Diversity and Function of Mycorrhizal Fungi

Abstract

With the progress of science and technology and the deepening of people’s understanding of mycorrhizal fungi, the diversity and function of mycorrhizal fungi have attracted attention of scholars, and the research on mycorrhizal application technology has been strengthened. In order to grasp the latest progress and current situation of mycorrhizal fungi diversity research, and clarify the achievements in the research and application fields of mycorrhizal fungi diversity and function, this study summarizes the latest research progress of mycorrhizal fungi diversity and function, which are discussed. The morphological characteristics, anatomical characteristics, host plant species and mycorrhizal fungal types, the species and genetic diversity of arbuscular mycorrhizal fungi, the origin of arbuscular mycorrhizal fungi, and the influencing factors of arbuscular mycorrhizal fungal diversity are discussed. A lot of work has been done on the species, geographical distribution, ecological characteristics, and resource investigation of ectomycorrhizal fungi. More and more ECM fungal resources have been detected and identified. The ecological function of mycorrhizal fungi is manifested in the aspects of plant community and plant ecosystem stability by improving ecosystem productivity. Mycorrhizal fungi can form symbionts with plants, enter the food web as food, and affect terrestrial ecosystems.

1. Introduction

Mycorrhiza is a symbiont formed by plant roots and some fungi in soil. It is widely found in terrestrial ecosystems, including forests, fields, and grasslands. Mycorrhiza plays an important role in the ecosystem [1, 2, 3]. More than 80% of terrestrial ecosystems can form mycorrhiza with mycorrhizal fungi. This symbiosis is beneficial to both host plants and fungi. According to the morphological characteristics, anatomical characteristics, host plant species, and mycorrhizal fungi types, mycorrhizal fungi can be divided into arbuscular mycorrhizal (AM) and ectomycorrhiza (ECM). Ectoendomycorrhiza, orchid mycorrhiza, ericoid mycorrhiza, monotropoid mycorrhiza, and arbutoid mycorrhiza. Among them, ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) are the most diverse and widely distributed mycorrhizal types.

As early as 1885, the German botanist and plant physiologist B. Frank first discovered mycorrhiza, that is, the symbiosis of special fungi (mycorrhizal fungi) in the soil and plant roots [4], which opened a series of studies on the structure, type, formation mechanism, physiological and ecological functions of mycorrhizal fungi, and mycorrhizal biotechnology, forming an independent discipline-mycorrhizology [5, 6].

2. Diversity of mycorrhizal fungi

According to different host types and symbiotic characteristics, mycorrhizal fungi are divided into seven categories [5]: ectomycorrhizal (ectomycorrhizal; eCM), arbuscular mycorrhiza (arbuscular mycorrhiza; aM), ectoendomycorrhizas (ectoendomycorrhizas; eEM), orchid mycorrhiza (orchid mycorrhiza; oRM), arbutoid mycorrhiza (arbutoid mycorrhiza; aRM), ericoid mycorrhiza (ericoid mycorrhiza; eRM), and monotropoid mycorrhiza; mM) seven categories. According to the criteria of root tissue morphological differentiation and host plant lineage, mycorrhiza is divided into four main types: arbuscular mycorrhiza (arbuscular mycorrhiza; aM), ectomycorrhizal (ectomycorrhizal; eCM), ericoid mycorrhiza (ericoid mycorrhiza; eRM), and orchid mycorrhiza (orchid mycorrhiza; oRM). In mycorrhizal symbionts, mycorrhizal fungi absorb water and mineral elements in the soil through mycelia and provide them to host plants. At the same time, host plants provide carbohydrates to mycorrhizal fungi in return. In addition, the functions of mycorrhiza in the ecosystem also include affecting the construction of aboveground plant communities, regulating soil carbon sequestration, nutrient cycling, ecosystem stability, and so on [7, 8, 9, 10, 11]. Mycorrhiza is widely distributed in the terrestrial environment. Under natural conditions, more than 80% (92% of the family level) of terrestrial plant roots can be infected by mycorrhizal fungi to form mycorrhiza [12]. Although most plants can form mycorrhizae, the distribution of mycorrhizae is not uniform. Ectomycorrhizae (2% of the total), arbuscular mycorrhizae (72%), and orchid mycorrhizae (10%) are the main mycorrhizae.

2.1 Diversity of arbuscular mycorrhizal fungi

Arbuscular mycorrhiza (AM) is the earliest and most widely distributed and is known as the mother of plant root symbionts. Therefore, it has attracted extensive attention from mycorrhizal scholars. Most (about 80%) terrestrial plant species can form this mycorrhiza [13]. Arbuscular mycorrhizal fungi (AM fungi), which can infect plant roots to form AM, are obligate nutritional fungi, and the carbohydrates required for their growth and development are all derived from host plants [6]. The structure of AM fungi is composed of two parts: inner root and outer root. The intraradical hyphae, vesicles, and arbuscules formed by AM fungi in the root cortex are located in the root cortex. The structures such as dendric or cauliflower-like branches belong to the root structure of AM fungi, while the extraradical hyphae and spores located in the root and soil together constitute its extra root structure.

2.1.1 Arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) is a symbiont formed by arbuscular mycorrhizal fungi (arbuscular mycorrhizas fungi, AMF) and plant roots. In addition to individual families such as Juncaceae, Caryophyllaceae, and Brassicaceae, which cannot or are not easy to form AM symbionts, most plants can be infected by AM fungi [6]. AM is also known as endomycorrhizas because it forms a symbiotic structure in the cortical tissue of plant roots. Its structural components include plant roots, soil hyphae outside the root, spores, auxiliary body, and intracellular and intercellular hyphae, vesicles, and arbuscules inside the root. AM was once known as vesicle-arbuscular mycorrhizas (VAM), but later studies found that only about 80% of AM fungi form vesicular structures, so the term VAM was discarded [14].

The species classification of AM fungi has always been an important aspect of AM fungal diversity research. By 2015, 254 AM fungal morphological species had been recorded worldwide (http://schuessler.userweb.mwn.de/amphylo/; statistics dated February 2015), which can be divided into 4 orders, 11 families, and 17 genera. Among them, there were 121 species of Glomeraceae, 40 species of Acaulosporaceae, 11 species of Diversisporaceae, 9 species of Ambisporaceae, 7 species of Pacisporaceae, 6 species of Claroideoglomeraceae, 5 species of Gigasporaceae, 4 species of Sacculosporaceae, 3 species of Paraglomeraceae, 2 species of Archaeosporaceae, and 1 species of Geosiphonaceae. At the same time, great progress has been made in the study of AM fungal species diversity in China. According to incomplete statistics, 113 species of AM fungi belonging to 7 genera have been isolated in China, including 22 species of Aeaulopora, 3 species of Entrophospora, 3 species of Archaeospora, 65 species of Glomus, 1 species of Paraglomus, 4 species of Gigaspora, and 15 species of Scutellospora [15].

Since 1974, the number of researchers on AM fungi has gradually increased. The corresponding researchers have described the morphological characteristics of some AM fungi and established the family Endogone. AM fungi are classified as Endogone [16]. In 1875, Berkeley and Broome [17] established the genus Sclerocystis. In 1889, the genus Sclerocystis and the genus Endocystis were placed under the family Endocystis established by Paoletti in 1889. In 1922, Bucholtz [18] indicated that the family Endogonaceae belongs to the order Mucormycota. In 1974, Gerdeman and Trappe [19] newly described Acaulospora and Megasporangium and reclassified Endomycaceae into seven genera: Glomus, Endogone, Modicella, Gigaspora, Acaulospora, Glaziella, and Sclerocystis. In 1979, Benjimin moved the family Enterobacteriaceae from Mucor to Endogonales [20]. The biggest change in the systematic classification of AM fungi was in 1999–2000, Morton and Benny [21] established Glomals, which consists of 2 suborders, 3 families, and 6 genera. Almeida and Schenck [22] transferred all the strains except Sclerocystis coremioides into Glomus. In 2000, Redecker [23] transferred them into Glomus and abolished Sclerocystis. Schublerr [24] analyzed the 18S r DNA sequence of AM fungi in 2002 and removed AM fungi from Zygomycota and reestablished Glomeromycot. The reason why the classification system of AM fungi changes so frequently is the limitation of morphological identification. The same species in different countries and regions have different life histories, and the results of morphological structure in different environments are quite different. It is this instability that promotes the direction of systematic classification gradually tends toward molecular methods. Oehl et al. [25] divided AM fungi into 1 class, 5 orders, 14 families, and 26 genera by combining modern molecular biology techniques and traditional morphological identification methods. So far, less than 250 species of AM fungi have been described, and only 341 species of AM fungi have been obtained by environmental DNA analysis. Such a small amount of AM fungi can form a symbiotic relationship with more than 200,000 plants on the earth, which means that AM fungi do not have strict host specificity [26]. Therefore, the classification of AM fungi is still under development, and the classification of taxonomy is inconclusive.

AMF belongs to Glomeromycota in the classification system. According to the latest classification system of AMF [27], there are 1 class (Glomeromycetes), 4 orders (Glomerales, Diversisporales, Paraglomerales, Archaeosporales), 11 families (Glomeraceae, Claroideoglomeraceae, Gigasporaceae, Acaulosporaceae, Pacisporaceae, Diversisporaceae, Sacculosporaceae, Paraglomeraceae, Geosiphonaceae, Ambisporaceae, Archaeosporaceae), 29 genera (Dominikia, Funneliformis, Glomus, Kamienskia, Rhizophagus, Sclerocystis, Septoglomus, Claroideoglomus, Bulbospora, Cetraspora, Dentiscutata, Gigaspora, Intraornatospora, Paradentiscutata, Racocetra, Scutellospora, Acaulospora, Pacis-pora, Corymbiglomus, Diversispora, Otospora, Redeckera, Tricispora, Sacculospora, Paraglomus, Geosiphon, Ambispora, Archaeospora, Entrophospora), and about 300 species.

2.1.2 Inheritance of arbuscular mycorrhizal fungi

The sum of the genetic information carried by arbuscular mycorrhizal fungi is the genetic diversity of AM fungi. It was found that there were a large number of genetic variations among AM fungal populations and within populations, even within a single bosom. This shows that AM fungi have rich genetic diversity [28]. The generation of genetic diversity is based on changes in DNA genetic material, mainly reflected in gene polymorphism [29, 30]. Sanders et al. first reported the genetic information of Glomus fungi: DNA sequence polymorphism [31]; Lloyd-Macgilp et al. [32] studied and analyzed the ITS gene sequences of rDNA among five Glomus mosseae strains with different geographical origins, and found that there were sequence polymorphisms. Redecker et al. [23] studied the molecular system of AM fungi by PCR amplification of the 18S rRNA gene of G.spinuosum and Scleroystis coremioides and RFLP analysis of the products by restriction endonuclease digestion. Developmental status; Antoniolli et al. [33] amplified the ITS region of G.mosseae and Gi.margarita isolated from pasture after a single culture, and found that there were differences between the ITS regions of the two strains, and the degree of genetic variation of G.mosseae was greater than that of Gi.margarita. Pawlowska and Taylor [34] found that there were three kinds of ITS region variations in each nucleus of G.etunicatum. The genetic variation of AM fungi exists not only in ribosomal genes but also in the gene coding region, which is universal [34]. Kuhn et al. [35] found that the highly conserved binding protein gene (BiP) in fungi was mutated in the single offspring of G.intraradices; Corradi et al. [30] found that there were differences in the number of copies of the gene.

Changes in genetic materials can cause changes in AM fungal metabolism and differences in morphological characteristics. Bever and Morton [36] showed that nuclear migration and hyphal fusion occurred during the division of the mycelium of Sc.pellucida, resulting in nuclear recombination, which further led to a variety of differences in the single culture and the shape and size of the five single-saturated offspring were not the same. Koch et al. [37] reported that the AFLP amplification bands of G.intraradices populations were quite different under the same environmental conditions. These genetic variations have led to differences in physiological and symbiotic functions within AM fungal populations, such as differences in the length of AM fungal hyphae that further translate into differences in nutrient uptake by symbiotic plants [38].

At present, there are two hypotheses about the genetic variation of AM fungi. Kuhn et al. [35] reported that the genetic variation of AM fungi is likely to be due to the fusion of hyphae in the asexual reproduction of AM fungi, resulting in the formation of new hyphae and spores containing nuclei with different genetic components, and the frequency of intranuclear gene recombination is not high, that is, the genetic variation of AM fungi is distributed in different nuclei. However, Pawlowska and Taylor [34] believed that the abundant genetic variation in AM fungi was due to ploidy, that is, all the variations were contained in the mononuclear cells of AM fungi. Therefore, the genetic structure of AM fungi needs to be further studied. These rich genetic diversity should be considered in the study of physiological and ecological molecular biology of AM fungi, because they may be converted into functional diversity [28]. At the same time, the genetic diversity of AM fungi laid the foundation for the application of molecular biology analysis technology in the classification and identification of AM fungi and made the classification of AM fungi more scientific and perfect.

The diversity of AM fungi is the result of the combined action of various factors. Host plant types, plant compounds, and environmental factors can affect the diversity of AM fungi. Tawaraya et al. [39] confirmed that the host plant type of AM fungi is one of the important factors affecting the distribution of AM fungal species. The composition and infection rate of AM fungal community vary with different host plants. Some scholars believe that the influence of plant species on the diversity of AM fungal community is mainly due to the effect of plant root metabolites. Alkaloids and flavonoid metabolites synthesized by plants will affect the germination of AM fungal spores. The research directions of AM fungal diversity at home and abroad have different emphases. Most of the articles focus on soil physical and chemical properties and AM fungal diversity. Zhang Haibo [40] RDA analysis showed that plant species had a certain effect on the difference of AM fungal community structure. The composition of AM fungal community in different plant rhizospheres was different, indicating that AM fungi had specific selection for host plants. Yang Xiuli et al. [41] found that the distribution of AM fungal species richness in different seasons in the same forest type was regular, and the highest species richness was in August. The distribution of AM fungal spore density in different seasons also showed a certain regularity, and the highest spore density was in September. Combined with the research on the diversity of AM fungi in different forest types, it was found that the composition of AM fungi in different forest types was different, the spore density was different, and the dominant species were different. AM fungi and forests have undergone a long period of evolution, and the entire ecosystem has now entered a complete and orderly positive feedback virtuous cycle. The use of AM fungi to promote plant colonization growth and vegetation restoration should consider its species preference. Therefore, studying the relationship between AM fungi and forest types is helpful in protecting AM fungi resources and forest resources.

2.1.3 Origin of arbuscular mycorrhizal fungi

Based on existing fossil evidence, AM symbionts originated in the Ordovician (Ordovician) at least 460 million years ago and play an important role in the early plant login process and subsequent evolution of terrestrial plants [12, 42, 43]. Plants face many challenges in the early stage of landing, such as arid terrestrial conditions, nutrient-poor soil, and strong ultraviolet radiation. With the help of AM fungi, plants can effectively absorb soil mineral nutrients and have the ability to resist adverse environmental conditions [6]. Therefore, the emergence of AM fungi is considered to be an important prerequisite for the successful early landing of plants [43]. Although this assertion can be corroborated by recent research, the study found that the earliest origin of a class of plant land money (liverwort; it originated about 430 million years ago) and can be infected by AM fungi, which significantly enhances the nitrogen and phosphorus absorption capacity of Marchantia polymorpha, and increases its growth rate and fitness [44]. However, how was the early plant-AM fungal mutualism established? This remains to be further explored and scientifically answered.

AM fungi is an ancient asexual reproduction of the biological groups [45], in its long-term evolution process, multinuclear genome slowly emerged, that is, there are multiple nuclei appear at the same time in its hyphae and asexual spores, a single spore can appear up to tens of thousands of nuclei [35, 46]. In the process of reproduction of AM fungi, many nuclei can be transferred to the mycelium or spores of the offspring, so that AM fungi have extremely high individual genetic diversity, and often have high genetic diversity in terms of individual spores [31, 34, 47]. It is precisely because of its asexual reproduction and multinuclear characteristics, although the species differentiation rate of AM fungi is slow, it also suggests that it may have a faster functional differentiation rate, and the intraspecific functional diversity of AM fungi should be high [48, 49, 50]. At the same time, the existence of multinuclear genomes is bound to bring greater resistance to the molecular identification of AM fungal species that are currently widely used [31]. High genetic variability to some extent hinders us from identifying AM fungal diversity through DNA sequences. Although the characteristics of asexual reproduction of AM fungi have been widely recognized, new research evidence shows that a large number of specific genes related to sexual reproduction are present in AM fungi [51, 52]. Although these sexual reproduction-related genes cannot prove that sexual reproduction exists in AM fungi, they provide new ideas and directions for future research, which will help us fully understand AM fungi and understand their genetic and evolutionary processes [53]. In the recently completed genome sequencing of the representative species of AM fungi (Rhizophagus irregularis), 28,232 genes were found [54]. Further annotation and analysis of these genes open the door for future research on the genetics, evolution, symbiotic mechanism, and physiological function of AM fungi.

2.1.4 Influencing factors of arbuscular mycorrhizal fungal diversity

Because of the mutual selection relationship between AM fungi and host plants, and the strong adaptability of AM fungi to different ecological environments, the diversity of AM fungi shows obvious differences in different ecosystems [26, 55, 56] and is affected by many factors at the same time.

2.1.4.1 Host plants

Because AM fungi rely on obtaining nutrients from host plants to maintain their own survival and reproduction, the structure and diversity of AM fungal communities are bound to be affected by host plant communities [57]. A large number of studies have shown that the community structure of host plants will affect the structure, function, and diversity of AM fungal community in root-soil to a certain extent [58, 59]. Hausmann et al. found that the nutritional needs of host plants at different stages of growth will affect the AM fungal community structure in their roots [60].

2.1.4.2 Soil factors

AM fungi exist in the root tissue and rhizosphere soil of host plants, so the salinity, pH, and nutrient elements of soil will affect the structure and diversity of AM fungal community. At the same time, AM fungi will also change the physical and chemical factors of rhizosphere soil [60]. Soil nutrients can affect the colonization, growth, distribution, and spore production of AM fungi. Among them, phosphorus is the most closely related to AM fungi in all soil nutrients. Studies have shown that when the content of available phosphorus in rhizosphere soil is low, that is, under the condition of phosphorus as a limiting condition, the addition of phosphorus fertilizer will appropriately promote the reproduction of AM fungi. However, when enough phosphorus fertilizer is added and the phosphorus limiting conditions of the soil are lifted, the competition of the aboveground part of the plant will be intensified, resulting in a decrease in the carbon content transported by the plant to the underground roots and mycorrhiza, thus inhibiting the spore production and colonization of AM fungi. In turn, it affects the structure and diversity of AM fungal communities [61, 62]. Similar to phosphorus, soil nitrogen also has an important effect on the community structure and diversity of AM fungi [63]. The simultaneous changes of nitrogen and phosphorus will change the distribution of carbon content in the aboveground and underground parts of the plant, which will reduce the diversity of AM fungal communities [64, 65].

2.1.4.3 Climatic factors

Climatic factors mainly affect the growth and reproduction of AM fungal community through temperature, light, and water conditions. Studies have shown that soil temperature can directly affect the ability of AM fungal community to infect host plants. When the ambient temperature is low, AM fungi will produce more vesicle structure. However, when the temperature increases, the colonization rate and mycelial structure of AM fungi increase significantly [66].

2.1.4.4 Geographical factors

There are significant differences in the structure and diversity of AM fungal communities in different environmental soils, mainly due to changes in altitude, elevation, and latitude. Geographical factors mainly affect AM fungi by indirectly affecting other environmental factors such as light, precipitation, and temperature. Cai Xiaobu et al. [67] found that the diversity and spore density of AM fungi increased with the increase of altitude in the Qinghai-Tibet Plateau at an altitude of 3700–5220 m.

2.2 Diversity of ectomycorrhizal fungi

ECM is an ectomycorrhizal fungus that forms mycorrhizal symbionts with the roots of host plants. The hyphae of ECM fungi will wrap the surface of the host root tip to form a mantle, but their hyphae do not penetrate the interior of the plant cells, but only extend and grow between the cells to form a Hartig net. Emanating hyphae are sparse or dense, with special mycorrhizal symbiotic structures such as fungal mantle, Hartig net, and epitaxial hyphae, which are called ectomycorrhizal fungi [68, 69]. These symbiotic fungi are called ectomycorrhizal fungi (EcMF). This symbiotic relationship of ECM fungi in plant roots has been widely studied. According to statistics, there are more than 20,000 fungi in the world, which can form ECM symbionts with 6200 plants [70]. This symbiotic relationship has a wide impact on forest ecosystems. Fagaceae [71] (Fagaceae), Pinaceae [72] (Pinaceae), Betulaceae [73] (Betulaceae), Myrtaceae [74] (Myrtaceae), Juglandaceae [75] (Juglandaceae), Salicaceae [76] (Salicaceae), and other dominant tree species in forest ecosystems have important economic and ecological values. The symbiotic relationship between these plants and ECM fungi is one of the key factors for their normal growth, production, and reproduction [77].

The formation of ectomycorrhizal (ECM) usually includes four stages: contact (recognition), invasion, expansion, and emergence. These stages are usually completed in a short time, and there is no obvious boundary between them [78]. In general, the formation of ECM is initiated by the proliferation of fungi on the root surface caused by the root exudates of the host plant. The mechanism of fungal invasion into plant tissues involves a variety of complex biological processes, including the stacking and interweaving of hyphae, as well as the expansion and enclosure of hyphal networks [79]. After the formation of the mantle, the fungi continue to expand inward, through the root epidermis into the cortex tissue and grow rapidly, forming a Hastelloy network, which marks the completion of the invasion stage. In this process, fungi inhibit the defense response of host plants by secreting effector proteins [80]. It was found that there was signal communication in the symbiosis process between plants and ECM fungi. Plants transmit signals by secreting flavonoids [81, 82], while ECM fungi transmit signals by secreting auxin [83, 84]. Auxin can enter root cells, induce plant auxin signaling pathway, inhibit root growth, and induce lateral root formation. In addition, symbiotic fungi can release sesquiterpenes to promote plant lateral root formation, which is independent of the host plant auxin signaling pathway. Therefore, the mechanism of ECM formation is very complex, involving the interaction of multiple biological processes and signaling pathways [85].

Researchers have conducted extensive research on ECM, mainly involving the morphology of ECM, the promoting effect of ECM fungi on the growth of host plants, the diversity of ECM fungal fruiting bodies, and taxonomy. At present, it is known that about 280 genera [86, 87, 88] more than 20,000 species [77] of fungi can form ECM with 250–300 genera and 6000–7000 species of plants [89], mainly distributed in Ascomycota (Turberaceae, Gloniaceae, Helvellaceae, and Pezizaceae). Russulaceae, Cantharellaceae, Thelephoraceae, Atheliaceae, Pisolithaceae, Sclerodermataceae, Cortinariaceae, Clavariaceae, Boletaceae, Inocybaceae, Sebacinaceae, Amanitaceae, and Hymenogastraceae of Basidiomycota [90].

2.2.1 Diversity of ectomycorrhizal fungi in China

Chinese scholars have made great progress in the investigation of ectomycorrhizal fungi resources. The species, geographical distribution, and ecological characteristics of ectomycorrhizal fungi in the main afforestation tree species and important timber tree species in various forest areas have been basically clarified [91]. According to incomplete statistics, nearly 600 species of ectomycorrhizal fungi have been reported in China, belonging to 2 subphyla, Ascomycotina and Basidiomycotina, 28 families, and 63 genera. Chinese scholars have carried out many investigations on pine species, mainly including Pinus yunnanensis, Pinus massoniana, Pinus densiflora, Pinus sylvestris var.mongolia, Pinus taiwanensis, and Pinus koraiensis. More than 184 species of ectomycorrhizal fungi have been reported. It belongs to 20 families and 41 genera. Among them, Russula, Inocybe, Tomentella, Cortinarius, Amanita, Boletus, and Pisolithus are common fungi. In addition, some fungi also form ectomycorrhiza with coniferous forest types, such as larch (Larix) and spruce (Picea) and some broad-leaved forest types, such as Quercus, Eucalyptus, and Populus. Wang et al. [92] used the method of field ecology to investigate the ectomycorrhizal fungi of important afforestation tree species such as Pinus tabulaeformis, Pinus massoniana, and Pinus sylvestris in Northeast China, Central China, South China, and East China. A total of 19 species of ectomycorrhizal fungi were found.

In Northeast China, Zhao et al. [93] identified 51 species of ectomycorrhizal fungi of Russulaceae in the Changbai Mountain area through specimen collection and investigation. Meng Fengrong and Shao Jingwen [91] investigated the ectomycorrhizal fungi of 12 main coniferous forest types in the main forest areas of Changbai Mountains, Xiaoxing ‘an Mountains, and Daxing ‘an Mountains by means of field investigation and specimen collection and identification. There were 163 species of ectomycorrhizal fungi belonging to 43 genera and 19 families. Wang Shuqing and Xu Lihua [94] classified and identified 76 species of ectomycorrhizal fungi associated with larch, Pinus sylvestris var.mongolica, Pinus koraiensis, Pinus tabulaeformis, poplar, and other major timber species in Northeast China through field investigation and indoor microscopic examination. Tu Liguer et al. [95] obtained 100 species of ectomycorrhizal fungi in five typical vegetation communities of broad-leaved Korean pine mixed forest in Changbai Mountain by sampling survey method and morphological classification method. Wang [96] found that there were 16 species of underground ectomycorrhizal fungi of Quercus mongolica in Bingla Mountain of Liaoning Province by combining morphological and molecular biological methods. Song Fuqiang et al. [97] preliminarily identified 21 species of ectomycorrhizal fungi associated with Pinus koraiensis in Xiaoxing ‘an Mountains, belonging to 5 families and 8 genera. Lang Qinglong et al. [98] investigated the ectomycorrhizal fungal resources of oak trees in Fengcheng area of Liaoning Province after more than 100 field investigations and specimen identification, and initially identified 35 species in this area. Wang Hui et al. [99] determined that there were 36 species of ectomycorrhizal fungi belonging to 7 families and 13 genera in Dandong area of Liaoning Province by fruiting body investigation and specimen identification. Among them, 29 species of ectomycorrhizal fungi were associated with Quercus mongolica. Cui and Mu [100] studied the resources of ectomycorrhizal fungi in the typical distribution area of Tilia amurensis in Heilongjiang Province by morphological and anatomical characteristics, and preliminarily divided them into 18 types.

In Northwest China, Tang et al. used the investigation and identification of fruiting bodies, observation of external morphology and anatomical structure to investigate and study 35 species of poplar in Shaanxi Province, and identified 9 species of ectomycorrhizal fungi [101]. Tian Maolin [102] investigated and identified 63 species of Russulaceae ectomycorrhizal fungi in Gansu Province. Wu et al. reported 78 species of ectomycorrhizal fungi in Shaanxi Province through field investigation and specimen collection [103]. Wu Chonghua et al. found that 28 species of ectomycorrhizal fungi were obtained by investigating the ectomycorrhizal fungi of the main tree species such as Abies fargesii and Larix chinensis in Taibai Mountain Nature Reserve [104]. Pu Xun reported that there were 77 species of ectomycorrhizal fungi symbiotic with trees in Longnan area of Gansu Province, belonging to 14 families and 28 genera [105]. Xun [106] used plant taxonomy and ecological methods to identify 37 species of ectomycorrhizal fungi in Helan Mountain, Ningxia [106]. Zhang Ruqin et al. [107] studied the ectomycorrhizal fungi under pine-oak mixed forest, larch forest, Pinus tabulaeformisforest, and Pinus armandii forest in Huoditang forest area of Qinling Mountains by field investigation, specimen collection, isolation, and identification. A total of 29 species, 16 genera, and 10 families [107] were identified. Qi et al. reported 28 species of ectomycorrhizal fungi in the middle part of Qinling Mountains (Shaanxi section of Qinling Mountains) by means of field investigation, specimen collection, culture, and isolation [108]. Ding et al. found that there were seven species of ectomycorrhizal fungi in Zibai Mountain Nature Reserve of Qinling Mountains by collecting specimens, morphological structure, and microscopic observation [109]. Through field investigation, specimen collection, and molecular biology, Guli Ahmati et al. [110] conducted a comprehensive investigation and study on ectomycorrhizal fungi in the Habahe plain area of Altay, Xinjiang, and identified 14 genera (21 species) of ectomycorrhizal fungi [110].

In Central China, Chen Zuohong et al. investigated the ectomycorrhizal fungi of Amanita in Mangshan Nature Reserve of Hunan Province by special investigation and collecting a large number of specimens. A total of 25 species have been identified [111]; Li Jianzong discovered two new records of ectomycorrhizal fungi (Amanita) in the mixed forest of Pinus massoniana and broad-leaved trees in Changsha, Hunan Province, and made specimens [112]; Cui et al. collected specimens and identified 31 species of ectomycorrhizal fungi of Russula in Henan mountainous area [113]. Zhang Linping et al. found 28 species of ectomycorrhizal fungi in Jiangxi Tongboshan Nature Reserve by macroscopic characteristics, microscopic observation, and specimen identification [114]. Li Jianzong et al. identified 25 species of ectomycorrhizal fungi in Shunhuangshan Nature Reserve by investigating and collecting specimens [115]. Li Jianzong et al. [116] found that there are 39 species of fungi that form ectomycorrhizal fungi with trees in Liubuxi Nature Reserve of Hunan Province by investigating and collecting specimens. Huang et al. [117] used morphological and molecular biological identification methods to investigate the ectomycorrhizal fungi of Pinus massoniana in lead-zinc mining areas in Central China and found that there were 47 species of ectomycorrhizal fungi symbiotic with Pinus massoniana [117].

In North China, Qin et al. [118] investigated Castanea mollissima in Huairou County, Yanshan Mountains, Beijing by collecting fruiting bodies, and identified 13 genera and 29 species of ectomycorrhizal fungi symbiotic with Castanea mollissima [118]. Cai Huaifu et al. collected specimens and indoor identification, and reported 7 species of ectomycorrhizal fungi collected in some areas of Beijing Songshan Nature Reserve [119].; Tian et al. identified 38 species of ectomycorrhizal fungi of Russulaceae in Chifeng, Inner Mongolia [120]. Fan Yongjun et al. used morphological anatomy and molecular biology methods to study the ectomycorrhizal fungi of Betula platyphylla in Inner Mongolia, and identified 13 species of ectomycorrhizal fungi associated with it [121].; Bai Shulan et al. identified 78 species (15 families, 31 genera) of ectomycorrhizal fungi in Daqing Mountain and 79 species (15 families, 32 genera) in Manhan Mountain by outdoor investigation and indoor observation [122]; Fan et al. studied and investigated Qinghai spruce in Helan Mountain area of Inner Mongolia from the perspective of morphology, anatomy, and molecular biology, and identified 11 different types of ectomycorrhizal fungi [123]. Bai et al. collected fruiting bodies for morphological and anatomical observation and initially believed that there were 163 species of ectomycorrhizal fungi distributed in Daqing Mountain, Inner Mongolia [124].

In South China, Gong Mingqin and Chen Yu [125] obtained 11 species of ectomycorrhizal fungi from Pinus and Anacardium in South China by combining outdoor investigation with indoor observation [125]. Rao Benqiang studied the ectomycorrhizal fungi of Pinus massoniana in Wuyishan Nature Reserve by macroscopic, microscopic, and ultrastructural methods. A total of 78 species of ectomycorrhizal fungi associated with Pinus massoniana were observed [126]. Qian et al. collected a total of 123 species of ectomycorrhizal fungi (84 species were identified) after 6 years of investigation on the plant community of Tsuga chinensis in Wuyishan Nature Reserve through macroscopic morphological observation, microscopic and ultrastructural observation [127].

In East China, Li Haibo et al. investigated the resources of ectomycorrhizal fungi in Lishui, Zhejiang Province through field investigation, collection, isolation, and culture, and identified 38 species of ectomycorrhizal fungi [128]. Ke Lixia and Liu Birong investigated the resources of ectomycorrhizal fungi under pine forests in Huangshan area by means of field ecological investigation, indoor observation, and microscopic examination, and identified 43 species of ectomycorrhizal fungi [129]. Chen Tianxing and Chen Shuanglin obtained 49 species of ectomycorrhizal fungi in Purple Mountain of Nanjing by collecting specimens and morphological taxonomy [130].

In Southwest China, Bi Guochang et al. investigated the ectomycorrhizal fungi in alpine coniferous forests in Northwest Yunnan by field investigation and collection of fruiting body specimens and identified more than 140 species of ectomycorrhizal fungi belonging to 3 genera [131]. He Shaochang has collected and identified 138 species of ectomycorrhizal fungi in Guizhou trees [132]. Tan Fanghe and Wang Yunzhang [133] investigated the resources of ectomycorrhizal fungi under pine forests in Sichuan Province by investigating fungal fruiting bodies, collecting specimens, indoor observation, and microscopic examination, and identified 9 families, 18 genera, and 50 species [133]. Yu Fuqiang et al. [134] collected fruiting body specimens and observed microscopic characteristics, and investigated the ectomycorrhizal fungi under the main Pinus yunnanensis forests in Yunnan Province. In total, 211 species of ectomycorrhizal fungi belonging to 39 genera and 27 families were identified.

Zhu Tianhui et al. conducted a systematic investigation and study on the species of ectomycorrhizal fungi of Eucalyptus in Sichuan Province through field investigation and indoor identification. It was found that there were 17 types of ectomycorrhizal fungi associated with Eucalyptus, belonging to 9 families and 11 genera [135]; Liu Xiaojiao et al. collected specimens and consulted relevant literature, and sorted out 23 species of ectomycorrhizal fungi of Russulaceae in Sejila Mountain, Tibet [136]; Zhou Guanglin et al. collected specimens and indoor identification, and found that there are 37 species of ectomycorrhizal fungi in Chongqing Jinyun Mountain National Nature Reserve [137]; Luo et al. found that there were 68 species of ectomycorrhizal fungi in Pinus massoniana in Kaili City, Guizhou Province through field investigation [138].

2.2.2 Taxonomy of ectomycorrhizal fungi

At first, scientists mainly identified the species of ECM fungi by morphology and studied their diversity. In recent years, molecular biology techniques have been widely used in the study of ectomycorrhizal fungi, and more and more ECM fungi have been identified. According to the latest statistics, there are about 20,000–25,000 species of ECM fungi worldwide, mainly from Basidiomycota and Ascomycota, and a small amount from Zygomycota [139].

According to the life form and phylogenetic characteristics of ECM fungi, scientists divided all ECM fungi into 66 evolutionary branches (lineages), including 37 evolutionary branches of Basidiomycetes, 27 evolutionary branches of Ascomycota, and 2 evolutionary branches of Zygomycota [140]. Wurzburger et al. identified that Suillus tomentosus, Cenococcum geophilum, and an ECM fungus from Russula were widely distributed in the swamp forest transition zone in southeastern Alaska [141]. Smith et al. studied the ECM fungal community of three leguminous tree species in tropical rainforests. A total of 118 ECM fungi were identified and divided into four branches [142]. Lamit et al. identified 367 ECM fungi in three different elevation gradients in the forests of Northern Iran, and the abundance of ectomycorrhizal fungi decreased with increasing altitude [143]. Bonito et al. identified 44 genera of ECM fungi in Carya illinoinensis (Carya illinoinensis) [144].

In recent years, Chinese scholars have also applied molecular biotechnology to the study of ECM fungal diversity. The dominant genera of ECM fungi in Pinus koraiensis pure forest in Northeast China were Sebacina, Suillus, Meliniomyces, Russula, Tomentella, Rhizopogon, and Amphinema [145]. The ECM fungi of natural Pinus sylvestris var.mongolica of different age classes in Honghuaerji belong to 2 phyla, 4 classes, 12 orders, 26 families, and 43 genera. The proportion of dominant genera of ECM fungi in different age classes is different [146]; the ECM fungi in the rhizosphere soil of Betula platyphylla in Inner Mongolia came from 2 phyla, 6 classes, 11 orders, 25 families, and 38 genera [147]; ECM fungi in 15 Pinus tabulaeformis forests in 9 provinces of Northern China belong to 2 phyla, 5 classes, 13 orders, 24 families, and 41 genera [148]; the ECM fungi of the main oaks in the Qinling Mountains and the Loess Plateau belong to 2 phyla, 6 classes, 13 orders, 28 families, and 48 genera [149]; the ECM fungi associated with Betula albo-sinensis in the Qinling Mountains belong to 6 orders, 8 families, and 9 genera, and the ECM fungi associated with Larix gmelinii come from 4 orders, 11 families, and 11 genera [150]; eCM fungi belong to 3 phyla, 8 classes, 18 orders, 23 families, and 18 genera in Fagaceae of tropical mountain rainforest [71].

2.2.3 Investigation of ectomycorrhizal fungi resources

2.2.3.1 Investigation of ectomycorrhizal fungi resources abroad

For more than a century, mycorrhizal researchers at home and abroad have done a lot of work in the investigation of ECM resources, and more and more ECM fungal resources have been detected and identified. The diversity of ECM fungi has been investigated abroad for a long time. As early as 1889, Noack tried to detect the resources of mycorrhizal fungi by tracing the mycelium of fruiting bodies [151]. Subsequently, Melin [152, 153] preliminarily speculated that there was a surprising number of ECM fungal diversity under natural conditions by investigating the occurrence of fruiting bodies in the field [152]. Since the 1990s, mycorrhizal research has become a hot spot. Systematic and large-scale investigations have confirmed the speculation of Melin et al. mycorrhizal researchers also continue to predict the global ECM fungal diversity based on the types of ECM that have been found. First, according to statistical analysis by Molina et al. [154], he predicted that there are about 5000–6000 ECM fungi in the world [153]. As many fungal species have been shown to form ECM with plants, Taylor and Alexander [155] reestimated the diversity of ECM fungi and believed that there are about 7000–10,000 ECM fungi in the world [154]. In recent years, with the use of molecular biology methods for ECM diversity research, this number has further expanded. The latest statistics show that about 20,000–25,000 fungi can form ECM with about 6000 higher plants around the world [139, 155]. The current research results show that ECM fungi are derived from three fungal phyla, namely Basidiomycota, Ascomycota, and Zygomycota [156, 157]. The proportion of Basidiomycota is the largest, reaching about 95%, followed by Ascomycota, about 4.8%, and only a small part of the fungi of Zygomycota can form ECM [154]. According to the life form and phylogenetic characteristics of ECM fungi, Tedersoo et al. divided all ECM fungi into 66 evolutionary branches (lineages), including 37 evolutionary branches of Basidiomycetes, 27 evolutionary branches of Ascomycota, and 2 evolutionary branches of Zygomycota [87].

2.2.3.2 Investigation of ectomycorrhizal fungi resources at home and abroad

The study of ECM fungi in China started late but developed rapidly. In the early stage, the investigation of ECM resources in China was mainly based on one or more tree species on the local scale. Bi Guochang et al. conducted a systematic investigation of ECM fungal resources in alpine coniferous forests in Northwestern Yunnan using a combination of standard plot surveys and route surveys. A total of 33 genera and 140 species of ECM fungi were identified [131]; Chen Lianqing found that a total of 27 fungi can form ectomycorrhizal fungi with Pinus massoniana [158]; Gong Mingqin and Chen Yu investigated ECM fungi from two hosts in South China, and found 11 ECM fungi [125]. Zhao Zhong et al. [159] investigated the ECM of Populus tomentosa in the central plains and found a total of 14 ECM fungi [159]. Tang et al. [101] investigated 35 ECM fungal resources of poplar in Shaanxi Province and identified 9 ECM fungi. Wu et al. [104] investigated the ectomycorrhizal resources in the Taibai Mountain Nature Reserve and identified 28 species of ectomycorrhizal fungi, belonging to 6 families and 18 genera [104]. Meng and Shao investigated the underground ECM of the main coniferous forests in Changbai Mountain, Lesser Khingan Mountains, and Greater Khingan Mountains in Northeast China, and finally identified 19 families, 43 genera, and 163 species of ECM fungi by means of indoor identification [91]. Bai Shulan conducted a systematic investigation of ECM fungal resources in Daqingshan area of Inner Mongolia. A total of 163 ECM fungi were found, belonging to 18 families and 41 genera [160]; Fan Yongjun analyzed ECM fungal resources of four host plants in Inner Mongolia from the perspective of morphology and anatomy and finally identified 48 mycorrhizal morphological types [161].

Later, with the development of molecular biology techniques, Chinese mycorrhizal researchers kept pace with the times and quickly used this technique to study ECM diversity, and the spatial scale of the study also expanded from the early local scale to the regional scale. Zhang found that 19 kinds of fungi can form ECM with Pinus sylvestris var.mongolica [162]. Wang Qin et al. studied the diversity of ECM fungi in temperate Quercus liaotungensis and subtropical Castanopsis fargesii, and detected 66 and 17 OTUs from each of the two host plants [163, 164]. Ding et al. found 36 ECM fungi in the subtropical evergreen broad-leaved forest in Southwest China [165]; Yi et al. used high-throughput sequencing to detect a total of 143 OTUs in Qinghai spruce forest in Helan Mountain, Inner Mongolia, belonging to 20 families and 25 genera [166]. Geng Rong et al. [167, 168] investigated the ECM fungi of Picea asperata and Quercus aliena var.acuteserrata in Xinjiashan forest area of Qinling Mountains by combining morphological and ITS sequencing methods. The results showed that 37 and 51 species of ECM fungi were detected, belonging to 10 families and 14 genera, respectively [167, 168]. Gao et al. used 454 pyrosequencing to find 393 OTUs in different succession stages of forests in Gutian Mountain, Zhejiang Province, belonging to 21 ECM fungal evolutionary lineages [169]. Wen Zhugui et al. conducted a systematic study on the ECM fungi of Pseudotsuga sinensis, an endangered species in China. A total of 66 fungi were identified to form ECM with Pseudotsuga sinensis [170]; Han Qisheng et al. detected a total of 60 ECM fungal OTUs in the alpine timberline species of Larix gmelinii forest in Taibai Mountain, belonging to 16 families and 28 genera [171]. Wang et al. combined morphological and molecular identification to carry out large-scale sampling and research on ECM fungal communities in five different habitat types of Quercus liaotungensis forests in Northern China. A total of 220 OTUs were detected to belong to 28 ECM fungal evolutionary branches [172].

3. Mycorrhizal fungal function

3.1 The ecological function of AM fungi

The ecological functions of AM fungi are reflected in the stability of plant communities and plant ecosystems and the improvement of ecosystem productivity [173]. The ecological functions of AM fungi are mainly applied to the restoration of degraded soil and the reconstruction of degraded vegetation [174]. Studies have shown that the inoculation of AM fungi significantly improved the ability of host plants to repair heavy metal-contaminated soil. AM fungi infect host plants to establish a symbiotic system, enhance plant tolerance to heavy metals, and promote plant growth and metabolism by chelating soil heavy metal ions with extracellular hyphae and expanding root absorption area [175]. It affects the morphology of heavy metals in soil and regulates the transport of heavy metals in plants by affecting the secretion effect of roots and changing the absorption kinetics of roots [176]. Wang Haijuan et al. used alfalfa to inoculate AM fungi. After inoculation, it was found that it could significantly improve the root growth of alfalfa and improve the rhizosphere soil microorganisms [177]; Zhang Zhongfeng et al. reported that inoculation of AM fungi in karst areas could promote the growth and biomass growth of Cyclobalanopsis glauca seedlings. The above studies have found that the inoculation of AM fungi has a certain potential effect on the stability of vegetation in the ecosystem and the recovery ability of the natural ecosystem [178].

3.2 The role of mycorrhizal fungi in terrestrial ecosystems

3.2.1 Symbiosis with plants

Mycorrhizal fungi can form symbionts with plants. Mycorrhizal fungi in addition to part of the fungal hyphae in the root surface of the formation of hyphal sheath or into the root cortex cells to form vesicles from the branches, there are a large number of hyphae stretching in the rhizosphere soil of plants, these stretching to the rhizosphere soil of the mycelium to expand the absorption area of plant roots, enhance the absorption capacity of mineral elements and water in plants, through isotope tracing and water determination, has proved that mycorrhizal fungi can promote the absorption and utilization of P, N, K, Cu, Zn, and other elements and water in plants [179, 180, 181]; on the other hand, fungi also obtain carbohydrates and other nutrients through the roots of plants, thus forming a mutually beneficial relationship in nutrition.

3.2.2 Entering the food web as food

Many fungi that form ectomycorrhizal fungi can form fruiting bodies with different shapes and large volumes on the surface or trunk in the wet rainy season, and some can form truffles in the ground. Many of these fruiting bodies or truffles are the food of arthropods, some small mammals and some birds in the soil ecosystem, and many of them are delicious foods for humans, such as fungi of the genera Cantharellus, Boletus, Lactarius, Tricholoma, Russula, and Ramaria. On the one hand, these fungi play a role in maintaining species diversity in the ecosystem as a food source for animals; on the other hand, the biomass of mycorrhizal fungi directly enters the food web through animals, thereby accelerating the circulation of substances in the ecosystem.

3.2.3 Affecting the succession of plant communities and the floristic composition of communities in terrestrial ecosystems

In the fifties and sixties of the last century, ecologists have noticed that in the process of natural restoration of man-made or natural damaged ecosystems, the plants that first settled in the damaged habitats are often not the dominant species before the destruction; the more serious the damage to the ecosystem, the slower the natural recovery. In 1965 Mayr argued: “Why can‘t settle successfully in unspoiled habitats where there is usually strong competition for settlers in damaged habitats?” Why is it particularly easy to settle in disturbed habitats? May’s own answer is that “ the adaptive advantages of these species under certain conditions are usually at the expense of losing the adaptability of some other adaptive components and types,” which is obviously a self-contradictory argument. After that, ecologists studied the flora composition and succession of newly formed volcanic islands, such as Long Island and Iceland near Guyana. It was found that most of the plants that first settled on the volcanic island were Polygonaceae, Amaranthaceae, Cruciferae, Urticaceae, and Caryophyllaceae. At any time, plants of other families gradually moved to settle [182]. These first settled families are recognized as families that do not form mycorrhiza under natural conditions [19, 183]. In 1979, Reeves et al. published their comparative study of mid-elevation sage communities in natural and damaged ecosystems and found that 99% of plants in natural communities had mycorrhiza [184], while in the damaged ecosystem adjacent to the natural community (road embankment), the number of plants with mycorrhiza was less than 1%. According to the results of this study, and with reference to the settlement and succession of plants on the volcanic island, Reeves et al. made a reasonable explanation for the problem raised by Mayr. They believed that in the damaged ecosystem, the disturbance to the soil may reduce or eliminate the propagules of mycorrhizal fungi (generally referring to surviving hyphae and spores); the reduction of mycorrhiza fungal propagules reduced the possibility of infection of new host plants, and non-mycorrhizal species successfully colonized and established communities (because plant species that require mycorrhizal nutrition died at the seedling stage due to colonization of mycorrhiza fungal propagules on their roots even after immigration). The non-mycorrhizal species further grew successfully, and further reduced the propagules of mycorrhizal fungi (the survival time of mycorrhizal fungi in the absence of host plants was limited). All mycorrhizal fungal propagules were eliminated, and competition with mycorrhizal plant species was also avoided. It is very slow to return to the system state before being disturbed (only by the slow invasion of mycorrhizal fungi). The more serious the interference, the greater the possibility of eliminating mycorrhizal fungal propagules, so the recovery of the system will take longer. This explanation of Reeves et al. was further confirmed by later studies. Janos ‘s study showed that in the humid tropics, the restoration of disturbed ecosystems was first settled by facultative mycorrhizal plants in the damaged environment, and then gradually replaced by mycorrhizal plants and developed into a climax ecosystem. These results indicate that mycorrhizal fungi play an important role in the succession type, succession speed, and flora composition of plant communities in terrestrial ecosystems.

3.2.4 Regulation of resource allocation in ecosystems

In natural terrestrial ecosystems, the vast majority of plants form mycorrhizas, and the fungi that form mycorrhizas with plants are usually less specific to their hosts [185]. This is especially true for mycorrhizal fungi from branches [186]. Therefore, in the undisturbed forest ecosystem, the hyphae that grow outward from the roots of infected plants can further infect the roots of other plants, so that some unrelated plants in the system can be linked together through the hyphae of different mycorrhizal fungi, and the material and information can be exchanged through the hyphal network or hyphal bridge or hyphal connection between plant roots. Ecologists have noticed this way of binding in ecosystems and referred to these plants bound together by hyphae as “Guilds.” It has been proved that plants in dependent plant groups can use hyphae as channels to transmit C, N, P, and other substances between plants in two directions [185], thus affecting the allocation of resources in the ecosystem. Grime et al. further studied the role of this hyphal network in maintaining species diversity in ecosystems, and through research under experimental conditions, it is believed that this hyphal network shared by different plant species can increase species diversity in ecosystems by bidirectionally transferring carbohydrates [187]. However, Bergelson et al. considered that the study under this experimental condition did not prove that there was a relationship between “source” and “sink” of carbohydrates between the two plants, nor did it indicate that there was a “net flow” of carbohydrates between plants [188]. Therefore, this study could not be of universal significance.

3.2.5 Promote the absorption and utilization of water and nutrients by host plants

ECM mainly promotes the absorption of nutrients and water by expanding the absorption area and absorption range of tree roots. A large number of studies have shown that under forest site conditions, plant mycorrhizal fungi have different types of hyphal morphology in the soil. These hyphae can form a large hyphal system. The hyphal systems from different hosts and different types of ECM fungi are interconnected, and an intricate common hyphal network (common mycorrhizal networks: CMNs) is constructed underground [155, 189, 190]. Once the roots of trees form mycorrhizas with ectomycorrhizal fungi, the whole plant is connected to these CMNs. The absorption area and absorption length of the huge CMNs are much larger than the roots, which can not only promote the absorption and utilization of water and nutrients by host plants but also exchange water and nutrients between different host individuals and even different hosts through CMNs [191, 192, 193, 194].

3.2.6 Improving the stress resistance of host plants

3.2.6.1 Enhance the drought resistance of the host

Compared with host trees, ECM fungi have stronger plasticity and ecological adaptability, and most of them have the characteristics of drought and high-temperature resistance. Therefore, under drought conditions, host plants can obtain water from the soil through the mycelium network of ECM fungi to maintain normal growth. At the same time, under drought conditions, ECM can increase the leaf water potential and water retention capacity of host trees and reduce the leaf water saturation deficit value [195, 196]. ECM can also enhance drought resistance by regulating some physiological functions of host plants. For example, ECM fungi enhance host drought resistance by affecting the content or activity of host plant enzymes, including superoxide dismutase (SOD), catalase, peroxidase, and polyphenol oxidase [197, 198, 199].

3.2.6.2 Enhance the host‘s resistance to saline-alkali

Ishida et al. investigated the ectomycorrhizal resources of S.mongolica in saline-alkali land of Northeast China and found that 11 ECM fungi could form ectomycorrhizal with S. mongolica, and the dominant species (Geopora sp.) was obvious [200]. A large number of studies have shown that under salt stress conditions, ECM can promote the absorption of water and various nutrients in the soil by tree roots on the one hand, and on the other hand, it can regulate the ion balance in plants and enhance the activity of the host root system, thereby alleviating the damage of saline-alkali stress to the host [201, 202]. In addition, it has been reported that ECM can also improve the respiration and water use efficiency of its host plants to enhance the host’s saline-alkali resistance [199].

3.2.6.3 Enhanced host resistance to heavy metals

In recent years, with the excessive development and utilization of mineral resources, heavy metal pollution is becoming more and more serious. These pollutions will cause great toxicity to plants and even cause plant death. Studies have found that mycorrhizal fungi can effectively reduce the toxicity of heavy metals. Huang et al. [203, 204] found that Pinus massoniana grown in manganese mining areas can form mycorrhizal fungi with a variety of ECM fungi. Using inoculation experiments, it was found that compared with aseptic seedlings, mycorrhizal seedlings can significantly improve the adaptability of the host, resist heavy metal toxicity, and ensure the biomass growth of the host [203, 204]. Huang et al. [203, 204] studied the effects of growth and heavy metal accumulation and distribution of Pinus tabulaeformis seedlings by planting mycorrhizal seedlings in copper and cadmium-contaminated soil. The results showed that inoculation of mycorrhizal fungi could promote the growth and biomass accumulation of host trees on the one hand, and significantly reduce the accumulation of heavy metals in Pinus tabulaeformis on the other hand, and effectively block the transport of heavy metals from roots to stems and leaves of plants [205, 206]. The current research results show that mycorrhizal fungi can secrete a series of organic acids (such as oxalic acid) or secrete mucus that can chelate heavy metals. These secretions play an important role in the process of plant resistance to heavy metal stress. For example, Ahonen-Jonnarth et al. found that under aluminum stress, ectomycorrhizal seedlings can secrete more oxalic acid than uninoculated seedlings, and oxalic acid can chelate aluminum well [207].

3.2.6.4 Enhance the host‘s disease resistance

Liu Runjin and Chen Yinglong found that; the sheath and Hatnet in the ECM structure can effectively prevent the mechanical damage of soil pathogens to plant roots [5]. Pine damping-off disease is a common disease on Pinus tabulaeformis seedlings, which causes serious damage to P. tabulaeformis seedlings. Studies have found that inoculation of ectomycorrhizal fungi can significantly prevent P. tabulaeformis damping-off disease [208, 209]. In addition, ectomycorrhizal fungi also secrete some secondary metabolites, such as antibiotics, which can effectively inhibit the invasion of harmful bacteria and fungi in soil to host plants [210].

3.2.6.5 Promote seedling growth and improve the survival rate of afforestation

Afforestation is generally carried out in habitat destruction or difficult site areas. The main reason for the failure of afforestation is the low survival rate of seedlings. A large number of studies have shown that mycorrhizal seedlings can increase the chlorophyll content of the host, increase the photosynthetic rate of the seedlings, and reduce transpiration water loss [211, 212, 213, 214, 215, 216]. On the other hand, many ECM fungi can produce plant endogenous hormones. For example, Amanita, Boletus and Rhizopogon can produce indole acetic acid (IAA) and cytokinin, and R. luteolus can produce zeatin [217, 218, 219]. These hormones can regulate the balance of hormones in plants and promote the growth of host plants, thereby improving the survival rate and preservation rate of afforestation [220].