Application of signaling molecules in reducing metal accumulation in alfalfa and alleviating metal-induced phytotoxicity in Pb/Cd-contaminated soil

Elucidating the downstream pathways triggered by H2S signaling in Arabidopsis thaliana under drought stress via transcriptome analysis

Hydrogen sulfide (H2S) is a crucial signaling molecule in plants. Recent studies have shown that H2S plays an equally important role as nitric oxide (NO) and hydrogen peroxide (H2O2) in plant signaling. Previous studies have demonstrated the involvement of H2S in regulating drought and other stressful environmental conditions, but the exact downstream molecular mechanisms activated by the H2S signaling molecule remain unclear. In this study, we conducted a comprehensive genome-wide transcriptomic analysis of both wild type (WT) and double mutant (lcd/des1). Arabidopsis thaliana plants were exposed to 40% polyethylene glycol (PEG) to induce drought stress and 20 µM sodium hydrosulfide (NaHS). The resulting transcriptome data were analyzed for differentially significant genes and their statistical enrichments in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. The results indicated significant upregulation of genes related to photosynthesis, carbon fixation, plant secondary metabolite biosynthesis, inositol and phosphatidylinositol signaling pathways, and stress-responsive pathways in mutant plants under drought stress. Mutant plants with impaired H2S signaling mechanisms displayed greater susceptibility to drought stress compared to wild-type plants. In summary, all findings highlight the pivotal role of H2S signaling in stimulating other drought-responsive signaling pathways.

NaHS immersion alleviates the stress effect of chromium(III) on alfalfa seeds by affecting active oxygen metabolism

OBJECTIVE

This study aimed to investigate the regulatory effects of exogenous hydrogen sulfide (H2S) on seed germination, seedling growth, and reactive oxygen species (ROS) homeostasis in alfalfa under chromium (Cr) ion (III) stress.

METHODS

The effects of 0-4 mM Cr(III) on the germination and seedling growth of alfalfa were first assessed. Subsequently, following seed NaHS immersion, the influence of H2S on alfalfa seed germination and seedling growth under 2 mM Cr(III) stress was investigated, and the substance contents and enzyme activities associated with ROS metabolism were quantified.

RESULTS

Compared to the control group, alfalfa plant germination was delayed under 2 mM Cr(III) stress for up to 48 h (p < 0.05). At 120 h, the total seedling length was approximately halved, and the root length was roughly one-third of the control. Treatment with 0.02-0.1 mM NaHS alleviated the delay in germination and root growth inhibition caused by 2 mM Cr(III) stress, resulting in an increased ratio of root length to hypocotyl length from 0.57 to 1 above. Additionally, immersion in 0.05 mM NaHS reduced hydrogen peroxide (H2O2) and oxygen-free radicals (O2· -) levels (p < 0.05), boosted glutathione (GSH) levels (p < 0.05), and notably enhanced catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR) activities (p < 0.05) compared to the 2 mM Cr(III) stress treatment group.

CONCLUSION

Seed immersion in NaHS mitigated the delay in germination and inhibition of root elongation under 2 mM Cr(III) stress. This effect is likely attributed to the regulation of intracellular ROS homeostasis and redox balance through enzymatic and non-enzymatic systems; thus, providing a potential mechanism for combating oxidative stress.

Building the physiological barrier: Suberin plasticity in response to environmental stimuli

In response to environmental changes, plant roots undergo two major differentiations: the formation of the Casparian strip and the suberin lamella, both of them are widely recognized as an apoplastic diffusion barrier for nutrient and water exchange between the soil and the root vascular bundle. Suberin is a complex biopolyester composed of glycerol esters and phenolic compounds deposited in the cell walls of specific tissues such as endodermis, exodermis, periderm, seed coat and other marginal tissues. Recently, significant progress has been made due to the development of biochemical and genetic techniques. In this review, we not only summarize the aspect of suberin biosynthesis, transport and polymerization, but also elucidate the molecular mechanisms regarding its regulatory network, as well as its adaptive role in abiotic or biotic stress. This will provide important theoretical references for improving crop growth by modifying their adaptive root suberin structure when exposed to environmental changes.

Transcriptome-wide m6A methylation profile reveals tissue specific regulatory networks in switchgrass (Panicum virgatum L.) under cadmium stress

The heavy metal cadmium (Cd), known for its high toxicity, poses a grave threat to human health through the food chain. N6-methyladenosine (m6A), the most abundant internal modification, regulates plant adaptation to various adversities, yet the panorama of m6A modifications in switchgrass under cadmium stress remains elusive. This study examines the physiological responses of switchgrass roots and shoots exposed to 50 μM CdCl2, alongside an overview of transcriptome-wide m6A methylation patterns. After cadmium treatment, methylation modifications are primarily enriched near stop codons and the 3'UTR region, with a negative correlation between m6A modification and gene expression levels. In shoots, approximately 58 % of DEGs with m6A modifications show upregulation in expression and decrease in m6A peaks, including zinc transporter 4-like (ZIP4). In roots, about 43 % of DEGs with m6A modifications exhibit downregulation in expression and increase in m6A peaks, such as the ABC transporter family member (ABCG25). We further validate the m6A enrichment, gene expression and mRNA stability of ZIP4 in response to Cd treatment. The results suggest that the negative correlation of m6A enrichment and gene expression is due to altered mRNA stability. Our study establishes an m6A regulatory network governing cadmium transport in switchgrass roots and shoots, offering new avenues for candidate gene manipulation in phytoremediation applications of heavy metal pollution.

Assembly patterns and key taxa of bacterial communities in the rhizosphere soil of moso bamboo (Phyllostachys pubescens) under different Cd and Pb pollution

Moso bamboo is excellent candidate for cadmium (Cd)/lead (Pb) phytoremediation, while rhizosphere microbiome has significant impact on phytoremediation efficiency of host plant. However, little is known about the rhizosphere bacterial communities of moso bamboo in Cd/Pb contaminated soils. Therefore, this study investigated the assembly patterns and key taxa of rhizosphere bacterial communities of moso bamboo in Cd/Pb polluted and unpolluted soils, by field sampling, chemical analysis, and 16S rRNA gene sequencing. The results indicated α-diversity between Cd/Pb polluted and unpolluted soils showed a similar pattern (p > 0.05), while β-diversity was significantly different (p < 0.05). The relative abundance analysis indicated α-proteobacteria (37%) and actinobacteria (31%) were dominant in Cd/Pb polluted soils, while γ-proteobacteria (40%) and α-proteobacteria (22%) were dominant in unpolluted soils. Co-occurrence network analysis indicated microbial networks were less complex and more negative in polluted soils than in unpolluted soils. Mantel analysis indicated soil available phosphorus, organic matter, and available Pb were the most important environmental factors affecting microbial community structure. Correlation analysis showed 11 bacterial genera were significantly positively related to Cd/Pb. Overall, this study identified the bacterial community composition of bamboo rhizosphere in responding to Cd/Pb contamination and provides a theoretical basis for microbe-assistant phytoremediation in the future.

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.

Utilization of phosphogypsum and red mud in alfalfa cultivation

In this work, the utilization of phosphogypsum (PG), a waste coming from the manufacture of phosphate fertilizers, as fertilizer for alfalfa (Medicago sativa L.) crops was investigated using pot experiments. The objective of this study was to evaluate the effects of both phosphogypsum and red mud (RM) in two soils representative of the pasture production area in Southern Spain. The morpho-physiological parameters of biomass, plant height, number of stems and number of leaves, as well as the chemical parameters of soil content, were measured. High doses of PG inhibited seed germination in some treatments. In addition, the treatment substrate (2550 g soil + 50 g kg-1 PG + 100 g kg-1 RM) also affected seed germination, possibly due to the large amount of RM. The application of PG and RM to the soil increased the availability of important nutrients for alfalfa, such as phosphorus (P), calcium (Ca2+) and magnesium (Mg2+). The results demonstrate that the treatment with PG significantly improved the uptake of P in alfalfa.

DNA-SIP delineates unique microbial communities in the rhizosphere of the hyperaccumulator Sedum alfredii which are beneficial to Cd phytoextraction

Rhizo-microbe recruited by hyperaccumulating plants are crucial for the extraction of metals from contaminated soils. It is important, but difficult, to identify the specific rhizosphere microbes of hyperaccumulators shaped by root exudation. Continuous 13CO2 labeling, microbial DNA-based stable isotope probing (DNA-SIP), and high throughput sequencing were applied to identify those rhizosphere microorganisms using exudates from the Cd hyperaccumulator Sedum alfredii. In contrast to its non-hyperaccumulating ecotype (NAE), the hyperaccumulating ecotype (HAE) of S. alfredii strongly changed the rhizosphere environment and extracted a 5-fold higher concentration of Cd from contaminated soil. Although both HAE and NAE harbored Streptomyces, Massilia, Bacillus, and WPS-2 Uncultured Bacteria with relative abundance of more than 1% in the rhizosphere associated with plant growth and immunity, the HAE rhizosphere specifically recruited Rhodanobacter (2.66%), Nocardioides (1.16%), and Burkholderia (1.01%) through exudates to benefit the extraction of Cd from soil. Different from the bacterial network with weak cooperation in the NAE rhizosphere, a closed-loop bacterial network shaped by exudates was established in the HAE rhizosphere to synergistically resist Cd. This research reveals a specific rhizosphere bacterial community induced by exudates assisted in the extraction of Cd by S. alfredii and provides a new perspective for plant regulation of the rhizo-microbe community beneficial for optimizing phytoremediation.

Cadmium levels and soil pH drive structure and function differentiation of endophytic bacterial communities in Sedum plumbizincicola: A field study

Sedum plumbizincicola is a promising hyperaccumulator for heavy metal phytoremediation. It grows in heavy metal polluted soil and stores specific endophyte resources with heavy metal tolerance or growth promotion characteristics. In this study, the endophyte communities of S. plumbizincicola, growing naturally in the field (two former mining locations and one natural location) were investigated, and their structure and function were comparatively studied. The bioaccumulation and translocation characteristics of cadmium (Cd) and selenium (Se) in S. plumbizincicola were also evaluated. The results showed that the heavy metal pollution reduced the richness and diversity of endophyte communities. Soil pH and Cd concentration could be the key factors affecting the composition of the endophyte community. Co-occurrence network analysis identified that 22 keystone taxa belonging to Actinobacteriota, Firmicutes, Myxococcota and Proteobacteria were positively correlated with Cd bioaccumulation and translocation. The predicted endophyte metabolic pathways were enriched in physiological metabolism, immune system, and genetic Information processing. These findings may help to understand how endophytes assist host plants to enhance their adaptability to harsh environments, and provide a basis for further exploration of plant-endophyte interactions and improvement in phytoremediation efficiency.

Mitigation of the mobilization and accumulation of toxic metal(loid)s in ryegrass using sodium sulfide

Remediation of soils contaminated with toxic metal(loid)s (TMs) and mitigation of the associated ecological and human health risks are of great concern. Sodium sulfide (Na2S) can be used as an amendment for the immobilization of TMs in contaminated soils; however, the effects of Na2S on the leachability, bioavailability, and uptake of TMs in highly-contaminated soils under field conditions have not been investigated yet. This is the first field-scale research study investigating the effect of Na2S application on soils with Hg, Pb and Cu contents 70-to-7000-fold higher than background values and also polluted with As, Cd, Ni, and Zn. An ex situ remediation project including soil replacement, immobilization with Na2S, and safe landfilling was conducted at Daiziying and Anle (China) with soils contaminated with As, Cd, Cu, Hg, Ni, Pb and Zn. Notably, Na2S application significantly lowered the sulfuric-nitric acid leachable TMs below the limits defined by Chinese regulations. There was also a significant reduction in the DTPA-extractable TMs in the two studied sites up to 85.9 % for Hg, 71.4 % for Cu, 71.9 % for Pb, 48.1 % for Cd, 37.1 % for Zn, 34.3 % for Ni, and 15.7 % for As compared to the untreated controls. Moreover, Na2S treatment decreased the shoot TM contents in the last harvest to levels lower than the TM regulation limits concerning fodder crops, and decreased the TM root-to-shoot translocation, compared to the untreated control sites. We conclude that Na2S has great potential to remediate soils heavily tainted with TMs and mitigate the associated ecological and human health risks.

Silicon Combined with Melatonin Reduces Cd Absorption and Translocation in Maize

Cadmium (Cd) is one of the most toxic and widely distributed heavy metal pollutants, posing a huge threat to crop production, food security, and human health. Corn is an important food source and feed crop. Corn growth is subject to Cd stress; thus, reducing cadmium stress, absorption, and transportation is of great significance for achieving high yields, a high efficiency, and sustainable and safe corn production. The use of silicon or melatonin alone can reduce cadmium accumulation and toxicity in plants, but it is unclear whether the combination of silicon and melatonin can further reduce the damage caused by cadmium. Therefore, pot experiments were conducted to study the effects of melatonin and silicon on maize growth and cadmium accumulation. The results showed that cadmium stress significantly inhibited the growth of maize, disrupted its physiological processes, and led to cadmium accumulation in plants. Compared to the single treatment of silicon or melatonin, the combined application of melatonin and silicon significantly alleviated the inhibition of the growth of maize seedlings caused by cadmium stress. This was demonstrated by the increased plant heights, stem diameters, and characteristic root parameters and the bioaccumulation in maize seedlings. Under cadmium stress, the combined application of silicon and melatonin increased the plant height and stem diameter by 17.03% and 59.33%, respectively, and increased the total leaf area by 43.98%. The promotion of corn growth is related to the reduced oxidative damage under cadmium stress, manifested in decreases in the malondialdehyde content and relative conductivity and increases in antioxidant enzyme superoxide dismutase and guaiacol peroxidase activities, as well as in soluble protein and chlorophyll contents. In addition, cadmium accumulation in different parts of maize seedlings and the health risk index of cadmium were significantly reduced, reaching 48.44% (leaves), 19.15% (roots), and 20.86% (health risk index), respectively. Therefore, melatonin and silicon have a significant synergistic effect in inhibiting cadmium absorption and reducing the adverse effects of cadmium toxicity.

MsYSL6, A Metal Transporter Gene of Alfalfa, Increases Iron Accumulation and Benefits Cadmium Resistance

Iron (Fe) is necessary for plant growth and development. The mechanism of uptake and translocation in Cadmium (Cd) is similar to iron, which shares iron transporters. Yellow stripe-like transporter (YSL) plays a pivotal role in transporting iron and other metal ions in plants. In this study, MsYSL6 and its promoter were cloned from leguminous forage alfalfa. The transient expression of MsYSL6-GFP indicated that MsYSL6 was localized to the plasma membrane and cytoplasm. The expression of MsYSL6 was induced in alfalfa by iron deficiency and Cd stress, which was further proved by GUS activity driven by the MsYSL6 promoter. To further identify the function of MsYSL6, it was heterologously overexpressed in tobacco. MsYSL6-overexpressed tobacco showed better growth and less oxidative damage than WT under Cd stress. MsYSL6 overexpression elevated Fe and Cd contents and induced a relatively high Fe translocation rate in tobacco under Cd stress. The results suggest that MsYSL6 might have a dual function in the absorption of Fe and Cd, playing a role in the competitive absorption between Fe and Cd. MsYSL6 might be a regulatory factor in plants to counter Cd stress. This study provides a novel gene for application in heavy metal enrichment or phytoremediation and new insights into plant tolerance to toxic metals.

Argon-stimulated nitric oxide production and its function in alfalfa cadmium tolerance

Recent results showed that argon may have great potential in both medicines (especially) and agriculture. However, how argon positively influences crop physiology remains elusive. Here, we observed that the stimulation of nitric oxide (NO) production upon cadmium (Cd) stress in hydroponic alfalfa root tissues was strengthened by argon-rich water and/or a NO-releasing compound. The pharmacological results further indicated that above potential source of NO stimulation achieved by argon might be attributed to NO synthase (NOS) and nitrate reductase (NR). Under hydroponic and pot conditions, the improvement of Cd tolerance elicited by argon, confirmed by the alleviation in the plant growth inhibition, oxidative damage, and Cd accumulation, was sensitive to the scavenger of NO. These results suggested a crucial role of argon-induced NO synthesis in response to Cd stress. Subsequent evidence showed that the improved iron homeostasis and increased S-nitrosylation were also dependent on argon-stimulated NO. Above results were matched with the transcriptional profiles of representative target genes involved in heavy metal detoxification, antioxidant defence, and iron homeostasis. Taken together, our results clearly indicated that argon-stimulated NO production contributes to Cd tolerance by favoring important defense strategies against heavy metal exposure.

Clean-Up of Heavy Metals from Contaminated Soil by Phytoremediation: A Multidisciplinary and Eco-Friendly Approach

Pollution from heavy metals is one of the significant environmental concerns facing the world today. Human activities, such as mining, farming, and manufacturing plant operations, can allow them access to the environment. Heavy metals polluting soil can harm crops, change the food chain, and endanger human health. Thus, the overarching goal for humans and the environment should be the avoidance of soil contamination by heavy metals. Heavy metals persistently present in the soil can be absorbed by plant tissues, enter the biosphere, and accumulate in the trophic levels of the food chain. The removal of heavy metals from contaminated soil can be accomplished using various physical, synthetic, and natural remediation techniques (both in situ and ex situ). The most controllable (affordable and eco-friendly) method among these is phytoremediation. The removal of heavy metal defilements can be accomplished using phytoremediation techniques, including phytoextraction, phytovolatilization, phytostabilization, and phytofiltration. The bioavailability of heavy metals in soil and the biomass of plants are the two main factors affecting how effectively phytoremediation works. The focus in phytoremediation and phytomining is on new metal hyperaccumulators with high efficiency. Subsequently, this study comprehensively examines different frameworks and biotechnological techniques available for eliminating heavy metals according to environmental guidelines, underscoring the difficulties and limitations of phytoremediation and its potential application in the clean-up of other harmful pollutants. Additionally, we share in-depth experience of safe removing the plants used in phytoremediation-a factor frequently overlooked when choosing plants to remove heavy metals in contaminated conditions.

Bioorganic fertilizers improve the adaptability and remediation efficiency of Puccinellia distans in multiple heavy metals-contaminated saline soil by regulating the soil microbial community

Soil salinization and heavy metal (HM) pollution are global environmental problems. Bioorganic fertilizers facilitate phytoremediation, but their roles and microbial mechanisms in natural HM-contaminated saline soils have not been explored. Therefore, greenhouse pot trials were conducted with three treatments: control (CK), manure bioorganic fertilizer (MOF), and lignite bioorganic fertilizer (LOF). The results showed that MOF and LOF significantly increased nutrient uptake, biomass, toxic ion accumulation in Puccinellia distans, soil available nutrients, SOC, and macroaggregates. More biomarkers were enriched in MOF and LOF. Network analysis confirmed that MOF and LOF increased the number of bacterial functional groups and fungal community stability and strengthened their positive association with plants; Bacteria have a more significant effect on phytoremediation. Most biomarkers and keystones play important roles in promoting plant growth and stress resistance in the MOF and LOF treatments. In summary, besides enrichment of soil nutrients, MOF and LOF can also improve the adaptability and phytoremediation efficiency of P. distans by regulating the soil microbial community, with LOF having a greater effect.

Signaling and Detoxification Strategies in Plant-Microbes Symbiosis under Heavy Metal Stress: A Mechanistic Understanding

Plants typically interact with a variety of microorganisms, including bacteria, mycorrhizal fungi, and other organisms, in their above- and below-ground parts. In the biosphere, the interactions of plants with diverse microbes enable them to acquire a wide range of symbiotic advantages, resulting in enhanced plant growth and development and stress tolerance to toxic metals (TMs). Recent studies have shown that certain microorganisms can reduce the accumulation of TMs in plants through various mechanisms and can reduce the bioavailability of TMs in soil. However, relevant progress is lacking in summarization. This review mechanistically summarizes the common mediating pathways, detoxification strategies, and homeostatic mechanisms based on the research progress of the joint prevention and control of TMs by arbuscular mycorrhizal fungi (AMF)-plant and Rhizobium-plant interactions. Given the importance of tripartite mutualism in the plant-microbe system, it is necessary to further explore key signaling molecules to understand the role of plant-microbe mutualism in improving plant tolerance under heavy metal stress in the contaminated soil environments. It is hoped that our findings will be useful in studying plant stress tolerance under a broad range of environmental conditions and will help in developing new technologies for ensuring crop health and performance in future.

Exogenously applied calcium regulates antioxidative system and reduces cadmium-uptake in Fagopyrum esculentum

Calcium (Ca) being macronutrient plays a prominent role in signal transduction during various abiotic stresses. However, their involvements to alleviate heavy metal stress in plants remain evasive. In the present investigation, we found that application of exogenous Ca to Cd-stressed common buckwheat plants reversed the toxic effects of Cd by enhancing root and shoot length, biomass accumulation and reduced Cd-uptake as revealed by the translocation factor (<1), indicating more Cd is restrained in the roots. Moreover, present data also revealed that exogenous Ca significantly alleviated the Cd-induced oxidative damage by enhancing proline by 66.12% and 47.20% respectively in roots and shoots than control. The decline in the total chlorophyll content upon Ca application in Cd-treated plants was found less (38.96%) compared to buckwheat plants treated with Cd-stress alone (80.2%). APX and POD activities increased by 1.97 and 1.44 times in shoots, respectively, and increased by 2.81and 1.33 times in roots, respectively compared to the Cd-treated plants alone. The mineral content (Ca, K, Mg, Fe, P and S) that were suppressed in Cd-treated plants in both root and shoot were restored upon exogenous Ca application. Further, the correlation analysis showed significant positive correlation among proline and GSH synthesis in the Ca + Cd treatment. The correlations of Ca revealed to be positive with enhanced levels of APX and POD activity. Our data showed that exogenous application of Ca minimizes the Cd-toxicity and modulates the physiological and biochemical pathway in common buckwheat to withstand Cd-induced oxidative stress.

Phytoremediation of potentially toxic elements (PTEs) contaminated soils using alfalfa (Medicago sativa L.): A comprehensive review

Soil contamination with potentially toxic elements (PTEs) is an increasing environmental problem, posing serious threats to the living organisms. Phytoremediation is a sustainable and highly accepted technology for remediation of PTE-contaminated soils. Alfalfa has been widely adopted for the phytoremediation of PTE-contaminated soils due to its large biomass productivity, high PTE tolerance, and strong capacity to take up PTEs. However, there are still no literature reviews systematically summarized the potential of alfalfa in the phytoremediation. Therefore, we review the available literatures that present its PTE uptake, phytotoxicity, tolerance mechanisms, and aided techniques improving the phytoremediation efficiency. In this review, alfalfa shows high amounts of PTEs accumulation, especially in their root tissue. Meanwhile, the inner mechanisms of PTE tolerance and accumulation in alfalfa are discussed including: (i) the activation of antioxidant enzyme system, (ii) subcellular localization, (iii) production of glutathione, phytochelatins, and proline, and (iv) regulation of gene expression. Indeed, excessive PTE can overcome the defense system, which causes oxidative damage in alfalfa plants, thereby inhibiting growth and physiological processes and weakening the ability of PTE uptake. Till now, several approaches have been developed to improve the tolerance and/or accumulation of PTE in alfalfa plants as follows: (i) selection of PTE tolerant cultivars, (ii) applying plant growth regulators, (iii) addition of chelating agents, fertilizer, and biochar materials, and (iv) inoculation of soil microbes. Finally, we indicate that the selection of PTE-tolerant cultivars along with inoculation of soil microbes may be an efficient and eco-friendly strategy of the soil PTE phytoremediation.

Cu phytoextraction and biomass utilization as essential trace element feed supplements for livestock

Copper (Cu), as an essential element, is added to animal feed to stimulate growth and prevent disease. The forage crop alfalfa (Medicago sativa L.) produced during Cu phytoextraction may be considered a biofortified crop to substitute the Cu feed additives for livestock production, beneficially alleviating Cu contamination in soils and reducing its input into agriculture systems. To assess this, alfalfa was grown in three similar soils with different Cu levels, i.e., 11, 439 and 779 mg kg^-1 for uncontaminated soil (A), moderately Cu-contaminated soil (B) and highly Cu-contaminated soil (C), respectively. EDDS (Ethylenediamine-N,N'-disuccinic acid) was applied to the soils seven days before the first cutting at four rates (0, 0.5, 2 and 5 mmol kg^-1) to enhance bioavailable Cu uptake. Alfalfa grew well in soils A and B but not in the highly Cu-contaminated soil. After applying EDDS, a significant biomass reduction of the first cutting shoot was only observed with 5 mmol kg^-1 EDDS in the highly Cu-contaminated soil, with a 45% (P < 0.05) decrease when compared to the control. Alfalfa grown in the three soils gradually wilted after the first cutting with 5 mmol kg^-1 EDDS, and Cu concentrations in the first cutting shoot were augmented strongly, by 250% (P < 0.05), 3500% (P < 0.05) and 6700% (P < 0.05) compared to the controls, respectively. Cu concentrations in alfalfa shoots were found to be higher in this study than in some fodder plants and further augmented in soils with higher Cu levels and with EDDS application. These findings suggest that alfalfa grown on clean soils or soils with up to 450 mg Cu kg^-1 (with appropriate EDDS dosages) has the potential to be considered as a partial Cu supplementation for livestock. This research laid the foundation for the integration between Cu-phytoextraction and Cu-biofortification for livestock.

Human health risk assessment of lead, cadmium, and mercury co-exposure from agricultural soils in the Tuzla Canton (Bosnia and Herzegovina)

The aim of this study was to assess the risk of human exposure to lead (Pb), cadmium (Cd), and mercury (Hg) through agricultural soil by considering both uncertainty and variability in key exposure parameters. For this reason we collected soil samples from 29 locations in the Tuzla Canton (Bosnia and Herzegovina) and measured their metal levels with inductively coupled plasma atomic emission or absorption spectrometry (ICP-AES and ICP-AAS, respectively). The levels of Pb ranged from 13.33 to 1692.33 mg/kg, of Cd from 0.05 to 3.67 mg/kg, and of Hg from 0.02 to 2.73 mg/kg. To estimate cancer and non-cancer risks we used deterministic and semi-probabilistic methods. Lead was found to involve higher health risk than the other two heavy metals. Its hazard index (HI) decreased between population groups (children>women>men) and exposure routes (ingestion>skin contact>inhalation). Our Monte Carlo simulations indicated that Pb HIs for both adult populations had a 0.6 % probability to exceed the threshold value of 1, while in children this probability was 14.2 %. Cd and Hg showed no probability to exceed the threshold in any scenario. Our simulation results raise concern about possible adverse health effects of heavy metals from soil, especially in children. It is very important to continue monitoring environmental pollution and assess human health risk, not only with respect to soil, but also with other important environmental compartments, such as air and water.

Jasmonic acid (JA) and gibberellic acid (GA3) mitigated Cd-toxicity in chickpea plants through restricted cd uptake and oxidative stress management

Cadmium stress is one of the chief environmental cues that can substantially reduce plant growth. In the present research, we studied the effect of jasmonic acid (JA) and gibberellic acid (GA₃) applied individually and/or in combination to chickpea (Cicer arietinum) plants exposed to 150 µM cadmium sulphate. Cadmium stress resulted in reduced plant growth and pigment contents. Moreover, chickpea plants under cadmium contamination displayed higher levels of electrolytic leakage, H₂O_2, and malonaldehyde, as well as lower relative water content. Plants primed with JA (1 nM) and those foliar-fed with GA₃ (10^-6 M) showed improved metal tolerance by reducing the accumulation of reactive oxygen species, malonaldehyde and electrolytic leakage, and increasing relative water content. . Osmoprotectants like proline and glycinebetaine increased under cadmium contamination. Additionally, the enzymatic activities and non-enzymatic antioxidant levels increased markedly under Cd stress, but application of JA as well as of GA₃ further improved these attributes. Enzymes pertaining to the ascorbate glutathione and glyoxylase systems increased significantly when the chickpea plants were exposed to Cd. However, JA and GA₃ applied singly or in combination showed improved enzymatic activities as well as nutrient uptake, whereas they reduced the metal accumulation in chickpea plants. Taken together, our findings demonstrated that JA and GA₃ are suitable agents for regulating Cd stress resistance in chickpea plants.

Intercropping improves heavy metal phytoremediation efficiency through changing properties of rhizosphere soil in bamboo plantation

Moso bamboo is considered a potential species for heavy metal (HM) phytoremediation; however, the effect of intercropping on rhizosphere and phytoextraction remains to be elucidated. We comparatively investigated rhizobacteria, soil properties, and phytoextraction efficiency of monoculture and intercropping of Moso bamboo and Sedum plumbizincicola in Cu/Zn/Cd-contaminated soil. Compared with monocultures, intercropping increased the bacterial α-diversity indices (Shannon, Chao1) and the number of biomarkers. Intercropping reduced the contents of soil organic matter (SOM), available nutrients, and Cd and Cu in rhizosphere soils, and reduced the Cd and Zn contents in tissues of sedum. By contrast, Cd and Zn contents in tissues of bamboo increased, and the increase of organic acid in root exudates from intercropping could facilitate the HM absorption. The total amount of Cu, Zn, and Cd removed from the soil in intercropping system was 1.2, 1.9, and 1.8 times than those in monoculture bamboo, respectively. The abundances of Proteobacteria, Acidobacteria, Verrucomicrobia and Actinobacteria were higher in intercropping, playing an important role in soil nutrient cycles and HM remediation. These bacterial communities were closely correlated (P < 0.01) with SOM, available nitrogen, available phosphorus, and HMs. The results suggested this intercropping pattern can increase HM removal efficiency from polluted soils.

Hydrogen sulfide (H2S) signaling in plant development and stress responses

ABSTRACT

Hydrogen sulfide (H₂S) was initially recognized as a toxic gas and its biological functions in mammalian cells have been gradually discovered during the past decades. In the latest decade, numerous studies have revealed that H₂S has versatile functions in plants as well. In this review, we summarize H₂S-mediated sulfur metabolic pathways, as well as the progress in the recognition of its biological functions in plant growth and development, particularly its physiological functions in biotic and abiotic stress responses. Besides direct chemical reactions, nitric oxide (NO) and hydrogen peroxide (H₂O₂) have complex relationships with H₂S in plant signaling, both of which mediate protein post-translational modification (PTM) to attack the cysteine residues. We also discuss recent progress in the research on the three types of PTMs and their biological functions in plants. Finally, we propose the relevant issues that need to be addressed in the future research.

GRAPHIC ABSTRACT

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s42994-021-00035-4.

Copper nanowires as nanofertilizers for alfalfa plants: Understanding nano-bio systems interactions from microbial genomics, plant molecular responses and spectroscopic studies

The recent application of nano copper (Cu) compounds in the agrosystem has shown potential to improve the physiological performance and agronomical parameters of crops. We grew alfalfa (Medicago sativa) in potting mix amended with bulk, nano, and ionic Cu compounds at 80 and 280 mg Cu/kg; then, we evaluated plant performance at physiological and molecular levels. Plants treated with bulk/nano Cu presented better agronomical responses. The P and S content was reduced in bulk and ionic Cu-exposed plants, compared to controls (p ≤ .05). All Cu forms increased the content of Fe and Zn in roots and Fe in leaves, compared to controls (p ≤ .05). Leaf-superoxide dismutase expression was augmented ~27-fold and rubisco mRNA was unaffected in bulk/nano Cu-treated plants, compared to controls (p ≤ .05). Bulk/nano Cu incremented the relative abundance of microorganisms involved in the elemental uptake. These results indicate that nano Cu improved the physiology of alfalfa and can be considered as potential nanofertilizers.

An overview of removing heavy metals from sewage sludge: Achievements and perspectives

The removal of heavy metals from sewage sludge (SS) is attracting increasing attention because the presence of toxic heavy metals in SS restricts its reuse or disposal, especially on land. This review presents an overview of research on the origin and chemical speciation of heavy metals in SS and describes methods for their removal. SS primarily absorbs heavy metals from wastewater via passive sorption and active uptake of biomass, resulting in the different chemical speciation. The advantages and disadvantages of the current methods for the removal of heavy metals from SS are analysed. The current methods focus on the removal efficiencies of heavy metals, which are high enough to meet the standard of land application, but the treatment cost, the change and retention of nutrients, and the effects on SS properties resulting from heavy metal removal are usually ignored. In this review, the main knowledge gaps are identified and proposals for future research are made. These should comprise determining the underlying mechanisms of current removal methods, optimising and integrating the removal methods, and establishing systematic evaluation standards for these methods. This review will help researchers develop new environmentally and economically friendly methods for the removal of heavy metals from SS.

Exogenous application of signaling molecules to enhance the resistance of legume-rhizobium symbiosis in Pb/Cd-contaminated soils

Being signaling molecules, nitric oxide (NO) and hydrogen sulfide (H₂S) can mediate a wide range of physiological processes caused by plant metal toxicity. Moreover, legume-rhizobium symbiosis has gained increasing attention in mitigating heavy metal stress. However, systematic regulatory mechanisms used for the exogenous application of signaling molecules to alter the resistance of legume-rhizobium symbiosis under metal stress are currently unknown. In this study, we examined the exogenous effects of sodium nitroprusside (SNP) as an NO donor additive and sodium hydrosulfide (NaHS) as a H₂S donor additive on the phytotoxicity and soil quality of alfalfa (Medicago sativa)-rhizobium symbiosis in lead/cadmium (Pb/Cd)-contaminated soils. Results showed that rhizobia inoculation markedly promoted alfalfa growth by increasing chlorophyll content, fresh weight, and plant height and biomass. Compared to the inoculated rhizobia treatment alone, the addition of NO and H₂S significantly reduced the bioaccumulation of Pb and Cd in alfalfa-rhizobium symbiosis, respectively, thus avoiding the phytotoxicity caused by the excessive presence of metals. The addition of signaling molecules also alleviated metal-induced phytotoxicity by increasing antioxidant enzyme activity and inhibiting the level of lipid peroxidation and reactive oxygen species (ROS) in legume-rhizobium symbiosis. Also, signaling molecules improved soil nutrient cycling, increased soil enzyme activities, and promoted rhizosphere bacterial community diversity. Both partial least squares path modeling (PLS-PM) and variation partitioning analysis (VPA) identified that using signaling molecules can improve plant growth by regulating major controlling variables (i.e., soil enzymes, soil nutrients, and microbial diversity/plant oxidative damage) in legume-rhizobium symbiosis. This study offers integrated insight that confirms that the exogenous application of signaling molecules can enhance the resistance of legume-rhizobium symbiosis under metal toxicity by regulating the biochemical response of the plant-soil system, thereby minimizing potential health risks.

Combined application of silicon and nitric oxide jointly alleviated cadmium accumulation and toxicity in maize

Cadmium (Cd) contamination is a serious threat to plants and humans. Application of silicon (Si) or nitric oxide (NO) could alleviate Cd accumulation and toxicity in plants, but whether they have joint effects on alleviating of Cd accumulation and toxicity are not known. Therefore, the combined effect of Si and NO application on maize growth, Cd uptake, Cd transports and Cd accumulation were investigated in a pot experiment. Here, we reported that Cd stress decreased growth, caused Cd accumulation in plants. The combined application of Si and NO triggered a significant response in maize, increasing plant growth and reducing Cd uptake, accumulation, translocation and bioaccumulation factors under Cd stress. The grain Cd concentration was decreased by 66 % in the Si and NO combined treatment than Cd treatment. Moreover, the combined application of Si and NO reduced Cd health risk index in maize more effectively than either treatment alone. This study provided new evidence that Si and NO have a strong joint effect on alleviating the adverse effects of Cd toxicity by decreasing Cd uptake and accumulation. We advocate for supplement of Cd-contaminated soil with Si fertilizers and treatment of crops with NO as a practical approach to alleviating Cd toxicity.