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

Esterification with a Long-Chain Fatty Acid Elevates the Exposure Toxicity of Tigliane Diterpenoids from Euphorbia fischeriana Roots against Nematodes

In this study, two tigliane diterpenoids, 12-deoxyphorbol-13-hexadecanoate and 12-deoxyphorbol-13-acetate (prostratin), were identified from the methanol extract of the roots of Euphorbia fischeriana and were found to have the ability to significantly reduce the survival of Caenorhabditis elegans. It was determined that exposure to these two compounds had toxic effects on the growth, reproduction, locomotion behavior, and accumulation of lipids and lipofuscin of the nematodes. Moreover, the transcription levels of the genes associated with lipid accumulation, apoptosis, insulin, and nuclear hormone synthesis in C. elegans were significantly influenced. Interestingly, 12-deoxyphorbol-13-hexadecanoate produced exposure toxicity at lower concentrations than that of prostratin. Pearson correlation analysis indicates that the elevated exposure toxicity of 12-deoxyphorbol-13-hexadecanoate may be the result of differing transcription levels, which result from the differential expression of fat-6, egl-38, and cep-1. These results reveal that esterification with a long-chain fatty acid elevates the exposure toxicity of this tigliane diterpenoid, thus providing a basis for the application of tigliane diterpenoids in plant-derived nematicides.

Function discovery of a non-ribosomal peptide synthetase-like encoding gene in the nematode-trapping fungus Arthrobotrys oligospora

In this study, the function of a non-ribosomal peptide synthetase-like (NRPS-like) encoding gene AOL_s00188g306 (g306) was investigated to reveal the association between NRPS and nematocidal activity in the nematode-trapping fungus Arthrobotrys oligospora. Sequence analysis indicated that the encoded product of g306 is an adenylation domain of non-ribosomal peptide synthetases and extended short-chain dehydrogenase/reductase domain-containing proteins, and displays a wide substrate spectrum. The Δg306 mutants were more sensitive to chemical stressors than the wild type. Disruption of g306 impeded the nematocidal efficiency of A. oligospora. Metabolomics analysis showed that secondary metabolite biosynthesis and lipid metabolism were altered in the mutants. The phenotypic changes in the mutants can be attributed to the down-regulation of various metabolites, including fatty acyls, prenol lipids, steroidsand steroid derivative, and amino acid derivatives, identified in the present study. This study investigated the association between the non-ribosomal polypeptide-encoding gene g306 and nematicidal activity in A. oligospora, providing a reference for resolving the predation mechanism of nematode-trapping fungus.

Molecular Defense Response of Bursaphelenchus xylophilus to the Nematophagous Fungus Arthrobotrys robusta

Bursaphelenchus xylophilus causes pine wilt disease, which poses a serious threat to forestry ecology around the world. Microorganisms are environmentally friendly alternatives to the use of chemical nematicides to control B. xylophilus in a sustainable way. In this study, we isolated a nematophagous fungus-Arthrobotrys robusta-from the xylem of diseased Pinus massoniana. The nematophagous activity of A. robusta against the PWNs was observed after just 6 h. We found that B. xylophilus entered the trap of A. robusta at 24 h, and the nervous system and immunological response of B. xylophilus were stimulated by metabolites that A. robusta produced. At 30 h of exposure to A. robusta, B. xylophilus exhibited significant constriction, and we were able to identify xenobiotics. Bursaphelenchus xylophilus activated xenobiotic metabolism, which expelled the xenobiotics from their bodies, by providing energy through lipid metabolism. When PWNs were exposed to A. robusta for 36 h, lysosomal and autophagy-related genes were activated, and the bodies of the nematodes underwent disintegration. Moreover, a gene co-expression pattern network was constructed by WGCNA and Cytoscape. The gene co-expression pattern network suggested that metabolic processes, developmental processes, detoxification, biological regulation, and signaling were influential when the B. xylophilus specimens were exposed to A. robusta. Additionally, bZIP transcription factors, ankyrin, ATPases, innexin, major facilitator, and cytochrome P450 played critical roles in the network. This study proposes a model in which mobility improved whenever B. xylophilus entered the traps of A. robusta. The model will provide a solid foundation with which to understand the molecular and evolutionary mechanisms underlying interactions between nematodes and nematophagous fungi. Taken together, these findings contribute in several ways to our understanding of B. xylophilus exposed to microorganisms and provide a basis for establishing an environmentally friendly prevention and control strategy.

Tools and basic procedures of gene manipulation in nematode-trapping fungi

Nematode-trapping fungi (NTF) are the majority of carnivorous microbes to capture nematodes through diverse and sophisticated trapping organs derived from hyphae. They can adopt carnivorous lifestyles in addition to saprophytism to obtain extra-nutrition from nematodes. As a special group of fungi, the NTF are not only excellent model organism for studying lifestyle transition of fungi but also natural resources of exploring biological control of nematodes. However, the carnivorous mechanism of NTF remains poorly understood. Nowadays, the omics studies of NTF have provided numerous genes and pathways that are associated with the phenotypes of carnivorous traits, which need molecular tools to verify. Here, we review the development and progress of gene manipulation tools in NTF, including methodology and strategy of transformation, random gene mutagenesis methods and target gene mutagenesis methods. The principle and practical approach for each method was summarized and discussed, and the basic operational flow for each tool was described. This paper offers a clear reference and instruction for researchers who work on NTF as well as other group of fungi.

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.

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.

Acetylation of Sesquiterpenyl Epoxy-Cyclohexenoids Regulates Fungal Growth, Stress Resistance, Endocytosis, and Pathogenicity of Nematode-Trapping Fungus Arthrobotrys oligospora via Metabolism and Transcription

Sesquiterpenyl epoxy-cyclohexenoids (SECs) that depend on a polyketide synthase-terpenoid synthase (PKS-TPS) pathway are widely distributed in plant pathogenic fungi. However, the biosynthesis and function of the acetylated SECs still remained cryptic. Here, we identified that AOL_s00215g 273 (273) was responsible for the acetylation of SECs in Arthrobotrys oligospora via the construction of Δ273, in which the acetylated SECs were absent and major antibacterial nonacetylated SECs accumulated. Mutant Δ273 displayed increased trap formation, and nematicidal and antibacterial activities but decreased fungal growth and soil colonization. Glutamine, a key precursor for NH₃ as a trap inducer, was highly accumulated, and biologically active phenylpropanoids and antibiotics were highly enriched in Δ273. The decreased endocytosis and increased autophagosomes, with the most upregulated genes involved in maintaining DNA and transcriptional stability and pathways related to coronavirus disease and exosome, suggested that lack of 273 might result in increased virus infection and the acetylation of SECs played a key role in fungal diverse antagonistic ability.

Regulatory Mechanism of Trap Formation in the Nematode-Trapping Fungi

Nematode-trapping (NT) fungi play a significant role in the biological control of plant- parasitic nematodes. NT fungi, as a predator, can differentiate into specialized structures called “traps” to capture, kill, and consume nematodes at a nutrient-deprived condition. Therefore, trap formation is also an important indicator that NT fungi transition from a saprophytic to a predacious lifestyle. With the development of gene knockout and multiple omics such as genomics, transcriptomics, and metabolomics, increasing studies have tried to investigate the regulation mechanism of trap formation in NT fungi. This review summarizes the potential regulatory mechanism of trap formation in NT fungi based on the latest findings in this field. Signaling pathways have been confirmed to play an especially vital role in trap formation based on phenotypes of various mutants and multi-omics analysis, and the involvement of small molecule compounds, woronin body, peroxisome, autophagy, and pH-sensing receptors in the formation of traps are also discussed. In addition, we also highlight the research focus for elucidating the mechanism underlying trap formation of NT fungi in the future.

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.

Nematicidal Activity of Grammicin Biosynthesis Pathway Intermediates in Xylaria grammica KCTC 13121BP against Meloidogyne incognita

Grammicin, a polyketide metabolite produced by the endolichenic fungus Xylaria grammica KCTC 13121BP, shows strong nematicidal activity against Meloidogyne incognita. This study was performed to elucidate the grammicin biosynthesis pathway of X. grammica KCTC 13121BP and to examine the nematicidal activity of the biosynthesis intermediates and derivatives against M. incognita. Two grammicin biosynthesis intermediates were isolated from a T-DNA insertion transformant (strain TR-74) of X. grammica KCTC 13121BP and identified as 2-(hydroxymethyl)cyclohexa-2,5-diene-1,4-dione (compound 1) and 2,5-dihydroxybenzaldehyde (compound 2), which were also reported to be intermediates in the biosynthesis pathway of patulin, an isomer of grammicin. This indicates that the grammicin biosynthesis pathway overlaps almost with that of patulin, except for the last few steps. Among 13 grammicin biosynthesis intermediates and their derivatives (except grammicin), toluquinol caused the highest M. incognita J2 mortality, with an LC_{50/72 h} value of 11.13 µg/mL, which is similar to grammicin with an LC_{50/72 h} value of 15.95 µg/mL. In tomato pot experiments, the wettable powder type formulations (WP) of toluquinol (17.78 µg/mL) and grammicin (17.78 µg/mL) also effectively reduced gall formation on the roots of tomato plants with control values of 72.22% and 77.76%, respectively, which are much higher than abamectin (16.67%), but lower than fosthiazate (100%). The results suggest that toluquinol can be used directly as a biochemical nematicide or as a lead molecule for the development of new synthetic nematicides for the control of root-knot nematode diseases.

Polyketide Synthase-Terpenoid Synthase Hybrid Pathway Regulation of Trap Formation through Ammonia Metabolism Controls Soil Colonization of Predominant Nematode-Trapping Fungus

Polyketide synthase-terpenoid synthase (PKS-TPS) hybrid pathways for biosynthesis of unique sesquiterpenyl epoxy-cyclohexenoids (SECs) have been found to be widely distributed in plant pathogenic fungi. However, the natural and ecological functions of these pathways and their metabolites still remain cryptic. In this study, the whole PKS-TPS hybrid pathway in the predominant nematode-trapping fungus Arthrobotrys oligospora was first proposed according to all the intermediates and their derivatives from all the A. oligospora mutants with a deficiency in each gene involved in SEC biosynthesis. Most mutants displayed significantly increased trap formation which was correlated with alteration of the ammonia level. Further analysis revealed that the main metabolites involved in ammonia metabolism were largely increased in most mutants. However, significantly retarded colonization in soil were observed in most mutants compared to the wild-type strain due to significantly decreased antibacterial activities. Our results suggested that A. oligospora used the PKS-TPS hybrid pathway for fungal soil colonization via decreasing fungal nematode-capturing ability. This also provided solid evidence that boosting fungal colonization in soil was the secondary metabolite whose biosynthesis depended on a PKS-TPS hybrid pathway.

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.

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.

Unexpected Biosynthesis of Fluorescein-Like Arthrocolins against Resistant Strains in an Engineered Escherichia coli

Here we provide an unprecedented biofactory where fluorescent dye-like complex xanthenes could be produced in an engineered Escherichia coli. Feeding the strain with toluquinol or hydroquinones resulted in production of novel "unnatural" natural products including four arthrocolins embedded with indolyltriphenyl quaternary carbons. Arthrocolins A-C potently inhibited various human cancer cell lines including paclitaxel-resistant cell line A549/Taxol and methicillin-resistant Staphylococcus aureus and immensely restored the sensitivity of intractable fluconazole-resistant human pathogen Candida albicans to fluconazole.

Selected Mutations Revealed Intermediates and Key Precursors in the Biosynthesis of Polyketide-Terpenoid Hybrid Sesquiterpenyl Epoxy-cyclohexenoids

Sesquiterpenyl epoxy-cyclohexenoids (SECs) show impressive biological activities. However, the key pathways for SECs still remain unambiguous. Unexpectedly, 11 new SECs and derivatives with diverse oxidation patterns were isolated after the deletion of gene 274. A high accumulation of toluquinol and its new glycosides in mutant Δ276 and further isolation of the most crucial precursors farnesyl hydroquinone, farnesyl quinone, and three new derivatives from mutant Δ278 confirm that farnesylation at toluquinol is the key step for SECs.

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.