong tolerance to heavy metals. However, the biological mechanism of heavy metal resistance is limited, especially for applications in the soil restoration of mining areas. Here, through the investigation of soil cadmium(Cd) in different mining areas and soil potted under Cd stress, the adsorption capacity of Miscanthus lutarioriparia was analyzed. The physiological and transcriptional effects of Cd stress on M. lutarioriparia leaves and roots under hydroponic conditions were analyzed. The results showed that M. lutarioriparia could reduce the Cd content in mining soil by 29.82%. Moreover, different Cd varieties have different Cd adsorption capacities in soils with higher Cd concentration. The highest cadmium concentrations in the aboveground and belowground parts of the plants were 185.65 mg/kg and 186.8 mg/kg, respectively. The total chlorophyll content, superoxide dismutase and catalase activities all showed a trend of increasing first and then decreasing. In total, 24,372 differentially expressed genes were obtained, including 7735 unique to leaves, 7725 unique to roots, and 8912 unique to leaves and roots, which showed differences in gene expression between leaves and roots. These genes were predominantly involved in plant hormone signal transduction, glutathione metabolism, flavonoid biosynthesis, ABC transporters, photosynthesis and the metal ion transport pathway. In addition, the number of upregulated genes was greater than the number of downregulated genes at different stress intervals, which indicated that M. lutarioriparia adapted to Cd stress mainly through positive regulation. These results lay a solid foundation for breeding excellent Cd resistant M. lutarioriparia and other plants. The results also have an important theoretical significance for further understanding the detoxification mechanism of Cd stress and the remediation of heavy metal pollution in mining soil.

Deciphering the mechanistic role of Bacillus paramycoides (PM51) and Bacillus tequilensis (PM52) in bio-sorption and phyto-assimilation of Cadmium via Linum usitatissimum L. Seedlings.

Three Cd2+ resistant bacterium's minimal inhibition concentrations were assessed and their percentages of Cd2+ accumulation were determined by measurements using an atomic absorption spectrophotometer (AAS). The results revealed that two isolates Bacillus paramycoides (PM51) and Bacillus tequilensis (PM52), identified by 16S rDNA gene sequencing, showed a higher percentage of Cd2+ accumulation i.e., 83.78% and 81.79%, respectively. Moreover, both novel strains can tolerate Cd2+ levels up to 2000 mg/L isolated from district Chakwal. Amplification of the czcD, nifH, and acdS genes was also performed. Batch bio-sorption studies revealed that at pH 7.0, 1 g/L of biomass, and an initial 150 mg/L Cd2+ concentration were the ideal bio-sorption conditions for Bacillus paramycoides (PM51) and Bacillus tequilensis (PM52). The experimental data were fit to Langmuir isotherm measurements and Freundlich isotherm model R2 values of 0.999 for each of these strains. Bio sorption processes showed pseudo-second-order kinetics. The intra-diffusion model showed Xi values for Bacillus paramycoides (PM51) and Bacillus tequilensis (PM52) of 2.26 and 2.23, respectively. Different surface ligands, was investigated through Fourier-transformation infrared spectroscopy (FTIR). The scanning electron microscope SEM images revealed that after Cd2+ adsorption, the cells of both strains became thick, adherent, and deformed. Additionally, both enhanced Linum usitatissimum plant seed germination under varied concentrations of Cd2+ (0 mg/L, 250 mg/L,350 mg/L, and 500 mg/L). Current findings suggest that the selected strains can be used as a sustainable part of bioremediation techniques.

Graphitic carbon nitride alleviates cadmium toxicity to soybeans through nitrogen supply.

Graphitic carbon nitride (g-C3N4) is a promising candidate for heavy metal remediation, primarily composed of carbon (C) and nitrogen (N). It has been demonstrated that g-C3N4 adjusts rhizosphere physicochemical conditions, especially N conditions, alleviating the absorption and accumulation of Cadmium (Cd) by soybeans. However, the mechanisms by which g-C3N4 induces N alterations to mitigates plant uptake of Cd remain unclear. This study investigated the impact of g-C3N4-mediated changes in N conditions on the accumulation of Cd by soybeans using pot experiments. It also explored the microbiological mechanisms underlying alterations in soybean rhizospheric N cycling induced by g-C3N4. It was found that g-C3N4 significantly increased N content in the soybean rhizosphere (p < 0.05), particularly in terms of available nitrogen (AN) of nitrate and ammonium. Plants absorbed more ammonium nitrogen (NH4⁺-N), the content of which in the roots showed a significant negative correlation with Cd concentration in plant (p < 0.05). Additionally, g-C3N4 significantly affected rhizospheric functional genes associated with N cycling (p < 0.05) by increasing the ratio of the N-fixation functional gene nifH and decreasing the ratios of functional genes amoA and nxrA involved in nitrification. This enhances soybean's N-fixing potential and suppresses denitrification potential in the rhizosphere, preserving NH4⁺-N. Niastella, Flavisolibacter, Opitutus and Pirellula may play a crucial role in the N fixation and preservation process. In summary, the utilization of g-C3N4 offers a novel approach to ensure safe crop production in Cd-contaminated soils. The results of this study provide valuable data and a theoretical foundation for the remediation of Cd polluted soils.

Strategies of soil microbial N-cycling in different cadmium contaminated soil with wheat straw return.

Cadmium contamination inevitably affects the microbially mediated transformation of nitrogen in soils with wheat straw return. The responses of nitrogen functional microorganisms to cadmium in acidic and alkaline soils under wheat straw returned are still unclear. In this study, quantitative polymerase chain reaction (qPCR) and sequencing of nitrifying and denitrifying bacteria were performed to investigate the effects of wheat straw application on nitrogen conversion in different Cd-contaminated soils during an incubation experiment. Results showed that the presence of Cd decreased the abundance of hao gene catalyzing nitrification and norB gene catalyzing denitrification process, resulting the accumulation of NH4+-N and reduction of NO3--N in the acidic soils. Additionally, Cd-contamination stimulates the nitrification catalyzed by bacterial amoA gene and thus reduced the NH4+-N content in the alkaline soils. Meanwhile, Cd dominated the decrease of NO3--N content by promoting denitrification process catalyzed by nirS gene. Among all nitrifying and denitrifying microorganisms, Nitrosospira are tolerant to Cd stress under alkaline condition but sensitive to acidic condition, which dominantly harbored hao gene in the acidic soils and bacterial amoA gene in the alkaline soils. This study aimed to provide reasonable information for the rational adoption of wheat straw returning strategies to realize nitrogen regulation in Cd-contaminated farmland soil.

Physiological response and molecular mechanism of Quercus variabilis under cadmium stress.

Heavy metal pollution is a global environmental problem, and Quercus variabilis has a stronger tolerance to Cd stress than do other species. We aimed to explore the physiological response and molecular mechanisms of Q. variabilis to Cd stress. In this study, the antioxidant enzyme activities of leaves were determined, while the photosynthetic parameters of leaves were measured using Handy PEA, and ion fluxes and DEGs in the roots were investigated using noninvasive microtest technology (NMT) and RNA sequencing techniques, respectively. Cd stress at different concentrations and for different durations affected the uptake patterns of Cd2+ and H+ by Q. variabilis and affected the photosynthetic efficiency of leaves. Moreover, there was a positive relationship between antioxidant enzyme (CAT and POD) activity and Cd concentration. Transcriptome analysis revealed that many genes, including genes related to the cell wall, glutathione metabolism, ion uptake and transport, were significantly upregulated in response to cadmium stress in Q. variabilis roots. WGCNA showed that these DEGs could be divided into eight modules. The turquoise and blue modules exhibited the strongest correlations, and the most significantly enriched pathways were the phytohormone signaling pathway and the phenylpropanoid biosynthesis pathway, respectively. These findings suggest that Q. variabilis can bolster plant tolerance by modulating signal transduction and increasing the synthesis of compounds, such as lignin, under Cd stress. In summary, Q. variabilis can adapt to Cd stress by increasing the activity of antioxidant enzymes, and regulating the fluxes of Cd2+ and H+ ions and the expression of Cd stress-related genes.

R3 strain and Fe-Mn modified biochar reduce Cd absorption capacity of roots and available Cd content of soil by affecting rice rhizosphere and endosphere key flora.

Microorganisms have a significant role in regulating the absorption and transportation of Cd in the soil-plant system. However, the mechanism by which key microbial taxa play a part in response to the absorption and transportation of Cd in rice under Cd stress requires further exploration. In this study, the cadmium-tolerant endophytic bacterium Herbaspirillum sp. R3 (R3) and Fe-Mn-modified biochar (Fe-Mn) were, respectively, applied to cadmium-contaminated rice paddies to investigate the effects of key bacterial taxa in the soil-rice system on the absorption and transportation of Cd in rice under different treatments. The results showed that both R3 and Fe-Mn treatments considerably decreased the content of cadmium in roots, stems and leaves of rice at the peak tillering stage by 17.24-49.28% in comparison to the control (CK). The cadmium content reduction effect of R3 treatment is better than that of Fe-Mn treatment. Further analysis revealed that the key bacterial taxa in rice roots under R3 treatment were Sideroxydans and Actinobacteria, and that their abundance showed a substantial positive correlation and a significant negative correlation with the capacity of rice roots to assimilate Cd from the surroundings, respectively. The significant increase in soil pH under Fe-Mn treatment, significant reduction in the relative abundances of Acidobacteria, Verrucomicrobia, Subdivision3 genera incertae sedis, Sideroxydans, Geobacter, Gp1, and Gp3, and the significant increase in the relative abundance of Thiobacillus among the soil bacterial taxa may be the main reasons for the decrease in available Cd content of the soil. In addition, both the R3 and Fe-Mn treatments showed some growth-promoting effects on rice, which may be related to their promotion of transformations of soil available nutrients. This paper describes the possible microbial mechanisms by which strain R3 and Fe-Mn biochar reduce Cd uptake in rice, providing a theoretical basis for the remediation of Cd contamination in rice and soil by utilizing key microbial taxa.

Cadmium phytoremediation potential of Houttuynia cordata: Insights from growth, uptake, and rhizosphere mechanisms.

Cadmium (Cd) pollutes 7.0 % of China's land area. This study examined the potential of Houttuynia cordata for Cd phytoremediation because of its ability to accumulate Cd in its growth matrix. H. cordata were planted in plastic pots filled with paddy field soils having low (LCd), medium (MCd), and high (HCd) Cd levels of 0.19, 0.69, and 2.91 mg/kg, respectively. After six months of growth, harvested plant parts were evaluated for Cd uptake and tolerance mechanisms. Metabolomics and metagenomics approaches were employed to investigate the soil rhizosphere mechanism. Results showed that the average plant biomass increased as soil Cd increased. The biomass Cd contents surpassed the allowable Cd limits for food (≤ 0.2 mg/kg) and medicinal uses (≤ 0.3 mg/kg). Cd contents were higher in H. cordata roots (30.59-86.27 mg/kg) than in other plant parts (0.63-2.90 mg/kg), with significantly increasing values as Cd soil level increased. Phenolic acids, lipids, amino acids and derivatives, organic acids, and alkaloids comprised the majority (69 in MCd vs HCd and 73 % in LCd vs HCd) of the shared upregulated metabolites. In addition, 13 metabolites specific to H. cordata root exudates were significantly increased. The top two principal metabolic pathways were arginine and proline metabolism, and beta-alanine metabolism. H. cordata increased the abundance of Firmicutes and Glomeromycota across all three Cd levels, and also stimulated the growth of Patescibacteria, Rozellomycota, and Claroideoglomus in HCd. Accordingly, H. cordata demonstrated potential for remediation of Cd-contaminated soils, and safety measures for its production and food use must be highly considered.

High temperature and nib acidification during cacao-controlled fermentation improve cadmium transfer from nibs to testa and the liquor's flavor.

Migration of nib Cd to the testa during fermentation can be achieved with high temperatures (> 45 °C) and low nib pH values (< 5.0) using spontaneous fermentation. However, this low pH can lead to low flavor quality. This study used three controlled temperature fermentation treatments on three cacao genotypes (CCN 51, ICS 95, and TCS 01) to test its effects on the nib pH, the migration of nib Cd to the testa, and the liquor flavor quality. All treatments were effective in reducing the total nib Cd concentration. Nevertheless, the treatment with the higher mean temperature (44.25 °C) and acidification (pH 4.66) reached the highest mean nib Cd reductions throughout fermentation, a 1.37 factor in TCS 01, promoting the development of fine-flavor cocoa sensorial notes. In unfermented beans, the Cd concentration of nibs was higher than that of the testa, and the Cd migration proceeded down the total concentration gradient. However, Cd migration was observed against the concentration gradient (testa Cd > nib Cd) from the fourth day. Cd migration could increase by extensive fermentation until the sixth day in high temperatures and probably by the adsorbent capacity of the testa. Genotype-by-treatment interactions were present for the nib Cd reduction, and a universal percentage of decrease of Cd for each genotype with fermentation cannot be expected. Selecting genotypes with highly adsorbent testa combined with controlled temperatures would help reduce the Cd concentration in the cacao raw material, improving its safety and quality.

Crosstalk between melatonin and nitric oxide restrains Cadmium-induced oxidative stress and enhances vinblastine biosynthesis in Catharanthus roseus (L) G Don.

KEY MESSAGE: Nitric oxide functions downstream of the melatonin in adjusting Cd-induced osmotic and oxidative stresses, upregulating the transcription of D4H and DAT genes, and increasing total alkaloid and vincristine contents. A few studies have investigated the relationship between melatonin (MT) and nitric oxide (NO) in regulating defensive responses. However, it is still unclear how MT and NO interact to regulate the biosynthesis of alkaloids and vincristine in leaves of Catharanthus roseus (L.) G. Don under Cd stress. Therefore, this context was explored in the present study. Results showed that Cd toxicity (200 µM) induced oxidative stress, decreased biomass, Chl a, and Chl b content, and increased the content of total alkaloid and vinblastine in the leaves. Application of both MT (100 µM) and sodium nitroprusside (200 µM SNP, as NO donor) enhanced endogenous NO content and accordingly increased metal tolerance index, the content of total alkaloid and vinblastine. It also upregulated the transcription of two respective genes (D4H and DAT) under non-stress and Cd stress conditions. Moreover, the MT and SNP treatments reduced the content of H2O2 and malondialdehyde, increased the activities of superoxide dismutase and ascorbate peroxidase, enhanced proline accumulation, and improved relative water content in leaves of Cd-exposed plants. The scavenging NO by 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxy l-3-oxide (cPTIO) averted the effects of MT on the content of total alkaloid and vinblastine and antioxidative responses. Still, the effects conferred by NO on attributes mentioned above were not significantly impaired by p-chlorophenylalanine (p-CPA as an inhibitor of MT biosynthesis). These findings and multivariate analyses indicate that MT motivated terpenoid indole alkaloid biosynthesis and mitigated Cd-induced oxidative stress in the leaves of periwinkle in a NO-dependent manner.

Industrial hemp (Cannabis sativa L.) can utilize and remediate soil strongly contaminated with Cu, As, Cd, and Pb by phytoattenuation.

Industrial hemp (Cannabis sativa L.) has great application potential in heavy metal-polluted soils owing to its safe non-food utilization. However, the fate of heavy metals in different varieties of hemp planted in strongly contaminated natural soils remains unknown. Here, we investigated the growth, heavy metal uptake, distribution, and transfer of nine hemp varieties in soils strongly contaminated with Cu, As, Cd, and Pb. Hemp variety and metal type were the main factors affecting the growth and heavy metal uptake in hemp. The nine hemp varieties grew well in the contaminated soils; however, differences existed among the varieties. The biomass of Z3 reached 5669.1 kg hm-1, whereas that of Yunma No. 1 was only 51.8 % of Z3. The plant height, stalk diameter, and stalk bark thickness of Z3 were greater than those of the other varieties, reaching 168 cm, 9.2 mm, and 0.56 mm, respectively. Permanova's analysis revealed that the total effects of Cu, As, Cd, and Pb on the growth of the nine hemp varieties reached 60 %, with leaf As having the greatest effect, reaching 16 %. , Even in strongly contaminated soils, the nine varieties showed poor Cu, As, Cd, and Pb uptake. Most of the Cu, As, Cd, and Pb were retained in the root, reaching 57.7-72.4, 47.6-64.7, 76.0-92.9, and 70.0-87.8 %, respectively. Overall, the Cu, As, Cd, and Pb uptake of Wanma No.1 was the highest among the nine varieties, whereas that of Guangxi Bama was the lowest. These results indicate that hemp is a viable alternative for phytoattenuation in soils contaminated with heavy metals because of its ability to tolerate and accumulate Cu, As, Cd, and Pb in its roots, and Guangxi Bama is superior to the other varieties considering the safe utilization of hemp products.

Mechanism for Fe(III) to decrease cadmium uptake of wheat plant: Rhizosphere passivation, competitive absorption and physiological regulation.

The presence of dissolved Fe(III) and Fe(III)-containing minerals has been found to alleviate cadmium (Cd) accumulation in wheat plants grown in Cd-contaminated soils, but the specific mechanism remains elusive. In this work, hydroponic experiments were conducted to dissect the mechanism for dissolved Fe(III) (0-2000 µmol L-1) to decrease Cd uptake of wheat plants and study the influence of Fe(III) concentration and Cd(II) pollution level (0-20 µmol L-1) on the Cd uptake process. The results indicated that dissolved Fe(III) significantly decreased Cd uptake through rhizosphere passivation, competitive absorption, and physiological regulation. The formation of poorly crystalline Fe(III) oxides facilitated the adsorption and immobilization of Cd(II) on the rhizoplane (over 80.4 %). In wheat rhizosphere, the content of CaCl2-extractable Cd decreased by 52.7 % when Fe(III) concentration was controlled at 2000 µmol L-1, and the presence of Fe(III) may reduce the formation of Cd(II)-organic acid complexes (including malic acid and succinic acid secreted by wheat roots), which could be attributed to competitive reactions. Down-regulation of Cd uptake genes (TaNramp5-a and TaNramp5-b) and transport genes (TaHMA3-a, TaHMA3-b and TaHMA2), along with up-regulation of the Cd efflux gene TaPDR8-4A7A, contributed much to the reduction of Cd accumulation in wheat plants in the presence of Fe(III). The inhibitory effect of Fe(III) on Cd uptake and transport in wheat plants declined with increasing Cd(II) concentration, particularly at 20 µmol L-1. This work provides important implications for remediating Cd-contaminated farmland soil and ensuring the safe production of wheat by using dissolved Fe(III) and Fe(III)-containing minerals.

Genome-wide identification of bHLH gene family and its response to cadmium stress in Populus × canescens.

The basic helix-loop-helix (bHLH) gene family is integral to various aspects of plant development and the orchestration of stress response. This study focuses on the bHLH genes within Populus × canescens, a poplar species noted for its significant tolerance to cadmium (Cd) stress. Through our comprehensive genomic analysis, we have identified and characterized 170 bHLH genes within the P. canescens genome. These genes have been systematically classified into 22 distant subfamilies based on their evolutionary relationships. A notable conservation in gene structure and motif compositions were conserved across these subfamilies. Further analysis of the promoter regions of these genes revealed an abundance of essential cis-acting element, which are associated with plant hormonal regulation, development processes, and stress response pathway. Utilizing quantitative PCR (qPCR), we have documented the differential regulation of PcbHLHs in response to elevated Cd concentrations, with distinct expression patterns observed across various tissues. This study is poised to unravel the molecular mechanism underpinning Cd tolerance in P. canescens, offering valuable insights for the development of new cultivars with enhanced Cd accumulation capacity and tolerance. Such advancements are crucial for implementing effective phytoremediation strategies to mitigate soil pollution caused by Cd.

Cryo-EM structure of cadmium-bound human ABCB6.

ATP-binding cassette transporter B6 (ABCB6), a protein essential for heme biosynthesis in mitochondria, also functions as a heavy metal efflux pump. Here, we present cryo-electron microscopy structures of human ABCB6 bound to a cadmium Cd(II) ion in the presence of antioxidant thiol peptides glutathione (GSH) and phytochelatin 2 (PC2) at resolutions of 3.2 and 3.1 Å, respectively. The overall folding of the two structures resembles the inward-facing apo state but with less separation between the two halves of the transporter. Two GSH molecules are symmetrically bound to the Cd(II) ion in a bent conformation, with the central cysteine protruding towards the metal. The N-terminal glutamate and C-terminal glycine of GSH do not directly interact with Cd(II) but contribute to neutralizing positive charges of the binding cavity by forming hydrogen bonds and van der Waals interactions with nearby residues. In the presence of PC2, Cd(II) binding to ABCB6 is similar to that observed with GSH, except that two cysteine residues of each PC2 molecule participate in Cd(II) coordination to form a tetrathiolate. Structural comparison of human ABCB6 and its homologous Atm-type transporters indicate that their distinct substrate specificity might be attributed to variations in the capping residues situated at the top of the substrate-binding cavity.

Enhancing remediation efficiency of hyperaccumulators through earthworm addition: Evidence from a pot study on cadmium-contaminated soil.

Soil cadmium (Cd) contamination is an urgent environmental problem, which endangers human health through the food chain. Bioremediation attracted extensive attention around the world due to the high cost-efficiency. However, the remediation efficiency of different plant and earthworm species of soil Cd pollution is still unclear, it is thus of great significance to explore the combined effects of different remediation plants and earthworm species to improve the bioremediation capacity. In the present study, we consequently selected three species of Cd hyperaccumulator plants (vetiver, P. vittata and S. emarginatum) and three species of earthworms (E. fetida P1, E. fetida P2, and P. guillelmi) to compare the differences in Cd accumulation among various earthworm-plant combinations. Results indicated that the changes of soil pH and SOM in plant-animal combined application induced the higher soil Cd removal efficiency. The Cd removal efficiency showed highest in combination groups P. vittata-E. fetida P2 and P. vittata-P. guillelmi. Meanwhile, the improvements of biomass of plants and animals also were consistent with the increasing of Cd concentration in both plants and earthworms after combined application. It showed that the Cd concentrations in P. vittata were the highest while the TFs of Cd in S. emarginatum displays significantly more than that in others. In conclusion, the recommended combined system of earthworm-plant (P. vittata-E. fetida P2 and P. vittata-P. guillelmi) to provide reference for soil Cd bioremediation system in practice.

Effects of nitrogen forms on Cd uptake and tolerance in wheat seedlings.

Hydroponic experiment was conducted to explore the effects of two nitrogen (N) levels with five nitrate nitrogen (NO3--N) and ammonium nitrogen (NH4+-N) ratios on the growth status and Cd migration patterns of wheat seedlings under Cd5 and Cd30 level. Results showed that higher Cd were detrimental to the growth, absorption of K and Ca, expression of genes mediating NO3--N and NH4+-N transport, which also increased the content of malondialdehyde (MDA) and hydrogen peroxide (H2O2) in shoots and roots of wheat seedlings. Higher N treatment alleviated the inhibitory effects of Cd stress on the biomass, root development, photosynthesis and increased the tolerance index of wheat seedlings. The ratio of NO3--N and NH4+-N was the main factor driving Cd accumulation in wheat seedlings, the combined application of NH4+-N and NO3--N was more conducive for the growth, nitrogen assimilation and Cd tolerance to the Cd stressed wheat seedlings. Increased NO3--N application rates significantly up-regulated the expression levels of TaNPF2.12, TaNRT2.2, increased NH4+-N application rates significantly up-regulated the expression levels of TaAMT1.1. The high proportion of NO3--N promoted the absorption of K, Ca and Cd in the shoots and roots of wheat seedlings, while NH4+-N was the opposite. Under low Cd conditions, the NO3--N to NH4+-N ratio of 1:1 was more conducive to the growth of wheat seedlings, under high Cd stress, the optimal of NO3--N to NH4+-N was 1:2 for inhibiting the accumulation of Cd in wheat seedlings. The results indicated that increasing NH4+-N ratio appropriately could inhibit wheat Cd uptake by increasing NH4+, K+ and Ca2+ for K and Ca channels, and promote wheat growth by promoting N assimilation process.

Rotation of Celosia argentea and Sedum plumbizincicola promotes Cd phytoextraction efficiency.

Most hyperaccumulators cannot maintain vigorous growth throughout the year, which may result in a low phytoextraction efficiency for a few months. In the present study, rotation of two hyperaccumulators is proposed to address this issue. An 18-month field experiment was conducted to evaluate the phytoextraction efficiency of Cd by the monoculture and rotation of Celosia argentea and Sedum plumbizincicola. The results showed that rotation increased amount of extracted Cd increased by 2.3 and 1.6 times compared with monoculture of C. argentea and S. plumbizincicola. In rotation system, the biomass of S. plumbizincicola and Cd accumulation in C. argentea increased by 54.4% and 40.7%, respectively. Rotation reduced fallow time and increased harvesting frequency, thereby enhancing Cd phytoextraction. Planting C. argentea significantly decreased soil pathogenic microbes and increased the abundances of plant growth-promoting rhizobacteria (PGPR) and 1-aminocyclopropane-1-carboxylate (ACC) deaminase genes, which may be beneficial for the growth of S. plumbizincicola. Planting S. plumbizincicola increased the abundance of sulfur oxidization (SOX) system genes and decreased soil pH (p < 0.05), thereby increasing the Cd uptake by C. argentea. These findings indicated that rotation of C. argentea and S. plumbizincicola is a promising method for promoting Cd phytoextraction.

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.