Evasion of host defense by in vivo-produced protoplast-like cells of the insect mycopathogen Beauveria bassiana

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.

Two sirtuin proteins, Hst3 and Hst4, modulate asexual development, stress tolerance, and virulence by affecting global gene expression in Beauveria bassiana

Beauveria bassiana is a widely used entomopathogenic fungus in insect biological control applications. In this study, we investigated the role of two sirtuin homologs, BbHst3 and BbHst4, in the biological activities and pathogenicity of B. bassiana. Our results showed that deletion of BbHst3 and/or BbHst4 led to impaired sporulation, reduced (~50%) conidial production, and decreased tolerance to various stresses, including osmotic, oxidative, and cell wall-disturbing agents. Moreover, BbHst4 plays dominant roles in histone H3-K56 acetylation and DNA damage response, while BbHst3 is more responsible for maintaining cell wall integrity. Transcriptomic analyses revealed significant changes (>1,500 differentially expressed genes) in gene expression patterns in the mutant strains, particularly in genes related to secondary metabolism, detoxification, and transporters. Furthermore, the ΔBbHst3, ΔBbHst4, and ΔBbHst3ΔBbHst4 strains exhibited reduced virulence in insect bioassays, with decreased (~20%) abilities to kill insect hosts through topical application and intra-hemocoel injection. These findings highlight the crucial role of BbHst3 and BbHst4 in sporulation, DNA damage repair, cell wall integrity, and fungal infection in B. bassiana. Our study provides new insights into the regulatory mechanisms underlying the biological activities and pathogenicity of B. bassiana and emphasizes the potential of targeting sirtuins for improving the efficacy of fungal biocontrol agents.IMPORTANCESirtuins, as a class of histone deacetylases, have been shown to play important roles in various cellular processes in fungi, including asexual development, stress response, and pathogenicity. By investigating the functions of BbHst3 and BbHst4, we have uncovered their critical contributions to important phenotypes in Beauveria bassiana. Deletion of these sirtuin homologs led to reduced conidial yield, increased sensitivity to osmotic and oxidative stresses, impaired DNA damage repair processes, and decreased fungal virulence. Transcriptomic analyses showed differential expression of numerous genes involved in secondary metabolism, detoxification, transporters, and virulence-related factors, potentially uncovering new targets for manipulation and optimization of fungal biocontrol agents. Our study also emphasizes the significance of sirtuins as key regulators in fungal biology and highlights their potential as promising targets for the development of novel antifungal strategies.

Selective protein self-deprivation by Mormon crickets following fungal attack

Immune responses to infection result in behavioral changes that affect resource acquisition, such as general starvation and compensatory feeding to offset changes in resource allocation. Mormon crickets aggregate and march in bands containing millions of insects. Some bands are comprised of insects seeking proteins. They are also low in circulating phenoloxidase (PO) and more susceptible to fungal attack, as we have demonstrated in the lab. Here, we ask: Do Mormon crickets elevate PO and consume protein in response to infection by the pathogenic fungus Beauveria bassiana? B. bassiana was applied topically (day 0), and mortality began on day 5. Total protein, PO, and prophenoloxidase (proPO) were assayed in hemolymph on day 1 and 4. On day 1, PO titers were not different between inoculated and control insects, whereas by day 4, PO was greater in the inoculated group. proPO activity was unchanged. Circulating protein declined in inoculated insects relative to controls. As predicted, PO titers were elevated as a result of fungal infection, and hemolymph protein was reduced, but the insects did not compensate behaviorally. Indeed, during the first three days post-infection, infected insects reduced protein consumption while maintaining carbohydrate consumption similar to the controls. Following day 3, a more general reduction in protein and carbohydrate intake was evident in infected insects. Survivorship to infection was associated with the amount of protein consumed and unrelated to carbohydrate consumption. Selective protein deprivation by the host seems counterintuitive, but it might limit growth and toxin production by the invading fungus. Alternatively, the fungus might control the host diet to compromise host immunity to infection. Abrupt changes in allocation resulting from an infection can lead to changes in acquisition that are not always intuitive. Because protein acquisition drives aggression between members of the migratory band, B. bassiana application may reduce cannibalism and slow band movement.

Impact of the Diet on the Mortality and on Gene Expression of the Antimicrobial Peptide Tenecin 3 in Tenebrio molitor Larvae Infected by Beauveria bassiana

Large-scale insect rearing can be subjected to microbial infections, leading to serious economic losses. When possible, the use of antibiotics is to be avoided for insects intended as feed or food and new strategies to preserve the health of the farmed insects are required. The effectiveness of insect immune system depends on several factors, including the nutritional composition of the diet. The possibility to modulate immune responses through the diet is currently a topic of great interest from an application point of view. We evaluated the effect of two different diets on the survival rate and gene expression of the antimicrobial peptide Tenecin 3 in uninfected and Beauveria bassiana-infected Tenebrio molitor L. larvae. A wheat bran diet, mixed 50% with brewers' spent grains, could positively influence the expression of Tenecin 3 gene when uninfected T. molitor is allowed to develop on such a substrate from early larval stages. Even if, in our trial, the diet with added brewers' spent grains could not reduce the mortality of the larvae infected with B. bassiana, higher transcriptional levels of the antifungal peptide were observed in insects fed this diet, depending on the timing of diet administration.

The Elongator Subunit Elp3 Regulates Development, Stress Tolerance, Cell Cycle, and Virulence in the Entomopathogenic Fungus Beauveria bassiana

Transcriptional activity is mediated by chromatin remodeling, which in turn is affected by post-translational modifications, including histone acetylation. Histone acetyltransferases (HATs) are capable of promoting euchromatin formation and then activating gene transcription. Here, we characterize the Elp3 GNAT family HAT, which is also a subunit of Elongator complex, in the environmentally and economically important fungal insect pathogen, Beauveria bassiana. BbElp3 showed high localization levels to mitochondria, with some nuclear and cytoplasmic localization also apparent. Targeted gene knockout of BbElp3 resulted in impaired asexual development and morphogenesis, reduced tolerances to multiple stress conditions, reduced the ability of the fungus to utilize various carbon/nitrogen sources, increased susceptibility to rapamycin, and attenuated virulence in bioassays using the greater wax moth, Galleria mellonella. The ΔBbElp3 mutant also showed disrupted cell cycle, abnormal hyphal septation patterns, and enlarged autophagosomes in vegetative hyphae. Transcriptome analyses revealed differential expression of 775 genes (DEGs), including 336 downregulated and 438 upregulated genes in the ΔBbElp3 strain as compared to the wild type. Downregulated genes were mainly enriched in pathways involved in DNA processing and transcription, cell cycle control, cellular transportation, cell defense, and virulence, including hydrophobins, cellular transporters (ABC and MFS multidrug transporters), and insect cuticular degrading enzymes, while upregulated genes were mainly enriched in carbohydrate metabolism and amino acid metabolism. These data indicate pleiotropic effects of BbElp3 in impacting specific cellular processes related to asexual development, cell cycle, autophagy, and virulence.

Transcription Factor Mavib-1 Negatively Regulates Conidiation by Affecting Utilization of Carbon and Nitrogen Source in Metarhizium acridum

Conidium is the main infection unit and reproductive unit of pathogenic fungi. Exploring the mechanism of conidiation and its regulation contributes to understanding the pathogenicity of pathogenic fungi. Vib-1, a transcription factor, was reported to participate in the conidiation process. However, the regulation mechanism of Vib-1 in conidiation is still unclear. In this study, we analyzed the function of Vib-1 and its regulation mechanism in conidiation through knocking out and overexpression of Vib-1 in entomopathogenic fungus Metarhizium acridum. Results showed that the colonial growth of Mavib-1 disruption mutant (ΔMavib-1) was significantly decreased, and conidiation was earlier compared to wild type (WT), while overexpression of Mavib-1 led to a delayed conidiation especially when carbon or nitrogen sources were insufficient. Overexpression of Mavib-1 resulted in a conidiation pattern shift from microcycle conidiation to normal conidiation on nutrient-limited medium. These results indicated that Mavib-1 acted as a positive regulator in vegetative growth and a negative regulator in conidiation by affecting utilization of carbon and nitrogen sources in M. acridum. Transcription profile analysis demonstrated that many genes related to carbon and nitrogen source metabolisms were differentially expressed in ΔMavib-1 and OE strains compared to WT. Moreover, Mavib-1 affects the conidial germination, tolerance to UV-B and heat stresses, cell wall integrity, conidial surface morphology and conidial hydrophobicity in M. acridum. These findings unravel the regulatory mechanism of Mavib-1 in fungal growth and conidiation, and enrich the knowledge to conidiation pattern shift of filamentous fungi.

The genus Entomophthora: bringing the insect destroyers into the twenty-first century

The fungal genus Entomophthora consists of highly host-specific pathogens that cause deadly epizootics in their various insect hosts. The most well-known among these is the "zombie fly" fungus E. muscae, which, like other Entomophthora species, elicits a series of dramatic behaviors in infected hosts to promote optimal spore dispersal. Despite having been first described more than 160 years ago, there are still many open questions about Entomophthora biology, including the molecular underpinnings of host behavior manipulation and host specificity. This review provides a comprehensive overview of our current understanding of the biology of Entomophthora fungi and enumerates the most pressing outstanding questions that should be addressed in the field. We briefly review the discovery of Entomophthora and provide a summary of the 21 recognized Entomophthora species, including their type hosts, methods of transmission (ejection of spores after or before host death), and for which molecular data are available. Further, we argue that this genus is globally distributed, based on a compilation of Entomophthora records in the literature and in online naturalist databases, and likely to contain additional species. Evidence for strain-level specificity of hosts is summarized and directly compared to phylogenies of Entomophthora and the class Insecta. A detailed description of Entomophthora's life-cycle and observed manipulated behaviors is provided and used to summarize a consensus for ideal growth conditions. We discuss evidence for Entomophthora's adaptation to growth exclusively inside insects, such as producing wall-less hyphal bodies and a unique set of subtilisin-like proteases to penetrate the insect cuticle. However, we are only starting to understand the functions of unusual molecular and genomic characteristics, such as having large > 1 Gb genomes full of repetitive elements and potential functional diploidy. We argue that the high host-specificity and obligate life-style of most Entomophthora species provides ample scope for having been shaped by close coevolution with insects despite the current general lack of such evidence. Finally, we propose six major directions for future Entomophthora research and in doing so hope to provide a foundation for future studies of these fungi and their interaction with insects.

A virulence-related lectin traffics into eisosome and contributes to functionality of cytomembrane and cell-wall in the insect-pathogenic fungus Beauveria bassiana

Lectins are characterized of the carbohydrate-binding ability and play comprehensive roles in fungal physiology (e.g., defense response, development and host-pathogen interaction). Beauveria bassiana, a filamentous entomopathogenic fungus, has a lectin-like protein containing a Fruit Body_domain (BbLec1). BbLec1 could bind to chitobiose and chitin in fungal cell wall. BbLec1 proteins interacted with each other to form multimers, and translocated into eisosomes. Further, the interdependence between BbLec1 and the eisosome protein PliA was essential for stabilizing the eisosome architecture. To test the BbLec1 roles in B. bassiana, we constructed the gene disruption and complementation mutants. Notably, the BbLec1 loss resulted in the impaired cell wall in mycelia and conidia as well as conidial formation capacity. In addition, disruption of BbLec1 led to the reduced cytomembrane integrity and the enhanced sensitivity to osmotic stress. Finally, ΔBbLec1 mutant strain displayed the weakened virulence when compared with the wild-type strain. Taken together, BbLec1 traffics into eisosome and links the functionality of eisosome to development and virulence of B. bassiana.

Mechanism of Metarhizium rileyi evading cellular immune responses in Helicoverpa armigera

Upon entry into the hemocoel of host insects, entomopathogenic fungi switch to yeast-like hyphal bodies that are not recognized by host hemocytes and replicate extensively in the hemolymph. The mechanism by which hyphal bodies evade host cellular immunity is not well understood. This study compares Metarhizium rileyi conidia and hyphal bodies with respect to elicitation of the immune response of Helicoverpa armigera and recognition by host pattern recognition receptors (PRRs). We found that the ability of host hemocytes to phagocytize and nodulate hyphal bodies was weaker than those responses against conidia, suggesting that hyphal bodies are more able to evade host cellular immunity. Additionally, we found that the binding affinity of H. armigera β-1,3-glucan recognition proteins was much lower for hyphal bodies than for conidia. We observed no agglutination response of H. armigera C-type lectin 3 (HaCTL3) against hyphal bodies, and HaCTL3 bound significantly less to hyphal bodies than to conidia, indicating that host PRRs have a lower affinity for hyphal bodies than for conidia. This study provides direct evidence that the mechanism whereby entomopathogenic fungi escape host cellular immunity involves the inability of host PRRs to sufficiently recognize hyphal bodies to elicit the cellular immune response.

The Polymycovirus-Mediated Growth Enhancement of the Entomopathogenic Fungus Beauveria bassiana Is Dependent on Carbon and Nitrogen Metabolism

Polymycoviridae is a growing family of mycoviruses whose members typically have non-conventional capsids and multi-segmented, double-stranded (ds) RNA genomes. Beauveria bassiana polymycovirus (BbPmV) 1 is known to enhance the growth and virulence of its fungal host, the entomopathogenic ascomycete and popular biological control agent B. bassiana. Here we report the complete sequence of BbPmV-3, which has six genomic dsRNA segments. Phylogenetic analysis of RNA-dependent RNA polymerase (RdRp) protein sequences revealed that BbPmV-3 is closely related to the partially sequenced BbPmV-2 but not BbPmV-1. Nevertheless, both BbPmV-3 and BbPmV-1 have similar effects on their respective host isolates ATHUM 4946 and EABb 92/11-Dm, affecting pigmentation, sporulation, and radial growth. Production of conidia and radial growth are significantly enhanced in virus-infected isolates as compared to virus-free isogenic lines on Czapek-Dox complete and minimal media that contain sucrose and sodium nitrate. However, this polymycovirus-mediated effect on growth is dependent on the carbon and nitrogen sources available to the host fungus. Both BbPmV-3 and BbPmV-1 increase growth of ATHUM 4946 and EABb 92/11-Dm when sucrose is replaced by lactose, trehalose, glucose, or glycerol, while the effect is reversed on maltose and fructose. Similarly, both BbPmV-3 and BbPmV-1 decrease growth of ATHUM 4946 and EABb 92/11-Dm when sodium nitrate is replaced by sodium nitrite, potassium nitrate, or ammonium nitrate. In conclusion, the effects of polymycoviruses on B. bassiana are at least partially mediated via its metabolic pathways.

Mitochondrial evolution in the entomopathogenic fungal genus Beauveria

Species in the fungal genus Beauveria are pathogens of invertebrates and have been commonly used as the active agent in biopesticides. After many decades with few species described, recent molecular approaches to classification have led to over 25 species now delimited. Little attention has been given to the mitochondrial genomes of Beauveria but better understanding may led to insights into the nature of species and evolution in this important genus. In this study, we sequenced the mitochondrial genomes of four new strains belonging to Beauveria bassiana, Beauveria caledonica and Beauveria malawiensis, and compared them to existing mitochondrial sequences of related fungi. The mitochondrial genomes of Beauveria ranged widely from 28,806 to 44,135 base pairs, with intron insertions accounting for most size variation and up to 39% (B. malawiensis) of the mitochondrial length due to introns in genes. Gene order of the common mitochondrial genes did not vary among the Beauveria sequences, but variation was observed in the number of transfer ribonucleic acid genes. Although phylogenetic analysis using whole mitochondrial genomes showed, unsurprisingly, that B. bassiana isolates were the most closely related to each other, mitochondrial codon usage suggested that some B. bassiana isolates were more similar to B. malawiensis and B. caledonica than the other B. bassiana isolates analyzed.

The greater wax moth Galleria mellonella: biology and use in immune studies

The greater wax moth Galleria mellonella is an invertebrate that is increasingly being used in scientific research. Its ease of reproduction, numerous offspring, short development cycle, and finally, its known genome and immune-related transcriptome provide a convenient research model for investigation of insect immunity at biochemical and molecular levels. Galleria immunity, consisting of only innate mechanisms, shows adaptive plasticity, which has recently become the subject of intensive scientific research. This insect serves as a mini host in studies of the pathogenicity of microorganisms and in vivo tests of the effectiveness of single virulence factors as well as new antimicrobial compounds. Certainly, the Galleria mellonella species deserves our attention and appreciation for its contribution to the development of research on innate immune mechanisms. In this review article, we describe the biology of the greater wax moth, summarise the main advantages of using it as a model organism and present some of the main techniques facilitating work with this insect.

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.

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.

Genome sequencing and analysis of fungus Hirsutella sinensis isolated from Ophiocordyceps sinensis

Ophiocordyceps sinensis has been used as a traditional medicine or healthy food in China for thousands of years. Hirsutella sinensis was reported as the only correct anamorph of O. sinensis. It is reported that the laboratory-grown H. sinensis mycelium has similar clinical efficacy and less associated toxicity compared to the wild O. sinensis. The research of the H. sinensis is becoming more and more important and urgent. To gain deeper insight into the biological and pharmacological mechanisms, we sequenced the genome of H. sinensis. The genome of H. sinensis (102.72 Mb) was obtained for the first time, with > 99% coverage. 10,200 protein-encoding genes were predicted based on the genome sequence. A detailed secondary metabolism analysis and structure verification of the main ingredients were performed, and the biosynthesis pathways of seven ingredients (mannitol, cordycepin, purine nucleotides, pyrimidine nucleotides, unsaturated fatty acid, cordyceps polysaccharide and sphingolipid) were predicted and drawn. Furthermore, infection process and mechanism of H. sinensis were studied and elaborated in this article. The enzymes involved in the infection mechanism were also predicted, cloned and expressed to verify the mechanism. The genes and proteins were predicted and annotated based on the genome sequence. The pathways of several active components in H. sinensis were predicted and key enzymes were confirmed. The work presented here would improve the understanding of the genetic basis of this organism, and contribute to further research, production and application of H. sinensis.

Adipokinetic hormone promotes infection with entomopathogenic fungus Isaria fumosorosea in the cockroach Periplaneta americana

The adipokinetic hormones (AKHs) are known to be involved in insect immunity, thus their role in the cockroach Periplaneta americana infected with the entomopathogenic fungus Isaria fumosorosea was examined in this study. The application of I. fumosorosea resulted in a significant increase in both Akh gene expression and AKH peptide levels. Further, co-application of I. fumosorosea with Peram-CAH-II significantly enhanced cockroach mortality compared with the application of I. fumosorosea alone. The mechanism of AKH action could involve metabolic stimulation, which was indicated by a significant increase in carbon dioxide production; this effect can increase the turnover and thus efficacy of toxins produced by I. fumosorosea in the cockroach's body. I. fumosorosea treatment resulted in a significant decrease in haemolymph nutrients (carbohydrates and lipids), but co-application with Peram-CAH-II restored control level of lipids or even further increased the level of carbohydrates. Such nutritional abundance could enhance the growth and development of I. fumosorosea. Further, both I. fumosorosea and Peram-CAH-II probably affected oxidative stress: I. fumosorosea alone curbed the activity of catalase in the cockroach's gut, but co-application with Peram-CAH-II stimulated it. Interestingly, the hormone alone had no effect on catalase activity. Taken together, the results of the present study demonstrate the interactions between the fungus and AKH activity; understanding this relationship could provide insight into AKH action and may have practical implications for insect pest control in the future.

Highly specific host-pathogen interactions influence Metarhizium brunneum blastospore virulence against Culex quinquefasciatus larvae

Entomopathogenic fungi are potential biological control agents of mosquitoes. Our group observed that not all mosquitoes were equally susceptible to fungal infection and observed significant differences in virulence of different spore types. Conidiospores and blastospores were tested against Culex quinquefasciatus larvae. Blastospores are normally considered more virulent than conidia as they form germ tubes and penetrate the host integument more rapidly than conidia. However, when tested against Cx. quinquefasciatus, blastospores were less virulent than conidia. This host-fungus interaction was studied by optical, electron and atomic force microscopy (AFM). Furthermore, host immune responses and specific gene expression were investigated. Metarhizium brunneum (formerly M. anisopliae) ARSEF 4556 blastospores did not readily adhere to Culex larval integument and the main route of infection was through the gut. Adhesion forces between blastospores and Culex cuticle were significantly lower than for other insects. Larvae challenged with blastospores showed enhanced immune responses, with increased levels of phenoloxidase, glutathione-S-transferase, esterase, superoxide dismutase and lipid peroxidase activity. Interestingly, M. brunneum pathogenicity/stress-related genes were all down-regulated in blastospores exposed to Culex. Conversely, when conidia were exposed to Culex, the pathogenicity genes involved in adhesion or cuticle degradation were up-regulated. Delayed host mortality following blastospore infection of Culex was probably due to lower adhesion rates of blastospores to the cuticle and enhanced host immune responses deployed to counter infection. The results here show that subtle differences in host-pathogen interactions can be responsible for significant changes in virulence when comparing mosquito species, having important consequences for biological control strategies and the understanding of pathogenicity processes.

The interplay between dose and immune system activation determines fungal infection outcome in the African malaria mosquito, Anopheles gambiae

The Toll pathway is a central regulator of antifungal immunity in insects. In mosquitoes, the Toll pathway affects infections with the fungal entomopathogen, Beauveria bassiana, which is considered a potential mosquito biopesticide. We report here the use of B. bassiana strain I93-825 in Anopheles gambiae to analyze the impact of Toll pathway modulation on mosquito survival. Exposure to a narrow dose range of conidia by direct contact decreased mosquito longevity and median survival. In addition, fungal exposure dose correlated positively and linearly with hazard ratio. Increased Toll signaling by knockdown of its inhibitor, cactus, decreased survivorship of uninfected females, increased mosquito survival after low dose B. bassiana exposure, but had little effect following exposure to higher doses. This observed trade-off could have implications for development of B. bassiana as a prospective vector control tool. On the one hand, selection for small increases in mosquito immune signaling across a narrow dose range could impair efficacy of B. bassiana. On the other hand, costs of immunity and the capacity for higher doses of fungus to overwhelm immune responses could limit evolution of resistance.

Microbiota in insect fungal pathology

Significant progress has been made in the biochemical and genetic characterization of the host-pathogen interaction mediated by insect pathogenic fungi, with the most widely studied being the Ascomycetes (Hypocrealean) fungi, Metarhizium robertsii and Beauveria bassiana. However, few studies have examined the consequences and effects of host (insect) microbes, whether compatible or antagonistic, on the development and survival of entomopathogenic fungi. Host microbes can act on the insect cuticular surface, within the gut, in specialized insect microbe hosting structures, and within cells, and they include a wide array of facultative and/or obligate exosymbionts and endosymbionts. The insect microbiome differs across developmental stages and in response to nutrition (e.g., different plant hosts for herbivores) and environmental conditions, including exposure to chemical insecticides. Here, we review recent advances indicating that insect-pathogenic fungi have evolved a spectrum of strategies for exploiting or suppressing host microbes, including the production of antimicrobial compounds that are expressed at discrete stages of the infection process. Conversely, there is increasing evidence that some insects have acquired microbes that may be specialized in the production of antifungal compounds to combat infection by (entomopathogenic) fungi. Consideration of the insect microbiome in fungal insect pathology represents a new frontier that can help explain previously obscure ecological and pathological aspects of the biology of entomopathogenic fungi. Such information may lead to novel approaches to improving the efficacy of these organisms in pest control efforts.

Essential role of Rpd3-dependent lysine modification in the growth, development and virulence of Beauveria bassiana

Rpd3 is a class I histone deacetylase that reverses lysine acetylation thus influencing cellular processes and functions. However, its role in fungal insect pathogens has not been explored yet. Here we show that Rpd3-dependent lysine modification and gene expression orchestrate growth, conidiation and virulence in Beauveria bassiana. Deletion of Rpd3 resulted in severe growth defects on various carbon/nitrogen sources, 97% reduction in conidiation capacity and drastic attenuation in virulence. These phenotypes concurred with differential expression of 1479 proteins and hyperacetylation or hypoacetylation of 2227 lysine residues on 1134 proteins. Many of these proteins fell into carbon/nitrogen metabolism and cell rescue/defence/virulence, indicating vital roles of Rpd3-dependent protein expression and lysine modification in the fungal growth and virulence. Intriguingly, lysine residues of four core histones (H2A, H2B, H3 and H4) and many histone acetyltransferases were also hyper- or hypoacetylated in Δrpd3, suggesting direct and indirect roles for Rpd3 in genome-wide lysine modification. However, crucial development activators were transcriptionally repressed and not found in either proteome or acetylome. Single/double-site-directed H3K9/K14 mutations for hyper/hypoacetylation exerted significant impacts on conidiation and dimorphic transition crucial for fungal virulence. Altogether, Rpd3 mediates growth, asexual development and virulence through transcriptional/translational regulation and posttranslational lysine modification in B. bassiana.

Pleiotropic effects of the histone deacetylase Hos2 linked to H4-K16 deacetylation, H3-K56 acetylation, and H2A-S129 phosphorylation in Beauveria bassiana

Histone acetyltransferases and deacetylases maintain dynamics of lysine acetylation/deacetylation on histones and nonhistone substrates involved in gene regulation and cellular events. Hos2 is a Class I histone deacetylases that deacetylates unique histone H4-K16 site in yeasts. Here, we report that orthologous Hos2 deacetylates H4-K16 and is also involved in the acetylation of histone H3-K56 and the phosphorylation of histone H2A-S129 and cyclin-dependent kinase 1 CDK1-Y15 in Beauveria bassiana, a filamentous fungal insect pathogen. These site-specific modifications are evidenced with hyperacetylated H4-K16, hypoacetylated H3-K56, and both hypophosphorylated H2A-S129 and CDK1-Y15 in absence of hos2. Consequently, the Δhos2 mutant suffered increased sensitivities to DNA-damaging and oxidative stresses, disturbed cell cycle, impeded cytokinesis, increased cell size or length, reduced conidiation capacity, altered conidial properties, and attenuated virulence. These phenotypic changes correlated well with dramatic repression of many genes that are essential for DNA damage repair, G₁ /S transition and DNA synthesis, hyphal septation, and asexual development. The uncovered ability for Hos2 to directly deacetylate H4-K16 and to indirectly modify H3-K56, H2A-S129, and CDK1-Y15 provides novel insight into more subtle regulatory role for Hos2 in genomic stability and diverse cellular events in the fungal insect pathogen than those revealed previously in nonentomophathogenic fungi.

Protein deficiency lowers resistance of Mormon crickets to the pathogenic fungus Beauveria bassiana

Little is known about the effects of dietary macronutrients on the capacity of insects to ward off a fungal pathogen. Here we tested the hypothesis that Mormon crickets fed restricted protein diets have lower enzymatic assays of generalized immunity, slower rates of encapsulation of foreign bodies, and greater mortality from infection by Beauveria bassiana, a fungal pathogen. Beginning in the last nymphal instar, Mormon crickets were fed a high, intermediate, or low protein diet with correspondingly low, intermediate, or high carbohydrate proportions. After they eclosed to adult, we drew hemolymph, topically applied B. bassiana, maintained them on diet treatments, and measured mortality for 21 days. Mormon crickets fed high protein diets had higher prophenoloxidase titers, greater encapsulation response, and higher survivorship to Beauveria fungal infection than those on low protein diets. We replicated the study adding very high and very low protein diets to the treatments. A high protein diet increased phenoloxidase titers, and those fed the very high protein diet had more circulating prophenoloxidase. Mormon crickets fed the very low protein diet were the most susceptible to B. bassiana infection, but the more concentrated phenoloxidase and prophenoloxidase associated with the highest protein diets did not confer the greatest protection from the fungal pathogen as in the first replicate. We conclude that protein-restricted diets caused Mormon crickets to have lower phenoloxidase titers, slower encapsulation of foreign bodies, and greater mortality from B. bassiana infection than those fed high protein diets. These results support the nutrition-based dichotomy of migrating Mormon crickets, protein-deficient ones are more susceptible to pathogenic fungi whereas carbohydrate-deficient ones are more vulnerable to bacterial challenge.

Daylight length-dependent translocation of VIVID photoreceptor in cells and its essential role in conidiation and virulence of Beauveria bassiana

The fungal insect pathogen Beauveria bassiana has the blue-light photoreceptor VIVID (VVD) but lacks a pigmentation pattern to trace its light responses. Here, we show that the fungal vvd is transcriptionally expressed, and linked to other blue/red photoreceptor genes, in a daylight length-dependent manner. GFP-tagged VVD fusion protein was localized to periphery, cytoplasm and vacuoles of hyphal cells in light/dark (L:D) cycles of 24:0 and 16:8 and aggregated in cytoplasm with shortening daylight until transfer into nuclei in full darkness. Deletion of vvd caused more reduced (91%) conidiation capacity in L:D 12:12 cycle of blue light (450/480 nm) than of yellow-to-red (540-760 nm) and white lights (∼70%). The conidiation defect worsened with shortened daylight in different L:D cycles of white light, coinciding well with drastic repression of key activator genes in central development pathway. Intriguingly, the deletion mutant displayed blocked secretion of cuticle-degrading Pr1 proteases, retarded dimorphic transition in insect haemocoel, and hence a lethal action twice longer than those for control strains against Galleria mellonella regardless of the infection passing or bypassing insect cuticle. Conclusively, VVD sustains normal conidiation in a daylight length-dependent manner and acts as a vital virulence factor in B. bassiana.

Central Nervous System Responses of the Oriental migratory, Locusta migratoria manilensis, to Fungal Infection

Responses of the central nervous system (CNS) to microbial challenge and the interplay between the CNS and the immune system are important for defending against pathogen attack. We have examined the CNS transcriptional response of Locusta migratoria manilensis to infection by the locust-specific fungal pathogen, Metarhizium acridum. CNS responses were examined during spore attachment, fungal germination and pre-penetration of the cuticle, and cuticle penetration/hemocoel ingress and proliferation. Effects were seen at the earliest time points (4 h post-infection) and the number of differentially expressed genes (DEGs) was highest during late mycosis (72 h post-infection). Significantly affected neurological pathways included genes involved in serotonergic, cholinergic, dopaminergic, GABAergic, and glutamergic synapse responses, as well as pathways responsible for synaptic vesicle cycle, long-term potentiation and depression, and neurotrophin and retrograde endocannabinoid signaling. In addition, a significant number of immune related DEGs were identified. These included components of the Toll, Imd and JAK/STAT pathways, consistent with interactions between the CNS and immune systems. The activation of immune response related CNS genes during early stage infection highlights the rapid detection of microbial pathogens and suggests an important role for the CNS in modulating immunity potentially via initiating behavioral adaptations along with innate immune responses.

Vital role for cyclophilin B (CypB) in asexual development, dimorphic transition and virulence of Beauveria bassiana

Cyclophilin B (CypB) was previously revealed as one of many putative secretory proteins in the transcriptome of Beauveria bassiana infection to a lepidopteran pest. Here we show a main localization of CypB in hyphal cell walls and septa and its essential role in the in vitro and in vivo asexual cycles of the fungal insect pathogen. Deletion of cypB reduced colony growth by 16-42% on two rich media and 30 scant media with different carbon or nitrogen sources. The deletion mutant suffered a delayed conidiation on a standard medium and a final 47% reduction in conidial yield, accompanied with drastic transcript depression of several key genes required for conidiation and conidial maturation. The mutant conidia required 10h longer to germinate 50% at optimal 25°C than wild-type conidia. Intriguingly, cultivation of the mutant conidia in a trehalose-peptone broth mimic to insect hemolymph resulted in 83% reduction in blastospore yield but only slight decrease in biomass level, indicating severe defects in transition of hyphae to blastospores. LT₅₀ for the deletion mutant against Galleria mellonella larvae through normal cuticle infection was prolonged to 7.4d from a wild-type estimate of 4.7d. During colony growth, additionally, the deletion mutant displayed hypersensitivity to Congo red, menadione, H₂O₂ and heat shock but increased tolerance to cyclosporine A and rapamycin. All of changes were restored by targeted gene complementation. Altogether, CypB takes part in sustaining normal growth, aerial conidiation, conidial germination, dimorphic transition, stress tolerance and pathogenicity in B. bassiana.

Differential Pathogenicity of Metarhizium Blastospores and Conidia Against Larvae of Three Mosquito Species

Biorational insecticides are being increasingly used in integrated pest management programs. In laboratory bioassays, the pathogenicity of blastospores and conidia of the entomopathogenic fungus Metarhizium brunneum ARSEF 4556 was evaluated against larvae of three mosquito species. Three propagule concentrations (1 × 106, 1 × 107, and 1 × 108 spores ml - 1) were used in the bioassays. Results showed that Aedes aegypti had lower survival rates when exposed to blastospores than when exposed to conidia, whereas the converse was true for Culex quinquefasciatus larvae. Anopheles stephensi larvae survival rates were similar when exposed to blastospores and conidia, except at the higher doses, where blastospores were more virulent. Several assays showed little difference in mortalities when using either 1 × 107 or 1 × 108 spores ml - 1, suggesting a threshold above which no higher control levels or economic benefit would be achieved. When tested at the lowest dose, the LT50 of Cx. quinquefasciatus using blastospores, wet conidia, and dry conidia was 3.2, 1.9, and 4.4 d, respectively. The LT50 of Ae. aegypti using blastospores, wet conidia, and dry conidia was 1.3, 3.3, and 6.2 d, respectively. The LT50 of An. stephensi using blastospores, wet conidia, and dry conidia was 2.0, 1.9, and 2.1 d, respectively. These observations suggest that for optimized control, two different formulations of the fungus may be needed when treating areas where there are mixed populations of Aedes, Anopheles, and Culex.

The production and uses of Beauveria bassiana as a microbial insecticide

Among invertebrate fungal pathogens, Beauveria bassiana has assumed a key role in management of numerous arthropod agricultural, veterinary and forestry pests. Beauveria is typically deployed in one or more inundative applications of large numbers of aerial conidia in dry or liquid formulations, in a chemical paradigm. Mass production is mainly practiced by solid-state fermentation to yield hydrophobic aerial conidia, which remain the principal active ingredient of mycoinsecticides. More robust and cost-effective fermentation and formulation downstream platforms are imperative for its overall commercialization by industry. Hence, where economics allow, submerged liquid fermentation provides alternative method to produce effective and stable propagules that can be easily formulated as dry stable preparations. Formulation also continues to be a bottleneck in the development of stable and effective commercial Beauveria-mycoinsecticides in many countries, although good commercial formulations do exist. Future research on improving fermentation and formulation technologies coupled with the selection of multi-stress tolerant and virulent strains is needed to catalyze the widespread acceptance and usefulness of this fungus as a cost-effective mycoinsecticide. The role of Beauveria as one tool among many in integrated pest management, rather than a stand-alone management approach, needs to be better developed across the range of crop systems. Here, we provide an overview of mass-production and formulation strategies, updated list of registered commercial products, major biocontrol programs and ecological aspects affecting the use of Beauveria as a mycoinsecticide.

Metarhizium brunneum Blastospore Pathogenesis in Aedes aegypti Larvae: Attack on Several Fronts Accelerates Mortality

Aedes aegypti is the vector of a wide range of diseases (e.g. yellow fever, dengue, Chikungunya and Zika) which impact on over half the world's population. Entomopathogenic fungi such as Metarhizium anisopliae and Beauveria bassiana have been found to be highly efficacious in killing mosquito larvae but only now are the underlying mechanisms for pathogenesis being elucidated. Recently it was shown that conidia of M. anisopliae caused stress induced mortality in Ae. aegypti larvae, a different mode of pathogenicity to that normally seen in terrestrial hosts. Blastospores constitute a different form of inoculum produced by this fungus when cultured in liquid media and although blastospores are generally considered to be more virulent than conidia no evidence has been presented to explain why. In our study, using a range of biochemical, molecular and microscopy methods, the infection process of Metarhizium brunneum (formerly M. anisopliae) ARSEF 4556 blastospores was investigated. It appears that the blastospores, unlike conidia, readily adhere to and penetrate mosquito larval cuticle. The blastospores are readily ingested by the larvae but unlike the conidia are able infect the insect through the gut and rapidly invade the haemocoel. The fact that pathogenicity related genes were upregulated in blastospores exposed to larvae prior to invasion, suggests the fungus was detecting host derived cues. Similarly, immune and defence genes were upregulated in the host prior to infection suggesting mosquitoes were also able to detect pathogen-derived cues. The hydrophilic blastospores produce copious mucilage, which probably facilitates adhesion to the host but do not appear to depend on production of Pr1, a cuticle degrading subtilisin protease, for penetration since protease inhibitors did not significantly alter blastospore virulence. The fact the blastospores have multiple routes of entry (cuticle and gut) may explain why this form of the inoculum killed Ae. aegypti larvae in a relatively short time (12-24hrs), significantly quicker than when larvae were exposed to conidia. This study shows that selecting the appropriate form of inoculum is important for efficacious control of disease vectors such as Ae. aegypti.

The Na+ /H+ antiporter Nhx1 controls vacuolar fusion indispensible for life cycles in vitro and in vivo in a fungal insect pathogen

The sole Na⁺ /H⁺ antiporter Nhx1 has been generally unexplored in filamentous fungi. We characterized Nhx1 in the entomopathogenic fungus Beauveria bassiana. An eGFP-tagged Nhx1 fusion accumulated in small punctuate structures, presumably endosomal and trans-Golgi network compartments, between septum and tubular vacuole of each wild-type cell stained with a vacuole-specific dye. Deletion of nhx1 resulted in significant acidification and severe fusion defect in vacuoles, which were fragmented and distinct from large or tubular wild-type vacuoles. The deletion also caused a drastic reduction in aerial conidiation or submerged blastospore production and more severe defect in vegetative growth than in conidial germination. The Δnhx1 mutant became more sensitive to high osmolarity, heat shock and several metal ions during growth but its conidia showed increased UV-B tolerance. Intriguingly, Δnhx1 was unable to infect a model insect through cuticle penetration or intrahaemocoel injection because it produced much less biomass and cuticle-degrading enzymes in a minimal broth and failed to form blastospores in the insect haemolymph. All changes were completely or largely restored by targeted nhx1 complementation. Our results provide novel insight into an indispensability of Nhx1 for not only vacuolar fusion but also life cycles in vitro and in vivo in B. bassiana.

Genome-Wide Host-Pathogen Interaction Unveiled by Transcriptomic Response of Diamondback Moth to Fungal Infection

Genome-wide insight into insect pest response to the infection of Beauveria bassiana (fungal insect pathogen) is critical for genetic improvement of fungal insecticides but has been poorly explored. We constructed three pairs of transcriptomes of Plutella xylostella larvae at 24, 36 and 48 hours post treatment of infection (hpt_I) and of control (hpt_C) for insight into the host-pathogen interaction at genomic level. There were 2143, 3200 and 2967 host genes differentially expressed at 24, 36 and 48 hpt_I/hpt_C respectively. These infection-responsive genes (~15% of the host genome) were enriched in various immune processes, such as complement and coagulation cascades, protein digestion and absorption, and drug metabolism-cytochrome P450. Fungal penetration into cuticle and host defense reaction began at 24 hpt_I, followed by most intensive host immune response at 36 hpt_I and attenuated immunity at 48 hpt_I. Contrastingly, 44% of fungal genes were differentially expressed in the infection course and enriched in several biological processes, such as antioxidant activity, peroxidase activity and proteolysis. There were 1636 fungal genes co-expressed during 24–48 hpt_I, including 116 encoding putative secretion proteins. Our results provide novel insights into the insect-pathogen interaction and help to probe molecular mechanisms involved in the fungal infection to the global pest.

Entomopathogenic Fungi: New Insights into Host-Pathogen Interactions

Although many insects successfully live in dangerous environments exposed to diverse communities of microbes, they are often exploited and killed by specialist pathogens. Studies of host-pathogen interactions (HPI) provide valuable insights into the dynamics of the highly aggressive coevolutionary arms race between entomopathogenic fungi (EPF) and their arthropod hosts. The host defenses are designed to exclude the pathogen or mitigate the damage inflicted while the pathogen responds with immune evasion and utilization of host resources. EPF neutralize their immediate surroundings on the insect integument and benefit from the physiochemical properties of the cuticle and its compounds that exclude competing microbes. EPF also exhibit adaptations aimed at minimizing trauma that can be deleterious to both host and pathogen (eg, melanization of hemolymph), form narrow penetration pegs that alleviate host dehydration and produce blastospores that lack immunogenic sugars/enzymes but facilitate rapid assimilation of hemolymph nutrients. In response, insects deploy an extensive armory of hemocytes and macromolecules, such as lectins and phenoloxidase, that repel, immobilize, and kill EPF. New evidence suggests that immune bioactives work synergistically (eg, lysozyme with antimicrobial peptides) to combat infections. Some proteins, including transferrin and apolipophorin III, also demonstrate multifunctional properties, participating in metabolism, homeostasis, and pathogen recognition. This review discusses the molecular intricacies of these HPI, highlighting the interplay between immunity, stress management, and metabolism. Increased knowledge in this area could enhance the efficacy of EPF, ensuring their future in integrated pest management programs.

Conidia of the insect pathogenic fungus, Metarhizium anisopliae, fail to adhere to mosquito larval cuticle

Adhesion of conidia of the insect pathogenic fungus, Metarhizium anisopliae, to the arthropod host cuticle initially involves hydrophobic forces followed by consolidation facilitated by the action of extracellular enzymes and secretion of mucilage. Gene expression analysis and atomic force microscopy were used to directly quantify recognition and adhesion between single conidia of M. anisopliae and the cuticle of the aquatic larval stage of Aedes aegypti and a representative terrestrial host, Tenebrio molitor. Gene expression data indicated recognition by the pathogen of both hosts; however, the forces for adhesion to the mosquito were approximately five times lower than those observed for Tenebrio. Although weak forces were recorded in response to Aedes, Metarhizium was unable to consolidate firm attachment. An analysis of the cuticular composition revealed an absence of long-chain hydrocarbons in Aedes larvae which are thought to be required for fungal development on host cuticle. This study provides, to our knowledge, the first evidence that Metarhizium does not form firm attachment to Ae. aegypti larvae in situ, therefore preventing the normal route of invasion and pathogenesis from occuring.

Microcyle conidiation in filamentous fungi

The typical life cycle of filamentous fungi commonly involves asexual sporulation after vegetative growth in response to environmental factors. The production of asexual spores is critical in the life cycle of most filamentous fungi. Normally, conidia are produced from vegetative hyphae (termed mycelia). However, fungal species subjected to stress conditions exhibit an extremely simplified asexual life cycle, in which the conidia that germinate directly generate further conidia, without forming mycelia. This phenomenon has been termed as microcycle conidiation, and to date has been reported in more than 100 fungal species. In this review, first, we present the morphological properties of fungi during microcycle conidiation, and divide microcycle conidiation into four simple categories, even though fungal species exhibit a wide variety of morphological differences during microcycle conidiogenesis. Second, we describe the factors that influence microcycle conidiation in various fungal species, and present recent genetic studies that have identified the genes responsible for this process. Finally, we discuss the biological meaning and application of microcycle conidiation.

Susceptibility of adults of the cerambycid beetle Hedypathes betulinus to the entomopathogenic fungi Beauveria bassiana, Metarhizium anisopliae, and Purpureocillium lilacinum

The cerambycid beetle Hedypathes betulinus (Klug) (Coleoptera: Cerambycidae) causes severe damage to yerba mate plants (Ilex paraguariensis (St. Hilaire) (Aquifoliales: Aquifoliaceae)), which results in large losses of production. In this study, the pathogenicity of entomopathogenic fungi of the species Beauveria bassiana (Balsamo-Crivelli) Vuillemin (Hypocreales: Cordycipitaceae), Metarhizium anisopliae sensu lato (Metschnikoff) Sorokin (Hypocreales: Clavicipitaceae), and Purpureocillium lilacinum (Thom) Luangsa-ard, Hywel-Jones, Houbraken and Samson (Hypocreales: Ophiocordycipitaceae) on yerba mate were evaluated. Fifteen isolates of B. bassiana, two of M. anisopliae, and seven of P. lilacinum on H. betulinus adults were analyzed under laboratory conditions. The raw mortality rate caused by B. bassiana isolates varied from 51.1 to 86.3%, and their LT50 values varied between 8.7 and 13.6 d. The isolates of M. anisopliae caused 69.6‒81.8% mortality, and their LT50 values varied between 7.4 and 7.9 d. In contrast, isolates of P. lilacinum were not pathogenic. M. anisopliae and B. bassiana isolates were pathogenic against H. betulinus adults, suggesting that they may be useful in biological control programs for insect pests of yerba mate.