CRISPR-Cas9-mediated enhancement of Beauveria bassiana virulence with overproduction of oosporein

Biocontrol agents play a pivotal role in managing pests and contribute to sustainable agriculture. Recent advancements in genetic engineering can facilitate the development of entomopathogenic fungi with desired traits to enhance biocontrol efficacy. In this study, a CRISPR-Cas9 ribonucleoprotein system was utilized to genetically improve the virulence of Beauveria bassiana, a broad-spectrum insect pathogen used in biocontrol of arthropod pests worldwide. CRISPR-Cas9-based disruption of the transcription factor-encoding gene Bbsmr1 led to derepression of the oosporein biosynthetic gene cluster resulting in overproduction of the red-pigmented dibenzoquinone oosporein involved in host immune evasion, thus increasing fungal virulence. Mutants defective for Bbsmr1 displayed a remarkable enhanced insecticidal activity by reducing lethal times and concentrations, while concomitantly presenting negligible or minor pleiotropic effects. In addition, these mutants displayed faster germination on the insect cuticle which correlated with higher density of free-floating blastospores in the hemolymph and accelerated mortality of the host. These findings emphasize the utility of genetic engineering in developing enhanced fungal biocontrol agents with customized phenotypic traits, and provide an efficient and versatile genetic transformation tool for application in other beneficial entomopathogenic fungi.

Transcriptional reprogramming in the entomopathogenic fungus Metarhizium brunneum and its aphid host Myzus persicae during the switch between saprophytic and parasitic lifestyles

BACKGROUND

The fungus Metarhizium brunneum has evolved a remarkable ability to switch between different lifestyles. It develops as a saprophyte, an endophyte establishing mutualistic relationships with plants, or a parasite, enabling its use for the control of insect pests such as the aphid Myzus persicae. We tested our hypothesis that switches between lifestyles must be accompanied by fundamental transcriptional reprogramming, reflecting adaptations to different environmental settings.

RESULTS

We combined high throughput RNA sequencing of M. brunneum in vitro and at different stages of pathogenesis to validate the modulation of genes in the fungus and its host during the course of infection. In agreement with our hypothesis, we observed transcriptional reprogramming in M. brunneum following conidial attachment, germination on the cuticle, and early-stage growth within the host. This involved the upregulation of genes encoding degrading enzymes and gene clusters involved in synthesis of secondary metabolites that act as virulence factors. The transcriptional response of the aphid host included the upregulation of genes potentially involved in antifungal activity, but antifungal peptides were not induced. We also observed the induction of a host flightin gene, which may be involved in wing formation and flight muscle development.

CONCLUSIONS

The switch from saprophytic to parasitic development in M. brunneum is accompanied by fundamental transcriptional reprogramming during the course of the infection. The aphid host responds to fungal infection with its own transcriptional reprogramming, reflecting its inability to express antifungal peptides but featuring the induction of genes involved in winged morphs that may enable offspring to avoid the contaminated environment.

Efficacy of entomopathogenic fungi against Philaenus spumarius, the vector of Xylella fastidosa

BACKGROUND

Xylella fastidiosa is an important causative agent of Olive Quick Decline Syndrome in the Apulia region of Italy. The current study evaluated the bioefficacy of three entomopathogenic fungal strains: Beauveria bassiana SGB7004, Metarhizium robertsii SGB1K, and Akanthomyces lecanii SGB4711 against Philaenus spumarius the main vector of this pathogen, under laboratory conditions. Pathogenicity bioassays were performed by dipping nymphs and adults of P. spumarius in an aqueous suspension of powdered fungal culture (PFC) or conidial suspension (CS) of the three fungal strains.

RESULTS

Both B. bassiana SGB7004 and M. robertsii SGB1K affected the viability of nymphs, resulting in more than 80% mortality at 48 h post treatment, while the effect of A. lecanii SGB4711 was not statistically significant. On adults, all three biocontrol strains were effective in a time- and concentration-dependent manner. The PFCs of B. bassiana SGB7004, M. robertsii SGB1K, and A. lecanii SGB4711 at the highest concentration tested (120 mg mL-1) resulted in 97%, 83% and 27% mortality at the trial endpoint (120 h), respectively. Mycelial growth was observed on 38.5%, 37.0% and 61.5% of dead insects treated with B. bassiana SGB7004 (2.3 × 108 CFU mL-1), M. robertsii SGB1K (3.8 × 106 CFU mL-1) and A. lecanii SGB4711 (5.4 × 108 CFU mL-1), respectively. None of the PFCs of the tested strains was pathogenic when injected into nymph spittle.

CONCLUSIONS

Beauveria bassiana SGB7004 and M. robertsii SGB1K significantly affected the survival of P. spumarius nymphs and adults, while A. lecanii SGB4711 was not effective on nymphs and only slightly effective against adults. © 2024 Society of Chemical Industry.

Bioactive Bioxanthracene and Cyclodepsipeptides from the Entomopathogenic Fungus Blackwellomyces roseostromatus BCC56290

In the course of our ongoing research targeting the identification of potential biocontrol agents from entomopathogenic fungi (EPF), we explored a solid-state rice fungal extract of Blackwellomyces roseostromatus BCC56290 derived from infected lepidopteran larvae. Chemical and biological prospections afforded four unprecedentedly reported natural products differentiated into a dimeric naphthopyran bioxanthracene ES-242 derivative (1) and three cyclodepsipeptides (2-4) in addition to two known cyclodepsipeptides, cardinalisamides B (5) and C (6). Chemical structures of the isolated compounds were elucidated through comprehensive 1D/2D NMR and HR-ESI-MS data together with comparisons to the reported literature. The absolute configuration of the isolated cyclodepsipeptides was determined using Marfey's method. All isolated compounds were assessed for their antimicrobial, cytotoxic, and nematicidal activities with some compounds revealing significant activities.

Metabolomic profiling of virulent and non-virulent Beauveria bassiana strains: insights into the pathogenicity of Tetranychus truncatus

In this study, the pathogenicity of local Beauveria bassiana strains was elucidated using molecular and metabolomics methodologies. Molecular verification of the B. bassiana-specific chitinase gene was achieved via phylogenetic analysis of the Bbchit1 region. Subsequent metabolomic analyses employing UPLC-Q-TOF-MS revealed a different number of non-volatile metabolite profiles among the six B. bassiana strains. Bb6 produced the most non-volatile compounds (17) out of a total of 18, followed by Bb15 (16) and Bb12 (15). Similarly, Bb5, Bb8, and Bb21, three non-virulent B. bassiana strains, produced 13, 14, and 14 metabolites, respectively. But unique secondary metabolites like bassianolide and beauvericin, pivotal for virulence and mite management, were exclusively found in the virulent strains (Bb6, Bb12, and Bb15) of B. bassiana. The distinctive non-volatile metabolomic profiles of these strains underscore their pathogenicity against Tetranychus truncatus, suggesting their promise in bio-control applications.

Computational Applications: Beauvericin from a Mycotoxin into a Humanized Drug

Drug discovery was initially attributed to coincidence or experimental research. Historically, the traditional approaches were complex, lengthy, and expensive, entailing costly random screening of synthesized compounds or natural products coupled with in vivo validation largely depending on the availability of appropriate animal models. Currently, in silico modeling has become a vital tool for drug discovery and repurposing. Molecular docking and dynamic simulations are being used to find the best match between a ligand and a molecule, an approach that could help predict the biomolecular interactions between the drug and the target host. Beauvericin (BEA) is an emerging mycotoxin produced by the entomopathogenic fungus Beauveria bassiana, being originally studied for its potential use as a pesticide. BEA is now considered a molecule of interest for its possible use in diverse biotechnological applications in the pharmaceutical industry and medicine. In this manuscript, we provide an overview of the repurposing of BEA as a potential therapeutic agent for multiple diseases. Furthermore, considerable emphasis is given to the fundamental role of in silico techniques to (i) further investigate the activity spectrum of BEA, a secondary metabolite, and (ii) elucidate its mode of action.

Bbhox2 is a key regulator for conidiation and virulence in Beauveria bassiana

Beauveria bassiana, a well-known filamentous biocontrol fungus, is the main pathogen of numerous field and forest pests. To explore the potential factors involved in the fungal pathogenicity, Bbhox2, an important and conserved functional transcription factor containing homeodomain was carried out by functional analysis. Homologous recombination was used to disrupt the Bbhox2 gene in B.bassiana. The conidia yield of the deletant fungal strain was significantly reduced. The conidial germination was faster, and stress tolerance to Congo red and high osmotic agents were decreased compared with that in the wildtype. Additionally, ΔBbhox2 showed a dramatic reduction in virulence no matter in topical inoculations or in intra-hemolymph injections against Galleria mellonella larvae, which is likely due to the failure of appressorium formation and the defect in producing hyphal body. These results indicate that the Bbhox2 gene markedly contributes to conidiation and pathogenicity in B. bassiana.

Multiomics Reveals Mechanisms of Alternaria oxytropis Inhibiting Pathogenic Fungi in Oxytropis ochrocephala

Endophytic fungi can benefit the host plant and increase the plant resistance. Now, there is no in-depth study of how Alternaria oxytropis (A. oxytropis) is enhancing the ability of inhibiting pathogenic fungi in Oxytropis ochrocephala (O. ochrocephala). In this study, the fungal community and metabolites associated with endophyte-infected (EI) and endophyte-free (EF) O. ochrocephala were compared by multiomics. The fungal community indicated that there was more A. oxytropis, less phylum Ascomycota, and less genera Leptosphaeria, Colletotrichum, and Comoclathris in the EI group. As metabolic biomarkers, the levels of swainsonine and apigenin-7-O-glucoside-4-O-rutinoside were significantly increased in the EI group. Through in vitro validation experiments, swainsonine and apigenin-7-O-glucoside-4-O-rutinoside can dramatically suppress the growth of pathogenic fungi Leptosphaeria sclerotioides and Colletotrichum americae-borealis by increasing the level of oxidative stress. This work suggested that O. ochrocephala containing A. oxytropis could increase the resistance to fungal diseases by markedly enhancing the content of metabolites inhibiting pathogenic fungi.

Cross-talk between immunity and behavior: insights from entomopathogenic fungi and their insect hosts

Insects are one of the most successful animals in nature, and entomopathogenic fungi play a significant role in the natural epizootic control of insect populations in many ecosystems. The interaction between insects and entomopathogenic fungi has continuously coevolved over hundreds of millions of years. Many components of the insect innate immune responses against fungal infection are conserved across phyla. Additionally, behavioral responses, which include avoidance, grooming, and/or modulation of body temperature, have been recognized as important mechanisms for opposing fungal pathogens. In an effort to investigate possible cross-talk and mediating mechanisms between these fundamental biological processes, recent studies have integrated and/or explored immune and behavioral responses. Current information indicates that during discrete stages of fungal infection, several insect behavioral and immune responses are altered simultaneously, suggesting important connections between the two systems. This review synthesizes recent advances in our understanding of the physiological and molecular aspects influencing cross-talk between behavioral and innate immune antifungal reactions, including chemical perception and olfactory pathways.

Effect of entomopathogenic fungus Beauveria bassiana on the growth characteristics and metabolism of black soldier fly larvae

Beauveria bassiana is an entomopathogenic fungus widely used in agriculture to reduce populations of various pests. However, when agricultural waste is utilized for organic recycling, B. bassiana has the potential to impact recycling performance, by affecting the survival, and body mass of decomposing organisms (such as insect's larvae). Additionally, in natural conditions where decayed organic matter contains a high load of different entomopathogenic organisms, larval growth may be affected when consumed or in contact. In a laboratory study, we aimed to comprehend the effects of B. bassiana on the growth characteristics and larval metabolism of the black soldier fly larvae, which is a known decomposing insect. The experiments used both feeding (mixing the spores with the diet, hereafter BF) and contact treatments (by dipping the larva in the spores solution, hereafter BD), and were compared to a water-treated control group. The BF treatment significantly reduced larval body weight, adult emergence, and adult weight compared to both the control and the BD treatment. Furthermore, an analysis of hemolymph metabolites, categorized by class, indicated a higher accumulation of metabolites belonging to the purine and purine derivative classes, as well as carboxylic acids and their derivatives, including peptides and oligopeptides, indicating potential disruption of protein synthesis or degradation caused by the BF treatment. Pathway enrichment analysis showed significant alterations in purine metabolism and D-Arginine and D-ornithine metabolism compared to the control. Taurine and hypotaurine metabolism were significantly altered in the BD treatment compared to the control but not significantly enriched in the BF treatment. Our results suggest that the BF treatment impairs protein synthesis or degradation, affecting larval growth characteristics. Future studies should explore innate immunity-related gene expression and antimicrobial peptide production in BSF larvae to understand their immunity to pathogens.

Entomopathogenic Fungi: An Eco-Friendly Synthesis of Sustainable Nanoparticles and Their Nanopesticide Properties

The agricultural industry could undergo significant changes due to the revolutionary potential of nanotechnology. Nanotechnology has a broad range of possible applications and advantages, including insect pest management using treatments based on nanoparticle insecticides. Conventional techniques, such as integrated pest management, are inadequate, and using chemical pesticides has negative consequences. As a result, nanotechnology would provide ecologically beneficial and effective alternatives for insect pest control. Considering the remarkable traits they exhibit, silver nanoparticles (AgNPs) are recognized as potential prospects in agriculture. Due to their efficiency and great biocompatibility, the utilization of biologically synthesized nanosilver in insect pest control has significantly increased nowadays. Silver nanoparticles have been produced using a wide range of microbes and plants, which is considered an environmentally friendly method. However, among all, entomopathogenic fungi (EPF) have the most potential to be used in the biosynthesis of silver nanoparticles with a variety of properties. Therefore, in this review, different ways to get rid of agricultural pests have been discussed, with a focus on the importance and growing popularity of biosynthesized nanosilver, especially silver nanoparticles made from fungi that kill insects. Finally, the review highlights the need for further studies so that the efficiency of bio-nanosilver could be tested for field application and the exact mode of action of silver nanoparticles against pests can be elucidated, which will eventually be a boon to the agricultural industry for putting a check on pest populations.

Evaluation of Metarhizium brunneum- and Metarhizium-Derived VOCs as Dual-Active Biostimulants and Pest Repellents in a Wireworm-Infested Potato Field

Wireworm, the larval stages of click beetles, are a serious pest of tubers, brassicas and other important commercial crops throughout the northern hemisphere. No effective control agent has been developed specifically for them, and many of the pesticides marketed as having secondary application against them have been withdrawn from EU and Asian markets. Metarhizium brunneum, an effective entomopathogenic fungus, and its derived volatile metabolites are known to be effective plant biostimulants and plant protectants, although field efficacy has yet to be validated. Field validation of a combined M. brunneum and derived VOC treatments was conducted in Wales, UK, to assess the effects of each as a wireworm control agent and biostimulant. Plots were treated with Tri-Soil (Trichoderma atroviridae), M. brunneum, 1-octen-3-ol or 3-octanone, or combinations thereof. Treatments were applied subsurface during potato seeding (n = 52), and potatoes were harvested at the end of the growing season. Each potato was weighed individually and scored for levels of wireworm damage. Applications of both the VOCs and the M. brunneum individually were found to significantly decrease wireworm burden (p < 0.001). Combinations of M. brunneum and 3-octanone were also found to significantly decrease wireworm damage (p < 0.001), while no effect on yield was reported, resulting in an increased saleable mass over controls (p < 0.001). Herein, we present a novel 'stimulate and deter' wireworm control strategy that can be used to significantly enhance saleable potato yields and control wireworm populations, even under high pest pressure densities.

Metabolomic Analysis Demonstrates the Impacts of Polyketide Synthases PKS14 and PKS15 on the Production of Beauvericins, Bassianolide, Enniatin A, and Ferricrocin in Entomopathogen Beauveria bassiana

Beauveria bassiana is a globally distributed entomopathogenic fungus that produces various secondary metabolites to support its pathogenesis in insects. Two polyketide synthase genes, pks14 and pks15, are highly conserved in entomopathogenic fungi and are important for insect virulence. However, understanding of their mechanisms in insect pathogenicity is still limited. Here, we overexpressed these two genes in B. bassiana and compared the metabolite profiles of pks14 and pks15 overexpression strains to those of their respective knockout strains in culture and in vivo using tandem liquid chromatography-mass spectrometry (LC-MS/MS) with Global Natural Products Social Molecular Networking (GNPS). The pks14 and pks15 clusters exhibited crosstalk with biosynthetic clusters encoding insect-virulent metabolites, including beauvericins, bassianolide, enniatin A, and the intracellular siderophore ferricrocin under certain conditions. These secondary metabolites were upregulated in the pks14-overexpressing strain in culture and the pks15-overexpressing strain in vivo. These data suggest that pks14 and pks15, their proteins or their cluster components might be directly or indirectly associated with key pathways in insect pathogenesis of B. bassiana, particularly those related to secondary metabolism. Information about interactions between the polyketide clusters and other biosynthetic clusters improves scientific understanding about crosstalk among biosynthetic pathways and mechanisms of pathogenesis.

AoSte12 Is Required for Mycelial Development, Conidiation, Trap Morphogenesis, and Secondary Metabolism by Regulating Hyphal Fusion in Nematode-Trapping Fungus Arthrobotrys oligospora

Nematode-trapping (NT) fungi are a unique group of carnivorous microorganisms that can capture and digest nematodes by producing ingenious trapping devices (traps). Arthrobotrys oligospora, a representative NT fungus, can develop adhesive three-dimensional networks for nematode predation. Hyphal fusion is indispensable for the trap formation of A. oligospora. Here, we characterized an orthologous Ste12 protein (AoSte12) in A. oligospora via gene disruption, DNA affinity purification sequencing (DAP-Seq), and multi-omics approaches. The disruption of the Aoste12 gene caused an increase in hyphal fusion and resulted in defects in mycelial growth, conidiation, trap morphology, and stress resistance, as well as reducing the number of nuclei and lipid droplet accumulation. Moreover, transcriptome and DAP-Seq analysis revealed that AoSte12 was involved in cellular processes associated with growth, cell fusion, the tricarboxylic acid cycle, vesicles, actin filaments, and lipid metabolism. In addition, combining metabolome with transcriptome and DAP-Seq analysis indicated that AoSte12 was involved in the mitogen-activated protein kinase signaling pathway, lipid metabolism, and secondary metabolites. A yeast two-hybrid assay revealed that AoSte12 can interact with diverse proteins, such as the MAK-2 orthologue protein Fus3, the vacuolar sorting protein Pep3, and UDP-glycosyltransferase. Our results suggest that AoSte12 plays an indispensable role in hyphal fusion and thus regulates sporulation and trap morphogenesis. These results provide deep insights into the connection between hyphal fusion and trap formation in NT fungi. IMPORTANCE Nematode-trapping (NT) fungi are an important natural enemy of nematodes and can capture their prey by producing traps. Hyphal anastomosis and fusion are important for mycelial growth and the colony morphological development of filamentous fungi and are also crucial for the trap morphogenesis of NT fungi. Arthrobotrys oligospora can form complex three-dimensional networks (traps) when sensing the presence of nematodes. This study revealed that AoSte12 is indispensable for hyphal fusion and that it regulates mycelial growth, conidiation, trap morphogenesis, stress resistance, the number of nuclei, and lipid droplet accumulation in A. oligospora. In addition, DNA affinity purification sequencing, transcriptome, and metabolome analyses further revealed that AoSte12 is involved in the mitogen-activated protein kinase pathway, lipid metabolism, and secondary metabolism. Overall, these findings expand the important role of AoSte12 in NT fungus A. oligospora and provide a broad foundation for elucidating the regulatory mechanism of trap development and the lifestyle transitions of pathogenic fungi.

Genomic Determinants of Entomopathogenic Fungi and Their Involvement in Pathogenesis

Entomopathogenic fungi offer an effective and eco-friendly alternative to curb insect populations in biocontrol strategy. The evolutionary history of selected entomopathogenic fungi indicates their ancestral relationship with plant endophytes. During this host shifting, entomopathogenic fungi must have acquired multiple mechanisms, including a combination of various biomolecules that make them distinguishable from other fungi. In this review, we focus on understanding various biochemical and molecular mechanisms involved in entomopathogenesis. In particular, we attempt to explain the indispensable role of enlarged gene families of various virulent factors, viz. chitinases, proteases, lipases, specialized metabolites, and cytochrome P450, in entomopathogenesis. Our analysis suggests that entomopathogenic fungi recruit a different set of gene products during the progression of pathogenesis. Knowledge of these bio-molecular interactions between fungi and insect hosts will allow researchers to execute pointed efforts towards the development of improved entomopathogenic fungal strains.

Bioactive Alpha-Pyrone and Phenolic Glucosides from the Marine-Derived Metarhizium sp. P2100

Glycoside compounds have attracted great interest due to their remarkable and multifarious bioactivities. In this study, four hitherto unknown 4-methoxy-β-D-glucosyl derivatives were obtained and identified from the marine-derived fungus Metarhizium sp. P2100, including three alpha-pyrone glycosides (1-3) and one phenolic glycoside (4). Their planar structures were elucidated by comprehensive spectroscopic analysis, including 1D/2D NMR and HRESIMS. The absolute configurations of 1-3 were determined by a single-crystal X-ray crystallographic experiment, a comparison of the experimental, and a calculated electronic circular dichroism (ECD) spectra, respectively. Compounds 2 and 3 are a pair of rare epimeric pyranoside glycosides at C-7 with a core of aglycone as 2H-pyrone. Compounds 1-4 exhibited weak anti-inflammatory activities. In particular, compounds 1-3 displayed inhibitory activities against α-amylase, showing a potential for the development of a new α-amylase inhibitor for controlling diabetes.

Transcription Factors BbPacC and Bbmsn2 Jointly Regulate Oosporein Production in Beauveria bassiana

The entomopathogenic fungus Beauveria bassiana can produce the secondary metabolite oosporein under alkaline conditions or in fungus-killed cadavers. However, the regulatory mechanism of oosporein synthesis is not fully understood. In thisstudy, we found that the pH signaling transcription factor BbPacC is involved in the regulation of oosporein production. Overexpression of BbPacC promotes oosporein production in B. bassiana at pH 6.0 or under alkaline conditions (pH 8.0), but deletion of this gene abolished oosporein production. Under acidic conditions (pH 4.0), no oosporein production was observed in the wild-type and BbPacC overexpression strains. Yeast one-hybrid assays and electrophoretic mobility shift assay (EMSA) confirmed the binding ability of BbPacC with 4 putative PacC-binding sites in the promoter region of BbOpS3, a transcription factor located in the oosporein synthetic gene cluster regulating the expression of oosporein synthetic genes. Overexpression of Bbmsn2, a previously reported negative regulator of oosporein synthesis, in OEPacC or wild-type strains abolished oosporein production in all tested conditions. However, deletion of Bbmsn2 in the BbPacC overexpression strain significantly improved oosporein production even at pH 4.0. These results indicated that BbPacC is a positive regulator of oosporein production and functions jointly with Bbmsn2 to regulate oosporein production in different environments and particularly under alkaline conditions. IMPORTANCE B. bassiana produces the red dibenzoquinone pigment oosporein under certain specific conditions, such as alkaline conditions and fungus-killed cadavers. Ooporein possesses antibiotic and insect immune inhibition activities and plays multiple roles during the infection process of B. bassiana against insect hosts. Several negative regulators involved in oosporein synthesis have been reported; however, we know little about the positive regulators outside the biosynthetic gene cluster. Here, we found that the pH signaling transcription factor BbPacC positively regulates oosporein production by binding to several PacC-binding sites. In addition, our results also indicate that BbPacC jointly acts with the negative regulator Bbmsn2 to regulate oosporein synthesis. Our results provide insight into understanding the regulatory mechanism of oosporein production as well as targets to engineer B. bassiana strains producing high levels of oosporein.

The SWC4 subunit of the SWR1 chromatin remodeling complex is involved in varying virulence of Metarhizium brunneum isolates offering role of epigenetic regulation of pathogenicity

The host – pathogen interaction is a multifactorial process subject to a co-evolutionary arms race consisting of rapid changes in both host and pathogen, controlled at the genetic and epigenetic levels. Previously, we showed intra-species variation in disease progression and pathogenicity in aphids for Metarhizium brunneum isolates MbK and Mb7. Herein, we compared genomic, epigenetic, and metabolomic variations between these isolates and their effects on pathogenicity. Genomic variation could not completely explain the observed differences between the isolates. However, differential N6-adenine methylation (6 mA) and its correlation to reduced expression of the essential SWC4 subunit of SWR1 chromatin-remodelling complex (SWR1-C) led us to hypothesize a role for swc4 in the varying pathogenicity. Mutagenesis of the essential swc4 gene in MbKisolate resulted in reduction of secondary-metabolite (SM) secretion and impaired virulence in Galleria mellonella. Our results suggest the role of SWC4 in the regulation of SMs and the role of both SWC4 and SWR1-C in virulence of M. brunneum isolates. A better understanding of epigenetic regulation of SM production and secretion in entomopathogenic fungi may enable theirmanipulation for better biocontrol performance, and expand possibilities for environmentally friendly pest control.

Bioactive Alkaloids from the Marine-Derived Fungus Metarhizium sp. P2100

The Metarhizium fungal species are considered the prolific producers of bioactive secondary metabolites with a variety of chemical structures. In this study, the biosynthetic potential of marine-derived fungus Metarhizium sp. P2100 to produce bioactive alkaloids was explored by using the one strain many compounds (OSMAC) strategy. From the rice solid medium (mixed with glucose peptone and yeast broth (GPY)), wheat solid medium (mixed with Czapek) and GPY liquid medium, one rare N-butenone spiroquinazoline alkaloid, N-butenonelapatin A (1), together with nine known compounds (2-10), were isolated and identified. Their structures were elucidated by analysis of the comprehensive spectroscopic data, including 1D and 2D NMR and HRESIMS, and the absolute configuration of 1 was determined by a single-crystal X-ray crystallographic experiment. N-butenonelapatin A (1) represents the first example of N-butenone spiroquinazoline with a rare α, β-unsaturated ketone side chain in the family of spiroquinazoline alkaloids. Compound 4 displayed antibacterial activity against Vibrio vulnificus MCCC E1758 with a minimum inhibitory concentration (MIC) value of 6.25 µg/mL. Compound 7 exhibited antibacterial activities against three aquatic pathogenic bacteria, including V. vulnificus MCCC E1758, V. rotiferianus MCCC E385 and V. campbellii MCCC E333 with the MIC values of 12.5, 12.5 and 6.25 μg/mL, respectively. Compounds 3 and 6 demonstrated anti-inflammatory activity against NO production induced by lipopolysaccharide (LPS) with the IC50 values of 37.08 and 37.48 μM, respectively. In addition, compound 1 showed weak inhibitory activity against the proliferation of tumor cell lines A-375 and HCT 116. These findings further demonstrated that fungi of the Metarhizium species harbor great potentials in the synthesis of a variety of bioactive alkaloids.

Evaluation of the Toxicity of Chemical and Biogenic Insecticides to Three Outbreaking Insects in Desert Steppes of Northern China

The locusts Oedales asiaticus (Bey-Bienko) and Myrmeleotettix palpalis (Zubovski) (Orthoptera Acrididae) and the leaf beetle Galeruca daurica (Joannis) (Coleoptera, Chrysomelidae) are economically devastating insect species in the desert steppes of Northern China. Control is mainly and frequently dependent on highly toxic chemicals. To date, there have been no complete and comprehensive reports of insecticide applications to these key pests. In this study, laboratory bioassays were carried out to determine and compare the toxicity of twelve insecticides to three outbreaking insects, O. asiaticus, M. palpalis, and G. daurica, from three typical desert steppe regions, SZWQ, XHQ and WLTQQ, respectively. The responses of the two locust species and the leaf beetle were evaluated by topical application and leaf dip bioassay techniques across a range of concentrations to develop dosage–mortality regressions. The insecticides tested included six chemical insecticides (β-cypermethrin, imidacloprid, phoxim, λ-cyhalothrin, methomyl, chlorantraniliprole) and six biogenic insecticides (spinosad, avermectin, rotenone, matrine, azadiracthin, and methoxyfenozide). The results showed that phoxim, λ-cyhalothrin, β-cypermethrin and spinosad showed highly toxic activity to O.asiaticus, M. palpalis, and G. daurica, while methonyl, chlorantraniliprole, and rotenone were moderately toxic to both locust species and the leaf beetle. The LC₅₀ values of matrine, azadiractin, and avermectin were more than 1 μg a.i./adult for O. asiaticus and M. palpalis, the LC₅₀ values of which were higher 2 g/L for G. daurica. Our findings complement information from previous similar studies and will inform future studies relating to the control of outbreaking insects, such as O.asiaticus, M. palpalis, and G. daurica in desert steppes of northern China. This study is also expected to provide basic data on the use of chemical and biogenic insecticides for application in desert steppes.

Biomembrane lipids: When physics and chemistry join to shape biological activity

Biomembranes constitute the first lines of defense of cells. While small molecules can often permeate cell walls in bacteria and plants, they are generally unable to penetrate the barrier constituted by the double layer of phospholipids, unless specific receptors or channels are present. Antimicrobial or cell-penetrating peptides are in fact highly specialized molecules able to bypass this barrier and even discriminate among different cell types. This capacity is made possible by the intrinsic properties of its phospholipids, their distribution between the internal and external leaflet, and their ability to mutually interact, modulating the membrane fluidity and the exposition of key headgroups. Although common phospholipids can be found in the membranes of most organisms, some are characteristic of specific cell types. Here, we review the properties of the most common lipids and describe how they interact with each other in biomembrane. We then discuss how their assembly in bilayers determines some key physical-chemical properties such as permeability, potential and phase status. Finally, we describe how the exposition of specific phospholipids determines the recognition of cell types by membrane-targeting molecules.

Fungal consortium of two Beauveria bassiana strains increases their virulence, growth, and resistance to stress: A metabolomic approach

The use of two or more microorganisms in a microbial consortium has been increasingly applied in the biological control of diseases and pests. Beauveria bassiana is one of the most widely studied fungal species in biological control, yet little is known about its role in fungal consortiums. In a previous study, our group found that a consortium formed by two strains of B. bassiana had significantly greater biocontrol potential against the polyphagous caterpillars Duponchelia fovealis (Lepidoptera: Crambidae) than either strain on its own. In this study, we use GC-MS and LC-MS/MS to evaluate and discuss the metabolomics of the consortium. A total of 21 consortium biomarkers were identified, corresponding to 14 detected by LC-MS/MS and seven by GC-MS. Antioxidant and anti-inflammatory mechanisms are the main properties of the metabolites produced by the consortium. These metabolites can depress the insect’s immune system, increasing its vulnerability and, hence, the fungal virulence of the consortium. In light of these results, we propose an action model of insect mortality due to the metabolites secreted by the consortium. The model includes the inhibition of defense mechanisms such as pro-inflammatory interleukin secretion, cell migration, cell aggregation, Dif, Dorsal and Relish gene transcription, and JAK/STAT and JNK signaling pathways. It also promotes the cleaning of oxidative molecules, like ROS, NOS, and H₂O₂, and the induction of virulence factors.

The Entomopathogenic Fungus Beauveria bassiana Shows Its Toxic Side within Insects: Expression of Genes Encoding Secondary Metabolites during Pathogenesis

Entomopathogenic fungi are extensively used for the control of insect pests worldwide. Among them, Beauveria bassiana (Ascomycota: Hypocreales) produce a plethora of toxic secondary metabolites that either facilitate fungal invasion or act as immunosuppressive compounds. These toxins have different chemical natures, such as nonribosomal peptides and polyketides. Even though their precise role is poorly understood, they are usually linked to virulence. These fungal secondary metabolites are produced by the expression of gene clusters encoding the various proteins needed for their biosynthesis. Each cluster includes synthetases for nonribosomal peptides (NRPS), polyketides (PKS), or hybrid NRPS–PKS genes. The aim of this review is to summarize the information available from transcriptomics and quantitative PCR studies related to the expression of B. bassiana NRPS and PKS genes inside different insects as the infection progresses; as for the host immune response, to help understand the mechanisms that these toxins trigger as virulence factors, antimicrobials, or immunosuppressives within the context of a fungus–insect interaction.

The secondary metabolites from Beauveria bassiana PQ2 inhibit the growth and spore germination of Gibberella moniliformis LIA

Fungal secondary metabolites with antimicrobial properties are used for biological pest control. Their production is influenced by several factors as environment, host, and culture conditions. In the present work, the secondary metabolites from fermented extracts of Beauveria bassiana PQ2 were tested as antifungal agents against Gibberella moniliformis LIA. The L18 (21 × 37) orthogonal array from Taguchi methodology was used to assess 8 parameters (pH, agitation, sucrose, yeast extract, KH2PO4, MgSO4, NH4NO3, and CaCl2) in B. bassiana PQ2 submerged fermentation. The ability of the fermented extracts to slow down the growth rate of G. moniliformis LIA was evaluated. The results from 18 trials were analyzed by Statistica 7 software by evaluating the signal-to-noise ratio (S/N) to find the lower-the-better condition. Optimal culture conditions were pH, 5; agitation, 250 rpm; sucrose, 37.5 g/L-1; yeast extract, 10 g/L-1; KH2PO4, 0.8 g/L-1; MgSO4, 1.2 g/L-1; NH4NO3, 0.1 g/L-1; and CaCl2, 0.4 g/L-1, being the agitation at the highest level the most significant factor. The optimal conditions were validated in a sparged bottle bioreactor resulting in a higher S/N value (12.48) compared to the estimate. The extract obtained has the capacity to inhibit the germination of G. moniliformis spores at 24 h. HPLC-ESI-MS2 allowed to identify the water-soluble red pigment as oosporein (m/z 304.9). The secondary metabolites from B. bassiana PQ2 are a suitable alternative to control the growth and sporulation of G. moniliformis.

Mutation of a prenyltransferase results in accumulation of subglutinols and destruxins and enhanced virulence in the insect pathogen, Metarhizium anisopliae

The insect pathogenic fungus, Metarhizium anisopliae is a commercialized microbial agent used in biological control efforts targeting a diverse range of agricultural and other insect pests. The second step in the synthesis of a group of M. anisopliae α-pyrone diterpenoids (termed subglutinols) involves the activity of a prenyltransferase family geranylgeranyl diphosphate synthase (product of the subD/MaGGPPS5 gene). Here, we show that targeted gene disruption of MaGGPPS5 results in earlier conidial germination and faster greater vegetative growth compared to the wild type (WT) parent and complemented strains. In addition, insect bioassays revealed that the ΔMaGGPPS5 mutant strain displayed significantly increased virulence, with a ~50% decrease in the mean lethal time (LT₅₀ , from 6 to 3 days) to kill (50% of) target insects, and an ~15-40-fold decrease in the mean lethal dose (LC₅₀ ). Metabolite profiling indicated increased accumulation in the ΔMaGGPPS5 mutant of select subglutinols (A, B and C) and destruxins (A, A2, B and B2), the latter a set of fungal secondary metabolites that act as insect toxins, with a concomitant loss of production of subglutinol 'analogue 45'. These data suggest that the increased virulence phenotype seen for the ΔMaGGPPS5 strain can, at least in part, be attributed to a combination of faster growth and increased insect toxin production, linking the production of two different secondary metabolite pathways, and represent a novel approach for the screening of isolates with enhanced virulence via modulation of terpenoid secondary metabolite biosynthesis.

General Limitations to Endophytic Entomopathogenic Fungi Use as Plant Growth Promoters, Pests and Pathogens Biocontrol Agents

The multiple roles of fungal entomopathogens in host plants' growth promotion, pest and pathogen management have drawn huge attention for investigation. Endophytic species are known to influence various activities of their associated host plants, and the endophyte-colonized plants have been demonstrated to gain huge benefits from these symbiotic associations. The potential application of fungal endophytes as alternative to inorganic fertilizers for crop improvement has often been proposed. Similarly, various strains of insect pathogenic fungi have been formulated for use as mycopesticides and have been suggested as long-term replacement for the synthetic pesticides that are commonly in use. The numerous concerns about the negative effects of synthetic chemical pesticides have also driven attention towards developing eco-friendly pest management techniques. However, several factors have been underlined to be militating the successful adoption of entomopathogenic fungi and fungal endophytes as plant promoting, pests and diseases control bio-agents. The difficulties in isolation and characterization of novel strains, negative effects of geographical location, vegetation type and human disturbance on fungal entomopathogens, are among the numerous setbacks that have been documented. Although, the latest advances in biotechnology and microbial studies have provided means of overcoming many of these problems. For instance, studies have suggested measures for mitigating the negative effects of biotic and abiotic stressors on entomopathogenic fungi in inundative application on the field, or when applied in the form of fungal endophytes. In spite of these efforts, more studies are needed to be done to achieve the goal of improving the overall effectiveness and increase in the level of acceptance of entomopathogenic fungi and their products as an integral part of the integrated pest management programs, as well as potential adoption as an alternative to inorganic fertilizers and pesticides.

Iron homeostasis in the absence of ferricrocin and its consequences in fungal development and insect virulence in Beauveria bassiana

The putative ferricrocin synthetase gene ferS in the fungal entomopathogen Beauveria bassiana BCC 2660 was identified and characterized. The 14,445-bp ferS encodes a multimodular nonribosomal siderophore synthetase tightly clustered with Fusarium graminearum ferricrocin synthetase. Functional analysis of this gene was performed by disruption with the bar cassette. ΔferS mutants were verified by Southern and PCR analyses. HPLC and TLC analyses of crude extracts indicated that biosynthesis of ferricrocin was abolished in ΔferS. Insect bioassays surprisingly indicated that ΔferS killed the Spodoptera exigua larvae faster (LT₅₀ 59 h) than wild type (66 h). Growth and developmental assays of the mutant and wild type demonstrated that ΔferS had a significant increase in germination under iron depletion and radial growth and a decrease in conidiation. Mitotracker staining showed that the mitochondrial activity was enriched in ΔferS under both iron excess and iron depletion. Comparative transcriptomes between wild type and ΔferS indicated that the mutant was increased in the expression of eight cytochrome P450 genes and those in iron homeostasis, ferroptosis, oxidative stress response, ergosterol biosynthesis, and TCA cycle, compared to wild type. Our data suggested that ΔferS sensed the iron excess and the oxidative stress and, in turn, was up-regulated in the antioxidant-related genes and those in ergosterol biosynthesis and TCA cycle. These increased biological pathways help ΔferS grow and germinate faster than the wild type and caused higher insect mortality than the wild type in the early phase of infection.

Selection of Endophytic Beauveria bassiana as a Dual Biocontrol Agent of Tomato Pathogens and Pests

Endophytic fungi (EF) can enhance both plant growth and defense barriers against pests and pathogens, contributing to the reduction of chemical pesticides and fertilizers use in agriculture. Beauveria bassiana is an entomopathogenic fungus showing endophytism in several crops, often associated with a good capacity to limit the development of pests and disease agents. However, the diversity of the protective efficacy and plant response to different strains can be remarkable and needs to be carefully assessed for the successful and predictable use of these beneficial microorganisms. This study aims to select B. bassiana strains able to colonize tomato plants as endophytes as well as to control two important disease agents, Botrytis cinerea and Alternaria alternata, and the pest aphid, Macrosiphum euphorbiae. Nine wild-type isolates and one commercial strain were screened for endophytism, then further characterized for plant-growth promotion plus inhibition of disease development and pest infestation. Four isolates proved to have a good control activity against the biotic stressors tested, but only Bb716 was also able to promote plant growth. This work provides a simple workflow for the selection of beneficial EF, paving the way towards more effective use of B. bassiana in Integrate Pest Management (IPM) of tomato.

Secondary metabolites from hypocrealean entomopathogenic fungi: genomics as a tool to elucidate the encoded parvome

Covering: 2014 up to the third quarter of 2019 Hypocrealean entomopathogenic fungi (HEF) produce a large variety of secondary metabolites (SMs) that are prominent virulence factors or mediate various interactions in the native niches of these organisms. Many of these SMs show insecticidal, immune system modulatory, antimicrobial, cytotoxic and other bioactivities of clinical or agricultural significance. Recent advances in whole genome sequencing technologies and bioinformatics have revealed many biosynthetic gene clusters (BGCs) potentially involved in SM production in HEF. Some of these BGCs are now well characterized, with the structures of the cognate product congeners elucidated, and the proposed biosynthetic functions of key enzymes validated. However, the vast majority of HEF BGCs are still not linked to SM products ("orphan" BGCs), including many clusters that are not expressed (silent) under routine laboratory conditions. Thus, investigations into the encoded parvome (the secondary metabolome predicted from the genome) of HEF allows the discovery of BGCs for known SMs; uncovers novel metabolites based on the BGCs; and catalogues the predicted SM biosynthetic potential of these fungi. Herein, we summarize new developments of the field, and survey the polyketide, nonribosomal peptide, terpenoid and hybrid SM BGCs encoded in the currently available 40 HEF genome sequences. Studying the encoded parvome of HEF will increase our understanding of the multifaceted roles that SMs play in biotic and abiotic interactions and will also reveal biologically active SMs that can be exploited for the discovery of human and veterinary drugs or crop protection agents.

The Chemical Ecology Approach to Reveal Fungal Metabolites for Arthropod Pest Management

Biorational insecticides (for instance, avermectins, spinosins, azadirachtin, and afidopyropen) of natural origin are increasingly being used in agriculture. The review considers the chemical ecology approach for the search for new compounds with insecticidal properties (entomotoxic, antifeedant, and hormonal) produced by fungi of various ecological groups (entomopathogens, soil saprotrophs, endophytes, phytopathogens, and mushrooms). The literature survey revealed that insecticidal metabolites of entomopathogenic fungi have not been sufficiently studied, and most of the well-characterized compounds show moderate insecticidal activity. The greatest number of substances with insecticidal properties was found to be produced by soil fungi, mainly from the genera Aspergillus and Penicillium. Metabolites with insecticidal and antifeedant properties were also found in endophytic and phytopathogenic fungi. It was noted that insect pests of stored products are mostly low sensitive to mycotoxins. Mushrooms were found to be promising producers of antifeedant compounds as well as insecticidal proteins. The expansion of the number of substances with insecticidal properties detected in prospective fungal species is possible by mining fungal genomes for secondary metabolite gene clusters and secreted proteins with their subsequent activation by various methods. The efficacy of these studies can be increased with high-throughput techniques of extraction of fungal metabolites and their analysis by various methods of chromatography and mass spectrometry.

Synergy in Efficacy of Artemisia sieversiana Crude Extract and Metarhizium anisopliae on Resistant Oedaleus asiaticus

In order to explore the synergistic control effect of crude extracts of Artemisia sieversiana and Metarhizium anisopliae on Oedaleus asiaticus, we used different doses of M. anisopliae and crude extracts of A. sieversiana singly and in combination, to determine their toxicities to fourth instar O. asiaticus. The results showed that the combination of 10% crude extract of A. sieversiana with 10⁷ and 10⁸ spores/g M. anisopliae concentrations and the combination of 20% crude extract of A. sieversiana with 10⁷ and 10⁸ spores/g M. anisopliae concentrations had significant effects on the mortality, body weight gain, body length gain, growth rate, and overall performance of O. asiaticus than those of the crude extract of A. sieversiana and M. anisopliae alone. Among them, the 20% A. sieversiana crude extract mixed with 10⁸ spores/g M. anisopliae and 10% A. sieversiana crude extract combined with 10⁷ spores/g M. anisopliae, had the best control efficacy. In order to clarify the biochemical mechanism underlying the immune responses of O. asiaticus to the pesticide treatments, we monitored the activities of four enzymes: superoxidase dismutase (SOD), peroxidase (POD), catalase (CAT), and polyphenol oxidase (PPO). The results showed that the activities of three enzymes (SOD, CAT, and PPO) were significantly increased from the treatment with the combination of M. anisopliae mixed with crude extract of A. sieversiana. Interestingly, compared to the crude extract, the combination treatment did not significantly induce the expression of POD enzyme activity, which may be a biochemical factor for increasing the control effect of the combination treatment. Our results showed that the combination treatment had synergistic and antagonistic effects on host mortality, growth, development, and enzyme activities in O. asiaticus.

Immunotranscriptome analysis of Plutella xylostella reveals differences in innate immune responses to low- and high-virulence Beauveria bassiana strain challenges

BACKGROUND

Entomopathogenic fungi have developed multiple strategies to overcome the immune defenses of their target insects, whereas insect pests have devised various defense mechanisms to combat fungal infection. However, differences in the molecular mechanisms of the innate immune defense strategies of insects upon infection with different fungal strains from the same species have not been reported.

RESULTS

Two Beauveria bassiana strains were obtained that significantly varied in their pathogenicity but were comparable in terms of growth, conidial yield, and cuticle penetration. To investigate the molecular mechanisms underlying the immune response of Plutella xylostella infected with these two strains, RNA-Seq was performed 48 h after infection. A total of 1027 differentially expressed genes (DEGs) were identified, and more than 200 DEGs were enriched in Kyoto Encyclopedia of Genes and Genome (KEGG) pathways involved in disease response, revealing differences in the immune response of P. xylostella to different B. bassiana infections at 48 h. Twenty-eight of the DEGs were related to innate immune functions, such as pathogen recognition, immune system activation and antimicrobial reactions. RNA interference (RNAi)-mediated gene silencing assays showed that PxApoLIII and PxCSP played critical roles in the P. xylostella immune response. PxApoLIII was expressed at higher levels during infection with the high-virulence strain, whereas PxCSP showed the opposite expression pattern during infection with the low-virulence strain, indicating that PxApoLIII and PxCSP might participate in P. xylostella innate immune defense against high- and low-virulence B. bassiana strains.

CONCLUSION

The present findings demonstrate that strains of a single species of pathogenic fungi that differ in virulence can induce the expression of different genes in P. xylostella. These results advance our knowledge of the molecular mechanisms underlying fungi-pest interactions.

Peniterpenoids A-C, new sesquiterpenoid metabolites from a wheat cyst nematode Penicillium janthinellum

Three new sesquiterpenoids, peniterpenoids A - C (1-3), together with six known metabolites (4-9) were isolated from an entomogenous fungus Penicillium janthinellum (LB1.20090001) collected from a wheat cyst nematode. The structures of the new compounds were elucidated based on NMR and HRESIMS spectroscopic analyses. The absolute configuration of the C-8 secondary alcohol of peniterpenoid B (2) was determined by [Rh₂(OCOCF₃)₄]-induced ECD experiment. Subsequently, the antimicrobial and DPPH scavenging activities were determined. Compounds 6-8 exhibited moderate antibacterial activities against Staphylococcus aureus (CGMCC1.2465) with MIC values of 25.0, 50.0 and 12.5 μg/mL, respectively.

The multifunctional lifestyles of Metarhizium: evolution and applications

The genus Metarhizium is comprised of a diverse group of common soil fungi that exhibit multifunctional lifestyles with varying degrees of saprotrophic, endophytic, and insect pathogenic modes of nutrient acquisition. The transcriptome of these species is modulated to reflect immediate needs of the fungus and availability of resources-a form of transcriptional plasticity that allows for physiological adaptation to environments with diverse and dynamic exploitable nutrient sources. In this review, we discuss the endophytic, insect pathogenic lifestyles of Metarhizium spp., including their symbiotic interface, origins, and evolution, and agricultural applications. Isotope labeling experiments have demonstrated that a mutually beneficial exchange of limiting nutrients occurs between the fungus and its host plant, with nitrogen derived via insect pathogenesis being translocated from Metarhizium to host plants in exchange for fixed carbon in the form of photosynthate. Thus, the endophytic and entomopathogenic abilities of Metarhizium spp. are not exclusive of one another, but rather are interdependent and reciprocal in nature. Although endophytic, insect pathogenic fungi (EIPF) could certainly have evolved from insect pathogenic fungi, phylogenomic evidence indicates that this genus is more closely related to plant-associated fungi than animal pathogens, suggesting that Metarhizium evolved from a lineage of plant symbionts, which subsequently acquired genes for insect pathogenesis. Entomopathogenicity may have been an adaptive trait, allowing for procurement of insect-derived nitrogen that could be translocated to host plants and bartered for fixed carbon, thereby improving the stability of fungal-plant symbioses. Given their ability to simultaneously parasitize soil insects, including a number of pests of agriculturally important crops, as well as promote plant health, growth, and productivity, Metarhizium spp. are considered promising alternatives to the chemical pesticides and fertilizers that have wreaked havoc on the health and integrity of ecosystems. KEY POINTS: • Metarhizium is a fungus that is an insect pathogen as well as a plant symbiont. • The genus Metarhizium has specialist and generalist insect pathogens. • Metarhizium is phylogenetically most closely related to plant endophytes.

Host-pathogen interaction after infection of Galleria mellonella with the filamentous fungus Beauveria bassiana

The filamentous fungus Beauveria bassiana is a natural pathogen of the greater wax moth Galleria mellonella. Infection with this fungus triggered systemic immune response in G. mellonella; nevertheless, the infection was lethal if spores entered the insect hemocel. We observed melanin deposition in the insect cuticle and walls of air bags, while the invading fungus interrupted tissue continuity. We have shown colonization of muscles, air bags, and finally colonization and complete destruction of the fat body-the main organ responsible for the synthesis of defense molecules in response to infection. This destruction was probably not caused by simple fungal growth, because the fat body was not destroyed during colonization with a human opportunistic pathogen Candida albicans. This may mean that the infecting fungus is able to destroy actively the insect's fat body as part of its virulence mechanism. Finally, we were unable to reduce the extremely high virulence of B. bassiana against G. mellonella by priming of larvae with thermally inactivated fungal spores.

Secondary metabolites from hypocrealean entomopathogenic fungi: novel bioactive compounds

Covering: 2014 up to the third quarter of 2019 Entomopathogens constitute a unique, specialized trophic subgroup of fungi, most of whose members belong to the order Hypocreales (class Sordariomycetes, phylum Ascomycota). These Hypocrealean Entomopathogenic Fungi (HEF) produce a large variety of secondary metabolites (SMs) and their genomes rank highly for the number of predicted, unique SM biosynthetic gene clusters. SMs from HEF have diverse roles in insect pathogenicity as virulence factors by modulating various interactions between the producer fungus and its insect host. In addition, these SMs also defend the carcass of the prey against opportunistic microbial invaders, mediate intra- and interspecies communication, and mitigate abiotic and biotic stresses. Thus, these SMs contribute to the role of HEF as commercial biopesticides in the context of integrated pest management systems, and provide lead compounds for the development of chemical pesticides for crop protection. These bioactive SMs also underpin the widespread use of certain HEF as nutraceuticals and traditional remedies, and allowed the modern pharmaceutical industry to repurpose some of these molecules as life-saving human medications. Herein, we survey the structures and biological activities of SMs described from HEF, and summarize new information on the roles of these metabolites in fungal virulence.

A New High-Quality Draft Genome Assembly of the Chinese Cordyceps Ophiocordyceps sinensis

Ophiocordyceps sinensis (Berk.) is an entomopathogenic fungus endemic to the Qinghai-Tibet Plateau. It parasitizes and mummifies the underground ghost moth larvae, then produces a fruiting body. The fungus-insect complex, called Chinese cordyceps or "DongChongXiaCao," is not only a valuable traditional Chinese medicine, but also a major source of income for numerous Himalayan residents. Here, taking advantage of rapid advances in single-molecule sequencing, we assembled a highly contiguous genome assembly of O. sinensis. The assembly of 23 contigs was ∼110.8 Mb with a N50 length of 18.2 Mb. We used RNA-seq and homologous protein sequences to identify 8,916 protein-coding genes in the IOZ07 assembly. Moreover, 63 secondary metabolite gene clusters were identified in the improved assembly. The improved assembly and genome features described in this study will further inform the evolutionary study and resource utilization of Chinese cordyceps.

When Appearance Misleads: The Role of the Entomopathogen Surface in the Relationship with Its Host

Currently, potentially harmful insects are controlled mainly by chemical synthetic insecticides, but environmental emergencies strongly require less invasive control techniques. The use of biological insecticides in the form of entomopathogenic organisms is undoubtedly a fundamental resource for the biological control of insect pests in the future. These infectious agents and endogenous parasites generally act by profoundly altering the host's physiology to death, but their success is closely related to the neutralization of the target insect's immune response. In general, entomopathogen parasites, entomopathogenic bacteria, and fungi can counteract immune processes through the effects of secretion/excretion products that interfere with and damage the cells and molecules typical of innate immunity. However, these effects are observed in the later stages of infection, whereas the risk of being recognized and neutralized occurs very early after penetration and involves the pathogen surface components and molecular architecture; therefore, their role becomes crucial, particularly in the earliest pathogenesis. In this review, we analyze the evasion/interference strategies that entomopathogens such as the bacterium Bacillus thuringiensis, fungi, nematocomplexes, and wasps implement in the initial stages of infection, i.e., the phases during which body or cell surfaces play a key role in the interaction with the host receptors responsible for the immunological discrimination between self and non-self. In this regard, these organisms demonstrate evasive abilities ascribed to their body surface and cell wall; it appears that the key process of these mechanisms is the capability to modify the surface, converting it into an immunocompatible structure, or interaction that is more or less specific to host factors.

Epigenetic manipulation of filamentous fungi for biotechnological applications: a systematic review

The study of the epigenetic regulation of gene function has reached pivotal importance in life sciences in the last decades. The mechanisms and effects of processes such as DNA methylation, histone posttranslational modifications and non-coding RNAs, as well as their impact on chromatin structure and dynamics, are clearly involved in physiology homeostasis in plants, animals and microorganisms. In the fungal kingdom, studies on the model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe contributed enormously to the elucidation of the eukaryote epigenetic landscape. Epigenetic regulation plays a central role in the expression of virulence attributes of human pathogens such as Candida albicans. In this article, we review the most recent studies on the effects of drugs capable of altering epigenetic states and on the impact of chromatin structure-related genes deletion in filamentous fungi. Emphasis is given on plant and insect pathogens, endophytes, secondary metabolites and cellulases/xylanases producing species.

Disruption of an adenylate-forming reductase required for conidiation, increases virulence of the insect pathogenic fungus Metarhizium acridum by enhancing cuticle invasion

BACKGROUND

Metarhizium acridum, is a specific acridid pathogen developed for use against the migratory locust (Locusta migratoria manilensis). Adenylate-forming reductases (AFRs) include enzymes that are involved in natural product biosynthesis. Here, we genetically characterize the functions of a class IV AFR in M. acridum (MaAfr^IV ) on fungal development and virulence.

RESULTS

Gene expression analyses indicated MaAfr^IV was induced on locust wings early during the infection process. Surprisingly, loss of MaAfr^IV increased virulence (25.20% decrease in the median lethal time) against the locust in topical bioassays but was no different than the wild type when the cuticle was bypassed by direct infection of conidia into the insect hemocoel. Virulence markers including protease (Pr1) expression and appressorial turgor pressure were higher in the mutant than the parent strain. No difference was seen in the expression of host immune genes (Toll pathway) or in polyphenol oxidase (PPO) activity in locusts infected by the ΔMaAfr^IV or wild type strains. However, the ΔMaAfr^IV strain was unable to successfully sporulate on dead cadavers.

CONCLUSION

Disruption of MaAfr^IV increased fungal virulence by promoting insect cuticle invasion without altering host immune response or fungal immune evasion. Although loss of MaAfr^IV conferred an apparent benefit to the fungus in terms of enhanced virulence, a significant trade-off was seen in the inability of the fungus to sporulate on the cadaver. As conidiation on the cadaver is essential for subsequent propagation in the environment, loss of MaAfr^IV can reduce the engineering strains survivability in the field and improve the safety. © 2019 Society of Chemical Industry.

Root colonization by endophytic insect-pathogenic fungi

Several ascomycetous insect-pathogenic fungi, including species in the genera Beauveria and Metarhizium, are plant root symbionts/endophytes and are termed as endophytic insect-pathogenic fungi (EIPF). The endophytic capability and insect pathogenicity of Metarhizium are coupled to provide an active method of insect-derived nitrogen transfer to plant hosts via fungal mycelia. In exchange for the insect-derived nitrogen, the plant provides photosynthate to the fungus. This symbiotic interaction offers other benefits to the plant-EIPF can improve plant growth, they are antagonistic to plant pathogens and herbivores and can enhance the plant tolerance to abiotic stresses. The mechanisms and underlying biochemical and genetic features of insect pathogenesis are generally well-established. However, there is a paucity of information regarding the underlying mechanisms in this plant-symbiotic association. Here we review five aspects of EIPF interactions with host plant roots: (i) rhizosphere colonization, (ii) signalling factors from the plant and EIPF, (iii) modulation of plant defence responses, (iv) nutrient exchange and (v) tripartite interactions with insects and other micro-organisms. The elucidation of these interactions is fundamental to understanding this symbiotic association for effective application of EIPF in an agricultural setting.

Genome-wide DNA methylation analysis of Metarhizium anisopliae during tick mimicked infection condition

BACKGROUND

The Metarhizium genus harbors important entomopathogenic fungi. These species have been widely explored as biological control agents, and strategies to improve the fungal virulence are under investigation. Thus, the interaction between Metarhizium species and susceptible hosts have been explored employing different methods in order to characterize putative virulence determinants. However, the impact of epigenetic modulation on the infection cycle of Metarhizium is still an open topic. Among the different epigenetic modifications, DNA methylation of cytosine bases is an important mechanism to control gene expression in several organisms. To better understand if DNA methylation can govern Metarhizium-host interactions, the genome-wide DNA methylation profile of Metarhizium anisopliae was explored in two conditions: tick mimicked infection and a saprophytic-like control.

RESULTS

Using a genome wide DNA methylation profile based on bisulfite sequencing (BS-Seq), approximately 0.60% of the total cytosines were methylated in saprophytic-like condition, which was lower than the DNA methylation level (0.89%) in tick mimicked infection condition. A total of 670 mRNA genes were found to be putatively methylated, with 390 mRNA genes uniquely methylated in the tick mimicked infection condition. GO terms linked to response to stimuli, cell wall morphogenesis, cytoskeleton morphogenesis and secondary metabolism biosynthesis were over-represented in the tick mimicked infection condition, suggesting that energy metabolism is directed towards the regulation of genes associated with infection. However, recognized virulence determinants known to be expressed at distinct infection steps, such as the destruxin backbone gene and the collagen-like protein gene Mcl1, were found methylated, suggesting that a dynamic pattern of methylation could be found during the infectious process. These results were further endorsed employing RT-qPCR from cultures treated or not with the DNA methyltransferase inhibitor 5-Azacytidine.

CONCLUSIONS

The set of genes here analyzed focused on secondary metabolites associated genes, known to be involved in several processes, including virulence. The BS-Seq pipeline and RT-qPCR analysis employing 5-Azacytidine led to identification of methylated virulence genes in M. anisopliae. The results provided evidences that DNA methylation in M. anisopliae comprises another layer of gene expression regulation, suggesting a main role of DNA methylation regulating putative virulence determinants during M. anisopliae infection cycle.

Interactions between Fungal-Infected Helicoverpa armigera and the Predator Chrysoperla externa

The aim of the present study was to evaluate the interactions between Chrysoperla externa (Hagen, 1861) and the eggs and first-instar larvae of Helicoverpa armigera (Hübner 1805) infected by entomopathogenic fungi. The H. armigera eggs and larvae were treated with sterile distilled water + 0.01% Tween 80 (T1, control), Beauveria bassiana (Bals.) Vuill (T2), Metarhizium anisopliae (Metsch.) Sorok (T3), or Metarhizium rileyi (Farlow) Samson. (T4) at different concentrations (1 × 107, 1 × 108, and 1 × 109 con. mL-1). For each treatment, a single third-instar C. externa was offered prey (a combination of 80 eggs and 50 first-instar H. armigera larvae) at 0, 24, and 48 h after inoculation. Ten trials were completed for each treatment, and the entire experiment was repeated three times. Neither the concentrations of fungi nor the application method affected consumption by C. externa. Because all the predator larvae reached the pupal phase, with 100% viability in adults, these results suggest that entomopathogenic fungi and C. externa are compatible and that the simultaneous use of these biological control agents is possible for managing H. armigera.