Despite the widespread use of Tillandsia usneoides as a biomonitor for atmospheric Hg-0 pollution, little is known about its dose-dependent physiological and molecular responses to Hg-0 exposure. Here, we integrated twenty physiological biomarkers, across five categories with transcriptomic and metabolomic analyses, to reveal the potential roles of metal-chelation partitioning, hormetic trade-offs, energy metabolism regulation, and membrane lipid remodeling in Hg-0 tolerance. We first identified O-2(& centerdot;-) and H2O2 as sensitive damage indicators, and CAT, POD, APX, MDHAR, and MT as sensitive response indicators. Among them, CAT and POD showed the most robust resistance across a wide concentration range. We further uncovered a trade-off-mediated hormesis pattern, where a low Hg-0 dose (5 ng & centerdot;m(-3)) enhances antioxidant and chelation defenses without evident costs. At concentrations <= 10 ng & centerdot;m(-3), Hg binding was consistently governed by MT-mediated sequestration (up to 1035-fold induction). The 10 ng & centerdot;m(-3) concentration represents a threshold at which stimulation and inhibition coexist likely due to metabolic resource reallocation. In contrast, higher doses (50-500 ng & centerdot;m(-3)) induced excessive ROS production, collapse of cellular redox balance, and membrane lipid peroxidation, marking transition from adaptation to injury. Multi-omics evidence also revealed dose-segmented regulatory strategies. At concentrations <= 50 ng & centerdot;m(-3), elevated terpenoid levels were observed, which may be associated with ROS detoxification. Conversely, exposure to 50-500 ng & centerdot;m(-3) Hg-0 triggered a shift toward PC-dominated chelation (285-307 fold increase) and suppression of photosynthetic carbon assimilation. It also activated MAPK cascades, phenylpropanoid biosynthesis, and cytoskeletal remodeling. In addition, ABC transporters mediated the transport of metal-thiol complexes collectively, potentially related to detoxification and structural defense. Overall, this study identifies a concentration-sensitive regulatory window governing redox balance, energy metabolism, and Hg chelation. This framework provides a new insight into potential mechanisms of Hg-0 detoxification.

Returning straw to soil can increase the soil organic matter (OM) content, thus lowering the bioavailability of lipophilic compounds. However, the effects of OM content and its fractions (particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)) on the toxicity of soil polycyclic aromatic hydrocarbons (PAHs) to soil invertebrates remain poorly understood. In this study, collembolan Folsomia candida was exposed to a range of phenanthrene (PHE) concentrations (0, 5, 10, 20, 30, 40 mg kg-1 dry soil) at four different OM addition rates (0%, 0.25%, 0.35%, 0.5%) for 28 days. At the end of the experiment, the life-history traits (survival and juvenile production), and the internal PHE concentration of F. candida and the fractions of OM (POC and MAOC) were determined. The results showed that increasing OM content reduced the internal PHE concentration at higher soil PHE concentrations. The survival and juvenile production was significantly increased by increasing OM content under PHE contamination (p < 0.05). Further, the concentration causing 50% inhibition of juvenile production (EC50) value for the effect of PHE on juvenile production was significantly increased by adding OM (p < 0.05). Partial least squares structural equation modeling demonstrated that different soil organic matter fractions differed in their ability to immobilize PHE, and MAOC showed a capacity to reduce the bioavailability and toxicity of PHE better than POC. These results show that the addition of OM could alleviate the toxicity of PHE to F. candida. Our findings highlight the potential of exogenous organic matter to reduce PAH toxicity to soil invertebrates and provide a theoretical basis for the ecological remediation of PAH-contaminated soils.

Climate change is intensifying daily temperature fluctuations (DTF), reshaping the thermal environment experienced by organisms. Most related studies have been conducted under constant high temperatures, and how intensified DTF affects pollutant toxicity and underlying mechanisms remains poorly understood. We exposed Enchytraeus albidus to phenanthrene (PHE; 0, 5, 10, 20, 40 and 80 mg kg-1) under four DTF regimes (mean 20 °C; amplitudes 0, 2, 5 and 7 °C) and measured internal PHE, the number of juveniles, and lipid composition. PHE and DTF showed a synergistic negative effect on the number of juveniles, with EC50 decreasing from 46 to 23 mg kg-1 as DTF amplitude increased, while internal PHE did not differ significantly among DTF regimes. Membrane phospholipid unsaturation and storage lipid reserves were reduced as PHE concentration and DTF amplitude increased. Correlation analysis further showed that variation in the number of juveniles was significantly associated with changes in membrane phospholipid unsaturation and storage lipid reserves, and their response patterns were generally consistent across treatments. Overall, by linking variation in the number of juveniles with concurrent changes in membrane phospholipids and storage lipids, these results provide possible mechanistic insight into how intensified DTF modulates PHE toxicity and further emphasize the need to explicitly consider DTF in risk assessment under climate change.

Cadmium (Cd) accumulation in rice poses a serious threat to food safety and human health. However, the molecular mechanisms underlying Cd uptake, enrichment and translocation across different rice varieties remain unclear. Here, we identified R66 and R41 as high- and low-Cd-accumulating cultivars, respectively, through screening of 110 rice cultivars. Phenotypic and physiological comparisons were conducted, and root transcriptomics analyzed to identify key genes responsible for Cd uptake and translocation. We identified two candidate genes, OsSultr1;1 and OsZIP4, as possible regulators of Cd accumulation in rice. In short-term hydroponic validation assays at the seedling stage under 5 μM Cd stress, knockout of OsSultr1;1 decreased Cd concentrations in roots by approximately 40%, suggesting its involvement in modulating Cd uptake. Knockout of OsZIP4 reduced shoot Cd concentrations by 45% and significantly lowered the translocation factor, suggesting that OsZIP4 is involved in the long-distance transport of Cd. Overall, this study further elucidates the candidate gene-mediated pathways influencing Cd uptake and translocation in different rice varieties at the early developmental stage, providing a crucial theoretical foundation for breeding low-Cd-accumulating rice to enhance food safety.

The symbiosis between Robinia pseudoacacia L. and rhizobia has been identified as an advanced remediation solution for Cd contamination. However, the mechanism involved in the regulatory interaction of nitric oxide (NO) and Robinia-rhizobia symbiosis in the remediation of Cd pollution remains unclear. We aimed to elucidate the mechanism by which NO mediates the enhancement of Cd resistance in the Robinia-rhizobia symbiosis. We analyzed plant growth, Cd content and Cd chemical form, antioxidant system, nitrogen fixation capacity and endophytic community structure of Robinia-rhizobia symbiosis using physiological and biochemical parameters and metagenomic sequencing. NO significantly reduced Cd content in the roots of symbiosis (14.9

Elevated atmospheric carbon dioxide (ECO2) is a key climatic factor influencing the resilience of plant–microbial symbiotic systems against heavy metal contamination. Robinia pseudoacacia–rhizobia symbiosis shows great potential for cadmium (Cd) remediation. However, the mechanism by which ECO2 regulates Cd phytostabilization in symbiosis remains unclear. This study conducted a 90-day experiment in growth chambers to investigate the effects of ECO2 on the growth, Cd accumulation and chemical forms, as well as nutrient uptake and antioxidant system in Robinia pseudoacacia–rhizobia symbiosis. Results indicated that ECO2 significantly increased plant biomass and photosynthetic efficiency while significantly raising Cd content in roots (34.5%, p < 0.001) and decreasing it in shoots (31.4%, p < 0.001). This resulted in a significant reduction in Cd translocation factor (TF). Meanwhile, ECO2 markedly increased Cd accumulation in roots (81.2%, p < 0.001) and reduced the bioavailability of Cd in the symbiosis. Moreover, ECO2 promoted the content of nutrients and stimulated the antioxidant system. The random forest model indicated that root weight, Cd and Mn contents are the core factors for ECO2-driven Cd phytostabilization. This study demonstrates that ECO2 enhanced Cd phytostabilization by optimizing the resistance of symbiosis to Cd, offering a novel perspective for predicting plant–microbe joint restoration of heavy metal pollution under global climate change scenarios.

Gaseous elemental mercury (Hg0) poses significant risks to ecosystems due to its high volatility and bioavailability. However, it remains largely unknown how the endophytes of Tillandsia usneoides (Spanish moss), a biological indicator responds to Hg0. In this study, after 14 days of exposure to the vapor from 0.22 mL of liquid Hg0, T. usneoides accumulated a markedly elevated Hg content of 164, 900 +/- 28, 900 mu g kg-1. High-throughput sequencing of 16S rRNA and ITS genes revealed that Hg0 exposure significantly reduced the alpha-diversity of endophytic bacteria and altered the R-diversity of both bacterial and fungal communities. Under Hg0 stress, taxonomic shifts included increased relative abundances of Pseudomonas, Enterobacter, and Acidiella. Functional predictions further indicated upregulated expression of key enzymes involved in Hg detoxification and antioxidant defense, such as mercuric reductase (MerA), glutathione S-transferase (GST), catalase (CAT), and superoxide dismutase (SOD). From Hg0-exposed T. usneoides, we isolated five endophytic bacteria (Staphylococcus sp. L3, Pseudomonas sp. L9, Enterobacter L19, Enterobacter L6, Bacillus LE) and two fungi (Aspergillus G1, G2), all of which demonstrated strong Hg2+ transformation and tolerance. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) confirmed morphological adaptations and Hg enrichment on microbial surfaces. Foliar inoculation with strains L6, LE, and G2 significantly alleviated Hg0-induced growth inhibition in Arabidopsis thaliana, enhancing biomass, leaf/root length, and root surface area, while mitigating auxin suppression. Overall, this study clarifies how T. usneoides endophytes respond to Hg0 and highlights their promising role in microbial-assisted phytoremediation of Hg0 pollution.

The Tibetan Plateau stores mercury (Hg) pollutants in its glaciers, which can be methylated by microorganisms into more neurotoxic methylmercury (MeHg) under climate warming, posing a significant threat to downstream ecosystems. However, microbial Hg methylation in glaciers across different climate zones on the plateau is poorly understood. To investigate the influence of climate zones on Hg methylation, we selected the Kuqionggangri Glacier (monsoon zone) and Tianshan Glacier No. 1 (westerly zone) for a comparative study. By measuring total mercury (THg) and MeHg concentrations, along with physicochemical parameters, in snow and ice, and by combining these measurements with high-throughput 16S rRNA sequencing, we explored the underlying mechanisms. The results show that although the THg concentration in the westerly-zone glacier (66.2 ng/L) was significantly higher than in the monsoon-zone glacier (24.3 ng/L), the MeHg/THg ratio was markedly lower (0.11% vs. 0.23%). This spatial differentiation is primarily governed by climate-zone-dominated material transport patterns: monsoon-transported, abundant organic matter (total organic carbon, 3.86 mg/L) promoted the development of Hg-methylating microorganisms (such as Nitrospira and Desulfovibrio), whose relative abundance (3.8%) was much higher than in the westerly zone (0.15%). Co-occurrence network analysis further corroborated the pivotal role of these key taxa in the microbial interaction network. This study systematically reveals the spatial heterogeneity of microbial Hg methylation across glaciers of the Tibetan Plateau, thereby providing a scientific basis for assessing the biogeochemical cycling and ecological risks of Hg in glacial ablation zones.

The Tibetan Plateau stores mercury (Hg) pollutants in its glaciers, which can be methylated by microorganisms into more neurotoxic methylmercury (MeHg) under climate warming, posing a significant threat to downstream ecosystems. However, microbial Hg methylation in glaciers across different climate zones on the plateau is poorly understood. To investigate the influence of climate zones on Hg methylation, we selected the Kuqionggangri Glacier (monsoon zone) and Tianshan Glacier No. 1 (westerly zone) for a comparative study. By measuring total mercury (THg) and MeHg concentrations, along with physicochemical parameters, in snow and ice, and by combining these measurements with high-throughput 16S rRNA sequencing, we explored the underlying mechanisms. The results show that although the THg concentration in the westerly-zone glacier (66.2 ng/L) was significantly higher than in the monsoon-zone glacier (24.3 ng/L), the MeHg/THg ratio was markedly lower (0.11% vs. 0.23%). This spatial differentiation is primarily governed by climate-zone-dominated material transport patterns: monsoon-transported, abundant organic matter (total organic carbon, 3.86 mg/L) promoted the development of potential Hg-methylating microorganisms (such as Nitrospira and Desulfovibrio), whose relative abundance (3.8%) was much higher than in the westerly zone (0.15%). Co-occurrence network analysis further corroborated the pivotal role of these key taxa in the microbial interaction network. This study systematically reveals the spatial heterogeneity of microbial Hg methylation across glaciers of the Tibetan Plateau, thereby providing a scientific basis for assessing the biogeochemical cycling and ecological risks of Hg in glacial ablation zones.

Interactive effects of elevated CO2 (eCO2) and heavy metal pollution on plant-microbe systems are critical for predicting ecosystem responses under future climate scenarios. We investigated how eCO2 modulates lead (Pb) toxicity in rapeseed (Brassica napus) and its rhizosphere microbiome using a two-factor experiment with three CO2 concentrations (400, 550, and 700 ppm) and three Pb levels (0, 70, and 400 mg·kg-1). The alleviating effect of eCO2 on Pb toxicity was strongly concentration-dependent. Moderate eCO2 (550 ppm) significantly increased biomass, photosynthetic rate, nutrient uptake, and non-enzymatic antioxidant capacity (glutathione and ascorbate), thereby mitigating Pb-induced oxidative damage. Conversely, 700 ppm eCO2 induced photosynthetic acclimation and provided limited protection. Concurrently, eCO2 reshaped rhizosphere bacterial community in a concentration-dependent manner, enriching stress-tolerant taxa including Proteobacteria and Paenibacillus, and enhancing predicted functional pathways related to carbon and energy metabolism. Mantel analysis revealed strong positive correlations between plant nutrient status and microbial functional potential. We propose a "plant-microbe interaction framework" in which moderate eCO2 increases photosynthetic carbon inputs, fostering beneficial microbial communities that in turn support plant tolerance to Pb stress. These findings highlight the non-linear nature of CO2-heavy metal interactions and provide new insights for microbe-assisted phytoremediation strategies under future climate conditions.

Gaseous elemental mercury [Hg(0)g] contamination has emerged as a critical global environmental threat owing to its persistent atmospheric mobility. Despite its ecological risks, the metabolic adaptation mechanisms of plants to Hg(0)g stress remain largely unexplored. Here, we investigated the physiological and metabolic responses of Tillandsia usneoide, a bioindicator for atmospheric Hg(0), to Hg(0)g exposure (∼300 μg m⁻³, an extreme concentration near point sources like Hg smelting, mining, and coal processing). Our results demonstrated that Hg(0)g induced severe oxidative stress in T. usneoide, evidenced by elevated malondialdehyde (MDA) and superoxide anion (O2-) levels, concomitant with suppressed potassium (K) and phosphorus (P) uptake and chlorophyll biosynthesis. To counteract oxidative damage, T. usneoide activated antioxidant defenses by modulating superoxide dismutase (SOD) and peroxidase (POD) activities. Untargeted metabolomics found Hg(0)g-responsive metabolites predominantly enriched in betalain, cutin, suberine and wax biosynthesis, and tyrosine metabolism. Notably, two key metabolites, liquiritin and L-Dopa, were functionally validated to alleviate Hg(0)g toxicity in Arabidopsis thaliana. Liquiritin and L-Dopa not only reduced Hg uptake but also enhanced POD activity and glutathione (GSH) synthesis, thereby mitigating H2O2 and MDA-driven oxidative stress while promoting plant growth. Taken together, this study provides the first evidence of Hg(0)-induced metabolic reprogramming in plants and proposes liquiritin and L-Dopa as promising candidates for phytoremediation strategies against Hg(0)g pollution.

Rhizobia can enhance legume growth in the presence of heavy metals, making the legume-rhizobium symbiosis promising for heavy metal phytoremediation. This study aimed to elucidate the molecular mechanisms by which rhizobia enhance cadmium (Cd) tolerance of Robinia pseudoacacia. We investigated two R. pseudoacacia genotypes with contrasting Cd accumulation capacity: SX (high-accumulating) and HB (low-accumulating). The study examined oxidative stress responses, reactive oxygen species (ROS) scavenging mechanisms, and changes in transcriptome and metabolome profiles following Cd exposure and rhizobium inoculation. Rhizobium inoculation effectively reduced H2O2 and MDA accumulation in both genotypes, enhancing their antioxidative substances, despite increased shoot Cd accumulation. Furthermore, rhizobium symbiosis promoted the accumulation of amino acids and carbohydrates, providing additional carbon resources and osmo-protective compounds to support Cd stress responses. Transcriptome analysis revealed elevated expression of metal transporter genes (ABC, OPT, HIP, DTX families) in SX shoots, leading to differential Cd accumulation between the genotypes. Under Cd exposure, SX plants exhibited significantly higher levels of flavonoid-related transcripts and metabolites, also antioxidant enzymes and substances compared to HB plants. Rhizobium inoculation alleviates cadmium toxicity to R. pseudoacacia by mitigating oxidative stress and activating amino acid and carbohydrate metabolism, enhancing the antioxidant defense system and Cd tolerance of R. pseudoacacia. These findings provide new insights into the Cd tolerance mechanisms of the Robinia-rhizobium symbiosis and its utilization for phytoremediation and heavy metal pollution management.

The consumption of rice enriched with methylmercury (MeHg) is one of the important exposure routes, posing threats to human health. However, metabolic responses of different rice varieties to soil MeHg remain unclear. Here, we analyzed the changes in root metabolites of high- (H699) and low-accumulating (H777) rice varieties under MeHg stress via untargeted metabolomics. MeHg (0.9 μg g-1) led to a significant increase in oxidative damage and antioxidant enzyme activities in leaves, suggesting leaves are more sensitive to MeHg. Distinct metabolite profiles occurred in rice roots under MeHg stress, with H699 predominantly involving lipid metabolism pathways, whereas H777 engaged amino acid metabolism pathways. Two key metabolites, silymarin and 12-hydroxylauric acid can effectively alleviate MeHg stress in rice by reducing MeHg accumulated in rice roots by ∼85 %. Overall, this study provides key targets and foundations for breeding low-MeHg, tolerant rice, crucial for reducing grain MeHg, ensuring safety, and protecting health.

Sulfate-driven anaerobic oxidation of methane (AOM) and anaerobic digestion (AD) with municipal wastewater sludge containing heavy metals may provide favorable conditions for the biogeochemical transformation of mercury (Hg) by methanogens and methanotrophs. However, it remains largely unclear what Hg-methylators functioned and what role Methanosarcina played in these processes. Here, we performed sulfate-driven AOM following AD with Hg-containing wastewater sludge and investigated the role of microbes, especially Methanosarcina, in the biogeochemical transformation of Hg based on 16S rRNA amplicon and metatranscriptomic sequencing. Results showed that methylmercury (MeHg) concentrations and MeHg/total Hg ratios increased significantly, implying mercuric Hg [Hg(II)] methylation predominated MeHg demethylation. Desulfovibrio, Desulfobulbus and Methanosarcina dominated and thus likely played important roles in Hg(II) methylation, while Methanosarcina dominated and functioned in methane metabolism. In the presence of sulfate, differentially-expressed genes (DEGs) related to Hg transporting ATPase increased significantly, indicating Methanosarcina absorbed a large amount of Hg(II) and likely further methylated it to MeHg. No Hg response DEGs were found in the absence of sulfate, further confirming sulfate played an essential role in Hg cycle. Overall, these results suggest that controlling sulfate levels and Methanosarcina abundances in municipal wastewater could potentially mitigate MeHg risks to humans.

Iron plaques on the root surface can promote or inhibit the absorption and accumulation of heavy metals by plants. However, the mechanism by which iron regulates the response of Robinia pseudoacacia to mercury (Hg) has not been elucidated, which hinders its application in divalent Hg (Hg2+) removal from Hg-contaminated soil. In this study, association analyses between transcriptome and metabolome were used to investigate effects of iron on the rhizosphere microenvironment and performance of R. pseudoacacia to assess its potential for Hg2+ removal. The results showed that the addition of 10 mg kg-1 iron significantly increased the development of iron plaques on the root surface and reduced the secretion of low-molecular-weight organic acids by roots, thereby changing rhizosphere soil characteristics and decreasing total Hg in roots. In addition, the secretion of choline supported signal transduction and enhanced the interaction between R. pseudoacacia and rhizobia, thereby inducing resistance to Hg2+. Anti-oxidative enzyme activities were increased and Hg2+ exposure of plants was reduced. Enhanced Hg2+ resistance was indicated by improved photosynthesis and growth, despite promoted xylem loading and transport of Hg2+, resulting in its accumulation in aboveground tissues, which is essential for Hg2+ removal. These results indicate that iron addition has a great potential to improve the growth of R. pseudoacacia in Hg-contaminated soil and promote the accumulation of Hg2+ in aboveground tissues for phytoremediation approaches.

Atmospheric hydrargyrum (Hg) is a neurotoxic heavy metal, and plant leaves are active Hg reservoirs. Tillandsia usneoides is an indicator plant for atmospheric Hg pollution; however, the uptake, transport modes, and redistribution mechanisms of Hg in T. usneoides are underexplored. Herein, we investigated these mechanisms and the influencing factors of Hg0 in T. usneoides at multiple levels. We found that Hg0 can be absorbed through both stomata and lipids, with higher Hg concentrations showing a greater tendency to be taken up by lipids. Hg passes through cell membranes via active transport, facilitated by Ca2+ ion channels and water channel proteins. Most Hg (50.1-97.9%) is retained in tissue cells in a low-toxicity and low-activity form (phosphate, pectinate, protein-bound and oxalate), with a small fraction located on leaf surfaces and in cuticular cells. After entering the cells, Hg was primarily retained in the cell wall (26.7-47.9%), with HC-2 demonstrating maximal retention (88.8-96.6%). As much as 61.3-91.5% of organelle-associated Hg was localized in chloroplasts. The -OH functional group in HC-2 might play an important role in Hg retention, closing a significant gap in our understanding of the underlying mechanisms. Furthermore, we discovered that after the removal of Hg stress, T. usneoides did not release Hg for a month. However, there was a tendency for Hg in the tissue and surface to be transported toward the cuticle. Our findings expand the understanding of plant leaf-atmosphere Hg interactions and reveal the intrinsic mechanisms of Hg detoxification in T. usneoides.

Cadmium (Cd) pollution is a growing concern worldwide, because it threatens human health through the food chain. Woody plants, such as the pioneer species black locust (Robinia pseudoacacia L.), are widely used in phytoremediation of Cd-contaminated soils, but strongly differ in Cd tolerance. Nitric oxide (NO), a highly reactive gas of biogenic and anthropogenic origin, has been shown to protect plants to Cd exposure. We investigated the protective mechanism of NO against Cd toxicity in black locust using physiological, transcriptomic and metabolomic approaches. We studied the correlation between cell wall traits, genes, and metabolites. The findings indicated that NO improved the growth of black locust under Cd exposure and elevated the fraction of Cd in the cell wall. NO increased cell wall thickness by stimulating the biosynthesis of pectin, cellulose, hemicellulose, and lignin. Transcriptomic and metabolomic analyses demonstrated that NO upregulated genes related to root cell wall biosynthesis and increased the accumulation of related metabolites, thereby increasing the Cd resistance of black locust. Our results elucidated a molecular mechanism underlying NO-mediated Cd tolerance in black locust and provided novel insights for phytoremediation of Cd-polluted soils by woody plants.

In recent years, the utilization of phosphorus-enriched biochar (PBC) has attracted significant attention due to its exceptional stability and surface reactivity. This review systematically summarizes the advancements in research related to the application of PBC as an adsorbent for remediating water contaminated with heavy metals. Initially, the precursors utilized in the production of PBC, encompassing biomass and phosphorus sources, are introduced. Subsequently, the distinct physicochemical properties and adsorption characteristics resulting from phosphorus doping on the biochar surface through various carbonization processes and parameters are elucidated. Additionally, the diverse adsorption mechanisms employed by PBC in removing heavy metals from water are analyzed. Lastly, future research prospects and associated challenges concerning PBC are presented. This paper aims to furnish comprehensive background information for the practical implementation of PBC in the purification of heavy metal-contaminated water environments.

Rhizobia and arbuscular mycorrhizal fungi (AMF) are symbiotic microorganisms important for plants grown in nutrient-deficient and heavy metal-contaminated soils. However, it remains unclear how plants respond to the coupled stress by heavy metal and nitrogen (N) deficiency under co-inoculation. Here, we investigated the synergistic effect of Mesorhizobium huakuii QD9 and Funneliformis mosseae on the response of black locust (Robinia pseudoacacia L.) grown in sand culture to cadmium (Cd) under N deficiency conditions. The results showed that single inoculation of AMF improved the growth and Cd resistance of black locust, co-inoculation improved the most. Compared to non-inoculated controls, co-inoculation mediated higher biomass and antioxidant enzyme activity, reduced oxidative stress, and promoted nodulation, mycorrhizal colonization, photosynthetic capacity, and N, P, Fe and Mg acquisition when exposed to Cd. This increase was significantly higher under N deficiency compared to N sufficiency. In addition, the uptake of Cd by co-inoculated black locust roots increased, but Cd translocation to the above-ground decreased under both N deficiency and sufficiency. Thus, in the tripartite symbiotic system, not merely metabolic processes but also Cd uptake increased under N deficiency. However, enhanced Cd detoxification in the roots and reduced allocation to the shoot likely prevent Cd toxicity and rather stimulated growth under these conditions.

Sulfate-reducing bacteria (SRB) are known to alter methylmercury (MeHg) production in paddy soil, but the effect of SRB on MeHg dynamics in rhizosphere and rice plants remains to be fully elucidated. The present study investigated the impact of SRB on MeHg levels in unsterilized and γ-sterilized mercury-polluted paddy soils, with the aim to close this knowledge gap. Results showed that the presence of SRB reduced MeHg production by ∼22 % and ∼17 % in the two soils, but elevated MeHg contents by approximately 55 % and 99 % in rice grains, respectively. Similar trend at smaller scales were seen in roots and shoots. SRB inoculation exerted the most profound impact on amino acid metabolism in roots, with the relative response of L-arginine positively linking to MeHg concentrations in rhizosphere. The SRB-induced enrichment of MeHg in rice plants may be interpreted by the stronger presence of endophytic nitrogen-related microbes (e.g. Methylocaldum, Hyphomicrobium and Methylocystis) and TGA transcription factors interacting with glutathione metabolism and calmodulin. Our study provides valuable insights into the complex effects of SRB inoculation on MeHg dynamics in rice ecosystems, and may help to develop strategies to effectively control MeHg accumulation in rice grains.