Improved tolerance of maize (Zea mays L.) to heavy metals by colonization of a dark septate endophyte (DSE) Exophiala pisciphila

Prevalent arbuscular mycorrhizae in roots and highly variable mycobiome in leaves of epiphytic subtropical fern Ophioderma pendulum

PREMISE

Endophytic and mycorrhizal fungi are crucial in facilitating plant nutrition acquisition and stress tolerance. In epiphytic habitats, plants face nutrition and water stress, but their roots are mostly nonmycorrhizal and especially lacking in arbuscular mycorrhizal associations. Ophioderma pendulum is an epiphytic fern with a partially mycoheterotrophic lifestyle, likely heavily reliant on symbiotic fungi. To characterize fungal associations in the sporophyte of O. pendulum, we focused on leaves and roots of O. pendulum, seeking to reveal the fungal communities in these organs.

METHODS

Roots and leaves from O. pendulum in a subtropical forest were examined microscopically to observe the morphology of fungal structures and determine the percentage of various fungal structures in host tissues. Fungal composition was profiled using metabarcoding techniques that targeted ITS2 of the nuclear ribosomal DNA.

RESULTS

Roots were consistently colonized by arbuscular mycorrhizal fungi (Glomeromycota), especially Acaulospora. Unlike previous findings on epiphytic ferns, dark septate endophytes were rare in O. pendulum roots. Leaves were predominantly colonized by Ascomycota fungi, specifically the classes Dothideomycetes (46.88%), Eurotiomycetes (11.51%), Sordariomycetes (6.23%), and Leotiomycetes (6.14%). Across sampling sites, fungal community compositions were similar in the roots but differed significantly in the leaves.

CONCLUSIONS

Ophioderma pendulum maintains stable, single-taxon-dominant communities in the roots, primarily featuring arbuscular mycorrhizal fungi, whereas the leaves may harbor opportunistic fungal colonizers. Our study underlines the significance of mycorrhizal fungi in the adaptation of epiphytic ferns.

Why different sugarcane cultivars show different resistant abilities to smut? : Comparisons of endophytic microbial compositions and metabolic functions in stems of sugarcane cultivars with different abilities to resist smut

To elucidate the mechanisms underlying the resistance to smut of different sugarcane cultivars, endophytic bacterial and fungal compositions, functions and metabolites in the stems of the sugarcane cultivars were analyzed using high-throughput sequencing techniques and nontargeted metabolomics. The results showed that the levels of ethylene, salicylic acid and jasmonic acid in sugarcane varieties that were not sensitive to smut were all higher than those in sensitive sugarcane varieties. Moreover, endophytic fungi, such as Ramichloridium, Alternaria, Sarocladium, Epicoccum, and Exophiala species, could be considered antagonistic to sugarcane smut. Additionally, the highly active arginine and proline metabolism, pentose phosphate pathway, phenylpropanoid biosynthesis, and tyrosine metabolism in sugarcane varieties that were not sensitive to smut indicated that these pathways contribute to resistance to smut. All of the above results suggested that the relatively highly abundant antagonistic microbes and highly active metabolic functions of endophytes in non-smut-sensitive sugarcane cultivars were important for their relatively high resistance to smut.

The Analysis of Microbial Community Characteristics Revealed that the Pathogens of Leaf Spot of Rosa roxburghii Originated from the Phyllosphere

Members of the plant mycobiota are all associated to varying degrees with the development of plant diseases. Although many reports on the plant mycobiota are well documented, the relationships between mycobiota of Rosa roxburghii and plant diseases are poorly understood. Mutual interactions and extent of the roles of microbial communities associated with R. roxburghii and the source of pathogens are still unclear, and more research is needed on the health benefits of this ecologically important population. Using high-throughput sequencing, we analyzed the mycobiota composition and ecological guilds of the rhizosphere, root, and phyllosphere of healthy and diseased R. roxburghii from the Tianfu R. roxburghii Industrial Park in Panzhou city, Guizhou province. Analysis of community composition showed that the relative abundance of pathogens of leaf spot, including Alternaria, Pestalotiopsis and Neofusicoccum in the phyllosphere of diseased plant (LD), were 1.15%, 0.15% and 0.06%, and the relative abundance of Alternaria and Pestalotiopsis were 0.96% and 0.58% in healthy plant (LH). The alpha diversity indices indicated that fungal diversity was higher in healthy plants compared to diseased plants in each compartment. The alpha diversity index of fungi in the phyllosphere (LH) of healthy R. roxburghii, including Shannon, Chao-1, and Faith-pd indices, was 1.02, 81.50 and 10.42 higher than that of the diseased (LD), respectively. The fungi in the rhizosphere of healthy was 1.03, 59.00 and 5.56 higher than the diseased, respectively. The Shannon index of fungi in the root of healthy was 0.29 higher than that of diseased. Principal Coordinate analysis and ANOSIM results showed that there were significant differences in mycobiota composition between healthy and diseased phyllospheres (P < 0.05), as well as rhizosphere fungal community, while there was no significant difference between healthy and diseased roots (P > 0.05). Linear discriminant analysis effect size revealed that, at different taxonomic levels, there were significantly different taxa between the healthy and diseased plants in each compartment. The ecological guilds differed between healthy and diseased plants according to the FUNGuild analysis. For example, of healthy compared to diseased plants, the percentages of "lichenized-undefined saprotroph" were increased by 2.34%, 0.44%, and 1.54% in the phyllosphere, root, and rhizosphere, respectively. In addition, the plant pathogens existed in each compartment of R. roxburghii, but the percentages of "plant pathogen" were increased by 1.16% in the phyllosphere of diseased compared to healthy plants. Together, the ecological guild and co-occurrence network indicated that the potential pathogens of leaf spot were mainly found in the phyllosphere. This study explained one of pathogen origin of leaf spots of R. roxburghii by the microbial community ecology, which will provide the new insights for identification of plant pathogens.

Core Microbiome and Microbial Community Structure in Coralloid Roots of Cycas in Ex Situ Collection of Kunming Botanical Garden in China

Endophytes are essential in plant succession and evolution, and essential for stress resistance. Coralloid root is a unique root structure found in cycads that has played a role in resisting adverse environments, yet the core taxa and microbial community of different Cycas species have not been thoroughly investigated. Using amplicon sequencing, we successfully elucidated the microbiomes present in coralloid roots of 10 Cycas species, representing all four sections of Cycas in China. We found that the endophytic bacteria in coralloid roots, i.e., Cyanobacteria, were mainly composed of Desmonostoc_PCC-7422, Nostoc_PCC-73102 and unclassified_f__Nostocaceae. Additionally, the Ascomycota fungi of Exophiala, Paraboeremia, Leptobacillium, Fusarium, Alternaria, and Diaporthe were identified as the core fungi taxa. The Ascomycota fungi of Nectriaceae, Herpotrichiellaceae, Cordycipitaceae, Helotiaceae, Diaporthaceae, Didymellaceae, Clavicipitaceae and Pleosporaceae were identified as the core family taxa in coralloid roots of four sections. High abundance but low diversity of bacterial community was detected in the coralloid roots, but no significant difference among species. The fungal community exhibited much higher complexity compared to bacteria, and diversity was noted among different species or sections. These core taxa, which were a subset of the microbiome that frequently occurred in all, or most, individuals of Cycas species, represent targets for the development of Cycas conservation.

Endophytic Bacillus sp. AP10 harboured in Arabis paniculata mediates plant growth promotion and manganese detoxification

Phytoremediation of heavy metal-polluted soils assisted by plant-associated endophytes, is a suitable method for plant growth and manganese (Mn) removal in contaminated soils. This investigation was conducted to evaluate the Mn-resistant endophytic resources of the Mn hyperaccumulator Arabis paniculata and their functions in the phytoremediation of Mn²⁺ toxicity. This study isolated an endophytic bacterium with high Mn resistance and indole-3-acetic acid (IAA) production form A. paniculata and identified it as Bacillus sp. AP10 using 16 S rRNA gene sequencing analysis. The effects of Bacillus sp. AP10 on the alleviation of Mn²⁺ toxicity in Arabidopsis thaliana seedlings and the molecular mechanisms were further investigated using biochemical tests and RNA-seq analysis. Under Mn²⁺ stress, Bacillus sp. AP10 increased the biomass, chlorophyll content and the translocation factor (TF) values of Mn in the aerial parts, while decreased the malondialdehyde (MDA) content of A. thaliana seedlings compared with that of control plants. The differentially expressed genes (DEGs) and enrichment analysis showed that Bacillus sp. AP10 could significantly increase the expression of key genes involved in cell-wall loosening, which may improve plant growth under Mn stress. Superoxide dismutase (SOD)-encoding genes were detected as DEGs after AP10 treatment. Moreover, AP10 regulated the expression of genes responsible for phenylpropanoid pathway, which may promote antioxidant flavonoids accumulation for reactive oxygen species (ROS) scavenging to improve Mn tolerance. The activation of ATP-binding cassette (ABC) transporter gene expression especially ABCB1 after AP10 stimulation, explained the elevation of metal ion binding or transport related to enhanced Mn accumulation in plants. Futhermore, AP10 might alleviate Mn toxicity through enhancing abscisic acid (ABA) responsive gene expression and ABA biosynthesis. These findings provide new insights into the functions and regulatory mechanism of Bacillus sp. AP10 in promoting plant growth, and tolerance, improving Mn accumulation and alleviating Mn²⁺ toxicity in plants. The application of Bacillus sp. AP10 as potential phytoremediators may be a promising strategy in Mn²⁺ contaminated fields. AVAILABILITY OF DATA AND MATERIALS: The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Miscanthus sinensis contributes to the survival of Pinus densiflora seedlings at a mining site via providing a possible functional endophyte and maintaining symbiotic relationship between P. densiflora and endophytes from high soil temperature stress

At a sedimentary site in an old mine site, Miscanthus sinensis formed patches, where Pinus densiflora seedlings could grow better compared with those outside the patches, indicating that M. sinensis would improve P. densiflora seedling establishment. The purpose of this study was to understand the mechanisms by which M. sinensis facilitates the survival of P. densiflora seedlings by considering the soil properties, heavy metal tolerance, and root endophytes in P. densiflora seedlings at the sedimentary site. The sedimentary site, which is a bare ground, contained high concentrations of Fe, indicating that plants should be exposed to Fe and high soil temperature stresses. Measurement of soil temperature revealed that M. sinensis suppressed sharp increases and alternation of soil temperature, resulting in reducing high soil temperature stress in P. densiflora seedlings. To adapt to the Fe stress environment, P. densiflora outside and inside the patches produced Fe detoxicants, including catechin, condensed tannin, and malic acid. Ceratobasidium bicorne and Aquapteridospora sp. were commonly isolated from P. densiflora seedlings outside and inside the patches as root endophytes, which might enhance Fe tolerance in the seedlings. Aquapteridospora sp., which is considered as a dark-septate endophyte (DSE), was also isolated from the roots of M. sinensis, suggesting that M. sinensis might play a source of a root endophyte to P. densiflora seedlings. Ceratobasidium bicorne could be classified into root endophytes showing symbiosis and weak pathogenicity to host plants. Therefore, high soil temperature stress would weaken P. densiflora seedlings, causing root endophytic C. bicorne to appear pathogenic. We suggested that P. densiflora could adapt to the Fe stress environment via producing Fe detoxicants, and M. sinensis would facilitate the establishment of P. densiflora seedlings in the sedimentary site by providing a DSE, Aquapteridospora sp., and maintaining symbiosis of C. bicorne from high soil temperature stress.

A Dark Septate Endophyte Improves Cadmium Tolerance of Maize by Modifying Root Morphology and Promoting Cadmium Binding to the Cell Wall and Phosphate

Dark septate endophytes (DSEs) can improve the performance of host plants grown in heavy metal-polluted soils, but the mechanism is still unclear. A sand culture experiment was performed to investigate the effects of a DSE strain (Exophiala pisciphila) on maize growth, root morphology, and cadmium (Cd) uptake under Cd stress at different concentrations (0, 5, 10, and 20 mg·kg^-1). The results indicated that the DSE significantly improved the Cd tolerance of maize, causing increases in biomass, plant height, and root morphology (length, tips, branch, and crossing number); enhancing the Cd retention in roots with a decrease in the transfer coefficient of Cd in maize plants; and increasing the Cd proportion in the cell wall by 16.0-25.6%. In addition, DSE significantly changed the chemical forms of Cd in maize roots, resulting in decreases in the proportions of pectates and protein-integrated Cd by 15.6-32.4%, but an increase in the proportion of insoluble phosphate Cd by 33.3-83.3%. The correlation analysis revealed a significantly positive relationship between the root morphology and the proportions of insoluble phosphate Cd and Cd in the cell wall. Therefore, the DSE improved the Cd tolerance of plants both by modifying root morphology, and by promoting Cd binding to the cell walls and forming an insoluble phosphate Cd of lower activity. These results of this study provide comprehensive evidence for the mechanisms by which DSE colonization enhances Cd tolerance in maize in root morphology with Cd subcellular distribution and chemical forms.

Dark septate endophyte Exophiala pisciphila promotes maize growth and alleviates cadmium toxicity

Dark septate endophytes (DSE) are typical root endophytes with the ability to enhance plant growth and tolerance to heavy metals, but the underlying mechanisms are unclear. Here, the physiological and molecular mechanisms of a DSE strain, Exophiala pisciphila, in mitigating cadmium (Cd, 20 mg/kg) toxicity in maize were investigated. Our results showed, under Cd stress, E. pisciphila inoculation enhanced the biomass of maize and reduced both inorganic and soluble forms of Cd (high toxicity) by 52.6% in maize leaves, which may be potentially contributing to Cd toxicity mitigation. Besides, E. pisciphila inoculation significantly affected the expression of genes involved in the signal transduction and polar transport of phytohormone, and then affected abscisic acid (ABA) and indole-3-acetic acid (IAA) contents in maize roots, which was the main reason for promoting maize growth. In addition, E. pisciphila also made a 27% increase in lignin content by regulating the expression of genes involved in the synthesis of it, which was beneficial to hinder the transport of Cd. In addition, E. pisciphila inoculation also activated glutathione metabolism by the up-regulation of genes related to glutathione S-transferase. This study helps to elucidate the functions of E. pisciphila under Cd stress, sheds light on the mechanism of detoxifying Cd and provides new insights into the protection of crops from heavy metals.

Effects of fungal seed endophyte FXZ2 on Dysphania ambrosioides Zn/Cd tolerance and accumulation

Metal-induced oxidative stress in contaminated soils affects plant growth. In the present study, we evaluated the role of seed endophyte FXZ2 on Dysphania ambrosioides Zn/Cd tolerance and accumulation. A series of pot experiments were conducted under variable Zn (500, 1,000, and 1,500 mg kg^–1) and Cd (5, 15, 30, and 60 mg kg^–1). The results demonstrated that FXZ2-inoculation significantly enhanced the growth of D. ambrosioides and improved its chlorophyll and GSH content. In the rhizosphere, FXZ2 inoculation changed the chemical speciation of Zn/Cd and thus affected their uptake and accumulation in host plants. The exchangeable and carbonate-bound fractions (F1 + F2) of Zn decreased in the rhizosphere of FXZ2-inoculated plants (E+) as compared to non-inoculated plants (E-) under Zn stress (500 and 1,000 mg kg^–1), correspondingly, Zn in the shoots of E+ decreased (p < 0.05). However, at Cd stress (30 and 60 mg kg^–1), the F1 + F2 fractions of Cd in E+ rhizospheric soils increased; subsequently, Cd in the shoots of E+ increased (p < 0.05). FXZ2 could exogenously secrete phytohormones IAA, GA, and JA. The study suggests that seed endophyte FXZ2 can increase Zn/Cd tolerance of host plant by altering Zn/Cd speciation in rhizospheric soils, as well as exogenous production of phytohormones to promote growth, lowering oxidative damage while enhancing antioxidant properties. For Zn/Cd accumulation, it has opposite effects: Zn uptake in E+ plants was significantly (p < 0.05) decreased, while Cd accumulation in E+ plants was significantly (p < 0.05) increased. Thus, FXZ2 has excellent application prospects in Cd phytoextraction and decreasing Zn toxicity in agriculturally important crops.

Plant association with dark septate endophytes: When the going gets tough (and stressful), the tough fungi get going

Dark septate endophytes (DSEs) comprise a diverse and ubiquitous group of fungal generalists with broad habitat niches that robustly colonize the roots of plants in stressful environments. DSEs possess adaptation strategies that determine their high tolerance to heavy metal (HM) contamination, drought, and salinity. Most DSEs developed efficient melanin-dependent and melanin-independent mechanisms of HM detoxification and osmoprotection, including intracellular immobilization and extracellular efflux of HMs and excess ions, and the scavenging of reactive oxygen species. DSEs form mutualistic relationship with plants according to the hypothesis of "habitat-adapted associations", supporting the survival of their hosts under stressful conditions. As saprophytes, DSEs mineralize a complex soil substrate improving plants' nutrition and physiological parameters. They can protect the host plant from HMs by limiting HM accumulation in plant tissues and causing their sequestration in root cell walls as insoluble compounds, preventing further HM translocation to shoots. The presence of DSE in drought-affected plants can substantially ameliorate the physiology and architecture of root systems, improving their hydraulic properties. Plant growth-promoting features, supported by the versatility and easy culturing of DSEs, determine their high potential to enhance phytoremediation and revegetation projects for HM-contaminated, saline, and desertic lands reclamation.

Endophytic fungi isolated from plants present on a mine tailing facility show a differential growth response to lead

Plants growing in metal polluted sites can be a source of microorganisms suitable for bio-assisted phytoremediation strategies. In this work, three endophytic fungi from the roots of Poa stuckertii and Poa pratensis, two grasses that naturally colonize a Lead-Zinc tailing storage facility in Southern Chile, were isolated and identified. The leachate of the tailing sands showed a Pb content of 1.36 ± 0.71 ppm, and a pH of 7.3. By amplifying the ITS1/ITS4 region of fungal ribosomal DNA, the isolates were identified as Bjerkandera sp., Microdochium sp. and Sarocladium sp. When the growth media was supplemented with 50 ppm of Pb at pH 4.5, Microdochium sp. showed an 80% decrease in the biomass, but the biomass production of Bjerkandera sp. and Sarocladium sp. was not affected by the same treatment. The accumulation of Pb in Microdochium sp. increased as a function of the concentration of the metal in the growth media, between 48.3 μM and 241.3 μM. We showed that two Poaceae plants growing on a Lead-Zinc tailing storage facility are a source of endophyte fungi, and that Pb had a differential effect on the growth of the isolated fungi independent of the plant of origin.

Insights into the beneficial roles of dark septate endophytes in plants under challenging environment: resilience to biotic and abiotic stresses

Dark septate endophytes (DSE) exert a plethora of effects in regulating plant growth, signalling and stress tolerance. The advent of metagenomics has led to the identification of various species of DSE to be associated with plant organs. They are known to modulate growth, nutrient uptake, phytohormone biosynthesis and production of active bioconstituents in several plants. The interactions between the DSE and host plants are mostly mutualistic but they can also be neutral or exhibit negative interactions. The DSE has beneficial role in removal/sequestration of toxic heavy metals from various environmental sites. Here, we discuss the beneficial role of DSE in enhancing plant tolerance to heavy metal stress, drought conditions, high salinity and protection from various plant pathogens. Furthermore, the underlying mechanism of stress resilience facilitated by DSE-plant interaction has also been discussed. The article also provides insights to some important future perspectives associated with DSE-mediated phytoremediation and reclamation of polluted land worldwide thus facilitating sustainable agriculture.

Harnessing an arbuscular mycorrhizal fungus to improve the adaptability of a facultative metallophytic poplar (Populus yunnanensis) to cadmium stress: Physiological and molecular responses

Populus yunnanensis Dode, a facultative metallophytic poplar, exhibits afforestation potential in barren mine tailing areas. However, the interactions and functional roles of arbuscular mycorrhizal fungus (AMF) in P. yunnanensis adaptability to heavy metal stress remain unclear. Physiological and molecular responses of P. yunnanensis plantlets to AMF (Funneliformis mosseae) under cadmium (Cd) stress (50 mg kg^-1) were investigated. Results showed attenuation of Cd phytotoxicity effects on cell organelles upon AMF inoculation, which also reduced the Cd concentration in the poplar leaves, stems, and roots. Under Cd stress, AMF-blocking of metal transporter (e.g., Ca²⁺ channel) activity occurred, decreasing root cell Cd influx by reducing H⁺ efflux. Bioaugmentation of rhizosphere sediments by AMF to stabilize metals with a decreasing DTPA-extractable Cd also occurred. The AMF inoculation promoted Cd conversion into inactive, less phytotoxic forms, and helped to maintain ion homeostasis and relieve nutritional ion (e.g., Ca, Mg) disorders caused by excessive Cd. Leaf enzyme and non-enzyme antioxidant systems were triggered. Root and leaf physiological response patterns differed. The AMF regulated the poplar functional genes, and nine metal-responsive gene clusters were identified. We suggest that AMF is a functional component of P. yunnanensis phenotype extension, contributing to strong adaptability to unfavorable mine tailings conditions.

Biodiversity of Root Endophytic Fungi from Oxyria sinensis Grown in Metal-Polluted and Unpolluted Soils in Yunnan Province, Southwestern China

Oxyria sinensis adopts a tolerant strategy as a metal excluder to survive toxic metal concentrations. Biodiversity and the endophytic fungal community colonizing the O. sinensis roots were assessed from a mining area (MA) and a neighboring non-mining area (nMA) in southwestern China. All O. sinensis roots formed fully developed dark septate endophytes (DSEs) and arbuscular mycorrhizal fungi (AMF). Total DSE colonization was higher for the MA versus nMA, in contrast to the total AMF colonization in the two sites. The DSE colonization was higher than AMF colonization regardless of the site. Pure-culture data showed that the fungi closely related to Exophiala, Cadophora and Phialophora dominantly colonized the O. sinensis roots. A total of 450 operational taxonomic units (OTUs) were identified showing the presence of a distinct fungal community in MA and nMA, which was shaped by soil physiochemical properties, including soil Zn concentrations and organic matter. We found that O. sinensis accumulates and adapts efficiently to local endophytic fungi to achieve the expansion of its community, including the spontaneously reclaimed DSE. This property may be targeted to achieve its colonization with a pioneer plant for phytoremediation in the restoration of a vegetation cover in a metal-contaminated area.

Effects of the Dark Septate Endophyte (DSE) Exophiala pisciphila on the Growth of Root Cell Wall Polysaccharides and the Cadmium Content of Zea mays L. under Cadmium Stress

This paper aims to investigate the mechanism by which dark septate endophytes (DSEs) enhance cadmium (Cd) tolerance in there host plants. Maize (Zea mays L.) was inoculated with a DSE, Exophiala pisciphila, under Cd stress at different concentrations (0, 5, 10, and 20 mg·kg⁻¹). The results show that, under 20 mg/kg Cd stress, DSE significantly increased maize biomass and plant height, indicating that DSE colonization can be utilized to increase the Cd tolerance of host plants. More Cd was retained in DSE-inoculated roots, especially that fixed in the root cell wall (RCW). The capability of DSE to induce a higher Cd holding capacity in the RCW is caused by modulation of the total sugar and uronic acid of DSE-colonized RCW, mainly the pectin and hemicellulose fractions. The fourier-transform spectroscopy analysis results show that carboxyl, hydroxyl, and acidic groups are involved in Cd retention in the DSE-inoculated RCW. The promotion of the growth of maize and improvement in its tolerance to Cd due to DSEs are related to restriction of the translocation of Cd from roots to shoots; resistance of Cd uptake Cd inside cells; and the increase in RCW-integrated Cd through modulating RCW polysaccharide components.

The Mycobiota of High Altitude Pear Orchards Soil in Colombia

Simple Summary

Soil fungi are extremely important in the agro-environment. They are among the main decomposers of organic matter, contributing to carbon, nitrogen, and phosphorous cycles. They often establish positive relationships with plants, protecting them from pathogens and abiotic stresses. This study aimed to uncover the soil fungal communities of two high altitude pear orchards with biomolecular techniques. We found a rich and diverse assemblage, dominated by fungi belonging to Ascomycota and Mortierellomycota. Most of the found species were novel records for soil fungi in Colombia. The most common fungal genera were Mortierella, Fusarium, Pseudaleria and Cylindrocarpon. Among the identified fungi, some species are known to be bioactive, with promising activities as biocontrol agents, plant-growth promoters, and producers of valuable substances. These results could contribute for a more attentive management of Colombian pear orchards in future and an enrichment of knowledge on Colombian biodiversity.

Abstract

In Colombia, the cultivation of deciduous fruit trees such as pear is expanding for socio-economic reasons and is becoming more and more important for the local population. Since organized cultivation is slowly replacing sustenance cultivation, scientific information on the present agro-environment is needed to proceed in this change in an organic and environmentally friendly way. In particular, this study is an accurate description of the mycobiota present in the bulk soil of two different high altitude pear orchards in the Colombian Andes. The metabarcoding of soil samples allowed an in-depth analysis of the whole fungal community. The fungal assemblage was generally dominated by Ascomycota and secondly by Mortierellomycota. As observed in other studies in Colombia, the genus Mortierella was found to be especially abundant. The soil of the different pear orchards appeared to host quite different fungal communities according to the soil physico-chemical properties. The common mycobiota contained 35 fungal species, including several species of Mortierella, Humicola, Solicoccozyma and Exophiala. Moreover, most of the identified fungal species (79%) were recorded for the first time in Colombian soils, thus adding important information on soil biodiversity regarding both Colombia and pear orchards.

Zn tolerance in the evergreen shrub, Aucuba japonica, naturally growing at a mine site: Cell wall immobilization, aucubin production, and Zn adsorption on fungal mycelia

Aucuba japonica Thunb. is an evergreen understory shrub that grows naturally at a mine site. The mine soil contains high concentrations of heavy metals, and A. japonica appears to maintain detoxification mechanisms against heavy metals in the study site’s understory. This study aimed to investigate the heavy metal tolerance mechanisms in A. japonica, considering the possible roles of arbuscular mycorrhizal and root-endophytic fungi. We conducted fieldwork in summer (canopy-foliation season) and winter (canopy-defoliation season) to measure the heavy metal concentrations in leaves, branches, and roots and analyze possible detoxicants in the roots. The infection rates of arbuscular mycorrhizal and root-endophytic fungi were evaluated via microscopic observation, and heavy metal (Zn) localization in A. japonica roots was observed using confocal laser scanning microscopy. Field analysis showed that A. japonica accumulated excessive Zn and produced aucubin and citric acid in the roots in both summer and winter. Zn localization observations clarified that Zn was distributed in thickened epidermal and cortical cell walls, suggesting that the cell walls functioned as Zn deposition sites, reducing Zn toxicity. It was further clarified that Zn was contained within cortical cells, indicating that Zn might be detoxified by aucubin and citric acid. Arbuscular mycorrhizal and root-endophytic fungi within cortical cells adsorbed Zn on fungal cell walls, indicating that these fungi would reduce Zn content within root cells and might alleviate Zn toxicity. Our results indicated that A. japonica would maintain Zn tolerance in both summer and winter via Zn immobilization in the cell walls and production of aucubin and citric acid, and that arbuscular mycorrhizal and root-endophytic fungi might play important roles in the Zn tolerance of A. japonica.

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.

Metal Accumulation and Tolerance in Artemisia indica var. maximowiczii (Nakai) H. Hara. and Fallopia sachalinensis (F.Schmidt) Ronse Decr., a Naturally Growing Plant Species at Mine Site

For growing plants at mine sites, plant species that accumulate metals in tissues and are tolerant to high metal concentrations should be selected from the perspective of phytostabilization. However, the eco-chemical or elemental information of the plant species at the mine sites is limited. The purpose of this study was to identify plants that can adapt to natural growth at mine sites, via: (1) vegetation survey, (2) elemental analysis in soil and plants, and (3) detoxicant detection in plant cells. Our vegetation survey indicated that plants growing at our study site are consistent with plant species confirmed at other mine sites in previous reports. A. indica var. maximowiczii and F. sachalinensis, present at the mine site, highly accumulated Fe, Al, and Cu in the roots, indicating their metal tolerance. Furthermore, A. indica var. maximowiczii produced detoxicants such as chlorogenic acid and 3,5-dicaffeoylquinic acid in the roots, which exhibited high antioxidative activity that would play an important role in metal tolerance in A. indica var. maximowiczii. This study will be effective in providing fundamental information on phytostabilization at mine sites.

The Different Faces of Arabidopsis arenosa-A Plant Species for a Special Purpose

The following review article collects information on the plant species Arabidopsis arenosa. Thus far, A. arenosa has been known as a model species for autotetraploidy studies because, apart from diploid individuals, there are also tetraploid populations, which is a unique feature of this Arabidopsis species. In addition, A arenosa has often been reported in heavy metal-contaminated sites, where it occurs together with a closely related species A. halleri, a model plant hyperaccumulator of Cd and Zn. Recent studies have shown that several populations of A. arenosa also exhibit Cd and Zn hyperaccumulation. However, it is assumed that the mechanism of hyperaccumulation differs between these two Arabidopsis species. Nevertheless, this phenomenon is still not fully understood, and thorough research is needed. In this paper, we summarize the current state of knowledge regarding research on A. arenosa.

Phialocephala fortinii increases aluminum tolerance in Miscanthus sinensis growing in acidic mine soil

Miscanthus sinensis growing in our study mine site contained a high concentration of Al in the adventitious roots. It has a root endophyte, Phialocephala fortinii, in its adventitious roots at a high frequency. The purpose of this study was to elucidate the effects of P. fortinii on Al tolerance mechanisms of M. sinensis and reveal potential underlying mechanisms. In the absence of P. fortinii, M. sinensis produced chlorogenic, citric, and malic acids that could act to alleviate Al toxicity in acidic mine soil. Up on fungal inoculation, the levels of these compounds were reduced, although the growth of seedlings and Mg concentration in the roots were increased. IAA production by the fungus may contribute to enhanced plant growth whereas an increase of Mg uptake could reduce toxicity of reactive oxygen species under Al stress. These actions of P. fortinii could promote growth and survival of M. sinensis in mine sites.

Dark septate endophytes isolated from non-hyperaccumulator plants can increase phytoextraction of Cd and Zn by the hyperaccumulator Noccaea caerulescens

Dark septate endophytes (DSEs) can improve plant stress tolerance by promoting growth and affecting element accumulation. Due to its ability to accumulate high Cd, Zn, and Ni concentrations in its shoots, Noccaea caerulescens is considered a promising candidate for phytoextraction in the field. However, the ability of DSEs to improve trace element (TE) phytoextraction with N. caerulescens has not yet been studied. The aim of this study was therefore to determine the ability of five DSE strains, previously isolated from poplar roots collected at different TE-contaminated sites, to improve plant development, mineral nutrient status, and metal accumulation by N. caerulescens during a pot experiment using two soils differing in their level of TE contamination. Microscopic observations revealed that the tested DSE strains effectively colonised the roots of N. caerulescens. In the highly contaminated (HC) soil, a threefold increase in root biomass was found in plants inoculated with the Leptodontidium sp. Pr30 strain compared to that in the non-inoculated condition; however, the plant nutrient status was not affected. In contrast, the two strains Phialophora mustea Pr27 and Leptodontidium sp. Me07 had positive effects on the mineral nutrient status of plants without significantly modifying their biomass. Compared to non-inoculated plants cultivated on HC soil, Pr27- and Pr30-inoculated plants extracted more Zn (+ 30%) and Cd (+ 90%), respectively. In conclusion, we demonstrated that the responses of N. caerulescens to DSE inoculation ranged from neutral to beneficial and we identified two strains (i.e. Leptodontidium sp. (Pr30) and Phialophora mustea (Pr27)) isolated from poplar that appeared promising as they increased the amounts of Zn and Cd extracted by improving plant growth and/or TE accumulation by N. caerulescens. These results generate interest in further characterising the DSEs that naturally colonise N. caerulescens and testing their ability to improve phytoextraction.

Melatonin confers heavy metal-induced tolerance by alleviating oxidative stress and reducing the heavy metal accumulation in Exophiala pisciphila, a dark septate endophyte (DSE)

Background

Melatonin (MT), ubiquitous in almost all organisms, functions as a free radical scavenger. Despite several reports on its role as an antioxidant in animals, plants, and some microorganisms, extensive studies in filamentous fungi are limited. Based upon the role of melatonin as an antioxidant, we investigated its role in heavy metal-induced stress tolerance in Exophiala pisciphila, a dark septate endophyte (DSE), by studying the underlying mechanisms in alleviating oxidative stress and reducing heavy metal accumulation.

Results

A significant decrease in malondialdehyde (MDA) and oxygen free radical (OFR) in E. pisciphila was recorded under Cd, Zn, and Pb stresses as compared to the control. Pretreatment of E. pisciphila with 200.0 μM exogenous melatonin significantly increased the activity of superoxide dismutase (SOD) under Zn and Pb stresses. Pretreatment with 200.0 μM melatonin also lowered Cd, Zn, and Pb concentrations significantly. Melatonin production was enhanced by Cd, Cu, and Zn after 2 d, and melatonin biosynthetic enzyme genes, E. pisciphila tryptophan decarboxylase (EpTDC1) and serotonin N-acetyltransferase (EpSNAT1), were transcriptionally upregulated. The overexpression of EpTDC1 and N-acetylserotonin O-methyltransferase (EpASMT1) in Escherichia coli and Arabidopsis thaliana enhanced its heavy metal-induced stress tolerance. The overexpression of EpTDC1 and EpASMT1 reduced the Cd accumulation in the whole A. thaliana plants, especially in the roots.

Conclusions

Melatonin conferred heavy metal-induced stress tolerance by alleviating oxidative stress, activating antioxidant enzyme SOD, and reducing heavy metal accumulation in E. pisciphila. Melatonin biosynthetic enzyme genes of E. pisciphila also played key roles in limiting excessive heavy metal accumulation in A. thaliana. These findings can be extended to understand the role of melatonin in other DSEs associated with economically important plants and help develop new strategies in sustainable agriculture practice where plants can grow in soils contaminated with heavy metals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02098-1.

The ameliorative effects of exogenous inoculation of Piriformospora indica on molecular, biochemical and physiological parameters of Artemisia annua L. under arsenic stress condition

Aim of the current study was to investigate the effect of exogenously inoculated root endophytic fungus, Piriformospora indica, on molecular, biochemical, morphological and physiological parameters of Artemisia annua L. treated with different concentrations (0, 50, 100 and 150 μmol/L) of arsenic (As) stress. As was significantly accumulated in the roots than shoots of P. indica-inoculated plants. As accumulation and immobilization in the roots is directly associated with the successful fungal colonization that restricts most of As as compared to the aerial parts. A total of 4.1, 11.2 and 25.6 mg/kg dry weight of As was accumulated in the roots of inoculated plants supplemented with 50, 100 and 150 μmol/L of As, respectively as shown by atomic absorption spectroscopy. P. indica showed significant tolerance in vitro to As toxicity even at high concentration. Furthermore, flavonoids, artemisinin and overall biomass were significantly increased in inoculated-stressed plants. Superoxide dismutase and peroxidase activities were increased 1.6 and 1.2 fold, respectively under 150 μmol/L stress in P. indica-colonized plants. Similar trend was followed by ascorbate peroxidase, catalase and glutathione reductase. Like that, phenolic acid and phenolic compounds showed a significant increase in colonized plants as compared to their respective control/un-colonize stressed plants. The real-time PCR revealed that transcriptional levels of artemisinin biosynthesis genes, isoprenoids, terpenes, flavonoids biosynthetic pathway genes and signal molecules were prominently enhanced in inoculated stressed plants than un-inoculated stressed plants.

Root fungal endophytes: identity, phylogeny and roles in plant tolerance to metal stress

Metal trace elements accumulate in soils mainly because of anthropic activities, leading living organisms to develop strategies to handle metal toxicity. Plants often associate with root endophytic fungi, including nonmycorrhizal fungi, and some of these organisms are associated with metal tolerance. The lack of synthetic analyses of plant-endophyte-metal tripartite systems and the scant consideration for taxonomy led to this review aiming (1) to inventory non-mycorrhizal root fungal endophytes described with respect to their taxonomic diversity and (2) to determine the mutualistic roles of these plant-fungus associations under metal stress. More than 1500 species in 100 orders (mainly Hypocreales and Pleosporales) were reported from a wide variety of environments and hosts. Most reported endophytes had a positive effect on their host under metal stress, but with various effects on metal uptake or translocation and no clear taxonomic consistency. Future research considering the functional patterns and dynamics of these associations is thus encouraged.

Combined application of Bacillus sp. MN-54 and phosphorus improved growth and reduced lead uptake by maize in the lead-contaminated soil

Lead (Pb) is considered an important environmental contaminant due to its considerable toxicity to living organisms. It can enter and accumulate in plant tissues and become part of the food chain. In the present study, individual and combined effects of Bacillus sp. MN-54 and phosphorus (P) on maize growth and physiology were evaluated in Pb-contaminated soil. A pristine soil was artificially contaminated with two levels of Pb (i.e., 250 and 500 mg kg^-1 dry soil) and was transferred to plastic pots. Bacillus sp. MN-54 treated and untreated maize (DK-6714) seeds were planted in pots. Recommended doses of nutrients (N and K) were applied in each pot while P was applied in selective pots. Results showed that Pb stress hampered the maize growth and physiological attributes in a concentration-dependent manner, and significant reductions in seedling emergence, shoot and root lengths, fresh and dry biomasses, leaf area, chlorophyll content, rate of photosynthesis, and stomatal conductance were recorded compared with control. Application of Bacillus sp. MN-54 or P particularly in combination significantly reduced the toxic effects of Pb on maize. At higher Pb level (500 mg kg^-1), the combined application effectively reduced Pb uptake up to 42.4% and 50% by shoots, 30.8% and 33.9% by roots, and 18.4% and 26.2% in available Pb content in soil after 45 days and 90 days, respectively compared with that of control. Moreover, the use of Bacillus sp. MN-54 significantly improved the P uptake by maize plants by 44.4% as compared with that of control. Our findings suggest that the combined use of Bacillus sp. MN-54 and P could be effective and helpful in improving plant growth and Pb immobilization in Pb-contaminated soil.

Fungal Endophytes for Grass Based Bioremediation: An Endophytic Consortium Isolated from Agrostis stolonifera Stimulates the Growth of Festuca arundinacea in Lead Contaminated Soil

Bioremediation is an ecologically-friendly approach for the restoration of heavy metal-contaminated sites and can exploit environmental microorganisms such as bacteria and fungi. These microorganisms are capable of removing and/or deactivating pollutants from contaminated substrates through biological and chemical reactions. Moreover, they interact with the natural flora, protecting and stimulating plant growth in these harsh conditions. In this study, we isolated a group of endophytic fungi from Agrostis stolonifera grasses growing on toxic waste from an abandoned lead mine (up to 47,990 Pb mg/kg) and identified them using DNA sequencing (nrITS barcoding). The endophytes were then tested as a consortium of eight strains in a growth chamber experiment in association with the grass Festuca arundinacea at increasing concentrations of lead in the soil to investigate how they influenced several growth parameters. As a general trend, plants treated with endophytes performed better compared to the controls at each concentration of heavy metal, with significant improvements in growth recorded at the highest concentration of lead (800 galena mg/kg). Indeed, this set of plants germinated and tillered significantly earlier compared to the control, with greater production of foliar fresh and dry biomass. Compared with the control, endophyte treated plants germinated more than 1-day earlier and produced 35.91% more plant tillers at 35 days-after-sowing. Our results demonstrate the potential of these fungal endophytes used in a consortium for establishing grassy plant species on lead contaminated soils, which may result in practical applications for heavy metal bioremediation.

Dark Septate Endophytic Fungi Associated with Sugarcane Plants Cultivated in São Paulo, Brazil

Dark septate endophytes (DSEs) constitute a polyphyletic group within the Ascomycota, with global distribution and a wide range of host plant species. The present study evaluated the diversity of DSE in sugarcane roots of the varieties RB867515, RB966928, and RB92579, and four varieties of not commercialized energy cane. A total of 16 DSE strains were isolated, mostly from the varieties RB966928 and RB867515, with six and five isolates, respectively. Just one of the four energy cane varieties had fungi with DSE appearance. The analyses of the DNA sequences from the internal transcribed spacer (ITS) and the large subunit (LSU), in association with the micromorphology of the isolates, allowed the differentiation of the 16 isolates in at least five species, within the families Periconiaceae, Pleosporaceae, Lentitheciaceae, Vibrisseaceae, and Apiosporaceae and the orders Pleosporales, Helotiales, and Xylariales. The order Pleosporales represented 80% of the isolates, and the species Periconia macrospinosa, with six isolates, accounted for the highest isolation frequency. The results confirm the natural occurrence of the DSE symbiosis in sugarcane varieties and the generalist character of these fungi as some of the detected species have already been reported associated with other host plants, ecosystems, and regions of the world.

Effects of Dark Septate Endophytes Strain A024 on Damping-off Biocontrol, Plant Growth and the Rhizosphere Soil Enviroment of Pinus sylvestris var. mongolica Annual Seedlings

Dark septate endophytes (DSEs) exert a vital role in promoting plant growth, improving mineral absorption, biological disease control, and enhancing plant stress resistance. The effects of dark septate endophyte strain, Phialocephala bamuru A024 on damping-off biocontrol, plant development, nutrients within the rhizosphere soil, as well as bacterial communities in the annual seedlings of P. sylvestris var. Mongolica were studied. According to our findings, following P. bamuru A024 inoculation, the damping-off disease morbidity decreased significantly compared with control, some physiological indices such as β-1,3-glucanase, chitinase enzyme activity as well as a soluble protein and proline content in P. sylvestris var. mongolica were elevated under R. solani stress. After inoculation with P. bamuru A024, the biomass in seedlings, nutrients in soil, root structure index, together with activities of soil enzymes were remarkably up-regulated relative to control (p < 0.05). As suggested by the results of high-throughput sequencing, the microbial structure in the rhizosphere soil of the P. sylvestris var. mongolica showed significant differences (p < 0.05) after P. bamuru A024 inoculation compared to control treatment and the rhizosphere soil bacterial community structure after DSE A024 inoculation was positively correlated to the main soil nutrition indices.

Role of Microorganisms in the Remediation of Wastewater in Floating Treatment Wetlands: A Review

This article provides useful information for understanding the specific role of microbes in the pollutant removal process in floating treatment wetlands (FTWs). The current literature is collected and organized to provide an insight into the specific role of microbes toward plants and pollutants. Several aspects are discussed, such as important components of FTWs, common bacterial species, rhizospheric and endophytes bacteria, and their specific role in the pollutant removal process. The roots of plants release oxygen and exudates, which act as a substrate for microbial growth. The bacteria attach themselves to the roots and form biofilms to get nutrients from the plants. Along the plants, the microbial community also influences the performance of FTWs. The bacterial community contributes to the removal of nitrogen, phosphorus, toxic metals, hydrocarbon, and organic compounds. Plant–microbe interaction breaks down complex compounds into simple nutrients, mobilizes metal ions, and increases the uptake of pollutants by plants. The inoculation of the roots of plants with acclimatized microbes may improve the phytoremediation potential of FTWs. The bacteria also encourage plant growth and the bioavailability of toxic pollutants and can alleviate metal toxicity.

Lead tolerant endophyte Trametes hirsuta improved the growth and lead accumulation in the vegetative parts of Triticum aestivum L

Rapid industrialization and increasing population are continuously adding contaminants to our environment. Among those, heavy metals are considered to be one of the serious threats to the ecosystem due to their persistent nature. Microbe assisted phytoremediation is an effective tool for metal remediation as microbes enhance the metal availability and uptake to the host plants or reduce it by binding them intracellularly or extracellularly. An endophytic fungus, Trametes hirsuta, was isolated from Chenopodium album L. plant growing in the lead (Pb) contaminated soil of an industrial area. This is the first study citing Trametes hirsuta, as a root endophyte of Chenopodium album L. This endophytic fungus was found to be tolerant to high concentration of Pb i.e., 1500 mg L^-1, when tested in-vitro. Wheat (Triticum aestivum L.) seedlings were infected by Trametes hirsuta and Pb tolerance was observed. With the fungal inoculation plants cumulative growth and total chlorophyll content increased by 24% and 18%, respectively as compared to their respective non-inoculated controls at 1000 mg kg^-1 Pb. Similary, 50% more Pb accumulation was measured in the shoots of fungal inoculated plants at 1500 mg kg^-1 Pb as compared to control. Thus, the results of the present study suggest that mutualism with endophytic fungi can improve the survival of host plants in metal contaminated soils, additionally it can also assist the phytoextraction of heavy metals from polluted sites by increasing their uptake by the host plants.

Significance of manganese resistant bacillus cereus strain WSE01 as a bioinoculant for promotion of plant growth and manganese accumulation in Myriophyllum verticillatum

Endophytic bacteria are generally helpful for plant growth and protection. Strain WSE01, which was identified as bacillus cereus, was isolated from the stem of Myriophyllum verticillatum and it displayed a high tolerance to Mn (1500 mg/L). The strain was found to be able to produce indole-3-acetic acid (IAA) and siderophores, fix the atmospheric nitrogen and dissolve potassium from insoluble K-bearing minerals. In hydroponic culture experiments, the inoculation of strain WSE01 significantly promoted the growth and increased the leaf enzyme activity in the inoculated plant M. verticillatum. Furthermore, the manganese content was increased by 36.4% in stems and by 54.7% in leaves of the inoculated plant under Mn stress at 400 mg/L, compared to the non-inoculated group. This study suggests that the strain WSE01 has the potential to be used as biocontrol and/or biofertilizing agents for application in macrophyte M. verticillatum and conduces to achieving more effective phytoremediation of metal-contaminated waters.

Dark septate endophytic fungi mitigate the effects of salt stress on cowpea plants

The association of plant with microorganisms, such as dark septate endophytic fungi, has mitigated the harmful effects of chemical, physical, and biological agents on the host. The objective of this work was to evaluate the interaction of the dark septate endophytic fungi with cowpea plants under salt stress. Endophytic fungi were isolated from Vochysia divergens root system, and molecular identification of fungi was performed by sequencing the ITS region. We selected and identified Sordariomycetes sp1-B'2 and Melanconiella elegans-21W2 for their ability to infect V. divergens root in vitro with development of typical dark septate fungi structures. Cowpea plants-inoculated or not inoculated with Sordariomycetes sp1-B'2 and M. elegans 21W2-were cultivated in 5-L pots under greenhouse conditions and submitted to four different electrical conductivities of irrigation water (1.2, 2.2, 3.6, and 5.0 dS m-1). The salinity caused decrease in leaf concentration of K and increased leaf concentration of calcium, sodium, and chlorine; and no influence of dark septate endophytic fungi was observed in these responses. On the other hand, root colonization with Sordariomycetes sp1-B'2 and M. elegans 21W2 resulted in improved nutrition with N and P in cowpea under salt stress, favoring the growth and rate of liquid photosynthesis. However, such positive responses were evident only at moderate levels of salinity.

Mercury resistance and bioremediation mediated by endophytic fungi

The present study proposes the use of endophytic fungi for mercury bioremediation in in vitro and host-associated systems. We examined mercury resistance in 32 strains of endophytic fungi grown in culture medium supplemented with toxic metal concentrations. The residual mercury concentrations were quantified after mycelial growth. Aspergillus sp. A31, Curvularia geniculata P1, Lindgomycetaceae P87, and Westerdykella sp. P71 were selected and further tested for mercury bioremediation and bioaccumulation in vitro, as well as for growth promotion of Aeschynomene fluminensis and Zea mays in the presence or absence of the metal. Aspergillus sp. A31, C. geniculata P1, Lindgomycetaceae P87 and Westerdykella sp. P71 removed up to 100% of mercury from the culture medium in a species-dependent manner and they promoted A. fluminensis and Z. mays growth in substrates containing mercury or not (Dunnett's test, p < 0.05). Lindgomycetaceae P87 and C. geniculata P1 are dark septate endophytic fungi that endophytically colonize root cells of their host plants. The increase of host biomass correlated with the reduction of soil mercury concentration due to the metal bioaccumulation in host tissues and its possible volatilization. The soil mercury concentration was decreased by 7.69% and 57.14% in A. fluminensis plants inoculated with Lindgomycetaceae P87 + Aspergillus sp. A31 and Lindgomycetaceae P87, respectively (Dunnet's test, p < 0.05). The resistance mechanisms of mercury volatilization and bioaccumulation in plant tissues mediated by these endophytic fungi can contribute to bioremediation programs. The biochemical and genetic mechanisms involved in bioaccumulation and volatilization need to be elucidated in the future.

Heavy metal tolerance of orchid populations growing on abandoned mine tailings: A case study in Sardinia Island (Italy)

Understanding how environmental pollutants influence plant occurrence, growth, and development is key for effective management plans and potential bioremediation. Rare plants, such as orchids, may occur in modified habitats and on soils containing heavy metals, yet their ecological and physiological responses to heavy metals is poorly understood. We investigated the influence of heavy metal pollution on orchid growth rates and interactions with soil fungal mutualists by comparing a large population of the orchid Epipactis helleborine (L.) Crantz subsp. tremolsii (Pau) E. Klein that grows on mine tailings in south-west Sardinia (Italy) with a population that grows on non-contaminated soils in central Sardinia. Soils of the contaminated site had high levels of heavy metals and low organic matter and nutritive elements content. We performed a morphological analysis on twenty individuals that have been subjected to measurement of bioaccumulation and translocation of heavy metals. Fungi associated with the roots of plants from the contaminated and uncontaminated site were grown and identified by DNA barcoding approach. Plants from the contaminated site were smaller than the ones growing in the uncontaminated site and were found to be able to tolerate heavy metals from the soil and to accumulate and translocate them into their organs. Fungi belonging to the genus Ilyonectria (Ascomycota) were found both in contaminated and uncontaminated sites, while an unidentified fungus was isolated from roots in the contaminated site only. These results are discussed in terms of orchids' tolerance to heavy metals and its physiological and ecological mechanisms. The role of contaminated habitats in harbouring orchids and peculiar taxa is also discussed.

Dark Septate Endophytes Improve the Growth and the Tolerance of Medicago sativa and Ammopiptanthus mongolicus Under Cadmium Stress

Although the ecological function of dark septate endophytes (DSEs) is well studied, little is known about the responses of the host plant to DSEs obtained from other plants, especially under conditions of heavy metal stress. This study aimed to investigate how DSEs from a heavy-metal habitat affect non-host plants in cadmium (Cd) stress soils, which then provides a basis for the application of DSEs in the cultivation of different plant and soil remediation strategies for polluted ecosystems. We isolated and identified two species of DSE (Acrocalymma vagum and Scytalidium lignicola) inhabiting the roots of Ilex chinensis (host plant) which are grown in metal-polluted habitats. Then, the Cd stress tolerance of the DSEs was tested using a pure culture of which the Cd concentration has been adjusted. Subsequently, we examined the performance of non-host plants (Medicago sativa and Ammopiptanthus mongolicus) which were inoculated with DSEs under Cd stress in a growth chamber. The results indicated that the two DSEs could grow under Cd stress in vitro, even when not exhibiting high levels of tolerance to Cd. The superoxide dismutase (SOD), malondialdehyde (MDA), glutathione (GSH), soluble protein, and melanin of the DSE fungi reached maximal levels at concentrations of 30-60 mg Cd/L, indicating the important preventive strategies adopted by the DSE fungi in environments contaminated by Cd. Despite a decreased biomass of DSE hyphae with enhanced Cd concentrations, the accumulation of Cd in the DSE hyphae tended to show an increasing trend. Both DSEs were effective colonizers of the non-host plants. A. vagum and S. lignicola inoculation significantly promoted the biomass and the root architecture of the two non-host plants under Cd stress. A. vagum inoculation increased the total nitrogen (TN) of A. mongolicus, whereas inoculation with S. lignicola significantly increased the organic carbon (OC) of M. sativa. In particular, the DSE inoculation significantly improved the accumulation of Cd in plant tissues under Cd stress, demonstrating a potential application in the bio-remediation of heavy-metal-pollution areas. Our findings suggest that the DSE inoculation improved the root growth and nutrient absorption of non-host plants, altered the soil Cd concentration, and facilitated plant growth and survival under Cd stress. These results contribute to a better understanding of DSE-plant interactions in habitats contaminated by heavy metals.

Contaminação versus manifestação endofítica: implicações no cultivo in vitro de plantas

Resumo A cultura de tecidos vegetais é imprescindível à propagação e multiplicação uniforme de plantas, à conservação de germoplasma, a programas de melhoramento e à transformação genética. Essa técnica tem exigido, cada vez mais, estudos que colaborem com o entendimento dos mecanismos envolvidos no crescimento dos microrganismos nos meios de cultivo, bem como as relações que eles estabelecem com a planta hospedeira. Dessa maneira, a presente revisão pretende esclarecer esses questionamentos e promover a distinção entre contaminação e manifestação endofítica que ocorrem no cultivo in vitro por diferentes causas. Tal distinção permite diminuir o pânico que se instala quando do seu aparecimento, além de auxiliar na adoção de medidas de prevenção e/ou controle desses eventos sem que haja descartes desnecessários de material de alto valor comercial e genético.

Biopriming with Piriformospora indica ameliorates cadmium stress in rice by lowering oxidative stress and cell death in root cells

Piriformospora indica is known for plant growth promotion and abiotic stress alleviation potential in several agricultural crops. However, a systemic analysis is warranted to explore potential application of this important fungus to augment heavy metal tolerance in rice. The present study explores potential of P. indica in ameliorating the effect of cadmium (Cd) stress in rice cultivars N22 and IR64. Seedlings inoculated with P. indica recorded significantly higher root-shoot length and biomass as compared to non-inoculated plants under control and Cd stress, respectively. Moreover, P. indica inoculated stressed roots accumulated more Cd as compared to non-inoculated stressed roots in both the varieties. Interestingly, cell death and reactive oxygen species (ROS) accumulation were significantly lower in the inoculated plant roots as compare with non-inoculated roots under Cd stress. The results emphasized significantly higher accumulation of Cd in fungal spores could reduce ROS accumulation in root cells resulting in lower cell death.

EpABC Genes in the Adaptive Responses of Exophiala pisciphila to Metal Stress: Functional Importance and Relation to Metal Tolerance

Exophiala pisciphila is one of the dominant dark septate endophytes (DSEs) colonizing metal-polluted slag heaps in southwest China. It shows numerous super-metal-tolerant characteristics, but the molecular mechanisms involved remain largely unknown. In the present study, the functional roles of a specific set of ATP-binding cassette (ABC) transporters in E. pisciphila were characterized. In total, 26 EpABC genes belonging to 6 subfamilies (ABCA to ABCG) were annotated in previous transcriptome sequencing libraries, and all were regulated by metal ions (Pb, Zn, and Cd), which was dependent on the metal species and/or concentrations tested. The results from the heterologous expression of 3 representative EpABC genes confirmed that the expression of EpABC2.1, EpABC3.1, or EpABC4.1 restored the growth of metal-sensitive mutant Saccharomyces cerevisiae strains and significantly improved the tolerance of Arabidopsis thaliana to Pb, Zn, and Cd. Interestingly, the expression of the 3 EpABC genes further altered metal (Pb, Zn, and Cd) uptake and accumulation and promoted growth by alleviating the inhibitory activity in yeast and thale cress caused by toxic ions. These functions along with their vacuolar location suggest that the 3 EpABC transporters may enhance the detoxification of vacuolar compartmentation via transport activities across their membranes. In conclusion, the 26 annotated EpABC transporters may play a major role in maintaining the homeostasis of various metal ions in different cellular compartments, conferring an extreme adaptative advantage to E. pisciphila in metal-polluted slag heaps.IMPORTANCE Many ABC transporters and their functions have been identified in animals and plants. However, little is known about ABC genes in filamentous fungi, especially DSEs, which tend to dominantly colonize the roots of plants growing in stressed environments. Our results deepen the understanding of the function of the ABC genes of a super-metal-tolerant DSE (E. pisciphila) in enhancing its heavy metal resistance and detoxification. Furthermore, the genetic resources of DSEs, e.g., numerous EpABC genes, especially from super-metal-tolerant strains in heavy metal-polluted environments, can be directly used for transgenic applications to improve tolerance and phytoextraction potential.

Effects of Dark Septate Endophytes on the Performance of Hedysarum scoparium Under Water Deficit Stress

Hedysarum scoparium, a species characterized by rapid growth and high drought resistance, has been used widely for vegetative restoration of arid regions in Northwest China that are prone to desertification. Desert soil is typically deficient in available water and the alleviation of drought stress to host plants by endophytes could be an efficient strategy to increase the success of desert restoration. With the objective to seek more beneficial symbionts that can be used in the revegetation strategies, we addressed the question whether H. scoparium can benefit from inoculation by dark septate endophytes (DSEs) isolated from other desert plants. We investigated the influences of four non-host DSE strains (Phialophora sp., Knufia sp., Leptosphaeria sp., and Embellisia chlamydospora) isolated from other desert plants on the performance of H. scoparium under different soil water conditions. Differences in plant performance, such as plant growth, antioxidant enzyme activities, carbon, nitrogen, and phosphorous concentration under all the treatments, were examined. Four DSE strains could colonize the roots of H. scoparium successfully, and they established a positive symbiosis with the host plants depending on DSE species and water availability. The greatest benefits of DSE inoculation occurred in water stress treatment. Specifically, Phialophora sp. and Leptosphaeria sp. improved the root biomass, total biomass, nutrient concentration, and antioxidant enzyme activities of host plants under water deficit conditions. These data contribute to the understanding of the ecological function of DSE fungi in drylands.

Endophyte infection enhances accumulation of organic acids and minerals in rice under Pb2+ stress conditions

This study aimed to assess the effects of endophyte infection on Pb accumulation, organic acids (OAs) and mineral element contents by comparing endophyte-uninfected (E-) and endophyte-infected (E+) rice exposed to 0, 10.36, 20.72, 31.08 and 41.44 mg/L Pb²⁺ for 10 days. Pb²⁺ stress decreased growth, dry weight, and chlorophyll (Chl) content in E- and E+ in a concentration-dependent manner. Pb²⁺ accumulation was similar in E- and E+. Under Pb²⁺ stress, E+ accumulated more malate and fumarate in the leaves compared to E-, as well as more tartrate, malate, succinate and fumarate in the roots. Furthermore, E+ secreted more malate and lactate under 41.44 mg/L Pb²⁺ stress. Malate accumulation was induced by endophyte infection under Pb²⁺ exposure suggesting that malate is the most obvious candidate ligand for Pb²⁺. Endophyte infection increased Ca, Mg, P, Fe and Ni contents in the leaves and Ni contents in the roots under Pb²⁺ stress, but reduced Fe content in the roots under high Pb²⁺ stress. Under same Pb²⁺ concentration, endophyte infection significantly increased plant height, the dry weight of the shoots, and Chl content. The effects of endophyte infection might be due to changes in OAs accumulation and exudation and improvements in mineral uptake under Pb²⁺ stress.

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

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

Root-endophytic Chaetomium cupreum chemically enhances aluminium tolerance in Miscanthus sinensis via increasing the aluminium detoxicants, chlorogenic acid and oosporein

Miscanthus sinensis Andersson is a pioneer plant species that grows naturally at mining sites. Miscanthus sinensis can detoxify aluminium (Al) by producing phytosiderophores, such as chlorogenic acid, citric acid, and malic acid, and localizing Al in cell walls. Root-endophytic Chaetomium cupreum, which produces microbial siderophores, enhances Al tolerance in M. sinensis. However, we could not determine whether the siderophores produced by C. cupreum actually enhance Al tolerance in M. sinensis, because the microbial siderophores have not yet been identified in previous research. The purpose of this study was to clarify how C. cupreum chemically increases Al tolerance in M. sinensis under acidic mining site conditions, especially considering siderophores. Using instrumental analyses, the siderophore produced by C. cupreum was identified as oosporein. Comparison of the stability constant between Al and phytosiderophores and oosporein indicated that oosporein could detoxify Al similarly to chlorogenic acid, which shows higher stability constant than citric acid and malic acid. Inoculation test of C. cupreum onto M. sinensis in acidic mine soil showed that C. cupreum promoted seedling growth, and enhanced Al tolerance via inducing chlorogenic-acid production and producing oosporein. These results suggested that C. cupreum could chemically enhance Al tolerance and might promote growth via reducing excessive Al in cell walls, the main site of Al accumulation. In addition, the chemical enhancement of Al tolerance by C. cupreum might be important for M. sinensis to adapt to acidic mining sites.

The effects of dark septate endophyte (DSE) inoculation on tomato seedlings under Zn and Cd stress

Dark septate endophytes (DSEs) are a heterogeneous group of endophytic fungi that frequently colonize the roots of plants growing in trace metal element-contaminated soils. However, the functional role of DSEs in host plants growing in metal-stressed environments remains to be elucidated. In this study, two DSE strains of Phialophora mustea Neerg. (K36 and Z48) were separately inoculated in tomato (Lycopersicon esculentum Miller) seedlings under metal stress conditions (0, 5, 10 mg kg^-1 Cd or 0, 300, 600 mg kg^-1 Zn) to evaluate the effects of DSE inoculation on tomato seedlings in pot cultures. The results showed that DSE colonization increased tomato seedling biomass whether or not there was metal addition. DSE-inoculated tomatoes had a lower Cd and Zn accumulation in both the shoots and roots compared with their respective non-inoculated controls. Under metal stress conditions, DSE inoculation significantly enhanced the activities of antioxidant enzymes, such as superoxide dismutase (SOD) and peroxidase (POD), thus relieving the membrane lipid peroxidation damage caused by metal stress, and reduced the leaf malondialdehyde (MDA) concentrations more than that of the non-inoculated treatments. The results revealed that DSE enhanced metal tolerance and improved tomato plant growth, both by the reduced metal uptake into root and shoot accumulation and by the enhanced activities of antioxidant enzymes to eliminate reactive oxygen species (ROS) stress induced by excessive metals.

Bioremediation of hexavalent chromium by endophytic fungi; safe and improved production of Lactuca sativa L

One of the main problems of the industrialized world is the accumulation of chromium (Cr) in soil, which is a serious threat to the crops. Complete removal of Cr from the contaminated soils poses a great challenge. However, this issue can be minimised by using plant growth promoting microbes as a bioremediation tool. In the present study, healthy plants established near the University campus in Mardan were selected for the isolation of Cr resistant endophytes. From the designated plants, 114 species of endophytic fungi were isolated. Among the 114 isolated strains, 4 strains have induced resistance in L. sativa against Cr. The strains were identified as Aspergillus fumigatus, Rhizopus sp., Penicillium radicum and Fusarium proliferatum based on ITS region (18 S rDNA) homology. The isolates have removed Cr from soil and culture media as well as bio-transformed it from highly toxic hexavalent to least toxic trivalent form, thus helped the Cr stressed L. sativa to restore its normal growth. The Rhizopus Sp. CUC23 has mainly accrued Cr and detoxified intracellularly, whereas A. fumigatus ML43 and P. radicum PL17 has detoxified up to 95% of Cr extracellularly. From the results, it is concluded that the selected endophytic strains might be used as biofertilizer for healthy and safe crop production in Cr contaminated soils.

Effect of microorganisms on reducing cadmium uptake and toxicity in rice (Oryza sativa L.)

This study analyzed the application of three microorganism inoculums, including Bacillus subtilis, Bacillus cereus, and commercial effective microorganism (EM) solution in order to determine cadmium (Cd) reduction in rice (Oryza sativa L.) and rice growth promotion. Rice was grown in Cd-contaminated soil (120 mg/kg) and selected microorganisms were inoculated. Cd concentration and rice weight were measured at 45 and 120 days of the experiment. The result showed that B. subtilis inoculation into rice can highly reduce Cd accumulation in every part of rice roots and shoots (45 days), and grains (120 days). This species can effectively absorb Cd compared to other inoculums, which might be the main mechanism to reduce Cd transportation in rice plants. Interestingly, plants that were inoculated with bacterial species individually harbored higher calcium (Ca) and magnesium (Mg) accumulation; B. subtilis-inoculated plants had the highest levels of Ca and Mg compared to other inoculated ones. Moreover, inoculating rice plants with these microorganisms could increase the dry weight of the plant and protect them from Cd stress because all the inoculums presented the ability to solubilize phosphate, produce indole-3-acetic acid (IAA), and control ethylene levels by 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. After 120 days, quantification of each inoculum by quantitative polymerase chain reaction (qPCR) confirmed the root colonization of bacterial species, with B. subtilis showing higher 16S rRNA gene copy numbers than the other species. B. subtilis was classified as a non-human pathogenic strain, reassuring the safe application of this plant growth-promoting bacterium as a crop inoculum.