Fungal Peptide Destruxin A Plays a Specific Role in Suppressing the Innate Immune Response in Drosophila melanogaster

Opportunities for optimizing fungal biological control agents for long-term and effective management of insect pests of orchards and vineyards: a review

Novel tactics for controlling insect pests in perennial fruit and nut crops are needed because target pests often display decreased susceptibility to chemical controls due to overreliance on a handful of active ingredients and regulatory issues. As an alternative to chemical controls, entomopathogenic fungi could be utilized as biological control agents to manage insect pest populations. However, development of field ready products is hampered by a lack of basic knowledge. Development of field ready products requires collecting, screening, and characterizing a greater variety of potential entomopathogenic fungal species and strains. Creation of a standardized research framework to study entomopathogenic fungi will aid in identifying the potential mechanisms of biological control activity that fungi could possess, including antibiotic metabolite production; strains and species best suited to survive in different climates and agroecosystems; and optimized combinations of entomopathogenic fungi and novel formulations. This mini review therefore discusses strategies to collect and characterize new entomopathogenic strains, test different potential mechanisms of biocontrol activity, examine ability of different species and strains to tolerate different climates, and lastly how to utilize this information to develop strains into products for growers.

Uncovering the immune evasion mechanism of microbial pathogens: lessons from anti-encapsulation strategies adopted by a fungus

Overview of Hemocytin-Mediated Cellular Encapsulation and Melanization Responses in Insects Against Fungal Pathogens.

Transfection of entomopathogenic Metarhizium species with a mycovirus confers hypervirulence against two lepidopteran pests

Although most known viruses infecting fungi pathogenic to higher eukaryotes are asymptomatic or reduce the virulence of their host fungi, those that confer hypervirulence to entomopathogenic fungus still need to be explored. Here, we identified and studied a novel mycovirus in Metarhizium flavoviride, isolated from small brown planthopper (Laodelphax striatellus). Based on molecular analysis, we tentatively designated the mycovirus as Metarhizium flavoviride partitivirus 1 (MfPV1), a species in genus Gammapartitivirus, family Partitiviridae. MfPV1 has two double-stranded RNAs as its genome, 1,775 and 1,575 bp in size respectively, encapsidated in isometric particles. When we transfected commercial strains of Metarhizium anisopliae and Metarhizium pingshaense with MfPV1, conidiation was significantly enhanced (t test; P-value < 0. 01), and the significantly higher mortality rates of the larvae of diamondback moth (Plutella xylostella) and fall armyworm (Spodoptera frugiperda), two important lepidopteran pests were found in virus-transfected strains (ANOVA; P-value < 0.05). Transcriptomic analysis showed that transcript levels of pathogenesis-related genes in MfPV1-infected M. anisopliae were obviously altered, suggesting increased production of metarhizium adhesin-like protein, hydrolyzed protein, and destruxin synthetase. Further studies are required to elucidate the mechanism whereby MfPV1 enhances the expression of pathogenesis-related genes and virulence of Metarhizium to lepidopteran pests. This study presents experimental evidence that the transfection of other entomopathogenic fungal species with a mycovirus can confer significant hypervirulence and provides a good example that mycoviruses could be used as a synergistic agent to enhance the biocontrol activity of entomopathogenic fungi.

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.

Comparative transcriptome analysis reveals disruption of Plutella xylostella immune system by fungal peptide cyclosporin C

The diamondback moth (Plutella xylostella), a major threat to crucifers across the globe, has developed resistance against the majority of insecticides enhancing the need for alternate control measures against this pest. Recently cyclosporin C, a secondary metabolite produced by the insect pathogenic fungus Purpeocillium lilacinum, has been reported to induce lethal and sub-lethal effects against P. xylostella. To date, little is known about the molecular mechanisms of interaction between cyclosporin C and P. xylostella immune systems. This study reports the transcriptome-based immune response of P. xylostella to cyclosprin C treatment. Our results showed differential expression of 322, 97, and 504 differentially expressed genes (DEGS) in P. xylostella treated with cyclosporin C compared to control 24, 48, and 72 h post-treatment, respectively. Thirteen DEGs were commonly expressed at different time intervals in P. xylostella larvae treated with cyclosporin C compared to control. Cyclosporin C treatment induced the down-regulated expression of majority of immune-related genes related to pattern recognition responses, signal modulation, Toll and IMD pathways, antimicrobial peptides and antioxidant responses confirming the ability to suppress immune response of P. xylostella. These results will further improve our knowledge of the infection mechanism and complex biochemical processes involved in interaction between cyclosporin C and insect immune systems.

Immune-Related Gene Profiles and Differential Expression in the Grey Garden Slug Deroceras reticulatum Infected with the Parasitic Nematode Phasmarhabditis hermaphrodita

The grey garden slug (Deroceras reticulatum), a common terrestrial slug native to Europe with a global distribution including North America, is commonly considered the most severe slug pest in agriculture. The nematode Phasmarhabditis hermaphrodita, which has been used in the U.K. and Europe as a commercial biocontrol agent since 1994, has also recently been collected in Oregon and California and has long been considered a candidate biocontrol agent for slug management in the U.S. In this study, we report differential gene expressions in nematode-infected slugs using RNA-seq to identify slug immune-related genes against nematodes. Comparison of gene expression levels between the whole bodies of a nematode-infected slug (N-S) and an uninfected control slug (C-S) revealed that there were a total of 39,380 regulated unigenes, of which 3084 (3%) were upregulated and 6761 (6%) were downregulated at greater than 2-fold change (FC > 2) in the nematode-infected slug. To further investigate the biological functions of differentially expressed genes (DEGs), gene ontology (GO) and functional enrichment analysis were performed to map the DEGs to terms in the GO, eukaryotic ortholog groups of proteins (KOG) and Kyoto Encyclopedia of Genes and Genome Pathway (KEGG) databases. Among these DEGs, approximately 228 genes associated with immunity or immune-related pathways were upregulated 2-fold or more in the N-S compared to C-S. These genes include toll, Imd, JNK, scavenger receptors (SCRs), C-type lectins (CTLs), immunoglobulin-like domains, and JAK/STAT63 signaling pathways. From the RNA-seq results, we selected 18 genes and confirmed their expression levels by qRT-PCR. Our findings provide insights into the immune response of slugs during nematode infection. These studies provide fundamental information that will be valuable for the development of new methods of pest slug control using pathogenic nematodes in the field.

A single amino acid polymorphism in natural Metchnikowin alleles of Drosophila results in systemic immunity and life history tradeoffs

Antimicrobial peptides (AMPs) are at the interface of interactions between hosts and microbes and are therefore expected to be rapidly evolving in a coevolutionary arms race with pathogens. In contrast, previous work demonstrated that insect AMPs tend to evolve more slowly than the genome average. Metchikowin (Mtk) is a Drosophila AMP that has a single amino acid residue that segregates as either proline (P) or arginine (R) in populations of four different species, some of which diverged more than 10 million years ago. These results suggest that there is a distinct functional importance to each allele. The most likely hypotheses are driven by two main questions: does each allele have a different efficacy against different specific pathogens (specificity hypothesis)? Or, is one allele a more potent antimicrobial, but with a host fitness cost (autoimmune hypothesis)? To assess their functional differences, we created D. melanogaster lines with the P allele, R allele, or Mtk null mutation using CRISPR/Cas9 genome editing and performed a series of life history and infection assays to assess them. In males, testing of systemic immune responses to a repertoire of bacteria and fungi demonstrated that the R allele performs as well or better than the P and null alleles with most infections. Females show some results that contrast with males, with Mtk alleles either not contributing to survival or with the P allele outperforming the R allele. In addition, measurements of life history traits demonstrate that the R allele is more costly in the absence of infection for both sexes. These results are consistent with both the specificity hypothesis (either allele can perform better against certain pathogens depending on context), and the autoimmune hypothesis (the R allele is generally the more potent antimicrobial in males, and carries a fitness cost). These results provide strong in vivo evidence that differential fitness with or without infection and sex-based functional differences in alleles may be adaptive mechanisms of maintaining immune gene polymorphisms in contrast with expectations of rapid evolution. Therefore, a complex interplay of forces including pathogen species and host sex may lead to balancing selection for immune genotypes. Strikingly, this selection may act on even a single amino acid polymorphism in an AMP.

Destruxin A inhibits the hemocytin-mediated hemolymph immunity of host insects to facilitate Metarhizium infection

Insects have an effective innate immune system to protect themselves against fungal invasion. Metarhizium employs a toxin-based strategy using a nonribosomal peptide called destruxin A (DA) to counteract the host immune response. However, the mechanism by which DA inhibits insect immunity is still unclear. Here, we identified 48 DA-binding proteins in silkworm hemolymph, with the binding affinity (KD) ranging from 2 to 420 μM. Among these proteins, hemocytin, an important immune factor, was determined to be the strongest DA-binding protein. DA binds to hemocytin and regulates its conformation in a multisite manner. Furthermore, DA exerts a significant inhibitory effect on hemocytin-mediated hemocyte aggregation. By disrupting the interaction between hemocytin, actin A3, and gelsolin, DA prevents the transformation of granules into vesicles in hemocytes. These vesicles are responsible for storing, maturing, and exocytosing hemocytin. Therefore, hemocytin secretion is reduced, and the formation of structures that promote aggregation in outer hemocytes is inhibited.

Pathogenicity analysis and comparative genomics reveal the different infection strategies between the generalist Metarhizium anisopliae and the specialist Metarhizium acridum

BACKGROUND

The fungal genera Metarhizium contain many important multiple species that are used as biocontrol agents and as model organisms for exploring insect-fungal interactions. Metarhizium spp. exhibit different traits of pathogenicity, suggesting that the pathogenesis can be quite distinctive. However, the underlying differences in their pathogenesis remain poorly understood.

RESULTS

Pathogenicity analysis showed that Metarhizium anisopliae (strain CQMa421) displayed higher virulence against oriental migratory locusts, Locusta migratoria manilensis (Meyen), than the acridid-specific specie Metarhizium acridum (strain CQMa102). Relative to M. acridum, M. anisopliae possessed a higher conidial hydrophobicity, increased ability to penetrate the host, accelerated growth under hypoxia and enhanced ability for the utilization of different carbon sources. Different distributions of carbohydrate epitopes at cell wall surface of M. anisopliae might also contribute to successful evasion of host immune defenses. Comparative genomics showed that M. anisopliae has 98 more virulence-related secreted proteins (133) than M. acridum (35), which can be functionally classified as hydrolases, virulence effectors, cell wall degradation and stress tolerance-related proteins, and helpful to the cuticle penetration and host internal environment adaption. In addition, differences in genomic clusters specifically related to secondary metabolites, including the clusters of Indole-NRPS hybrid, T1PKS-NRPS like hybrid, Betalactone, Fungal-Ripp and NRPS-Terpene hybrid, may lead to differences in core virulence-related secondary metabolite genes in M. acridum (18) and M. anisopliae (36).

CONCLUSION

The comparative study provided new insights into the different infection strategies between M. anisopliae and M. acridum, and further facilitate the identification of virulence-related genes for the improvement of mycoinsecticides. © 2023 Society of Chemical Industry.

Limited effects of plant-beneficial fungi on plant volatile composition and host-choice behavior of Nesidiocoris tenuis

Biological control using plant-beneficial fungi has gained considerable interest as a sustainable method for pest management, by priming the plant for enhanced defense against pathogens and insect herbivores. However, despite promising outcomes, little is known about how different fungal strains mediate these beneficial effects. In this study, we evaluated whether inoculation of tomato seeds with the plant-beneficial fungi Beauveria bassiana ARSEF 3097, Metarhizium brunneum ARSEF 1095 and Trichoderma harzianum T22 affected the plant's volatile organic compound (VOC) profile and the host-choice behavior of Nesidiocoris tenuis, an emerging pest species in NW-European tomato cultivation, and the related zoophytophagous biocontrol agent Macrolophus pygmaeus. Results indicated that fungal inoculation did not significantly alter the VOC composition of tomato plants. However, in a two-choice cage assay where female insects were given the option to select between control plants and fungus-inoculated plants, N. tenuis preferred control plants over M. brunneum-inoculated plants. Nearly 72% of all N. tenuis individuals tested chose the control treatment. In all other combinations tested, no significant differences were found for none of the insects. We conclude that inoculation of tomato with plant-beneficial fungi had limited effects on plant volatile composition and host-choice behavior of insects. However, the observation that N. tenuis was deterred from the crop when inoculated with M. brunneum and attracted to non-inoculated plants may provide new opportunities for future biocontrol based on a push-pull strategy.

A Drosophila melanogaster model shows that fast growing Metarhizium species are the deadliest despite eliciting a strong immune response

We used Drosophila melanogaster to investigate how differences between Metarhizium species in growth rate and mechanisms of pathogenesis influence the outcome of infection. We found that the most rapid germinators and growers in vitro and on fly cuticle were the fastest killers, suggesting that pre-penetration competence is key to Metarhizium success. Virulent strains also induced the largest immune response, which did not depend on profuse growth within hosts as virulent toxin-producing strains only proliferated post-mortem while slow-killing strains that were specialized to other insects grew profusely pre-mortem. Metarhizium strains have apparently evolved resistance to widely distributed defenses such as the defensin Toll product drosomycin, but they were inhibited by Bomanins only found in Drosophila spp. Disrupting a gene (Dif), that mediates Toll immunity has little impact on the lethality of most Metarhizium strains (an exception being the early diverged M. frigidum and another insect pathogen Beauveria bassiana). However, disrupting the sensor of fungal proteases (Persephone) allowed rapid proliferation of strains within hosts (with the exception of M. album), and flies succumbed rapidly. Persephone also mediates gender differences in immune responses that determine whether male or female flies die sooner. We conclude that some strain differences in growth within hosts depend on immune-mediated interactions but intrinsic differences in pathogenic mechanisms are more important. Thus, Drosophila varies greatly in tolerance to different Metarhizium strains, in part because some of them produce toxins. Our results further develop D. melanogaster as a tractable model system for understanding insect-Metarhizium interactions.

Transcriptomic Analysis Reveals the Impact of the Biopesticide Metarhizium anisopliae on the Immune System of Major Workers in Solenopsis invicta

The red imported fire ant (Solenopsis invicta Buren, 1972) is a globally significant invasive species, causing extensive agricultural, human health, and biodiversity damage amounting to billions of dollars worldwide. The pathogenic fungus Metarhizium anisopliae (Metchnikoff) Sorokin (1883), widely distributed in natural environments, has been used to control S. invicta populations. However, the interaction between M. anisopliae and the immune system of the social insect S. invicta remains poorly understood. In this study, we employed RNA-seq to investigate the effects of M. anisopliae on the immune systems of S. invicta at different time points (0, 6, 24, and 48 h). A total of 1313 differentially expressed genes (DEGs) were identified and classified into 12 expression profiles using short time-series expression miner (STEM) for analysis. Weighted gene co-expression network analysis (WGCNA) was employed to partition all genes into 21 gene modules. Upon analyzing the statistically significant WGCNA model and conducting Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis on the modules, we identified key immune pathways, including the Toll and Imd signaling pathways, lysosomes, autophagy, and phagosomes, which may collectively contribute to S. invicta defense against M. anisopliae infection. Subsequently, we conducted a comprehensive scan of all differentially expressed genes and identified 33 immune-related genes, encompassing various aspects such as recognition, signal transduction, and effector gene expression. Furthermore, by integrating the significant gene modules derived from the WGCNA analysis, we constructed illustrative pathway diagrams depicting the Toll and Imd signaling pathways. Overall, our research findings demonstrated that M. anisopliae suppressed the immune response of S. invicta during the early stages while stimulating its immune response at later stages, making it a potential biopesticide for controlling S. invicta populations. These discoveries lay the foundation for further understanding the immune mechanisms of S. invicta and the molecular mechanisms underlying its response to M. anisopliae.

Effects of destruxin A on hemocytes of the domestic silkworm, Bombyx mori

Introduction

Destruxin A (DA) is a mycotoxin isolated from the entomopathogenic fungus Metarhizium anisopliae which has demonstrated inhibitory activity against various insect species. However, the mechanism of inhibition on target sites in insects remains unknown.

Methods

In this research, the dose-response relationship between DA and morphological changes in body tissues and organs of domestic silkworm, Bombyx mori, were investigated by histopathological methods to identify the target sites that responded to DA.

Results and Discussion

The results showed that responses of individual tissues and organs varied with DA dosage and treatment time. At low doses (i.e., 0.01μg/g), the hemocytes were the most sensitive to DA with morphological changes apparent at 6 h after treatment. However, the muscle cells, fat body, and Malpighian tubules were unaltered. At higher doses (i.e., > 0.1μg/g), morphological changes were observed in muscle cells, fat body, and Malpighian tubules at 24 h after treatment. The results indicated that DA can be an immunosuppressant by damaging host cells like hemocytes, and at higher doses may potentially impact other physiological processes, including muscle function, metabolism, and excretion. The information presented in the current study will facilitate development of mycopesticides and novel immunosuppressants.

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

BACKGROUND

The insect hemolymph (blood-equivalent fluid), composed of a large number of hemocytes (blood cells) and a variety of soluble immune effectors, is hostile for pathogens including fungi. In order to survive in the insect hemocoel (body cavity), the entomopathogenic fungus (EPF) has evolved two classical coping strategies, namely evasion and suppression of the host immune reactions. However, it remains unclear whether EPF has other ways of coping with host immunity.

RESULTS

In this study, we demonstrated that Metarhizium rileyi (an EPF) infection by injection of blastospores into the hemocoel enhanced the plasma antibacterial activity of cotton bollworm (Helicoverpa armigera), which was partially due to the enhanced expression of antimicrobial peptides (AMPs). The early stage of M. rileyi infection induced the translocation of gut bacteria into the hemocoel, where they were subsequently cleared due to the enhanced plasma antibacterial activity. Further, we showed that the enhanced plasma antibacterial activity and AMP expression were attributable to M. rileyi but not the invasive gut bacteria (opportunistic bacteria). Elevated ecdysone (major steroid hormone in insects) levels in the hemolymph at 48 h post-M. rileyi infection might contribute to the enhanced expression of AMPs. The fungus-elicited AMPs, such as cecropin 3 or lebocin, exhibited potent inhibitory activity against the opportunistic bacteria but not against hyphal bodies. In addition, the opportunistic bacteria competed with hyphal bodies for amino acid nutrients.

CONCLUSIONS

M. rileyi infection induced the translocation of gut bacteria, and then the fungi activated and exploited its host humoral antibacterial immunity to eliminate opportunistic bacteria, preventing them from competing for nutrients in the hemolymph. Unlike the classical strategies, EPF utilizes to evade or suppress host immunity, our findings reveal a novel strategy of interaction between EPF and host immunity. Video Abstract.

Temporal Exposure to Bt Insecticide Causes Oxidative Stress in Larval Midgut Tissue

Bacillus thuringiensis (Bt) three-domain Cry toxins are highly successful biological pesticides; however, the mechanism through which they cause death to targeted larval midgut cells is not fully understood. Herein, we challenged transgenic Bt-susceptible Drosophila melanogaster larvae with moderate doses of activated Cry1Ac toxin and assessed the midgut tissues after one, three, and five hours using transmission electron microscopy and transcriptome sequencing. Larvae treated with Cry1Ac showed dramatic changes to their midgut morphology, including shortened microvilli, enlarged vacuoles, thickened peritrophic membranes, and swelling of the basal labyrinth, suggesting water influx. Transcriptome analysis showed that innate immune responses were repressed, genes involved with cell death pathways were largely unchanged, and mitochondria-related genes were strongly upregulated following toxin exposure. Defective mitochondria produced after toxin exposure were likely to contribute to significant levels of oxidative stress, which represent a common physiological response to a range of toxic chemicals. Significant reductions in both mitochondrial aconitase activity and ATP levels in the midgut tissue supported a rapid increase in reactive oxygen species (ROS) following exposure to Cry1Ac. Overall, these findings support the role of water influx, midgut cell swelling, and ROS activity in response to moderate concentrations of Cry1Ac.

Binding proteins of destruxin A from Metarhizium against insect cell

Destruxin A (DA) is a cyclo-hexadepsipeptidic insecticidal mycotoxin isolated from the entomopathogenic fungi, Metarhizium spp. However, its mode of action is unknown. In this study, we isolated 149 candidate DA-binding proteins by drug affinity response target stability, and determined the interactions of 80 canditates with DA in vitro by surface plasmon resonance. The affinity coefficients (K_D) ranged from 24 to 469 μM. Binding proteins were functionally diverse and included cytoskeletal components and cell motility, protein transcription and translation pathways, ubiquitin dependent protein metabolic processes, nucleus pore entry and exit, and endoplasmic reticulum vesicle transport and etc. Electron microscopy revealed that DA damaged the cytoskeleton and multiple organelles, disrupted cell adhesion and motility, and led to cell death. DA appeared to have a multi-targeted approach to cellular structures and multiple life processes, leading to cell death. The results of this study could provide molecular evidence for the analysis of the insecticidal toxicology of DA and further improve the study of the pathogenic insect mechanism of Metarhizium.

Metabolomics of the interaction between a consortium of entomopathogenic fungi and their target insect: Mechanisms of attack and survival

One of the most concerning pests that attack strawberries in Brazil is Duponchelia fovealis (Zeller), a non-native moth with no registered control methods to date. Our group recently observed that a fungal consortium formed by two strains of Beauveria bassiana (Balsamo) increased the mortality of D. fovealis more than inoculation with each strain on its own. However, the molecular interaction between the fungal consortium and the caterpillars is unknown. Thus, in this work, we sought to pioneer the evaluation of the molecular interaction between a fungal consortium of B. bassiana and D. fovealis caterpillars. We aimed to understand the biocontrol process involved in this interaction and the defense system of the caterpillar. Seven days after D. fovealis were inoculated with the consortium, the dead and surviving caterpillars were analyzed using GC-MS and LC-MS. Some of the metabolites identified in dead caterpillars have primarily antioxidant action. Other metabolites may have insecticidal potential, such as diltiazem-like and tamsulosin-like compounds, as well as 2,5-dimethoxymandelic acid. In surviving caterpillars, the main mechanisms are pro-inflammatory from 2-Palmitoylglycerol metabolite and the antifungal action of the metabolite Aegle marmelos Alkaloid-C. The metabolites identified in dead caterpillars may explain the increased mortality caused by the consortium due to its antioxidant mechanism, which can suppress the caterpillars' immune system, and insecticide action. In surviving caterpillars, the main resistance mechanisms may involve the stimulus to the immunity and antifungal action.

A comparison between the effectiveness of the fungi Beauveria bassiana and Metarhizium anisopliae for the control of Argas persicus with the emphasis of histopathological changes in the integument

Argas persicus is an important ectoparasite of domestic fowl that causes heavy economic losses to the poultry industry. The present study was carried out to compare and assess the effects induced by spraying the fungi Beauveria bassiana and Metarhizium anisopliae separately, on the mobility and viability of semifed adult A. persicus, also to follow the histopathological effect induced by a selected concentration of 10¹⁰ conidia/ml of B. bassiana on the integument. Biological studies revealed a more or less similar pattern of response in adults treated with either of the two fungi (Increasing concentration resulting in more death along with examined period). As the estimated LC50 and LC95 of B. bassiana were recorded 5 × 10⁹ and 4.6 × 10¹² conidia/ml, respectively, and for M. anisopliae were 3 × 10¹¹ and 2.7 × 10¹⁶ conidia/ml respectively, the fungus B. bassiana was more efficient than M. anisopliae when applied at the same concentrations. The study revealed that spraying of Beauveria at 10¹² conidia/ml is sufficient to control A. persicus as it recorded 100% efficacy, so it might be selected to be the effective dose. Histological investigation of the integument treated with B. bassiana revealed the dispersal of the hyphal network on the after 11 days of treatment, accompanied by other changes. Results from our study verify the susceptibility of A. persicus to the pathogenic effect induced by spraying B. bassiana, which is sufficient for its control with the recording better results.

A single amino acid polymorphism in natural Metchnikowin alleles of Drosophila results in systemic immunity and life history tradeoffs

Antimicrobial peptides (AMPs) are at the interface of interactions between hosts and microbes and are therefore expected to be fast evolving in a coevolutionary arms race with pathogens. In contrast, previous work demonstrated that one AMP, Metchikowin (Mtk), has a single residue that segregates as either proline (P) or arginine (R) in populations of four different Drosophila species, some of which diverged more than 10 million years ago. The recurrent finding of this polymorphism regardless of geography or host species, coupled with evidence of balancing selection in Drosophila AMPs, suggest there is a distinct functional importance to each allele. The most likely hypotheses involve alleles having specificity to different pathogens or the more potent allele conferring a cost on the host. To assess their functional differences, we created D. melanogaster lines with the P allele, R allele, or Mtk null mutation using CRISPR/Cas9 genome editing. Here, we report results from experiments assessing the two hypotheses using these lines. In males, testing of systemic immune responses to a repertoire of bacteria and fungi demonstrated that the R allele performs as well or better than the P and null alleles with most infections. With some pathogens, however, females show results in contrast with males where Mtk alleles either do not contribute to survival or where the P allele outperforms the R allele. In addition, measurements of life history traits demonstrate that the R allele is more costly in the absence of infection for both sexes. These results provide strong in vivo evidence that differential fitness with or without infection and sex-based functional differences in alleles may be adaptive mechanisms of maintaining immune gene polymorphisms in contrast with expectations of rapid evolution. Therefore, a complex interplay of forces including pathogen species and host sex may lead to balancing selection for immune genotypes. Strikingly, this selection may act on even a single amino acid polymorphism in an AMP.

Beauveria bassiana interacts with gut and hemocytes to manipulate Aedes aegypti immunity

BACKGROUND

Mosquito-borne diseases affect millions of people. Chemical insecticides are currently employed against mosquitoes. However, many cases of insecticide resistance have been reported. Entomopathogenic fungi (EPF) have demonstrated potential as a bioinsecticide. Here, we assessed the invasion of the EPF Beauveria bassiana into Aedes aegypti larvae and changes in the activity of phenoloxidase (PO) as a proxy for the general activation of the insect innate immune system. In addition, other cellular and humoral responses were evaluated.

METHODS

Larvae were exposed to blastospores or conidia of B. bassiana CG 206. After 24 and 48 h, scanning electron microscopy (SEM) was conducted on the larvae. The hemolymph was collected to determine changes in total hemocyte concentration (THC), the dynamics of hemocytes, and to observe hemocyte-fungus interactions. In addition, the larvae were macerated to assess the activity of PO using L-DOPA conversion, and the expression of antimicrobial peptides (AMPs) was measured using quantitative Real-Time PCR.

RESULTS

Propagules invaded mosquitoes through the midgut, and blastopores were detected inside the hemocoel. Both propagules decreased the THC regardless of the time. By 24 h after exposure to conidia the percentage of granulocytes and oenocytoids increased while the prohemocytes decreased. By 48 h, the oenocytoid percentage increased significantly (P < 0.05) in larvae exposed to blastospores; however, the other hemocyte types did not change significantly. Regardless of the time, SEM revealed hemocytes adhering to, and nodulating, blastospores. For the larvae exposed to conidia, these interactions were observed only at 48 h. Irrespective of the propagule, the PO activity increased only at 48 h. At 24 h, cathepsin B was upregulated by infection with conidia, whereas both propagules resulted in a downregulation of cecropin and defensin A. At 48 h, blastospores and conidia increased the expression of defensin A suggesting this may be an essential AMP against EPF.

CONCLUSION

By 24 h, B. bassiana CG 206 occluded the midgut, reduced THC, did not stimulate PO activity, and downregulated AMP expression in larvae, all of which allowed the fungus to impair the larvae to facilitate infection. Our data reports a complex interplay between Ae. aegypti larvae and B. bassiana CG 206 demonstrating how this fungus can infect, affect, and kill Ae. aegypti larvae.

Interaction of Destruxin A with Three Silkworm Proteins: BmCRT, BmDPP3, and BmPDIA5

Destruxin A (DA), a hexa-cyclodepsipeptidic mycotoxin produced by the entomopathogenic fungus Metarhizium anisopliae, has insecticidal activity, but its molecular mechanism of action is still not clear. Three proteins with modification-related functions, calreticulin (BmCRT), dipeptidyl peptidase Ⅲ (BmDPP3), and protein disulfide isomerase A5 (BmPDIA5), were selected to verify the interactions with DA in this study. The kinetic data of the interactions were measured by surface plasmon resonance (SPR) and bio-layer interferometry (BLI) in vitro. The K_D values of DA with BmCRT, BmDPP3, and BmPDIA5 ranged from 10^-4 to 10^-5 mol/L, which suggested that the three proteins all had fairly strong interactions with DA. Then, it was found that DA in a dose-dependent manner affected the interactions of the three proteins with their partners in insect two-hybrid tests in SF-9 cells. Furthermore, the results of enzyme activities by ELISA indicated that DA could inhibit the activity of BmDPP3 but had no significant effect on BmPDIA5. In addition, DA induced the upregulation of BmDPP3 and the downregulation of BmCRT. The results prove that BmCRT, BmDPP3, and BmPDIA5 are all binding proteins of DA. This study might provide new insights to elucidate the molecular mechanism of DA.

Profiling Destruxin Synthesis by Specialist and Generalist Metarhizium Insect Pathogens during Coculture with Plants

Metarhizium is a genus of endophytic, insect-pathogenic fungi that is used as a biological control agent. The dual lifestyles of these fungi combine the parasitism of insect pests with the symbiotic association with plant roots. A major class of secreted metabolites by Metarhizium are cyclic depsipeptides called destruxins (DTXs). As prominent insecticidal compounds, their role during plant interactions is still largely unknown. Here, we examined the metabolomic profile of Metarhizium, with special emphasis on DTX production, using untargeted, liquid chromatography-tandem mass spectrometry (LC-MS/MS). Four Metarhizium species, two insect generalists (M. robertsii and M. brunneum), and two insect specialists (M. flavoviride and M. acridum) were inoculated onto agar plate cultures containing either bean (Phaseolus vulgaris) or corn (Zea mays) and grown for four and seven days. After methanol extraction, feature-based molecular networking (FBMN) was used to obtain DTX identification as defined by the Global Natural Products Social Molecular Networking (GNPS). A total of 25 DTX analogs were identified, with several DTX-like compounds in coculture that could not be identified. Metarhizium species differed in the amount and type of DTXs they produced, with the insect specialists producing far fewer amounts and types of DTXs than the insect generalists. The production of these metabolites varied between cultures of different ages and plant hosts. Conditions that influence the production of DTXs are discussed. As the genetic arsenal of natural products relates to the lifestyle of the organism, uncovering conditions with an ecological context may reveal strategies for producing novel compounds or precursors suitable for synthetic biology. IMPORTANCE The development of an intimate and beneficial association between fungi and plants requires an exchange of a complex mixture of chemical cues. These compounds are a means of communication, promoting or limiting the interaction, but can have numerous other biological and ecological functions. Determining how the metabolome, or a subset thereof, is linked to plant host preference and colonization has implications for future functional studies and may uncover novel therapeutic compounds whose production is elicited only under cocultivation. In this study, we performed an untargeted metabolomic analysis of plate cocultures with individual plant-fungal pairs. The identification of a major group of fungal metabolites, the destruxins, was examined for their role in plant specificity. The diversity of these metabolites and the production of numerous unidentified, structural analogs are evidence of the sensitivity of the methodology and the potential for future mining of this living data set.

Meta-Analysis of Immune Induced Gene Expression Changes in Diverse Drosophila melanogaster Innate Immune Responses

Simple Summary

Organisms can be infected by a wide range of pathogens, including bacteria, viruses, and parasites. Following infection, the host mounts an immune response to attempt to eliminate the pathogen. These responses are often specific to the type of pathogen and mediated by the expression of specialized genes. We have characterized the expression changes induced in host Drosophila fruit flies following infection by multiple types of pathogens, and identified a small number of genes that show expression changes in each infection. This includes genes that are known to be involved in pathogen resistance, and others that have not been previously studied as immune response genes. These findings provide new insight into transcriptional changes that accompany Drosophila immunity. They may suggest possible roles for the differentially expressed genes in innate immune responses to diverse classes of pathogens, and serve to identify candidate genes for further empirical study of these processes.

Abstract

Organisms are commonly infected by a diverse array of pathogens and mount functionally distinct responses to each of these varied immune challenges. Host immune responses are characterized by the induction of gene expression, however, the extent to which expression changes are shared among responses to distinct pathogens is largely unknown. To examine this, we performed meta-analysis of gene expression data collected from Drosophila melanogaster following infection with a wide array of pathogens. We identified 62 genes that are significantly induced by infection. While many of these infection-induced genes encode known immune response factors, we also identified 21 genes that have not been previously associated with host immunity. Examination of the upstream flanking sequences of the infection-induced genes lead to the identification of two conserved enhancer sites. These sites correspond to conserved binding sites for GATA and nuclear factor κB (NFκB) family transcription factors and are associated with higher levels of transcript induction. We further identified 31 genes with predicted functions in metabolism and organismal development that are significantly downregulated following infection by diverse pathogens. Our study identifies conserved gene expression changes in Drosophila melanogaster following infection with varied pathogens, and transcription factor families that may regulate this immune induction.

Destruxin A inhibits scavenger receptor B mediated melanization in Aphis citricola

BACKGROUND

Destruxin A (DA) is a mycotoxin secreted by entomogenous fungi, such as Metarhizium anisopliae, which has broad-spectrum insecticidal activity. Insect innate immunity provides resistance against the invasion of entomopathogenic fungi. Previous studies have shown that DA could inhibit the immune response, however, the suppressive mechanism of DA on prophenoloxidase system is still unknown.

RESULTS

Based on the transcriptome of Aphis citricola, we screened the scavenger receptor class B(AcSR-B)and identified that it significantly responds to DA. Spatio-temporal expression analysis showed that AcSR-B is highly expressed in adult stage and is mainly distributed in the abdominal region. We further revealed that both M. anisopliae and Escherichia coli could suppress the expression of AcSR-B at 24 h, and that the expressed recombinant protein rAcSR-B possessed agglutination activity to M. anisopliae and E. coli. DA could suppress the protein expression of AcSR-B. In addition, RNA interference of AcSR-B caused death of A. citricola in a dose-dependent manner, and RNA interference of AcSR-B increased mortality in A. citricola under the same lethal concentration of DA. The inhibiting effect of AcSR-B silencing was similar with the DA treatment upon phenol oxidase (PO) activity of A. citricola hemolymph. DA could not decrease PO activity further after AcSR-B silencing.

CONCLUSION

Destruxin A inhibits melanization by suppressing AcSR-B in A. citricola. Our findings are helpful in understanding the underlying molecular mechanism of the DA suppressing immune system, and uncover a potential molecular target for double-stranded RNA (dsRNA) insecticides.

Integrative Alternative Tactics for Ixodid Control

Simple Summary

Hard ticks are important for economic and health reasons, and control has mainly relied upon use of synthetic acaricides. Contemporary development of resistance and concerns relating to health and environmental safety have elicited exploration into alternative tactics for hard tick management. Some examples of alternative tactics involve biological control, desiccant dusts, growth regulators, vaccines, cultural methods, and ingested medications.

Abstract

Ixodids (hard ticks), ectoparasitic arthropods that vector the causal agents of many serious diseases of humans, domestic animals, and wildlife, have become increasingly difficult to control because of the development of resistance against commonly applied synthetic chemical-based acaricides. Resistance has prompted searches for alternative, nonconventional control tactics that can be used as part of integrated ixodid management strategies and for mitigating resistance to conventional acaricides. The quest for alternative control tactics has involved research on various techniques, each influenced by many factors, that have achieved different degrees of success. Alternative approaches include cultural practices, ingested and injected medications, biological control, animal- and plant-based substances, growth regulators, and inert desiccant dusts. Research on biological control of ixodids has mainly focused on predators, parasitoid wasps, infective nematodes, and pathogenic bacteria and fungi. Studies on animal-based substances have been relatively limited, but research on botanicals has been extensive, including whole plant, extract, and essential oil effects on ixodid mortality, behavior, and reproduction. The inert dusts kaolin, silica gel, perlite, and diatomaceous earth are lethal to ixodids, and they are impervious to environmental degradation, unlike chemical-based toxins, remaining effective until physically removed.

Carbohydrates Metabolic Signatures in Immune Cells: Response to Infection

INTRODUCTION

Metabolic reprogramming in immune cells is diverse and distinctive in terms of complexity and flexibility in response to heterogeneous pathogenic stimuli. We studied the carbohydrate metabolic changes in immune cells in different types of infectious diseases. This could help build reasonable strategies when understanding the diagnostics, prognostics, and biological relevance of immune cells under alternative metabolic burdens.

METHODS

Search and analysis were conducted on published peer-reviewed papers on immune cell metabolism of a single pathogen infection from the four known types (bacteria, fungi, parasites, and viruses). Out of the 131 selected papers based on the PIC algorithm (pathogen type/immune cell/carbohydrate metabolism), 30 explored immune cell metabolic changes in well-studied bacterial infections, 17 were on fungal infections of known medical importance, and 12 and 57 were on parasitic and viral infections, respectively.

RESULTS AND DISCUSSION

While carbohydrate metabolism in immune cells is signaled by glycolytic shift during a bacterial or viral infection, it is widely evident that effector surface proteins are expressed on the surface of parasites and fungi to modulate metabolism in these cells.

CONCLUSIONS

Carbohydrate metabolism in immune cells can be categorized according to the pathogen or the disease type. Accordingly, this classification can be used to adopt new strategies in disease diagnosis and treatment.

Microbial Response to Fungal Infection in a Fungus-Growing Termite, Odontotermes formosanus (Shiraki)

The crosstalk between gut microbiota and host immunity has emerged as one of the research foci of microbiome studies in recent years. The purpose of this study was to determine how gut microbes respond to fungal infection in termites, given their reliance on gut symbionts for food intake as well as maintaining host health. Here, we used Metarhizium robertsii, an entomopathogenic fungus, to infect Odontotermes formosanus, a fungus-growing termite in the family Termitidae, and documented changes in host gut microbiota via a combination of bacterial 16S rDNA sequencing, metagenomic shotgun sequencing, and transmission electron microscopy. Our analyses found that when challenged with Metarhizium, the termite gut showed reduced microbial diversity within the first 12 h of fungal infection and then recovered and even surpassed pre-infection flora levels. These combined results shed light on the role of gut flora in maintaining homeostasis and immune homeostasis in the host, and the impact of gut flora dysbiosis on host susceptibility to infection.

Acaricidal and pathogenic effects of the entomopathogenic fungus Beauveria bassiana on engorged females of the fowl tick, Argas persicus (Argasidae)

This study examined the acaricidal, histopathological and genotoxic effects of the entomopathogenic fungus Beauveria bassiana on engorged females of the fowl tick, Argas persicus. The acaricidal effect of B. bassiana (at 10⁶-10¹⁰ conidia/mL) was concentration- and time-dependent. At 10⁶ conidia/mL, the accumulated mortality of treated females ranged from 3.33 ± 3.3 to 13.33 ± 3.3% over 21 days. However, the mortality was remarkably increased after the application of 10¹⁰ conidia/mL, ranging from 6.67 ± 3.3 to 80 ± 5.8%. Females treated with various conidial concentrations exhibited complete inhibition of oviposition. Light and electron microscopic examination of the ovary of engorged female ticks at 3, 6 and 9 days after treatment with 10⁷ conidia/mL revealed that the oocytes exhibited drastic changes that affected their growth and development. The overall damage observed in the ovary included loss of grape-like appearance, distortion of oocytes, cytoplasmic vacuolation, degeneration of organelles and myelin figure formation. Previtellogenic oocyte progression was completely inhibited as the vitellogenic phase was not observed. Random amplified polymorphic DNA (RAPD-PCR) and inter simple sequence repeat (ISSR) methods were used to assess the genotoxic effects of B. bassiana at 10⁷ conidia/mL on engorged female A. persicus over 3 weeks. The lowest percentage of genomic template stability was recorded in paralyzed ticks after 3 weeks. The study demonstrated the efficacy of B. bassiana as a biocontrol agent against A. persicus as it interfered with its reproduction, movement and viability and disrupted its normal tissue and DNA integrity.

Fungal social influencers: secondary metabolites as a platform for shaping the plant-associated community

Fungal secondary metabolites (FSMs) are capable of manipulating plant community dynamics by inhibiting or facilitating the establishment of co-habitating organisms. Although production of FSMs is not crucial for survival of the producer, their absence can indirectly impair growth and/or niche competition of these fungi on the plant. The presence of FSMs with no obvious consequence on the fitness of the producer leaves questions regarding ecological impact. This review investigates how fungi employ FSMs as a platform to mediate fungal-fungal, fungal-bacterial and fungal-animal interactions associated with the plant community. We discuss how the biological function of FSMs may indirectly benefit the producer by altering the dynamics of surrounding organisms. We introduce several instances where FSMs influence antagonistic- or alliance-driven interactions. Part of our aim is to decipher the meaning of the FSM 'language' as it is widely noted to impact the surrounding community. Here, we highlight the contribution of FSMs to plant-associated interaction networks that affect the host either broadly or in ways that may have previously been unclear.

Do Mast Cells Contribute to the Antifungal Host Defense?

The fungal kingdom includes a group of microorganisms that are widely distributed in the environment, and therefore the exposure to them is almost constant. Furthermore, fungal components of the microbiome, i.e., mycobiome, could serve as a reservoir of potentially opportunistic pathogens. Despite close encounters with fungi, defense mechanisms that develop during fungal infections remain unexplored. The strategic location of mast cells (MCs) close to the external environment places them among the first cells to encounter pathogens along with the other innate immune cells. MCs are directly involved in the host defense through the ability to destroy pathogens or indirectly by activating other immune cells. Most available data present MCs’ involvement in antibacterial, antiviral, or antiparasitic defense mechanisms. However, less is known about their contribution in defense mechanisms against fungi. MCs may support immune responses to fungi or their specific molecules through initiated degranulation, synthesis and release of cytokines, chemokines, mediators, and generation of reactive oxygen species (ROS), as well as immune cells’ recruitment, phagocytosis, or provision of extracellular DNA traps. This review summarizes current knowledge on host defense mechanisms against fungi and MCs’ involvement in those processes. It also describes the effects of fungi or fungus-derived constituents on MCs’ activity.

Insect-fungal-interactions: A detailed review on entomopathogenic fungi pathogenicity to combat insect pests

Global food security is threatened by insect pests of economically important crops. Chemical pesticides have been used frequently for the last few decades to manage insect pests throughout the world. However, these chemicals are hazardous for human health as well as the ecosystem. In addition, several pests have evolved resistance to many chemicals. Finding environment friendly alternatives lead the researchers to introduce biocontrol agents such as entomopathogenic fungi (EPF). These fungi include various genera that can infect and kill insects efficiently. Moreover, EPFs have considerable host specificity with a mild effect on non-target organisms and can be produced in bulk quantity quickly. However, insights into the biology of EPF and mechanism of action are of prime significance for their efficient utilization as a biocontrol agent. This review focuses on EPF-mediated insect management by explaining particular EPF strains and their general mode of action. We have comprehensively discussed which criteria should be used for the selection of pertinent EPF, and which aspects can impact the EPF efficiency. Finally, we have outlined various advantages of EPF and their limitations. The article summarizes the prospects related to EPF utilization as biocontrol agents. We hope that future strategies for the management of insects will be safer for our planet.

Destruxin A Interacts with Aminoacyl tRNA Synthases in Bombyx mori

Destruxin A (DA), a hexa-cyclodepsipeptidic mycotoxin produced by the entomopathogenic fungus Metarhizium anisopliae, exhibits insecticidal activities in a wide range of pests and is known as an innate immunity inhibitor. However, its mechanism of action requires further investigation. In this research, the interactions of DA with the six aminoacyl tRNA synthetases (ARSs) of Bombyx mori, BmAlaRS, BmCysRS, BmMetRS, BmValRS, BmIleRS, and BmGluProRS, were analyzed. The six ARSs were expressed and purified. The BLI (biolayer interferometry) results indicated that DA binds these ARSs with the affinity indices (K_D) of 10⁻⁴ to 10⁻⁵ M. The molecular docking suggested a similar interaction mode of DA with ARSs, whereby DA settled into a pocket through hydrogen bonds with Asn, Arg, His, Lys, and Tyr of ARSs. Furthermore, DA treatments decreased the contents of soluble protein and free amino acids in Bm12 cells, which suggested that DA impedes protein synthesis. Lastly, the ARSs in Bm12 cells were all downregulated by DA stress. This study sheds light on exploring and answering the molecular target of DA against target insects.

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.

The Metarhizium anisopliae Toxin, Destruxin A, Interacts with the SEC23A and TEME214 Proteins of Bombyx mori

Destruxin A (DA), a mycotoxin isolated from the entomopathogenic fungus Metarhizium anisopliae, has good insecticidal and immune-inhibitory activity, but the action mechanism has not yet been elucidated. In order to identify the DA-binding proteins, we conducted drug affinity responsive target stability (DARTS) experiments, which indicated that the silkworm’s (Bombyx mori) transmembrane protein 214 (BmTEME214) and protein transport protein SEC23A isoform X2 (BmSEC23) are the potential DA-binding proteins. The current research was focused on validation of the interaction between DA and these two proteins via bio-layer interferometry (BLI) in vitro, insect two-hybrid (I2H) in Sf9 cells, and RNAi in the insect. The results of the BLI tests showed that DA has strong affinity to bind BmTEME214 and BmSEC23 proteins with a K_D value of 0.286 and 0.291 µM, respectively. In the I2H experiments, DA inhibited (at 0.02 µg/mL) and activated (at 0.002–0.0002 µg/mL) the protein interactions of BmSEC23–BmSEC13, but it only inhibited the BmTMEM214–BmSEC13L interaction. Furthermore, in the RNAi tests, an apparent increase in the silkworm’s mortality was recorded in the joint treatment of DA with dsBmSEC23 or dsBmTMEM214 when compared with the single treatment of DA (1.5 µg/g body), 40 µg/g body dsBmSEC23, or dsBmTMEM214. This research confirmed that BmSEC23 and BmTMEM214 are the DA-binding proteins and provided new insights to understand the action mechanism of DA.

Expression of a phenoloxidase cascade inhibitor enhances the virulence of the fungus Beauveria bassiana against the insect Helicoverpa armigera

Entomopathogenic fungi have high potential for controlling insect pests, although the slow killing speed has blocked their widespread application. To increase the virulence of entomopathogenic fungi, genetic modification can be employed. Egf1.0 is an immunosuppressive protein encoded by polydnavirus, carried by parasitoid wasp Microplitis demolitor, which blocks the prophenoloxidase (PPO) activation response of host insects. In this study, we explored the feasibility of genetically modifying entomopathogenic fungi with increased virulence by expressing Egf1.0. In comparison with the wild-type parents, the median lethal concentration (LC₅₀) of Beauveria bassiana expressing Egf1.0 against Helicoverpa armigera was reduced by 2.7-fold, and the median lethal time (LT₅₀) was reduced by 22.8%. In vitro assay showed that recombinant Egf1.0 was able to inhibit the PPO activation response of H. armigera. In vivo assay revealed that the expression of Egf1.0 in B. bassiana caused a higher degree of suppression to PPO activation response of H. armigera. These assays suggested that the increased virulence of the transgenic fungi is due to the increased ability to suppress the host insect's immune response. Moreover, colony growth, conidia yield, and germination assays revealed that the expression of Egf1.0 in B. bassiana had no effect on its growth and development. In conclusion, the expression of Egf1.0 can significantly enhance the pathogenicity of B. bassiana against host insects.

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.

Antifungal immune responses in mosquitoes (Diptera: Culicidae): A review

Mosquitoes transmit many parasites and pathogens to humans that cause significant morbidity and mortality. As such, we are constantly looking for new methods to reduce mosquito populations, including the use of effective biological controls. Entomopathogenic fungi are excellent candidate biocontrol agents to control mosquitoes. Understanding the complex ecological, environmental, and molecular interactions between hosts and pathogens are essential to create novel, effective and safe biocontrol agents. Understanding how mosquitoes recognize and eliminate pathogens such as entomopathogenic fungi may allow us to create insect-order specific biocontrol agents to reduce pest populations. Here we summarize the current knowledge of fungal infection, colonization, development, and replication within mosquitoes and the innate immune responses of the mosquitoes towards the fungal pathogens, emphasizing those features required for an effective mosquito biocontrol agent.

Synergistic Effect of Beauveria bassiana and Trichoderma asperellum to Induce Maize (Zea mays L.) Defense against the Asian Corn Borer, Ostrinia furnacalis (Lepidoptera, Crambidae) and Larval Immune Response

Ostrinia furnacalis, is the major pest of maize causing significant yield losses. So far, many approaches have been used to increase the virulence of entomopathogenic fungal isolates. The current study is an attempt to estimate synergistic effect of Beauveria bassiana and Trichoderma asperellum in order to explore larval immune response through RNA sequencing and differentially expression analysis. In vivo synergism was examined in seven proportions (B. bassiana: T. asperellum = 1:1, 1:2, 1:3, 1:4, 4:1, 3:1, 2:1) and in the in vitro case, two inoculation methods were applied: seed coating and soil drenching. Results revealed significant decrease in plant damage and high larval mortality in fungal treatments. Fungal isolates mediated the plant defense by increasing proline, superoxide dismutase (SOD), peroxidase (POD), polyphenol oxidase (PPO) and protease activities. Seed coating method was proved to be the most effective in case of maize endophytic colonization. In total, 59 immune-related differentially expressed genes DEGs were identified including, cytochrome P450, heat shock protein, ABC transporter, cadherin, peptidoglycan recognition protein (PGRP), cuticlular protein, etc. Further, transcriptomic response was confirmed by qRT-PCR. Our results concluded that, coculture of B. bassiana and T. asperellum has the synergistic potential to suppress the immune response of O. furnacalis and can be used as sustainable approach to induce plant resistance through activation of defense-related enzymes.

Characterization of mycotoxins from entomopathogenic fungi (Cordyceps fumosorosea) and their toxic effects to the development of asian citrus psyllid reared on healthy and diseased citrus plants

Entomopathogenic fungi (EPF) produce multiple mycotoxins, which play an essential role in improving fungal pathogenesis and virulence. To characterize various mycotoxins from the crude methanol extract of Cordyceps fumosorosea, a major EPF against various insect pests, we performed ultra-performance liquid chromatography coupled to quadrupole time of flight mass spectrometer (UPLC-QTOF MS) technique, and all compounds were identified through molecular mass and formulae. Bassianolide was assessed against the nymphs and adults of Diaphorina citri reared on healthy and Huánglóngbìng (HLB)-diseased Citrus spp. Plants under laboratory conditions. Overall, 17 compounds were identified from the fungal extract and categorized into three groups, i.e. (1) alkaloids (Isariotins A-C), (2) peptides (Bassianolide, Beauverolides, Beauvericin A, Isaridins and Destruxin E) and (3) polyketide (Tenuipyrone). The detected beauverolides (B, C, F, I, Ja) from C. fumosorosea were novel mycotoxins, and their detection intensity was the highest in the fungal extract. Furthermore, bassianolide caused more than 70% and 80% mortality of D. citri nymphs and adults after two days of application, respectively. After three days of chemical application, all nymphal and adult populations of D. citri were killed by bassianolide. However, the mortality rates of both populations, nymphs and adults, were higher on HLB-diseased plants as compared to healthy plants.

Cordycepin, a metabolite of Cordyceps militaris, reduces immune-related gene expression in insects

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High doses of cordycepin are lethal to G. mellonella.

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Cordycepin interacts with EPF to increase the rate of G. mellonella mortality.

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Cordycepin reduces immune-related gene expression in G. mellonella and S2r+ cells.

Hypocrealean entomopathogenic fungi (EPF) (Sordariomycetes, Ascomycota) are natural regulators of insect populations in terrestrial environments. Their obligately-killing life-cycle means that there is likely to be strong selection pressure for traits that allow them to evade the effects of the host immune system. In this study, we quantified the effects of cordycepin (3′-deoxyadenosine), a secondary metabolite produced by Cordyceps militaris (Hypocreales, Cordycipitaceae), on insect susceptibility to EPF infection and on insect immune gene expression. Application of the immune stimulant curdlan (20 µg ml⁻¹, linear beta-1,3-glucan, a constituent of fungal cell walls) to Drosophila melanogaster S2r+ cells resulted in a significant increase in the expression of the immune effector gene metchnikowin compared to a DMSO-only control, but there was no significant increase when curdlan was co-applied with 25 µg ml⁻¹ cordycepin dissolved in DMSO. Injection of cordycepin into larvae of Galleria mellonella (Lepidoptera: Pyralidae) resulted in dose-dependent mortality (LC₅₀ of cordycepin = 2.1 mg per insect 6 days after treatment). Incubating conidia of C. militaris and Beauveria bassiana (Hypocreales, Cordycipitaceae; an EPF that does not synthesize cordycepin) with 3.0 mg ml⁻¹ cordycepin had no effect on the numbers of conidia germinating in vitro. Co-injection of G. mellonella with a low concentration of cordycepin (3.0 mg ml⁻¹) plus 10 or 100 conidia per insect of C. militaris or B. bassiana caused a significant decrease in insect median survival time compared to injection with the EPF on their own. Analysis of predicted vs. observed mortalities indicated a synergistic interaction between cordycepin and the EPF. The injection of C. militaris and B. bassiana into G. mellonella resulted in increased expression of the insect immune effector genes lysozyme, IMPI and gallerimycin at 72 h post injection, but this did not occur when the EPF were co-injected with 3.0 mg ml⁻¹ cordycepin. In addition, we observed increased expression of IMPI and lysozyme at 48 h after injection with C. militaris, B. bassiana and sham injection (indicating a wounding response), but this was also prevented by application of cordycepin. These results suggest that cordycepin has potential to act as a suppressor of the immune response during fungal infection of insect hosts.

A Drosophila model of oral peptide therapeutics for adult intestinal stem cell tumors

Peptide therapeutics, unlike small-molecule drugs, display crucial advantages of target specificity and the ability to block large interacting interfaces, such as those of transcription factors. The transcription co-factor of the Hippo pathway, YAP/Yorkie (Yki), has been implicated in many cancers, and is dependent on its interaction with the DNA-binding TEAD/Sd proteins via a large Ω-loop. In addition, the mammalian vestigial-like (VGLL) proteins, specifically their TONDU domain, competitively inhibit YAP-TEAD interaction, resulting in arrest of tumor growth. Here, we show that overexpression of the TONDU peptide or its oral uptake leads to suppression of Yki-driven intestinal stem cell tumors in the adult Drosophila midgut. In addition, comparative proteomic analyses of peptide-treated and untreated tumors, together with chromatin immunoprecipitation analysis, reveal that integrin pathway members are part of the Yki-oncogenic network. Collectively, our findings establish Drosophila as a reliable in vivo platform to screen for cancer oral therapeutic peptides and reveal a tumor suppressive role for integrins in Yki-driven tumors.This article has an associated First Person interview with the first author of the paper.

Ultrastructural changes induced by the entomopathogenic fungus Beauveria bassiana in the ovary of the tick Argas (Persicargas) persicus (Oken)

The present study was carried out to assess the effects on the ovary of fed female Argas persicus following spraying of the ticks with spores of the fungus Beauveria bassiana suspended in triton X100 at a concentration of 10⁷ conidia/mL. Scanning and transmission electron microscopy observations provided evidence that B. bassiana invaded the ovary, causing extensive morphological damage and deterioration of the developing oocytes. Destruction of the shape and internal organelles of young and previtellogenic oocytes and complete inhibition of vitellogenesis was evident. This histopathological study is the first demonstration of ultrastructural damage in the ovaries of A. persicus after infection with B. bassiana. The data presented confirm that B. bassiana affects the ovary either directly by entering the oocytes and/or indirectly by producing toxins in the haemolymph that interfere with the development of oocytes, thus potentially contributing to the control of this tick in a way that is safe for its host and the environment.

Beauveria bassiana ribotoxin inhibits insect immunity responses to facilitate infection via host translational blockage

Entomopathogenic fungi are promising bio-pesticides. To facilitate infection, fungi recruit multiple virulence factors and deploy different molecular strategies to evade host immunity. Fungal ribotoxins are extracellular secreted ribonucleases (RNases) with ribotoxic cytotoxicity and insecticidal activity. However, it remains unclear whether they have further biological functions. Here we show that the entomopathogenic fungus Beauveria bassiana ribotoxin (Rib) contributes to fungal virulence by inhibiting insect host immunity. Gene deletion of Rib (ΔRib) resulted in attenuated fungal virulence during infection. Pathogenesis analysis demonstrated that Rib mainly inhibits insect immunity through modulating the reactive oxygen species (ROS) response, suppressing antimicrobial peptides (AMPs) production and retarding hyphae penetration from insect cuticles. To further confirm this immunosuppressive function, recombinant ribotoxin (rRib) protein was purified and co-injected with living or heat-killed bacteria, bacteria-derived peptidoglycan (PGN) and lipopolysaccharide (LPS) separately, which also significantly inhibited the AMPs production in Drosophila fat bodies. Furthermore, co-injection of rRib with Escherichia coli or Staphylococcus aureus significantly enhanced bacterial pathogenicity and facilitated infection. In addition, rRib injection resulted in a global inhibition of protein expression in different tissues of Drosophila adults. This work identified B. bassiana ribotoxin as a key virulence factor that inhibits insect immunity.

Tryptamine accumulation caused by deletion of MrMao-1 in Metarhizium genome significantly enhances insecticidal virulence

Metarhizium is a group of insect-pathogenic fungi that can produce insecticidal metabolites, such as destruxins. Interestingly, the acridid-specific fungus Metarhizium acridum (MAC) can kill locusts faster than the generalist fungus Metarhizium robertsii (MAA) even without destruxin. However, the underlying mechanisms of different pathogenesis between host-generalist and host-specialist fungi remain unknown. This study compared transcriptomes and metabolite profiles to analyze the difference in responsiveness of locusts to MAA and MAC infections. Results confirmed that the detoxification and tryptamine catabolic pathways were significantly enriched in locusts after MAC infection compared with MAA infection and that high levels of tryptamine could kill locusts. Furthermore, tryptamine was found to be capable of activating the aryl hydrocarbon receptor of locusts (LmAhR) to produce damaging effects by inducing reactive oxygen species production and immune suppression. Therefore, reducing LmAhR expression by RNAi or inhibitor (SR1) attenuates the lethal effects of tryptamine on locusts. In addition, MAA, not MAC, possessed the monoamine oxidase (Mao) genes in tryptamine catabolism. Hence, deleting MrMao-1 could increase the virulence of generalist MAA on locusts and other insects. Therefore, our study provides a rather feasible way to design novel mycoinsecticides by deleting a gene instead of introducing any exogenous gene or domain.

The Aspergilli and Their Mycotoxins: Metabolic Interactions With Plants and the Soil Biota

Species of the highly diverse fungal genus Aspergillus are well-known agricultural pests, and, most importantly, producers of various mycotoxins threatening food safety worldwide. Mycotoxins are studied predominantly from the perspectives of human and livestock health. Meanwhile, their roles are far less known in nature. However, to understand the factors behind mycotoxin production, the roles of the toxins of Aspergilli must be understood from a complex ecological perspective, taking mold-plant, mold-microbe, and mold-animal interactions into account. The Aspergilli may switch between saprophytic and pathogenic lifestyles, and the production of secondary metabolites, such as mycotoxins, may vary according to these fungal ways of life. Recent studies highlighted the complex ecological network of soil microbiotas determining the niches that Aspergilli can fill in. Interactions with the soil microbiota and soil macro-organisms determine the role of secondary metabolite production to a great extent. While, upon infection of plants, metabolic communication including fungal secondary metabolites like aflatoxins, gliotoxin, patulin, cyclopiazonic acid, and ochratoxin, influences the fate of both the invader and the host. In this review, the role of mycotoxin producing Aspergillus species and their interactions in the ecosystem are discussed. We intend to highlight the complexity of the roles of the main toxic secondary metabolites as well as their fate in natural environments and agriculture, a field that still has important knowledge gaps.

Insecticidal Gene Silencing by RNAi in the Neotropical Region

Insecticidal gene silencing by RNA interference (RNAi) involves a post-transcriptional mechanism with great potential for insect control. Here, we aim to summarize the progress on RNAi research toward control of insect pests in the Neotropical region and discuss factors determining its efficacy and prospects for pest management. We include an overview of the available RNAi information for Neotropical pests in the Lepidoptera, Coleoptera, Diptera, and Hemiptera orders. Emphasis is put on significant findings in the use of RNAi against relevant Neotropical pests, including diamondback moth (Plutella xylostella L.), Asian citrus psyllid (Diaphorina citri Kuwayama), and the cotton boll weevil (Anthonomus grandis Boheman). We also examine the main factors involved in insecticidal RNAi efficiency and major advances to improve screening of lethal genes, formulation, and delivery. Few studies detail resistance mechanisms to RNAi, demonstrating a need for more research. Advances in formulation, delivery, and resistance management tools for insecticidal RNAi in the Neotropics can provide a basis for efficient field application.