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

Enhancing soil quality in soybean cultivation: Mycorrhizal technology combined with intercropping under high cadmium stress

Cadmium (Cd) contamination presents a serious challenges for sustainable agriculture. This study evaluated the combined impact of arbuscular mycorrhizal fungi (AMF) inoculation and intercropping with Solanum nigrum on soil microbial diversity, enzyme activity, and environmental factors in soybean cultivation under high Cd stress. The combined treatment effectively reduced bioavailable Cd in soil, with the acid-soluble Cd fraction at 19.57 mg/kg and the reducible Cd fraction at 61.35 %, resulting in safe soybean grain Cd levels (2.63 mg/kg, below the 3 mg/kg organic standard). Illumina NovaSeq sequencing analysis revealed that key bacterial taxa, including Bradyrhizobium and PMMR1, were correlated with reduced Cd uptake in grains. Although bacterial α diversity increased, microbial network stability decreased in response to Cd, AMF inoculation, and intercropping. The combined treatment also enhanced soil enzyme activity by regulating the relative abundance of dominant or key genera such as Subgroup_6, Rokubacteriales and Pseudarthrobacter. Notably, catalase activity was 97.25 % higher in the combined treatment compared to monoculture without AMF colonization under high Cd conditions. These findings demonstrate the synergistic potential of AMF inoculation and S. nigrum intercropping as a sustainable approach to mitigate Cd contamination in crops while improving soil health in Cd-contaminated environments.

Arbuscular Mycorrhizal Fungi-Assisted Phytoremediation: A Promising Strategy for Cadmium-Contaminated Soils

Arbuscular mycorrhizal fungi (AMF) have been shown to play a major role in regulating the accumulation, transport, and toxicity of cadmium (Cd) in plant tissues. This review aims to highlight the current understanding of the mechanisms by which AMF alleviate Cd toxicity in plants. Cd accumulation in agricultural soils has become an increasing global concern due to industrial activities and the use of phosphatic fertilizers. Cd toxicity disrupts various physiological processes in plants, adversely affecting growth, photosynthesis, oxidative stress responses, and secondary metabolism. AMF alleviate Cd stress in plants through multiple mechanisms, including reduced Cd transport into plant roots, improved plant nutritional status, modulation of organic acid and protein exudation, enhanced antioxidant capacity, and maintenance of ion homeostasis. AMF colonization also influences Cd speciation, bioavailability, and compartmentalization within plant tissues. The expression of metal transporter genes, as well as the synthesis of phytochelatins and metallothioneins, are modulated by AMF during Cd stress. However, the efficacy of AMF in mitigating Cd toxicity depends on several factors, such as soil properties, plant species, AMF taxa, and experimental duration. Further knowledge of the intricate plant-AMF-Cd interactions is crucial for optimizing AMF-assisted phytoremediation strategies and developing Cd-tolerant and high-yielding crop varieties for cultivation in contaminated soils.

The role of arbuscular mycorrhizal fungi in micronutrient homeostasis and cadmium uptake and transfer in rice under different flooding intensities

Flooding intensity significantly alters the availability of iron (Fe), zinc (Zn), and cadmium (Cd) in paddy soil. However, the influence of arbuscular mycorrhizal fungi (AMF) on the uptake and transfer of Cd and micronutrients (Fe and Zn) under Cd stress in varying flooding conditions is not well understood. A pot experiment was conducted to investigate the micronutrient homeostasis and Cd uptake and transfer in rice cultivated in Cd-contaminated soil with AMF inoculation under continuous and intermittent flooding conditions. Compared to non-inoculation controls, mycorrhizal inoculation decreased Cd concentration in rice plants under continuous and intermittent flooding, and improved grain yield by 39.2 % for early season rice and 21.1 % for late season rice under continuous flooding. Mycorrhizal inoculation balanced the availability of Zn and Fe and decreased the availability of Cd in soil, lowering the ratios of soil-available Cd to both soil-available Zn and soil Fe2+. These changes led to a redistribution of Zn and Fe concentrations in rice, thereby reducing Cd acquisition in a soil-rice system. Structural equation model (SEM) analysis revealed that mycorrhizal inoculation had a strong direct negative effect on the expression of Zn and Fe-related genes OsNRAMP1, OsIRT1, and OsIRT2 in the roots of rice, which in turn directly affected root Cd concentration. Furthermore, mycorrhizal colonization decreased Cd transfer coefficients from leaves to grains under continuous flooding and from nodes and leaves to grains under intermittent flooding. In the nodes, the Fe concentration and the expression of genes OsIRT1 and OsHMA2 were associated with Cd transfer from the nodes to grains. Similarly, in the leaves, the expression of genes OsZIP1 and OsMTP1 corresponded with Cd transfer from leaves to grains. This study provides insights into the role of AMF in affecting micronutrient concentrations and Cd uptake in rice under varying flooding conditions.

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.

Integrated physiological, transcriptomic and metabolomic analyses provide insights into phosphorus-mediated cadmium detoxification in Salix caprea roots

Phosphorus (P) plays a crucial role in facilitating plant adaptation to cadmium (Cd) stress. However, the molecular mechanisms underlying P-mediated responses to Cd stress in roots remain elusive. This study investigates the effects of P on the growth, physiology, transcriptome, and metabolome of Salix caprea under Cd stress. The results indicate that Cd significantly inhibits plant growth, while sufficient P alleviates this inhibition. Under Cd exposure, P sufficiency resulted in increased Cd accumulation in roots, along with reduced oxidative stress levels (superoxide anion and hydrogen peroxide contents were reduced by 16.8% and 30.1%, respectively). This phenomenon can be attributed to the enhanced activities of antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT), as well as increased levels of antioxidants including ascorbic acid (AsA) and flavonoids under sufficient P conditions. A total of 4208 differentially expressed genes (DEGs) and 552 differentially accumulated metabolites (DAMs) were identified in the transcriptomic and metabolomic analyses, with 2596 DEGs and 113 DAMs identified among treatments with different P levels under Cd stress, respectively. Further combined analyses reveal the potential roles of several pathways in P-mediated Cd detoxification, including flavonoid biosynthesis, ascorbate biosynthesis, and plant hormone signal transduction pathways. Notably, sufficient P upregulates the expression of genes including HMA, ZIP, NRAMP and CAX, all predicted to localize to the cell membrane. This may elucidate the heightened Cd accumulation under sufficient P conditions. These findings provide insights into the roles of P in enhancing plant resistance to Cd stress and improving of phytoremediation.

Phosphorus deficiency-induced cell wall pectin demethylesterification enhances cadmium accumulation in roots of Salix caprea

Regions affected by heavy metal contamination frequently encounter phosphorus (P) deficiency. Numerous studies highlight crucial role of P in facilitating cadmium (Cd) accumulation in woody plants. However, the regulatory mechanism by which P affects Cd accumulation in roots remains ambiguous. This study aims to investigate the effects of phosphorus (P) deficiency on Cd accumulation, Cd subcellular distribution, and cell wall components in the roots of Salix caprea under Cd stress. The results revealed that under P deficiency conditions, there was a 35.4% elevation in Cd content in roots, coupled with a 60.1% reduction in Cd content in shoots, compared to the P sufficiency conditions. Under deficient P conditions, the predominant response of roots to Cd exposure was the increased sequestration of Cd in root cell walls. The sequestration of Cd in root cell walls increased from 37.1% under sufficient P conditions to 66.7% under P deficiency, with pectin identified as the primary Cd binding site under both P conditions. Among cell wall components, P deficiency led to a significant 31.7% increase in Cd content within pectin compared to P sufficiency conditions, but did not change the pectin content. Notably, P deficiency significantly increased pectin methylesterase (PME) activity by regulating the expression of PME and PMEI genes, leading to a 10.4% reduction in the degree of pectin methylesterification. This may elucidate the absence of significant changes in pectin content under P deficiency conditions and the concurrent increase in Cd accumulation in pectin. Fourier transform infrared spectroscopy (FTIR) results indicated an increase in carboxyl groups in the root cell walls under P deficiency compared to sufficient P treatment. The results provide deep insights into the mechanisms of higher Cd accumulation in root mediated by P deficiency.

Role of phosphorus on the biogeochemical behavior of cadmium in the contaminated soil under leaching and pot experiments

Phosphorus (P) is involved in various biochemical reactions in plant growth, so it is beneficial to plants growing in soils contaminated by metals, including cadmium (Cd). However, few studies have reported on the mechanistic roles of P in mitigating Cd toxicity to ryegrass root, and especially in alleviating the disruption of the mitochondrial function of living cells. In this study, the physiological and biochemical mechanisms associated with ryegrass growth under various Cd and P treatments were investigated using leaching and pot systems. The concentration of Cd in soil leachates showed a significantly positive relationship with redox potential (P < 0.05), but negative relationship (P < 0.05) with leachate pH values and dissolved organic carbon (DOC), indicating that exogenous P addition (as H2PO4-) may decrease Cd leaching from contaminated soil. Compared to the control (without P addition), the cumulative Cd content was reduced by 53.3% and 64.5% in the soil leachate with exogenous P application (20 mg/L and 80 mg/L), respectively. Notably, application of P decreased the Cd concentrations in the symplastic fractions and increased the Cd concentrations in the apoplastic fractions in root tips, which may help to alleviate Cd stress to the protoplast. Moreover, exogenous P was found to play a positive role in mitochondrial function and Ca2+ variation in root cells under Cd stress, which provides novel insights into the mechanisms of exogenous P in alleviating plant Cd injury.

The impacts of exogenous phosphorus on Cd absorption in perennial ryegrass root cell: Kinetic and mechanism study

Phosphorus (P) is critical to plants in metal-contaminated soils because it participates in various biochemical reactions during plant growth. However, the mechanisms of P in mitigating the toxicity of heavy metals to ryegrass root is still veiled. In this study, the physiological and biochemical dynamics of the ryegrass root under various cadmium (Cd) and P conditions were investigated in a hydroponic system. Cd stress decreased the length of the ryegrass root, but P application enhanced the root elongation to reduce the Cd concentration in the root. Both Cd and P dosages were positively correlated with hemicellulose 1 content, pectin content, and PME activity, while having a negative effect on cellulose content. Moreover, the addition of 80 mg L-1 P increased the contents of pectin and hemicellulose 1 by 2.5 and 5.8% even with 4 mg L-1 Cd. In addition, P supply increased pectin methylesterbase activity under Cd stress, which further changed the extra-cytoplasmic structures and cell wall composition. Thus, exogenous P promoted the immobilization of Cd onto the cell wall and protected protoplast primarily through indirectly regulating the binding capacity of the root cell wall for Cd.

The alleviation mechanisms of cadmium toxicity in Broussonetia papyrifera by arbuscular mycorrhizal symbiosis varied with different levels of cadmium stress

The alleviation of cadmium (Cd) toxicity in Broussonetia papyrifera by arbuscular mycorrhizal (AM) fungi are still not completely elucidated. This study investigated the effects of Rhizophagus irregularis on physiological and biochemical characteristics, and molecular regulation in B. papyrifera under different levels of Cd (0, 30, 90 and 270 mg kg-1 Cd) stress. Results showed that (1) AM symbiosis improved the growth and photosynthesis, enhanced ROS levels as stress signaling and maintained ROS balance under low and medium Cd stress. (2) AM symbiosis regulated AsA-GSH cycle to mitigate ROS overproduction under high Cd stress. (3) AM fungus can chelate more Cd under high Cd stress, increasing soil pH and GRSP content. (4) AM plants can fix or chelate more Cd by P in leaves and reserve more P in stems under high Cd stress. (5) AM symbioses increased root net Cd2+ influx and uptake under medium Cd stress but inhibited under high Cd stress, with upregulation of genes related heavy metals (HMs) transport under medium Cd stress and inhibited the transcription of genes related HMs transport under high Cd stress. Therefore, the alleviation mechanisms of Cd toxicity in B. papyrifera by R. irregularis symbiosis depends on the levels of Cd stress.

NaCl-mediated strategies for the trade-off between Cd bioconcentration and translocation in Solanum nigrum L

Salt interference significantly affects the behavior of heavy metals in the environment. This study compared and analyzed the response process, migration, and transformation of cadmium (Cd) in the hyperaccumulator Solanum nigrum (S. nigrum) under different NaCl levels to reveal the interference mechanisms of salt in plant remediation of Cd-contaminated soil. The results showed that Cd and salt stress significantly inhibited the growth of plants. The stress effect had more potent growth inhibition at the root than aboveground, thus inducing changes in the spatial configuration of the plants (decreased root-to-aboveground biomass ratio). Salt could activate Cd in plants, enhancing the inhibitory effect on plant growth. Salt increased Cd bioavailability due to the rhizosphere acidification effect, increasing plants' Cd accumulation. The Cd bioconcentration factor in plant roots peaked during the high Cd-high salt treatment (117.10), but the Cd accumulation of plants peaked during the high Cd-low salt treatment (233.04 μg plant-1). Salt additions and increased Cd concentrations enhanced root compartmentalization, inhibiting Cd transport to the aboveground. Changes in Fourier-transform infrared spectroscopy (FTIR) measurements confirmed that the functional groups in plants provided binding sites for Cd. These findings can help guide the phytoremediation of Cd contamination under saline soil conditions.

Revealing the effects and mechanisms of arbuscular mycorrhizal fungi on manganese uptake and detoxification in Rhus chinensis

Arbuscular mycorrhizal fungi (AMF) can alleviate heavy metal phytotoxicity and promote plant growth, while the underlying mechanisms of AMF symbiosis with host plants under manganese (Mn) stress remain elusive. A pot experiment was carried out to investigate the plant growth, micro-structure, Mn accumulation, subcellular distribution, chemical forms, and physiological and biochemical response of Rhus chinensis inoculated with Funneliformis mosseae (FM) under different Mn treatments. The results showed that compared with plants without FM, FM-associated plants exhibited higher growth status, photosynthetic pigments, and photosynthesis under Mn stress. FM-associated plants were able to maintain greater integrity in mesophyll structure, higher thickness of leaf, upper epidermis, and lower epidermis under Mn treatment, and promote leaf growth. Mn accumulation in leaves (258.67-2230.50 mg kg-1), stems (132.67-1160.00 mg kg-1), and roots (360.92-2446.04 mg kg-1) of the seedlings inoculated with FM was higher than non-inoculated ones. FM-associated plants exhibited higher osmotic regulating substances and antioxidant enzymes' activities under Mn exposure, suggesting lower Mn toxicity in FM inoculated seedlings, despite the augment in Mn accumulation. After FM inoculation, Mn concentration (151.04-1211.32 mg kg-1) and percentage (64.41-78.55%) enhanced in the cell wall, whilst the transport of Mn to aerial plant organs decreased. Furthermore, FM symbiosis favored the conversion of Mn from high toxic forms (2.17-15.68% in FEthanol, 11.37-24.52% in Fdeionized water) to inactive forms (28.30-38.15% in FNaCl, 18.07-28.59% in FHAc, 4.41-17.99% in FHCl) with low phytotoxicity. Our study offers a theoretical basis for remediation of the FM- R. chinensis symbiotic system in Mn-contaminated environments.

Endophytic Fungus Talaromyces sp. MR1 Promotes the Growth and Cadmium Uptake of Arabidopsis thaliana L. Under Cadmium Stress

Endophytes play essential roles in plant growth under metal(loid)s stress. An endophytic fungus strain MR1 was isolated from the roots of Miscanthus floridulus collected from a lead-zinc mining area (Huayuan, China), which could produce indole-3-acetic acid and have Cadmium (Cd) tolerance. Further 18S rRNA sequencing analysis showed that it was highly similar (99.83%) to Talaromyces pinophilus. In pot experiments, we explored the effects of strain MR1 on the growth and Cd uptake of a wide-type Arabidopsis thaliana under low (LC) and high (HC) Cd concentrations. The results showed that MR1 effectively increased the dry weight of aboveground and underground tissues by 25.95-107.21% in both LC and HC groups. Due to MR1 inoculation, the Cd content in the underground tissues was significantly (p < 0.05) decreased by 39.28% under low Cd concentration, while it was significantly (p < 0.05) increased by 28.28% under high Cd concentration. Besides, MR1 inoculations significantly (p < 0.05) increased the total content of removed Cd (17.080 μg) and BCF (0.064) by 129.77% and 153.95% under high Cd concentration. Therefore, we speculated that MR1 might be selected as the effective microbial agent to increase crop yield and control Cd content in the crop in light Cd-contaminated soil. Besides, MR1 could potentially enhance the phytoremediation efficiency of extremely Cd-contaminated soil.

How does phosphorus influence Cd tolerance strategy in arbuscular mycorrhizal - Phragmites australis symbiotic system?

To clarify how phosphorus (P) influences arbuscular mycorrhizal fungi (AMF) interactions with host plants, we measured the effects of variation in environmental P levels and AMF colonization on photosynthesis, element absorption, ultrastructure, antioxidant capacity, and transcription mechanisms in Phragmites australis (P. australis) under cadmium (Cd) stress. AMF maintained photosynthetic stability, element balance, subcellular integrity and enhanced antioxidant capacity by upregulating antioxidant gene expression. Specifically, AMF overcame Cd-induced stomatal limitation, and mycorrhizal dependence peaked in the high Cd-moderate P treatment (156.08%). Antioxidants and compatible solutes responded to P-level changes: the primary driving forces of removing reactive oxygen species (ROS) and maintaining osmotic balance were superoxide dismutase, catalase, and sugars at limited P levels and total polyphenol, flavonoid, peroxidase, and proline at abundant P levels, we refer to this phenomenon as "functional link." AMF and phosphorus enhanced Cd tolerance in P. australis, but the regulation of AMF was P-dependent. Phosphorus prevented increases in total glutathione content and AMF-induced GSH/GSSG ratio (reduced to oxidized glutathione ratio) by inhibiting the expression of assimilatory sulfate reduction and glutathione reductase genes. The AMF-induced flavonoid synthesis pathway was regulated by P, and AMF activated Cd-tolerance mechanisms by inducing P-dependent signaling.

AMF Inoculation Alleviates Molybdenum Toxicity to Maize by Protecting Leaf Performance

The use of arbuscular mycorrhizal fungi (AMF) is a vital strategy for enhancing the phytoremediation of heavy metals. However, the role of AMF under molybdenum (Mo) stress is elusive. A pot culture experiment was conducted to explore the effects of AMF (Claroideoglomus etunicatum and Rhizophagus intraradices) inoculation on the uptake and transport of Mo and the physiological growth of maize plants under different levels of Mo addition (0, 100, 1000, and 2000 mg/kg). AMF inoculation significantly increased the biomass of maize plants, and the mycorrhizal dependency reached 222% at the Mo addition level of 1000 mg/kg. Additionally, AMF inoculation could induce different growth allocation strategies in response to Mo stress. Inoculation significantly reduced Mo transport, and the active accumulation of Mo in the roots reached 80% after inoculation at the high Mo concentration of 2000 mg/kg. In addition to enhancing the net photosynthetic and pigment content, inoculation also increased the biomass by enhancing the uptake of nutrients, including P, K, Zn, and Cu, to resist Mo stress. In conclusion, C. etunicatum and R. intraradices were tolerant to the Mo stress and could alleviate the Mo-induced phytotoxicity by regulating the allocation of Mo in plants and improving photosynthetic leaf pigment contents and the uptake of nutrition. Compared with C. etunicatum, R. intraradices showed a stronger tolerance to Mo, which was manifested by a stronger inhibition of Mo transport and a higher uptake of nutrient elements. Accordingly, AMF show potential for the bioremediation of Mo-polluted soil.

Regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi

Endophytic fungi exist widely in plants and play an important role in the growth and adaptation of plants. They could be used in phytoremediation techniques against heavy metal contaminated soil since beneficial microbial symbionts can endow plants with resistance to external heavy metal stresses. This review summarized the regulation mechanism of plant response to heavy metal stress mediated by endophytic fungi. Potential endophytic fungi in enhancing plant's adaption to heavy metal stresses include arbuscular mycorrhizal fungi, dark septate endophytic fungi, plant growth promoting endophytic fungi. The mechanisms involve coevolution strategy, immune regulation and detoxification transport to improve the ability of plants to adapt to heavy metal stress. They can increase the synthesis of host hormones and maintaining the balance of endogenous hormones, strengthen osmotic regulation, regulate carbon and nitrogen metabolism, and increase immune activity, antioxidant enzyme and glutathione activity. They also help to improve the detoxification transport and heavy metal emission capacity of the host by significantly producing iron carrier, metallothionein and 1-aminocyclopropane-1-carboxylic acid deaminase. The combination of endophytic fungi and hyperaccumulation plants provides a promising technology for the ecological restoration of heavy metal contaminated soil. Endophytic fungi reserves further development on enhancing host plant's adaptability to heavy metal stresses.