Arthrobotrys oligospora: a model organism for understanding the interaction between fungi and nematodes

AoPrdx2 Regulates Oxidative Stress, Reactive Oxygen Species, Trap Formation, and Secondary Metabolism in Arthrobotrys oligospora

Prdx2 is a peroxiredoxin (Prx) family protein that protects cells from attack via reactive oxygen species (ROS), and it has an important role in improving the resistance and scavenging capacity of ROS in fungi. Arthrobotrys oligospora is a widespread nematode-trapping fungus that can produce three-dimensional nets to capture and kill nematodes. In this study, AoPrdx2, a homologous protein of Prx5, was investigated in A. oligospora via gene disruption, phenotypic analysis, and metabolomics. The deletion of Aoprdx2 resulted in an increase in the number of mycelial septa and a reduction in the number of nuclei and spore yield. Meanwhile, the absence of Aoprdx2 increased sensitivity to oxidative stresses, whereas the ∆Aoprdx2 mutant strain resulted in higher ROS levels than that of the wild-type (WT) strain. In particular, the inactivation of Aoprdx2 severely influenced trap formation and pathogenicity; the number of traps produced by the ∆Aoprdx2 mutant strain was remarkably reduced and the number of mycelial rings of traps in the ∆Aoprdx2 mutant strain was less than that of the WT strain. In addition, the abundance of metabolites in the ∆Aoprdx2 mutant strain was significantly downregulated compared with the WT strain. These results indicate that AoPrdx2 plays an indispensable role in the scavenging of ROS, trap morphogenesis, and secondary metabolism.

Isolation, Identification, and Evaluation of the Predatory Activity of Chinese Arthrobotrys Species towards Economically Important Plant-Parasitic Nematodes

The current investigation aimed to isolate and identify predatory fungal strains and evaluate their efficacy in mitigating the effects of plant-parasitic nematodes. We successfully isolated three distinct nematophagous fungal strains from soil samples, identified as Arthrobotrys megalosporus, A. oligospora, and A. sinensis, using conventional and molecular identification methodologies. In vitro trials illustrated the high capture efficiency of these fungi against plant-parasitic nematodes. Over an exposure period of 48 h to Aphelenchoides besseyi, Bursaphelenchus xylophilus, and Ditylenchus destructor, A. megalosporus (GUCC220044) displayed predation rates of 99.7%, 83.0%, and 21.1%, respectively. A. oligospora (GUCC220045) demonstrated predation rates of 97.3%, 97.3%, and 54.6%, and A. sinensis (GUCC220046) showed rates of 85.1%, 68.3%, and 19.0% against the same cohort of nematodes. The experimental outcomes substantiate that all three identified fungal strains demonstrate predatory activity against the tested nematodes, albeit with varying efficiencies.

Study of a Mexican isolate of Arthrobotrys musiformis (Orbiliales): Predatory behavior and nematocidal activity of liquid culture filtrates against Haemonchus contortus (Trichostrongylidae), protein profile and myco-constituent groups

A Mexican isolate of the nematophagous fungus Arthrobotrys musiformis was obtained from a soil sample from the Chapultepec ecological reserve zone, in Cuernavaca, Morelos, Mexico. This isolate demonstrated an important predatory activity (74.9%) against the parasitic nematode Haemonchus contortus (L3) and its fungal liquid culture filtrates (LCF) grown in two media showed the following highest nematocidal activities (NA): In Czapek-DoxBroth (CzDoxB) 80.66% and potato-dextrose broth (PDB) 49.84%. Additionally, two major compounds derived from carboxylic acids and two derivates from alkane group were identified by GC-MS. These compounds have been associated to many biological activities. On the other hand, the protein profile analysis by SDS-electrophoresis followed by a zymogram revealed a 10 kDa protein with protease activity. This study provides important information for future experiments focused to explore the potential use of this protein as well as the identified bioactive compounds presents in the LCF as potential candidates against sheep haemonchosis.

Uncovering the biogeographic pattern of the widespread nematode-trapping fungi Arthrobotrys oligospora: watershed is the key

Studies of biogeographic patterns of fungi have long been behind those of plants and animals. The presence of worldwide species, the lack of systematic sampling design and adequate sampling effort, and the lack of research units are responsible for this status. This study investigates the biogeographical patterns of Arthrobotrys oligospora, the most widespread globally distributed nematode-trapping fungi (NTF), by stratified collecting and analyzing 2,250 samples from 228 sites in Yunnan Province, China. The A. oligospora was isolated, and 149 strains were subjected to ITS, TUB, TEF and RPB2 gene sequencing and multi-gene association phylogeographic analysis. The results show that at population level A. oligospora is randomly distributed throughout Yunnan Province and has no biogeographical distribution pattern. At the genetic level, the phylogenetic tree of A. oligospora diverges into five major evolutionary clades, with a low degree of gene flow between the five clades. However, the correlation between the phylogenetic diversity of A. oligospora and geographical factors was low. There was no clear pattern in the phylogenetic clades distribution of A. oligospora either without dividing the study unit or when the grid was used as the study unit. When watersheds were used as the study unit, 67.4%, 63.3%, 65.9%, 83.3%, and 66.7% of clade 1-5 strains were distributed in the Jinsha river, Red river, Peal river, Lancang river, and Nujiang-Irawaddy river watersheds, respectively. The clades distribution of A. oligospora was highly consistent with the watersheds distribution. Training predictions of the clades distributions using randomly generated polygons were also less accurate than watersheds. These results suggest that watersheds are key to discovering the biogeographic distribution patterns of A. oligospora. The A. oligospora populations are blocked by mountains in the watershed, and gene flow barriers have occurred, which may have resulted in the formation of multiple cryptic species. Watersheds are also ideal for understanding such speciation processes, explaining factors affecting biodiversity distribution and coupling studies of plant and animal and microbial diversity.

Recent Advances in Life History Transition with Nematode-Trapping Fungus Arthrobotrys oligospora and Its Application in Sustainable Agriculture

Parasitic nematodes cause great annual loss in the agricultural industry globally. Arthrobotrys oligospora is the most prevalent and common nematode-trapping fungus (NTF) in the environment and the candidate for the control of plant- and animal-parasitic nematodes. A. oligospora is also the first recognized and intensively studied NTF species. This review highlights the recent research advances of A. oligospora as a model to study the biological signals of the switch from saprophytism to predation and their sophisticated mechanisms for interacting with their invertebrate hosts, which is of vital importance for improving the engineering of this species as an effective biocontrol fungus. The application of A. oligospora in industry and agriculture, especially as biological control agents for sustainable purposes, was summarized, and we discussed the increasing role of A. oligospora in studying its sexual morph and genetic transformation in complementing biological control research.

Roles of the Fungal-Specific Lysine Biosynthetic Pathway in the Nematode-Trapping Fungus Arthrobotrys oligospora Identified through Metabolomics Analyses

In higher fungi, lysine is biosynthesized via the α-aminoadipate (AAA) pathway, which differs from plants, bacteria, and lower fungi. The differences offer a unique opportunity to develop a molecular regulatory strategy for the biological control of plant parasitic nematodes, based on nematode-trapping fungi. In this study, in the nematode-trapping fungus model Arthrobotrys oligospora, we characterized the core gene in the AAA pathway, encoding α-aminoadipate reductase (Aoaar), via sequence analyses and through comparing the growth, and biochemical and global metabolic profiles of the wild-type and Aoaar knockout strains. Aoaar not only has α-aminoadipic acid reductase activity, which serves fungal L-lysine biosynthesis, but it also is a core gene of the non-ribosomal peptides biosynthetic gene cluster. Compared with WT, the growth rate, conidial production, number of predation rings formed, and nematode feeding rate of the ΔAoaar strain were decreased by 40-60%, 36%, 32%, and 52%, respectively. Amino acid metabolism, the biosynthesis of peptides and analogues, phenylpropanoid and polyketide biosynthesis, and lipid metabolism and carbon metabolism were metabolically reprogrammed in the ΔAoaar strains. The disruption of Aoaar perturbed the biosynthesis of intermediates in the lysine metabolism pathway, then reprogrammed amino acid and amino acid-related secondary metabolism, and finally, it impeded the growth and nematocidal ability of A. oligospora. This study provides an important reference for uncovering the role of amino acid-related primary and secondary metabolism in nematode capture by nematode-trapping fungi, and confirms the feasibility of Aoarr as a molecular target to regulate nematode-trapping fungi to biocontrol nematodes.

Trichoderma Species: Our Best Fungal Allies in the Biocontrol of Plant Diseases-A Review

Biocontrol agents (BCA) have been an important tool in agriculture to prevent crop losses due to plant pathogens infections and to increase plant food production globally, diminishing the necessity for chemical pesticides and fertilizers and offering a more sustainable and environmentally friendly option. Fungi from the genus Trichoderma are among the most used and studied microorganisms as BCA due to the variety of biocontrol traits, such as parasitism, antibiosis, secondary metabolites (SM) production, and plant defense system induction. Several Trichoderma species are well-known mycoparasites. However, some of those species can antagonize other organisms such as nematodes and plant pests, making this fungus a very versatile BCA. Trichoderma has been used in agriculture as part of innovative bioformulations, either just Trichoderma species or in combination with other plant-beneficial microbes, such as plant growth-promoting bacteria (PGPB). Here, we review the most recent literature regarding the biocontrol studies about six of the most used Trichoderma species, T. atroviride, T. harzianum, T. asperellum, T. virens, T. longibrachiatum, and T. viride, highlighting their biocontrol traits and the use of these fungal genera in Trichoderma-based formulations to control or prevent plant diseases, and their importance as a substitute for chemical pesticides and fertilizers.

The Multifaceted Gene 275 Embedded in the PKS-PTS Gene Cluster Was Involved in the Regulation of Arthrobotrisin Biosynthesis, TCA Cycle, and Septa Formation in Nematode-Trapping Fungus Arthrobotrys oligospora

The predominant nematode-trapping fungus Arthrobotrys oligospora harbors a unique polyketide synthase-prenyltransferase (PKS-PTS) gene cluster AOL_s00215g responsible for the biosynthesis of sesquiterpenyl epoxy-cyclohexenoids (SECs) that are involved in the regulation of fungal growth, adhesive trap formation, antibacterial activity, and soil colonization. However, the function of one rare gene (AOL_s00215g275 (275)) embedded in the cluster has remained cryptic. Here, we constructed two mutants with the disruption of 275 and the overexpression of 275, respectively, and compared their fungal growth, morphology, resistance to chemical stress, nematicidal activity, transcriptomic and metabolic profiles, and infrastructures, together with binding affinity analysis. Both mutants displayed distinct differences in their TCA cycles, SEC biosynthesis, and endocytosis, combined with abnormal mitochondria, vacuoles, septa formation, and decreased nematicidal activity. Our results suggest that gene 275 might function as a separator and as an integrated gene with multiple potential functions related to three distinct genes encoding the retinoic acid induced-1, cortactin, and vacuolar iron transporter 1 proteins in this nematode-trapping fungus. Our unexpected findings provide insight into the intriguing organization and functions of a rare non-biosynthetic gene in a biosynthetic gene cluster.

Amphiphysin AoRvs167-Mediated Membrane Curvature Facilitates Trap Formation, Endocytosis, and Stress Resistance in Arthrobotrysoligospora

Bin1/Amphiphysin/Rvs (BAR) domain-containing proteins mediate fundamental cellular processes, including membrane remodeling and endocytosis. Nematode-trapping (NT) fungi can differentiate to form trapping structures through highly reorganized cell membranes and walls. In this study, we identified the NT fungus Arthrobotrys oligospora ortholog of yeast Rvs167 and documented its involvement in membrane bending and endocytosis. We further confirmed that the deletion of AoRvs167 makes the fungus more hypersensitive to osmotic salt (Nacl), higher temperatures (28 to 30 °C), and the cell wall perturbation agent Congo red. In addition, the disruption of AoRvs167 reduced the trap formation capacity. Hence, AoRvs167 may regulate fungal pathogenicity through the integrity of plasma membranes and cell walls.

The role of WSC domain-containing protein encoding gene AOL_s00043g401 in the growth and nematode trapping of Arthrobotrys oligospora

Arthrobotrys oligospora is a model nematode-trapping fungus that has been extensively investigated to understand the interactions between fungi and nematodes. Nematode capture by A. oligospora is a complex process in which recognition of nematodes is generally believed to be mediated by lectins from the fungi. Lectins are a group of carbohydrate-binding proteins that widely exist in microorganisms and contain specific glycosylation recognition domains. In this work, we report the effect of a putative WSC domain-containing protein encoding gene AOL_s00043g401 (g401) on the growth and nematode-trapping of A. oligospora. The g401 gene was knocked out using the homologous recombination approach, and the △g401 mutant strain was then evaluated for its growth rate, conidial yield and germination rate, adaptation to environmental stresses, and nematocidal activity. Interestingly, the deletion of the putative lectin gene g401 had no significant effect on saprophytic growth, conidial yield and germination rate, responses to high salt, surfactant, and strong oxidative environments, as well as nematode-trapping efficiency of A. oligospora. We speculate that this phenomenon might have been caused by an intrinsic genetic compensation of the g401 in this fungus. For instance, more copies of WSC domain encoding genes or PQQ-DH domain encoding genes were found in the genome of A. oligospora. These findings provide further experimental evidence on the effect of lectin-related functional proteins in A. oligospora on nematode capture and will help further analyze their potential roles in the biological control of nematodes in the future.

Isolation of nematophagous fungi from soil samples collected from three different agro-ecologies of Ethiopia

Background

Several species of nematophagous fungi exist in nature that can capture and kill nematodes as natural predators of soil-dwelling worms. These are important in agriculture and animal husbandry as biological control agents. The diversity of nematophagous fungi found from soil had not been studied in Ethiopia.

Objective

This study aimed to isolate Nematophagous Fungi from Soil Samples Collected From three Different Agro-Ecologies of Ethiopia.

Methods

Cross-sectional study was conducted and samples were collected from three different agro-climatic zones of Ethiopia; Debre-Berhan (highland), Bishoftu (mid-altitude), and Awash (lowland). Twenty-seven soil samples were randomly taken from each of the three different agro-ecological climates (9 from each agro-ecological climatic zone). For each study site, samples were collected from the soil of decomposed animal feces/dung, agricultural/farmlands, and forest lands in triplicates.

Results

The present study disclosed that nematophagous fungi were widespread from the study area. A total of 33 species of nematophagous fungi belonging to four genera, Arthrobotryes, Paecilomyces, Monacrosporium, and Harposporium were identified. Arthrobotrys were the most commonly isolated genera followed by Paecilomyces. The six identified species were Arthrobotrys oligospora, Paecilomyces lilacinus, Arthrobotryes dactyloides, Monacosporum eudermatum, Harposporium helicoides, and Monacosporum cionopagum.

Conclusion

This study indicated that Arthrobothryes oligospora was the most common species in Bishoftu and Awash whereas. In Debre-Berhan, Paecilomyces lilacinus was the most prevalent species. Monacosporum cionapagum was not isolated from dung soil and agricultural soil whereas Harposporium helicoides and Arthrobothryes dactyloides were not found from dung and forest soil respectively.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02572-4.

The fucose-specific lectin gene AOL_s00054g276 affects trap formation and nematocidal activity of the nematophagous fungus Arthrobotrys oligospora

Nematode-trapping fungi are natural enemies of nematodes in nature. Arthrobotrys oligospora, a typical nematode-trapping fungus with a clear genetic background, can capture and infect nematodes by forming adhesive three-dimensional networks. Lectins, a class of glycoproteins containing glycosyl-specific recognition domains, play an important role in biological recognition. However, the fucose-specific lectins have rarely been studied regarding the process of preying on nematodes. In this study, we characterized the biological role of the fucose-specific lectin encoding gene AOL_s00054g276 (g276) in A. oligospora. The gene g276 was first deleted based on homologous recombination, then the phenotype and nematocidal activity of the Δg276 mutant was evaluated. The results showed that the deletion of gene g276 delayed trap formation and weakened its nematocidal activity; however, mycelial growth, conidia production, conidial germination rates, and adaption to environmental stresses were not affected. Our results suggest that the fucose-specific lectin encoding gene g276 might be associated with the morphogenesis of this fungus, and its deletion resulted in a significantly low density of three-dimensional traps (P < 0.05) and a significantly low nematode-tapping efficiency (P < 0.001). These findings provide a basis for further elucidating the mechanism of A. oligospora preying on nematodes and lay a foundation for the development and utilization of fungal-derived lectins for nematode control in future.

Development of a water refractive index-matched microneedle integrated into a light sheet microscopy system for continuous embryonic cell imaging

In this study, microneedle-integrated light sheet microscopy (LSM) was developed for trapping and continuously imaging embryos of Caenorhabditis elegans with subcellular resolution. To reduce aberrations when the light sheet was propagated into the device, a microneedle was fabricated using a transparent, water refractive index-matched polymer. It was proven that when the light sheet emerged from the water-immersed objective and penetrated through the microneedle with a circular surface, even with a non-perpendicular incident angle, fewer aberrations were found. An embryo was injected into and trapped at the tip of the microneedle, which was positioned at the interrogation window of the LSM apparatus with the image plane perpendicular to the light sheet, and this setup was used to sequentially acquire embryo images. By applying the light sheet, higher-resolution, higher-contrast images were obtained. The system also showed low photobleaching and low phototoxicity to embryos of C. elegans. Furthermore, three-dimensional embryo images with a whole field of view of the microneedle could be achieved by stitching together images and reconstructing sequential two-dimensional embryo images.

Individual and Combined Application of Nematophagous Fungi as Biological Control Agents against Gastrointestinal Nematodes in Domestic Animals

Gastrointestinal nematodes (GINs) are a group of parasites that threaten livestock yields, and the consequent economic losses have led to major concern in the agricultural industry worldwide. The high frequency of anthelmintic resistance amongst GINs has prompted the search for sustainable alternatives. Recently, a substantial number of both in vitro and in vivo experiments have shown that biological controls based on predatory fungi and ovicidal fungi are the most promising alternatives to chemical controls. In this respect, the morphological characteristics of the most representative species of these two large groups of fungi, their nematicidal activity and mechanisms of action against GINs, have been increasingly studied. Given the limitation of the independent use of a single nematophagous fungus (NF), combined applications which combine multiple fungi, or fungi and chemical controls, have become increasingly popular, although these new strategies still have antagonistic effects on the candidates. In this review, we summarize both the advantages and disadvantages of the individual fungi and the combined applications identified to date to minimize recurring infections or to disrupt the life cycle of GINs. The need to discover novel and high-efficiency nematicidal isolates and the application of our understanding to the appropriate selection of associated applications are discussed.

Aolatg1 and Aolatg13 Regulate Autophagy and Play Different Roles in Conidiation, Trap Formation, and Pathogenicity in the Nematode-Trapping Fungus Arthrobotrys oligospora

Autophagy is a conserved cellular recycling and trafficking pathway in eukaryotes that plays an important role in cell growth, development, and pathogenicity. Atg1 and Atg13 form the Atg1–Atg13 complex, which is essential for autophagy in yeast. Here, we characterized the roles of the Aolatg1 and Aolatg13 genes encoding these autophagy-related proteins in the nematode-trapping fungus Arthrobotrys oligospora. Investigation of the autophagy process by using the AoAtg8-GFP fusion protein showed that autophagosomes accumulated inside vacuoles in the wild-type (WT) A. oligospora strain, whereas in the two mutant strains with deletions of Aolatg1 or Aolatg13, GFP signals were observed outside vacuoles. Similar results were observed by using transmission electron microscopy. Furthermore, deletion of Aolatg1 caused severe defects in mycelial growth, conidiation, conidial germination, trap formation, and nematode predation. In addition, transcripts of several sporulation-related genes were significantly downregulated in the ΔAolatg1 mutant. In contrast, except for the altered resistance to several chemical stressors, no obvious differences were observed in phenotypic traits between the WT and ΔAolatg13 mutant strains. The gene ontology analysis of the transcription profiles of the WT and ΔAolatg1 mutant strains showed that the set of differentially expressed genes was highly enriched in genes relevant to membrane and cellular components. The Kyoto Encyclopedia of Genes and Genomes analysis indicated that differentially expressed genes were highly enriched in those related to metabolic pathways, autophagy and autophagy-related processes, including ubiquitin-mediated proteolysis and SNARE interaction in vesicular transport, which were enriched during trap formation. These results indicate that Aolatg1 and Aolatg13 play crucial roles in the autophagy process in A. oligospora. Aolatg1 is also involved in the regulation of asexual growth, trap formation, and pathogenicity. Our results highlight the importance of Aolatg1 in the growth and development of A. oligospora, and provide a basis for elucidating the role of autophagy in the trap formation and pathogenicity of nematode-trapping fungi.

The STRIPAK component SipC is involved in morphology and cell-fate determination in the nematode-trapping fungus Duddingtonia flagrans

The striatin-interacting phosphatase and kinase (STRIPAK) complex is a highly conserved eukaryotic signaling hub involved in the regulation of many cellular processes. In filamentous fungi, STRIPAK controls multicellular development, hyphal fusion, septation, and pathogenicity. In this study, we analyzed the role of the STRIPAK complex in the nematode-trapping fungus Duddingtonia flagrans which forms three-dimensional, adhesive trapping networks to capture Caenorhabditis elegans. Trap networks consist of several hyphal loops which are morphologically and functionally different from vegetative hyphae. We show that lack of the STRIPAK component SipC (STRIP1/2/HAM-2/PRO22) results in incomplete loop formation and column-like trap structures with elongated compartments. The misshapen or incomplete traps lost their trap identity and continued growth as vegetative hyphae. The same effect was observed in the presence of the actin cytoskeleton drug cytochalasin A. These results could suggest a link between actin and STRIPAK complex functions.

Identification, Characterization, and Evaluation of Nematophagous Fungal Species of Arthrobotrys and Tolypocladium for the Management of Meloidogyne incognita

Root-knot nematodes belonging to the genus Meloidogyne are agriculturally important pests, and biocontrol strategies offer safer alternatives for their management. In the present study, two fungal species from Indian soils were identified as Arthrobotrys thaumasia and Tolypocladium cylindrosporum based on morphological characteristics and further confirmed using molecular markers. In vitro evaluation of A. thaumasia against M. incognita and Caenorhabditis elegans showed 82 and 73% parasitism, respectively, whereas T. cylindrosporum gave 65.2 and 57.7% parasitism, respectively. Similarly, culture filtrates of A. thaumasia caused 57.7 and 53.7% mortality of M. incognita and C. elegans, respectively, whereas T. cylindrosporum caused higher mortality of 87.3 and 64%, respectively. Besides, greenhouse evaluation of both fungi against M. incognita infecting tomato significantly reduced nematode disease burden reflecting parasitic success measured as the total number of galls, egg masses, eggs per egg mass, and derived nematode multiplication factor. Application of A. thaumasia and T. cylindrosporum reduced nematode multiplication factor by 80 and 95%, respectively, compared with control. General metabolite profiling of tested fungi using gas chromatography–mass spectrometry and ultra-performance liquid chromatography–quadrupole/time of flight mass spectrometry reported for the first time here showed presence of various volatile and non-volatile compounds with nematicidal activity, viz., trimethyl-heptadiene, methyl-hexadecanol, dodecadienal, decane, terpendole E, dodecane, acetamido-6-anthraquinone, and hexadecanol. Also, other compounds such as undecane, dibutyl-disulfide, octadecenal, paganin, talathermophilin, dactylarin, tolypyridone A, tolypyridone B, pyridoxatin, and destruxin were identified, reported in the literature to possess antibacterial, antifungal, and insecticidal properties. This is the first report of the occurrence of both fungi from India and pioneer demonstration of T. cylindrosporum for root-knot nematode management.

A putative F-box-domain-encoding gene AOL_s00076g207 regulates the development and pathogenicity of Arthrobotrys oligospora

F-box protein is a key component of the Skp1-cullin-F-box-type ubiquitin ligase complex (SCF-ULC) that marks its target proteins with ubiquitin for proteasomal degradation. In this study, we explored the potential role of AOL_s00076g207 (Aog207) in Arthrobotrys oligospora, a model fungus for studying nematodes-fungi interactions. The Aog207 gene encodes a putative F-box protein of the SCF-ULC. Deletion of Aog207 could inhibit mycelial growth in TYGA and PDA media. More importantly, the conidial germination rate of ΔAog207 mutants was remarkably declined compared to that of wild-type (WT) strain, and the mutant strains were more sensitive toward chemical stressors than the WT strain. In addition, ΔAog207 mutants generated fewer traps and captured fewer nematodes than WT strain. In summary, Aog207 disruption significantly affected the pathogenicity, mycelial growth, conidial germination, environmental adaptation and trap formation of A. oligospora. These findings may facilitate a better understanding of the nematode predation mechanism of A. oligospora and provide an experimental basis for developing biological control agents against nematodes.

Fatal attraction of Caenorhabditis elegans to predatory fungi through 6-methyl-salicylic acid

Salicylic acid is a phenolic phytohormone which controls plant growth and development. A methyl ester (MSA) derivative thereof is volatile and involved in plant-insect or plant-plant communication. Here we show that the nematode-trapping fungus Duddingtonia flagrans uses a methyl-salicylic acid isomer, 6-MSA as morphogen for spatiotemporal control of trap formation and as chemoattractant to lure Caenorhabditis elegans into fungal colonies. 6-MSA is the product of a polyketide synthase and an intermediate in the biosynthesis of arthrosporols. The polyketide synthase (ArtA), produces 6-MSA in hyphal tips, and is uncoupled from other enzymes required for the conversion of 6-MSA to arthrosporols, which are produced in older hyphae. 6-MSA and arthrosporols both block trap formation. The presence of nematodes inhibits 6-MSA and arthrosporol biosyntheses and thereby enables trap formation. 6-MSA and arthrosporols are thus morphogens with some functions similar to quorum-sensing molecules. We show that 6-MSA is important in interkingdom communication between fungi and nematodes.

Historical Differentiation and Recent Hybridization in Natural Populations of the Nematode-Trapping Fungus Arthrobotrys oligospora in China

Maintaining the effects of nematode-trapping fungi (NTF) agents in order to control plant-parasitic nematodes (PPNs) in different ecological environments has been a major challenge in biological control applications. To achieve such an objective, it is important to understand how populations of the biocontrol agent NTF are geographically and ecologically structured. A previous study reported evidence for ecological adaptation in the model NTF species Arthrobotrys oligospora. However, their large-scale geographic structure, patterns of gene flow, their potential phenotypic diversification, and host specialization remain largely unknown. In this study, we developed a new panel of 20 polymorphic short tandem repeat (STR) markers and analyzed 239 isolates of A. oligospora from 19 geographic populations in China. In addition, DNA sequences at six nuclear gene loci and strain mating types (MAT) were obtained for these strains. Our analyses suggest historical divergence within the A. oligospora population in China. The genetically differentiated populations also showed phenotypic differences that may be related to their ecological adaptations. Interestingly, our analyses identified evidence for recent dispersion and hybridization among the historically subdivided geographic populations in nature. Together, our results indicate a changing population structure of A. oligospora in China and that care must be taken in selecting the appropriate strains as biocontrol agents that can effectively reproduce in agriculture soil while maintaining their nematode-trapping ability.

The role of Jacalin-related lectin gene AOL_s00083g511 in the development and pathogenicity of the nematophagous fungus Arthrobotrys oligospora

Arthrobotrys oligospora is a model species of nematophagous fungi and has great potential for the biological control of nematode diseases. Lectin is a protein that binds to carbohydrates and their complexes with high specificity, which mediates recognition events in various physiological and pathological processes. This study aimed to investigate the role of the Jacalin-related lectin (JRL) gene, AOL_s00083g511, in A. oligospora development. Through a homology recombination approach, we obtained the AOL_s00083g511 knockout mutant strain (Ag511). Next, the biological characteristics of the Ag511 mutant strain, including growth rate, conidia germination rate, adaptation to environmental stresses, and nematocidal activity, were compared with those of the wild-type (WT) strain. The results showed that the JRL gene AOL_s00083g511 did not affect fungal growth, conidia germination, 3D-trap formation, and the ability of A. oligospora to prey on nematodes significantly. We speculate that this phenomenon may be caused by a loss of the key β1-β2 loops in the AOL_ s00083g511-encoded JRL domain and an intrinsic genetic compensation of AOL_s00083g511 in this fungus. The growth rates of both strains on high salt or surfactant media were similar; however, in the strong oxidation medium, the growth rate of the Ag511 mutant was significantly lower than that of the WT strain, indicating that AOL_s00083g511 might play a role in oxidative stress resistance. These findings provide a basis for further analysis of the related functions of the JRL gene in A. oligospora and their potential roles in the biological control of nematodes in the future.

High Predatory Capacity of a Novel Arthrobotrys oligospora Variety on the Ovine Gastrointestinal Nematode Haemonchus contortus (Rhabditomorpha: Trichostrongylidae)

With the worldwide development of anthelmintic resistance, new alternative approaches for controlling gastrointestinal nematodes in sheep are urgently required. In this work, we identified and characterized native nematode-trapping fungi. We collected seven isolates of fungi with the capacity to form adhesive, three-dimensional networks as the main mechanism to capture, kill, and consume nematodes. The nematode-trapping fungi were classified into two groups; the first group includes the R2-13 strain, showing faster growth, abundant aerial hyphae, scarce conidia production, bigger conidia, and it formed a clade with Arthrobotrys oligospora sensu stricto. The second comprises the A6, A12, A13, R2-1, R2-6, and R2-14 strains, showing a growth adhering to the culture medium, forming little aerial hyphae, smaller conidia, and these formed a sister clade to A. oligospora. Except for the R2-6 strain, conidia production was induced by light. In all the strains, the predatory capacity against the sheep gastrointestinal nematode Haemonchus contortus was greater than 58% compared with the control group. The A6 and A13 strains were the most active against the infective H. contortus third instar (L₃) larvae, with an average capture capacity of 91%. Altogether, our results support evidence for a novel A. oligospora variety with high nematode-trapping activity and promissory in helminthic control.

Effect of an Arthrobotrys musiformis (Fungi: Orbiliales) culture filtrate on the population of gastrointestinal parasitic nematode eggs in faeces of grazing lambs

This study assessed the anthelmintic activity of the oral administration of a free-spore culture filtrate of the nematophagous fungus (NF) Arthrobotrys musiformis (M-10) on gastrointestinal parasitic nematodes (GIN) in naturally infected lambs. The fungus was grown on potato-dextrose agar plates (PDA) and transferred to a fermented rice medium (FRM). After 40-day incubation the total amount of FRM with the growing fungi was transferred to a flask shaker with distilled water for a 24 h period. The fungus was centrifuged and filtered. Three groups of six naturally-infected lambs (>1000 epg) each were treated once as follows: Group 1) 63.8 mg/kg A. musiformis culture filtrate (CF) (per os); Group 2) Levamisole 7.5 mg/ml (intramuscularly), Group 3) 15 ml of distilled water (per os). Faecal samples were individually collected on days -2, 0, 2, 4, 6, 8 and 10 after treatment. For each experimental group, mean egg shedding was calculated and transformed (log 10 [epg + 1]). Means between the fungal filtrate group and the negative control were analysed using a T-Student Test. The faecal egg count reduction test (FECRT) was performed in groups treated with CF and Levamisole in relation to the control group (water) were 36.8-57.4% and 89-95.4%, respectively., although due to the difference between groups, no statistical significance was found (p > 0.05). The use of A. musiformis CF appears to be a good alternative treatment, although, more studies should be performed to establish the use of these fungal products as potential tools for GIN control.

Suppression of root-knot nematode Meloidogyne incognita on tomato plants using the nematode trapping fungus Arthrobotrys oligospora Fresenius

AIM

The aims of the study were to isolate and characterize the nematode trapping fungus, Arthrobotrys oligospora, to investigate the suppressive and predacious activities of the fungus against Meloidogyne incognita and to study the potentiality of A. oligospora in controlling root-knot caused by M. incognita on tomato plants.

METHODS AND RESULTS

Arthrobotrys oligospora (MRDS 300) was isolated from sandy soil samples collected from Al-Beheira, Egypt. In vitro experiments revealed a high efficiency of the fungus in capturing and suppressing M. incognita second juveniles (J₂ ). Microscopic observations showed that the fungus develops adhesive traps consisting of loops of hyphae. Moreover, an in vitro experiment showed that the culture filtrate of A. oligospora had a high toxic effect on the nematode. Pot experiments carried out in two seasons (2018-2019) showed that A. oligospora significantly suppressed root knot on tomato plants caused by M. incognita. The number of females, galls and nematodes in different developing stages were reduced significantly. The treatment with A. oligospora had a prominent effect on enhancing plant growth.

CONCLUSION

Arthrobotrys oligospora had significant suppressive and predacious effects against root-knot nematode, M. incognita. The fungus developed different forms of trapping devices in addition to secreting toxic metabolites to M. incognita. The fungus had a plant-growth promoting effect.

SIGNIFICANCE AND IMPACT OF THE STUDY

Arthrobotrys oligospora (MRDS 300) is a potential biological control agent that can be utilized in controlling the root-knot diseases caused by M. incognita.

Assessing the predatory activity of Arthrobotrys oligosporus strain C-2197 as biocontrol of the root-knot nematode Meloidogyne spp

The root-knot nematode, Meloidogyne spp., is an endoparasite that infects plants' root system and causes yield losses in several important crops. Meloidogyne is one of the most devastating pests, so searching for effective biological agents is needed to mitigate its damage. In this study, the predatory activity of Arthrobotrys oligosporus Fresen strain C-2197, obtained from a tropical dry forest of Ecuador, was evaluated as a biocontrol alternative for root-knot caused by Meloidogyne spp. Our results showed that A. oligosporus C-2197 has predatory activity against juvenile nematodes, 72.31%, and 79% efficacy, for in vitro and greenhouse conditions. Besides, the studied strain showed growth-promoting activity, increasing leaf and root area of inoculated plants. Growth promoting activity was also observed in field tests. The present study validates the potential use of A. oligosporus as a biocontrol of Meloidogyne spp. in tomato production systems under greenhouse. It also presents useful information on the use of different cultivation media and substrates for massive A. oligosporus spore concentrates.

Novel Polyketide-Terpenoid Hybrid Metabolites from a Potent Nematicidal Arthrobotrys oligospora Mutant ΔAOL_s00215g278

Here, we reported that detailed investigation on trace targeted metabolites from nematode-trapping fungus Arthrobotrys oligospora mutant with deletion of P450 gene AOL_s00215g278 led to isolation of 9 new polyketide-terpenoid hybrid derivatives, including four new glycosides of the key precursor farnesyl hydrotoluquinol (1) and, surprisingly, four new sesquiterpenyl epoxy-cyclohexenoids (SECs) analogues. Among them, two major target metabolites 1 and 14 displayed moderate nematode inhibitory ability. Moreover, the mutant lacking AOL_s00215g278 could form far more nematode-capturing traps within 6 h in contact with nematodes and show rapid potent nematicidal activity with killing 93.7% preys, though deletion of the P450 gene resulted in dramatic decrease in fungal colony growth and failure to produce fungal conidia. The results unequivocally revealed that gene AOL_s00215g278 should be involved in not only the SEC biosynthetic pathway in the nematode-trapping fungus A. oligospora but also fungal conidiation and nematicidal activity.

Effect of Heterodera schachtii female age on susceptibility to three fungal hyperparasites in the genus Hyalorbilia

Three closely related nematophagous fungi in the genus Hyalorbilia were compared for their ability to parasitize females and eggs of Heterodera schachtii at different developmental stages. DoUCR50, StM, and ARF were originally isolated from Heterodera schachtii, Meloidogyne incognita, and Heterodera glycines, respectively. Phylogenetic analysis and pairwise sequence analysis showed that DoUCR50 and StM are more closely related to each other than they are to ARF. DoUCR50 parasitism suppressed 100% of the J2 hatch from 3-week-old H. schachtii females and 75% of the hatch from 4-week-old females. Eggs within 5-week-old females were resistant to parasitism, and hatch of J2 was unaffected by exposure to DoUCR50. StM and ARF did not reduce the hatch of J2 from H. schachtii females of any age. Eggs removed from females and spread onto water agar cultures of the fungi were mostly resistant to parasitism. DoUCR50 parasitized only 16% of such eggs from 3-week-old females. Extracellular hydrolytic enzyme production by the three fungal strains grown on PDA or parasitized H. schachtii females was evaluated using API ZYM (bioMérieux) test strips. All three fungi produced extracellular hydrolytic enzymes when grown on PDA or H. schachtii females. Trypsin-like protease activity was uniquely detected in DoUCR50 grown on PDA and H. schachtii females, with the highest activity associated with the fungus grown on parasitized females.

Novel Polyketide-Terpenoid Hybrid Metabolites and Increased Fungal Nematocidal Ability by Disruption of Genes 277 and 279 in Nematode-Trapping Fungus Arthrobotrys oligospora

Nematode-trapping fungus Arthrobotrys oligospora can produce a type of sesquiterpenyl epoxy-cyclohexenoid (SEC) metabolites that are regarded as characteristic chemtaxonomic markers. Here, we reported investigation on the functions of a putatively cupin-like family gene 277 and a dehydrogenase gene 279 by gene engineering, chemical metabolite profiling and phenotype analysis. Ten targeted metabolites were isolated from two mutants Δ277 and Δ279 and four novel metabolites including three polyketide-terpenoid (PK-TP) hybrid ones were characterized. Metabolite C_277-1 from mutant Δ277 shared the characteristic feature of the first and simplest PK-TP hybrid precursor, prenyl toluquinol, and metabolites C_279-1 and C_279-2 from mutant Δ279 shared the basic carbon skeleton of the key PK-TP hybrid precursor, farnesyl toluquinol, for biosynthesis of SEC metabolites. These results suggested that gene 277 should be involved in biosynthesis of the second prenyl unit for farnesyl toluquinol precursor, and gene 279 might be responsible for the diagnostic epoxy formation. Further analysis revealed that genes 277 and 279 might play roles in fungal conidiation, predatory trap formation, and nematode-capturing ability.

Structural insights into the fungi-nematodes interaction mediated by fucose-specific lectin AofleA from Arthrobotrys oligospora

Fungal lectin can bind specific carbohydrate structures of the host and work in recognition and adhesion or as a toxic factor. AofleA, as a fucose-specific lectin from widely studied nematode predatory fungus Arthrobotrys oligospora, possibly plays a key role in the event of capturing nematodes, but the mechanism remains unknown. Here we report the crystal structure of AofleA, which exists as a homodimer with each subunit folds as a six-bladed β-propeller. Our structural and biological results revealed that three of the six putative binding sites of AofleA had fucose-binding abilities. In addition, we found that AofleA could bind to the pharynx and intestine of the nematode in a fucose-binding-dependent manner. Our results facilitate the understanding of the mechanism that fucose-specific lectin mediates fungi-nematodes interaction, and provide structural information for the development of potential applications of AofleA.

Expansion of Adhesion Genes Drives Pathogenic Adaptation of Nematode-Trapping Fungi

Understanding how fungi interact with other organisms has significant medical, environmental, and agricultural implications. Nematode-trapping fungi (NTF) can switch to pathogens by producing various trapping devices to capture nematodes. Here we perform comparative genomic analysis of the NTF with four representative trapping devices. Phylogenomic reconstruction of these NTF suggested an evolutionary trend of trapping device simplification in morphology. Interestingly, trapping device simplification was accompanied by expansion of gene families encoding adhesion proteins and their increasing adhesiveness on trap surfaces. Gene expression analysis revealed a consistent up-regulation of the adhesion genes during their lifestyle transition from saprophytic to nematophagous stages. Our results suggest that the expansion of adhesion genes in NTF genomes and consequential increase in trap surface adhesiveness are likely the key drivers of fungal adaptation in trapping nematodes, providing new insights into understanding mechanisms underlying infection and adaptation of pathogenic fungi.

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Expansion of subtilisin, adhesion protein, and polygalacturonase gene families

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Trap simplification during evolution of nematode-trapping fungi

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Connection between trap simplification and expansion of adhesion genes

Nematode-Trapping Fungi Produce Diverse Metabolites during Predator-Prey Interaction

Nematode-trapping fungi are natural antagonists of nematodes. These predatory fungi are capable of switching their lifestyle from a saprophytic to predatory stage in the presence of nematodes by developing specialized trapping devices to capture and consume nematodes. The biochemical mechanisms of such predator–prey interaction have become increasingly studied given the potential application of nematode-trapping fungi as biocontrol agents, but the involved fungal metabolites remain underexplored. Here, we report a comprehensive liquid–chromatography mass spectrometry (LC–MS) metabolomics study on one hundred wild isolates of nematode-trapping fungi in three different species, Arthrobotrysoligospora, Arthrobotrys thaumasia, and Arthrobotrys musiformis. Molecular networking analysis revealed that the fungi were capable of producing thousands of metabolites, and such chemical diversity of metabolites was notably increased as the fungi switched lifestyle to the predatory stage. Structural annotations by tandem mass spectrometry revealed that those fungal metabolites belonged to various structural families, such as peptide, siderophore, fatty alcohol, and fatty acid amide, and their production exhibited species specificity. Several small peptides (<1.5 kDa) produced by A.musiformis in the predatory stage were found, with their partial amino acid sequences resolved by the tandem mass spectra. Four fungal metabolites (desferriferrichrome, linoleyl alcohol, nonadecanamide, and citicoline) that were significantly enriched in the predatory stage were identified and validated by chemical standards, and their bioactivities against nematode prey were assessed. The availability of the metabolomics datasets will facilitate comparative studies on the metabolites of nematode-trapping fungi in the future.

Sesquiterpenyl Epoxy-Cyclohexenoids and their Signaling Functions in Nematode-Trapping Fungus Arthrobotrys oligospora

In this study, we purified three new sesquiterpenyl epoxy-cyclohexenoid (SEC) analogues, arthrobotrisin D (11) and its two derivatives, from nematode-trapping fungus Arthrobotrys oligospora. Our results revealed that arthrobotrisin type SEC metabolites could be detected in all the test fungal strains from geographically distinct regions grown on different nutrient media, indicative of unique diagnostic character as chemical indicators for A. oligospora. The time course designs over short-term intervals of the fungus under direct contact and indirect contact with living or dead nematodes revealed that arthrobotrisin B and D (6 and 11) displayed significant relationships (positive or negative correlation) with fungal saprophytic and pathogenic stages during a nematode predation event. Interestingly, fungus on nutrient-limiting medium conducive to fungal trap formation could rapidly drop the concentration levels of arthrobotrisins B and D within 6 h when dead nematodes were around, in great contrast to that for living nematodes. Moreover, only in the fungal strain under direct contact with living dominant soil bacteria, arthrobotrisins B and D exhibited significant increase in amounts. Among them, the new SEC, arthrobotrisin D (11) was found to be a key unique metabolic signal for fungal colony growth and fungal interaction with prey and bacteria. Our study suggested that chemical analysis of SEC metabolites in A. oligospora provides a window into the fungal growth status and much valuable information about ecological environments associated with the nematode infections.

The Velvet Proteins VosA and VelB Play Different Roles in Conidiation, Trap Formation, and Pathogenicity in the Nematode-Trapping Fungus Arthrobotrys oligospora

The velvet family proteins VosA and VelB are involved in growth regulation and differentiation in the model fungus Aspergillus nidulans and other filamentous fungi. In this study, the orthologs of VosA and VelB, AoVosA, and AoVelB, respectively, were characterized in the nematode-trapping fungus Arthrobotrys oligospora, which captures nematodes by producing trapping devices (traps). Deletion of the AovelB gene resulted in growth defects in different media, and the aerial hyphae from the ΔAovelB mutant lines were fewer in number and their colonies were less dense than those from the wild-type (WT) strain. The ΔAovelB mutants each displayed serious sporulation defects, and the transcripts of several sporulation-related genes (e.g., abaA, flbC, rodA, and vosA) were significantly down-regulated compared to those from the WT strain. Furthermore, the ΔAovelB mutant strains became more sensitive to chemical reagents, including sodium dodecyl sulfate and H₂O₂. Importantly, the ΔAovelB mutants were unable to produce nematode-capturing traps. Similarly, extracellular proteolytic activity was also lower in the ΔAovelB mutants than in the WT strain. In contrast, the ΔAovosA mutants displayed no obvious differences from the WT strain in these phenotypic traits, whereas conidial germination was lower in the ΔAovosA mutants, which became more sensitive to heat shock stress. Our results demonstrate that the velvet protein AoVelB is essential for conidiation, trap formation, and pathogenicity in A. oligospora, while AoVosA plays a role in the regulation of conidial germination and heat shock stress.

Appressorium: The Breakthrough in Dikarya

Phytopathogenic and mycorrhizal fungi often penetrate living hosts by using appressoria and related structures. The differentiation of similar structures in saprotrophic fungi to penetrate dead plant biomass has seldom been investigated and has been reported only in the model fungus Podospora anserina. Here, we report on the ability of many saprotrophs from a large range of taxa to produce appressoria on cellophane. Most Ascomycota and Basidiomycota were able to form appressoria. In contrast, none of the three investigated Mucoromycotina was able to differentiate such structures. The ability of filamentous fungi to differentiate appressoria no longer belongs solely to pathogenic or mutualistic fungi, and this raises the question of the evolutionary origin of the appressorium in Eumycetes.

Role of Low-Affinity Calcium System Member Fig1 Homologous Proteins in Conidiation and Trap-Formation of Nematode-trapping Fungus Arthrobotrys oligospora

Arthrobotrys oligospora is a typical nematode-trapping fungus capturing free-living nematodes by adhesive networks. Component of the low-affinity calcium uptake system (LACS) has been documented to involve in growth and sexual development of filamentous fungi. Bioassay showed incapacity of trap formation in A. oligospora on Water Agar plate containing 1 mM ethylene glycol tetraacetic acid (EGTA) due to Ca²⁺ absorbing block. The functions of homologous proteins (AoFIG_1 and AoFIG_2) of LACS were examined on conidiation and trap formation of A. oligospora. Compared with wild type, ΔAoFIG_1 (AOL_s00007g566) resulted in 90% of trap reduction, while ΔAoFIG_2 (AOL_s00004g576) reduced vegetative growth rate up to 44% and had no trap and conidia formed. The results suggest that LACS transmembrane protein fig1 homologs play vital roles in the trap-formation and is involved in conidiation and mycelium growth of A. oligospora. Our findings expand fig1 role to include development of complex trap device and conidiation.

Signal pathways involved in microbe-nematode interactions provide new insights into the biocontrol of plant-parasitic nematodes

Plant-parasitic nematodes (PPNs) cause severe damage to agricultural crops worldwide. As most chemical nematicides have negative environmental side effects, there is a pressing need for developing efficient biocontrol methods. Nematophagous microbes, the natural enemies of nematodes, are potential biocontrol agents against PPNs. These natural enemies include both bacteria and fungi and they use diverse methods to infect and kill nematodes. For instance, nematode-trapping fungi can sense host signals and produce special trapping devices to capture nematodes, whereas endo-parasitic fungi can kill nematodes by spore adhesion and invasive growth to break the nematode cuticle. By contrast, nematophagous bacteria can secrete virulence factors to kill nematodes. In addition, some bacteria can mobilize nematode-trapping fungi to kill nematodes. In response, nematodes can also sense and defend against the microbial pathogens using strategies such as producing anti-microbial peptides regulated by the innate immunity system. Recent progresses in our understanding of the signal pathways involved in microbe-nematode interactions are providing new insights in developing efficient biological control strategies against PPNs. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

Utilization of Nematode Destroying Fungi for Management of Plant-Parasitic Nematodes-A Review

Nematode destroying fungi are potential biocontrol agent for management of plant-parasitic nematodes. They inhibit nematode population through trapping devices or by means of enzymes and metabolic products. They regulate nematode behavior by interfering plant-nematode recognition, and promote plant growth. For more effective biocontrol, thorough understanding of the biology of nematode destroying fungi, targeted nematode pest and the soil ecology and environmental condition in the field is necessary. This review highlights different types of nematode destroying fungi, their mode of action as well as commercial products based on reports published in this area of research.

Integrated Metabolomics and Morphogenesis Reveal Volatile Signaling of the Nematode-Trapping Fungus Arthrobotrys oligospora

The adjustment of metabolic patterns is fundamental to fungal biology and plays vital roles in adaptation to diverse ecological challenges. Nematode-trapping fungi can switch their lifestyle from saprophytic to pathogenic by developing specific trapping devices induced by nematodes to infect their prey as a response to nutrient depletion in nature. However, the chemical identity of the specific fungal metabolites used during the switch remains poorly understood. We hypothesized that these important signal molecules might be volatile in nature. Gas chromatography-mass spectrometry was used to carry out comparative analysis of fungal metabolomics during the saprophytic and pathogenic lifestyles of the model species Arthrobotrys oligospora. Two media commonly used in research on this species, cornmeal agar (CMA) and potato dextrose agar (PDA), were chosen for use in this study. The fungus produced a small group of volatile furanone and pyrone metabolites that were associated with the switch from the saprophytic to the pathogenic stage. A. oligospora fungi grown on CMA tended to produce more traps and employ attractive furanones to improve the utilization of traps, while fungi grown on PDA developed fewer traps and used nematode-toxic furanone metabolites to compensate for insufficient traps. Another volatile pyrone metabolite, maltol, was identified as a morphological regulator for enhancing trap formation. Deletion of the gene AOL_s00079g496 in A. oligospora led to increased amounts of the furanone attractant (2-fold) in mutants and enhanced the attractive activity (1.5-fold) of the fungus, while it resulted in decreased trap formation. This investigation provides new insights regarding the comprehensive tactics of fungal adaptation to environmental stress, integrating both morphological and metabolomic mechanisms.

IMPORTANCE Nematode-trapping fungi are a unique group of soil-living fungi that can switch from the saprophytic to the pathogenic lifestyle once they come into contact with nematodes as a response to nutrient depletion. In this study, we investigated the metabolic response during the switch and the key types of metabolites involved in the interaction between fungi and nematodes. Our findings indicate that A. oligospora develops multiple and flexible metabolic tactics corresponding to different morphological responses to nematodes. A. oligospora can use similar volatile furanone and pyrone metabolites with different ecological functions to help capture nematodes in the fungal switch from the saprophytic to the pathogenic lifestyle. Furthermore, studies with A. oligospora mutants with increased furanone and pyrone metabolites confirmed the results. This investigation reveals the importance of volatile signaling in the comprehensive tactics used by nematode-trapping fungi, integrating both morphological and metabolomic mechanisms.

Predator-prey interactions of nematode-trapping fungi and nematodes: both sides of the coin

Nematode-trapping fungi develop complex trapping devices to capture and consume nematodes. The dynamics of these organisms is especially important given the pathogenicity of nematodes and, consequently, the potential application of nematode-trapping fungi as biocontrol agents. Furthermore, both the nematodes and nematode-trapping fungi can be easily grown in laboratories, making them a unique manipulatable predator-prey system to study their coevolution. Several different aspects of these fungi have been studied, such as their genetics and the different factors triggering trap formation. In this review, we use the nematode-trapping fungus Arthrobotrys oligospora (which forms adhesive nets) as a model to describe the trapping process. We divide this process into several stages; namely attraction, recognition, trap formation, adhesion, penetration, and digestion. We summarize the latest findings in the field and current knowledge on the interactions between nematodes and nematode-trapping fungi, representing both sides of the predator-prey interaction.

Alleviatory effect of spent Pleurotus eryngii Quel substrate on replant problem of Rehmannia glutinosa Libosch

Rehmannia glutinosa Libosch. is a medicinal plant cultivated at a commercial scale in China. However, replanting problems result in a severe decline in both the biomass and quality of its roots, which are of greatest medicinal value. This study attempted to remediate the replant soil using spent Pleurotus eryngii Quel substrate for alleviating this issue, and to investigate the underlying mechanisms. Results showed that R. glutinosa grew successfully in fresh soil and remedial replant soil, while no roots were harvested in the unremedied replant soil. Overall, the nutritional status in the remedial soil was higher than that of the unremedied and fresh soil, while the concentration of allelopathic phenolic acids was lower. When planted in unremedied soil, the growth of five plant pathogens was induced and one beneficial fungus was suppressed. When planted in remedied soil, four out of the five pathogens were suppressed, while two beneficial fungi were identified in the remedial soil. This study suggests that the spent P. eryngii substrate significantly alleviates the replant problem of R. glutinosa, and that the alleviatory function reflects a synergetic effect, including the supplementation of soil nutrition, the degradation of allelochemicals, and the remediation of unbalanced microbial community.

The NADPH oxidase AoNoxA in Arthrobotrys oligospora functions as an initial factor in the infection of Caenorhabditis elegans

Reactive oxygen species (ROS) produced by NADPH oxidases can serve as signaling molecules to regulate a variety of physiological processes in multi-cellular organisms. In the nematophagous fungus Arthrobotrys oligospora, we found that ROS were produced during conidial germination, hyphal extension, and trap formation in the presence of nematodes. Generation of an AoNoxA knockout strain demonstrated the crucial role of NADPH oxidase in the production of ROS in A. oligospora, with trap formation impaired in the AoNoxA mutant, even in the presence of the nematode host. In addition, the expression of virulence factor serine protease P186 was up-regulated in the wild-type strain, but not in the mutant strain, in the presence of Caenorhabditis elegans. These results indicate that ROS derived from AoNoxA are essential for full virulence of A. oligospora in nematodes.

Potent Nematicidal Activity and New Hybrid Metabolite Production by Disruption of a Cytochrome P450 Gene Involved in the Biosynthesis of Morphological Regulatory Arthrosporols in Nematode-Trapping Fungus Arthrobotrys oligospora

Types of polyketide synthase-terpenoid synthase (PKS-TPS) hybrid metabolites, including arthrosporols with significant morphological regulatory activity, have been elucidated from nematode-trapping fungus Arthrobotrys oligospora. A previous study suggested that the gene cluster AOL_s00215 in A. oligospora was involved in the production of arthrosporols. Here, we report that disruption of one cytochrome P450 monooxygenase gene AOL_s00215g280 in the cluster resulted in significant phenotypic difference and much aerial hyphae. A further bioassay indicated that the mutant showed a dramatic decrease in the conidial formation but developed numerous traps and killed 85% nematodes within 6 h in contact with prey, in sharp contrast to the wild-type strain with no obvious response. Chemical investigation revealed huge accumulation of three new PKS-TPS epoxycyclohexone derivatives with different oxygenated patterns around the epoxycyclohexone moiety and the absence of arthrosporols in the cultural broth of the mutant ΔAOL_s00215g280. These findings suggested that a study on the biosynthetic pathway for morphological regulatory metabolites in nematode-trapping fungus would provide an efficient way to develop new fungal biocontrol agents.

The roles of actin cytoskeleton and actin-associated protein Crn1p in trap formation of Arthrobotrys oligospora

Nematode-trapping fungi include a variety of species capable of generating specific trapping devices to capture nematodes and the production of devices is also an indicator of a switch from saprophytic to predacious lifestyles. Traps are developed from vegetative mycelia, but they are quite different from hyphae in both morphological and physiological characteristics. Therefore, the molecular mechanisms underlying their formation have attracted much attention. In this investigation, Arthrobotrys oligospora, a nematode-trapping fungus, has three-dimensional networks and genomics and proteomics were recently performed, so as to reveal the relationship between actin cytoskeleton and trap formation. Both actin staining via FITC-phalloidin and treatment of actin polymerization inhibitor Lat-B illustrated that the actin cytoskeleton played an important role in trap development. Furthermore, absence of the conserved actin-associated protein Crn1p caused a structural defect of traps and failure to infect nematodes. It was observed that mutant Δcrn1 represented a reduced number of rings and a lower complexity of three-dimensional networks, likely due to the disturbance of actin branching. Collectively, our study confirmed the involvement of the actin cytoskeleton as well as the conserved actin-associated protein Crn1p in trap formation. It further suggested the manners in which Crn1p influences the development of three-dimensional networks in A. oligospora.

GPH1 is involved in glycerol accumulation in the three-dimensional networks of the nematode-trapping fungus Arthrobotrys oligospora

Turgor is very important for the invasive growth of fungal pathogens. Glycerol, a highly osmotic solvent, is considered to play an important role in turgor generation. The nematophagous fungus Arthrobotrys oligospora mainly lives as a saprophyte. In the presence of nematodes, A. oligospora enters the parasitic stage by forming three-dimensional networks (traps) to capture nematodes. In A. oligospora, we found that glycerol accumulated during nematode-induced trap formation. We demonstrated that deleting gph1, which encodes glycogen phosphorylase, decreased the glycerol content, compared with that of a wild-type strain. Although the number of traps induced by nematodes was not affected in the Δgph1 mutant, the capture rate was lower. Meanwhile, deleting gph1 also affected the growth rate and conidiation capacity of the fungus. These results indicate that glycerol derived from GPH1 is essential for the full virulence of A. oligospora against nematodes.

Nematicidal Key Precursors for the Biosynthesis of Morphological Regulatory Arthrosporols in the Nematode-Trapping Fungus Arthrobotrys oligospora

Arthrobotrys oligospora is the first recognized nematode-trapping fungus and by far the most abundant in the environment. Our recent study revealed the polyketide synthase (PKS) gene AOL_s00215g283 in A. oligospora involved in the production of many secondary metabolites and the trap formation of the fungus. Here we report that the disruption of two genes in the upstream flanking region of the gene AOL_s00215g283, AOL_s00215g281 and AOL_s00215g282, which putatively encoded one amidohydrolase and one cytochrome P450 monooxygenase, respectively, both resulted in significant nematicidal activity of the cultural broths of the mutants and loss of morphological regulatory arthrosporols. Chemical investigation revealed the huge accumulation of 6-methylsalicylic acid in the cultural broth of the mutant ΔAOL_s00215g281 and the high production of m-cresol in the mutant ΔAOL_s00215g282, respectively. Further bioassay revealed that 6-methylsalicylic acid and m-cresol displayed significant nematicidal activity toward root-knot nematodes Meloidogyne incognita with IC₉₀ values of 300 and 100 μg/mL, respectively. The mutant ΔAOL_s00215g282 displayed a more complex metabolite profile than the mutant ΔAOL_s00215g281, suggesting that m-cresol was a more versatile key precursor than 6-methylsalicylic acid. These findings not only demonstrated that the gene AOL_s00215g283 encodes the 6-methylsalicylic acid synthase and the gene AOL_s00215g281 encodes the decarboxylase for 6-methylsalicylic acid but also provided evidence for the potential functions of the precursors in fungal complex biosynthetic pathways and had more implications for the establishment of efficient fungal biocontrol agents.