Arbuscular mycorrhizal fungi in phytoremediation of contaminated areas by trace elements: mechanisms and major benefits of their applications

Non-phytoremediation and phytoremediation technologies of integrated remediation for water and soil heavy metal pollution: A comprehensive review

Since the 1980s, there has been increasing concern over heavy metal pollution remediation. However, most research focused on the individual remediation technologies for heavy metal pollutants in either soil or water. Considering the potential migration of these pollutants, it is necessary to explore effective integrated remediation technologies for soil and water heavy metals. This review thoroughly examines non-phytoremediation technologies likes physical, chemical, and microbial remediation, as well as green remediation approaches involving terrestrial and aquatic phytoremediation. Non-phytoremediation technologies suffer from disadvantages like high costs, secondary pollution risks, and susceptibility to environmental factors. Conversely, phytoremediation technologies have gained significant attention due to their sustainable and environmentally friendly nature. Enhancements through chelating agents, biochar, microorganisms, and genetic engineering have demonstrated improved phytoremediation remediation efficiency. However, it is essential to address the environmental and ecological risks that may arise from the prolonged utilization of these materials and technologies. Lastly, this paper presents an overview of integrated remediation approaches for addressing heavy metal contamination in groundwater-soil-surface water systems and discusses the reasons for the research gaps and future directions. This paper offers valuable insights for comprehensive solutions to heavy metal pollution in water and soil, promoting integrated remediation and sustainable development.

Integrating waste valorization and symbiotic microorganisms for sustainable bioremediation of metal(loid)-polluted soils

Remediation strategies for metal(loid)-polluted soils vary among the wide range of approaches, including physical, chemical, and biological remediation, or combinations of these. In this study, we assessed the effectiveness of a set of soil remediation treatments based on the combined application of inorganic (marble sludge) and organic amendments (vermicompost, and dry olive residue [DOR] biotransformed by the saprobic fungi Coriolopsis rigida and Coprinellus radians) and inoculation with arbuscular mycorrhizal fungi (AMFs) (Rhizophagus irregularis and Rhizoglomus custos). The treatments were applied under greenhouse conditions to soil residually polluted by potentially toxic elements (PTEs) (Pb, As, Zn, Cu, Cd, and Sb), and wheat was grown in the amended soils to test the effectiveness of the treatments in reducing soil toxicity and improving soil conditions and plant performance. Therefore, we evaluated the influence of the treatments on the main soil properties and microbial activities, as well as on PTE availability and bioaccumulation in wheat plants. Overall, the results showed a positive influence of all treatments on the main soil properties. Treatments consisting of a combination of marble and organic amendments, especially biotransformed DOR amendments, showed the greatest effectiveness in improving the soil biological status, promoting plant growth and survival, and reducing PTE availability and plant uptake. Furthermore, AMF inoculation further enhanced the efficacy of DOR amendments by promoting the immobilization of PTEs in soil and stimulating the phytostabilization mechanisms induced by AMFs, thus playing an important bioprotective role in plants. Therefore, our results highlight that biotransformed DOR may represent an efficient product for use as a soil organic amendment when remediating metal(loid)-polluted soils, and that its application in combination with AMFs may represent a promising sustainable bioremediation strategy for recovering soil functions and reducing toxicity in polluted areas.

Mycorrhizae in mine wasteland reclamation

Mycorrhizae are found on about 70-80 % of the roots of all plant species; ectomycorrhizae (ECM) are mostly found on woody plants and gymnosperms, whereas arbuscular mycorrhizal fungi (AMF) are found on 80-90 % of all plant species. In abandoned mining sites, woody plants dominate, while non-woody species remain scarce. However, this pattern depends on the specific mine site and its ecological context. This review article explores the potential of using mycorrhizae-plant associations to enhance and facilitate the remediation of mine wastelands and metal-polluted sites. In this review, we employed reputable databases to collect articles and relevant information on mycorrhizae and their role in plant growth and soil fertility spanning from the 1990s up to 2024. Our review found that the abilities of plants selected for minewasteland reclamation can be harnessed effectively if their mycorrhizae utilization is known and considered. Our findings indicate that AMF facilitates plant cohabitation by influencing species richness, feedback effects, shared mycelial networks, and plant-AMF specificity. Several types of mycorrhizae have been isolated from mine wastelands, including Glomus mosseae, which reduces heavy metal accumulation in plants, and Rhizophagus irregularis, which enhances plant growth and survival in revegetated mine sites. Additionally, studies on ECM in surface mine spoil restoration stands highlight their role in enhancing fungal biodiversity and providing habitats for rare and specialized fungal species. Recent research shows that ECM and AMF fungi can interact synergistically to enhance plant growth, with ECM improving plant nitrogen absorption and AMF increasing nitrogen use efficiency. Our review also found that despite their critical role in improving plant growth and resilience, there remains limited knowledge about the specific mechanisms by which mycorrhizae communicate with each other and other microorganisms, such as bacteria, root-associated fungi, soil protozoa, actinomycetes, nematodes, and endophytes, within the soil matrix. This article highlights the connection between mycorrhizae and plants and other microorganisms in mine wastelands, their role in improving soil structure and nutrient cycling, and how mycorrhizae can help restore soil fertility and promote plant growth, thus improving the overall environmental quality of mine wasteland sites.

Sustainable Remediation of Soil and Water Utilizing Arbuscular Mycorrhizal Fungi: A Review

Phytoremediation is recognized as an environmentally friendly technique. However, the low biomass production, high time consumption, and exposure to combined toxic stress from contaminated media weaken the potential of phytoremediation. As a class of plant-beneficial microorganisms, arbuscular mycorrhizal fungi (AMF) can promote plant nutrient uptake, improve plant habitats, and regulate abiotic stresses, and the utilization of AMF to enhance phytoremediation is considered to be an effective way to enhance the remediation efficiency. In this paper, we searched 520 papers published during the period 2000-2023 on the topic of AMF-assisted phytoremediation from the Web of Science core collection database. We analyzed the author co-authorship, country, and keyword co-occurrence clustering by VOSviewer. We summarized the advances in research and proposed prospective studies on AMF-assisted phytoremediation. The bibliometric analyses showed that heavy metal, soil, stress tolerance, and growth promotion were the research hotspots. AMF-plant symbiosis has been used in water and soil in different scenarios for the remediation of heavy metal pollution and organic pollution, among others. The potential mechanisms of pollutant removal in which AMF are directly involved through hyphal exudate binding and stabilization, accumulation in their structures, and nutrient exchange with the host plant are highlighted. In addition, the tolerance strategies of AMF through influencing the subcellular distribution of contaminants as well as chemical form shifts, activation of plant defenses, and induction of differential gene expression in plants are presented. We proposed that future research should screen anaerobic-tolerant AMF strains, examine bacterial interactions with AMF, and utilize AMF for combined pollutant removal to accelerate practical applications.

Heavy metals drive microbial community assembly process in farmland with long-term biosolids application

Biosolids are considered an alternative to chemical fertilizers due to their rich nutrients. However, long-term biosolids application can lead to heavy metals accumulation, which severely affects soil microbial community compositions. The factors influencing soil microbial community assembly were explored under a 16-year long-term experiment with biosolids applications. Our results indicated that biosolids application significantly increased fungal richness while not for bacterial and arbuscular mycorrhizal (AM) fungal richness. Besides, biosolids application significantly affected soil bacterial, fungal compositions and AM fungal community. Soil microorganisms were clustered into different modules with bacterial and AM fungal communities were affected by both organic matter and heavy metals, while fungal communities were affected by heavy metals (Cr, Ni, and As). The soil bacterial community assembly was dominated by stochastic processes while the fungal and AM fungal community assemblies were mainly driven by deterministic processes. Random forest analysis showed that heavy metals were identified as major drivers (Hg, Cu, Cd, and Zn for bacteria, Pb and Cr for fungi, and As and Ni for AM fungi) of the community assembly process. Overall, our study highlights the significant role of heavy metals in shaping microbial community dynamics and gives a guide for controlling biosolids application.

Effects of arbuscular mycorrhizal fungi on the reduction of arsenic accumulation in plants: a meta-analysis

Arsenic (As) accumulation in plants is a global concern. Although the application of arbuscular mycorrhizal fungi (AMF) has been suggested as a potential solution to decrease As concentration in plants, there is currently a gap in a comprehensive, quantitative assessment of the abiotic and biotic factors influencing As accumulation. A meta-analysis was performed to quantitatively investigate the findings of 76 publications on the impacts of AMF, plant properties, and soil on As accumulation in plants. Results showed a significant dose-dependent As reduction with higher mycorrhizal infection rates, leading to a 19.3% decrease in As concentration. AMF reduced As(V) by 19.4% but increased dimethylarsenic acid (DMA) by 50.8%. AMF significantly decreased grain As concentration by 34.1%. AMF also improved plant P concentration and dry biomass by 33.0% and 62.0%, respectively. The most significant reducing effects of As on AMF properties were seen in single inoculation and experiments with intermediate durations. Additionally, the benefits of AMF were significantly enhanced when soil texture, soil organic carbon (SOC), pH level, Olsen-P, and DTPA-As were sandy soil, 0.8%-1.5%, ≥7.5, ≥9.1 mg/kg, and 30-60 mg/kg, respectively. AMF increased easily extractable glomalin-related soil protein (EE-GRSP) and total glomalin-related soil protein (T-GRSP) by 23.0% and 28.0%, respectively. Overall, the investigated factors had significant implications in developing AMF-based methods for alleviating the negative effects of As stress on plants.

Removal of sulfamethoxazole and Cu, Cd compound pollution by arbuscular mycorrhizal fungi enhanced vertical flow constructed wetlands

The combined pollution of antibiotics and heavy metals (HMs) has a serious impact on the water ecological environment. Previous researches mainly focused on the removal of antibiotics or HMs as single pollutants, with limited investigation into the treatment efficiencies and underlying mechanisms associated with their co-occurring pollution. In this study, 16 micro vertical flow constructed wetlands (MVFCWs) were constructed to treat composite wastewater consisting of sulfamethoxazole (SMX), copper (Cu) and cadmium (Cd), involving two different inoculation treatments (arbuscular mycorrhizal fungi (AMF) inoculated and uninoculated) and eight kinds of pollutant exposure (Control Check (CK), SMX, Cu, Cd, SMX + Cu, SMX + Cd, Cu + Cd, SMX + Cu + Cd). The findings of this study demonstrated that the inoculation of AMF in MVFCWs resulted in removal efficiencies of SMX, Cu, and Cd ranging from 18.70% to 80.52%, 75.18% to 96.61%, and 40.50% to 89.23%, respectively. Cu and CuCd promoted the degradation of SMX in the early stage and inhibited the degradation of SMX in the later stage. Cd did not demonstrate a comparable promotive impact on SMX degradation, and its addition hindered Cu removal. However, comparatively, the presence of Cu exerted a more pronounced inhibitory effect on Cd removal. Furthermore, the addition of Cu augmented the abundances of Proteobacteria, Bacteroidetes (at the phylum level) and Rhodobacter, Lacunisphaera and Flavobacterium (at the genus level), and Cu exposure showed a substantially stronger influence on the microbial community than that of Cd and SMX. AMF might confer protection to plants against HMs and antibiotics by enriching Nakamurella and Lacunisphaera. These findings proved that AMF-C. indica MVFCW was a promising system, and the inoculation of AMF effectively enhanced the simultaneous removal of compound pollution.

Effect of EDDS on the rhizosphere ecology and microbial regulation of the Cd-Cr contaminated soil remediation using king grass combined with Piriformospora indica

The negative impacts of soil heavy metals composite pollution on agricultural production and human health are becoming increasingly prevalent. The applications of green chelating agents and microorganisms have emerged as promising alternate methods for enhancing phytoremediation. The regulatory effects of root secretion composition, microbial carbon source utilization, key gene expression, and soil microbial community structure were comprehensively analyzed through a combination of HPLC, Biolog EcoPlates, qPCR, and high-throughput screening techniques. The application of EDDS resulted in a favorable rhizosphere ecological environment for the king grass Piriformospora indica, characterized by a decrease in soil pH by 0.41 units, stimulation of succinic acid and fumaric acid secretion, and an increase in carbon source metabolic activity of amino acids and carbohydrates. Consequently, this improvement enhanced the bioavailability of Cd/Cr and increased the biomass of king grass by 25.7%. The expression of dissimilatory iron-reducing bacteria was significantly upregulated by 99.2%, while there was no significant difference in Clostridium abundance. Furthermore, the richness of the soil rhizosphere fungal community (Ascomycota: 45.8%, Rozellomycota: 16.7%) significantly increased to regulate the proportion of tolerant microbial dominant groups, promoting the improvement of Cd/Cr removal efficiency (Cd: 23.4%, Cr: 18.7%). These findings provide a theoretical basis for the sustainable development of chelating agent-assisted plants-microorganisms combined remediation of heavy metals in soil.

Synergistic effects of arbuscular mycorrhizal fungi and biochar are highly beneficial to Ligustrum lucidum seedlings in Cd-contaminated soil

Cadmium (Cd) contamination is a widespread environmental issue. There is a lack of knowledge about the impacts of applying arbuscular mycorrhizal fungi (AMF) and biochar, either alone or in their combination, on alleviating Cd phytotoxicity in Ligustrum lucidum. Therefore, a pot experiment was conducted in a greenhouse, where L. lucidum seedlings were randomly subjected to four regimes of AMF treatments (inoculation with sterilized AMF, with Rhizophagus irregularis, Diversispora versiformis, alone or a mixture of these two fungi), and two regimes of biochar treatments (with or without rice-husk biochar), as well as three regimes of Cd treatments (0, 15, and 150 mg kg-1), to examine the responses of growth, photosynthetic capabilities, soil enzymatic activities, nutritional concentrations, and Cd absorption of L. lucidum plants to the interactive effects of AMF, biochar, and Cd. The results demonstrated that under Cd contaminations, AMF alone significantly increased plant total dry weight, soil pH, and plant nitrogen (N) concentration by 84%, 3.2%, and 13.2%, respectively, and inhibited soil Cd transferring to plant shoot by 42.2%; biochar alone significantly enhanced net photosynthetic rate, soil pH, and soil catalase of non-mycorrhizal plants by 16.4%, 9%, and 11.9%, respectively, and reduced the soil Cd transferring to plant shoot by 44.7%; the additive effect between AMF and biochar greatly enhanced plant total dry weight by 101.9%, and reduced the soil Cd transferring to plant shoot by 51.6%. Furthermore, dual inoculation with D. versiformis and R. irregularis conferred more benefits on plants than the single fungal species did. Accordingly, amending Cd-contaminated soil with the combination of mixed-fungi inoculation and biochar application performed the best than either AMF or biochar alone. These responses may have been attributed to higher mycorrhizal colonization, soil pH, biomass accumulation, and biomass allocation to the roots, as well as photosynthetic capabilities. In conclusion, the combined use of mixed-fungi involving D. versiformis and R. irregularis and biochar addition had significant synergistic effects on enhancing plant performance and reducing Cd uptake of L. lucidum plants in Cd-contaminated soil.

Integrating phytoremediation and mycoremediation with biosurfactant-producing fungi for hydrocarbon removal and the potential production of secondary resources

Treatment of petroleum-contaminated soil to a less toxic medium via physical and chemical treatment is too costly and requires posttreatment. This review focuses on the employment of phytoremediation and mycoremediation technologies in cleaning hydrocarbon-contaminated soil which is currently rare. It is considered environmentally beneficial and possibly cost-effective as it implements the synergistic interaction between plants and biosurfactant producing mycorrhiza to degrade hydrocarbon contaminants. This review also covers possible sources of hydrocarbon pollution in water and soil, toxicity effects, and current technologies for hydrocarbon removal and degradation. In addition to these problems, this review also discusses the challenges and opportunities of transforming the resultant treated sludge and treating plants into potential by-products for a higher quality of life for future generations.

The role of iron-rich organic fertilizer in promoting the growth of Chinese cabbage and inhibiting the transformation of cadmium

The issue of heavy metal pollution caused by human production and living activities is progressively worsening. This study explored the effect of iron-rich organic fertilizer on the growth, quality, and cadmium (Cd) absorption of Chinese cabbage under Cd stress. The results showed that iron-rich organic fertilizer could increase the soluble protein content and root length of Chinese cabbage. Meanwhile, it could change the form of Cd to inhibit the enrichment of Cd in Chinese cabbage. The alkali hydrolyzed nitrogen (AN), total potassium (TK), organic matter (OM), and moisture content (MC) of the Z3 treatment group (2 % ferrous sulfate heptahydrate) were significantly higher than those of other treatment groups. The microbial network of Z3 was more complex than the other three groups. PICRUSt analysis and correlation analysis showed that the genes related to protein synthesis (e.g., glutathione S-transferase, zinc and Cd transporter, outer membrane protein, ArsR family transcriptional regulator, catalase, etc.) can also promote microbial absorption. This study aims to provide theoretical insights into soil Cd pollution immobilization techniques.

Diversity of arbuscular mycorrhizal fungi in seleniferous soils and their role in plant growth promotion

The present study aimed to investigate the molecular diversity of arbuscular mycorrhizal fungi (AMF) in natural seleniferous soils and their role in protecting plants from Se toxicity. The genomic DNA extracted from maize roots grown in seleniferous and non-seleniferous regions was amplified using AMF-specific primers by nested PCR. The 1.5 kb amplicon spanning pSSU-ITS-pLSU of 18S rRNA of AMF was deciphered using the Illumina Miseq Next Generation Sequencing (NGS) technique. A total of 17 AMF species from the seleniferous region and 18 AMF species from the non-seleniferous region were identified. The number of reads of Glomus irregularis, G. custos, and G. intraradices was higher in seleniferous soil than in non-seleniferous soil, indicating their tolerance to Se. A consortium of Se-tolerant AMF inoculum was prepared and inoculated to maize plants, grown in natural seleniferous soils. AMF-inoculated plants had healthy growth with higher root, shoot, and grain biomass than non-AMF-inoculated plants. AMF inoculation leads to higher Se accumulation in roots but lesser Se accumulation in shoots and seeds of inoculated maize plants as compared to control plants. Present study results suggest that AMF species from seleniferous soils have the potential to be used as biofertilizers to improve plant growth and tolerate Se toxicity in seleniferous soils.

Boron-resistant Alternaria alternata (OG14) mitigates boron stress by improving physiological and antioxidative response in wheat (Triticum aestivum L.)

Effect of Alternaria alternata (OG14) isolated from a rock lichen (Xanthoria sp.) was investigated on the relief of boron stress in wheat. To determine the tolerance level to B stress, the fungus was grown at increasing boric acid (BA) concentrations in the range of 0.0-2.5 M. No significant change in colony development of the fungus was observed up to 1 M BA application compared to the control but after this dose, it decreased depending on the increase in the BA dose. When the element content of wheat seedlings was evaluated by ICP-MS, BA application increased B content together with Mg, P, K, Fe contents of the seedlings to very high levels compared to the control. However, fungus + BA treatments decreased the content of B and the other elements in the seedlings. The BA applications resulted in an increase in the levels of reactive oxygen species, including H2O2 and O2.-as well as lipid peroxidation in the seedlings. However, when the fungal inoculation was performed under the same BA conditions, the levels of these parameters decreased. The fungus inoculation stimulated the activity of all studied enzymes compared to BA applications. BA applications alone increased non - enyzmatic the oxidized ascorbate level more than the reducing ascorbate, leading to a decrease in the AsA/DHA ratio. The results show that A. alternata treatment can mitigate the negative effects of B stress on wheat seedlings by reducing ROS, LPO, B content, increasing the capacity of enzymatic and non-enzymatic antioxidants, and improving root and shoot length.

Recent Advances in Microbial-Assisted Remediation of Cadmium-Contaminated Soil

Soil contamination with cadmium (Cd) is a severe concern for the developing world due to its non-biodegradability and significant potential to damage the ecosystem and associated services. Industries such as mining, manufacturing, building, etc., rapidly produce a substantial amount of Cd, posing environmental risks. Cd toxicity in crop plants decreases nutrient and water uptake and translocation, increases oxidative damage, interferes with plant metabolism and inhibits plant morphology and physiology. However, various conventional physicochemical approaches are available to remove Cd from the soil, including chemical reduction, immobilization, stabilization and electro-remediation. Nevertheless, these processes are costly and unfriendly to the environment because they require much energy, skilled labor and hazardous chemicals. In contrasting, contaminated soils can be restored by using bioremediation techniques, which use plants alone and in association with different beneficial microbes as cutting-edge approaches. This review covers the bioremediation of soils contaminated with Cd in various new ways. The bioremediation capability of bacteria and fungi alone and in combination with plants are studied and analyzed. Microbes, including bacteria, fungi and algae, are reported to have a high tolerance for metals, having a 98% bioremediation capability. The internal structure of microorganisms, their cell surface characteristics and the surrounding environmental circumstances are all discussed concerning how microbes detoxify metals. Moreover, issues affecting the effectiveness of bioremediation are explored, along with potential difficulties, solutions and prospects.

Bioelimination of Phytotoxic Hydrocarbons by Biostimulation and Phytoremediation of Soil Polluted by Waste Motor Oil

Soils contaminated by waste motor oil (WMO) affect their fertility, so it is necessary to recover them by means of an efficient and safe bioremediation technique for agricultural production. The objectives were: (a) to biostimulate the soil impacted by WMO by applying crude fungal extract (CFE) and Cicer arietinum as a green manure (GM), and (b) phytoremediation using Sorghum vulgare with Rhizophagus irregularis and/or Rhizobium etli to reduce the WMO below the maximum value according to NOM-138 SEMARNAT/SS or the naturally detected one. Soil impacted by WMO was biostimulated with CFE and GM and then phytoremediated by S. vulgare with R. irregularis and R. etli. The initial and final concentrations of WMO were analyzed. The phenology of S. vulgare and colonization of S. vulgaris roots by R. irregularis were measured. The results were statistically analyzed by ANOVA/Tukey's HSD test. The WMO in soil that was biostimulated with CFE and GM, after 60 days, was reduced from 34,500 to 2066 ppm, and the mineralization of hydrocarbons from 12 to 27 carbons was detected. Subsequently, phytoremediation with S. vulgare and R. irregularis reduced the WMO to 86.9 ppm after 120 days, which is a concentration that guarantees the restoration of soil fertility for safe agricultural production for human and animal consumption.

Effect of Glomus mosseae, cadmium, and elevated air temperature on main flavonoids and phenolic acids contents in alfalfa

Global warming and heavy metal-contaminated soils co-occur in natural ecosystems. Flavonoids and phenolic acids in plants have significant antioxidant activity and free radical scavenging ability, which can quickly increase under adverse environments. Arbuscular mycorrhizal fungi (AMF) colonization can affect the synthesis of flavonoids and phenolic acids in host plants. This study focused on the main effect of Glomus mosseae, cadmium (Cd, 8 mg kg^-1 dry soils), and elevated temperature (ET, + 3 °C) on main flavonoids and phenolic acids in 120-d Medicago sativa L. (alfalfa). Elevated temperature decreased G. mosseae colonization ratio by 49.5% under Cd exposure. Except for p-hydroxybenzoic acid, flavonoids and phenolic acids content in shoots increased (p < 0.05) under G. mosseae + Cd relative to Cd only. G. mosseae and Cd showed significant effects on rutin, quercetin, apigenin, liquiritigenin, gallic acid, p-hydroxybenzoic acid, p-coumaric acid, and ferulic acid, and G. mosseae colonization led to increases in these compounds by 41.7%, 35.4%, 32.2%, 267.8%, 84.7%, 33.5%, 102.8%, and 89.4%, respectively, under ET + Cd. Carbon, N, and Cd in alfalfa and G. mosseae colonization rate were significant factors on flavonoids and phenolic acids accumulation. Additionally, P content in shoots significantly influenced flavonoids content. G. mosseae inoculation significantly stimulated the synthesis of main flavonoids and phenolic acids in alfalfa shoots under ET + Cd, which was helpful to understand the regulation of AMF on non-enzyme antioxidant system of plants grown in heavy metal-contaminated soils under global change scenarios.

Physio-Biochemical and Transcriptomic Features of Arbuscular Mycorrhizal Fungi Relieving Cadmium Stress in Wheat

Arbuscular mycorrhizal fungi (AMF) can improve plant cadmium (Cd) tolerance, but the tolerance mechanism in wheat is not fully understood. This study aimed to examine the physiological properties and transcriptome changes in wheat inoculated with or without Glomus mosseae (GM) under Cd stress (0, 5, and 10 mg·kg^-1 CdCl₂) to understand its role in wheat Cd tolerance. The results showed that the Cd content in shoots decreased while the Cd accumulation in roots increased under AMF symbiosis compared to the non-inoculation group and that AMF significantly promoted the growth of wheat seedlings and reduced Cd-induced oxidative damage. This alleviative effect of AMF on wheat under Cd stress was mainly attributed to the fact that AMF accelerated the ascorbate-glutathione (AsA-GSH) cycle, promoted the production of GSH and metallothionein (MTs), improved the degradation of methylglyoxal (MG), and induced GRSP (glomalin-related soil protein) secretion. Furthermore, a comparative analysis of the transcriptomes of the symbiotic group and the non-symbiotic group revealed multiple differentially expressed genes (DEGs) in the 'metal ion transport', 'glutathione metabolism', 'cysteine and methionine metabolism', and 'plant hormone signal transduction' terms. The expression changes of these DEGs were basically consistent with the changes in physio-biochemical characteristics. Overall, AMF alleviated Cd stress in wheat mainly by promoting immobilization and sequestration of Cd, reducing ROS production and accelerating their scavenging, in which the rapid metabolism of GSH may play an important role.

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

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

The Anatomical Basis of Heavy Metal Responses in Legumes and Their Impact on Plant-Rhizosphere Interactions

Rapid industrialization, urbanization, and mine tailings runoff are the main sources of heavy metal contamination of agricultural land, which has become one of the major constraints to crop growth and productivity. Finding appropriate solutions to protect plants and agricultural land from heavy metal pollution/harmful effects is important for sustainable development. Phytoremediation and plant growth-promoting rhizobacteria (PGPR) are promising methods for this purpose, which both heavily rely on an appropriate understanding of the anatomical structure of plants. Specialized anatomical features, such as those of epidermis and endodermis and changes in the root vascular tissue, are often associated with heavy metal tolerance in legumes. This review emphasizes the uptake and transport of heavy metals by legume plants that can be used to enhance soil detoxification by phytoremediation processes. Moreover, the review also focuses on the role of rhizospheric organisms in the facilitation of heavy metal uptake, the various mechanisms of enhancing the availability of heavy metals in the rhizosphere, the genetic diversity, and the microbial genera involved in these processes. The information presented here can be exploited for improving the growth and productivity of legume plants in metal-prone soils.

Exploring the significance of nanomaterials and organic amendments - Prospect for phytoremediation of contaminated agroecosystem

Emerging micro-pollutants have rapidly contaminated the agro-ecosystem, posing serious challenges to a sustainable future. The vast majority of them have infiltrated the soil and damaged agricultural fields and crops after being released from industry. These pollutants and their transformed products are also transported in vast quantities which further exacerbate the damage. Sustainable remediation techniques are warranted for such large amounts of contaminants. As aforementioned, many of them have been detected at very high concentrations in soil and water which adversely affect crop physiology by disrupting different metabolic processes. To combat this situation, nanomaterials and other organic amendments assisted phytoremediation ware considered as a viable alternative. It is a potent synergistic activity between the biological system and the supplied organic or nanomaterial material to eliminate emerging contaminants and micropollutants from crop fields. This can be effectively be applied to degraded crop fields and could potentially embody a green technology for sustainable agriculture.

Arbuscular Mycorrhiza Extraradical Mycelium Promotes Si and Mn Subcellular Redistribution in Wheat Grown under Mn Toxicity

Manganese (Mn) and aluminum (Al) toxicities are serious edaphic limitations to crop production in acidic soils. Excess Mn can be countered using a stress-adapted soil microbiota that establish symbiotic relationships with native plants. The arbuscular mycorrhizal fungi (AMF) associated with Lolium rigidum L. develop extraradical mycelia (ERM) that quickly colonize wheat and lead to greater shoot growth by promoting stress-evading mechanisms that are not yet completely explained. In the present study, wheat growth was assessed after 3 weeks on disturbed and undisturbed (intact ERM) acidic soil where the native non-mycotrophic Silene gallica L. or strongly mycotrophic L. rigidum were previously developed. The physiological and biochemical mechanisms responsible for increased growth were analyzed by assessing wheat leaf chlorophyll content, photosystem II quantum yield and performance index, enzymatic activity of ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR), guaiacol peroxidase (GPX), superoxide dismutase (SOD) and contents and subcellular localization of Mn, Mg, Si and K. The soil from native plants had a beneficial effect on shoot weight and chlorophyll levels. The highest benefits were obtained for wheat grown in soil with intact ERM associated with L. rigidum. In this condition, where earlier mycorrhization was favored, the Mn content decreased, alongside the content of Si, while the Mg/Mn ratio increased. Mn was redirected to the apoplast, while Si was redirected to the symplast. The activity of APX, GPX and SOD increased, probably due to increased metabolic growth (higher shoot weight and chlorophyll content). Understanding the mechanisms induced by native AMF responsible for increasing wheat performance can contribute to the establishment of sustainable approaches for crop production in acidic soils with Mn toxicity. The use of native plant AMF developers can improve the sustainable use of natural resources in the scope of greener agricultural practices.

Plants-Microorganisms-Based Bioremediation for Heavy Metal Cleanup: Recent Developments, Phytoremediation Techniques, Regulation Mechanisms, and Molecular Responses

Rapid industrialization, mine tailings runoff, and agricultural activities are often detrimental to soil health and can distribute hazardous metal(loid)s into the soil environment, with harmful effects on human and ecosystem health. Plants and their associated microbes can be deployed to clean up and prevent environmental pollution. This green technology has emerged as one of the most attractive and acceptable practices for using natural processes to break down organic contaminants or accumulate and stabilize metal pollutants by acting as filters or traps. This review explores the interactions between plants, their associated microbiomes, and the environment, and discusses how they shape the assembly of plant-associated microbial communities and modulate metal(loid)s remediation. Here, we also overview microbe-heavy-metal(loid)s interactions and discuss microbial bioremediation and plants with advanced phytoremediation properties approaches that have been successfully used, as well as their associated biological processes. We conclude by providing insights into the underlying remediation strategies' mechanisms, key challenges, and future directions for the remediation of metal(loid)s-polluted agricultural soils with environmentally friendly techniques.

Field Evaluation of Arbuscular Mycorrhizal Fungal Colonization in Miscanthus × giganteus and Seed-Based Miscanthus Hybrids Grown in Heavy-Metal-Polluted Areas

Understanding the behavior of arbuscular mycorrhizal fungi (AMF) associated with plants is essential for optimizing plant cultivation to the phytoremediation of degraded soils. The objective of the present study was to evaluate the differences in AMF root colonization between novel seed-based interspecific Miscanthus hybrids (M. sacchariflorus × M. sinensis) and the standard M. × giganteus when grown in soils contaminated with heavy metals (Pb, Cd, and Zn). During the third and fourth growing seasons, higher concentration of metals in the roots and a limited transfer of metals from the roots to the shoots were observed in all the plants studied. After the third growing season, the lowest values of AMF colonization rates were observed for the GNT34 hybrid. After the fourth growing season, AMF colonization decreased, which could be due to the drought that occurred during that season. GNT34 showed a lower tendency to develop mycorrhizal structures on heavy-metal (HM)-contaminated soils than GNT41 and M × g; however, this hybrid was insensitive to changes in colonization rates during the dry growing season.

Impact of arbuscular mycorrhiza on maize P1B-ATPases gene expression and ionome in copper-contaminated soils

Arbuscular mycorrhizal (AM) fungi, symbionts of most land plants, increase plant fitness in metal contaminated soils. To further understand the mechanisms of metal tolerance in the AM symbiosis, the expression patterns of the maize Heavy Metal ATPase (HMA) family members and the ionomes of non-mycorrhizal and mycorrhizal plants grown under different Cu supplies were examined. Expression of ZmHMA5a and ZmHMA5b, whose encoded proteins were predicted to be localized at the plasma membrane, was up-regulated by Cu in non-mycorrhizal roots and to a lower extent in mycorrhizal roots. Gene expression of the tonoplast ZmHMA3a and ZmHMA4 isoforms was up-regulated by Cu-toxicity in shoots and roots of mycorrhizal plants. AM mitigates the changes induced by Cu toxicity on the maize ionome, specially at the highest Cu soil concentration. Altogether these data suggest that in Cu-contaminated soils, AM increases expression of the HMA genes putatively encoding proteins involved in Cu detoxification and balances mineral nutrient uptake improving the nutritional status of the maize plants.

Biochar application for the remediation of trace metals in contaminated soils: Implications for stress tolerance and crop production

In modern agriculture and globalization, the release of trace metals from manufacturing effluents hinders crop productivity by polluting the atmosphere and degrading food quality. Sustaining food safety in polluted soils is critical to ensure global food demands. This review describes the negative effects of trace metals stress on plant growth, physiology, and yield. Furthermore, also explains the potential of biochar in the remediation of trace metal's contaminations in plants by adoption of various mechanisms such as reduction, ion exchange, electrostatic forces of attraction, precipitation, and complexation. Biochar application enhances the overall productivity, accumulation of biomass, and photosynthetic activity of plants through the regulation of various biochemical and physiological mechanisms of plants cultivated under trace metals contaminated soil. Moreover, biochar scavenges the formation of reactive oxygen species, by activating antioxidant enzyme production i.e., ascorbate peroxidase, catalase, superoxide dismutase, peroxidase, etc. The application of biochar also improves the synthesis of stressed proteins and proline contents in plants thus maintaining the osmoprotectant and osmotic potential of the plant under contaminates stress. Integrated application of biochar with other amendments i.e., microorganisms and plant nutrients to improve trace metal remediation potential of biochar and improving crop production was also highlighted in this review. Moreover, future research needs regarding the application of biochar have also been addressed.

The intercropping and arbuscular mycorrhizal fungus decrease Cd accumulation in upland rice and improve phytoremediation of Cd-contaminated soil by Sphagneticola calendulacea (L.) Pruski

Little is known about the impact of the combined application of intercropping and arbuscular mycorrhizal fungus (AMF) on the plant growth and Cd accumulation in the two intercropped plants. A greenhouse pot experiment was performed to investigate the effects of intercropping (IC) and AMF-Glomus versiforme (GV) on the growth, photosynthesis, Cd accumulation and antioxidant activities in the two intercropped plants-upland rice and Cd hyperaccumulator Sphagneticola calendulacea (L.) Pruski in the soils added with 5 mg Cd kg^-1. It was found that the GV inoculation and the combined treatment of IC and GV (IC + GV) significantly (p < 0.05) increased the biomasses and the P contents of upland rice and S. calendulacea. In addition, the Cd concentrations and uptakes of plants in IC, GV and IC + GV treatments were significantly (p < 0.05) dropped in upland rice but increased in S. calendulacea compared with the monocropping control, and the compound treatment showed better effect on decreasing Cd accumulation in upland rice (especially grains) and increasing Cd uptake by S. calendulacea compared with the single intercropping or AMF treatment. Moreover, IC, GV and IC + GV treatments significantly (p < 0.05) improved the net photosynthetic rate, stomatal conductance and transpiration rate of the two intercropped plants. Finally, IC, GV and IC + GV treatments all significantly increased the catalase activities and total antioxidant capacities, while decreased the malondialdehyde contents in upland rice and S. calendulacea. The present work could provide a feasible strategy for safe production of upland rice and phytoremediation of Cd contaminated soils.

Remediation of copper-contaminated soils using Tagetes patula L., earthworms and arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) and earthworms have potential uses in the bioremediation of contaminated soils. In recent years, heavy metal-contaminated sites have been remediated by adding plants and AMF or earthworms to the soil. However, there are few studies on remediation using combinations of plants, animals, and microbes, especially for the remediation of Cu-contaminated soil. The present study investigated the separate and combined effects of AMF and earthworms on Cu-contaminated soil in which Tagetes patula L. was grown. The results show that the combined application of AMF and earthworms markedly increased the biomass of plant shoots and roots by more than 100%. It also increased Cu extraction by T. patula by 270%. The combined treatment was effective in increasing the CEC, contents of OM, and available Cu, P and K, but reduced the soil pH. Furthermore, the combined treatment significantly increased the abundance and diversity of the soil microbial community. In particular, the abundances of the bacteria Bacteroides, Proteobacteria, and Actinobacteria were increased, with the genera Flavobacterium, Pedobacter, Algoriphagus, Gaetbulibacter, Pseudomonas, Luteimonas, and Arthrobacter dominating. Meanwhile, the abundance of the fungus Zygomycota was increased, with Mortierella dominating. Moreover, inoculation with earthworms greatly improved the structure of the soil microbial community.

Plant response to mycorrhizal inoculation and amendments on a contaminated soil

Understanding the combined effects of soil amendments and inoculation of mycorrhizal fungi on the response of different plant species during the phytostabilization process of trace elements contaminated soils is a challenge. This task is more difficult but more realistic when studied under field conditions. We assess the combined effects of two amendment doses and mycorrhizal inoculation on the response of saplings of two tree species planted in a contaminated field. The amendments were a mix of sugar beet lime and biosolid compost. The inoculation treatments were made with a commercial inoculum of arbuscular mycorrhizal fungi for wild olive and ectomycorrhizal fungi for stone pine. Results showed a weak or null effect of the mycorrhizal inoculation on plant growth, survival and trace element accumulation. There was a significant increase on P nutrition for stone pine, growing on non-amended conditions. Soil amendments were very effective reducing trace elements availability and their accumulation in both plant species, especially in roots. However, the effects on plant biomass were species-dependent and contrasted; low-dose amendments increased the biomass of wild olive by 33.3%, but reduced by 28% that of pine. The high doses of amendments (60 T ha^-1) produced some negative effects on plant growth and nutrition, probably related to the increase of soil salinity. Both plant species, stone pine and wild olive, have been proved to be adequate for phytostabilization of contaminated soils under Mediterranean climate, due to their drought tolerance and the low transfer of trace elements from root to shoot, thus reducing toxicity for the food web. To implement microbial-assisted phytoremediation approaches, a better understanding of the diversity and ecology of plant-associated microorganisms is needed. The use of indigenous fungi, locally adapted and tolerant to contamination, would be more suitable for phytostabilization purposes.

Effects of Exogenous Application of Plant Growth Regulators (SNP and GA3) on Phytoextraction by Switchgrass (Panicum virgatum L.) Grown in Lead (Pb) Contaminated Soil

Soil lead (Pb) contamination is a major environmental and public health risk. Switchgrass (Panicum virgatum), a second-generation biofuel crop, is potentially useful for the long-term phytoremediation and phytoextraction of Pb contaminated soils. We evaluated the efficacy of a coordinated foliar application of plant growth regulators and soil fungicide and a chelator in order to optimize phytoextraction. Plants were grown in soil culture under controlled conditions. First, three exogenous nitric oxide (NO) donors were evaluated at multiple concentrations: (1) S-nitroso-N-acetylpenicillamine (SNAP); (2) sodium nitroprusside (SNP); and (3) S-nitrosoglutathione (GSNO). Second, the effect of SNP (0.5 μM) was examined further with the model chelate EDTA and the soil fungicide propicanazole. Third, a combined foliar application of SNP and gibberellic acid (GA3) was examined with EDTA and propicanazole. The soil application of propiconazole (a broad-spectrum fungicides) reduced AMF colonization and allowed greater Pb phytoextraction. The foliar application of SNP resulted in similar concentrations of Pb (roots and foliage) to plants that were challenged with chelates and soil fungicides. The combined foliar application of SNP and GA3 resulted in significantly greater average Pb concentration (243 mg kg−1) in plant foliage in comparison to control plants (182 mg kg−1) and plants treated with GA3 alone (202 mg kg−1). The combined foliar application of SNP and GA3 resulted in the greatest phytoextraction efficiency and could therefore potentially improve phytoextraction by switchgrass grown in Pb contaminated soils.

Differential strategies of two species of arbuscular mycorrhizal fungi in the protection of maize plants grown in chromium-contaminated soils

Chromium (Cr) is a nonessential element for plants that is extremely toxic at high concentrations. Zea mays L. is a species of plant that has developed adaptive mechanisms to increase its tolerance and absorption capacity for this metal. One effective mechanism is to form associations with arbuscular mycorrhizal fungi (AMF), which help the plant cope with stress from heavy metals such as Cr. However, it is still not clear which species of AMF are more efficient as bioremediating agents for plants of agricultural importance. Here, we evaluated the effect of Gigaspora gigantea and Rhizophagus irregularis as protective agents of maize plants in soils contaminated with Cr at concentrations of 0, 48.5, 97 and 194 mg kg^-1 under greenhouse conditions. Maize tolerance to Cr was corroborated, as well as increased absorption of this element by plants associated with both species of AMF. G. gigantea caused an increase in nitrogen content and greater translocation of Cr to the aerial part of the plant; R. irregularis registered an increase in the formation of arbuscules and vesicles with increasing metal concentration and greater retention of Cr in the roots of the plants. Based on these results, we can conclude that the analyzed species of fungi use different strategies, with similar effectiveness, to enhance the absorption capacity of Cr by the plant and influence the differential deposition of this metal in various parts of the plant.

Comprehensive Assessment of Ameliorative Effects of AMF in Alleviating Abiotic Stress in Tomato Plants

Population growth and food necessity envisaged the dire need for supplementation to a larger community balance in food production. With the advent of the green revolution, agriculture witnessed the insurrection of horticultural fruit crops and field crops in enormous modes. Nevertheless, chemical fertilizer usage foresees soil pollution and fertility loss. Utilization of biocontrol agents and plant growth promotion by microbial colonization enrooted significant restoration benefits. Constant reliability for healthy foods has been emancipated across the globe stressing high nutritive contents among indigenous field crops like tomato (Solanum lycopersicum). However, stress tolerance mechanisms and efficient abatement require deeper insights. The applicability of arbuscular mycorrhizal fungi (AMF) poses as an ultimate strategy to minimize the deleterious consequences of abiotic stress such as salt, drought, temperature and heavy metal stress sustainably. The rational modality employing the application of AMF is one of significant efforts to lessen cell damages under abiotic stress. The novelty of the compilation can be redressed to cohesive literature for combating stress. The literature review will provide agricultural scientists worldwide in providing a rational approach that can have possible implications in not only tomato but also other vegetable crops.

Cadmium toxicity in plants: Impacts and remediation strategies

Cadmium (Cd) is an unessential trace element in plants that is ubiquitous in the environment. Anthropogenic activities such as disposal of urban refuse, smelting, mining, metal manufacturing, and application of synthetic phosphate fertilizers enhance the concentration of Cd in the environment and are carcinogenic to human health. In this manuscript, we reviewed the sources of Cd contamination to the environment, soil factors affecting the Cd uptake, the dynamics of Cd in the soil rhizosphere, uptake mechanisms, translocation, and toxicity of Cd in plants. In crop plants, the toxicity of Cd reduces uptake and translocation of nutrients and water, increases oxidative damage, disrupts plant metabolism, and inhibits plant morphology and physiology. In addition, the defense mechanism in plants against Cd toxicity and potential remediation strategies, including the use of biochar, minerals nutrients, compost, organic manure, growth regulators, and hormones, and application of phytoremediation, bioremediation, and chemical methods are also highlighted in this review. This manuscript may help to determine the ecological importance of Cd stress in interdisciplinary studies and essential remediation strategies to overcome the contamination of Cd in agricultural soils.

Phytoremediation: a sustainable environmental technology for heavy metals decontamination

Toxic metal contamination of soil is a major environmental hazard. Chemical methods for heavy metal's (HMs) decontamination such as heat treatment, electroremediation, soil replacement, precipitation and chemical leaching are generally very costly and not be applicable to agricultural lands. However, many strategies are being used to restore polluted environments. Among these, phytoremediation is a promising method based on the use of hyper-accumulator plant species that can tolerate high amounts of toxic HMs present in the environment/soil. Such a strategy uses green plants to remove, degrade, or detoxify toxic metals. Five types of phytoremediation technologies have often been employed for soil decontamination: phytostabilization, phytodegradation, rhizofiltration, phytoextraction and phytovolatilization. Traditional phytoremediation method presents some limitations regarding their applications at large scale, so the application of genetic engineering approaches such as transgenic transformation, nanoparticles addition and phytoremediation assisted with phytohormones, plant growth-promoting bacteria and AMF inoculation has been applied to ameliorate the efficacy of plants as candidates for HMs decontamination. In this review, aspects of HMs toxicity and their depollution procedures with focus on phytoremediation are discussed. Last, some recent innovative technologies for improving phytoremediation are highlighted.

Arbuscular mycorrhizal fungi-induced mitigation of heavy metal phytotoxicity in metal contaminated soils: A critical review

The heavy metal pollution is a worldwide problem and has received a serious concern for the ecosystem and human health. In the last decade, remediation of the agricultural polluted soil has attracted great attention. Phytoremediation is one of the technologies that effectively alleviate heavy metal toxicity, however, this technique is limited to many factors contributing to low plant growth rate and nature of metal toxicities. Arbuscular mycorrhizal fungi (AMF) assisted alleviation of heavy metal phytotoxicity is a cost-effective and environment-friendly strategy. AMF have a symbiotic relationship with the host plant. The bidirectional exchange of resources is a hallmark and also a functional necessity in mycorrhizal symbiosis. During the last few years, a significant progress in both physiological and molecular mechanisms regarding roles of AMF in the alleviation of heavy metals (HMs) toxicities in plants, acquisition of nutrients, and improving plant performance under toxic conditions of HMs has been well studied. This review summarized the current knowledge regarding AMF assisted remediation of heavy metals and some of the strategies used by mycorrhizal fungi to cope with stressful environments. Moreover, this review provides the information of both molecular and physiological responses of mycorrhizal plants as well as AMF to heavy metal stress which could be helpful for exploring new insight into the mechanisms of HMs remediation by utilizing AMF.

High Aluminum Drives Different Rhizobacterial Communities Between Aluminum-Tolerant and Aluminum-Sensitive Wild Soybean

Aluminum (Al)-resistant plant cultivars can recruit beneficial microbes to alleviate the stresses. However, the mechanism of how rhizobacterial communities strengthen Al tolerance of wild soybean has not been addressed. The aim of this study was to investigate the bacterial community structure in the rhizosphere of Al-tolerant (BW69) and Al-sensitive (W270) wild soybean germplasm subjected to three Al concentrations. We analyzed the rhizobacterial communities of the two genotypes by high-throughput sequencing of 16S rRNA genes. The results showed that high Al stress recruited different rhizobacterial communities between two genotypes. In total, 49 OTUs, such as OTU15 (Gammaproteobacteria_KF-JG30-C25_norank), OTU23 (Mizugakiibacter), and OTU93 (Alkanibacter), were enriched in the rhizosphere of BW69 at the low and high Al concentrations. Moreover, bacterial community in the rhizosphere of BW69 had a more complex co-occurrence network than did W270 at the high Al concentration. Overall, our findings highlighted that high Al concentration magnified the difference in rhizobacterial community structure between two genotypes. However, the lower modularity of the co-occurrence network in rhizosphere of BW69 than W270 under Al stress may cause the rhizobacterial community to be less resistant and more influenced by disturbance. This study emphasizes the possibility of using rhizobacteria as an improved crop breeding or gene to produce crops that are more resistant to the toxicity of heavy metal.

Promises and potential of in situ nano-phytoremediation strategy to mycorrhizo-remediate heavy metal contaminated soils using non-food bioenergy crops (Vetiver zizinoides & Cannabis sativa)

Heavy metals (HMs) in soil, air, and water environments effect human health. These HMs cannot be degraded in soil and they can only be transformed from one state to another. Food and energy resources such as coal, oil, petrol, etc. are gradually diminishing due to ever increasing demand and consumption, world faces crisis. There is an urgent need to address these problems by reclaiming the waste/polluted land for food and energy production. Various physicochemical remediation strategies are being proposed, developed, and tested but they are all very costly and only applicable to small contaminated sites. During the past two decades or so, plant-based phytoremediation technology is rapidly evolving as a promising new tool to address the issue with the potential to remediate HM contaminated soils in a sustainable manner. Plants, labeled as phyto-tolerant or phyto-accumulators, surviving on such contaminated soils reduce the toxicity by preventing their translocation or destroying the contaminants by sequestration by synthesizing thiol-containing HM-binding proteins (nano-molecules) and peptides (phytochelators or PCs) which modulate internal levels of metal concentration between deficient and toxic levels. But such plants are very slow growing, producing small biomass, and the process taking a long time to effectively remediate such soils. To overcome limitations of using such plants, plants capable of high biomass production and tolerating multiple HMs, such as non-food bioenergy crops (Vetiver and Hamp), are required. This plant-based remediation strategy can further be enhanced with the use of both plants and rhizosphere microbes like arbuscular mycorrhizal fungi (AMF) and plant growth-promoting bacteria. The combination of three components, i.e. high biomass producing plant, soil, and its rhizosphere harboring plant growth-promoting rhizobial (PGPR) microbiota, particularly AMF, will further improve the process of nano-phytoremediation of HM contaminated soils. This mini review focuses on how phytoremediation, nanotechnology, AMF and PGPR technologies can be merged together to form an integrated nano-mycorrhizo-phytoremediation (NMPR) strategy which synergistically achieve the goal of remediation of soil contaminants and improve the phytoremediation performance of bioenergy plants grown on HM polluted soils. This review also identifies the urgent need to conduct field-scale application of this strategy and use it as potential tool for reestablishing plant cover and population diversity during restoration of derelict land post-industrial/mining activities.

Mercury Phytoremediation with Lolium perenne-Mycorrhizae in Contaminated Soils

The symbiotic association between the roots of a plant and the mycelium of some fungi is identified as mycorrhizae. Symbiosis helps the plant to obtain nutrients from the soil more efficiently, and may favor the phytoremediation capacity of plants such as Lolium perenne, in soils contaminated with mercury. In this study, the morphological and molecular response was evaluated, as well as the variation in mercury accumulation in the different structures of L. perenne when associated with arbuscular mycorrhizal fungi. Association tests were performed to determine the optimal concentration of the biological inoculant and it was found that the best results were given with the proportion of one part of inoculant in three parts of soil (w/w ratio). The differential expression of the glutathione-S-transferase GST gene was evaluated through real-time PCR and the concentration of heavy metals inside and outside the plant was evaluated with inductively coupled plasma atomic emission spectroscopy (ICP). It was found that the plants that were inoculated with mycorrhizae developed longer stems and shorter roots; in the same way, the GST gene had greater expression in the stem than in the root, largely because the roots help the filtration of nutrients to the stem, retaining metals and detoxifying by GST-catalyzed glutathione.

The knowledge domain and emerging trends in phytoremediation: a scientometric analysis with CiteSpace

As a cost-effective, environmentally friendly remediation technology, phytoremediation is defined as the use of green plants to remove pollutants from the environment or render them harmless and has been applied to a variety of contaminated sites throughout the world. There is a prominent phenomenon in which publications about phytoremediation increase each year and involve an increasing number of subject categories. This paper adopts the scientometric analysis method to assess the current state and explore the trends of phytoremediation research based on the bibliographic records retrieved from the Web of Science Core Collection (WoSCC). The results of this paper clearly answer the following questions. (1) What are the publishing characteristics of research on the topic of phytoremediation? What are the characteristics of academic collaboration in phytoremediation research? (2) What are the characteristics and development trends of phytoremediation research? (3) What are the hotspots and frontiers of phytoremediation research? Overall, the research method provides a new approach for the assessment of the performance of phytoremediation research. These results may help new researchers quickly integrate into the field of phytoremediation, as they can easily grasp the frontiers of phytoremediation research and obtain more valuable scientific information. This study also provides references for the follow-up research of relevant researchers.

Arbuscular Mycorrhizal Fungi Improve the Performance of Sweet Sorghum Grown in a Mo-Contaminated Soil

Arbuscular mycorrhizal fungi are among the most ubiquitous soil plant-symbiotic fungi in terrestrial environments and can alleviate the toxic effects of various contaminants on plants. As an essential micronutrient for higher plants, molybdenum (Mo) can cause toxic effects at excess levels. However, arbuscular mycorrhizal fungal impacts on plant performance and Mo accumulation under Mo-contamination still require to be explored. We first studied the effects of Claroideoglomus etunicatum BEG168 on plant biomass production and Mo accumulation in a biofuel crop, sweet sorghum, grown in an agricultural soil spiked with different concentrations of MoS2. The results showed that the addition of Mo produced no adverse effects on plant biomass, N and P uptake, and root colonization rate, indicating Mo has no phytotoxicity and fungitoxicity at the test concentrations. The addition of Mo did not increase and even decreased S concentrations in plant tissues. Arbuscular mycorrhizal inoculation significantly enhanced plant biomass production and Mo concentrations in both shoots and roots, resulting in increased Mo uptake by mycorrhizal plants. Overall, arbuscular mycorrhizal inoculation promoted the absorption of P, N and S by sweet sorghum plants, improved photosystem (PS) II photochemical efficiency and comprehensive photosynthesis performance. In conclusion, MoS2 increased Mo accumulation in plant tissues but produced no toxicity, while arbuscular mycorrhizal inoculation could improve plant performance via enhancing nutrient uptake and photochemical efficiency. Sweet sorghum, together with arbuscular mycorrhizal fungi, shows a promising potential for phytoremediation of Mo-contaminated farmland and revegetation of Mo-mine disturbed areas, as well as biomass production on such sites.

Phytoremediation of Heavy Metal-Contaminated Sites: Eco-environmental Concerns, Field Studies, Sustainability Issues, and Future Prospects

Environmental contamination due to heavy metals (HMs) is of serious ecotoxicological concern worldwide because of their increasing use at industries. Due to non-biodegradable and persistent nature, HMs cause serious soil/water pollution and severe health hazards in living beings upon exposure. HMs can be genotoxic, carcinogenic, mutagenic, and teratogenic in nature even at low concentration. They may also act as endocrine disruptors and induce developmental as well as neurological disorders, and thus, their removal from our natural environment is crucial for the rehabilitation of contaminated sites. To cope with HM pollution, phytoremediation has emerged as a low-cost and eco-sustainable solution to conventional physicochemical cleanup methods that require high capital investment and labor alter soil properties and disturb soil microflora. Phytoremediation is a green technology wherein plants and associated microbes are used to remediate HM-contaminated sites to safeguard the environment and protect public health. Hence, in view of the above, the present paper aims to examine the feasibility of phytoremediation as a sustainable remediation technology for the management of metal-contaminated sites. Therefore, this paper provides an in-depth review on both the conventional and novel phytoremediation approaches; evaluates their efficacy to remove toxic metals from our natural environment; explores current scientific progresses, field experiences, and sustainability issues; and revises world over trends in phytoremediation research for its wider recognition and public acceptance as a sustainable remediation technology for the management of contaminated sites in the twenty-first century.

Transcriptome analysis reveals comprehensive responses to cadmium stress in maize inoculated with arbuscular mycorrhizal fungi

Biological strategy of utilization of plants-microbe's interactions to remediate cadmium (Cd) contaminated soils is effective and practical. However, limited evidence at transcriptome level is available about how microbes work with host plants to alleviate Cd stress. In the present study, comparative transcriptomic analysis was performed between maize seedlings inoculated with arbuscular mycorrhizal (AM) fungi and non-AM fungi inoculation under distinct concentrations of CdCl₂ (0, 25, and 50 mg per kg soil). Significantly higher levels of Cd were found in root tissues of maize colonized by AM fungi, whereas, Cd content was reduced as much as 50% in leaf tissues when compared to non-AM seedlings, indicating that symbiosis between AM fungi and maize seedlings can significantly block translocation of Cd from roots to leaf tissues. Moreover, a total of 5827 differentially expressed genes (DEG) were determined and approximately 68.54% DEGs were downregulated when roots were exposed to high Cd stress. In contrast, 67.16% (595) DEGs were significantly up-regulated when seedlings were colonized by AM fungi under 0 mg CdCl₂. Based on hierarchical clustering analysis, global expression profiles were split into eight distinct clusters. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that hundreds of genes functioning in plant hormone signal transduction, mitogen-activated protein kinase (MAPK) signaling pathway and glutathione metabolism were enriched. Furthermore, MapMan pathway analysis indicated a more comprehensive overview response, including hormone metabolism, especially in JA, glutathione metabolism, transcription factors and secondary metabolites, to Cd stress in mycorrhizal maize seedlings. These results provide an overview, at the transcriptome level, of how inoculation of maize seedlings by AM fungi could facilitate the relief of Cd stress.

SlZRT2 Encodes a ZIP Family Zn Transporter With Dual Localization in the Ectomycorrhizal Fungus Suillus luteus

Ectomycorrhizal (ECM) fungi are important root symbionts of trees, as they can have significant effects on the nutrient status of plants. In polluted environments, particular ECM fungi can protect their host tree from Zn toxicity by restricting the transfer of Zn while securing supply of essential nutrients. However, mechanisms and regulation of cellular Zn homeostasis in ECM fungi are largely unknown, and it remains unclear how ECM fungi affect the Zn status of their host plants. This study focuses on the characterization of a ZIP (Zrt/IrtT-like protein) transporter, SlZRT2, in the ECM fungus Suillus luteus, a common root symbiont of young pine trees. SlZRT2 is predicted to encode a plasma membrane-located Zn importer. Heterologous expression of SlZRT2 in yeast mutants with impaired Zn uptake resulted in a minor impact on cellular Zn accumulation and growth. The SlZRT2 gene product showed a dual localization and was detected at the plasma membrane and perinuclear region. S. luteus ZIP-family Zn uptake transporters did not show the potential to induce trehalase activity in yeast and to function as Zn sensors. In response to excess environmental Zn, gene expression analysis demonstrated a rapid but minor and transient decrease in SlZRT2 transcript level. In ECM root tips, the gene is upregulated. Whether this regulation is due to limited Zn availability at the fungal-plant interface or to developmental processes is unclear. Altogether, our results suggest a function for SlZRT2 in cellular Zn redistribution from the ER next to a putative role in Zn uptake in S. luteus.

Functionally dissimilar soil organisms improve growth and Pb/Zn uptake by Stachys inflata grown in a calcareous soil highly polluted with mining activities

Micro- and macro-organisms are key components of sustainable soil-plant systems; and are involved in plant growth stimulation and accumulation of heavy metals in the plant, with great contribution to phytoremediation of heavy metal-contaminated soils. However, the combined effect of arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR) and earthworms on plant growth and metal uptake is not yet clear. The main objective of this study was to examine the combined influences of these soil organisms on the growth and metal uptake by a native plant species (Stachys inflata), and subsequently on potential phytoremediation in a soil highly polluted with Pb/Zn mining activities. Metal tolerant AMF, PGPR and earthworms were used either alone or in combination in a factorial pot experiment under greenhouse conditions for 4 months. Inoculation of AMF, PGPR and earthworm led to an increase of soil Pb/Zn availability with the greatest Zn availability (about 2 times) with triple inoculation of all the organisms and the highest Pb availability (about 3 times) with co-inoculation of PGPR and earthworms. Triple inoculation of soil organisms increased the total plant biomass 3 times, total Pb uptake 6 times and total Zn uptake 5 times as compared with the uninoculated plants. The growth-stimulating effect of combined soil organisms was much greater than that of individual or dually-inoculated organisms. These results showed that it is possible to use the combination of metal-tolerant soil organisms as a potential bioaugmentation tool to accelerate metal phytoremediation rate in calcareous soils polluted by Pb/Zn mining activity under arid conditions. This could occur as a consequence of the synergistic effects of AMF, PGPR and earthworms on metal availability in the soil, S. inflata growth, metal tolerance and uptake. However, it would practically require more than hundreds of cropping cycles (360 for Pb and 250 for Zn) to achieve metal maximum permissible limits under the conditions of this pot experiment.

A non-toxic polymer enhances sorghum-mycorrhiza symbiosis for bioremediation of Cd

In this study, the effect of a mycorrhizal symbiosis on the translocation of Cd from Cd-polluted soil to sorghum roots was investigated using rhizoboxes. A factorial experiment (two factors including fungus inoculation and Cd contamination) in a completely randomized design with three replicates was performed. In the rhizobox rhizosphere compartment, plants were cultivated in uncontaminated soil and mycorrhizal inoculation (inoculated with Claroideoglomus etunicatum or non-inoculated) was performed, and in the other compartment, the soil was contaminated with Cadmium (Cd) at one of three levels (0, 100 mg kg^-1 using a non-toxic organic polymer (poly (N-vinyl succinate))-Cd, or 100 mg kg^-1 using Cd-nitrate). Cd pollution resulted in a significant decrease in shoot dry weight (from 7.52 to 6.18 and 6.68 g pot^-1, from control to polymer-Cd and nitrate-Cd respectively), root mycorrhizal colonization (from 32.33% to 8.16% and 8.33%), shoot phosphorus concentration (from 3.14 to 2.80 and 2.76 g kg^-1), and soil carbohydrate (from 12.05 to 10.74 and 10.24 mg g^-1), and also resulted in significant increases in soil glomalin (from 595.55 to 660.52 and 690.39 μg g^-1). The use of mycorrhizal fungi increased the glomalin content of the soil and improved the studied parameters. The results revealed the key role of Claroideoglomus etunicatum in translocation of Cd in the rhizobox and also in precise control of Cd concentration of plant tissues (increase or decrease of them depending on Cd composition and Cd availability). Poly(N-vinyl succinate) increased Cd availability and Cd concentration of shoot tissue (5.19 mg kg^-1) compared to nitrate-Cd (3.68 mg kg^-1) and could be recommended for improving phytoremediation.

Are Fungal Endophytes Merely Mycorrhizal Copycats? The Role of Fungal Endophytes in the Adaptation of Plants to Metal Toxicity

The contamination of soil with toxic metals is a worldwide problem, resulting in the disruption of plant vegetation and subsequent crop production. Thus, remediation techniques for contaminated soil and water remain a constant interest of researchers. Phytoremediation, which utilizes plants to remove or stabilize contaminants, is perceived to be a promising strategy. However, phytoremediation's use to date is limited because of constraints associated with such factors as slow plant growth rates or metal toxicity. Microbial-assisted phytoremediation serves as an alternative solution, since the impact of the microbial symbionts on plant growth and stress tolerance has frequently been described. Endophytic fungi occur in almost every plant in the natural environment and contribute to plant growth and tolerance to environmental stress conditions. Although this group of symbiotic fungi was found to form association with a wide range of hosts, including the non-mycorrhizal Brassicaceae metallophytes, their role in the response of plants to metal toxicity has not been thoroughly elucidated to date. This review summarizes the current knowledge regarding the role of endophytic fungi in the tolerance of plants to toxic metals and highlights the similarities and differences between this group of symbiotic fungi and mycorrhizal associations in terms of the survival of the plant during heavy metal stress.

Effects of arbuscular mycorrhizal fungi, biochar and cadmium on the yield and element uptake of Medicago sativa

The synergistic effects of arbuscular mycorrhizal fungi (AMF) inoculation and biochar application on plant growth and heavy metal uptake remain unclear. A pot experiment was carried out to investigate the influence of AMF inoculation, biochar and cadmium (Cd) addition on the growth, nutrient and cadmium uptake of Medicago sativa, as well as soil biological and chemical characteristics. In comparison to the non-Cd pollution treatment, Cd addition significantly decreased mycorrhizal colonization, biomass, and N, P, Ca and Mg contents of shoots and roots in the absence of biochar. Biochar amendment did not increase mycorrhizal colonization at either Cd levels. Regardless of the biochar amendment, AMF inoculation significantly promoted contents of N and P in plant shoots grown in the Cd-contaminated soils. Nevertheless, in the presence of Cd pollution, biochar dramatically elevated the biomass and N, P, K and Ca contents of plant tissues in both AMF inoculation treatments. Biochar addition significantly reduced soil DTPA-extracted Cd. The treatments with AMF inoculation and biochar amendment showed the lowest shoot Cd concentrations and contents, highest plant tissue N and P contents in the Cd addition group. These results suggested that combined use of AMF inoculation and biochar amendment had significant synergistic effects not only on nutrient uptake but also on the reduction in cadmium uptake of alfalfa grown in Cd-polluted soil.

Alleviation of cadmium stress by arbuscular mycorrhizal symbiosis

Owing to the realization of the harmful effect of cadmium on the environment and plants and as the plants are sessile organisms, they need to increase the protective mechanisms to cope with Cd stress. Inoculation the plant with soil microbes at the place of their growing is an important strategy to support the plants against stresses. In this study, trigonella plants were inoculated with arbuscular mycorrhizal (AM) fungi under different CdCl₂ concentrations (0, 2.25, and 6.25 mM). AM inoculation increased growth parameters, chlorophyll, and protein contents. Root colonization was significantly increased at low Cd concentration (2.25 mM) and decreased at high one (6.25 mM). Also, with AM fungal inoculation, the translocation factor of trigonella plants significantly decreased as compared to non-AM ones at both low and high Cd concentrations. In addition, it was clearly that malondialdehyde content of trigonella plants increased significantly at both Cd concentrations and with AM fungal inoculation its content decreased compared to those of non-AM ones. AM inoculation significantly increased antioxidant enzymes activities compared to non-AM ones. Consequently, this study showed a tolerance strategy of AM trigonella plants against Cd stress, thus mycorrhizal symbiosis becomes a promising and suitable as phytostabilizers of Cd stressed soil.

Pb accumulation in spores of arbuscular mycorrhizal fungi

Heavy metal (HM) pollution of soils is one of the most important and unsolved environmental problems affecting the world, with alternative solutions currently being investigated through different approaches. Arbuscular mycorrhizal fungi (AMF) are soil inhabitants that form symbiotic relationships with plants. This alleviates HM toxicity in the host plant, thereby enhancing tolerance. However, the few investigations that have addressed the presence of metals in the fungus structures were performed under experimental conditions, with there being no results reported for Pb. The current study represents a first approximation concerning the capability of spores to accumulate Pb in the AMF community present in a Pb polluted soil under field conditions. Micro X-ray fluorescence was utilized to obtain a direct observation of Pb in spores, and the innovation of total reflection X-ray fluorescence was applied to obtain Pb quantification in spores. The AMF community included species of Ambisporaceae, Archaeosporaceae, Gigasporacea, Glomeraceae and Paraglomeraceae, and was tolerant to high Pb concentrations in soil. Pb accumulation in AMF spores was demonstrated at the community level and corroborated by direct observation of the most abundant spores, which belonged to the Gigasporaceae group. Spore Pb accumulation is possibly dependent on the AMF and host plant species.