Genetically Engineering Entomopathogenic Fungi

Genetically Engineered Microorganisms and Their Impact on Human Health

The emergence of antibiotic-resistant strains, the decreased effectiveness of conventional therapies, and the side effects have led researchers to seek a safer, more cost-effective, patient-friendly, and effective method that does not develop antibiotic resistance. With progress in synthetic biology and genetic engineering, genetically engineered microorganisms effective in treatment, prophylaxis, drug delivery, and diagnosis have been developed. The present study reviews the types of genetically engineered bacteria and phages, their impacts on diseases, cancer, and metabolic and inflammatory disorders, the biosynthesis of these modified strains, the route of administration, and their effects on the environment. We conclude that genetically engineered microorganisms can be considered promising candidates for adjunctive treatment of diseases and cancers.

Thielaviopsis paradoxa and cultivable mycobiota associated with carapace of Rhynchophorus palmarum L. (Coleoptera: Curculionidae) in the state of Alagoas, Brazil

Rhynchophorus palmarum (Coleoptera: Curculionidae) is a significant agricultural pest in palm plantations across tropical America, playing a critical role as a vector of the fungus Thielaviopsis paradoxa, which is the causative agent of stem bleeding disease in coconut palms. This disease has raised concerns due to its rapid spread and subsequent reduction in coconut production in northeastern Brazil. Additionally, this insect can establish mutualistic interactions with various fungi, including saprophytic, phytopathogenic, and entomopathogenic fungi, underscoring the importance of identifying its external mycobiota. The aim of this study was to assess the presence of T. paradoxa in the digestive tract and identify the cultivable mycobiota associated with the carapace of R. palmarum. To achieve this, a mycological study was conducted by culturing the external surface and digestive tract of field-caught adult insects (10 males and 10 females) on potato dextrose agar (PDA) in Maceió, Alagoas, Brazil. Fungal identification was performed by correlating microscopic features with the macroscopic characteristics of the obtained colonies. The results showed that T. paradoxa was detected in 15.0% of carapace isolates but was not found in the insects' intestinal tract. Additionally, nine fungal genera frequently associated with saprophytic or phytopathogenic behaviors were identified on the carapace. Eight of these genera belong to the Ascomycota phylum, while one is classified in the Basidiomycota phylum. The ubiquitous presence of Paecilomyces spp. and the occurrence of Trichosporon spp. in 95% of the assessed insects stand out. Furthermore, other potentially phytopathogenic fungi such as Penicillium spp., Fusarium spp., and Aspergillus spp., as well as fungi with entomopathogenic potential like Paecilomyces spp., Trichoderma spp., Metarhizium spp., and Beauveria bassiana, were detected. These findings enhance the understanding of the complex interactions between R. palmarum and its fungal hosts, providing insights for integrated pest management strategies.

Histopathological observations and comparative transcriptome analysis of Ophiocordyceps sinensis infection of Hepialus xiaojinensis in the early stage

Hepialus xiaojinensis is a Lepidopteran insect and one of the hosts for the artificial cultivation of Cordyceps. Ophiocordyceps sinensis can infect and coexist with H. xiaojinensis larvae for a long time. Little studies focused on the interaction process through its early infection stage. In this research, we particularly study the interaction of infected and uninfected larvae in the 3rd (OS-3, CK-3) and 4th (OS-4, CK-4) instars. O. sinensis was distributed within the larvae and accompanied by pathological changes in some tissue structures. In response to O. sinensis infection, OS-3 enhanced the antioxidant defense ability, while OS-4 decreased. The transcriptome analysis showed that OS-3 resisted the invasion of O. sinensis by the immune and nervous systems. Correspondingly, OS-4 reduced immune response and utilized more energy for growth and development. This study provides a comprehensive resource for analyzing the mechanism of H. xiaojinensis and O. sinensis interaction.

The transcriptome of the entomopathogenic fungus Culicinomyces clavisporus contains an ortholog of the insecticidal ribotoxin Hirsutellin

The entomopathogenic fungus Culicinomyces clavisporus is known to infect and kill mosquito larvae and therefore has been seen as a potential biological control agent against disease vector mosquitoes. Whereas most fungal entomopathogens infect hosts by penetrating the external cuticle, C. clavisporus initiates infection through ingestion (per os). This unique infection strategy suggests that the C. clavisporus genome may be mined for novel pathogenicity factors with potential for vector control. To this end, an Isoseq-based transcriptome analysis was initiated, and resulted in a total of 3,512,145 sequences, with an average length of 1,732 bp. Transcripts assembly and annotation suggested that the C. clavisporus transcriptome lacked the cuticle-degrading proteins that have been associated with other entomopathogenic fungi, supporting the per os pathogenicity process. Furthermore, mining of the sequence data unexpectedly revealed C. clavisporus transcripts homologous to the Hirsutellin toxin. Comparative sequence analyses indicated that the C. clavisporus Hirsutellin predicted protein has retained the canonical molecular features that have been associated with the ribotoxic and insecticidal properties of the original toxin isolated from Hirsutella thompsonii. The identification of an Hirsutellin ortholog in C. clavisporus was supported by phylogenetic analyses demonstrating that Culicinomyces and Hirsutella were closely related genera in the Ophiocordycipitaceae family. Validation of the mosquitocidal activity of this novel C. clavisporus protein has yet to be performed but may help position Hirsutellin orthologs as prime candidates for the development of alternative biocontrol approaches complementing the current toolbox of vector mosquito management strategies.

Debilitation of Galleria mellonella hemocytes using CytCo a cytolytic-like protein derived from the entomopathogen Conidiobolus obscurus

Cytolytic (Cyt)-like genes are present in both pathogenic bacteria and fungi. Bacterial Cyt proteins can destroy insect midgut epithelial cells after ingestion by hosts and some of them have been developed as biopesticides; however, few studies have investigated their functions in fungal pathogens. This study investigated the effects of a Cyt-like protein (CytCo) derived from Conidiobolus obscurus (Entomophthoromycotina) on the hemocytes of the greater wax moth Galleria mellonella larvae. The results showed a significant decline in hemocyte viability after treatment with CytCo in vivo or in vitro. The hemocyte density in the hemolymph was reduced by 65.2% and 50.2% after 12 h in vivo and 6 h in vitro treatments, respectively. Apoptosis/necrosis tests using fluorescence microscopy demonstrated that CytCo-treated hemocytes displayed apoptosis, and many of them also showed necrosis after 6 h in vitro treatment. Based on transcriptome analysis, several genes involved in the programmed cell death signaling pathway were upregulated in the CytCo-treated hemocytes. Meanwhile, the differentially expressed genes related to energy production, signal transduction, transcription regulation, and melanization were upregulated, demonstrating activated immune responses; those putatively related to hemocyte adhesion were downregulated, possibly in response to the reduction of hemocytes in hemolymph. In conclusion, CytCo as a virulence factor, could irreversibly incapacitate host hemocytes, playing an important role in debilitating insect immunity. This novel insecticidal protein holds a potential to develop biopesticide for controlling agroforestry pests.

Enhanced Cd efflux capacity and physiological stress resistance: The beneficial modulations of Metarhizium robertsii on plants under cadmium stress

Due to the high migration capacity in agricultural soil-crop systems, cadmium (Cd) is accumulated in various crops and severely inhibits plant growth. In this study, we showed that, under Cd stress, the plant-symbiotic fungus Metarhizium robertsii reduced Cd accumulation in Arabidopsis thaliana shoots and roots by 21.8 % and 23.8 %, respectively. This is achieved by M. robertsii colonization-induced elevation of Cd efflux capacity via upregulation of three PCR genes, which is confirmed by the fact that the extent to which M. robertsii reduced Cd accumulation in the WT plants was greater than the inactivating mutants of the PCR genes. M. robertsii also alleviated Cd-induced leaf etiolation in A. thaliana by increasing the chlorophyll amount and modified plant physiological status to increase Cd stress tolerance via increasing production of catalase, peroxidase and glutathione and upregulating multiple HIPP proteins involved in sequestration of Cd. Notably, consistent with that in A. thaliana, the colonization of M. robertsii also reduced the Cd accumulation in Oryza sativa seedlings by upregulating the PCR gene OsPCR1, and increased chlorophyll amount and alleviated oxidative stress. Therefore, M. robertsii colonization reduced Cd accumulation in plants, and promoted plant growth and health by elevating Cd efflux capacity and modifying physiological status.

Selection and characterization of thermotolerant Beauveria bassiana isolates and with insecticidal activity against the cotton-melon aphid Aphis gossypii (Glover) (Hemiptera: Aphididae)

BACKGROUND

Cotton-melon aphid Aphis gossypii (Glover) causes severe damage mainly to cucurbits. Twenty-two Beauveria sp. isolates were simultaneously assessed for their pathogenicity and heat tolerance. The selected isolates were identified molecularly and characterized in terms of conidial germination rate, mycelial growth, conidial yield and endophytic activity.

RESULTS

Screening bioassays showed that the B. bassiana isolates B3, B7, B9 and B12 were the most toxic, inducing mortality equal to or slightly higher than the commercialized strain B. bassiana BNat (70.7%). Median lethal concentration (LC₅₀ ) bioassays revealed that only isolate B12 had a significantly lower LC₅₀ value (5.4 × 10⁵ conidia ml^-1 ) than strain BNat (5 × 10⁶ conidia ml^-1 ). The heat tolerance screening test (1 h of exposure to 45°C) allowed us to select isolates B3, B7, B9 and B12 with germination rates of 57.5% to 80.1% after 24 h incubation at 25°C, all significantly higher than strain BNat (22.1%). The germination rates of all isolates decreased significantly after 2 h of exposure to 45°C, with the exception of isolate B12 which displayed the highest thermotolerance (72% germination). The four selected isolates were able to endophytically colonize cucumber leaves when applied to the foliage. Inoculation of cucumber plants with isolate B12 did not affect cucumber plant growth. However, several plant growth parameters were improved 5 weeks after root inoculation.

CONCLUSION

On the basis of its potent toxicity and thermotolerance, isolate B12 is a good candidate for further development as a biopesticide for use in integrated pest management strategies for aphid control. © 2022 Society of Chemical Industry.

New Downstream Signaling Branches of the Mitogen-Activated Protein Kinase Cascades Identified in the Insect Pathogenic and Plant Symbiotic Fungus Metarhizium robertsii

Fungi rely on major signaling pathways such as the MAPK (Mitogen-Activated Protein Kinase) signaling pathways to regulate their responses to fluctuating environmental conditions, which is vital for fungi to persist in the environment. The cosmopolitan Metarhizium fungi have multiple lifestyles and remarkable stress tolerance. Some species, especially M. robertsii, are emerging models for investigating the mechanisms underlying ecological adaptation in fungi. Here we review recently identified new downstream branches of the MAPK cascades in M. robertsii, which controls asexual production (conidiation), insect infection and selection of carbon and nitrogen nutrients. The Myb transcription factor RNS1 appears to be a central regulator that channels information from the Fus3- and Slt2-MAPK cascade to activate insect infection and conidiation, respectively. Another hub regulator is the transcription factor AFTF1 that transduces signals from the Fus3-MAPK and the membrane protein Mr-OPY2 for optimal formation of the infection structures on the host cuticle. Homologs of these newly identified regulators are found in other Metarhizium species and many non-Metarhizium fungi, indicating that these new downstream signaling branches of the MAPK cascades could be widespread.

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

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

Genetic and non-genetic tailoring of microalgae for the enhanced production of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) - A review

The myriad health benefits associated with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) laid the path for their application in the functional foods and nutraceutical industries. Fish being primarily exploited for extraction of EPA and DHA are unsustainable sources; thus, oleaginous microalgae turn out to be an alternative sustainable source. This review paper aims to provide the recent developments in the context of enhancing EPA and DHA production by utilising non-genetic tailoring and genetic tailoring methods. We have also summarized the legislation, public perception, and possible risks associated with the usage of genetically modified microalgae focusing on EPA and DHA production.

Slt2-MAPK/RNS1 Controls Conidiation via Direct Regulation of the Central Regulatory Pathway in the Fungus Metarhizium robertsii

Ascomycete fungi usually produce small hydrophobic asexual conidia that are easily dispersed and essential for long-term survival under a variety of environmental conditions. Several upstream signaling regulators have been documented to control conidiation via regulation of the central regulatory pathway that contains the transcription factors BrlA, AbaA and WetA. Here, we showed that the Slt2-MAPK signaling pathway and the transcription factor RNS1 constitute a novel upstream signaling cascade that activates the central regulatory pathway for conidiation in the Ascomycetes fungus M. robertsii. The BrlA gene has two overlapping transcripts BrlAα and BrlAβ; they have the same major ORF, but the 5' UTR of BrlAβ is 835 bp longer than the one of BrlAα. During conidiation, Slt2 phosphorylates the serine residue at the position 306 in RNS1, which self-induces. RNS1 binds to the BM2 motif in the promoter of the BrlA gene and induces the expression of the transcript BlrAα, which in turn activates the expression of the genes AbaA and WetA. In conclusion, the Slt2/RNS1 cascade represents a novel upstream signaling pathway that initiates conidiation via direct activation of the central regulatory pathway. This work provides significant mechanistic insights into the production of asexual conidia in an Ascomycete fungus.

Transcriptional Differences Guided Discovery and Genetic Identification of Coprogen and Dimerumic Acid Siderophores in Metarhizium robertsii

Siderophores are small molecular iron chelators and participate in the multiple cellular processes in fungi. In this study, biosynthesis gene clusters of coprogens and dimerumic acids were identified by transcriptional level differences of genes related to iron deficiency conditions in Metarhizium robertsii. This leads to the characterization of new coprogen metachelin C (1) and five known siderophores metachelin A (2), metachelin A-CE (3), metachelin B (4), dimerumic acid 11-mannoside (5), and dimerumic acid (6). The structure of metachelin C (1) was elucidated by NMR spectroscopy and HR-ESI-MS analysis. Genetic deletions of mrsidA, and mrsidD abolished the production of compounds 1–6 that implied their involvement in the biosynthesis of coprogen and dimerumic acid. Interestingly, NRPS gene mrsidD is responsible for biosynthesis of both coprogen and dimerumic acid, thus we proposed plausible biosynthetic pathways for the synthesis of coprogen and dimerumic acid siderophores. Therefore, our study provides the genetic basis for understanding the biosynthetic pathway of coprogen and dimerumic acid in Metarhizium robertsii.

The Threat of Pests and Pathogens and the Potential for Biological Control in Forest Ecosystems

Forests are an essential component of the natural environment, as they support biodiversity, sequester carbon, and play a crucial role in biogeochemical cycles—in addition to producing organic matter that is necessary for the function of terrestrial organisms. Forests today are subject to threats ranging from natural occurrences, such as lightning-ignited fires, storms, and some forms of pollution, to those caused by human beings, such as land-use conversion (deforestation or intensive agriculture). In recent years, threats from pests and pathogens, particularly non-native species, have intensified in forests. The damage, decline, and mortality caused by insects, fungi, pathogens, and combinations of pests can lead to sizable ecological, economic, and social losses. To combat forest pests and pathogens, biocontrol may be an effective alternative to chemical pesticides and fertilizers. This review of forest pests and potential adversaries in the natural world highlights microbial inoculants, as well as research efforts to further develop biological control agents against forest pests and pathogens. Recent studies have shown promising results for the application of microbial inoculants as preventive measures. Other studies suggest that these species have potential as fertilizers.

Comprehensive insights into host-pathogen interaction between brown planthopper and a fungal entomopathogen by dual RNA sequencing

BACKGROUND

The brown planthopper (BPH) is one of the most destructive pests of rice, causing tremendous yield and economic losses every year. The fungal entomopathogen Metarhizium anisopliae was previously proved to have great potential for BPH biocontrol. Genome-wide insight into the insect-fungus interaction is crucial for genetic improvement of M. anisopliae to enhance its virulence to BPH but still has been poorly explored.

RESULTS

Using dual RNA-seq approach, we present here a global view of host and fungal gene expressions in BPH adults during the fungal infection. The results revealed that BPH could initiate strong defense responses against the fungal attack by upregulating the expressions of a large number of genes, including genes involved in cuticle formation, immune response, cell detoxification and biomacromolecule metabolism. Correspondingly, the fungal entomopathogen could induce a series of genes to infect and modulate BPH, including genes involved in fungal penetration, invasive growth, stress resistance and virulence. Three host defense-related genes (NlPCE4, NlPOD1 and NlCYP4DE1) were chosen for further function analysis. RNAi-mediated knockdown of NlPCE4 caused a significant decrease in BPH survival, but no obvious effects on the survival rates were detected by the suppression of NlPOD1 and NlCYP4DE1. Combination of dsRNA injection and fungal infection could significantly enhance the BPH-killing speed, as synergistic mortalities were observed in co-treatments of RNAi and M. anisopliae infection.

CONCLUSION

Our study provides a comprehensive insight into molecular mechanisms of host-pathogen interaction between BPH and M. anisopliae and contributes to future development of new efficient biocontrol strategies for BPH biocontrol.

A novel cascade allows Metarhizium robertsii to distinguish cuticle and hemocoel microenvironments during infection of insects

Pathogenic fungi precisely respond to dynamic microenvironments during infection, but the underlying mechanisms are not well understood. The insect pathogenic fungus Metarhizium robertsii is a representative fungus in which to study broad themes of fungal pathogenicity as it resembles some major plant and mammalian pathogenic fungi in its pathogenesis. Here we report on a novel cascade that regulates response of M. robertsii to 2 distinct microenvironments during its pathogenesis. On the insect cuticle, the transcription factor COH2 activates expression of cuticle penetration genes. In the hemocoel, the protein COH1 is expressed due to the reduction in epigenetic repression conferred by the histone deacetylase HDAC1 and the histone 3 acetyltransferase HAT1. COH1 interacts with COH2 to reduce COH2 stability, and this down-regulates cuticle penetration genes and up-regulates genes for hemocoel colonization. Our work significantly advances the insights into fungal pathogenicity in insects.

Pathogenic fungi respond precisely to dynamic microenvironments during infection, but the underlying mechanisms are not well understood. This study identifies a regulatory cascade in a fungal pathogen of insects that acts as a switch to turn genes on or off in response to two distinct host microenvironments; the insect cuticle and the hemocoel.

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.

A Novel Nitrogen and Carbon Metabolism Regulatory Cascade Is Implicated in Entomopathogenicity of the Fungus Metarhizium robertsii

The entomopathogenic fungus Metarhizium robertsii can switch among parasitic, saprophytic, and symbiotic lifestyles in response to changing nutritional conditions, which is attributed to its extremely versatile metabolism. Here, we found that the Fus3–mitogen-activated protein kinase (MAPK) and the transcription factor regulator of nutrient selection 1 (RNS1) constitute a novel fungal cascade that regulates the degradation of insect cuticular lipids, proteins, and chitin to obtain nutrients for hyphal growth and enter the insect hemocoel for subsequent colonization. On the insect cuticle, Fus3-MAPK physically contacts and phosphorylates RNS1, which facilitates the entry of RNS1 into nuclei. The phosphorylated RNS1 binds to the DNA motif BM2 (ACCAGAC) in its own promoter to self-induce expression, which then activates the expression of genes for degrading cuticular proteins, chitin, and lipids. We further found that the Fus3-MAPK/RNS1 cascade also activates genes for utilizing complex and less-favored nitrogen and carbon sources (casein, colloid chitin, and hydrocarbons) that were not derived from insects, which is repressed by favored organic carbon and nitrogen nutrients, including glucose and glutamine. In conclusion, we discovered a novel regulatory cascade that controls fungal nitrogen and carbon metabolism and is implicated in the entomopathogenicity of M. robertsii.

IMPORTANCE Penetration of the cuticle, the first physical barrier to pathogenic fungi, is the prerequisite for fungal infection of insects. In the entomopathogenic fungus Metarhizium robertsii, we found that the Fus3–mitogen-activated protein kinase (MAPK) and the transcription factor regulator of nutrient selection 1 (RNS1) constitute a novel cascade that controls cuticle penetration by regulating degradation of cuticular lipids, proteins, and chitin to obtain nutrients for hyphal growth and entry into the insect hemocoel. In addition, during saprophytic growth, the Fus3-MAPK/RNS1 cascade also activates utilization of complex and less-favored carbon and nitrogen sources that are not derived from insects. The homologs of Fus3-MAPK and RNS1 are widely found in ascomycete filamentous fungi, including saprophytes and pathogens with diverse hosts, suggesting that the regulation of utilization of nitrogen and carbon sources by the Fus3-MAPK/RNS1 cascade could be widespread. Our work provides significant insights into regulation of carbon and nitrogen metabolism in fungi and fungal pathogenesis in insects.

Manipulation of host ecdysteroid hormone levels facilitates infection by the fungal insect pathogen, Metarhizium rileyi

Entomopathogenic fungi such as Metarhizium rileyi and Beauveria bassiana are widely used insect biological control agents. Little, however, is known concerning genetic or enzymatic factors that differentiate the mechanisms employed by these two fungal pathogens to infect target hosts. Infection by either of these organisms is known to increase levels of the growth and molting hormone, ecdysone, which also regulates the expression of a number of innate immune pathways. M. rileyi, but not B. bassiana, has apparently evolved an ecdysteroid-22-oxidase (MrE22O) that inactivate ecdysone. We show that deletion of MrE22O impaired virulence compared with the wild-type strain, with an increase in ecdysone titer seen in hosts that was coupled to an increase in the expression of antimicrobial genes. An M. rileyi strain engineered to overexpress MrE22O (MrE22O^OE ), as well as trans-expression in B. bassiana (Bb::MrE220^OE ) resulted, in strains displaying enhanced virulence and dampening of host immune responses compared with their respective wild-type parental strains. These results indicate that ecdysone plays an important role in mediating responses to fungal infection and that some insect pathogenic fungi have evolved mechanisms for targeting this hormone as a means for facilitating infection.

GATA-type transcription factor MrNsdD regulates dimorphic transition, conidiation, virulence and microsclerotium formation in the entomopathogenic fungus Metarhizium rileyi

The GATA-type sexual development transcription factor NsdD has been implicated in virulence, secondary metabolism and asexual development in filamentous fungi. However, little is known about its function in the yeast-to-hypha transition and in microsclerotium formation. In the current study, the orthologous NsdD gene MrNsdD in the entomopathogenic fungus Metarhizium rileyi was characterized. Transcriptional analysis indicated that MrNsdD was involved in yeast-to-hypha transition, conidiation and microsclerotium formation. After targeted deletion of MrNsdD, dimorphic transition, conidiation, fungal virulence and microsclerotium formation were all impaired. Compared with the wild-type strain, the ΔMrNsdD mutants were hypersensitive to thermal stress. Furthermore, transcriptome sequencing analysis revealed that MrNsdD regulated a distinct signalling pathway in M. rileyi during the yeast-to-hypha transition or microsclerotium formation, but exhibited overlapping regulation of genes during the two distinct developmental stages. Taken together, characterization of the MrNsdD targets in this study will aid in the dissection of the molecular mechanisms of dimorphic transition and microsclerotium development.

New discoveries and applications of mosquito fungal pathogens

Mosquitoes are a major threat to human health globally because they transmit infectious diseases, such as malaria, lymphatic filariasis, and arboviruses. The conventional mosquito control efforts, based on synthetic insecticides, have been compromised owing to the eventual development of insecticide resistance and the adverse environmental impacts of insecticides. Alternative eco-friendly approaches using entomopathogenic fungi to alleviate vector-borne disease burden have gained an increasing interest because of their selective specificity and environmental safety. Existing literature revealed an enormous potential of microbial agents for the biocontrol of mosquitoes. With the advances in genetic recombination and transformation techniques, genetically engineered fungal biopesticides showed promising efficacy against insecticide-resistant mosquitoes. In this article, we elaborate on the important mosquito fungal and oomycota pathogens as potential biocontrol agents and infection mechanism through oral ingestion. Recent advances on the secreted effectors for suppression of host immunity and progress on the development of transgenic mosquito-killing fungi were discussed.

Selection of Beauveria bassiana (Hypocreales: Cordycipitaceae) strains to control Xyleborus affinis (Curculionidae: Scolytinae) females

BACKGROUND

Xyleborus affinis Eichhoff (Coleoptera: Curculionidae) is an ambrosia beetle reported to affect avocado trees (Persea americana Mill.). The use of the entomopathogenic fungus (EPF) Beauveria bassiana (Bals.-Criv.) Vuill. for ambrosia beetle control represents an alternative to insecticides.

METHODS

This study was designed in two stages to select B. bassiana strains with potential to control X. affinis females. In the first stage, 19 B. bassiana Mexican strains from EPF collection, isolated from Coleoptera (CHE-CNRCB, http://www.gob.mx/senasica/documentos/coleccion-de-hongos-entomopatogenos), were tested. Analyses included radial growth rate, conidial yield, spore germination, and germ tube length. Results were analysed by Principal Component Analysis (PCA) to identify clusters within favourable growth phenotypes. For the second stage, 10 selected strains were re-analysed for virulence-related metabolic characteristic, including cell wall-bound cuticle-degrading enzymes-Pr1-like proteases and β-N-acetyl glucosaminidases (NAGase) chitinases, conidial hydrophobicity and monopolar germination parameters. A second PCA analysis was run for those virulence parameters analysed, and upon results strains CHE-CNRCB 44, 171, 431 and 485 were selected and tested against X. affinis females. Females were treated with a 1 × 108 conidia mL-1 suspension (recommended rate), using a Potter Tower.

RESULTS

All strains showed insecticidal activity, inducing up to 58% mortality; about 30% dead beetles developed aerial mycelia (CHE-CNRCB 485) and the fastest mortality rate was t0 = 1.95 (CHE-CNRCB 44).

CONCLUSION

Since all selected strains showed virulence against X. affinis females, results indicated the possibility of selecting B. bassiana strains based on multiple metabolic attributes, as a preliminary test to perform bioassays against order-related target insects.

H2A.Z and chromatin remodelling complexes: a focus on fungi

Chromatin is a highly dynamic structure that closely relates with gene expression in eukaryotes. ATP-dependent chromatin remodelling, histone post-translational modification and DNA methylation are the main ways that mediate such plasticity. The histone variant H2A.Z is frequently encountered in eukaryotes, and can be deposited or removed from nucleosomes by chromatin remodelling complex SWR1 or INO80, respectively, leading to altered chromatin state. H2A.Z has been found to be involved in a diverse range of biological processes, including genome stability, DNA repair and transcriptional regulation. Due to their formidable production of secondary metabolites, filamentous fungi play outstanding roles in pharmaceutical production, food safety and agriculture. During the last few years, chromatin structural changes were proven to be a key factor associated with secondary metabolism in fungi. However, studies on the function of H2A.Z are scarce. Here, we summarize current knowledge of H2A.Z functions with a focus on filamentous fungi. We propose that H2A.Z is a potential target involved in the regulation of secondary metabolite biosynthesis by fungi.

Phenotypic and molecular insights into heat tolerance of formulated cells as active ingredients of fungal insecticides

Formulated conidia of insect-pathogenic fungi, such as Beauveria and Metarhizium, serve as the active ingredients of fungal insecticides but are highly sensitive to persistent high temperatures (32-35 °C) that can be beyond their upper thermal limits especially in tropical areas and during summer months. Fungal heat tolerance and inter- or intra-specific variability are critical factors and limitations to field applications of fungal pesticides during seasons favoring outbreaks of pest populations. The past decades have witnessed tremendous advances in improving fungal pesticides through selection of heat-tolerant strains from natural isolates, improvements and innovations in terms of solid-state fermentation technologies for the production of more heat-tolerant conidia, and the use of genetic engineering of candidate strains for enhancing heat tolerance. More recently, with the entry into a post-genomic era, a large number of signaling and effector genes have been characterized as important sustainers of heat tolerance in both Beauveria and Metarhizium, which represent the main species used as fungal pesticides worldwide. This review focuses on recent advances and provides an overview into the broad molecular basis of fungal heat tolerance and its multiple regulatory pathways. Emphases are placed on approaches for screening of heat-tolerant strains, methods for optimizing conidial quality linked to virulence and heat tolerance particularly involving cell wall architecture and optimized trehalose/mannitol contents, and how molecular determinants can be exploited for genetic improvement of heat tolerance and pest-control potential. Examples of fungal pesticides with different host spectra and their appropriateness for use in apiculture are given. KEY POINTS: • Heat tolerance is critical for field stability and efficacy of fungal insecticides. • Inter- and intra-specific variability exists in insect-pathogenic fungi. • Optimized production technology and biotechnology can improve heat tolerance. • Fungal heat tolerance is orchestrated by multiple molecular pathways.

Disruption of a C69-Family Cysteine Dipeptidase Gene Enhances Heat Shock and UV-B Tolerances in Metarhizium acridum

In fungi, peptidases play a crucial role in adaptability. At present, the roles of peptidases in ultraviolet (UV) and thermal tolerance are still unclear. In this study, a C69-family cysteine dipeptidase of the entomopathogenic fungus Metarhizium acridum, MaPepDA, was expressed in Escherichia coli. The purified enzyme had a molecular mass of 56-kDa, and displayed a high activity to dipeptide substrate with an optimal Ala-Gln hydrolytic activity at about pH 6.0 and 55°C. It was demonstrated that MaPepDA is an intracellular dipeptidase localized in the cytosol, and that it is expressed during the whole fungal growth. Disruption of the MaPepDA gene increased conidial germination, growth rate, and significantly improved the tolerance to UV-B and heat stress in M. acridum. However, virulence and conidia production was largely unaffected in the ΔMaPepDA mutant. Digital gene expression data revealed that the ΔMaPepDA mutant had a higher UV-B and heat-shock tolerance compared to wild type by regulating transcription of sets of genes involved in cell surface component, cell growth, DNA repair, amino acid metabolism, sugar metabolism and some important signaling pathways of stimulation. Our results suggested that disruption of the MaPepDA could potentially improve the performance of fungal pesticides in the field application with no adverse effect on virulence and conidiation.

Analogous and Diverse Functions of APSES-Type Transcription Factors in the Morphogenesis of the Entomopathogenic Fungus Metarhizium rileyi

APSES-type transcription factors (TFs) have analogous and diverse functions in the regulation of fungal morphogenesis processes. However, little is known about these functions in microsclerotium formation. In this study, we characterized two orthologous APSES genes (MrStuA and MrXbp) in the entomopathogenic fungus Metarhizium rileyi Deletion of either MrStuA or MrXbp impaired dimorphic transition, conidiation, fungal virulence, and microsclerotium formation. Compared with the wild-type strain, ΔMrStuA and ΔMrXbp mutants were hypersensitive to thermal and oxidative stress. Furthermore, transcriptome sequencing analysis revealed that MrStuA and MrXbp independently regulate their own distinctive subsets of signaling pathways during dimorphic transition and microsclerotium formation, but they also show an overlapping regulation of genes during these two distinct morphogenesis processes. These results provide a global insight into vital roles of MrStuA and MrXbp in M. rileyi and aid in dissection of the interacting regulatory mechanisms of dimorphism transition and microsclerotium development.IMPORTANCE Transcription factors (TFs) are core components of the signaling pathway and play an important role in transcriptional regulation of gene expression during fungal morphogenesis processes. A prevailing theory suggests an interplay between different TFs regulating microsclerotial differentiation; however, the persisting issue remains that these interplay mechanisms are not clear. Here, we analyzed two members of the APSES-type TFs in Metarhizium rileyi using a gene deletion strategy and transcriptome analysis. Mutants were significantly impaired in microsclerotium formation and dimorphic transition. Transcriptome analysis provided evidence for interacting regulatory mechanisms by the two TFs in microsclerotium formation and dimorphic transition. Furthermore, we investigated their overlapping roles in mediating the expression of genes required for different fungal morphogenesis processes. Characterization of TFs in this study will aid in dissecting the interplay between regulatory mechanisms in fungal morphogenesis processes.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Characterization of trehalose-6-phosphate phosphatase in trehalose biosynthesis, asexual development, stress resistance and virulence of an insect mycopathogen

Biological control potential of entomopathogenic fungi depending on conidiation capacity, conidial stress tolerance and virulence can be improved through genetic engineering. To explore a possible role of trehalose biosynthesis pathway in improving fungal pest-control potential, we characterized biological functions of trehalose-6-phosphate phosphatase (BbTPP) in Beauveria bassiana, an insect mycopathogen that serves as a main source of fungal insecticides. Deletion of BbTPP resulted in abolished trehalose biosynthesis, reduced conidiation capacity, decreases in conidial thermotolerance and UV-B resistance, increased hyphal sensitivities to chemical stresses, and attenuated virulence. By contrast, over-expression of BbTPP led to increased trehalose accumulation, decreased T6P accumulation, and enhanced stress tolerance and virulence despite little impact on growth and conidiation under normal conditions. These results indicate that BbTPP serves as not only a key player in control of trehalose biosynthesis required for multiple cellular functions but also a potential candidate to be exploited for genetic improvement of fungal potential against insect pests.

Comparative Transcriptome Analysis of Thitarodes Armoricanus in Response to the Entomopathogenic Fungi Paecilomyces Hepiali and Ophiocordyceps Sinensis

Thitarodes armoricanus is a medicinal and economically important Lepidopteran insect species. The larvae infected by Paecilomyces hepiali survive no more than four days, while those infected by Ophiocordyceps sinensis can survive for several months before mummification. This provides a valuable comparative system to study interactions between an insect host and different pathogenic fungi. By using the T. armoricanus genome, a time-course transcriptome analysis of the whole larvae without guts was performed to explore the larvae response to P. hepiali and O. sinensis infection. A total of 3106 differentially expressed genes in five clusters were identified. The genes involved in coagulation and multiple metabolisms were both suppressed after P. hepiali or O. sinensis infection, whereas those related to environmental information responses, cell processes, biotic stimulus, and immunity (such as cecropin (CEC)) were elevated. The rapid death of T. armoricanus after P. hepiali infection might be caused by osmotic imbalance, immunocompromise (such as DEFs and GLVs), and nervous system dysfunction (glutamatergic synapse). Up-regulation of the genes related to cuticle structure, nervous system (such as neurotrophin signal pathway and dopaminergic synapse) and immune effectors (such as attacin (ATT) and proline-rich antimicrobial peptide 1 (PRAMP1)) in T. armoricanus, may contribute to the co-existence of T. armoricanus and O. sinensis. This study provides a global view and potential key genes of the interaction between T. armoricanus and two fungal entomopathogens.

A review of progress toward field application of transgenic mosquitocidal entomopathogenic fungi

In Africa, adult mosquito populations are primarily controlled with insecticide-impregnated bed nets and residual insecticide sprays. This coupled with widespread applications of pesticides in agriculture has led to increasing insecticide resistance in mosquito populations. We have developed multiple alternative strategies for exploiting transgenic Metarhizium spp. directed at: (i) shortening the lifespan of adult mosquitoes; (ii) reducing transmission potential of Plasmodium spp.; (iii) reducing vector competence via pre-lethal effects. The present challenge is to convert this promising strategy into a validated public health intervention by resolving outstanding issues related to the release of genetically modified organisms. © 2019 Society of Chemical Industry.

Increasing Pyruvate Concentration Enhances Conidial Thermotolerance in the Entomopathogenic Fungus Metarhizium robertsii

The fungal entomopathogens Metarhizium spp. have been developed as environmentally friendly mycoinsecticides. However, heat stress severely reduces the viability of Metarhizium conidia in the field, which is an important obstacle to the successful use of these mycoinsecticides. Heat treatment induces rapid accumulation of pyruvate, which timely scavenges heat-induced ROS (reactive oxygen species) in hyphal cells of M. robertsii. However, in heat-treated conidia, pyruvate accumulation occurs later than the rapid production of ROSs, which could harm the conidial cells. In the present study, a transgenic M. robertsii strain was constructed with the pyruvate kinases gene overexpressed during conidiation. Two independent transformants of the transgenic strain produced conidia under optimal conditions with elevated pyruvate concentration. This inhibits the rapid heat-induced ROS production and prevents the collapse of mitochondrial membrane potential, thereby increasing conidial tolerance to heat stress. In conclusion, the tolerance of M. robertsii conidia to heat stress was improved by increasing the conidial pyruvate concentration, which could be translated into a more effective pest control.

Truncated, strong inducible promoter Pmcl1 from Metarhizium anisopliae

In this study, Metarhizium collagen -like protein (MCL1) promoter from Metarhizium anisopliae was analysed and truncated into different sizes through series of targeted and random deletions based on the presence of various transcription factor-binding sites. Synthetic Green Fluorescent Protein (sGFP) was being utilized as a reporter gene to study the relative expression driving capability of unmodified and truncated promoters. Conserved promoter sequence analysis revealed similarity between the paralogous promoters from M. brunneum and M. acridum. sGFP expression in the haemolymph was directed with the help of mcl1 signal peptide sequence. Deleting the promoter region from - 2764 to - 1583 bp increases the promoter mcl1 (Pmcl1) activity by twofolds, while deletions of the regions upstream of - 1150 bp and - 840 bp caused a decrease of sGFP expression level (80% and 70%, respectively). Transcriptional binding sites predicted for the deleted region revealed the loss of upstream repressing sequences such as Matalpha2 along with ROX1 and Rap1 repressor-binding sites located - 2234 bp, - 1754 bp and - 1724 bp from the TSS. Compared with Pmcl1-wild type (2.7 kbp), Pmcl1-1583 bp had a shorter sequence and showed statistically significant expression in M. anisopliae. This study introduces a highly efficient strong inducible promoter for over-expression of target genes in M. anisopliae.

Transgenic Metarhizium pingshaense synergistically ameliorates pyrethroid-resistance in wild-caught, malaria-vector mosquitoes

Transgenic Metarhizium pingshaense expressing the spider neurotoxin Hybrid (Met-Hybrid) kill mosquitoes faster and at lower spore doses than wild-type strains. In this study, we demonstrate that this approach dovetails with the cornerstone of current malaria control: pyrethroid-insecticides, which are the cornerstone of current malaria control. We used World Health Organization (WHO) tubes, to compare the impact on insecticide resistance of Met-Hybrid with red fluorescent M. pingshaense (Met-RFP), used as a proxy for the wild-type fungus. Insecticides killed less than 20% of Anopheles coluzzii and Anopheles gambiae s.s. mosquitoes collected in a malaria endemic region of Burkina Faso where pyrethroid use is common. Seven days post-infection, mortality for insecticide-sensitive and resistant mosquitoes averaged 94% with Met-Hybrid and 64% with Met-RFP, with LT80 values of 5.32±0.199 days and 7.76±0.183 days, respectively. Eighty nine percent of insecticide-resistant mosquitoes exposed to permethrin five days post-infection with Met-Hybrid died within 24 hours: only 22% died from Met-Hybrid alone over this 24-hour period. Compared to Met-RFP, Met-Hybrid also significantly reduced flight capacity of mosquitoes 3 to 5 days post-infection. Based on WHOPES phase I laboratory susceptibility bioassays, transgenic Met-Hybrid provides effective biological control for adult African malaria vectors that may be used to synergistically manage insecticide resistance with current methods.

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.

Insect Pathogenic Fungi as Endophytes

In this chapter, we explore some of the evolutionary, ecological, molecular genetics, and applied aspects of a subset of insect pathogenic fungi that also have a lifestyle as endophytes and we term endophytic insect pathogenic fungi (EIPF). We focus particularly on Metarhizium spp. and Beauveria bassiana as EIPF. The discussion of the evolution of EIPF challenges a view that these fungi were first and foremost insect pathogens that eventually evolved to colonize plants. Phylogenetic evidence shows that the lineages of EIPF are most closely related to grass endophytes that diverged c. 100MYA. We discuss the relationship between genes involved in "insect pathogenesis" and those involved in "endophytism" and provide examples of genes with potential importance in lifestyle transitions toward insect pathogenicity. That is, some genes for insect pathogenesis may have been coopted from genes involved in endophytic colonization. Other genes may be multifunctional and serve in both lifestyle capacities. The interactions of EIPF with their host plants are discussed in some detail. The genetic basis for rhizospheric competence, plant communication, and nutrient exchange is examined and we highlight, with examples, the benefits of EIPF to plants, and the potential reservoir of secondary metabolites hidden within these beneficial symbioses.