An entomopathogenic fungus exploits its host humoral antibacterial immunity to minimize bacterial competition in the hemolymph

The gut microbial community structure of the oriental armyworm Mythimna separata (Walker) (Lepidoptera: Noctuidae) affects the the virulence of the entomopathogenic fungus Metarhizium rileyi

Mythimna separata, the oriental armyworm, is a lepidopteran pest that threatens cereal crops. In the current study, two strains (XSBN200920 and JHML200710) of entomopathogenic fungus Metarhizium rileyi were tested for virulence against oriental armyworms. When treated with spore suspensions of both strains at a concentration of 1.0 × 108 spores/mL, the 3rd instar larvae's survival rate was considerably different (P < 0.01). The median lethal time of the insects exposed to XSBN200920 was about 3 d longer than that of JHML200710. The results of 16S ribosomal RNA sequencing showed that Chao1 richness in the JHML200710 treatment group was significantly decreased compared with the CK ( 0.02% Tween 80). The dominant gut bacteria species at the phylum level were Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidota in the three groups. The CK group had a much higher associated abundance of cyanobacteria than the other two fungal treatment groups. Sixteen genera revealed significant variations in the gut bacteria of the insects at the genus level. The Kyoto Encyclopedia of Genes and Genomes (KEGG) functional gene and enzyme analysis showed that when compared with the CK group, the XSBN200920 treatment group showed a significant reduction in six aspects, including betalain biosynthesis, spliceosome, and neuroactive ligand-receptor interaction. These findings suggested that healthy and fungus-infected insects' intestinal microbial community structure differed significantly. And the virulence of M. rileyi is closely linked to its ability to alter the structure of the intestinal microbiome of insects. The results offer a starting point for examining the relationship between the gut microbial diversity of oriental armyworms and variations in the virulence of pathogenic fungi.

A C-type lectin of Helicoverpa armigera maintains the stability of the hemolymph microbiota by regulating the expression of lysozyme

An increasing body of evidence suggests that the insect hemolymph is not a sterile environment and that various nonpathogenic microorganisms can stably or transiently inhabit the hemolymph in many insect species. However, little is currently known about how the insect immune system maintains microbial homeostasis within the hemolymph. In this study, a C-type lectin of Helicoverpa armigera (HaCTL6) was shown to be involved in maintaining the stability of the hemolymph microbiota. The expression of H. armigera antimicrobial peptide (AMP) genes was down-regulated after RNAi of HaCTL6. Moreover, the knockdown of HaCTL6 resulted in a decrease in the antibacterial activity and an increase in the total bacterial load of the hemolymph. Transcriptome analysis showed that a lysozyme (HaLysozyme-like) was significantly down-regulated after HaCTL6 RNAi. Moreover, the knockdown of HaLysozyme-like led to a decrease in the antibacterial activity and an increase in the total bacterial load of the hemolymph. Furthermore, the injection of recombinant HaLysozyme-like into the hemocoel caused a significant reduction in the total number of bacteria in the hemolymph. These results indicate that HaCTL6 may regulate the homeostasis of bacteria in the hemolymph by utilizing HaLysozyme-like as an effector.

Involvement of bacteria in the development of fungal infections in the Colorado potato beetle

Entomopathogenic fungi may interact with insects' symbiotic bacteria during infection. We hypothesized that topical infection with Beauveria bassiana may alter the microbiota of the Colorado potato beetle (CPB) and that these modifications may alter the course of mycoses. We used a model with two concentrations of conidia: (1) high concentration that causes rapid (acute) pathogenesis with fast mortality followed by bacterial decomposition of insects; (2) lower concentration that leads to prolonged pathogenesis ending in conidiation on cadavers. The fungal infections increased loads of enterobacteria and bacilli on the cuticle surface and in hemolymph and midgut, and the greatest increase was detected during the acute mycosis. By contrast, stronger activation of IMD and JAK-STAT signaling pathways in integuments and fat body was observed during the prolonged mycosis. Relatively stable (nonpathogenic) conditions remained in the midgut during both scenarios of mycosis with slight changes in bacterial communities, the absence of mesh and stat expression, a decrease in reactive oxygen species production, and slight induction of Toll and IMD pathways. Oral administration of antibiotic and predominant CPB bacteria (Enterobacteriaceae, Lactococcus, Pseudomonas) led to minor and mainly antagonistic effects in survival of larvae infected with B. bassiana. We believe that prolonged mycosis is necessary for successful development of the fungus because such pathogenesis allows the host to activate antibacterial reactions. Conversely, after infection with high concentrations of the fungus, the host's resources are insufficient to fully activate antibacterial defenses, and this situation makes successful development of the fungus impossible.

Establishment of Agrobacterium tumefaciens-mediated genetic transformation of the entomopathogenic fungus Hirsutella satumaensis

Hirsutella satumaensis, an endoparasitic fungus that targets Lepidoptera insects, holds significant potential for biocontrol applications. However, its molecular study has been limited due to the absence of an efficient genetic transformation system. In this study, an optimized Agrobacterium tumefaciens-mediated transformation protocol was developed for H. satumaensis using binary vectors pBARGPE1-GFP and pK2-bar, which carry the green fluorescent protein (eGFP) and phosphinothricin resistance (bar) genes, respectively. The optimal transformation conditions included a conidial concentration of 10⁵ conidia/mL, an A. tumefaciens (strain AGL-1) concentration of OD660 = 0.6, and a 3-day co-cultivation period with 200 μM acetosyringone, resulting in an average of 121 ± 5.07 transformants. Successful integration was confirmed by green fluorescence in the transformants. Furthermore, the ribotoxin gene hirsutellin A (HtA), specific to the genus Hirsutella, was successfully overexpressed using this system. Insect bioassays demonstrated that the gpdA promoter effectively drives HtA expression in H. satumaensis. The transformation system exhibited stable gene integration, strong fluorescence, and bioactivity. This study establishes the first genetic transformation protocol for H. satumaensis, providing a valuable tool for exploring insect-pathogen interactions and the functional roles of key genes in this entomopathogenic fungus.

Isolation and Characterization of Beauveria caledonica (Ascomycota: Hypocreales) Strains for Biological Control of Odoiporus longicollis Oliver (Coleoptera: Curculionidae)

The banana pseudostem weevil (BPW), Odoiporus longicollis (Oliver), is one of the most destructive pests of bananas that is seriously affecting the yield and quality of bananas. We isolated pathogens from banana pseudostem weevils in Xishuangbanna and Dongchuan, Yunnan, China, and explored their biological characteristics. The pathogenicity of the strains was verified through laboratory and greenhouse inoculation experiments. The results showed that four strains of fungi were identified and confirmed as Beauveria caledonica (Bc) via ITS-rDNA sequencing. Optimal in vitro culture conditions were found to be a photoperiod of 24 h light, 25 °C temperature, and 18 days on potato dextrose agar (PDA) medium with insect meal. Under these conditions, the Cs-1 strain achieved a colony diameter of 65.17 ± 0.74 mm and spore production of 1.24 × 108 cfu/cm2. The Cs-1 strain had the shortest lethal time (LT50) of 9.36 days at an inoculum of 1.00 × 109 cfu/mL, with a lethality of 86.67% after 20 days. The Cs-3 strain showed 77.78% lethality at 1.00 × 108 cfu/mL after 20 days. Despite variations in virulence, lethality did not correlate with major cuticle-degrading enzymes. The Cs-3 strain demonstrated effective biocontrol in greenhouse tests. Banana plants suffered significant damage without Bc-treated BPW, while the treated plantlets thrived. The mortality rate reached 82.78% after 35 days. This study marks the first identification of these entomopathogenic fungi (EPF) in Yunnan, China, highlighting B. caledonica's potential for biocontrol application.

The fungus Escovopsis (Ascomycota: Hypocreales): a critical review of its biology and parasitism of attine ant colonies

Two biological phenomena that contribute to increasing complexity in biological systems are mutualistic symbiotic interactions and the evolution of sociality. These two phenomena are also of fundamental importance to our understanding of the natural world. An organism that poses a threat to one or both of these is therefore also of great interest as it represents a challenge that mutualistic symbioses and social organisms have to overcome. This is the case with the fungus Escovopsis (Ascomycota: Hypocreales), which attacks the fungus garden of attine ants (Formicidae: Attina) such as the leaf cutters. This parasite has attracted much high-profile scientific interest for considerable time, and its study has been fruitful in understanding evolutionary, ecological and behavioural processes. Despite this, much of the biology and ecology of this organism remains unknown. Here we discuss this fungus and three sister genera (Escovopsioides, Luteomyces and Sympodiorosea) that until recently were considered as a single group. We first describe its position as the most highly specialised microbial symbiont in this system other than the mutualistic fungal cultivar itself and as that of greatest scientific interest. We then review the taxonomic history of the group and its macroevolution and biogeography. We examine what we know of its life cycle in the field - surprisingly little is known of how it is transmitted between colonies, but we explain what is known to date. We then review how it interacts with its host(s), first at the level of its direct interaction with the basidiomycete host fungi wherein we show the evidence for it being a mycoparasite; then at the colony level where empirical evidence points towards it being a parasite with a very low virulence or even merely a opportunist. Finally, we offer directions for future research.

Research progress of aphid immunity system: Potential effective target for green pest management

Due to the absence of acquired immunity, insects primarily rely on their innate immune system to resist pathogenic microorganisms and parasitoids in natural habitats. This innate immune system can be classified into cellular immunity and humoral immunity. Cellular immunity is mediated by hemocytes, which perform phagocytosis, aggregation, and encapsulation to fight against invaders, whereas the humoral immunity primarily activates the immune signaling pathways and induces the generation of immune effectors. Existing studies have revealed that the hemipteran aphids lack some crucial immune genes compared to other insect species, indicating the different immune mechanisms in aphids. The current review summarizes the adverse impacts of pathogenic microorganisms and parasitoids on aphids, introduces the cellular and humoral immune systems in insects, and analyzes the differences between aphids and other insect species. Furthermore, our review also discussed the existing prospects and challenges in aphid immunity research, and proposed the potential application of immune genes in green pest management.

An iron-binding protein of entomopathogenic fungus suppresses the proliferation of host symbiotic bacteria

BACKGROUND

Entomopathogenic fungal infection-induced dysbiosis of host microbiota offers a window into understanding the complex interactions between pathogenic fungi and host symbionts. Such insights are critical for enhancing the efficacy of mycoinsecticides. However, the utilization of these interactions in pest control remains largely unexplored.

RESULTS

Here, we found that infection by the host-specialist fungus Metarhizium acridum alters the composition of the symbiotic microbiota and increases the dominance of some bacterial symbionts in locusts. Meanwhile, M. acridum also effectively limits the overgrowth of the predominant bacteria. Comparative transcriptomic screening revealed that the fungus upregulates the production of MaCFEM1, an iron-binding protein, in the presence of bacteria. This protein sequesters iron, thereby limiting its availability. Functionally, overexpression of MaCFEM1 in the fungus induces iron deprivation, which significantly suppresses bacterial growth. Conversely, MaCFEM1 knockout relieves the restriction on bacterial iron availability, resulting in iron reallocation. Upon ΔMaCFEM1 infection, some host bacterial symbionts proliferate uncontrollably, turning into opportunistic pathogens and significantly accelerating host death.

CONCLUSIONS

This study elucidates the critical role of pathogenic fungal-dominated iron allocation in mediating the shift of host microbes from symbiosis to pathogenicity. It also highlights a unique biocontrol strategy that jointly exploits pathogenic fungi and bacterial symbionts to increase host mortality. Video Abstract.

The secretory protein COA1 enables Metarhizium robertsii to evade insect immune recognition during cuticle penetration

The interplay between the insect immune system and entomopathogenic fungi during cuticle penetration is not yet fully understood. Here, we show that a secretory protein COA1 (coat of appressorium 1) from Metarhizium robertsii, an entomopathogenic fungus causing diseases in a wide range of insects, is required to avoid host immune recognition during cuticle penetration. COA1 is highly expressed on the cuticle and translocated to the cell surface, where it directly binds with and masks carbohydrates of the fungal cell wall to avoid provoking the host's intense immune response. Deletion of Coa1 results in a robust immune response, leading to a reduction in bacterial load in both the gut and hemocoel and ultimately attenuating fungal virulence. Our work reveals a novel cell surface protein indispensable for fungal pathogenicity via masking cell wall carbohydrates to avert a hypersensitive response from the host.

Engineered fungus containing a caterpillar gene kills insects rapidly by disrupting their ecto- and endo-microbiomes

Similar to the physiological importance of gut microbiomes, recent works have shown that insect ectomicrobiotas can mediate defensive colonization resistance against fungal parasites that infect via cuticle penetration. Here we show that engineering the entomopathogenic fungus Metarhizium robertsii with a potent antibacterial moricin gene from silkworms substantially enhances the ability of the fungus to kill mosquitos, locusts, and two Drosophila species. Further use of Drosophila melanogaster as an infection model, quantitative microbiome analysis reveals that engineered strains designed to suppress insect cuticular bacteria additionally disrupt gut microbiomes. An overgrowth of harmful bacteria such as the opportunistic pathogens of Providencia species is detected that can accelerate insect death. In support, quantitative analysis of antimicrobial genes in fly fat bodies and guts indicates that topical fungal infections result in the compromise of intestinal immune responses. In addition to providing an innovative strategy for improving the potency of mycoinsecticides, our data solidify the importance of both the ecto- and endo-microbiomes in maintaining insect wellbeing.

The basic leucine zipper domain (bZIP) transcription factor BbYap1 promotes evasion of host humoral immunity and regulates lipid homeostasis contributing to fungal virulence in Beauveria bassiana

Basic leucine zipper domain transcription factors (TFs), of which yeast activator protein (Yap) is a significant class, are crucial for the development of sclerotia, the stress response, vegetative growth, and spore adhesion. Nevertheless, nothing is known about how Yap TFs contribute to the pathogenicity of entomopathogenic fungus. In this work, Beauveria bassiana was used to identify and knock out the yeast gene BbYap1, which is similar to Yap. The BbYap1 gene deletion has an impact on lipid homeostasis of B. bassiana; oleic acid, for example, dropped by 95.69%. The BbYap1 mutant exhibited much less virulence and vegetative development in comparison to the wild strain, while demonstrating a greater sensitivity to chemical stress. It is noteworthy that the physiological abnormalities brought on by BbYap1 deletion were largely repaired by the addition of exogenous oleic acid, as seen by the notable increase in insect survival in the blood cavity injection group. Following infection with the BbYap1 mutant, the host exhibits a considerable down-regulation of the expression of β-1,3-glucan recognition protein, gallerimycin, gloverin, and moricin-like protein genes. Likewise, the introduction of exogenous oleic acid markedly increased the host's expression of the aforementioned genes. In summary, BbYap1 regulates cellular enzyme lipid homeostasis and fungal virulence by eluding host humoral defense, which contributes to fungal chemical stress and vegetative development.

IMPORTANCE

Entomopathogenic fungi (EPF) offer an effective and eco-friendly alternative to curb insect populations in biocontrol strategy. When EPF enter the hemolymph of their host, they encounter a variety of stress reactions, such as immunological and oxidative stress. Basic leucine zipper domain transcription factors, of which yeast activator protein (Yap) is a significant class, have diverse biological functions related to metabolism, development, reproduction, conidiation, stress responses, and pathogenicity. This study demonstrates that BbYap1 of Beauveria bassiana regulates cellular enzyme lipid homeostasis and fungal virulence by eluding host humoral defense, which contributes to fungal chemical stress and vegetative development. These findings offer fresh perspectives for comprehending molecular roles of YAP in EPF.

A parasitoid regulates 20E synthesis and antibacterial activity of the host for development by inducing host nitric oxide production

Parasitoids are important components of the natural enemy guild in the biological control of insect pests. They depend on host resources to complete the development of a specific stage or whole life cycle and thus have evolved towards optimal host exploitation strategies. In the present study, we report a specific survival strategy of a fly parasitoid Exorista sorbillans (Diptera: Tachinidae), which is a potential biological control agent for agricultural pests and a pest in sericulture. We found that the expression levels of nitric oxide synthase (NOS) and nitric oxide (NO) production in host Bombyx mori (Lepidoptera: Bombycidae) were increased after E. sorbillans infection. Reducing NOS expression and NO production with an NOS inhibitor (NG-nitro-L-arginine methyl ester hydrochloride) in infected B. mori significantly impeded the growth of E. sorbillans larvae. Moreover, the biosynthesis of 20-hydroxyecdysone (20E) in infected hosts was elevated with increasing NO production, and inhibiting NOS expression lowered 20E biosynthesis. More importantly, induced NO synthesis was required to eliminate intracellular bacterial pathogens that presumably competed for shared host resources. Inhibiting NOS expression down-regulated the transcription of antimicrobial peptide genes and increased the number of bacteria in parasitized hosts. Collectively, this study revealed a new perspective on the role of NO in host-parasitoid interactions and a novel mechanism for parasitoid regulation of host physiology to support its development.

Gut Microbial Diversity Reveals Differences in Pathogenicity between Metarhizium rileyi and Beauveria bassiana during the Early Stage of Infection in Spodoptera litura Larvae

Beauveria bassiana and Metarhizium rileyi are extensively utilized to investigate fungal pathogenic mechanisms and to develop biological control agents. Notwithstanding, notable distinctions exist in their pathogenicity against the same host insect. This study aimed to elucidate the pathogenic differences between M. rileyi and B. bassiana by examining the impact of various ratios of B. bassiana strain AJS91881 and M. rileyi strain SXBN200920 on fifth instar larvae of Spodoptera litura, focusing on early infection stages and intestinal microbial community structure. The lethal time 50 (LT50) for B. bassiana was significantly lower than that for M. rileyi, indicating greater efficacy. Survival analyses in mixed groups (ratios of 1:9, 1:1, and 9:1 M. rileyi to B. bassiana) consistently demonstrated higher virulence of B. bassiana. Intestinal microbial diversity analysis revealed a significant increase in Achromobacter and Pseudomonas in larvae infected with M. rileyi, whereas Weissella was notably higher in those infected with B. bassiana. Additionally, significant shifts in microbial genera abundances were observed across all mixed infection groups. KEGG pathway enrichment analysis indicated that M. rileyi and B. bassiana employ distinct pathogenic strategies during early infection stages. In vitro tests confirmed the superior growth and stress resistance of B. bassiana compared to M. rileyi, but the antifungal ability of M. rileyi was better than that of B. bassiana. In conclusion, our findings provide preliminary insights into the differential pathogenic behaviors of M. rileyi and B. bassiana during the early infection stages in S. litura larvae, enhancing our understanding of their mechanisms and informing biological pest control strategies in agriculture and forestry.

Entomopathogenic fungi in crops protection with an emphasis on bioactive metabolites and biological activities

Plant pathogens with their abundance are harmful and cause huge damage to different agricultural crops and economy of a country as well as lead towards the shortage of food for humans. For their management, the utilization of entomopathogenic fungi is an eco-friendly technique, sustainable to the environment, safe for humans and has promising effect over chemical-based pesticides. This process requires a biochemical mechanism, including the production of enzymes, toxins, and other metabolites that facilitate host infection and invasion. Essential enzymes such as chitinase, proteinase, and lipase play a direct role in breaking down the host cuticle, the primary barrier to EPF (Entomopathogenic Fungi) infection. Additionally, secondary metabolites such as destruxins in Metarhizium, beauvericin in Beauveria, hirsutellides in Hirsutella, isarolides in Isaria, cordyols in Cordyceps, and vertihemipterins in Verticillium, among others, act both directly and indirectly to disable the defense mechanisms of insect hosts, thereby accelerating the EPF infection process. The chemical composition of these secondary metabolites varies, ranging from simple non-peptide pigments such as oosporine to highly complex piperazine derivatives such as vertihemiptellides. The biocontrol efficacy of EPF is extensively studied, with numerous fungal strains commercially available on a large scale for managing arthropod pests. This review emphasizes the role of proteins and enzymes against crop pathogens, detailing their mode of action, and describing the metabolites from entomopathogenic fungi and their biological activities. In doing so, these findings contribute to establishing a symbiotic equilibrium between agricultural productivity and environmental conservation.

Chronic bee paralysis virus exploits host antimicrobial peptides and alters gut microbiota composition to facilitate viral infection

The significance of gut microbiota in regulating animal immune response to viral infection is increasingly recognized. However, how chronic bee paralysis virus (CBPV) exploits host immune to disturb microbiota for its proliferation remains elusive. Through histopathological examination, we discovered that the hindgut harbored the highest level of CBPV, and displayed visible signs of damages. The metagenomic analysis showed that a notable reduction in the levels of Snodgrassella alvi and Lactobacillus apis, and a significant increase in the abundance of the opportunistic pathogens such as Enterobacter hormaechei and Enterobacter cloacae following CBPV infection. Subsequent co-inoculation experiments showed that these opportunistic pathogens facilitated the CBPV proliferation, leading to accelerated mortality in bees and exacerbation of bloated abdomen symptoms after CBPV infection. The expression level of antimicrobial peptide (AMP) was found to be significantly up-regulated by over 1000 times in response to CBPV infection, as demonstrated by subsequent transcriptome and quantitative real-time PCR investigations. In particular, through correlation analysis and a bacteriostatic test revealed that the AMPs did not exhibit any inhibitory effect against the two opportunistic pathogens. However, they did demonstrate inhibitory activity against S. alvi and L. apis. Our findings provide different evidence that the virus infection may stimulate and utilize the host's AMPs to eradicate probiotic species and facilitate the proliferation of opportunistic bacteria. This process weakens the intestinal barrier and ultimately resulting in the typical bloated abdomen.

Influence of symbiotic bacteria on the susceptibility of Plagiodera versicolora to Beauveria bassiana infection

The symbiotic bacterial microbiota of insects has been shown to play essential roles in processes related to physiology, metabolism, and innate immunity. In this study, the symbiotic microbiome of Plagiodera versicolora at different developmental stages was analyzed using 16S rRNA high-throughput sequencing. The result showed that symbiotic bacteria community in P. versicolora was primarily made up of Actinobacteriota, Proteobacteria, Firmicutes, Bacteroidota, and Dependentiae. The bacterial composition among different age individuals were highly diverse, while 65 core genera were distributed in all samples which recommend core bacterial microbiome. The 8 species core bacteria were isolated from all samples, and all of them were classified as Pseudomonas sp. Among them, five species have been proven to promote the vegetable growth of Beauveria bassiana. Moreover, the virulence of B. bassiana against nonaxenic larvae exceeded B. bassiana against axenic larvae, and the introduction of the Pseudomonas sp. to axenic larvae augmented the virulence of fungi. Taken together, our study demonstrates that the symbiotic bacteria of P. versicolora are highly dissimilar, and Pseudomonas sp. core bacteria can promote host infection by entomopathogenic fungus. This result emphasizes the potential for harnessing these findings in the development of effective pest management strategies.

Partner of neuropeptide bursicon homodimer pburs mediates a novel antimicrobial peptide Ten3LP via Dif/Dorsal2 in Tribolium castaneum

Bursicon is a cystine knot family neuropeptide, composed of two subunits, bursicon (burs) and partner of burs (pburs). The subunits can form heterodimers to regulate cuticle tanning and wing maturation and homodimers to signal different biological functions in innate immunity, midgut stem cell proliferation and energy homeostasis, and reproductive physiology in the model insects Drosophila melanogaster or Tribolium castaneum. Here, we report on the role of the pburs homodimer in signaling innate immunity in T. castaneum larvae. Through transcriptome analysis we identified a set of immune-related genes that respond to pburs RNAi. Treating larvae with recombinant-pburs protein led to up-regulation of antimicrobial peptide (AMP) genes in vivo and in vitro. The upregulation of most AMP genes was dependent on the NF-κB transcription factor Relish. Most importantly, we identified a novel AMP, Tenecin 3-like peptide (Ten3LP), regulated by pburs via NF-κB transcription factor Dorsal-related immunity factor (Dif)/Dorsal2, but not Relish. We conducted Ten3LP RNAi, synthesized recombinant Ten3LP protein for microbial inhibition assays and functionally characterized Ten3LP as an AMP specific for fungi and Gram-positive bacteria. We demonstrate that expression of Ten3LP is activated by pburs via the Toll pathway. These findings identify new molecular targets for development of potential antibiotics for treating microbial infections and perhaps for RNAi based pest management technology.