Metabolic alterations elicited by Cd and Zn toxicity in Zea mays with the association of Claroideoglomus claroideum

The concentrations of cadmium (Cd) and zinc (Zn) in arable lands exceed the maximum permissible levels due to the excessive use of phosphorus fertilizers and fungicides by farmers. The increasing issues related to the application of agrochemicals have lead to the demand for the implementation of sustainable agricultural approaches. Association of arbuscular mycorrhizae with crop plants is an appropriate strategy due to the potential of these microorganisms to augment the metals tolerance of plants through the immobilization of Cd and Zn in an eco-friendly manner. In the present study, 45 d old Zea mays (var. CoHM6) plants inoculated with AM fungi (Claroideoglomus claroideum) were exposed to 1.95 g Zn Kg^-1 soil and 0.45 g Cd Kg^-1 soil. The major objective of this study was to determine the metabolic alterations in the leaves and roots of mycorrhizal and non-mycorrhizal plants exposed to CdCl₂ and ZnSO₄. Both non AM and AM plants exhibited alterations in the quantity of primary and secondary metabolites on exposure to Zn and Cd toxicity. Moreover, Zn and Cd-induced accumulation of γ-sitosterol reduced the quantity of neophytadiene (a well-known terpenoid) and aided the production of 3-β-acetoxystigmasta-4,6,22-triene in maize leaves. Mycorrhization and heavy metal toxicity induced significant metabolic changes in the roots by producing 4,22-stigmastadiene-3-one, eicosane, 9,19-cyclolanost-24-en-3-ol, pentacosane, oxalic acid, heptadecyl hexyl ester, l-norvaline, and n-(2-methoxyethoxycarbonyl). In addition, the metal-induced variations in leaf and root lignin composition were characterized with the aid of the FTIR technique. Mycorrhization improved the tolerance of maize plants to Cd and Zn toxicity by stabilizing these metal ions in the soil and/or limiting their uptake into the plants, thus ensuring normal metabolic functions of their roots and shoots.

Radiocaesium in Tricholoma spp. from the Northern Hemisphere in 1971-2016

A considerable amount of data has been published on the accumulation of radiocaesium (¹³⁴Cs and particularly, ¹³⁷Cs) in wild fungi since the first anthropogenically influenced releases into the environment due to nuclear weapon testing, usage and subsequently from major accidents at nuclear power plants in Chernobyl (1986) and Fukushima (2011). Wild fungi are particularly susceptible to accumulation of radiocaesium and contamination persists for decades after pollution events. Macromycetes (fruiting bodies, popularly called mushrooms) of the edible fungal species are an important part of the human and forest animal food-webs in many global locations. This review discusses published occurrences of ¹³⁴Cs and ¹³⁷Cs in twenty four species of Tricholoma mushrooms sourced from the Northern Hemisphere over the last five decades, but also includes some recent data from Italy and Poland. Tricholoma are an ectomycorrhizal species and the interval for contamination to permeate to lower soils layers which host their mycelial networks, results in a delayed manifestation of radioactivity. Available data from Poland, over similar periods, may suggest species selective differences in accumulation, with some fruiting bodies, e.g. T. portentosum, showing lower activity levels relative to others, e.g. T. equestre. Species like T. album, T. sulphurescens and T. terreum also show higher accumulation of radiocaesium, but reported observations are few. The uneven spatial distribution of the data combined with a limited number of observations make it difficult to decipher any temporal contamination patterns from the observations in Polish regions. When data from other European sites is included, a similar variability of ¹³⁷Cs activity is apparent but the more recent Ukrainian data appears to show relatively lower activities. ⁴⁰K activity in mushrooms which is associated with essential potassium, remains relatively constant. Further monitoring of ¹³⁷Cs activity in wild mushrooms would help to consolidate these observations.

Ozone does not diminish the beneficial effects of arbuscular mycorrhizas on Medicago sativa L. in a low phosphorus soil

Enriched surface ozone (O₃) can impose harmful effects on plants. Conversely, arbuscular mycorrhizal (AM) symbiosis can enhance plant tolerance to various environmental stresses and facilitate plant growth. The interaction of AM fungi and O₃ on plant performance, however, seldom has been investigated. In this study, alfalfa (Medicago sativa L.) was used as a test plant to study the effects of O₃ and AM symbiosis on plant physiology and growth under two O₃ levels (ambient air and elevated O₃ with 60 nmol·mol^-1 O₃ enrichment) and three AM inoculation treatments (inoculation with exogenous or indigenous AM fungi and non-inoculation control). The results showed that elevated O₃ decreased plant net photosynthetic rate and biomass, and increased malondialdehyde concentration, while AM inoculation (with both exogenous and indigenous AM fungi) could promote plant nutrient acquisition and growth irrespective of O₃ levels. The positive effects of AM symbiosis on plant nutrient acquisition and antioxidant enzyme (superoxide dismutase and peroxidase) activities were most likely offset by increased stomatal conductance and O₃ intake. As a result, AM inoculation and O₃ generally showed no significant interactions on plant performance: although elevated O₃ did not diminish the beneficial effects of AM symbiosis on alfalfa plants, AM symbiosis also did not alleviate the harmful effects of O₃ on plants.

Citric acid and AMF inoculation combination-assisted phytoextraction of vanadium (V) by Medicago sativa in V mining contaminated soil

The use of citric acid (CA) chelator to facilitate metal bioavailability is a promising approach for the phytoextraction of heavy metal contaminants. However, the role of the CA chelator associated with arbuscular mycorrhizal fungi (AMF) inoculation on phytoextraction of vanadium (V) has not been studied. Therefore, in this study, a greenhouse pot experiment was conducted to evaluate the combined effect of CA chelator and AMF inoculation on growth performance and V phytoextraction of plants in V-contaminated soil. The experiment was performed via CA (at 0, 5, and 10 mM kg^-1 soil levels) application alone or in combination with AMF inoculation by Medicago sativa Linn. (M. sativa). Plant biomass, root mycorrhizal colonization, P and V accumulation, antioxidant enzyme activity in plants, and soil chemical speciation of V were evaluated. Results depicted (1) a marked decline in plant biomass and root mycorrhizal colonization in 5- and 10-mM CA treatments which were accompanied by a significant increased V accumulation in plant tissues. The effects could be attributed to the enhanced acid-soluble V fraction transferring from the reducible fraction. (2) The presence of CA significantly enhanced P acquisition while the P/V concentration ratio in plant shoots and roots decreased, owing to the increased V translocation from soil to plant. (3) In both CA-treated soil, AMF-plant symbiosis significantly improved dry weight (31.4-73.3%) and P content (37.3-122.5%) in shoots and roots of M. sativa. The combined treatments also showed markedly contribution in reduction of malondialdehyde (MDA) content (12.8-16.2%) and higher antioxidants (SOD, POD, and CAT) activities in the leaves. This suggests their combination could promote growth performance and stimulate antioxidant response to alleviate V stress induced by CA chelator. (4) Taken together, 10 mM kg^-1 CA application and AMF inoculation combination exhibited a higher amount of extracted V both in plant shoots and roots. Thus, citric acid-AMF-plant symbiosis provides a novel remediation strategy for in situ V phytoextraction by M. sativa in V-contaminated soil.

Mycorrhiza and Iron Tailings Synergistically Enhance Maize Resistance to Arsenic on Medium Arsenic-Polluted Soils Through Increasing Phosphorus and Iron Uptake

Agricultural arsenic (As, CAS. No. 7440-38-2) over the issue of pollution has been related to people's livelihood, security and moderate use of As contaminated soil is an important aspect of contaminated soil remediation. In this potted plant experiment, synergistic effects of arbuscular mycorrhizal fungi (AMF) Funneliformis mosseae and iron (Fe, CAS. No. 7439-89-6) oxides on plant growth and phosphorus (P, CAS. No. 7723-14-0), As and Fe uptake by maize (Zea mays L.) were studied on simulating medium As-polluted soils in greenhouse. Different amounts (0, 5, 10, 20, 40 g kg^- 1) of iron tailings (IT) were added. The results showed that IT20 and IT40 addition significantly increased mycorrhizal infection rate, plant biomass, root length and P, Fe uptake under FM treatment; IT40 addition decreased As concentration in roots. In addition, FM inoculation increased biomass, root length and P uptake by shoots, but decreased Fe and As concentration in shoots. Therefore, the combined FM inoculation and IT40 addition promoted maize growth and decreased As concentration in shoots by decreasing As absorption efficiency, increasing P and Fe uptake and P/As ratio.

Impacts of biochar-based fertilization on soil arbuscular mycorrhizal fungal community structure in a karst mountainous area

The application of biochar-based fertilizer can improve soil properties in part by stimulating microbial activity and growth. Karst ecosystems, which make up large areas of Southwest China, are prone to degradation. Understanding the response of arbuscular mycorrhizal fungal (AMF) community structure to biochar-based fertilizer application is of great significance to karst soil restoration. A field experiment was conducted in a typical karst soil (calcareous sandy loam) in Southwest China. A high-throughput sequencing approach was used to investigate the effect of biochar-based fertilization on AMF community structure in the karst soil. With the control (CK), compost with NPK fertilizer (MF), biochar (B), a lower amount of biochar with compost and NPK fertilizer (B1MF), biochar with compost and NPK fertilizer (BMF), and a higher amount of biochar with compost and NPK fertilizer (B4MF), the field trials were set up for 24 months. Soil amendments increased soil nutrient content and AMF diversity. The composition and structure of the AMF community varied among the treatments. AMF community composition was significantly impacted by soil chemical properties such as TC (total carbon), TN (total nitrogen), TP (total phosphorus), and AP (available phosphorus). Furthermore, network analysis showed that biochar-based fertilization increased the scale and complexity of the microbial co-occurrence network. Biochar-based fertilization enabled more keystone species (such as order Diversisporales and Glomerales) in the soil AMF network to participate in soil carbon resource management and soil nutrient cycling, indicating that biochar-based fertilizer is beneficial for the restoration of degraded karst soils.

Unraveling the effects of arbuscular mycorrhizal fungi on cadmium uptake and detoxification mechanisms in perennial ryegrass (Lolium perenne)

Cadmium (Cd) is a major environmental pollutant and one of the most toxic metals in the environment. Arbuscular mycorrhizal fungi (AMF) assisted phytoremediation can be used to remove Cd from polluted soils but the role of AMF, which mediate in Cd accumulation and tolerance, remains poorly understood. Here we inoculated Lolium perenne with two different AMF species (Glomus etunicatum and Glomus mosseae). Mycorrhizal L. perenne and non-mycorrhizal controls were exposed to Cd stress and we tested the effects of AMF mycorrhization on Cd uptake and subsequent tolerance, as well as the underlying mechanisms. Mycorrhizal infection increased root Cd²⁺ uptake and we observed that net Cd²⁺ influx was coupled with net Ca²⁺ influx. The inactivation of Ca²⁺ transporter channels decreased Cd²⁺ uptake in non-inoculated roots to a greater extent than in inoculated roots, indicating that AMF activates additional ion transport channels. In consequence, inoculated plants exhibited higher Cd accumulation in both roots and shoots than non-inoculated controls. However, AMF-inoculated plants showed higher chlorophyll concentrations, photosynthesis, and growth under Cd, indicating lower Cd toxicity in AMF-inoculated plants, despite the increase in Cd uptake. We observed that AMF-inoculated favored the isolation of Cd within cell walls and vacuoles, and had higher concentrations of superoxide dismutase activity and glutathione concentration in roots than non-inoculated plants, consequently experiencing less stress upon Cd exposure. Our results highlight the potential and mechanism of AMF for enhancing phytoremediation of L. perenne in heavy metal contaminated environments.

Enrichment of nosZ-type denitrifiers by arbuscular mycorrhizal fungi mitigates N2 O emissions from soybean stubbles

Hotspots of N2 O emissions are generated from legume residues during decomposition. Arbuscular mycorrhizal fungi (AMF) from co-cultivated intercropped plants may proliferate into the microsites and interact with soil microbes to reduce N2 O emissions. Yet, the mechanisms by which or how mycorrhizal hyphae affect nitrifiers and denitrifiers in the legume residues remain ambiguous. Here, a split-microcosm experiment was conducted to assess hyphae of Rhizophagus aggregatus from neighbouring maize on overall N2 O emissions from stubbles of nodulated or non-nodulated soybean. Soil microbes from fields intercropped with maize/soybean amended with fertilizer nitrogen (SS-N1) or unamended (SS-N0) were added to the soybean chamber only. AMF hyphae consistently reduced N2 O emissions by 20.8%-61.5%. Generally, AMF hyphae promoted the abundance of N2 O-consuming (nosZ-type) denitrifiers and altered their community composition. The effects were partly associated with increasing MBC and DOC. By contrast, AMF reduced the abundance of nirK-type denitrifiers in the nodulated SS-N0 treatment only and that of AOB in the non-nodulated SS-N1 treatment. Taken together, our results show that AMF reduced N2 O emissions from soybean stubbles, mainly through the promotion of N2 O-consuming denitrifiers. This holds promise for mitigating N2 O emissions by manipulating the efficacious AMF and their associated microbes in cereal/legume intercropping systems.

Combined effect of putrescine and mycorrhizal fungi in phytoremediation of Lallemantia iberica in Pb-contaminated soils

As soil contamination with heavy metals is increasing and polyamines have roles in the growth of mycorrhiza and plants, it is important to study phytoremediation, growth, tolerance, and mycorrhization in Lallemantia iberica as a multi-purpose plant, by the application of putrescine along with mycorrhiza in Pb-contaminated soils. For this purpose, the study was performed in a factorial arrangement with Pb (0, 300, 600, and 900 mg Pb/kg soil), mycorrhiza (non-inoculation, Funneliformis mosseae (Fm), and Rhizophagus intraradices (Ri)), and putrescine (0, 0.5, and 1 mM) in a greenhouse. Results showed that antioxidant activities, plant Pb, and mycorrhizal features enhanced, while transfer factor (TF), biomass, and tolerance decreased under Pb levels. Mycorrhiza improved growth, greenness, defense, and tolerance and reduced TF, Pb, and H₂O₂ content under Pb stress. Putrescine (0.5 mM) increased catalase activity, biomass, and colonization and reduced Pb content and TF under Pb levels. Combination of 0.5 mM putrescine with Fm increased shoot biomass (13%), peroxidase (17.2%), root P (7.5%), shoot tolerance (14.4%), colonization (5.1%), and hyphal width (5.5%) and decreased malondialdehyde (20.5%) and shoot Pb content (28.1%). Putrescine (1 mM) had negative effects on all traits in combination with Ri but not with Fm. Combination of putrescine and Fm showed more efficiency in decreasing Pb content in L. iberica and was effective in phytostabilization. It is generally concluded that 0.5 mM putrescine was the beneficial concentration in combination with mycorrhiza, Pb stress, and single use to improve plant performance, and Fm was a useful species for improving the growth and tolerance of L. iberica under Pb levels.

Demonstrational gardens with EDTA-washed soil. Part II: Soil quality assessment using biological indicators

In this study, we evaluated the impact of washing of Pb, Zn and Cd contaminated soil using EDTA-based technology (ReSoil®) on soil biological properties by measuring some of the most commonly used/sensitive biological indicators of soil perturbation. We estimated the temporal dynamics of the soil respiration, the activities of soil enzymes (dehydrogenase, β-glucosidase, urease, acid and alkaline phosphatase), and the effect of the remediation process on arbuscular mycorrhizal (AM) fungi in original (Orig), remediated (Rem) and remediated vitalized (Rem+V) soils during a more than one-year garden experiment. ReSoil® technology initially affected the activity level of soil microbial respiration and all enzyme activities except urease and reduced AM fungal potential in the soil. However, after one year of vegetable cultivation and standard gardening practices, soil microbial respiration, acid and alkaline phosphatase in the Rem and Rem+V reached similar activities as in the Orig. Only the activities of dehydrogenase and β-glucosidase remained lower in the remediated soil compared to the Orig. The frequency of arbuscular mycorrhiza in the root system, arbuscular density in the colonized root fragment, and the intensity of mycorrhizal colonization in the colonized root fragments in the remediated treatments increased with time; at the end of the experiment, no consistent differences in these parameters of mycorrhizal colonization were found among the treatments. Our results suggest a restored biological functioning of the remediated soil after one year of vegetable cultivation. In general, no differences were found between the Rem and Rem+V treatments, indicating that simple common garden practices are sufficient to restore soil functioning after remediation.

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.

Differential responses of 23 maize cultivar seedlings to an arbuscular mycorrhizal fungus when grown in a metal-polluted soil

Modern breeding efforts have been accelerating crop improvement and yielding numerous cultivars with distinct genetic traits; however, interactions between different cultivars and their root-associated arbuscular mycorrhizal fungi (AMF) are not clear. Herein, we selected the 22 most common commercial maize (Zea mays) varieties in China and an inbred line (B73) to study the differential responses of these 23 cultivars to mycorrhizal inoculation when grown in an arable soil polluted by multiple metals (Pb, Zn, and Cd). We found that the different cultivars exhibited significant variations in plant metal accumulation, ranging from strong metal exclusion (ZYY9) to strong metal accumulation (B73). Mycorrhizal colonization substantially altered metal uptake and repartitioning, while bioaugmenting the inherent characteristics of metal accumulation; for example, the AMF enhanced leaf accumulation of the metal-accumulator B73, and markedly reduced the root uptake of the metal-excluder ZYY9. However, such AMF-induced alterations were also substantially dependent on plant organs (roots and shoots) and metal species. We found that the extent of the AMF-induced leaf alterations was substantially greater than that of the root alterations. Similarly, the number of instances where the AMF significantly altered the Zn and Cd accumulation was far higher than the number of instances where Pb accumulation was significantly altered by AMF. In addition, the presence of AMF appeared to trigger the maize antioxidant systems, which may have alleviated the toxicity of excessive Cd, increased the leaf chlorophyll content, augmented the net photosynthetic rate, and promoted the growth of 17.39% of the maize cultivars. Our results suggest that a future crop breeding challenge is to produce cultivars for safe production or phytoremediation, thereby optimizing the combinations of crop cultivars and their root-associated AMF in slightly to moderately metal-polluted arable soils.

Nitrogen isotopes in the soil-to-tree continuum - Tree rings express the soil biogeochemistry of boreal forests exposed to moderate airborne emissions

Anthropogenic N emissions represent a potential threat for forest ecosystems, and environmental indicators that provide insight into the changing forest N cycle are needed. Tree ring N isotopic ratios (δ¹⁵N) appear as a contentious choice for this role as the exact mechanisms behind tree-ring δ¹⁵N changes seldom benefit from a scrutiny of the soil-to-tree N continuum. This study integrates the results from the analysis of soil chemistry, soil microbiome genomics, and δ¹⁵N values of soil N compounds, roots, ectomycorrhizal (EcM) fungi and recent tree rings of thirteen white spruce trees sampled in five stands, from two regions exposed to moderate anthropogenic N emissions (3.9 to 8.1 kg/ha/y) with distinctive δ¹⁵N signals. Our results reveal that airborne anthropogenic N with distinct δ¹⁵N signals may directly modify the NO₃^- δ¹⁵N values in surface soils, but not the ones of NH₄⁺, the preferred N form of the studied trees. Hence, the tree-ring δ¹⁵N values reflect specific soil N conditions and assimilation modes by trees. Along with a wide tree-ring δ¹⁵N range, we report differences in: soil nutrient content and N transformation rates; δ¹⁵N values of NH₄⁺, total dissolved N (TDN) and EcM mantle enveloping the root tips; and bacterial and fungal community structures. We combine EcM mantle and root δ¹⁵N values with fungal identification to infer that hydrophobic EcM fungi transfer N from the dissolved organic N (DON) pool to roots under acidic conditions, and hydrophilic EcM fungi transfer various N forms to roots, which also assimilate N directly under less acidic conditions. Despite the complexities of soil biogeochemical properties and processes identified in the studied sites, in the end, the tree-ring δ¹⁵N averages inversely correlate with soil pH and anthropogenic N inputs, confirming white spruce tree-ring δ¹⁵N values as a suitable indicator for environmental research on forest N cycling.

Effect of cadmium on sorghum root colonization by glomeral fungi and its impact on total and easily extractable glomalin production

Glomalin is a specific glycoprotein compound synthesized by glomeral fungi and its amount varies according to soil conditions. Due to the symbiosis of these fungi with a multitude of plants, a remarkable amount of glomalin is produced inside the hypha wall of these fungi. Furthermore, while increasing the symbiosis stability, glomalin plays a critical role in reducing the availability of potentially toxic elements (PTEs) through their fixation. In this regard, soil contamination with PTEs such as cadmium (Cd) affects the glomalin production of mycorrhiza fungi. In order to investigate the effect of different levels of Cd on glomalin production of three species of arbuscular mycorrhiza fungi in the presence of sorghum, a greenhouse experiment was conducted in a completely randomized design with factorial arrangement and three repetitions. Factors include four levels of Cd (0, 5, 10, 20 milligrams per kilogram of soil) and second factor included these three types of mycorrhiza: Funneliformis mosseae (FM), Rhizophagus intraradices (RI), and Claroideoglomus etunicatum (CE). The results of this study showed that when increasing soil Cd concentration, shoot dry weight, root colonization percentage, total and easily extractable glomalin decreased while leaf proline concentration, shoot, and root Cd concentration increased. The presence of mycorrhiza in comparison with the control increased the dry weight of shoots and plant height. The results showed that mycorrhizal treatments increased total and easily extractable glomalin compared to the absence of mycorrhiza. Also, the highest amount of glomalin production was observed in two species of FM and CE. Complexation of Cd by total glomalin and easily extractable glomalin was higher in two species of FM and CE rather than RI. The results showed that the use of three species of mycorrhizae reduced the adverse effects of high levels of Cd. Also, the concentration of leaf proline, soluble sugars, shoot, and root Cd concentration was higher in presence of mycorrhizal fungal species than in plant not inoculated with mycorrhizal fungal species. This increase was significant in both FM and CE species. Finally, a notable positive correlation was also observed between glomalin measured by Bradford method and percentage of root colonization.

Arbuscular mycorrhizal fungi potentially regulate N2O emissions from agricultural soils via altered expression of denitrification genes

Agricultural soils are an important source of nitrous oxide (N₂O), a potent greenhouse gas involved in the destruction of the protective ozone layer that contributes to global warming. During N₂O production, soil microorganisms play important driving and regulating roles. A few recent studies have revealed the potential effects of arbuscular mycorrhizal fungi (AMF), a widely distributed soil fungi, on controlling N₂O emissions. However, how AMF regulate N₂O production from soils remains poorly understood. To address the knowledge gap, we manipulated two independent soil environments, which were either allowed (AM) or prevented (NM) access by AMF hyphae in a microcosm experiment (n = 5). Soil physicochemical properties, N₂O flux, the diversity of bacterial communities, and the abundance of key genes responsible for N₂O production were assessed in both treatments over three months. Results showed that the presence of AMF significantly decreased N₂O emissions from agricultural soils in the 1st month, and the abundance of key genes responsible for denitrification (nirK and nosZ) significantly decreased in AM treatments, indicating that the regulation of N₂O emissions is transmitted by AMF-induced changes in the denitrification process. A structural equation model further revealed that AMF indirectly influenced N₂O emissions by altering the abundance of N metabolism-related genes, rather than by altering soil chemical properties or the diversity of bacterial communities. Thus, we proposed a possible mechanism by which AMF can regulate denitrification activities and therefore N₂O emissions from agricultural soils.

Improvement of alfalfa resistance against Cd stress through rhizobia and arbuscular mycorrhiza fungi co-inoculation in Cd-contaminated soil

Rhizobia and arbuscular mycorrhiza fungi (AMF) are important symbiotic microbes that are advantageous to plants growing in metal-contaminated soil. However, it remains unclear how inoculated microbes affect rhizosphere microbial communities or whether subsequent changes in rhizosphere microbiomes contribute to improving plant resistance under metal stress. This study investigated the effects of rhizobia and AMF inoculation on alfalfa resistance to Cd stress. The response of rhizosphere microbial communities to inoculation and its role in increasing alfalfa' ability to cope with stress were further analyzed using high-throughput sequencing of 16S and ITS rRNA genes. Results showed that single rhizobia or AMF inoculation significantly improved alfalfa resistance to Cd stress, while their co-inoculation resulted in the greatest overall improvement. Improved resistance was reflected by the significant mitigation of Cd-induced lipid peroxidation and reactive oxygen species (ROS) stress caused by increases in antioxidant enzyme activities along with co-inoculation. Furthermore, co-inoculation significantly altered the rhizosphere microbial community structure by decreasing fungal community diversity and increasing bacterial community diversity. Results of partial least squares path modeling (PLS-PM) and variation partitioning analysis (VPA) showed that the rhizosphere bacterial community predominated over the fungal community with respected to improvements in resistance to Cd stress under the co-inoculation treatments. This improvement was specifically seen in the enrichment of certain key bacterial taxa (including Proteobacteria, Actinobacteria, Acidobacteria, and Chloroflexi) induced by the rhizobia and AMF co-inoculation, enhancing alfalfa' ability to uptake rhizosphere nutrients and reduce its release of photosynthetically-derived carbon (C) into soil. Our findings revealed that the co-inoculation of multiple symbiotic microbes can assist plants to effectively cope with Cd stress, providing a greater understanding of rhizosphere bacterial taxa in the microbe-induced phytomanagement.

Effects of elevated CO2 on arbuscular mycorrhizal fungi associated with Robinia pseudoacacia L. grown in cadmium-contaminated soils

As symbionts capable of reciprocal rewards, arbuscular mycorrhizal fungi (AMF) can alleviate heavy metal toxicity to host plants and are easily influenced by elevated CO₂ (ECO₂). Although the individual effects of ECO₂ and cadmium (Cd) on AMF have been widely reported, the response of AMF to ECO₂ + Cd receives little attention. We evaluated the combined effects of ECO₂ and Cd on AMF in the rhizosphere soil and roots of Robinia pseudoacacia L. seedlings. Under ECO₂ + Cd relative to Cd, AMF gene copies and richness in rhizosphere soils increased (p < 0.05) and the diversity reduced (p < 0.05) at 4.5 mg Cd kg^-1 dry soil; whereas root AMF abundance at 4.5 mg Cd kg^-1 dry soil and the diversity and richness reduced (p < 0.05). Elevated CO₂ caused obvious differences in the dominant genera abundance between rhizosphere soils and roots upon Cd exposure. Responses of C, water-soluble organic nitrogen (WSON), pH, and diethylene triamine penta-acetic acid (DTPA)-Cd in rhizosphere soils and root N to ECO₂ shaped dominant genera in Cd-polluted rhizosphere soils. Levels of DTPA-Cd, WSON, C and pH in rhizosphere soils and C/N ratio, N, and Cd in roots to ECO₂ affected (p < 0.05) dominant genera in roots under Cd exposure. AMF richness and diversity were lower in roots than in rhizosphere soils. Elevated CO₂ altered AMF communities in rhizosphere soils and roots of R. pseudoacacia seedlings exposed to Cd. AMF associated with R. pseudoacacia may be useful/interesting to be used for improving the phytoremediation of Cd under ECO₂.

Partial and full mycoheterotrophy in green and albino phenotypes of the slipper orchid Cypripedium debile

Most green orchids form mycorrhizal associations with rhizoctonia fungi, a polyphyletic group including Serendipitaceae, Ceratobasidiaceae, and Tulasnellaceae. Although accumulating evidence indicated that partial mycoheterotrophy occurs in such so-called rhizoctonia-associated orchids, it remains unclear how much nutrition rhizoctonia-associated orchids obtain via mycoheterotrophic relationships. We investigated the physiological ecology of green and albino individuals of a rhizoctonia-associated orchid Cypripedium debile, by using molecular barcoding of the mycobionts and stable isotope (¹³C and ¹⁵ N) analysis. Molecular barcoding of the mycobionts indicated that the green and albino individuals harbored Tulasnella spp., which formed a clade with the previously reported C. debile mycobionts. In addition, stable isotope analysis showed that both phenotypes were significantly enriched in ¹³C but not in ¹⁵ N. Therefore, green and albino individuals were recognized as partial and full mycoheterotrophs, respectively. The green variants were estimated to obtain 42.5 ± 8.2% of their C from fungal sources, using the ¹³C enrichment factor of albino individuals as a mycoheterotrophic endpoint. The proportion of fungal-derived C in green C. debile was higher than that reported in other rhizoctonia-associated orchids. The high fungal dependence may facilitate the emergence of albino mutants. Our study provides the first evidence of partial mycoheterotrophy in the subfamily Cypripedioideae. Partial mycoheterotrophy may be more general than previously recognized in the family Orchidaceae.

Physiological and biochemical responses of soybean plants inoculated with Arbuscular mycorrhizal fungi and Bradyrhizobium under drought stress

BACKGROUND

The present study aims to study the effects of biofertilizers potential of Arbuscular Mycorrhizal Fungi (AMF) and Bradyrhizobium japonicum (B. japonicum) strains on yield and growth of drought stressed soybean (Giza 111) plants at early pod stage (50 days from sowing, R3) and seed development stage (90 days from sowing, R5).

RESULTS

Highest plant biomass, leaf chlorophyll content, nodulation, and grain yield were observed in the unstressed plants as compared with water stressed-plants at R3 and R5 stages. At soil rhizosphere level, AMF and B. japonicum treatments improved bacterial counts and the activities of the enzymes (dehydrogenase and phosphatase) under well-watered and drought stress conditions. Irrespective of the drought effects, AMF and B. japonicum treatments improved the growth and yield of soybean under both drought (restrained irrigation) and adequately-watered conditions as compared with untreated plants. The current study revealed that AMF and B. japonicum improved catalase (CAT) and peroxidase (POD) in the seeds, and a reverse trend was observed in case of malonaldehyde (MDA) and proline under drought stress. The relative expression of the CAT and POD genes was up-regulated by the application of biofertilizers treatments under drought stress condition. Interestingly a reverse trend was observed in the case of the relative expression of the genes involved in the proline metabolism such as P5CS, P5CR, PDH, and P5CDH under the same conditions. The present study suggests that biofertilizers diminished the inhibitory effect of drought stress on cell development and resulted in a shorter time for DNA accumulation and the cycle of cell division. There were notable changes in the activities of enzymes involved in the secondary metabolism and expression levels of GmSPS1, GmSuSy, and GmC-INV in the plants treated with biofertilizers and exposed to the drought stress at both R3 and R5 stages. These changes in the activities of secondary metabolism and their transcriptional levels caused by biofertilizers may contribute to increasing soybean tolerance to drought stress.

CONCLUSIONS

The results of this study suggest that application of biofertilizers to soybean plants is a promising approach to alleviate drought stress effects on growth performance of soybean plants. The integrated application of biofertilizers may help to obtain improved resilience of the agro ecosystems to adverse impacts of climate change and help to improve soil fertility and plant growth under drought stress.

A review on metal-based nanoparticles and their toxicity to beneficial soil bacteria and fungi

The unregulated deposition of metal-based nanoparticles in terrestrial ecosystems particularly in agricultural systems has alarmingly threatened the sustainability of the environment and diversity of beneficial microbial populations such as soil bacteria and fungi. This occurs due to the poor treatment of biosolids during wastewater treatment and their application in agricultural fields to enhance the fertility of soils. Continuous deposition, low biodegradability, and longer persistence of metal nanoparticles in soils adversely impact the population of soil beneficial bacteria and fungi. The current literature suggests the toxic outcome of nanoparticle-fungi and nanoparticle-bacteria interactions based on various toxicity endpoints. Therefore, due to the extreme importance of beneficial soil bacteria and fungi for soil fertility and plant growth, this review summarizes the production, application, release of metal nanoparticles in the soil system and their impact on various soil microbes specifically plant growth-promoting rhizobacteria, cellular toxicity and impact of nanoparticles on bioactive molecule production by microbes, destructive nanoparticle impact on unicellular, mycorrhizal, and cellulose/lignin degrading fungi. This review also highlights the molecular alterations in fungi and bacteria-induced by nanoparticles and suggests a plausible toxicity mechanism. This review advances the understanding of the nano-toxicity aspect as a common outcome of nanoparticles and fungi/bacteria interactions.

Arbuscular mycorrhizal fungus-induced decrease in nitrogen concentration in pore water and nitrogen leaching loss from red soil under simulated heavy rainfall

Arbuscular mycorrhizal fungus (AMF) is generally colonized in plant roots and influences the migration of mineral elements such as nitrogen (N) in soils. However, its effect on N leaching loss in red soils is limited. In the present study, red soils were collected from wasteland, farmland, and slopeland in the Yunnan Plateau. Maize, as a host plant, was cultured in a dual-compartment cultivation system. There were mycorrhizal and hyphal compartments for the AMF inoculation treatment and root and soil compartments for the non-inoculation treatment. The N concentration and uptake in maize, N concentration in pore water within two depth (0-20 and 20-40 cm), and N leaching losses from soil under simulated heavy rainfall (40 and 80 mm/h) were analyzed. Results showed that AMF inoculation significantly enhanced the biomass and N uptake in maize. Compared with the root and soil compartments, the N concentrations in pore water and their leaching losses from the mycorrhizal and hyphal compartments were decreased by 48-77% and 51-74%, respectively. Moreover, significant or extremely significantly positive correlations were observed between the N concentrations in pore water with the N leaching losses from soil. The three-way ANOVA showed that AMF highly significantly decreased N concentrations in pore water and their leaching losses from wasteland, farmland, and slopeland; rainfall intensity had strong influences on the N concentration in pore water from farmland and N leaching losses from wasteland and farmland, whereas the maize root's effect was insignificant. The study indicated that the AMF-induced decreases in the N leaching loss from red soils were caused by increased N uptake by maize and decreased N concentrations in pore water. These results have implications for reducing nutrient leaching loss through the management of beneficial microorganisms in soils.

Suppression of arbuscular mycorrhizal fungi decreases the temporal stability of community productivity under elevated temperature and nitrogen addition in a temperate meadow

Global change alters how terrestrial ecosystems function and makes them less stable over time. Global change can also suppress the development and effectiveness of arbuscular mycorrhizal fungi (AMF). This is concerning, as AMF have been shown to alleviate the negative influence of global changes on plant growth and maintain species coexistence. However, how AMF and global change interact and influence community temporal stability remains poorly understood. Here, we conducted a 4-year field experiment and used structural equation modeling (SEM) to explore the influence of elevated temperature, nitrogen (N) addition and AMF suppression on community temporal stability (quantified as the ratio of the mean community productivity to its standard deviation) in a temperate meadow in northern China. We found that elevated temperature and AMF suppression independently decreased the community temporal stability but that N addition had no impact. Community temporal stability was mainly driven by elevated temperature, N addition and AMF suppression that modulated the dominant species stability; to a lesser extent by the elevated temperature and AMF suppression that modulated AMF richness associated with community asynchrony; and finally by the N addition and AMF suppression that modulated mycorrhizal colonization. In addition, although N addition, AMF suppression and elevated temperature plus AMF suppression reduced plant species richness, there was no evidence that changes in community temporal stability were linked to changes in plant richness. SEM further showed that elevated temperature, N addition and AMF suppression regulated community temporal stability by influencing both the temporal mean and variation in community productivity. Our results suggest that global environmental changes may have appreciable consequences for the stability of temperate meadows while also highlighting the role of belowground AMF status in the responses of plant community temporal stability to global change.

Factors that influence the absorption of uranium by indigenous plants on the spoil tip of an abandoned mine in western Spain

The purpose of this work was to study the factors affecting the absorption of U by plants growing on the spoil tip of an abandoned mine in western Spain. The plant species were selected based on how palatable they were to livestock and were sampled for four consecutive years during which, we also recorded rainfall data. The factors related to the plants studied were the leaf size and the percentage and characteristics of the arbuscular mycorrhizae (AM) fungi present in their roots. Our results showed a correlation between the annual rainfall and the U concentration in the plants. The percentage of mycorrhization and AM vesicles is a predominant factor in the uptake of U by plants. Spergularia rubra (L.) J.Presl & C.Presl, which is resistant to mycorrhization, contained higher U concentrations relative to the plants that grew with AM mycorrhization. The absorption curves of the different plants studied indicated that these plants were tolerant to ²³⁸U from 875 Bq kg^-1 (70 mg kg^-1), with a hormesis effect below that concentration. The annual U removal was 0.068%, suggesting that AM are responsible for limiting the incorporation of U into the food chain, favouring its retention in the soil and preventing its dispersion.

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.