Effects of arbuscular mycorrhizal symbiosis on growth, nutrient and metal uptake by maize seedlings (Zea mays L.) grown in soils spiked with Lanthanum and Cadmium

Rhizophagus Irregularis regulates flavonoids metabolism in paper mulberry roots under cadmium stress

Broussonetia papyrifera is widely found in cadmium (Cd) contaminated areas, with an inherent enhanced flavonoids metabolism and inhibited lignin biosynthesis, colonized by lots of symbiotic fungi, such as arbuscular mycorrhizal fungi (AMF). However, the physiological and molecular mechanisms by which Rhizophagus irregularis, an AM fungus, regulates flavonoids and lignin in B. papyrifera under Cd stress remain unclear. Here, a pot experiment of B. papyrifera inoculated and non-inoculated with R. irregularis under Cd stress was carried out. We determined flavonoids and lignin concentrations in B. papyrifera roots by LC-MS and GC-MS, respectively, and measured the transcriptional levels of flavonoids- or lignin-related genes in B. papyrifera roots, aiming to ascertain the key components of flavonoids or lignin, and key genes regulated by R. irregularis in response to Cd stress. Without R. irregularis, the concentrations of eriodictyol, quercetin and myricetin were significantly increased under Cd stress. The concentrations of eriodictyol and genistein were significantly increased by R. irregularis, while the concentration of rutin was significantly decreased. Total lignin and lignin monomer had no alteration under Cd stress or with R. irregularis inoculation. As for flavonoids- or lignin-related genes, 26 genes were co-regulated by Cd stress and R. irregularis. Among these genes, BpC4H2, BpCHS8 and BpCHI5 were strongly positively associated with eriodictyol, indicating that these three genes participate in eriodictyol biosynthesis and were involved in R. irregularis assisting B. papyrifera to cope with Cd stress. This lays a foundation for further research revealing molecular mechanisms by which R. irregularis regulates flavonoids synthesis to enhance tolerance of B. papyrifera to Cd stress.

Changes in microRNAs expression of flax (Linum usitatissimum L.) planted in a cadmium-contaminated soil following the inoculation with root symbiotic fungi

Cadmium is one of the most harmful heavy metals that harm agricultural products. Evaluating microRNAs expression is a new and accurate method to study plant response in various environmental conditions. So this study aimed to evaluate the contribution of two symbiotic fungi in improving flax tolerance in a Cd-polluted soil using microRNAs and their target gene expression. A factorial pot experiment in a completely randomized design was conducted with different levels of Cd (0, 20, and 40 mg kg-1) on non-inoculated and inoculated flax with Claroideoglomus etunicatum and Serendipita indica. The results presented that increasing Cd levels caused a constant decline of alkaline phosphatase of soil (from 243 to 210 and 153 μg PNP g-1 h-1), respectively, from control (Cd0) to 20 and 40 mg Cd kg-1. However, the inoculation of flax with fungi significantly enhanced these properties. A negative correlation was observed between the expression level of microRNA 167 and microRNA 398 with their corresponding target genes, auxin response factor 8 and superoxide dismutase zinc/copper 1, respectively. The expression level of both microRNAs and their targets indicated that the inoculation with symbiont fungi could diminish Cd stress and enhance the growth of flax.

Recent advances in phyto-combined remediation of heavy metal pollution in soil

The global industrialization and modernization have witnessed a rapid progress made in agricultural production, along with the issue of soil heavy metal (HM) pollution, which has posed severe threats to soil quality, crop yield, and human health. Phytoremediation, as an alternative to physical and chemical methods, offers a more cost-effective, eco-friendly, and aesthetically appealing means for in-situ remediation. Despite its advantages, traditional phytoremediation faces challenges, including variable soil physicochemical properties, the bioavailability of HMs, and the slow growth and limited biomass of plants used for remediation. This study presents a critical overview of the predominant plant-based HM remediation strategies. It expounds upon the mechanisms of plant absorption, translocation, accumulation, and detoxification of HMs. Moreover, the advancements and practical applications of phyto-combined remediation strategies, such as the addition of exogenous substances, genetic modification of plants, enhancement by rhizosphere microorganisms, and intensification of agricultural technologies, are synthesized. In addition, this paper also emphasizes the economic and practical feasibility of some strategies, proposing solutions to extant challenges in traditional phytoremediation. It advocates for the development of cost-effective, minimally polluting, and biocompatible exogenous substances, along with the careful selection and application of hyperaccumulating plants. We further delineate specific future research avenues, such as refining genetic engineering techniques to avoid adverse impacts on plant growth and the ecosystem, and tailoring phyto-combined strategies to diverse soil types and HM pollutants. These proposed directions aim to enhance the practical application of phytoremediation and its integration into a broader remediation framework, thereby addressing the urgent need for sustainable soil decontamination and protection of ecological and human health.

Co-inoculation of rhizobia and AMF improves growth, nutrient uptake, and cadmium resistance of black locust grown in sand culture

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

Effects of the promoting bacterium on growth of plant under cadmium stress

Cadmium (Cd) pollution is a huge threat to ecosystem health. In the manuscript, pot experiments were conducted to investigate the changes in plant biomass and antioxidant indicators under different cadmium pollution levels (0, 25, 50, and 100 mg/kg) of inoculation of plant growth-promoting bacteria ZG7 on sugar beet. The results showed that the accumulation of excess Cd in sugar beet exhibited different symptoms, including reduced biomass (p < 0.05). Compared with the group treated with uninoculated strain ZG7, inoculation of strain ZG7 significantly reduced the toxicity of sugar beet to Cd and enhanced its antioxidant capacity, with no significant differences in root biomass and increases in leaf biomass of 15.71, 5.84, and 74.12 under different Cd concentration treatments (25, 50, and 100 mg/kg), respectively. The root enrichment of Cd was reduced by 49.13, 47.26, and 21.50%, respectively (p < 0.05). The leaf fraction was reduced by 59.35, 29.86, and 30.99%, respectively (p < 0.05). In addition, the enzymatic activities of sucrase, urease, catalase, and neutral phosphatase were significantly enhanced in the soil (p < 0.05). This study helps us to further investigate the mechanism of cadmium toxicity reduction by inoculated microorganisms and provides a theoretical reference for growing plants in cadmium-contaminated agricultural fields.

Mycorrhized wheat and bean plants tolerate bismuth contaminated soil via improved metal detoxification and antioxidant defense systems

Contamination of agricultural fields with bismuth (Bi) reduces crop yield and quality. Arbuscular mycorrhizal fungi (AMF) are known to enhance plant growth and crop production, even under stressful conditions such as soil contamination with heavy metals. The objective of this study was to investigate the effect of AMF on the mitigation of Bi-phytotoxicity in wheat (Triticum aestivum) and beans (Phaseolus vulgaris) and to provide a comprehensive evaluation of the physiological and biochemical basis for the growth and development of AMF-induced plants under Bi stress conditions. Wheat and bean were treated by Bi and AMF individually and in combination. Then the physiological and biochemical responses in the shoot and roots of the two crop species were studied. Evident retardations in plant growth and key photosynthesis-related parameters and accumulation of MDA, H2O2, as markers of oxidative stress, were observed in plants subjected to Bi. AMF colonization reduced the uptake and translocation of Bi in the plant organs by enhancing the exudation of polyphenols and organic acids into the rhizospheric soil. Mycorrhized wheat and bean plants were able to attenuate the effects of Bi by improving metal detoxification (phytochelatins, metallothionein, total glutathione, and glutathione-S-transferase activity) and antioxidant defense systems (both enzymatic and non-enzymatic) and maintaining C assimilation and nutrient status. The current results suggest the manipulation of AMF as a powerful approach to alleviate the phytotoxicity of Bi in legumes and grasses.

Endophytic Bacillus sp. R1 and Its Roles in Assisting Phytoremediation and Alleviating the Toxicity of Aluminum Combined Phenanthrene Contaminations in Brassica napus

The release of organic and inorganic contaminants into soil from industry, agriculture, and urbanization has become a major issue of international concern, particularly the heavy metals such as aluminum (Al) and the chemical phenanthrene (PHE). Due to their potential toxicity and non-biodegrade in the environment, efficient remediation methods are urgently needed. Recently, research has comprehensively discussed using plants and their endophytes in bioremediation efforts. Endophytic Bacillus sp. R1, isolated from Brassica napus permanently contaminated with Al and PHE, has growth-promoting properties and can efficiently detoxify these contaminants. The pot experiment indicated that compared to the Al combined PHE contaminated soil alone treatment, the R1 treatment led to increased Al accumulation in canola roots across different levels of PHE, Al, and combined PHE and Al contamination. However, Al accumulation in canola shoots and seeds remained unchanged for all treatments. Moreover, PHE in canola roots and shoots was decreased by R1 inoculation and thereby reducing 26.12-60.61% PHE translocated into canola seeds. Additionally, R1 inoculation significantly increased the proportion of extractable Al and, decreased the proportion of acid-soluble inorganic Al and humic-acid Al, but did not affect the concentration of organically complexed Al. In summary, endophyte R1 can degrade PHE, improve canola roots' Al uptake by increasing soil available Al, and scavenge the reactive oxygen species through production of antioxidant enzymes to help alleviate the toxicity of canola co-contaminated with aluminum and phenanthrene.

Multiomics reveals Claroideoglomus etunicatum regulates plant hormone signal transduction, photosynthesis and La compartmentalization in maize to promote growth under La stress

Rare earth elements (REEs) have been widely used in traditional and high-tech fields, and high doses of REEs are considered a risk to the ecosystem. Although the influence of arbuscular mycorrhizal fungi (AMF) in promoting host resistance to heavy metal (HM) stress has been well documented, the molecular mechanism by which AMF symbiosis enhances plant tolerance to REEs is still unclear. A pot experiment was conducted to investigate the molecular mechanism by which the AMF Claroideoglomus etunicatum promotes maize (Zea mays) seedling tolerance to lanthanum (La) stress (100 mg·kg^-1 La). C. etunicatum symbiosis significantly improved maize seedling growth, P and La uptake and photosynthesis. Transcriptome, proteome, and metabolome analyses performed alone and together revealed that differentially expressed genes (DEGs) related to auxin /indole-3-acetic acid (AUX/IAA) and the DEGs and differentially expressed proteins (DEPs) related to ATP-binding cassette (ABC) transporters, natural resistance-associated macrophage proteins (Nramp6), vacuoles and vesicles were upregulated. In contrast, photosynthesis-related DEGs and DEPs were downregulated, and 1-phosphatidyl-1D-myo-inositol 3-phosphate (PI(3)P) was more abundant under C. etunicatum symbiosis. C. etunicatum symbiosis can promote plant growth by increasing P uptake, regulating plant hormone signal transduction, photosynthesis and glycerophospholipid metabolism pathways and enhancing La transport and compartmentalization in vacuoles and vesicles. The results provide new insights into the promotion of plant REE tolerance by AMF symbiosis and the possibility of utilizing AMF-maize interactions in REE phytoremediation and recycling.

Arbuscular mycorrhizal fungi improve growth and tolerance of Platycladus orientalis under lead stress

Platycladus orientalis is a significant woody plant for phytoremediation in heavy metals contaminated soils. The growth and tolerance of host plants under the lead (Pb) stress were enhanced by arbuscular mycorrhizal fungi (AMF). To evaluate the adjustment by AMF on growth and activity of antioxidant system of P. orientalis under Pb stress. The two-factor pot experiment was conducted with three AM fungal treatments (noninoculated, Rhizophagus irregularis, and Funneliformis mosseae) and four Pb levels (0, 500, 1000, and 2000 mg kg-1). AMF increased dry weight, phosphorus uptake, root vitality, and total chlorophyll content of P. orientalis in spite of Pb stress. Compared with nonmycorrhizal treatments, mycorrhizal P. orientalis had lower H2O2 and malondialdehyde (MDA) contents under Pb stress. AMF increased Pb uptake in roots and decreased the Pb translating to the shoots yet under Pb stress. Total glutathione and ascorbate in roots of P. orientalis were decreased by AMF inoculation. Mycorrhizal P. orientalis had higher superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and glutathione S-transferase (GST) activities in shoots and roots than nonmycorrhizal counterparts. Mycorrhizal P. orientalis under Pb stress showed higher expression of PoGST1 and PoGST2 in roots than that in CK treatments. Future studies will explore the function of induced tolerance genes by AMF of P. orientalis under Pb stress.

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.

Reconditioning of plant metabolism by arbuscular mycorrhizal networks in cadmium contaminated soils: Recent perspectives

Cadmium (Cd) is one of the most perilous nonessential heavy metal for plants, owing to its high water solubility and obstruction with various physiological and biochemical processes. It enters food chain via plant uptake from contaminated soil, posing a grave menace to ecosystem and mankind. Green remediation comprises approaches intended at prudent use of natural resources for increasing profits to humans and environment. Arbuscular mycorrhizal (AM) fungi are considered a promising green technological tool for remedial of Cd-polluted soils. They are naturally associated with root system of plants in Cd-contaminated soils, evidencing their tolerance to Cd. AM can decrease Cd uptake by plants broadly through two strategies: (1) extracellular mechanisms involving Cd chelation by root exudates, binding to fungal cell wall/structures or to the glycoprotein glomalin; (2) intracellular means involving transfer via hyphal network, detoxification and vacuolar sequestration mediated by complexation of Cd with glutathione (GSH), phytochelatins (PCs), metallothioneins (MTs) and polyphosphate granules. Additionally, mycorrhizal symbiosis facilitates reconditioning of plants' metabolism primarily through dilution effect, increased water and mineral uptake. Recently, AM-induced remodelling of root cell wall synthesis has been reported to improve plant vigor and survival under Cd stressed environments. The present article highlights Cd impacts on AM growth, its diversity in Cd contaminated soils, and variations among diverse AM fungal species for imparting plant Cd tolerance. The most recent perspectives on AM-mediated Cd tolerance mechanisms in plants, including cellular and molecular studies have also been reviewed for successful utilization of these beneficial microbes in sustainable agriculture.

Effects of Arbuscular Mycorrhizal Fungi on Alleviating Cadmium Stress in Medicago truncatula Gaertn

Heavy metal contamination is a global problem for ecosystems and human health. Remediation of contaminated soils has received much attention in the last decade. Aided mitigation of heavy metal phytotoxicity by arbuscular mycorrhizal fungi (AMF) is a cost-effective and environmentally friendly strategy. This study was carried out to investigate the mitigation effect of AMF inoculation on heavy metal toxicity in Medicago truncatula under soil cadmium stress. Therefore, a pot experiment was designed to evaluate the growth, chlorophyll fluorescence, Cd uptake and distribution, malondialdehyde (MDA) content, root soil physicochemical properties, and metabolite profile analysis of M. truncatula with/without AMF inoculation in Cd (20 mg/Kg)-contaminated soil. The results showed that inoculating AMF under Cd stress might enhance photosynthetic efficiency, increase plant biomass, decrease Cd and MDA content, and improve soil physicochemical properties in M. truncatula. Non-targeted metabolite analysis revealed that inoculation with AMF under Cd stress significantly upregulated the production of various amino acids in inter-root metabolism and increase organic acid and phytohormone synthesis. This study provides information on the physiological responses of mycorrhizal plants to heavy metal stress, which could help provide deeper insight into the mechanisms of heavy metal remediation by AMF.

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.

Effects of Rhizophagus intraradices on soybean yield and the composition of microbial communities in the rhizosphere soil of continuous cropping soybean

Soybean (Glycine max L.) is an important oil and economic crop in the world. However, soybean continuous cropping may lead to the decline of soybean yield and quality. The purpose of this study was to investigate the effects of Rhizophagus intraradices on soybean growth/yield, root rot disease index, and the composition of microbial communities in the rhizosphere soil of continuous cropping soybean at the R8 stage. The results showed that the 100-seed weight, seed-yield per plant, yield per 0.04 hectare, pods per plant, seed number per plant, branch number, plant height, and fresh weight of root and shoot, and disease index of soybean root rot were significantly affected by the inoculation of R. intraradices and soybean continuous cropping. The growth/yield indexes of soybean were the highest in the inoculated soybean plants under non-continuous cropping. Inoculation of R. intraradices and soybean continuous cropping significantly decreased and increased the disease index of soybean root rot, respectively. Bacterial diversity levels in the rhizosphere soil of continuous cropping soybean were lower than those in non-continuous cropping soybean. Furthermore, it also showed that inoculation of R. intraradices could increase the bacterial and fungal diversity in rhizosphere soil of soybean. It also showed that both inoculation and soybean continuous cropping had effects on the composition of microbial communities in the rhizosphere soil of soybean. Proteobacteria and Ascomycota were the most dominant bacterial and fungal phylum in all samples, respectively. The results would contribute to evaluating the biocontrol potential of R. intraradices against soybean root rot disease, increase soybean yield and improve the composition of microbial communities in the rhizosphere soil of continuous cropping soybean.

Claroideoglomus etunicatum affects the structural and functional genes of the rhizosphere microbial community to help maize resist Cd and La stresses

Arbuscular mycorrhizal fungi (AMF) and plant rhizosphere microbes reportedly enhance plant tolerance to abiotic stresses and promote plant growth in contaminated soils. The co-contamination of soil by heavy metals (e.g., Cd) and rare earth elements (e.g., La) represents a severe environmental problem. Although the influence of AMF in the phytoremediation of contaminated soils is well documented, the underlying interactive mechanisms between AMF and rhizosphere microbes are still unclear. We conducted a greenhouse pot experiment to evaluate the effects of AMF (Claroideoglomus etunicatum) on maize growth, nutrient and metal uptake, rhizosphere microbial community, and functional genes in soils with separate and combined applications of Cd and La. The purpose of this experiment was to explore the mechanism of AMF affecting plant growth and metal uptake via interactions with rhizosphere microbes. We found that C. etunicatum (i) significantly enhanced plant nutritional level and biomass and decreased metal concentration in the co-contaminated soil; (ii) significantly altered the structure of maize rhizosphere bacterial and fungal communities; (iii) strongly enriched the abundance of carbohydrate metabolism genes, ammonia and nitrate production genes, IAA (indole-3-acetic acid) and ACC deaminase (1-aminocyclopropane-1-carboxylate) genes, and slightly altered the abundance of P-related functional genes; (iv) regulated the abundance of microbial quorum sensing system and metal membrane transporter genes, thereby improving the stability and adaptability of the rhizosphere microbial community. This study provides evidence of AMF improving plant growth and resistance to Cd and La stresses by regulating plant rhizosphere microbial communities and aids our understanding of the underlying mechanisms.

Effects of an Arbuscular Mycorrhizal Fungus on the Growth of and Cadmium Uptake in Maize Grown on Polluted Wasteland, Farmland and Slopeland Soils in a Lead-Zinc Mining Area

Arbuscular mycorrhizal fungi (AMF) exist widely in soil polluted by heavy metals and have significant effects on plant growth and cadmium (Cd) uptake. Cd contents differ among wasteland, farmland and slopeland soils in a lead-zinc mining area in Yunnan Province, Southwest China. The effects of AMF on maize growth, root morphology, low-molecular-weight organic acid (LMWOA) concentrations and Cd uptake were investigated via a root-bag experiment. The results show that AMF increased maize growth on Cd-polluted soils, resulting in increases in root length, surface area, volume and branch number, with the effects being stronger in farmland than in wasteland and slopeland soils; increased malic acid and succinic acid secretion 1.3-fold and 1.1-fold, respectively, in roots on farmland soil; enhanced the iron- and manganese-oxidized Cd concentration by 22.6%, and decreased the organic-bound Cd concentration by 12.9% in the maize rhizosphere on farmland soil; and increased Cd uptake 12.5-fold and 1.7-fold in shoots and by 25.7% and 86.6% in roots grown on farmland and slopeland soils, respectively. Moreover, shoot Cd uptake presented significant positive correlations with root surface area and volume and LMWOA concentrations. Thus, these results indicated the possible mechanism that the increased maize Cd uptake induced by AMF was closely related to their effect on root morphology and LMWOA secretion, with the effects varying under different Cd pollution levels.

Cadmium Phytotoxicity, Tolerance, and Advanced Remediation Approaches in Agricultural Soils; A Comprehensive Review

Cadmium (Cd) is a major environmental contaminant due to its widespread industrial use. Cd contamination of soil and water is rather classical but has emerged as a recent problem. Cd toxicity causes a range of damages to plants ranging from germination to yield suppression. Plant physiological functions, i.e., water interactions, essential mineral uptake, and photosynthesis, are also harmed by Cd. Plants have also shown metabolic changes because of Cd exposure either as direct impact on enzymes or other metabolites, or because of its propensity to produce reactive oxygen species, which can induce oxidative stress. In recent years, there has been increased interest in the potential of plants with ability to accumulate or stabilize Cd compounds for bioremediation of Cd pollution. Here, we critically review the chemistry of Cd and its dynamics in soil and the rhizosphere, toxic effects on plant growth, and yield formation. To conserve the environment and resources, chemical/biological remediation processes for Cd and their efficacy have been summarized in this review. Modulation of plant growth regulators such as cytokinins, ethylene, gibberellins, auxins, abscisic acid, polyamines, jasmonic acid, brassinosteroids, and nitric oxide has been highlighted. Development of plant genotypes with restricted Cd uptake and reduced accumulation in edible portions by conventional and marker-assisted breeding are also presented. In this regard, use of molecular techniques including identification of QTLs, CRISPR/Cas9, and functional genomics to enhance the adverse impacts of Cd in plants may be quite helpful. The review's results should aid in the development of novel and suitable solutions for limiting Cd bioavailability and toxicity, as well as the long-term management of Cd-polluted soils, therefore reducing environmental and human health hazards.

Transfer of La, Ce, Sm and Yb to alfalfa and ryegrass from spiked soil and the role of Funneliformis mosseae

Rare earth elements (REEs) are widely used in high-tech industries, and REE waste emissions have become a concern for ecosystems, food quality and human beings. Arbuscular mycorrhizal fungi (AMF) have repeatedly been reported to alleviate plant stress in metal-contaminated soils. To date, little information is available concerning the role of AMF in REE-contaminated soils. We recently showed that there was no transfer of Sm to alfalfa by Funneliformis mosseae, but only a single REE was examined, while light and heavy REEs are present in contaminated soils. To understand the role of AMF on the transfer of REEs to plants, we carried out an experiment using alfalfa (Medicago sativa) and ryegrass (Lolium perenne) in compartmented pots with separate bottom compartments that only were accessible by F. mosseae fungal hyphae. The bottom compartments contained a mixture of four REEs at equal concentrations (La, Ce, Sm and Yb). The concentration of REEs in plants was higher in roots than in shoots with higher REE soil-root than root-shoot transfer factors. Moreover, significantly higher light-REEs La and Ce were transferred to ryegrass shoots than Sm and the heavy-REE Yb, but this was not observed for alfalfa. Alfalfa dry weight was significantly increased by F. mosseae inoculation, but not ryegrass dry weight. For both plant species, there was significantly higher P uptake by the mycorrhizal plants than the nonmycorrhizal plants, but there was no significant transfer of La, Ce, Sm or Yb to alfalfa and ryegrass roots or shoots due to F. mosseae inoculation.

Combined Bioremediation of Bensulfuron-Methyl Contaminated Soils With Arbuscular Mycorrhizal Fungus and Hansschlegelia zhihuaiae S113

Over the past decades, because of large-scale bensulfuron-methyl (BSM) application, environmental residues of BSM have massively increased, causing severe toxicity in rotation-sensitive crops. The removal of BSM from the environment has become essential. In this study, the combined bioremediation of the arbuscular mycorrhizal fungi (AMF) Rhizophagus intraradices and BSM-degrading strain Hansschlegelia zhihuaiae S113 of BSM-polluted soil was investigated. BSM degradation by S113 in the maize rhizosphere could better promote AMF infection in the roots of maize, achieving an infection rate of 86.70% on the 36th day in the AMF + S113 + BSM group. Similarly, AMF enhanced the colonization and survival of S113 in maize rhizosphere, contributing 4.65 × 105 cells/g soil on the 15th day and 3.78 × 104 cells/g soil on the 20th day to a population of colonized-S113 (based possibly on the strong root system established by promoting plant-growth AMF). Both S113 and AMF coexisted in rhizosphere soil. The BSM-degrading strain S113 could completely remove BSM at 3 mg/kg from the maize rhizosphere soil within 12 days. AMF also promoted the growth of maize seedlings. When planted in BSM-contaminated soil, maize roots had a fresh weight of 2.59 ± 0.26 g in group S113 + AMF, 2.54 ± 0.20 g in group S113 + AMF + BSM, 2.02 ± 0.16 g in group S113 + BSM, and 2.61 ± 0.25 g in the AMF group, all of which exceeded weights of the control group on the 36th day except for the S113 + BSM group. Additionally, high-throughput sequencing results indicated that simultaneous inoculation with AMF and strain S113 of BSM-polluted maize root-soil almost left the indigenous bacterial community diversity and richness in maize rhizosphere soil unaltered. This represents a major advantage of bioremediation approaches resulting from the existing vital interactions among local microorganisms and plants in the soil. These findings may provide theoretical guidance for utilizing novel joint-bioremediation technologies, and constitute an important contribution to environmental pollution bioremediation while simultaneously ensuring crop safety and yield.

Arbuscular Mycorrhizal Fungi Are an Influential Factor in Improving the Phytoremediation of Arsenic, Cadmium, Lead, and Chromium

The increasing expansion of mines, factories, and agricultural lands has caused many changes and pollution in soils and water of several parts of the world. In recent years, metal(loid)s are one of the most dangerous environmental pollutants, which directly and indirectly enters the food cycle of humans and animals, resulting in irreparable damage to their health and even causing their death. One of the most important missions of ecologists and environmental scientists is to find suitable solutions to reduce metal(loid)s pollution and prevent their spread and penetration in soil and groundwater. In recent years, phytoremediation was considered a cheap and effective solution to reducing metal(loid)s pollution in soil and water. Additionally, the effect of soil microorganisms on increasing phytoremediation was given special attention; therefore, this study attempted to investigate the role of arbuscular mycorrhizal fungus in the phytoremediation system and in reducing contamination by some metal(loid)s in order to put a straightforward path in front of other researchers.

Synergistic interaction of fungal endophytes, Paecilomyces formosus LHL10 and Penicillium funiculosum LHL06, in alleviating multi-metal toxicity stress in Glycine max L

Heavy metal accumulation in crop grains due to hazardous metal contamination is considered a great concern. However, phytobeneficial fungi are reported to have important abilities for the biosafety of crops grown in contaminated soil. Therefore, the current study was undertaken to explore the mutualistic association of plant growth-promoting endophytic fungi in reducing heavy metal concentration in the seeds of soybean plants subsequently grown in contaminated soil, without comprising seed quality and biochemical profile. The results revealed that endophytic Paecilomyces formosus LHL10 and Penicillium funiculosum LHL06 synergistically produced higher amounts of GAs and IAA in a co-cultured medium. Moreover, the co-inoculation of LHL06 and LHL10 to soybean plants grown under multi-metal toxic conditions significantly mitigated the adverse effects of heavy metal toxicity and increased the seed production (number of pods per plants, number of seeds per pod, and 100 seed weight) of soybean plants grown under control and multi-metal toxic conditions. Moreover, the levels of carbohydrates (glucose, sucrose, and fructose), minerals (iron, calcium, magnesium, and potassium), amino acids (serine, glutamic acids, glycine, methionine, lysine, arginine, and proline), and antioxidants (superoxide dismutase, catalase, and peroxidase) were significantly enhanced in sole and co-inoculated plants under control and stress conditions. Whereas organic acids (citric acid, tartaric acid, malic acid, and succinic acid), lipid peroxidation (MDA) products, multi-metal accumulation (nickel, cadmium, copper, lead, chromium, and aluminum), and stress-responsive endogenous abscisic acid levels were significantly decreased in seeds of soybean plants grown under control and multi-metal toxic conditions upon LHL06 and LHL10 sole and co-inoculation. The current results suggested the positive biochemical regulation in seeds for improving the nutritional status and making it safe for human consumption.

Arbuscular Mycorrhizal Fungi Increase Pb Uptake of Colonized and Non-Colonized Medicago truncatula Root and Deliver Extra Pb to Colonized Root Segment

Arbuscular mycorrhizal (AM) fungi establish symbiosis and improve the lead (Pb) tolerance of host plants. The AM plants accumulate more Pb in roots than their non-mycorrhizal counterparts. However, the direct and long-term impact of AM fungi on plant Pb uptake has been rarely reported. In this study, AM fungus (Rhizophagus irregularis) colonized and non-colonized roots of Medicago truncatula were separated by a split-root system, and their differences in responding to Pb application were compared. The shoot biomass accumulation and transpiration were increased after R. irregularis inoculation, whereas the biomass of both colonized and non-colonized roots was decreased. Lead application in the non-colonized root compartment increased the R. irregularis colonization rate and up-regulated the relative expressions of MtPT4 and MtBCP1 in the colonized root compartments. Rhizophagus irregularis inoculation increased Pb uptake in both colonized and non-colonized roots, and R. irregularis transferred Pb to the colonized root segment. The Pb transferred through the colonized root segment had low mobility and might be sequestrated and compartmented in the root by R. irregularis. The Pb uptake of roots might follow water flow, which is facilitated by MtPIP2. The quantification of Pb transfer via the mycorrhizal pathway and the involvement of MtPIP2 deserve further study.

Gene expression analysis for selection and validation of suitable housekeeping gene(s) in cadmium exposed pigeonpea plants inoculated with arbuscular mycorrhizae

The expression stability of six commonly used housekeeping genes (18S rRNA-18S ribosomal RNA, EF1α-elongation factor 1α, ACT1-Actin 1, GAPDH-Glyceraldehyde-3-phosphate dehydrogenase, TUB6-Tubulin/FtsZ family and UBC-Ubiquitin-conjugating enzyme) were scrutinized in leaves and roots of Cd stressed pigeonpea plants inoculated with arbuscular mycorrhizal (AM) species- Rhizoglomus intraradices (Ri), Funneliformis mosseae (Fm), Claroideoglomus etunicatum (Ce), C. claroideum (Cc). The stability profile of each gene was assessed using ΔC_t, BestKeeper, NormFinder, RefFinder and geNorm algorithmic programs, which ranked different genes as most and least stable according to the tissues analysed. All the statistical algorithms ranked TUB6 as most stable and EF1α least stable housekeeping (HK) genes in both the plant tissues. The selected HK genes were verified using metallothionein (CcMT1) i.e. a stress responsive gene, whose expression altered under conditions of metal stress and AM inoculation. The expression pattern of CcMT1 varied highly when least stable reference gene was used for normalization as compared to most stable gene, under different treatments. Thus, there is a need of selecting suitable reference gene to achieve reliable results in gene expression studies using quantitative real time PCR (qRT-PCR). The study conducted will help future gene expression analysis in pigeonpea under specific stress.

Arbuscular mycorrhizal fungi alter microbiome structure of rhizosphere soil to enhance maize tolerance to La

Rhizosphere microbes are essential partners for plant stress tolerance. Recent studies indicate that arbuscular mycorrhizal fungi (AMF) can facilitate the revegetation of soils contaminated by heavy metals though interacting with rhizosphere microbiome. However, it is unclear how AMF affect rhizosphere microbiome to improve the growth of plant under rare earth elements (REEs) stress. AMF (Claroideoglomus etunicatum) was inoculated to maize grown in soils spiked with Lanthanum (0 mg kg^-1, La0; 10 mg kg^-1, La10; 100 mg kg^-1, La100; 500 mg kg^-1, La500). Plant biomass, nutrient uptake, REE uptake and rhizosphere bacterial and fungal community were evaluated. The results indicated that La100 and La500 decreased significantly root colonization rates and nutrition uptake (K, P, Ca and Mg content). La500 decreased significantly α-diversity indexes of bacterial and fungal community. AMF enhanced significantly the shoot and root fresh and dry weight of maize in all La treatments (except for the root fresh and dry weight of La0 and La10 treatment). For La100 and La500 treatments, AMF increased significantly nutrition uptake (K, P, Ca and Mg content) in shoot of maize by 27.40-441.77%. For La500 treatment, AMF decreased significantly shoot La concentration by 51.53% in maize, but increased significantly root La concentration by 30.45%. In addition, AMF decreased bacterial and fungal Shannon index in La0 treatment, but increased bacterial Shannon index in La500 treatment. Both AMF and La500 affected significantly the bacterial and fungal community composition, and AMF led to more influence than La. AMF promoted the enrichment of bacteria, including Planomicrobium, Lysobacter, Saccharothrix, Agrococcus, Microbacterium, Streptomyces, Penicillium and other unclassified genus, and fungi (Penicillium) in La500, which showed the function for promoting plant growth and tolerance of heavy metal. The study revealed that AMF can regulate the rhizosphere bacterial and fungal composition and foster certain beneficial microbes to enhance the tolerance of maize under La stress. Phytoremediation assisted by AMF is an attractive approach to ameliorate REEs-contaminated soils.

Combined effects of arbuscular mycorrhizae fungus and composted pig manure on the growth of ryegrass and uptake of Cd and Zn in the soil from an e-waste recycling site

Little attention has been paid to the combined effects of arbuscular mycorrhizae (AM) fungus and composted manure on heavy metal bioavailability and its uptake by plants grown in heavy metal-contaminated soils from electronic-waste (e-waste) recycling sites. A greenhouse pot experiment was conducted to investigate the effects of AM fungus, composted pig manure (CM) and AM fungus + CM (ACM) on the growth of ryegrass and uptake of Cd and Zn in the soil collected from an e-waste recycling site. The calcium chloride (CaCl₂) and Tessier sequential extraction procedure were adopted to evaluate the bioavailability and chemical speciation of Cd and Zn in the soil. Results showed that the application of CM and ACM significantly increased the pH but decreased the CaCl₂-extractable Cd and Zn concentrations in the rhizosphere and bulk soils. ACM treatment significantly shifted Cd from exchangeable fraction to other more stable fractions, and transformed the exchangeable Zn fraction to the carbonate-bound and reducible iron and manganese-bound fractions. Furthermore, the application of ACM can enhance the growth of plant shoots, and decrease the uptake of Cd and Zn in the ryegrass plants. This work suggests that AM fungus in combination with CM amendment may be a potential method for not only remediation of soil Cd and Zn pollution, but also reduction of Cd and Zn uptake by ryegrass grown in the soil from e-waste recycling sites.

Earthworm and arbuscular mycorrhiza interactions: Strategies to motivate antioxidant responses and improve soil functionality

Earthworms and arbuscular mycorrhizal fungi (AMF) act synergistically in the rhizosphere and may increase host plant tolerance to Cd. However, mechanisms by which earthworm-AMF-plant partnerships counteract Cd phytotoxicity are unknown. Thus, we evaluated individual and interactive effects of these soil organisms on photosynthesis, antioxidant capacity, and essential nutrient uptake by Solanum nigrum, as well as on soil quality following Cd exposure (0-120 mg kg^-1). Decreases in biomass and photosynthetic activity, as well as nutrient imbalances were observed in Cd-stressed plants; however, the addition of AMF and earthworms reversed these effects. Cd exposure increased superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, whereas inoculation with Rhizophagus intraradices decreased those. Soil enzymatic activity decreased by 15-60% with increasing Cd concentrations. However, Cd-mediated toxicity was partially reversed by soil organisms. Earthworms and AMF ameliorated soil quality based on soil enzyme activity. At 120 mg kg^-1 Cd, the urease, catalase, and acid phosphatase activities were 1.6-, 1.4-, and 1.2-fold higher, respectively, in soils co-incubated with earthworms and AMF than in uninoculated soil. Cd inhibited shoot Fe and Ca phytoaccumulation, whereas AMF and earthworms normalized the status of essential elements in plants. Cd detoxification by earthworm-AMF-S. nigrum symbiosis was manifested by increases in plant biomass accumulation (22-117%), chlorophyll content (17-63%), antioxidant levels (SOD 10-18%, POD 9-25%, total polyphenols 17-22%, flavonoids 15-29%, and glutathione 7-61%). It also ameliorated the photosynthetic capacity, and macro- and micronutrient statuses of plants; markedly reduced the levels of malondialdehyde (20-27%), superoxide anion (29-36%), and hydrogen peroxide (19-30%); and upregulated the transcription level of FeSOD. Thus, the combined action of earthworms and AMF feasibly enhances metal tolerance of hyperaccumulating plants and improves the quality of polluted soil.

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.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

No significant transfer of the rare earth element samarium from spiked soil to alfalfa by Funneliformis mosseae

Rare earth elements including samarium have been widely used in modern technologies in recent decades. Following over-exploitation and soil contamination, they can accumulate in plants and be toxic at high concentrations. Arbuscular mycorrhizae benefit plants in metal-contaminated soils by improving their survival and growth and alleviating metal toxicity, but little information is available about soil contaminated by rare earth elements. We performed two experiments using samarium to study the role of arbuscular mycorrhizal fungi on plant growth and samarium transfer to alfalfa in a samarium-spiked soil. A pot experiment was conducted in a soil spiked with two concentrations of samarium and a non-spiked control, inoculated or not with a metal-tolerant Funneliformis mosseae. A compartmented pot experiment was then performed with a separated compartment containing samarium-spiked sand only accessible by F. mosseae fungal hyphae to further study the transport of samarium from the soil to alfalfa. The biomass of alfalfa grown on samarium-spiked soil was reduced, while it was significantly higher following arbuscular mycorrhiza inoculation in the pot experiment, both in the control and samarium-spiked soil. Although mycorrhizal plants had a higher phosphorus content than non-mycorrhizal ones, there was no significant difference in samarium concentrations between mycorrhizal and non-mycorrhizal plants. The compartment experiment confirmed that there was no significant samarium transfer to the plant by F. mosseae. Other fungi and plants should be tested, and field experiments performed, but our results suggest that arbuscular mycorrhizal plants might be considered in phytorestoration of rare-earth-contaminated soils.

Prospect of phytoremediation combined with other approaches for remediation of heavy metal-polluted soils

Accumulation of heavy metals in agricultural soils due to human production activities-mining, fossil fuel combustion, and application of chemical fertilizers/pesticides-results in severe environmental pollution. As the transmission of heavy metals through the food chain and their accumulation pose a serious risk to human health and safety, there has been increasing attention in the investigation of heavy metal pollution and search for effective soil remediation technologies. Here, we summarized and discussed the basic principles, strengths and weaknesses, and limitations of common standalone approaches such as those based on physics, chemistry, and biology, emphasizing their incompatibility with large-scale applications. Moreover, we explained the effects, advantages, and disadvantages of the combinations of common single repair approaches. We highlighted the latest research advances and prospects in phytoremediation-chemical, phytoremediation-microbe, and phytoremediation-genetic engineering combined with remediation approaches by changing metal availability, improving plant tolerance, promoting plant growth, improving phytoextraction and phytostabilization, etc. We then explained the improved safety and applicability of phytoremediation combined with other repair approaches compared to common standalone approaches. Finally, we established a prospective research direction of phytoremediation combined with multi-technology repair strategy.

Lanthanum Prolongs Vase Life of Cut Tulip Flowers by Increasing Water Consumption and Concentrations of Sugars, Proteins and Chlorophylls

We evaluated the effect of separately adding two sources of lanthanum (La), LaCl₃ and La(NO₃)₃ × 6H₂O at a concentration of 40 µM each, to the preservative solution of 15 cut tulip flower varieties. Ascorbic acid (AsA; 0.2 g/L) was used as a reference solution, while distilled water was used as control. The variety Laura Fygi recorded the longest vase life with 13 days. The highest water consumption per gram of stem fresh biomass weight (FBW) (2.5 mL) was observed in the variety Violet Beauty, whereas the lowest (1.098 mL) was recorded in Pink Impression. At the end of the vase life period, higher concentrations of total soluble sugars in petals and total soluble proteins in leaves were recorded in La-treated stems, compared to the AsA treatment and the control. Additionally, La(NO₃)₃ × 6H₂O supply increased the fresh weight of stems in vase and prolonged vase life. Moreover, this treatment resulted in the highest foliar concentration of chlorophylls at the end of vase life. Therefore, La increases tulip flower vase life as a consequence of improving the concentrations of some vital biomolecules.

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.

Streptomyces pactum combined with manure compost alters soil fertility and enzymatic activities, enhancing phytoextraction of potentially toxic metals (PTMs) in a smelter-contaminated soil

The effect of manure compost alone and combined with Streptomyces pactum (Act12) applied in the smelter-contaminated soil was investigated. The soil fertility, enzymatic activities, potentially toxic metals (PTMs) solubility, and phytoremediation efficiency of potherb mustard (Brassica juncea, Coss.) were assessed. Results showed that the application of compost reduced the soil pH, while significantly increased the soil electrical conductivity (EC) (7.0 folds), available phosphorus (AP) (10.8 folds), available potassium (AK) (2.81 folds), dissolved organic carbon (DOC) (5.22 folds), organic matter (OM) (4.93 folds), together with soil enzymatic activities viz. urease (UR) (4.39 folds), dehydrogenase (DEH) (45.0 folds) and alkaline phosphatase (ALP) (123.9 folds) in comparison with control. The inoculation of Act12 increased AP, AK, DOC, OM and UR values, but reduced EC, DEH and ALP values compared to corresponding lone compost amendment. Additionally, Act12 solubilized PTMs (Cd and Zn) in the soil, and accordingly enhanced the PTMs uptake in the plant. The phytoextraction indices viz. biological concentration factor (BCF), translocation factor (TF) and metal extraction amount (MEA) indicated that compost and Act12 had a synergistic role in enhancing the phytoremediation efficiency, among which MEA values of Cd and Zn maximally increased by 9.64 and 11.4 folds, respectively, compared to control. Redundancy analysis (RDA) indicated that phytoextraction indices correlated well with soil parameters. Our results suggested that manure compost associated with Act12 is a potential strengthening strategy in phytoremediation of PTMs contaminated soil.

Arbuscular mycorrhizal fungi alleviate Cd phytotoxicity by altering Cd subcellular distribution and chemical forms in Zea mays

Arbuscular mycorrhizal fungus (AMF) can relieve Cd phytotoxicity and improve plant growth, but the mechanisms involved in this process have still been not completely known. In the present work, a pot experiment was conducted to examine productions of glutathione (GSH) and phytochelatins (PCs), and absorption, chemical forms and subcellular distribution of Cd in maize (Zea mays) inoculated with or without AMF (Rhizophagus intraradices (Ri) and Glomus versiforme (Gv)) in Cd-amended soils (0, 1 and 5 mg Cd kg^-1 soil). In general, both Ri and Gv inoculation dramatically enhanced biomass production and reduced Cd concentrations in shoots and roots of maize when compared to the non-mycorrhizal treatment. Moreover, both Ri and Gv symbiosis obviously increased contents of GSH and PCs, both in shoots and roots. Subcellular distribution of Cd in maize indicated that most of Cd (more than 90%) was accumulated in cell wall and soluble fraction. In addition, Cd proportions in soluble fractions in shoots of maize inoculated with Gv or Ri were considerably increased, but reduced in cell wall fractions compared to non-mycorrhizal maize, indicating that mycorrhizal symbiosis promoted Cd transfer to vacuoles. Furthermore, proportions of Cd in inorganic and water-soluble forms were declined, but elevated in pectates and proteins-integrated forms in mycorrhizal maize, which suggested that Gv and Ri could convert Cd into inactive forms. These observations could provide a further understanding of potential Cd detoxification mechanism in maize inoculated with AMF.

Combined application of arbuscular mycorrhizal fungi and steel slag improves plant growth and reduces Cd, Pb accumulation in Zea mays

Little attention has been paid to the combined use of arbuscular mycorrhizal fungus (AMF) and steel slag (SS) for ameliorating heavy metal polluted soils. A greenhouse pot experiment was conducted to study the effects of SS and AMF-Funneliformis mosseae (Fm), Glomus versiforme (Gv) and Rhizophagus intraradices (Ri) on plant growth and Cd, Pb uptake by maize grown in soils added with 5 mg Cd kg^-1 and 300 mg Pb kg^-1 soil. The combined usage of AMF and SS (AMF + SS) promoted maize growth, and Gv + SS had the most obvious effect. Meanwhile, single SS addition and AMF + SS decreased Cd, Pb concentrations in maize, and the greater reductions were found in combined utilization, and the lowest Cd, Pb concentrations of maize appeared in Gv + SS. Single SS amendment and AMF + SS enhanced soil pH and decreased soil diethylenetriaminepentaacetic acid (DTPA)-extractable Cd, Pb concentrations. Furthermore, alone and combined usage of AMF and SS increased contents of soil total glomalin. Our research indicated a synergistic effect between AMF and SS on enhancing plant growth and reducing Cd, Pb accumulation in maize, and Gv + SS exerted the most pronounced effect. This work suggests that AMF inoculation in combination with SS addition may be a potential method for not only phytostabilization of Pb-Cd-contaminated soil but maize safety production.

Mycoextraction: Rapid Cadmium Removal by Macrofungi-Based Technology from Alkaline Soil

Fungi are promising materials for soil metal bioextraction and thus biomining. Here, a macrofungi-based system was designed for rapid cadmium (Cd) removal from alkaline soil. The system realized directed and rapid fruiting body development for subsequent biomass harvest. The Cd removal efficiency of the system was tested through a pot culture experiment. It was found that aging of the added Cd occurred rapidly in the alkaline soil upon application. During mushroom growth, the soil solution remained considerably alkaline, though a significant reduction in soil pH was observed in both Cd treatments. Cd and dissolved organic carbon (DOC) in soil solution generally increased over time and a significant correlation between them was detected in both Cd treatments, suggesting that the mushroom‒substratum system has an outstanding ability to mobilize Cd in an alkaline environment. Meanwhile, the growth of the mushrooms was not affected relative to the control. The estimated Cd removal efficiency of the system was up to 12.3% yearly thanks to the rapid growth of the mushroom and Cd enrichment in the removable substratum. Transcriptomic analysis showed that gene expression of the fruiting body presented considerable differences between the Cd treatments and control. Annotation of the differentially expressed genes (DEGs) indicated that cell wall sorption, intracellular binding, and vacuole storage may account for the cellular Cd accumulation. In conclusion, the macrofungi-based technology designed in this study has the potential to become a standalone biotechnology with practical value in soil heavy metal removal, and continuous optimization may make the system useful for biomining.