**2. Antagonistic Interactions**

Antagonistic interactions between fungi and nematodes are as numerous as they are varied. For example, many nematodes, such as *Aphelenchus avenae*, *Aphelenchoides* spp., and *Paraphelenchus acontioides*, can feed on a diversity of fungi. These are commonly referred to as fungivorous nematodes [7]. In contrast, a number of fungal species such as *Arthrobotrys oligospora* can prey on nematodes and their eggs, consuming them as food. Such fungi are known as nematophagous fungi [8].

#### *2.1. Nematodes Feeding on and Antagonizing Fungi*

Many of the nematodes have fungi in their diets or feed exclusively on fungi [9]. Thus, as a major component of the soil food web, nematodes can influence both the fungal diversity and abundance and community structure, including crop growth and tolerance to soil pollution. Fungivorous nematodes commonly exist in soil containing many di fferent fungal species. Nematodes in the genera *Aphelenchus*, *Aphelenchoides*, *Ditylenchus*, and *Tylenchus* are among the most common fungivorous nematodes [10]. Generally, fungivorous nematodes feed on a diversity of soil fungi, including saprophytic, plant-pathogenic, and plant-beneficial (such as mycorrhizal) fungi and are known as polyphagous nematodes [11]. While the population densities of fungivorous nematodes in soil may be lower than those of phytoparasitic nematodes and bacterivorous nematodes, the population densities of fungivorous nematodes can increase rapidly in the presence of suitable fungal food [10]. Depending on the soil microbiome, nematode feeding on soil fungi could have significant impacts on soil ecology and crop productivity. For example, if fungivorous nematodes were to feed on plant-pathogenic fungi, the phytopathogen population in the soil could be suppressed. However, if mycopathogenic fungi (e.g., species in the genera *Gliocladium* and *Trichoderma*) antagonistic to plant-pathogenic fungi are found to be the food of nematodes, then the beneficial e ffects of these antagonist fungi to plants would be reduced due to the actions of these nematodes. All these fungi with di fferent relationships to nematodes and to each other can be present in the same ecological niches. In addition, the food fungi for nematodes are not all identical. Di fferent food fungi may present di fferent attractiveness to the fungivorous nematodes and that attractiveness may vary depending on the environmental conditions. Furthermore, both fungi and nematodes are mobile, in di fferent ways, to allow them to disperse across ecological niches [2,9].

One group of fungi that nematodes like to feed on is the mycorrhizal fungi. Indeed, the interactions between mycorrhizal fungi and fungivorous nematodes have been the subject of intensive investigations because of the potential e ffects of grazing on the function of the mycorrhiza in nutrient uptake and growth of the host plants. Indeed, surveys have found that fungal fruiting bodies (mushrooms) produced by ectomycorrhizal fungi often contain nematodes. Such fungal grazing by nematodes can have other e ffects, such as the release of nutrients immobilized in fungal biomass as resources for bacteria [12]. Aside from ectomycorrhizal fungi, endomycorrhizal fungi also interact with nematodes. For example, the reproduction of *Aphelenchoides* sp. nematodes can be triggered by the co-inoculation of arbuscular mycorrhizal (AM) fungi, which lead plants to achieve further growth and greater arsenic (As) tolerance at low As-polluted soil. [13]. This could have significant implications for changing the composition and infectivity of field assemblages of AM fungi. On the other hand, nematodes of the genus *Aphelenchus* can prevent the symbiosis between endomycorrhizal fungi of the genus *Glomus* with pine roots. In such instances, fumigant nematicides need to be used to disinfest the soil before pine seedling planting can be successful. Indeed, fumigation not only increases the endomycorrhizal infection of pine roots but also enables the pine trees to utilize the dead nematodes as an excellent pabulum [6].

Fungivorous nematodes could be multifunctional. *A. avenae* is a non-parasitic fungivorous nematode that can control plant-pathogenic fungi [7]. For example, both *A. avenae* and *Aphelenchoides* spp. suppressed *Rhizoctonia solani* and reduced the damping of disease in cauliflower seedlings [14]. In addition, *A. avenae* can suppress the propagation of the plant parasitic nematode *Ditylenchus destructor,* suggesting that it is a potential biocontrol agen<sup>t</sup> against both certain plant-pathogenic fungi and plant parasitic nematodes [15]. Genetic analyses of cell wall-degrading enzymes from *A. avenae* support the roles of these enzymes in feeding on both plant pathogenic fungi and a plant parasitic nematode [16]. Interestingly, in the pinewood nematode *Bursaphelenchus xylophilus*, a cellulase similar to those in fungi and associated with the ability to parasitize living plants was identified as most likely the result of horizontal gene transfer, acquired during its evolution of plant parasitism [17].

In the fungal prey–nematode predator relationship, just like other types of prey–predator relationships, the fungal prey can develop resistance mechanisms against nematode predation. One type of fungal prey defense is the production and secretion of toxic secondary metabolites and toxic proteins [18]. For example, the model mushroom *Coprinopsis cinerea* produces a toxic substance on its mycelial surface that can kill nematodes upon contact [19]. Indeed, upon predation by nematodes, *C. cinerea* exhibited a comprehensive set of di fferentially expressed genes (DEGs) and the production of a bacterial cytolysin-like toxin. Some of these DEGs in *C. cinerea* represent a novel type of fungal effector protein against nematodes [20].

#### *2.2. Fungi Antagonizing Nematodes*

The above examples show nematodes antagonizing and feeding on fungi; the opposite can also happen and is known to be quite common in nature. The interaction between nematophagous fungi and nematodes has played a crucial role in understanding broad fungi–nematode interactions. Nematophagous fungi, including those that are variously called predaceous, nematode-trapping, and nematode-destroying fungi, possess amazing abilities to capture nematodes and reduce the population size of plant-parasitic nematodes. Such abilities have significant applied interests in agriculture [21,22]. Studies of nematophagous fungi and their interactions with nematodes have revealed several mechanisms of their interactions at the molecular, cellular, organismal, and ecological levels. Indeed, such studies have propelled their interactions as models for studying inter-kingdom interactions in predator–prey coevolution, and for biocontrol applications [23–29].

#### 2.2.1. Diversity and Evolution of Fungal Predation Structures

Nematophagous fungi have been traditionally divided into four main groups based on the mechanisms that they use to attack nematodes: (i) nematode-trapping fungi, producing extensive hyphal networks, knobs, and constricting rings as trapping devices to catch and hold live nematodes; (ii) endoparasitic fungi, as obligate parasites that exist as conidia in the environment and infect nematodes by either adhering to the surface of the prey or by directly being ingested by the nematodes followed by germination, growth, and nematode killing; (iii) egg- and cyst-parasitic fungi, as facultative parasites that grow on and parasitize the sedentary stages of nematodes such as eggs; and (iv) toxin-producing fungi, producing toxic compounds that are active against nematodes [30,31]. Except for the egg stage, most nematodes at other life stages are capable of moving through their environments, which presents a challenge for relatively slow-growing and immobile fungal parasites. However, many fungi have evolved to parasitize mobile stages of nematodes by employing complex and sophisticated predation structures, including (1) trapping structures to immobilize nematodes; (2) adhesive conidia to attach and colonize the nematodes' pseudocoeloms; (3) acanthocytes, spiny balls, and stephanocysts to damage the cuticle of nematodes and then consume them; and (4) gun cells to launch finger-like tubes directly at the target nematodes [8,21,32].

The interaction between nematophagous fungi and nematodes induces morphogenesis and virulence gene expression in these fungi, signaling a transition from their saprobic stage to phagocytic stage. Evolutionarily, nematophagous fungi are widely distributed across many phylogenetically-independent taxonomic groups, indicating that the ability to phagocytize nematodes has evolved multiple times [23,33]. Among the nematode-trapping fungi, there are also multiple types of trapping structures, including constricting rings and five types of adhesive traps (sessile adhesive knobs, stalked adhesive knobs, adhesive nets, adhesive columns, and non-constricting rings), all of which were originated from the vegetative hyphae [34]. Consistent with frequent and independent origins of nematode-trapping devices, members of the Orbiliaceae produce five types of traps, among them *Arthrobotrys dactyloides, Arthrobotrys superba, Arthrobotrys oligospora*, and *Monacrosporium gephyropagum* are capable of forming conidial traps—traps formed directly from the asexual spore, the conidia. At a low nutrient level, competition for nutrients among microorganisms can be intense; thus, the ability of fungal spores to directly germinate into the traps could be highly advantageous [35,36]. Consistent with convergen<sup>t</sup> evolution in some trapping structures, two groups of fungi from two di fferent phyla, namely *Zoophagus* species of Zygomycota and *Nematoctonus* species of Basidiomycota, can both produce adhesive knobs [37]. However, traps based on adhesive hyphae are restricted to the fungal genera *Stylopage* and *Cystopage* of Zygomycetes [38], while the prominent fungi parasitizing cys<sup>t</sup> nematode juveniles, *Hirsutella rhossiliensis* and *Hirsutella minnesotensis*, are representatives of adhesive spores [30]. Some species of endoparasites have developed morphologically-adapted conidia that, when eaten by the nematodes, become lodged in either its buccal cavity or esophagus. These species belong almost exclusively to the genus *Harposporium* [39]. Among other nematode-trapping fungal structures, stephanocysts are restricted to the genus *Hyphoderma* of Basidiomycota [40]. Spiny balls and acanthocytes are known only by *Coprinus comatus* and *Stropharia rugosoannulata*, respectively, in Agaricales of Basidiomycota [41,42]. Finally, a very peculiar attack device called the "gun cell" is produced by endoparasitic fungi in the genus *Haptoglossa* (Oomycete fungi) [43].

Among the broad groups of nematophagous fungi, those that form specialized morphological adaptations to capture nematodes are especially interesting. These nematode-trapping fungi (NTF) can switch their lifestyle from saprophytes to predators under certain cues. Such transitions have made them good models for studying inter-kingdom communication with regard to the mechanisms of fungal pathogenesis and adaptation [23,27,44]. In recent years, -omics studies have significantly improved our understanding of host–microbe interactions, especially in those cases where the microorganisms are di fficult to grow under laboratory conditions [45]. In the case of fungi–nematode interactions, sequencing of the genomes of the nematode female and egg parasite *Pochonia chlamydosporia* [46]; the nematode-trapping fungi *Arthrobotrys oligospora* [44], *Monacrosporium haptotylum* [25], and *Drechslerella stenobrocha* [47]; and the facultative nematode endoparasite *H. minnesotensis* have greatly contributed to our understanding of the evolutionarily distinct strategies of fungal pathogenesis against nematodes [48].

Using NTF in the phylum Ascomycota as models, phylogenies based on genes and genomes from both the nuclei and the mitochondria support that, within the Orbiliales, the nematode-trapping mechanisms have evolved along two major lineages. In one lineage, the species form constricting rings. In the second, the species form adhesive traps, including three-dimensional hyphal networks, adhesive hyphal branches, and adhesive knobs [23,33,49]. Furthermore, a combined five-gene phylogeny and molecular clock calibration based on two fossil records revealed that the organismic interactions between NTF and nematodes likely dates back to more than 419 million years of co-evolution, with the active carnivores (fungi with constricting rings) and passive carnivores (fungi with adhesive traps) diverged from each other around 246 Mya, shortly after the occurrence of the Permian–Triassic extinction event about 251.4 Mya [23]. However, no major carnivorous ascomycete divergence has been correlated to the Cretaceous–Tertiary extinction event. More research is needed to identify if the evolution of fungal carnivorism was a response to mass extinction events.

Despite the diverse morphogenesis, di fferent kinds of nematode traps share two structural features that are di fferent from vegetative hyphae. The first one is the presence of numerous cytosolic organelles, commonly known as dense bodies [50]. These dense bodies are peroxisomal in nature and only detected in nematode-trapping fungi, but not in endoparasitic nematophagous fungi that infected their host with adhesive or non-adhesive spores [51]. Their functions seemed to be involved in adhering to nematodes and supplying energy and/or structural components to the invading hyphae [51]. A recent study showed that disruption of the gene *Aoime2* caused reductions in both trap formation and electron-dense bodies in trap cells [52], with substantially fewer nematodes captured by the mutants. The second feature common to the adhesive traps (columns, networks, and knobs) is the presence of extensive layers of extracellular polymers, which are thought to be important for adhesion of the traps to the surface of nematodes [53]. Recent genome comparisons and surface structural analyses revealed evidence for expansion of adhesion genes in NTF genomes and with associated increase in trap surface adhesiveness. Both of these can enhance the ability of the fungi to penetrate and digest the nematodes and likely represent the key drivers of fungal adaptation in trapping nematodes [27].

#### 2.2.2. Host Recognition, Adhesion, Host Specificity, and Infection Process

As a shared characteristic among all types of nematophagous fungi, recognition of the hosts and adhesion to the cuticle of the nematodes or eggshells by the fungi are the first steps in infection. The nematode cuticle is a solid exoskeleton that mainly consists of proteins. The exoskeleton acts as a barrier against environment stresses and potential pathogen attacks [54]. At present, how NTF penetrates the nematode exoskeleton has not been fully elucidated. Current research results sugges<sup>t</sup> that secreted enzymes from NTF play a major role during invasion of the nematodes by the fungi. Specifically, genetic, ultrastructural, and histochemical studies showed that the presence of extracellular hydrolytic enzymes such as chitinases, collagenases, and proteases are essential for nematode cuticle penetration [55]. Indeed, phylogenetic analysis of the pathogenicity-related serine proteases from nematophagous and entomopathogenic fungi showed that they evolved from a common ancestor [56]. Penetration is typically followed by content digestion, resulting in the formation of a new fungal biomass inside and later outside the nematodes. Table 1 shows the four main steps of infection from the four main groups of nematophagous fungi, plus the producers of special attack devices (structures which mechanically damage the cuticle of nematodes, as the fifth group) [21,30].


Nematode-trapping fungi (NTF) are usually not host specific and can trap many types of soil-dwelling nematodes [80]. In contrast, there is some host specificity among endoparasitic fungi. The endoparasitic fungi are obligate parasites and mostly exist as conidia in the environment. The conidial attachment to a particular nematode species does not always lead to infection, but specific recognition signals for adhesion are required, as shown by the endoparasitic fungus *Drechmeria coniospora* [71,81]. Fungi that parasitize nematodes are common soil saprophytes, attacking primarily the sedentary stages (female and egg stages) of nematodes or sedentary nematodes, such as *Heterodera*, *Globodera*, and *Meloidogyne* [30]. Nematode-toxic fungi have nematode-immobilizing activity and can kill their nematode hosts by producing toxins. The success and e fficiency of nematode attacks by producers of special attacking devices are also sometimes linked to the toxins produced [41]. Special attacking devices are similar to a sharp sword or acanthocytes, spiny balls, and stephanocysts, like real medieval weapons, causing damage to the nematode cuticle, resulting in extravasation of the inner contents of the nematodes and allowing complete colonization of the nematode body by fungal hyphae.

#### 2.2.3. Competition between Nematode-Trapping Fungi and Nematodes

Evolutionary arms races are common between pathogens and hosts. Evidence for such arms races has been found between nematodes and NTF. In these arms races, fungal predators continuously evolve predatory strategies to secure food from nematodes. In turn, the prey nematodes evolve counter measures, such as enhanced innate immunity and sophisticated nervous systems to sense and avoid their predator fungi. Many factors can influence such arms races. For example, in soil environments, the populations of NTF and their target nematodes not only interact with each other as predators and prey but also with other fungi and nematodes nearby, respectively. In addition, biotic factors such as other microbes and plants as well as abiotic factors such as nutrient levels can also influence NTF–nematode interactions. Systematic studies on the bitrophic (NTF and nematode) or multitrophic (plant, soil microorganisms, nematode, and NTF) interactions under natural conditions are required to obtain a broad understanding of the factors influencing the ecology and evolution of such interactions. Below we summarize our current understanding of the potential mechanisms involved in the arms race between NTFs and nematodes.

#### Innate Immune Defense Responses in Nematodes

The epidermis and the collagen-rich cuticle that surrounds the nematode provide a physical barrier to fungal pathogens. Nematodes can also sense and defend against fungal pathogens using strategies such as producing antimicrobial peptides regulated by the innate immunity system. To cope with bacterial and fungal pathogen attacks from the intestine or the cuticle, the innate immune response of *Caenorhabditis elegans*is accompanied by an increase of reactive oxygen species (ROS) [82]. The nematode genomes also contain many antimicrobial peptide (AMP)-coding genes that play important roles in their innate immunity. In one study, when *C. elegans* was infected, one of the AMPs, NLP-31, showed strong activities against several fungi, including *Drechmeria coniospora*, *Neurospora crassa*, and *Aspergillus fumigatus* [83]. The recent expansion of the AMP-encoding *nlp* genes as revealed by genome sequencing, together with the evidence for their in vivo role and the signatures of positive selection of the *nlp29* gene cluster, sugges<sup>t</sup> that these genes are important for the survival of *C. elegans* when they interact with *D. coniospora* spores [84]. The FOXO transcription factor DAF-16, which lies downstream of the conserved insulin/IGF-1 signaling (IIS) pathway, is required for survival after fungal infection and wounding [85]. RNA-seq analysis further identified shared and unique signaling pathways regulated by DAF-16/FOXO and highlighted the intestinal DAF-16 regulatory components and roles of the innate immune system countering fungal pathogenesis [86].

#### Competition between Different Fungal Species and Nematodes

To survive and reproduce, nematodes and NTF need to successfully cope with many stressors and competing demands in soil. Competition can be among different fungal predators, among different nematodes, and between NTF and nematodes [87]. Surveys have found that multiple NTF species often coexist in the same niche, suggesting that they likely compete for the same prey in their natural environments. For example, *Arthrobotrys* species are sympatrically distributed and are generalist predators of nematodes. Two species in *Arthrobotrys*, namely *A. thaumasia* and *A. musiformis*, are sympatric with nematodes in more than 63% surveyed natural sites. In addition, the ability to sense prey among wild isolates of *Arthrobotrys oligospora* varied greatly [28]. Some nematodes are trapped/colonized by more than one NTF at the same time (e.g., colonized from opposite ends of the nematodes). In some of those cases, evidence for competition between NTFs has been found. For example, the hyphae of *A. oligospora* were often observed to be dead or degenerated when placed in close proximity to live mycelia of the endoparasitic fungus *D. coniospora*, consistent with the latter being an antagonist against *A. oligospora* under the specific conditions [88]. El-Borai et al. [89] indicated that the tested nematodes were repelled by activated *Arthrobotrys* species but were attracted to activated endoparasitic fungi from the genera *Myzocytium* and *Catenaria*.

Antagonistic interactions between NTF and nematodes have been detected in the soil environments. As expected, density-dependent parasitism has been reported, demonstrating that an increase in NTF density would lead to a decrease in nematode prey density, which subsequently would lead to a decrease in NTF density and an increase in nematode density. This negative frequency-dependent selection between NTF and nematodes regulates the densities of both groups of organisms [90]. This model successfully described changes in parasitism of the nematode *Heterodera schachtii* by the nematophagous fungus *Hirsutella rhossiliensis* as a function of host density, with the disease dynamics in soil microcosms exhibiting both a temporal density-dependent parasitism and a host threshold density [91]. Suppression of the root-knot nematode *Meloidogyne javanica* by NTFs *Monacrosporium cionopagum* and *H. rhossiliensis* was positively related to the nematode host *Steinenema glaseri* density, and the dynamics of suppression varied among different species [92]. In addition, spatial sampling of the nematophagous fungus *H. rhossiliensis* revealed a relationship between numbers of hosts (*Criconentella xenoplax*) and the degree of parasitism [93], with evidence of the two interacting partners possessing similar density-dependent dynamics among tested patches of agricultural fields [94]. Recent greenhouse trials also showed that parasitism of *H. rhossiliensis* was strongly correlated with the density of the soybean cys<sup>t</sup> nematode [30]. At broader geographic scales, in a survey of 53 citrus orchards in central ridge and flatwood ecoregions of Florida, the spatial patterns of entomopathogenic nematode species were found to be modulated by variations in their susceptibilities to nematophagous fungal species (*Catenaria* sp., *A. musiformis, Arthrobotrys dactyloides, Paecilomyces lilacinus, A. oligospora*, and *Gamsylella gephyropaga*) across habitats [95]. However, strong and diverse top-down control effects on the nematode community in coastal sand dunes were found in a recent study, where three microbial enemies of nematodes (*Catenaria* spp., *H. rhossiliensis*, and *Pasteuria penetrans*) were correlated, either positively or negatively, with plant parasitic nematode population size [96]. Together, these results are consistent with some species-specific effects for both the fungal and the nematode partners in natural and agricultural ecosystems. Aside from these individualized surveys, metagenomic methods have also been used to analyze the relationships between fungi and nematodes (as well as bacteria) in field settings [97] and revealed a diversity of spatial associations similar to those described above between plant parasitic nematodes and NTFs [98,99].

#### **3. Synergistic Interactions between Phytophagous Nematodes and Phytopathogenic Fungi against Host Plants**

#### *3.1. Interactions between Phytophagous Nematodes and Soil-borne Fungal Pathogens*

In the soil environment, opportunities exist for interactions between soil-borne pathogens and pests of plants when they occupy the same ecological niche. While antagonism can occur between them in their competitions for space and resources, synergistic interactions between them are also possible to cause greater damage to plants, including crops. For example, in the rhizosphere, nematode attacks can lower the resistance of plants to pathogens and increase their susceptibility to infection by soil-borne fungal pathogens. In these situations, the physiological states of all three interacting partners play a very important role in the outcome of such tripartite interactions.

The first example of a nematode–fungi disease complex in plants was described by Atkinson, in 1892, where he reported that the fusarium wilt of cotton (caused by *Fusarium oxysporum* f. sp. *vasinfectum*) was more severe in the presence of root-knot nematodes (*Meloidogyne* spp.) [100]. Subsequently, many other cases of synergistic interactions between nematodes and fungi have been reported, involving sedentary endoparasitic root-knot and cys<sup>t</sup> nematodes, and increasing disease severity caused by *Fusarium* or *Verticillium* wilt fungi. *Meloidogyne* spp. has been shown to interact with *Fusarium* wilt to negatively impact a number of crops, with cys<sup>t</sup> nematodes acting in a similar manner to increase wilt diseases. Meanwhile, entomopathogenic nematodes and pathogenic fungi have been shown capable of generating additive interactions to increase insect pest mortality [101]. In these cases, an initial fungal infection plays a key role in weakening the larvae and increasing the pest insects' susceptibility to nematodes by generating a stressful condition and altering the insects' behavior [102]. Table 2 summarizes recent examples of nematode–fungi pathogen disease complexes reported in crops and insects.


**Table 2.** Examples of nematode–pathogen disease complexes reported in crops and insects.

#### *3.2. Factors Influencing Interactions between Phytophagous Nematodes and Phytopathogenic Fungi*

As shown above, the nature of interactions between phytophagous nematodes and phytopathogenic fungi varies among the di fferent fungal and nematode species. For example, some plant pathogenic nematodes can induce physical damage, such as small wounds, to their host plants. Such wounds may allow fungal pathogens easy access to plant tissues to cause infections. Alternatively, some nematodes may induce physiological changes in their host plants, triggering changes

in fungal pathogen populations around the host plants and making them more likely to increase their population size and/or pathogenicity [121]. In addition, other biotic and abiotic factors such as host plant genotype, organic matter and nutrient availability, and other microbes could all a ffect the outcome of infections by nematode pests and plant fungal pathogens [117].

In agriculture fields, the fungal species composition can vary, depending on whether the fields are infested by root-knot nematodes. *Fusarium oxysporum* (11%) followed by *Fusarium solani* (6%) were found to be the most frequent fungal species associated with the presence of *Meloidogyne* spp., and fungal diversity plays an important role in the interactions between host plants and soil microorganisms [111]. For example, inoculation of certain bacterial and fungal combinations together had an inhibitory e ffect on each other and reduced crop disease severity [112]. On the other hand, abiotic factors such as soil moisture and soil physicochemical properties also play important roles for infection by plant fungal pathogens and nematode pests [112].

Another interesting interaction between nematodes and fungi that could have important e ffects on agriculture is that between entomopathogenic nematodes and entomopathogenic fungi. Together, these entomopathogenic pests and pathogens could help control pest insect populations in agricultural fields. However, to realize their potential, it is important to understand the life cycles of both the entomopathogenic nematodes and entomopathogenic fungi, and to develop strategies to allow them to grow in the same ecological niches with minimal negative impacts on each other [102]. Indeed, a previous study has shown that the virulence of both the entomopathogenic nematode *Steinernema riobravae* and entomopathogenic fungus *Beauveria bassiana* against last larval instars of *Galleria mellonella* could be synergistic or additive, depending on environmental conditions and application strategies [122].

#### **4. Fungi and Nematodes Interact through a Third Party**

In most natural soil ecosystems, fungal species co-occur with nematodes, and both often actively interact with plants. This cross-kingdom interaction between fungi and nematodes in the plant rhizosphere is often called a tripartite interaction. Other organisms, such as bacteria, may also be involved in this network of interactions. These interactions may involve direct cell–cell contacts. Alternatively, they may interact indirectly, using chemical signals. Indeed, chemical signals such as volatile organic compounds released by organisms such as bacteria, nematodes, fungi, or plants have been detected to initiate interactions between fungi and nematodes. Due to the ubiquitous distributions of these organisms in natural environments and agricultural fields, their interactions have significant ecological and economic impacts. Thus, it is important to develop a comprehensive understanding of such interactions involving all partners in terrestrial and agricultural ecosystems.

### *4.1. Induced Resistance*

There is a broad range of detrimental microbes and nematodes that can challenge the plant's capability for growth and survival. However, their e ffects on plants vary depending on other microbes in the same ecological niches. For example, colonization of plant roots by beneficial endophytic and mycorrhizal fungi can protect plants against a wide range of plant-parasitic nematodes through plant mediated mechanisms [123,124]. One example of a beneficial endophyte is the fungus *Trichoderma harzianum*. This fungus can induce jasmonic acid (JA)- and salicylic acid (SA)-regulated defense pathways in tomato (*Solanum lycopersicum*), causing resistance to the root-knot nematode *Meloidogyne incognita*. Similarly, mycorrhizal fungi represent an inextricable part of almost every plant system. Their role in suppression of plant pathogenic nematodes (PPNS) has been extensively studied and reviewed [125]. For example, plants associated with arbuscular mycorrhizal fungi (AMF) showed decreased damages caused by sedentary endoparasites than those without AMF colonization. The antagonistic action of mycorrhizal fungi against PPNS may be achieved directly, e.g., by competition for nutrients and space, or indirectly, by increasing plant tolerance, mediating induced

systemic resistance (ISR) in plants, changing rhizosphere interactions by altering root exudations, or all of the above [1].

Interestingly, some nematophagous fungi can colonize plants as endophytes. Thus, they are also capable of mediating ISR against nematodes in situ. Most studies investigating fungi–nematode interactions are conducted outside of the plant hosts; few have considered the e ffects of these fungi in the context of plant endophytes. The soil borne *Phialemonium inflatum* is a known nematode egg-parasite fungus, which can secrete extracellular proteases and chitinases and significantly reduce hatching of *M. javanica* juveniles [126]. A recent study revealed a novel role for this fungus. Specifically, a foliar-isolated *P. inflatum* strain was shown to be endophytic in cotton, and part of a plant-fungal defensive symbiosis in cotton. Using a seed treatment inoculation, this isolate showed significant inhibitory e ffects against the root-knot nematode *M. javanica* as an endophyte in cotton. This was the first study demonstrating antagonistic e ffects of endophytic *P. inflatum* against root-knot nematodes [127]. Similarly, compared to treatments with only the nematode *M. incognita* or with neither the nematode nor the *A. oligospora,* treatment containing endophytic and rhizospheric populations of *A. oligospora* showed reduced nematode population size and increased defense-related enzymes in tomatoes [87]. The inoculation of *Drechslerella dactyloides* and *D. brochopaga* also significantly increased plants' resistance against *M. incognita* [128].

Another example is an endophytic strain of *Pochonia chlamydosporia* that caused a moderately induced expression of genes involved in ISR in barley (*Hordeum vulgare*) [129]. However, in this study, the plants were not challenged with plant-parasitic nematodes to conclusively demonstrate priming for resistance to nematodes [129]. *Arabidopsis thaliana* root colonization by *P. chlamydosporia* showed modulated jasmonate signaling that resulted in accelerated flowering and improved yield [130]. Further studies showed that the e ffects were due to chitosan-mediated increases in root colonization by *P. chlamydosporia* [131,132]. Overall, these studies contribute to potential future applications of endophytes to increase plant tolerance/resistance to RKN.

#### *4.2. Alteration of Root Exudates*

Plant roots typically have a close association with mutualistic rhizosphere microorganisms. Together, they exude a wide range of both primary metabolites and secondary metabolites. Such metabolites can modify the surface properties of nematodes and a ffect microbial attachment to nematode surfaces [133]. In parallel, the success of the nematode infection and inhibition by the nematophagous microbes depends on how the plant roots and their associated microbes perceive the signaling molecules on the nematode surfaces. Indeed, metabolites exuded from plant roots a ffect not only the communication between plants and nematodes, but also the nematode–fungi interactions by modulating the surface properties of nematodes. One of the mechanisms of nematode suppression by *Fusarium* endophytes appears to be through altering root exudates [134]. A similar mechanism was proposed for AMF-mediated nematode suppression. In both tomatoes and bananas, AMF colonization of roots altered root exudates, leading to fewer nematodes penetrating AMF compared to roots with non-AMF colonization [135]. Specifically, root exudates altered by the nematophagous fungus *Pochonia indica* stimulated the hatching of mobile second-stage juveniles (J2s) that were dormant in nematode cysts [136], which subsequently caused a major inhibitory e ffect on the development of *Heterodera schachtii* in *Arabidopsis* roots [137]. *H. schachtii* is a plant pathogenic nematode capable of infecting more than 200 di fferent plants including economically important crops such as sugar beets, tomatoes, bananas, cabbage, broccoli, and radish.

The modes of action of mycorrhizal fungi against PPN may be exhibited through a direct e ffect, by competition for nutrients and space, or indirect e ffect, by increasing plant tolerance, mediating ISR in plants, altering rhizosphere interactions due to changed root exudations, or all of these combined, depending on the species of both AMF and nematodes [125]. Further research on the nematode/microbial selectivity in the attachment and the influence of plants on these interactions could open up possibilities for manipulating these interactions to improve plant health.
