Molecular biodiversity of arbuscular mycorrhizal fungi in trace metal-polluted soils

Mycorrhizal fungal communities associated with three metal accumulator plants growing in an abandoned Pb smelting factory

Plants associated with mycorrhizal fungi has the ability to establish on metal-contaminated soils playing an important role in phytoremediation programs. The objective of this study was to examine the presence of arbuscular mycorrhizal fungi (AMF) (spores density, diversity, indicator species, and root colonization) and dark septate endophytic fungi (DSE fungal root colonization) in three metal accumulator plants (Sorghum halepense, Bidens pilosa, and Tagetes minuta) growing in soils with high Pb content. The Pb content in AMF spores and plant biomass were also assessed. Rhizosphere soil samples were taken from the three dominant plant species at six study sites surrounding the abandoned Pb smelter and one uncontaminated site. The three studied plants were colonized by AMF and DSE fungi. A total of 24 AMF morphospecies were present in the Pb-contaminated areas. The AMF indicator species in the control site (non-contaminated area) was Funneliformis mosseae and in the most contaminated site were Gigaspora decipiens and Denticustata biornata. There was an increase in mycorrhizal variables such as the number of AMF vesicles, spore number, Pb content in AMF spores and plant biomass and DSE colonization (in Sorghum) with increasing soil Pb contamination, but a decrease in AMF diversity and richness was found. For upcoming soil restoration projects, it is crucial to understand the mycorrhizal fungi as well as the plant community that has adapted to the highly contaminated environment.

Cd contamination determined assembly processes and network stability of AM fungal communities in an urban green space ecosystem

The effects of cadmium (Cd) contamination on the assembly mechanism and co-occurrence patterns of arbuscular mycorrhizal (AM) fungal communities remain unclear, especially in urban green spaces. This study sequenced AM fungal communities in greenbelt soils in Zhengzhou (China). The effects of Cd contamination on the AM fungal diversity, community assembly processes, and co-occurrence patterns were explored. We found that (1) an increase in Cd contamination changed the community composition, which resulted in a significant improvement in the diversity of specialists of AM fungi and a significant decrease in the diversity of generalists. (2) Deterministic processes dominated the community assembly of specialists and stochastic processes dominated the community assembly of generalists. (3) Specialists played a more important role than generalists in maintaining the stability of AM fungal networks under Cd contamination. Overall, Cd contamination affected the ecological processes of AM fungi in urban green space ecosystems. However, the effects on the assembly processes and network stability of different AM fungi taxa (specialists and generalists) differed significantly. The present study provides deeper insight into the effect of Cd contamination on the ecological processes of AMF and is helpful in further exploring the ecological risk of Cd contamination in urban green spaces.

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

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

The function and community structure of arbuscular mycorrhizal fungi in ecological floating beds used for remediation of Pb contaminated wastewater

Arbuscular mycorrhizal fungi (AMF) have been demonstrated to be ubiquitous in aquatic ecosystems. However, their distributions and ecological functions are rarely studied. To date, a few studies have combined sewage treatment facilities with AMF to improve removal efficiency, but appropriate and highly tolerant AMF strains have not been explored, and the purification mechanisms remain unclear. In this study, three ecological floating-bed (EFB) installations inoculated with different AMF inocula (mine AMF inoculum, commercial AMF inoculum and non-AMF inoculated) were constructed to investigate their removal efficiency for Pb-contaminated wastewater. The AMF community structure shifts in the roots of Canna indica inhabiting EFBs during the three phases (pot culture phase, hydroponic phase and hydroponic phase with Pb stress) were tracked utilizing quantitative real-time polymerase chain reaction and Illumina sequencing techniques. Furthermore, transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were used to detect the Pb location in mycorrhizal structures. The results showed that AMF could promote host plant growth and enhance the Pb removal efficiency of the EFBs. The higher the AMF abundance, the better the effect of the AMF on Pb purification by EFBs. Both flooding and Pb stress decreased the AMF diversity but did not significantly inhibit the abundance. The three inoculation treatments showed different community compositions with different dominant AMF taxa in different phases, and an uncultured Paraglomus species (Paraglomus sp. LC516188.1) was found to be the most dominant (99.65 %) AMF in the hydroponic phase with Pb stress. The TEM and EDS analysis results showed that the Paraglomus sp. could accumulate Pb in plant roots through their fungal structures (intercellular mycelium, intracellular mycelium, etc.), which alleviated the toxic effect of Pb on plant cells and limited Pb translocation. The new findings provide a theoretical basis for the application of AMF in plant-based bioremediation of wastewater and polluted waterbodies.

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.

Generalist arbuscular mycorrhizal fungi dominated heavy metal polluted soils at two artisanal and small - scale gold mining sites in southeastern Ecuador

BACKGROUND

Artisanal and small-scale gold mining activities are producing contamination with heavy metals and metalloids (HMM) into soils and water worldwide. The HMM are considered as one of the major abiotic stresses due to their long-term persistence in soil. In this context, arbuscular mycorrhizal fungi (AMF) confer resistance to a variety of abiotic plant stressors including HMM. However, little is known regarding the diversity and composition of AMF communities in heavy metal polluted sites in Ecuador.

METHODS

In order to investigate the AMF diversity, root samples and associated soil of six plant species were collected from two sites polluted by heavy metals, located in Zamora-Chinchipe province, Ecuador. The AMF 18S nrDNA genetic region was analyzed and sequenced, and fungal OTUs were defined based on 99% sequence similarity. Results were contrasted with AMF communities from a natural forest and from reforestation sites located in the same province and with available sequences in GenBank.

RESULTS

The main pollutants in soils were Pb, Zn, Hg, Cd and Cu with concentrations exceeding the soil reference value for agricultural use. Molecular phylogeny and OTU delimitation showed 19 OTUs, the family Glomeraceae was the most OTU-rich followed by Archaeosporaceae, Acaulosporaceae, Ambisporaceae and Paraglomeraceae. Most of the OTUs (11 of 19) have been found at other locations worldwide, 14 OTUs were proven from nearby non-contaminated sites in Zamora-Chinchipe.

CONCLUSION

Our study showed that there are no specialized OTUs at the studied HMM polluted sites, but rather generalists adapted to a wide variety of habitats. Their potential role in phytoremediation approaches remains to be investigated.

Long-Term Persistence of Arbuscular Mycorrhizal Fungi in the Rhizosphere and Bulk Soils of Non-host Brassica napus and Their Networks of Co-occurring Microbes

Arbuscular mycorrhizal fungi (AMF) are obligate plant symbionts that improve the nutrition and health of their host. Most, but not all the crops form a symbiosis with AMF. It is the case for canola (Brassica napus), an important crop in the Canadian Prairies that is known to not form this association. From 2008 to 2018, an experiment was replicated at three locations of the Canadian Prairies and it was used to assess the impact of canola on the community of AMF naturally occurring in three cropping systems, canola monoculture, or canola in two different rotation systems (2-years, canola-wheat and 3-years, barley-pea-canola). We sampled canola rhizosphere and bulk soils to: (i) determine diversity and community structure of AMF, we expected that canola will negatively impact AMF communities in function of its frequency in crop rotations and (ii) wanted to assess how these AMF communities interact with other fungi and bacteria. We detected 49 AMF amplicon sequence variants (ASVs) in canola rhizosphere and bulk soils, confirming the persistence of a diversified AMF community in canola-planted soil, even after 10 years of canola monoculture, which was unexpected considering that canola is among non-mycorrhizal plants. Network analysis revealed a broad range of potential interactions between canola-associated AMF and some fungal and bacterial taxa. We report for the first time that two AMF, Funneliformis mosseae and Rhizophagus iranicus, shared their bacterial cohort almost entirely in bulk soil. Our results suggest the existence of non-species-specific AMF-bacteria or AMF-fungi relationships that could benefit AMF in absence of host plants. The persistence of an AMF community in canola rhizosphere and bulk soils brings a new light on AMF ecology and leads to new perspectives for further studies about AMF and soil microbes interactions and AMF subsistence without mycotrophic host plants.

Root colonization and rhizospheric community structure of Arbuscular Mycorrhizal Fungi in BADH transgenic maize BZ-136 and its recipient under salt stress and neutral soil

Betaine aldehyde dehydrogenase (BADH) transgenic maize has a capability to grow under drought and salt stress; the risk of planting BADH transgenic maize on symbiotic microorganisms remains problematic, however. A pot experiment was carried out to assess the impact of BADH transgenic maize BZ-136 on arbuscular mycorrhizal fungi (AMF) colonization in root and community structure in rhizosphere soil compared with that of parental maize Zheng58 in neutral and saline-alkaline soil. Microscope observation found that BZ-136 only had a significantly reduced effect on AMF colonization at the elongation stage (9-14%). High-throughput sequencing analysis revealed that the AMF taxonomic composition kept consistency at the genus level between transgenic BZ-136 and non-transgenic parental Zheng58. NMDS analysis verified the slight difference in community structure between BZ-136 and Zheng58 presented an agrotype-dependent pattern. AMF community indices showed that BZ-136 had a higher richness at the flowering stage in saline-alkaline soil and had a higher diversity at the mature stage in neutral soil. Heatmap analysis also illuminated AMF community structure of transgenic maize at species level was similar to that of non-transgenic maize. In summary, cropping transgenic BADH maize has minor or transient effects on AMF colonization and rhizospheric soil AMF community structure, while agrotype has a stronger effect on AMF community structure.

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.

Biogeographic Patterns of Arbuscular Mycorrhizal Fungal Communities Along a Land-Use Intensification Gradient in the Subtropical Atlantic Forest Biome

Information concerning arbuscular mycorrhizal (AM) fungal geographical distribution in tropical and subtropical soils from the Atlantic Forest (a global hotspot of biodiversity) are scarce and often restricted to the evaluation of richness and abundance of AM fungal species at specific ecosystems or local landscapes. In this study, we hypothesized that AM fungal diversity and community composition in subtropical soils would display fundamental differences in their geographical patterns, shaped by spatial distance and land-use change, at local and regional scales. AM fungal community composition was examined by spore-based taxonomic analysis, using soil trap cultures. Acaulospora koskei and Glomus were found as generalists, regardless of mesoregions and land uses. Other Acaulospora species were also found generalists within mesoregions. Land-use change and intensification did not influence AM fungal composition, partially rejecting our first hypothesis. We then calculated the distance-decay of similarities among pairs of AM fungal communities and the distance-decay relationship within and over mesoregions. We also performed the Mantel test and redundancy analysis to discriminate the main environmental drivers of AM fungal diversity and composition turnover. Overall, we found significant distance-decays for all land uses. We also observed a distance-decay relationship within the mesoregion scale (< 104 km) and these changes were correlated mainly to soil type (not land use), with the secondary influence of both total organic carbon and clay contents. AM fungal species distribution presented significant distance-decays, regardless of land uses, which was indicative of dispersal limitation, a stochastic neutral process. Although, we found evidence that, coupled with dispersal limitation, niche differentiation also played a role in structuring AM fungal communities, driven by long-term historical contingencies, as represented by soil type, resulting from different soil origin and mineralogy across mesoregions.

Long-term effects of mixed planting on arbuscular mycorrhizal fungal communities in the roots and soils of Juglans mandshurica plantations

Background

Establishing mixed plantations is an effective way to improve soil fertility and increase forest productivity. Arbuscular mycorrhizal (AM) fungi are obligate symbiotic fungi that can promote mineral nutrient absorption and regulate intraspecific and interspecific competition in plants. However, the effects of mixed plantations on the community structure and abundance of AM fungi are still unclear. Illumina MiSeq sequencing was used to investigate the AM fungal community in the roots and soils of pure and mixed plantations (Juglans mandshurica × Larix gmelinii). The objective of this study is to compare the differential responses of the root and rhizosphere soil AM fungal communities of Juglans mandshurica to long-term mixed plantation management.

Results

Glomus and Paraglomus were the dominant genera in the root samples, accounting for more than 80% of the sequences. Compared with that in the pure plantation, the relative abundance of Glomus was higher in the mixed plantation. Glomus, Diversispora and Paraglomus accounted for more than 85% of the sequences in the soil samples. The relative abundances of Diversispora and an unidentified genus of Glomeromycetes were higher and lower in the pure plantation, respectively. The Root_P samples (the roots in the pure plantation) had the highest number of unique OTUs (operational taxonomic units), which belonged mainly to an unidentified genus of Glomeromycetes, Paraglomus, Glomus and Acaulospora. The number of unique OTUs detected in the soil was lower than that in the roots. In both the root and soil samples, the forest type did not have a significant effect on AM fungal diversity, but the Sobs value and the Shannon, Chao1 and Ace indices of AM fungi in the roots were significantly higher than those in the soil.

Conclusions

Mixed forest management had little effect on the AM fungal community of Juglans mandshurica roots and significantly changed the community composition of the soil AM fungi, but not the diversity.

Alta presencia de cadmio resulta en baja diversidad de hongos formadores de micorrizas arbusculares asociados a cacao (Theobroma cacao L.)

Los hongos formadores de micorrizas arbusculares (HFMA) son simbiontes obligados presentes en la rizósfera de plantas de cacao y la diversidad de sus comunidades se modifica, dependiendo de diversos factores como la presencia de cadmio (Cd) en el suelo. La persistencia de HFMA en suelos enriquecidos naturalmente con Cd podría ser un indicador de su capacidad para tolerar esta condición. Esta investigación caracterizó la estructura de la comunidad de HFMA locales presentes en la rizósfera de plantas de cacao en dos suelos con baja (B-Cd: 0,1 mg kg-1) y alta (A-Cd: 20,9 mg kg-1) concentración de Cd. Esporas de HFMA se identificaron mediante claves taxonómicas y su abundancia, riqueza y diversidad se determinó en muestras de suelo originales y después de su multiplicación mediante cultivos trampa. Las comunidades de HFMA se compararon usando análisis de componentes principales (ACP) e índices de diversidad alfa y beta. Los resultados indican que A-Cd presentó valores significativamente menores de abundancia (21 %), riqueza (20 %) y diversidad (11 %) de morfoespecies de HFMA con respecto a B-Cd. Las dos comunidades de HFMA presentaron cinco de siete géneros en común, pero solo cuatro de las 23 morfoespecies descritas se encontraron en ambas comunidades.El análisis de diversidad beta y el ACP determinaron baja similaridad y tasa de recambio entre las comunidades de HFMA. La dominancia de Diversispora spurca, Rhizoglomus sp. yClaroideoglomus etunicatumen A-Cd sugiere que estas morfoespecies son estrés-tolerantes y candidatos potenciales para el desarrollo de estrategias de mitigación en suelos con Cd.

Arbuscular Mycorrhizal Fungal Communities of Native Plant Species under High Petroleum Hydrocarbon Contamination Highlights Rhizophagus as a Key Tolerant Genus

Arbuscular mycorrhizal fungi (AMF) have been shown to play an important role in increasing plant fitness in harsh conditions. Therefore, AMF are currently considered to be effective partners in phytoremediation. However, AMF communities in high levels of petroleum pollution are still poorly studied. We investigated the community structures of AMF in roots and rhizospheric soils of two plant species, Eleocharis elliptica and Populus tremuloides, growing spontaneously in high petroleum-contaminated sedimentation basins of a former petrochemical plant (91,000 μg/Kg of C10–C50 was recorded in a basin which is 26-fold higher than the threshold of polluted soil in Quebec, Canada). We used a PCR cloning, and sequencing approach, targeting the 18S rRNA gene to identify AMF taxa. The high concentration of petroleum-contamination largely influenced the AMF diversity, which resulted in less than five AMF operational taxonomical units (OTUs) per individual plant at all sites. The OTUs detected belong mainly to the Glomerales, with some from the Diversisporales and Paraglomerales, which were previously reported in high concentrations of metal contamination. Interestingly, we found a strong phylogenetic signal in OTU associations with host plant species identity, biotopes (roots or soils), and contamination concentrations (lowest, intermediate and highest). The genus Rhizophagus was the most dominant taxon representing 74.4% of all sequences analyzed in this study and showed clear association with the highest contamination level. The clear association of Rhizophagus with high contamination levels suggests the importance of the genus for the use of AMF in bioremediation, as well as for the survey of key AMF genes related to petroleum hydrocarbon resistance. By favoring plant fitness and mediating its soil microbial interactions, Rhizophagus spp. could enhance petroleum hydrocarbon pollutant degradation by both plants and their microbiota in contaminated sites.

Heavy Metal Pollutions: State of the Art and Innovation in Phytoremediation

Mineral nutrition of plants greatly depends on both environmental conditions, particularly of soils, and the genetic background of the plant itself. Being sessile, plants adopted a range of strategies for sensing and responding to nutrient availability to optimize development and growth, as well as to protect their metabolisms from heavy metal toxicity. Such mechanisms, together with the soil environment, meaning the soil microorganisms and their interaction with plant roots, have been extensively studied with the goal of exploiting them to reclaim polluted lands; this approach, defined phytoremediation, will be the subject of this review. The main aspects and innovations in this field are considered, in particular with respect to the selection of efficient plant genotypes, the application of improved cultural strategies, and the symbiotic interaction with soil microorganisms, to manage heavy metal polluted soils.

Vermicompost addition influences symbiotic fungi communities associated with leek cultivated in metal-rich soils

In the context of urban agriculture, where soils are frequently contaminated with metal(loid)s (TM), we studied the influence of vermicompost amendments on symbiotic fungal communities associated with plants grown in two metal-rich soils. Leek (Allium porrum L.) plants were grown with or without vermicompost in two metal-rich soils characterized by either geogenic or anthropogenic TM sources, to assess the influence of pollutant origin on soil-plant transfer. Fungal communities associated with the leek roots were identified by high throughput Illumina MiSeq and TM contents were measured using mass spectrometry. Vermicompost addition led to a dramatic change in the fungal community with a loss of diversity in the two tested soils. This effect could partially explain the changes in metal transfer at the soil-AMF-plant interface. Our results suggest being careful while using composts when growing edibles in contaminated soils. More generally, this study highlights the need for further research in the field of fungal communities to refine practical recommendations to gardeners. Graphical abstract.

Soil lead pollution modifies the structure of arbuscular mycorrhizal fungal communities

The impact of lead (Pb) pollution on native communities of arbuscular mycorrhizal fungi (AMF) was assessed in soil samples from the surroundings of an abandoned Pb smelting factory. To consider the influence of host identity, bulk soil surrounding plant roots soil samples of predominant plant species (Sorghum halepense, Bidens pilosa, and Tagetes minuta) growing in Pb-polluted soils and in an uncontaminated site were selected. Molecular diversity was assessed by sequencing the 18S rDNA region with primers specific to AMF (AMV4.5NF/AMDGR) using Illumina MiSeq. A total of 115 virtual taxa (VT) of AMF were identified in this survey. Plant species did not affect AMF diversity patterns. However, soil Pb content was negatively correlated with VT richness per sample. Paraglomeraceae and Glomeraceae were the predominant families while Acaulosporaceae, Ambisporaceae, Archaeosporaceae, Claroideoglomeraceae, Diversisporaceae, and Gigasporaceae were less abundant. Acaulosporaceae and Glomeraceae were negatively affected by soil Pb, but Paraglomeraceae relative abundance increased under increasing soil Pb content. Overall, 26 indicator taxa were identified; four of them were previously reported in Pb-polluted soils (VT060; VT222; VT004; VT380); and five corresponded to cultured spores of Scutellospora castaneae (VT041), Diversispora spp. and Tricispora nevadensis (VT060), Diversispora epigaea (VT061), Glomus proliferum (VT099), and Gl. indicum (VT222). Even though AMF were present in Pb-polluted soils, community structure was strongly altered via the differential responses of taxonomic groups of AMF to Pb pollution. These taxon-specific differences in tolerance to soil Pb content should be considered for future phytoremediation strategies based on the selection and utilization of native Glomeromycota.

Pb accumulation in spores of arbuscular mycorrhizal fungi

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

Ecotoxicity evaluation and human risk assessment of an agricultural polluted soil

The present study aims to evaluate the nature and level of chemical pollution as well as the potential toxicity and ecotoxicity of an agricultural soil irrigated by the water of Litani River. Our findings showed that the soil was mainly contaminated by alkanes (hentriacontane, octadecane, hexadecane) and metal trace elements (nickel, vanadium, chromium, and manganese). Soil organic extracts showed high cytotoxicity against human hepatic (HepG2) and bronchial epithelial cells (Beas-2B). Soil ecotoxicity was revealed by seed germination inhibition of several plant species (wheat, clover, alfalfa, tall fescue, and ryegrass) ranging from 7 to 30% on the polluted soil compared to non-polluted one. In addition, significant decreases in telluric microbial biomasses (bacterial and fungal biomasses), quantified by phospholipid fatty acids (PLFA) analysis were observed in polluted soil compared to non-contaminated soils. The density of the arbuscular mycorrhizal fungal (AMF) spores isolated from the polluted soil was about 316 spores/100 g. Three main AMF species were identified as Funelliformis mosseae, Septoglomus constrictum, and Claroideoglomus lamellosum. Moreover, 16 indigenous plant species were inventoried with Silybum marianum L. as the dominant one. Plant biodiversity indices (Shannon, Simpson, Menhinick, and Margaleff) were lower than those found in other contaminated soils. Finally, it was found that all the present plant species on this polluted site were mycorrhized, suggesting a possible protection of these plants against encountered pollutants, and the possibility to use AMF-assisted phytoremediation to clean-up such a site.

Co-inoculation of Lolium perenne with Funneliformis mosseae and the dark septate endophyte Cadophora sp. in a trace element-polluted soil

The presence of dark septate endophytes (DSEs) or arbuscular mycorrhizal fungi (AMF) in plant roots and their effects on plant fitness have been extensively described. However, little is known about their interactions when they are simultaneously colonizing a plant root, especially in trace element (TE)-polluted soils. We therefore investigated the effects of Cadophora sp. and Funneliformis mosseae on ryegrass (Lolium perenne) growth and element uptake in a Cd/Zn/Pb-polluted soil. The experiment included four treatments, i.e., inoculation with Cadophora sp., inoculation with F. mosseae, co-inoculation with Cadophora sp. and F. mosseae, and no inoculation. Ryegrass biomass and shoot Na, P, K, and Mg concentrations significantly increased following AMF inoculation as compared to non-inoculated controls. Similarly, DSE inoculation increased shoot Na concentration, whereas dual inoculation significantly decreased shoot Cd concentration. Moreover, oxidative stress determined by ryegrass leaf malondialdehyde concentration was alleviated both in the AMF and dual inoculation treatments. We used quantitative PCR and microscope observations to quantify colonization rates. They demonstrated that DSEs had no effect on AMF colonization, while AMF colonization slightly decreased DSE frequency. We also monitored fluorescein diacetate (FDA) hydrolysis and alkaline phosphatase (AP) activity in the rhizosphere soils. FDA hydrolysis remained unchanged in the three inoculated treatments, but AMF colonization increased AP activity and P mobility in the soil whereas DSE colonization did not alter AP activity. In this experiment, we unveiled the interactions between two ecologically important fungal groups likely to occur in roots which involved a decrease of oxidative stress and Cd accumulation in shoots. These results open promising perspectives on the fungal-based phytomanagement of TE-contaminated sites by the production of uncontaminated and marketable plant biomass.

Phytostabilization potential of ornamental plants grown in soil contaminated with cadmium

In a greenhouse experiment, five ornamental plants, Osmanthus fragrans (OF), Ligustrum vicaryi L. (LV), Cinnamomum camphora (CC), Loropetalum chinense var. rubrum (LC), and Euonymus japonicas cv. Aureo-mar (EJ), were studied for the ability to phytostabilization for Cd-contaminated soil. The results showed that these five ornamental plants can grow normally when the soil Cd content is less than 24.6 mg·kg^-1. Cd was mainly deposited in the roots of OF, LV, LC and EJ which have grown in Cd-contaminated soils, and the maximum Cd contents reached 15.76, 19.09, 20.59 and 32.91 mg·kg^-1, respectively. For CC, Cd was mainly distributed in the shoots and the maximum Cd content in stems and leaves were 12.5 and 10.71 mg·kg^-1, however, the total amount of Cd in stems and leaves was similar with the other ornamental plants. The enzymatic activities in Cd-contaminated soil were benefited from the five tested ornamental plants remediation. Soil urease and sucrase activities were improved, while dehydrogenase activity was depressed. Meanwhile, the soil microbial community was slightly influenced when soil Cd content is less than 24.6 mg·kg^-1 under five ornamental plants remediation. The results further suggested that ornamental plants could be promising candidates for phytostabilization of Cd-contaminated soil.

Glomeromycota communities survive extreme levels of metal toxicity in an orphan mining site

Abandoned tailing basins and waste heaps of orphan mining sites are of great concern since extreme metal contamination makes soil improper for any human activity and is a permanent threat for nearby surroundings. Although spontaneous revegetation can occur, the process is slow or unsuccessful and rhizostabilisation strategies to reduce dispersal of contaminated dust represent an option to rehabilitate such sites. This requires selection of plants tolerant to such conditions, and optimization of their fitness and growth. Arbuscular mycorrhizal fungi (AMF) can enhance metal tolerance in moderately polluted soils, but their ability to survive extreme levels of metal contamination has not been reported. This question was addressed in the tailing basin and nearby waste heaps of an orphan mining site in southern France, reaching in the tailing basin exceptionally high contents of zinc (ppm: 97,333 total) and lead (ppm: 31,333 total). In order to contribute to a better understanding of AMF ecology under severe abiotic stress and to identify AMF associated with plants growing under such conditions, that may be considered in future revegetation and rhizostabilisation of highly polluted areas, nine plant species were sampled at different growing seasons and AMF root colonization was determined. Glomeromycota diversity was monitored in mycorrhizal roots by sequencing of the ribosomal LSU. This first survey of AMF in such highly contaminated soils revealed the presence of several AMF ribotypes, belonging mainly to the Glomerales, with some examples from the Paraglomerales and Diversisporales. AMF diversity and root colonization in the tailing basin were lower than in the less-contaminated waste heaps. A Paraglomus species previously identified in a polish mining site was common in roots of different plants. Presence of active AMF in such an environment is an outstanding finding, which should be clearly considered for the design of efficient rhizostabilisation processes.

Barcoded NS31/AML2 primers for sequencing of arbuscular mycorrhizal communities in environmental samples

PREMISE OF THE STUDY

Arbuscular mycorrhizal fungi (AMF) are globally important root symbioses that enhance plant growth and nutrition and influence ecosystem structure and function. To better characterize levels of AMF diversity relevant to ecosystem function, deeper sequencing depth in environmental samples is needed. In this study, Illumina barcoded primers and a bioinformatics pipeline were developed and applied to study AMF diversity and community structure in environmental samples.

METHODS

Libraries of small subunit ribosomal RNA fragment amplicons were amplified from environmental DNA using a single-step PCR reaction with barcoded NS31/AML2 primers. Amplicons were sequenced on an Illumina MiSeq sequencer using version 2, 2 × 250-bp paired-end chemistry, and analyzed using QIIME and RDP Classifier.

RESULTS

Sequencing captured 196 to 6416 operational taxonomic units (OTUs; depending on clustering parameters) representing nine AMF genera. Regardless of clustering parameters, ∼20 OTUs dominated AMF communities (78-87% reads) with the remaining reads distributed among other OTUs. Analyses also showed significant biogeographic differences in AMF communities and that community composition could be linked to specific edaphic factors.

DISCUSSION

Barcoded NS31/AML2 primers and Illumina MiSeq sequencing provide a powerful approach to address AMF diversity and variations in fungal assemblages across host plants, ecosystems, and responses to environmental drivers including global change.

Phytoremediation: State-of-the-art and a key role for the plant microbiome in future trends and research prospects

Phytoremediation is increasingly adopted as a more sustainable approach for soil remediation. However, significant advances in efficiency are still necessary to attain higher levels of environmental and economic sustainability. Current interventions do not always give the expected outcomes in field settings due to an incomplete understanding of the multicomponent biological interactions. New advances in -omics are gradually implemented for studying microbial communities of polluted land in situ. This opens new perspectives for the discovery of biodegradative strains and provides us new ways of interfering with microbial communities to enhance bioremediation rates. This review presents retrospectives and future perspectives for plant microbiome studies relevant to phytoremediation, as well as some knowledge gaps in this promising research field. The implementation of phytoremediation in soil clean-up management systems is discussed, and an overview of the promoting factors that determine the growth of the phytoremediation market is given. Continuous growth is expected since elimination of contaminants from the environment is demanded. The evolution of scientific thought from a reductionist view to a more holistic approach will boost phytoremediation as an efficient and reliable phytotechnology. It is anticipated that phytoremediation will prove the most promising for organic contaminant degradation and bioenergy crop production on marginal land.

Atmospheric Deposition-Carried Zn and Cd from a Zinc Smelter and Their Effects on Soil Microflora as Revealed by 16S rDNA

In this study, we investigated the influence of heavy metals (HM) on total soil bacterial population and its diversity pattern from 10 km distance of a Zinc smelter in Feng County, Qinling Mountain, China. We characterized and identified the bacterial community in a HM polluted soil using 16S rDNA technology. Out results indicated that the maximum soil HM concentration and the minimum bacterial population were observed in S2 soil, whereas bacterial diversity raised with the sampling distance increased. The bacterial communities were dominated by the phyla Proteobacteria, Acidobacteria and Actinobacteria in cornfield soils, except Fimicutes phylum which dominated in hilly area soil. The soil CEC, humic acid (HA)/fulvic acid (FA) and microbial OTUs increased with the sampling distance increased. Shewanella, Halomonas and Escherichia genera were highly tolerant to HM stress in both cultivated and non-cultivated soil. Finally, we found a consistent correlation of bacterial diversity with total HM and SOM along the sampling distance surrounding the zinc smelter, which could provide a new insight into the bacterial community-assisted and phytoremediation of HM contaminated soils.

The large (134.9 kb) mitochondrial genome of the glomeromycete Funneliformis mosseae

Funneliformis mosseae is among the most ecologically and economically important glomeromycete species and occurs both in natural and disturbed areas in a wide range of habitats and climates. In this study, we report the sequencing of the complete mitochondrial (mt) genome of F. mosseae isolate FL299 using 454 pyrosequencing and Illumina HiSeq technologies. This mt genome is a full-length circular chromosome of 134,925 bp, placing it among the largest mitochondrial DNAs (mtDNAs) in the fungal kingdom. A comparative analysis with publically available arbuscular mycorrhizal fungal mtDNAs revealed that the mtDNA of F. mosseae FL299 contained a very large number of insertions contributing to its expansion. The gene synteny was completely reshuffled compared to previously published glomeromycotan mtDNAs and several genes were oriented in an anti-sense direction. Furthermore, the presence of different types of introns and insertions in rnl (14 introns) made this gene very distinctive in Glomeromycota. The presence of alternative genetic codes in both initiation (GUG) and termination (UGA) codons was another new feature in this mtDNA compared to previously published glomeromycotan mt genomes. The phylogenetic analysis inferred from the analysis of 14 protein mt genes confirmed the position of the Glomeromycota clade as a sister group of Mortierellomycotina. This mt genome is the largest observed so far in Glomeromycota and the first mt genome within the Funneliformis clade, providing new opportunities to better understand their evolution and to develop molecular markers.

Organic amendments increase phylogenetic diversity of arbuscular mycorrhizal fungi in acid soil contaminated by trace elements

In 1998, a toxic mine spill polluted a 55-km(2) area in a basin southward to Doñana National Park (Spain). Subsequent attempts to restore those trace element-contaminated soils have involved physical, chemical, or biological methodologies. In this study, the restoration approach included application of different types and doses of organic amendments: biosolid compost (BC) and leonardite (LEO). Twelve years after the last addition, molecular analyses of arbuscular mycorrhizal (AM) fungal communities associated with target plants (Lamarckia aurea and Chrysanthemum coronarium) as well as analyses of trace element concentrations both in soil and in plants were performed. The results showed an improved soil quality reflected by an increase in soil pH and a decrease in trace element availability as a result of the amendments and dosages. Additionally, the phylogenetic diversity of the AM fungal community increased, reaching the maximum diversity at the highest dose of BC. Trace element concentration was considered the predominant soil factor determining the AM fungal community composition. Thereby, the studied AM fungal community reflects a community adapted to different levels of contamination as a result of the amendments. The study highlights the long-term effect of the amendments in stabilizing the soil system.

Arbuscular mycorrhizal wheat inoculation promotes alkane and polycyclic aromatic hydrocarbon biodegradation: Microcosm experiment on aged-contaminated soil

Very few studies reported the potential of arbuscular mycorrhizal symbiosis to dissipate hydrocarbons in aged polluted soils. The present work aims to study the efficiency of arbuscular mycorrhizal colonized wheat plants in the dissipation of alkanes and polycyclic aromatic hydrocarbons (PAHs). Our results demonstrated that the inoculation of wheat with Rhizophagus irregularis allowed a better dissipation of PAHs and alkanes after 16 weeks of culture by comparison to non-inoculated condition. These dissipations observed in the inoculated soil resulted from several processes: (i) a light adsorption on roots (0.5% for PAHs), (ii) a bioaccumulation in roots (5.7% for PAHs and 6.6% for alkanes), (iii) a transfer in shoots (0.4 for PAHs and 0.5% for alkanes) and mainly a biodegradation. Whereas PAHs and alkanes degradation rates were respectively estimated to 12 and 47% with non-inoculated wheat, their degradation rates reached 18 and 48% with inoculated wheat. The mycorrhizal inoculation induced an increase of Gram-positive and Gram-negative bacteria by 56 and 37% compared to the non-inoculated wheat. Moreover, an increase of peroxidase activity was assessed in mycorrhizal roots. Taken together, our findings suggested that mycorrhization led to a better hydrocarbon biodegradation in the aged-contaminated soil thanks to a stimulation of telluric bacteria and hydrocarbon metabolization in mycorrhizal roots.

Soil Characteristics Driving Arbuscular Mycorrhizal Fungal Communities in Semiarid Mediterranean Soils

We investigated communities of arbuscular mycorrhizal fungi (AMF) in the roots and the rhizosphere soil of Brachypodium retusum in six different natural soils under field conditions. We explored phylogenetic patterns of AMF composition using indicator species analyses to find AMF associated with a given habitat (root versus rhizosphere) or soil type. We tested whether the AMF characteristics of different habitats or contrasting soils were more closely related than expected by chance. Then we used principal-component analysis and multivariate analysis of variance to test for the relative contribution of each factor in explaining the variation in fungal community composition. Finally, we used redundancy analysis to identify the soil properties that significantly explained the differences in AMF communities across soil types. The results pointed out a tendency of AMF communities in roots to be closely related and different from those in the rhizosphere soil. The indicator species analyses revealed AMF associated with rhizosphere soil and the root habitat. Soil type also determined the distribution of AMF communities in soils, and this effect could not be attributed to a single soil characteristic, as at least three soil properties related to microbial activity, i.e., pH and levels of two micronutrients (Mn and Zn), played significant roles in triggering AMF populations.

IMPORTANCE

Communities of arbuscular mycorrhizal fungi (AMF) are main components of soil biota that can determine the productivity of ecosystems. These fungal assemblages vary across host plants and ecosystems, but the main ecological processes that shape the structures of these communities are still largely unknown. A field study in six different soil types from semiarid areas revealed that AMF communities are significantly influenced by habitat (soil versus roots) and soil type. In addition, three soil properties related to microbiological activity (i.e., pH and manganese and zinc levels) were the main factors triggering the distribution of AMF. These results contribute to a better understanding of the ecological factors that can shape AMF communities, an important soil microbial group that affects multiple ecosystem functions.

Concentration of Petroleum-Hydrocarbon Contamination Shapes Fungal Endophytic Community Structure in Plant Roots

Plant-root inhabiting fungi are a universal phenomenon found in all ecosystems where plants are able to grow, even in harsh environments. Interactions between fungi and plant roots can vary widely from mutualism to parasitism depending on many parameters. The role of fungal endophytes in phytoremediation of polluted sites, and characterization of the endophytic diversity and community assemblages in contaminated areas remain largely unexplored. In this study, we investigated the composition of endophytic fungal communities in the roots of two plant species growing spontaneously in petroleum-contaminated sedimentation basins of a former petro-chemical plant. The three adjacent basins showed a highly heterogeneous pattern of pollutant concentrations. We combined a culture-based isolation approach with the pyrosequencing of fungal ITS ribosomal DNA. We selected two species, Eleocharis erythropoda Steud. and Populus balsamifera L., and sampled three individuals of each species from each of three adjacent basins, each with a different concentration of petroleum hydrocarbons. We found that contamination level significantly shaped endophytic fungal diversity and community composition in E. erythropoda, with only 9.9% of these fungal Operational Taxonomic Units (OTUs) retrieved in all three basins. However, fungal community structure associated with P. balsamifera remained unaffected by the contamination level with 28.2% of fungal OTUs shared among all three basins. This could be explained by the smaller differences of pollutant concentrations in the soil around our set of P. balsamifera sampless compared to that around our set of E. erythropoda samples. Our culture-based approach allowed isolation of 11 and 30 fungal endophytic species from surface-sterilized roots of E. erythropoda and P. balsamifera, respectively. These isolates were ribotyped using ITS, and all were found in pyrosequensing datasets. Our results demonstrate that extreme levels of pollution reduce fungal diversity and shape community composition in E. erythropoda. Our findings shed light on the effect of soil petroleum contamination on fungal endophytic communities and could help to develop strategies for improving phytoremediation using fungal endophytes.

Arbuscular mycorrhizal fungal responses to abiotic stresses: A review

The majority of plants live in close collaboration with a diversity of soil organisms among which arbuscular mycorrhizal fungi (AMF) play an essential role. Mycorrhizal symbioses contribute to plant growth and plant protection against various environmental stresses. Whereas the resistance mechanisms induced in mycorrhizal plants after exposure to abiotic stresses, such as drought, salinity and pollution, are well documented, the knowledge about the stress tolerance mechanisms implemented by the AMF themselves is limited. This review provides an overview of the impacts of various abiotic stresses (pollution, salinity, drought, extreme temperatures, CO2, calcareous, acidity) on biodiversity, abundance and development of AMF and examines the morphological, biochemical and molecular mechanisms implemented by AMF to survive in the presence of these stresses.

Impact of soil salinity on arbuscular mycorrhizal fungi biodiversity and microflora biomass associated with Tamarix articulata Vahll rhizosphere in arid and semi-arid Algerian areas

Soil salinization is an increasingly important problem in many parts of the world, particularly under arid and semi-arid areas. Unfortunately, the knowledge about restoration of salt affected ecosystems using mycorrhizae is limited. The current study aims to investigate the impact of salinity on the microbial richness of the halophytic plant Tamarix articulata rhizosphere. Soil samples were collected from natural sites with increasing salinity (1.82-4.95 ds.m(-1)). Six arbuscular mycorrhizal fungi (AMF) species were isolated from the different saline soils and identified as Septoglomus constrictum, Funneliformis mosseae, Funneliformis geosporum, Funneliformis coronatum, Rhizophagus fasciculatus, and Gigaspora gigantea. The number of AMF spores increased with soil salinity. Total root colonization rate decreased from 65 to 16% but remained possible with soil salinity. Microbial biomass in T. articulata rhizosphere was affected by salinity. The phospholipid fatty acids (PLFA) C16:1ω5 as well as i15:0, a15:0, i16:0, i17:0, a17:0, cy17:0, C18:1ω7 and cy19:0 increased in high saline soils suggesting that AMF and bacterial biomasses increased with salinity. In contrast, ergosterol amount was negatively correlated with soil salinity indicating that ectomycorrhizal and saprotrophic fungal biomasses were reduced with salinity. Our findings highlight the adaptation of arbuscular and bacterial communities to natural soil salinity and thus the potential use of mycorrhizal T. articulata trees as an approach to restore moderately saline disturbed arid lands.

Arbuscular mycorrhizal fungi community structure, abundance and species richness changes in soil by different levels of heavy metal and metalloid concentration

Arbuscular Mycorrhizal Fungi (AMF) play major roles in ecosystem functioning such as carbon sequestration, nutrient cycling, and plant growth promotion. It is important to know how this ecologically important soil microbial player is affected by soil abiotic factors particularly heavy metal and metalloid (HMM). The objective of this study was to understand the impact of soil HMM concentration on AMF abundance and community structure in the contaminated sites of South Korea. Soil samples were collected from the vicinity of an abandoned smelter and the samples were subjected to three complementary methods such as spore morphology, terminal restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis (DGGE) for diversity analysis. Spore density was found to be significantly higher in highly contaminated soil compared to less contaminated soil. Spore morphological study revealed that Glomeraceae family was more abundant followed by Acaulosporaceae and Gigasporaceae in the vicinity of the smelter. T-RFLP and DGGE analysis confirmed the dominance of Funneliformis mosseae and Rhizophagus intraradices in all the study sites. Claroideoglomus claroideum, Funneliformis caledonium, Rhizophagus clarus and Funneliformis constrictum were found to be sensitive to high concentration of soil HMM. Richness and diversity of Glomeraceae family increased with significant increase in soil arsenic, cadmium and zinc concentrations. Our results revealed that the soil HMM has a vital impact on AMF community structure, especially with Glomeraceae family abundance, richness and diversity.

Arbuscular mycorrhizal fungal diversity associated with Eleocharis obtusa and Panicum capillare growing in an extreme petroleum hydrocarbon-polluted sedimentation basin

Arbuscular mycorrhizal fungi (AMF) have been extensively studied in natural and agricultural ecosystems, but little is known about their diversity and community structure in highly petroleum-polluted soils. In this study, we described an unexpected diversity of AMF in a sedimentation basin of a former petrochemical plant, in which petroleum hydrocarbon (PH) wastes were dumped for many decades. We used high-throughput PCR, cloning and sequencing of 18S rDNA to assess the molecular diversity of AMF associated with Eleocharis obtusa and Panicum capillare spontaneously inhabiting extremely PH-contaminated sediments. The analyses of rhizosphere and root samples over two years showed a remarkable AMF richness comparable with that found in temperate natural ecosystems. Twenty-one taxa, encompassing the major families within Glomeromycota, were detected. The most abundant OTUs belong to genera Claroideoglomus, Diversispora, Rhizophagus and Paraglomus. Both plants had very similar overall community structures and OTU abundances; however, AMF community structure differed when comparing the overall OTU distribution across the two years of sampling. This could be likely explained by variations in precipitations between 2011 and 2012. Our study provides the first view of AMF molecular diversity in soils extremely polluted by PH, and demonstrated the ability of AMF to colonize and establish in harsh environments.

Bacteria associated with arbuscular mycorrhizal fungi within roots of plants growing in a soil highly contaminated with aliphatic and aromatic petroleum hydrocarbons

Arbuscular mycorrhizal fungi (AMF) belong to phylum Glomeromycota, an early divergent fungal lineage forming symbiosis with plant roots. Many reports have documented that bacteria are intimately associated with AMF mycelia in the soil. However, the role of these bacteria remains unclear and their diversity within intraradical AMF structures has yet to be explored. We aim to assess the bacterial communities associated within intraradical propagules (vesicles and intraradical spores) harvested from roots of plant growing in the sediments of an extremely petroleum hydrocarbon-polluted basin. Solidago rugosa roots were sampled, surface-sterilized, and microdissected. Eleven propagules were randomly collected and individually subjected to whole-genome amplification, followed by PCRs, cloning, and sequencing targeting fungal and bacterial rDNA. Ribotyping of the 11 propagules showed that at least five different AMF OTUs could be present in S. rugosa roots, while 16S rRNA ribotyping of six of the 11 different propagules showed a surprisingly high bacterial richness associated with the AMF within plant roots. Most dominant bacterial OTUs belonged to Sphingomonas sp., Pseudomonas sp., Massilia sp., and Methylobacterium sp. This study provides the first evidence of the bacterial diversity associated with AMF propagules within the roots of plants growing in extremely petroleum hydrocarbon-polluted conditions.

Contrasting the community structure of arbuscular mycorrhizal fungi from hydrocarbon-contaminated and uncontaminated soils following willow (Salix spp. L.) planting

Phytoremediation is a potentially inexpensive alternative to chemical treatment of hydrocarbon-contaminated soils, but its success depends heavily on identifying factors that govern the success of root-associated microorganisms involved in hydrocarbon degradation and plant growth stimulation. Arbuscular mycorrhizal fungi (AMF) form symbioses with many terrestrial plants, and are known to stimulate plant growth, although both species identity and the environment influence this relationship. Although AMF are suspected to play a role in plant adaptation to hydrocarbon contamination, their distribution in hydrocarbon-contaminated soils is not well known. In this study, we examined how AMF communities were structured within the rhizosphere of 11 introduced willow cultivars as well as unplanted controls across uncontaminated and hydrocarbon-contaminated soils at the site of a former petrochemical plant. We obtained 69 282 AMF-specific 18S rDNA sequences using 454-pyrosequencing, representing 27 OTUs. Contaminant concentration was the major influence on AMF community structure, with different AMF families dominating at each contaminant level. The most abundant operational taxonomic unit in each sample represented a large proportion of the total community, and this proportion was positively associated with increasing contamination, and seemingly, by planting as well. The most contaminated soils were dominated by three phylotypes closely related to Rhizophagus irregularis, while these OTUs represented only a small proportion of sequences in uncontaminated and moderately contaminated soils. These results suggest that in situ inoculation of AMF strains could be an important component of phytoremediation treatments, but that strains should be selected from the narrow group that is both adapted to contaminant toxicity and able to compete with indigenous AMF species.

Differential effects of abiotic factors and host plant traits on diversity and community composition of root-colonizing arbuscular mycorrhizal fungi in a salt-stressed ecosystem

Arbuscular mycorrhizal fungi (AMF) were investigated in roots of 18 host plant species in a salinized south coastal plain of Laizhou Bay, China. From 18 clone libraries of 18S rRNA genes, all of the 22 AMF phylotypes were identified into Glomus, of which 18 and 4 were classified in group A and B in the phylogenetic tree, respectively. The phylotypes related to morphologically defined Glomus species occurred generally in soil with higher salinity. AMF phylotype richness, Shannon index, and evenness were not significantly different between root samples from halophytes vs. non-halophytes, invades vs. natives, or annuals vs. perennials. However, AMF diversity estimates frequently differed along the saline gradient or among locations, but not among pH gradients. Moreover, UniFrac tests showed that both plant traits (salt tolerance, life style or origin) and abiotic factors (salinity, pH, or location) significantly affected the community composition of AMF colonizers. Redundancy and variation partitioning analyses revealed that soil salinity and pH, which respectively explained 6.9 and 4.2 % of the variation, were the most influential abiotic variables in shaping the AMF community structure. The presented data indicate that salt tolerance, life style, and origin traits of host species may not significantly affect the AMF diversity in roots, but do influence the community composition in this salinized ecosystem. The findings also highlight the importance of soil salinity and pH in driving the distribution of AMF in plant and soil systems.

The role of metal nanoparticles in influencing arbuscular mycorrhizal fungi effects on plant growth

A knowledge gap still remains concerning the in situ influences of nanoparticles on plant systems, partly due to the absence of soil microorganisms. Arbuscular mycorrhizal fungi (AMF) can form a mutualistic symbiosis with the roots of over 90% of land plants. This investigation sought to reveal the responses of mycorrhizal clover (Trifolium repens) to silver nanoparticles (AgNPs) and iron oxide nanoparticles (FeONPs) along a concentration gradient of each. FeONPs at 3.2 mg/kg significantly reduced mycorrhizal clover biomass by 34% by significantly reducing the glomalin content and root nutrient acquisition of AMF. In contrast, no negative effects of AgNPs at concentrations over 0.1 mg/kg were observed; however, AgNPs at 0.01 mg/kg inhibited mycorrhizal clover growth. In response to the elevated AgNPs content, the ability of AMF to alleviate AgNPs stress (via increased growth and ecological behaviors) was enhanced, which decreased Ag content and the activities of antioxidant enzymes in plants. These results were further supported by X-ray microcomputed tomography. Our findings suggest that in soil ecosystem, the influence of nanometals on plant systems would be more complicated than expected, and more attention should be focused on plant responses in combination with those of soil microorganisms.