Ectomycorrhizal fungi enhance the tolerance of phytotoxicity and cadmium accumulation in oak (Quercus acutissima Carruth.) seedlings: modulation of growth properties and the antioxidant defense responses

Genetic and Epigenetic Views of the Adaptive Evolution of Oaks (Quercus L.)

The long-lived angiosperm Quercus L. (Oaks) have emerged as promising model organisms for investigating adaptive divergence and ecological environmental interactions due to their longevity and large genetic diversity as well as recurrent gene flow among species with diverse natural habitats. Until recently, numerous genomic studies by new high-throughput sequencing platforms have provided access to link genes with ecological and physiological traits. However, the genetic and epigenetic mechanisms underlying the adaptive evolution of oak trees are still poorly understood. In this review, we summarise the current progress of reported oak genomes and inheritance systems, analysing the epigenetics and genetic structure of oaks. We review evidence regarding the genetic mechanism linking introgressive hybridisation and reproductive isolation for a better understanding of adaptive divergence and defining speciation in oaks. Furthermore, we also discuss the interaction and evolution between oaks, other organisms and the environment to explore the adaptive strategies and coevolutionary mechanisms among them. Through the impact of this article, hopefully, a distinctive avenue could be established to further study the inheritance, ecology and multidimensional evolution of oaks.

Transcriptome analysis unveils the functional effects of ectomycorrhizal fungal colonization on cadmium tolerance of willow saplings

Introduction

Ectomycorrhizal fungus (ECMF) could enhance plant tolerance to heavy metal toxicity by altering metal accumulation and protecting plants from oxidative injury. However, the molecular mechanisms underlying ECMF-mediated detoxification of cadmium (Cd) in willow sapling are not well known. This study aimed to unveil the roles of Cenococcum geophilum (CG) and Suillus luteus (SL) in regulating Cd toxicity tolerance in willow (Salix psammophila 'Huangpi1') saplings.

Methods

This study systematically evaluated physiological and biochemical parameters in the leaf and root tissues of 18 willow saplings, while concurrently conducting transcriptomic analysis of the roots under Cd stress. The specific treatments were labeled as follows: NF (no ECMF inoculation and no Cd addition), CG (CG colonization only), SL (SL colonization only), NF+Cd (no ECMF inoculation with 100 μM Cd addition), CG+Cd (CG colonization with 100 μM Cd addition), and SL+Cd (SL colonization with 100 μM Cd addition).

Results

The results showed the growth, photosynthesis, antioxidant system and transcriptome of 2-month-old willow saplings responded differently to ECMFs colonization under Cd stress. S. luteus markedly increased the aerial parts biomass, while C. geophilum significantly enhanced the root property indices of willow saplings under Cd stress. The highest number of differentially expressed genes (DEGs) was observed in the comparison between CG+Cd (CG colonization with 100 μM Cd addition) and NF+Cd (no ECMF inoculation with 100 μM Cd addition). C. geophilum colonization activated plant hormone signal transduction and carbohydrate metabolism pathways, while S. luteus enhanced the synthesis of secondary metabolites.

Discussion

This study provides a molecular perspective on the mechanism of interaction between ECMFs and willow saplings under Cd stress and supports the application of ECMFs for phytoremediation of Cd-contaminated soil.

Ectomycorrhizal fungal community varies across broadleaf species and developmental stages

Ectomycorrhizal fungi (EMF) play pivotal roles in determining temperate forest ecosystem processes. We tracked root EMF community succession across saplings, juveniles, and adults of three temperate broadleaf trees (Acer mono, Betula platyphylla, and Quercus mongolica) in Northeast China. Adult stages showed higher alpha diversity but lower community dissimilarity compared to earlier stages. In particular, the EMF alpha diversity of Quercus mongolica marginally increased along with host developmental stages and ranked as sapling < juvenile < adult. Unlike those of Acer mono and Quercus mongolica, the EMF community composition of Betula platyphylla showed greater variation between the sapling and juvenile stages than between the sapling and adult stages. Cooccurrence networks revealed increasing interconnectivity with host maturity, dominated by positive correlations (> 99%). LEfSe was employed to identify stage- and/or host-specific EMF indicators. This study highlighted the assembly of EMF community during the development of broadleaf trees in temperate forests, thereby advancing understanding of the succession and coevolution of symbiotic relationships.

Cenococcum geophilum impedes cadmium toxicity in Pinus massoniana by modulating nitrogen metabolism

Nitrogen (N) is of great significance to the absorption, distribution and detoxification of cadmium (Cd). Ectomycorrhizal fungi (EMF) are able to affect the key processes of plant N uptake to resist Cd stress, while the mechanism is still unclear. Therefore, we explored potential strategies of Cenococcum geophilum (C. geophilum) symbiosis to alleviate Cd stress in Pinus massoniana (P. massoniana) from the perspective of plant N metabolism and soil N transformation. The results showed that inoculation of C. geophilum significantly increased the activities of NR, NiR and GS in the shoots and roots of P. massoniana, thereby promoting the assimilation of NO3- and NH4+ into amino acids. Moreover, C. geophilum promoted soil urease and protease activities, but decreased soil NH4+ content, indicating that C. geophilum might increase plant uptake of soil inorganic N. qRT-PCR results showed that C3 symbiosis significantly up-regulated the expression of genes encoding functions involved in NH4+ uptake (AMT3;1), NO3- uptake (NRT2.1, NRT2.4, NRT2.9), as well as Cd resistance (ABCC1 and ABCC2), meanwhile down-regulated the expression of NRT7.3, Cd transporter genes (HMA2 and NRAMP3) in the roots of P. massoniana seedlings. These results demonstrated that C. geophilum was able to alleviate Cd stress by increasing the absorption and assimilation of inorganic N in plants and inhibiting the transport of Cd from roots to shoots, which provided new insights into how EMF improved host resistance to abiotic stress.

The ectomycorrhizal fungus Paxillus involutus positively modulates Castanea sativa Miller (var. Marsol) responses to heat and drought co-exposure

Castanea sativa Miller, a high-valuable crop for Mediterranean countries, is facing frequent and prolonged periods of heat and drought, severely affecting chestnut production. Aiming to tackle this problem, this study unraveled the influence of mycorrhizal association with the fungi Paxillus involutus (Batsch) on young chestnut plants' responses to combined heat (42 °C; 4 h/day) and drought (no irrigation until soil moisture reached 25%) over 21 days of stress exposure. Heat stress had no harmful effects on growth, photosynthesis, nor induced oxidative stress in either mycorrhizal (MR) or non-mycorrhizal (NMR) chestnut plants. However, drought (alone or combined) reduced the growth of NMR plants, affecting water content, leaf production, and foliar area, while also hampering net CO2 assimilation and carbon relations. The mycorrhizal association, however, mitigated the detrimental effects of both stresses, resulting in less susceptibility and fewer growth limitations in MR chestnut plants, which were capable of ensuring a proper carbon flow. Evaluation of the oxidative metabolism revealed increased lipid peroxidation and hydrogen peroxide levels in NMR plants under water scarcity, supporting their higher susceptibility to stress. Conversely, MR plants activated defense mechanisms by accumulating antioxidant metabolites (ascorbate, proline and glutathione), preventing oxidative damage, especially under the combined stress. Overall, drought was the most detrimental condition for chestnut growth, with heat exacerbating stress susceptibility. Moreover, mycorrhizal association with P. involutus substantially alleviated these effects by improving growth, water relations, photosynthesis, and activating defense mechanisms. Thus, this research highlights mycorrhization's potential to enhance C. sativa resilience against climate change, especially at early developmental stages.

Suillusgrevillei and Suillus luteus promote lead tolerance of Pinus tabulaeformis and biomineralize lead to pyromorphite

Lead (Pb) is a hazardous heavy metal that accumulates in many environments. Phytoremediation of Pb polluted soil is an environmentally friendly method, and a better understanding of mycorrhizal symbiosis under Pb stress can promote its efficiency and application. This study aims to evaluate the impact of two ectomycorrhizal fungi (Suillus grevillei and Suillus luteus) on the performance of Pinus tabulaeformis under Pb stress, and the biomineralization of metallic Pb in vitro. A pot experiment using substrate with 0 and 1,000 mg/kg Pb2+ was conducted to evaluate the growth, photosynthetic pigments, oxidative damage, and Pb accumulation of P. tabulaeformis with or without ectomycorrhizal fungi. In vitro co-cultivation of ectomycorrhizal fungi and Pb shots was used to evaluate Pb biomineralization. The results showed that colonization by the two ectomycorrhizal fungi promoted plant growth, increased the content of photosynthetic pigments, reduced oxidative damage, and caused massive accumulation of Pb in plant roots. The structural characteristics of the Pb secondary minerals formed in the presence of fungi demonstrated significant differences from the minerals formed in the control plates and these minerals were identified as pyromorphite (Pb5(PO4)3Cl). Ectomycorrhizal fungi promoted the performance of P. tabulaeformis under Pb stress and suggested a potential role of mycorrhizal symbiosis in Pb phytoremediation. This observation also represents the first discovery of such Pb biomineralization induced by ectomycorrhizal fungi. Ectomycorrhizal fungi induced Pb biomineralization is also relevant to the phytostabilization and new approaches in the bioremediation of polluted environments.

Metal-non-tolerant ecotypes of ectomycorrhizal fungi can protect plants from cadmium pollution

The application of mycorrhizal fungi as a bioaugmentation technology for phytoremediation of heavy metal (HM) contaminated soil has attracted widespread attention. In order to explore whether the adaptation of Pinus massoniana (P. massoniana) to metal polluted soil depends on the metal adaptation potential of their associated ectomycorrhizal fungi (ECMF), we evaluated the cadmium (Cd) tolerance of 10 ecotypes of Cenococcum geophilum (C. geophilum) through a membership function method, and P. massoniana seedlings were not (NM) or inoculated by Cd non-tolerant type (JaCg144), low-tolerant (JaCg32, JaCg151) and high-tolerant (JaCg205) isolates of C. geophilum were exposed to 0 and 100 mg·kg-1 for 3 months. The result showed that, each ecotype of C. geophilum significantly promoted the growth, photosynthesis and chlorophyll content, proline (Pro) content and the activity of peroxidase (POD) of P. massoniana seedlings, and decreased malonaldehyde (MDA) content and catalase (CAT) and superoxide dismutase (SOD) activity. The comprehensive evaluation D value of the tolerance to Cd stress showed that the order of the displaced Cd resistance of the four ecotypic mycorrhizal P. massoniana was: JaCg144 > JaCg151 > JaCg32 > JaCg205. Pearson correlation analysis showed that the Sig. value of the comprehensive evaluation (D) values of the strains and mycorrhizal seedlings was 0.077 > 0.05, indicating that the Cd tolerance of the the C. geophilum isolates did not affect its regulatory effect on the Cd tolerance of the host plant. JaCg144 and JaCg151 which are non-tolerant and low-tolerant ecotype significantly increased the Cd content in the shoots and roots by about 136.64-181.75% and 153.75-162.35%, indicating that JaCg144 and JaCg151 were able to effectively increase the enrichment of Cd from the soil to the root. Transcriptome results confirmed that C. geophilum increased the P. massoniana tolerance to Cd stress through promoting antioxidant enzyme activity, photosynthesis, and lipid and carbohydrate synthesis metabolism. The present study suggests that mental-non-tolerant ecotypes of ECMF can protect plants from Cd pollution, providing more feasible strategies for ectomycorrhizal-assisted phytoremediation.

Comparative transcriptome analysis of ectomycorrhizal fungus Pisolithus albus in response to individual and combined stress of copper and cadmium

An ectomycorrhizal fungus Pisolithus albus establishes the natural symbiosis with plant roots on extreme heavy metal (HM)-rich soil and enables their survival in toxic metal concentrations. Understanding P. albus key genes and pathways behind strong metal tolerance is crucial for its successful application in the rehabilitation of metal-contaminated barren lands. Therefore, this study aimed to analyze the whole transcriptome profile of P. albus under individual and combined metal stress of copper (Cu) and cadmium (Cd). At 480 µM Cu and 16 µM Cd toxic concentrations, P. albus has shown growth and survival and accumulated high metal (1.46 µg Cu and 1.13 µg Cd per mg of dry mycelia). The study found a stronger response of P. albus to single-metal stress in high concentration as compared to multi-metal stress in relatively lower concentration. Hence, the intensity of fungal response to HM stress is mainly determined by the metal concentration involved in stress. We have found a total of 11 pathways significantly associated with HM stress, among which amino acid, lipid, and carbohydrate metabolisms were highly affected. The functional enrichment of differentially expressed genes has shown the induced biosynthesis of arginine, melanin, metal chelating agents, membrane phospholipids, fatty acids, folate, pantothenate, ergothioneine, and other antioxidant agents; upregulation of zinc ion uptake, potassium transporters, and lysine degradation; and reduction of phosphatidylcholine degradation, incorrect protein folding, iron uptake, and potassium efflux as the top efficient tolerance mechanisms of P. albus against HM stress. The current study would contribute to understanding fungal HM tolerance and its further utilization in the bioremediation of metal-contaminated abandoned lands. The validation of RNA-sequencing analysis with RT-qPCR of selected genes showed the high credibility of the presented data.

Increased Tolerance of Massion's pine to Multiple-Toxic-Metal Stress Mediated by Ectomycorrhizal Fungi

Pinus massoniana (Massion's pine), a pioneer tree species, exhibits restoration potential in polluted mining areas. However, the physiological and molecular mechanisms of ectomycorrhizal (ECM) fungi in Massion's pine adaptability to multiple-toxic-metal stress are still unclear. Hence, Massion's pine seedlings inoculated with two strains of C. geophilum, which were screened and isolated from a polluted mine area, were cultivated in mine soil for 90 days to investigate the roles of EMF in mediating toxic metal tolerance in host plants. The results showed that compared with the non-inoculation control, C. geophilum (CG1 and CG2) significantly promoted the biomass, root morphology, element absorption, photosynthetic characteristics, antioxidant enzyme activities (CAT, POD, and SOD), and proline content of Massion's pine seedlings in mine soil. C. geophilum increased the concentrations of Cr, Cd, Pb, and Mn in the roots of Massion's pine seedlings, with CG1 significantly increasing the concentrations of Pb and Mn by 246% and 162% and CG2 significantly increasing the concentrations of Cr and Pb by 102% and 78%. In contrast, C. geophilum reduced the concentrations of Cr, Cd, Pb, and Mn in the shoots by 14%, 33%, 27%, and 14% on average, respectively. In addition, C. geophilum significantly reduced the transfer factor (TF) of Cr, Cd, Pb, and Mn by 32-58%, 17-26%, 68-75%, and 18-64%, respectively, and the bio-concentration factor (BF) of Cd by 39-71%. Comparative transcriptomic analysis demonstrated that the differently expressed genes (DEGs) were mainly encoding functions involved in "transmembrane transport", "ion transport", "oxidation reduction process", "oxidative phosphorylation", "carbon metabolism", "glycolysis/gluconeogenesis", "photosynthesis", and "biosynthesis of amino acids." These results indicate that C. geophilum is able to mitigate toxic metals stress by promoting nutrient uptake, photosynthesis, and plant growth, thereby modulating the antioxidant system to reduce oxidative stress and reducing the transport and enrichment of toxic metals from the root to the shoot of Massion's pine seedlings.

Boron-resistant Alternaria alternata (OG14) mitigates boron stress by improving physiological and antioxidative response in wheat (Triticum aestivum L.)

Effect of Alternaria alternata (OG14) isolated from a rock lichen (Xanthoria sp.) was investigated on the relief of boron stress in wheat. To determine the tolerance level to B stress, the fungus was grown at increasing boric acid (BA) concentrations in the range of 0.0-2.5 M. No significant change in colony development of the fungus was observed up to 1 M BA application compared to the control but after this dose, it decreased depending on the increase in the BA dose. When the element content of wheat seedlings was evaluated by ICP-MS, BA application increased B content together with Mg, P, K, Fe contents of the seedlings to very high levels compared to the control. However, fungus + BA treatments decreased the content of B and the other elements in the seedlings. The BA applications resulted in an increase in the levels of reactive oxygen species, including H2O2 and O2.-as well as lipid peroxidation in the seedlings. However, when the fungal inoculation was performed under the same BA conditions, the levels of these parameters decreased. The fungus inoculation stimulated the activity of all studied enzymes compared to BA applications. BA applications alone increased non - enyzmatic the oxidized ascorbate level more than the reducing ascorbate, leading to a decrease in the AsA/DHA ratio. The results show that A. alternata treatment can mitigate the negative effects of B stress on wheat seedlings by reducing ROS, LPO, B content, increasing the capacity of enzymatic and non-enzymatic antioxidants, and improving root and shoot length.

Contributions of ectomycorrhizal fungi in a reclaimed poplar forest (Populus yunnanensis) in an abandoned metal mine tailings pond, southwest China

Reclamation using fast-growing trees has great potential for agroforestry development on former non-ferrous metal mining areas. However, the functional traits of ectomycorrhizal fungi (ECMF) and the relationship between ECMF and reclaimed trees remain unknown. Here, the restoration of ECMF and their functions in reclaimed poplar (Populus yunnanensis) growing in a derelict metal mine tailings pond were investigated. We identified ECMF belonging to 15 genera in 8 families, suggesting the occurrence of spontaneous diversification as poplar reclamation progressed. We described a previously unknown ectomycorrhizal relationship between poplar roots and Bovista limosa. Our results showed that B. limosa PY5 alleviated the phytotoxicity of Cd and enhanced poplar heavy metal tolerance, resulting in increased plant growth due to reduced Cd accumulation in host tissues. As part of the improved metal tolerance mechanism, PY5 colonization activated antioxidant systems, enhanced the conversion of Cd into inactive chemical forms, and promoted the compartmentalization of Cd into host cell walls. These results suggest that introducing adaptative ECMF may be an alternative to bioaugmenting reforestation and phytomanagement programs of fast-growing native trees in the barren metal mining and smelting areas.

The bianchetto truffle (Tuber borchii) a lead-resistant ectomycorrhizal fungus increases Quercus cerris phytoremediation potential

Tuber borchii is a European edible truffle which forms ectomycorrhizas with several soft- and hardwood plants. In this article, the effects of high level of Pb on the in vitro growth of five T. borchii strains and the molecular mechanisms involved in Pb tolerance were studied. Moreover, the effects of the Pb treatment on T. borchii ectomycorrhizas and on the growth, element uptake and distribution in different organs of Quercus cerris seedlings were investigated. The results showed an extraordinary tolerance of T. borchii mycelium to Pb: all the tested strains were able to grow at Pb concentration over 4000 mg L^-1 . The mechanisms of tolerance seem related to Pb sequestration in the vacuole and its immobilization as crystal of Pb oxalate outside the hyphae rather than detoxification processes, considering the low expression of glutaredoxin and thioredoxin genes. T. borchii-Q. cerris mycorrhizas tolerate a soil concentration of Pb from 1869 to 4030 mg kg^-1 although, at these Pb concentrations, T. borchii showed a reduced ability to colonize roots. T. borchii mycorrhization increased the uptake of Pb by Q. cerris. Mycorrhization and Pb treatment also significantly influenced the uptake and translocation in the plant of other elements.

Comparative Physiological and Transcriptome Analysis Provide Insights into the Response of Cenococcum geophilum, an Ectomycorrhizal Fungus to Cadmium Stress

Cadmium (Cd) displays strong toxicity, high mobility, and cannot be degraded, which poses a serious threat to the environment. Cenococcum geophilum (C. geophilum) is one of the most common ectomycorrhizal fungi (ECMF) in the natural environment. In this study, three Cd sensitive and three Cd tolerant strains of C. geophilum were used to analyze the physiological and molecular responses to Cd exposure. The results showed that Cd inhibited the growth of all strains of C. geophilum but had a less toxic effect on the tolerant strains, which may be correlated to a lower content of Cd and higher activity of antioxidant enzymes in the mycelia of tolerant strains. Comparative transcriptomic analysis was used to identify differentially expressed genes (DEGs) of four selected C. geophilum strains after 2 mg/L Cd treatment. The results showed that the defense response of C. geophilum strain to Cd may be closely related to the differential expression of functional genes involved in cell membrane ion transport, macromolecular compound metabolism, and redox pathways. The results were further confirmed by RT-qPCR analysis. Collectively, this study provides useful information for elucidation of the Cd tolerance mechanism of ECMF.

Role of Ectomycorrhizal Symbiosis Behind the Host Plants Ameliorated Tolerance Against Heavy Metal Stress

Soil heavy metal (HM) pollution, which arises from natural and anthropogenic sources, is a prime threat to the environment due to its accumulative property and non-biodegradability. Ectomycorrhizal (ECM) symbiosis is highly efficient in conferring enhanced metal tolerance to their host plants, enabling their regeneration on metal-contaminated lands for bioremediation programs. Numerous reports are available regarding ECM fungal potential to colonize metal-contaminated lands and various defense mechanisms of ECM fungi and plants against HM stress separately. To utilize ECM–plant symbiosis successfully for bioremediation of metal-contaminated lands, understanding the fundamental regulatory mechanisms through which ECM symbiosis develops an enhanced metal tolerance in their host plants has prime importance. As this field is highly understudied, the present review emphasizes how plant’s various defense systems and their nutrient dynamics with soil are affected by ECM fungal symbiosis under metal stress, ultimately leading to their host plants ameliorated tolerance and growth. Overall, we conclude that ECM symbiosis improves the plant growth and tolerance against metal stress by (i) preventing their roots direct exposure to toxic soil HMs, (ii) improving plant antioxidant activity and intracellular metal sequestration potential, and (iii) altering plant nutrient uptake from the soil in such a way to enhance their tolerance against metal stress. In some cases, ECM symbiosis promotes HM accumulation in metal stressed plants simultaneous to improved growth under the HM dilution effect.