Systematic contributions of CFEM domain-containing proteins to iron acquisition are essential for interspecies interaction of the filamentous pathogenic fungus Beauveria bassiana

Beauveria bassiana transcriptomics reveal virulence-associated shifts during insect lipid assimilation

Insect cuticular lipids, especially epicuticular hydrocarbons (CHC), have a significant role in insect ecology and interactions with other organisms, including fungi. The CHC composition of a specific insect species may influence the outcome of the interaction with a specific fungal strain. Some insects, such as Piezodorus guildinii, have low susceptibility towards fungal infections seemingly due to their CHC composition. The entomopathogenic fungus Beauveria bassiana can assimilate CHC and incorporate them as building blocks via cytochrome P450 monooxygenases (CYPs). However, little is known about other enzymes that promote the degradation/assimilation of these cuticular components. In this study, we performed a transcriptomic analysis to evaluate the in vitro response of two virulence-contrasting B. bassiana strains when grown on three different P. guildinii CHC sources. We found a different expression profile of virulence-related genes, as well as different GO and KEGG parameters enriched at 4 days post-inoculation, which could help account for the intrinsic virulence and for an alkane-priming virulence enhancement effect. The hypovirulent strain predominantly showed higher expression of cuticle penetration genes, including chitinases, proteases, and CYPs, with GO term categories of "heme binding," "monooxygenase activity," and "peroxisome" pathways enriched. The hypervirulent strain showed higher expression of cell wall remodeling and cell cycle genes, and cuticle adhesion and a distinct set of CYPs, with GO categories of "DNA-binding transcription factor activity" and KEGG pathways corresponding to "meiosis-yeast" and "cell cycle" enriched. These results suggest a delay and alternate routes in pathogenicity-related metabolism in the hypovirulent strain in comparison with the hypervirulent strain. KEY POINTS: •Transcriptomics of two B. bassiana strains grown in P. guildinii cuticular components •Virulence-related genes correlated with virulence enhancement towards P. guildinii •Differentially expressed genes, GOs and KEGGs showed different metabolic timelines associated with virulence.

Role of cell membrane homeostasis in the pathogenicity of pathogenic filamentous fungi

The cell membrane forms a fundamental part of all living cells and participates in a variety of physiological processes, such as material exchange, stress response, cell recognition, signal transduction, cellular immunity, apoptosis, and pathogenicity. Here, we review the mechanisms and functions of the membrane structure (lipid components of the membrane and the biosynthesis of unsaturated fatty acids), membrane proteins (transmembrane proteins and proteins contributing to membrane curvature), transcriptional regulation, and cell wall components that influence the virulence and pathogenicity of filamentous fungi.

Unlocking the Siderophore Biosynthesis Pathway and Its Biological Functions in the Fungal Insect Pathogen Beauveria bassiana

Siderophores are small molecule iron chelators. The entomopathogenic fungus Beauveria bassiana produces a plethora of siderophores under iron-limiting conditions. In this study, a siderophore biosynthesis pathway, akin to the general pathway observed in filamentous fungi, was revealed in B. bassiana. Among the siderophore biosynthesis genes (SID), BbSidA was required for the production of most siderophores, and the SidC and SidD biosynthesis gene clusters were indispensable for the production of ferricrocin and fusarinine C, respectively. Biosynthesis genes play various roles in siderophore production, vegetative growth, stress resistance, development, and virulence, in which BbSidA plays the most important role. Accordingly, B. bassiana employs a cocktail of siderophores for iron metabolism, which is essential for fungal physiology and host interactions. This study provides the initial network for the genetic modification of siderophore biosynthesis, which not only aims to improve the efficacy of biocontrol agents but also ensures the efficient production of siderophores.

Two ferrous iron transporter-like proteins independently participate in asexual development under iron limitation and virulence in Beauveria bassiana

Reductive assimilation pathway involves ferric reductase and ferrous iron transporter, which is integral for fungal iron acquisition. A family of ferric reductase-like proteins has been functionally characterized in the filamentous entomopathogenic fungus Beauveria bassiana. In this investigation, two ferrous iron transporter-like proteins (Ftr) were functionally annotated in B. bassiana. BbFtr1 and BbFtr2 displayed high similarity in structure and were associated with the plasma and nuclear membrane. Their losses had no negatively influence on fungal growth on various nutrients and development under the iron-replete condition. Single mutants of BbFTR1 and BbFTR2 displayed the iron-availability dependent developmental defects, and double mutant exhibited the significantly impaired developmental potential under the iron-limited conditions. In insect bioassay, the double mutant also showed the weaker virulence than either of two single disruption mutants. These results suggested that two ferrous iron transporter-like proteins function independently in fungal physiologies under the iron-deficient condition. Intriguingly, a bZIP transcription factor BbHapX was required for expression of BbFTR1 and BbFTR2 under iron-depleted conditions. This study enhances our understanding of the iron uptake system in the filamentous entomopathogenic fungi.

Bioinformatics-Driven mRNA-Based Vaccine Design for Controlling Tinea Cruris Induced by Trichophyton rubrum

Tinea cruris, a dermatophyte fungal infection predominantly caused by Trichophyton rubrum and Epidermophyton floccosum, primarily affects the groin, pubic region, and adjacent thigh. Its recurrence is frequent, attributable to repeated fungal infections in susceptible individuals, especially those with onychomycosis or tinea pedis, which act as reservoirs for dermatophytes. Given the persistent nature of tinea cruris, vaccination emerges as a promising strategy for fungal infection management, offering targeted, durable protection against various fungal species. Vaccines stimulate both humoral and cell-mediated immunity and are administered prophylactically to prevent infections while minimizing the risk of antifungal resistance development. Developing fungal vaccines is challenging due to the thick fungal cell wall, similarities between fungal and human cells, antigenic variation, and evolutionary resemblance to animals, complicating non-toxic target identification and T-cell response variability. No prior research has shown an mRNA vaccine for T. rubrum. Hence, this study proposes a novel mRNA-based vaccine for tinea cruris, potentially offering long-term immunity and reducing reliance on antifungal medications. This study explores the complete proteome of T. rubrum, identifying potential protein candidates for vaccine development through reverse vaccinology. Immunogenic epitopes from these candidates were mapped and integrated into multitope vaccines and reverse translated to construct mRNA vaccines. Then, the mRNA was translated and computationally assessed for physicochemical, chemical, and immunological attributes. Notably, 1,3-beta-glucanosyltransferase, CFEM domain-containing protein, cell wall galactomannoprotein, and LysM domain-containing protein emerged as promising vaccine targets. Antigenic, immunogenic, non-toxic, and non-allergenic cytotoxic T lymphocyte, helper T lymphocyte, and B lymphocyte epitopes were selected and linked with appropriate linkers and Toll-like receptor (TLR) agonist adjuvants to formulate vaccine candidates targeting T. rubrum. The protein-based vaccines underwent reverse translation to construct the mRNA vaccines, which, after inoculation, were translated again by host ribosomes to work as potential components for triggering the immune response. After that, molecular docking, normal mode analysis, and molecular dynamic simulation confirmed strong binding affinities and stable complexes between vaccines and TLR receptors. Furthermore, immune simulations of vaccines with and without adjuvant demonstrated activation of immune responses, evidenced by elevated levels of IgG1, IgG2, IgM antibodies, cytokines, and interleukins. There was no significant change in antibody production between vaccines with and without adjuvants, but adjuvants are crucial for activating the innate immune response via TLRs. Although mRNA vaccines hold promise against fungal infections, further research is essential to assess their safety and efficacy. Experimental validation is crucial for evaluating their immunogenicity, effectiveness, and safety.

Essential roles of ferric reductase-like proteins in growth, development, stress response, and virulence of the filamentous entomopathogenic fungus Beauveria bassiana

In yeasts, ferric reductase catalyzes reduction of ferric ion to ferrous form, which is essential for the reductive iron assimilation system. However, the physiological roles of ferric reductases remain largely unknown in the filamentous fungi. In this study, genome-wide annotation revealed thirteen ferric reductase-like (Fre) proteins in the filamentous insect pathogenic fungus Beauveria bassiana, and all their functions were genetically characterized. Ferric reductase family proteins exhibit different sub-cellular distributions (e.g., cell periphery and vacuole), which was due to divergent domain architectures. Fre proteins had a synergistic effect on fungal virulence, which was ascribed to their distinct functions in different physiologies. Ten Fre proteins were not involved in reduction of ferric ion in submerged mycelia, but most proteins contributed to blastospore development. Only two Fre proteins significantly contributed to B. bassiana vegetative growth under the chemical-induced iron starvation, but most Fre proteins were involved in resistance to osmotic and oxidative stresses. Notably, a bZIP-type transcription factor HapX bound to the promoter regions of all FRE genes in B. bassiana, and displayed varying roles in the transcription activation of these genes. This study reveals the important role of BbFre family proteins in development, stress response, and insect pathogenicity, as well as their distinctive role in the absorption of ferric iron from the environment.

Rad6, a ubiquitin conjugator required for insect-pathogenic lifestyle, UV damage repair, and genomic expression of Beauveria bassiana

The E2 ubiquitin conjugator Rad6 is required for DNA damage bypass in budding yeast but remain functionally unknown in filamentous fungi. Here, we report pleiotropic effect of Rad6 ortholog in Beauveria bassiana, a wide-spectrum fungal insecticide. Global ubiquitination signal was greatly attenuated in the absence of rad6. The blocked ubiquitination led to severe growth defect, blocked asexual development, and abolished infectivity/insect pathogenicity, which correlated with compromised conidial quality (including viability, hydrophobicity, adherence to insect cuticle, and thermotolerance) and blocked secretion of cuticle-degrading enzymes including Pr1 family proteases. Importantly, Rad6 played much greater role in photoreactivation of UVB-impaired conidia by a 3- or 5-h light plus 9- or 7-h dark incubation than in dark reactivation of those impaired conidia by a 12-h dark incubation. The high activity of Rad6 in photoreactivation in vivo was derived from its link to a protein complex cored by the photolyase regulators WC1 and WC2 via the strong interactions of Rad6 with the E3 partner Rad18 and Rad18 with WC2 revealed in yeast two-hybrid assays. Transcriptomic analysis resulted in identification of 2700 differentially regulated genes involved in various function categories and metabolism pathways, indicating a regulatory role of Rad6-mediated ubiquitination in gene expression networks and genomic stability. Conclusively, Rad6 is required for asexual and insect-pathogenic lifecycles, solar UV damage repair, and genomic expression of B. bassiana. The primary dependence of its strong anti-UV role on photoreactivation in vivo unveils a scenario distinct from the core role of its yeast ortholog in DNA damage bypass.

MARVEL family proteins contribute to vegetative growth, development, and virulence of the insect fungal pathogen Beauveria bassiana

Beauveria bassiana is one of the most extensively studied entomopathogenic fungi (EPF) and is widely used as a biocontrol agent against various insect pests. Proteins containing the MARVEL domain are conserved in eukaryotes, typically with four transmembrane structures. In this study, we identified the five MARVEL domain proteins in B. bassiana. Five MARVEL domain proteins were localized to cytomembrane and vacuoles in B. bassiana, but had different roles in maintaining the lipid-droplet homeostasis. These proteins were required for fungal virulence, but differentially contributed to fungal utilization of nutrients, stress tolerance, and development under aerial and submerged conditions. Notably, BbMARVEL2 was essential for conidial surface morphology. Additionally, these five MARVEL domain proteins contributed to fungal interaction with the host immune defense. This study provides new mechanistic insights into the life cycle of B. bassiana as a biocontrol agent.

Multifunctionality of AsCFEM6 and AsCFEM12 effectors from the potato early blight pathogen Alternaria solani

Plant pathogens secrete fungal-specific common in several fungal extracellular membrane (CFEM) effectors to manipulate host immunity and contribute to their virulence. Little is known about effectors and their functions in Alternaria solani, the necrotrophic fungal pathogen causing potato early blight. To identify candidate CFEM effector genes, we mined A. solani genome databases. This led to the identification of 12 genes encoding CFEM proteins (termed AsCFEM1-AsCFEM12) and 6 of them were confirmed to be putative secreted effectors. In planta expression revealed that AsCFEM6 and AsCFEM12 have elicitor function that triggers plant defense response including cell death in different botanical families. Targeted gene disruption of AsCFEM6 and AsCFEM12 resulted in a change in spore development, significant reduction of virulence on potato and eggplant susceptible cultivars, increased resistance to fungicide stress, variation in iron acquisition and utilization, and the involvement in 1,8-dihydroxynaphthalene (DHN) melanin biosynthesis pathway. Using maximum likelihood method, we found that positive selection likely caused the polymorphism within AsCFEM6 and AsCFEM12 homologs in different Alternaria spp. Site-directed mutagenesis analysis indicated that positive selection sites within their CFEM domains are required for cell death induction in Nicotiana benthamiana and are critical for response to abiotic stress in yeast. These results demonstrate that AsCFEM effectors possess additional functions beyond their roles in host plant immune response and pathogen virulence.

Genetic Diversity and Virulence Variation of Metarhizium rileyi from Infected Spodoptera frugiperda in Corn Fields

Metarhizium rileyi is an entomopathogenic fungus that naturally infects the larvae of Spodoptera frugiperda, and has biocontrol potential. To explore more natural entomopathogenic fungi resources, a total of 31 strains were isolated from 13 prefectures in Yunnan Province. All the strains were identified using morphology and molecular biology. The genetic diversity of the 31 isolates of M. rileyi was analyzed using inter-simple sequence repeat (ISSR) techniques. Seven primers with good polymorphism were selected, and fifty-four distinct amplification sites were obtained by polymerase chain reaction amplification. Among them, 50 were polymorphic sites, and the percentage of polymorphic sites was 94.44%. The thirty-one strains were divided into eight subpopulations according to the regions. The Nei's gene diversity was 0.2945, and the Shannon information index was 0.4574, indicating that M. rileyi had rich genetic diversity. The average total genetic diversity of the subpopulations in the different regions was 0.2962, the gene diversity within the populations was 0.1931, the genetic differentiation coefficient was 0.3482 (>0.25), and the gene flow was 0.9360 (<1). The individual cluster analysis showed that there was no obvious correlation between the genetic diversity of the strains and their geographical origin, which also indicated that the virulence of the strains was not related to their phylogeny. Thus, the genetic distance of the different populations of M. rileyi in Yunnan Province was not related to the geographical distance. The virulence of those 32 strains against the 3rd-instar larvae of S. frugiperda were varied with the differences in geographical locations. On the 10th day of inoculation, seventeen strains had an insect mortality rate of 70.0%, and seven strains had an insect mortality rate of 100%. The half-lethal times of the M. rileyi SZCY201010, XSBN200920, and MDXZ200803 strains against the S. frugiperda larvae were less than 4 d. Thus, they have the potential to be developed into fungal insecticidal agents.

Hypovirulence-associated mycovirus epidemics cause pathogenicity degeneration of Beauveria bassiana in the field

BACKGROUND

The entomogenous fungus Beauveria bassiana is used as a biological insecticide worldwide, wild B. bassiana strains with high pathogenicity in the field play an important role in controlling insect pests via not only screening of highly virulent strains but also natural infection, but the pathogenicity degeneration of wild strains severely affected aforementioned effects. Previous studies have showed that multiple factors contributed to this phenomenon. It has been extensively proved that the mycovirus infection caused hypovirulence of phytopathogenic fungi, which has been used for plant disease biocontrol. However, it remains unknown whether the mycovirus epidemics is a key factor causing hypovirulence of B. bassiana naturally in the field.

METHODS

Wild strains of B. bassiana were collected from different geographic locations in Jilin Province, China, to clarify the epidemic and diversity of the mycoviruses. A mycovirus Beauveria bassiana chrysovirus 2 (BbCV2) we have previously identified was employed to clarify its impact on the pathogenicity of host fungi B. bassiana against the larvae of insect pest Ostrinia furnacalis. The serological analysis was conducted by preparing polyclonal antibody against a BbCV2 coat protein, to determine whether it can dissociate outside the host fungal cells and subsequently infect new hosts. Transcriptome analysis was used to reveal the interactions between viruses and hosts.

RESULTS

We surprisingly found that the mycovirus BbCV2 was prevalent in the field as a core virus in wild B. bassiana strains, without obvious genetic differentiation, this virus possessed efficient and stable horizontal and vertical transmission capabilities. The serological results showed that the virus could not only replicate within but also dissociate outside the host cells, and the purified virions could infect B. bassiana by co-incubation. The virus infection causes B. bassiana hypovirulence. Transcriptome analysis revealed decreased expression of genes related to insect epidermis penetration, hypha growth and toxin metabolism in B. bassiana caused by mycovirus infection.

CONCLUSION

Beauveria bassiana infected by hypovirulence-associated mycovirus can spread the virus to new host strains after infecting insects, and cause the virus epidemics in the field. The findings confirmed that mycovirus infection may be an important factor affecting the pathogenicity degradation of B. bassiana in the field.

Autophagy-Related Gene 4 Participates in the Asexual Development, Stress Response and Virulence of Filamentous Insect Pathogenic Fungus Beauveria bassiana

Autophagy is a conserved mechanism for the turnover of intracellular components. Among the 'core' autophagy-related genes (ATGs), the cysteine protease Atg4 plays an important role in the activation of Atg8 by exposing the glycine residue at its extreme carboxyl terminus. In the insect fungal pathogen Beauveria bassiana, a yeast ortholog of Atg4 was identified and functionally analyzed. Ablation of the BbATG4 gene blocks the autophagic process during fungal growth under aerial and submerged conditions. Gene loss did not affect fungal radial growth on various nutrients, but ΔBbatg4 exhibited an impaired ability to accumulate biomass. The mutant displayed increased sensitivity to stress caused by menadione and hydrogen peroxide. ΔBbatg4 generated abnormal conidiophores with reduced production of conidia. Additionally, fungal dimorphism was significantly attenuated in gene disruption mutants. Disruption of BbATG4 resulted in significantly weakened virulence in topical and intrahemocoel injection assays. Our study indicates that BbAtg4 contributes to the lifecycle of B. bassiana via its autophagic roles.

Dual Transcriptomics Reveals Interspecific Interactions between the Mycoparasite Calcarisporium cordycipiticola and Its Host Cordyceps militaris

Calcarisporium cordycipiticola is a mycoparasite of the edible fungus Cordyceps militaris, and mycoparasitism causes devastating diseases of mushrooms. In this study, dual-transcriptomic analysis was performed to reveal interspecific interactions between the mycoparasite C. cordycipiticola and its host C. militaris. At 4 and 8 days postinfection (dpi), 2,959 and 2,077 differentially expressed genes (DEGs) of C. cordycipiticola and 914 and 1,548 DEGs of C. militaris were identified compared with the mycelial stage, respectively, indicating that C. cordycipiticola responded more quickly than C. militaris. Lectins of the pathogen may play a role in the recognition of fungal prey. Both Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that primary metabolism was vigorous for the pathogen to colonize the host and that the pathogen's attack substantially altered C. militaris' primary metabolism. C. cordycipiticola upregulated some carbohydrate-active enzyme (CAZyme) genes, including CBM18, GH18, GH16, and GH76, for degrading the host cell wall and defending against host immunity. C. militaris produced excessive reactive oxygen species (ROS) to respond to the infection. The GO term "heme binding" was the only shared term enriched at both stages at 4 and 8 dpi, indicating that iron was important for both the pathogen and the host. The uptake of iron by pathogens through multiple pathways promoted colonization and removed high ROS levels produced by the host. The transcription levels of Cmhsp78, Cmhsp70, and Cmhyd1 in C. militaris responded quickly, and these genes have potential as candidates for the breeding of resistant varieties. This study provides clues for understanding the interactions between a mycoparasite and its mushroom host and will be helpful for the breeding of resistant varieties and disease prevention and control for this edible fungus. IMPORTANCE White mildew disease caused by Calcarisporium cordycipiticola is devastating for the fruiting body cultivation of Cordyceps militaris, a popular and highly valued edible fungus. Here, the pathogenic mechanisms of C. cordycipiticola, the responses of C. militaris to the infection, and the interaction of these two phylogenetically close species were revealed by time course dual-transcriptome profiles. In general, the host C. militaris responds more slowly than the pathogen C. cordycipiticola. For the first time, we found that iron was important for both the mycoparasite and the host. C. cordycipiticola takes up iron by multiple pathways to promote colonization and remove high ROS levels produced by the host. The rapidly responding genes Cmhsp70, Cmhsp78, and Cmhyd1 in C. militaris may have the potential as candidate genes for the breeding of resistant varieties. This study expands our understanding of the mycoparasitic interactions of two species from sister families and will be helpful for the breeding of and disease prevention and control in mushrooms.

The Conserved Cysteine-Rich Secretory Protein MaCFEM85 Interacts with MsWAK16 to Activate Plant Defenses

Metarhizium anisopliae is an entomopathogenic fungus which may enhance plant growth and resistance when acting as an endophyte in host plants. However, little is known about the protein interactions nor their activating mechanisms. Common in fungal extracellular membrane (CFEM) proteins have been identified as plant immune regulators that suppress or activate plant resistance responses. Here, we identified a CFEM domain-containing protein, MaCFEM85, which was mainly localized in the plasma membrane. Yeast two-hybrid (Y2H), glutathione-S-transferase (GST) pull-down, and bimolecular fluorescence complementation assays demonstrated that MaCFEM85 interacted with the extracellular domain of a Medicago sativa (alfalfa) membrane protein, MsWAK16. Gene expression analyses showed that MaCFEM85 and MsWAK16 were significantly upregulated in M. anisopliae and M. sativa, respectively, from 12 to 60 h after co-inoculation. Additional yeast two-hybrid assays and amino acid site-specific mutation indicated that the CFEM domain and 52th cysteine specifically were required for the interaction of MaCFEM85 with MsWAK16. Defense function assays showed that JA was up-regulated, but Botrytis cinerea lesion size and Myzus persicae reproduction were suppressed by transient expression of MaCFEM85 and MsWAK16 in the model host plant Nicotiana benthamiana. Collectively, these results provide novel insights into the molecular mechanisms underlying interactions of M. anisopliae with host plants.

Molecular Regulatory Mechanism of the Iron-Ion-Promoted Asexual Sporulation of Antrodia cinnamomea in Submerged Fermentation Revealed by Comparative Transcriptomics

Antrodia cinnamomea is a precious edible and medicinal fungus with activities of antitumor, antivirus, and immunoregulation. Fe²⁺ was found to promote the asexual sporulation of A. cinnamomea markedly, but the molecular regulatory mechanism of the effect is unclear. In the present study, comparative transcriptomics analysis using RNA sequencing (RNA-seq) and real time quantitative PCR (RT-qPCR) were conducted on A. cinnamomea mycelia cultured in the presence or absence of Fe²⁺ to reveal the molecular regulatory mechanisms underlying iron-ion-promoted asexual sporulation. The obtained mechanism is as follows: A. cinnamomea acquires iron ions through reductive iron assimilation (RIA) and siderophore-mediated iron assimilation (SIA). In RIA, ferrous iron ions are directly transported into cells by the high-affinity protein complex formed by a ferroxidase (FetC) and an Fe transporter permease (FtrA). In SIA, siderophores are secreted externally to chelate the iron in the extracellular environment. Then, the chelates are transported into cells through the siderophore channels (Sit1/MirB) on the cell membrane and hydrolyzed by a hydrolase (EstB) in the cell to release iron ions. The O-methyltransferase TpcA and the regulatory protein URBS1 promote the synthesis of siderophores. HapX and SreA respond to and maintain the balance of the intercellular concentration of iron ions. Furthermore, HapX and SreA promote the expression of flbD and abaA, respectively. In addition, iron ions promote the expression of relevant genes in the cell wall integrity signaling pathway, thereby accelerating the cell wall synthesis and maturation of spores. This study contributes to the rational adjustment and control of the sporulation of A. cinnamomea and thereby improves the efficiency of the preparation of inoculum for submerged fermentation.

Genome-wide identification of BCS1 domain-containing proteins reveals the mitochondrial bcs1 essential for growth, stress response, and virulence of the filamentous entomopathogenic fungus Beauveria bassiana

In yeasts, bcs1 is a mitochondrial AAA protein (ATPase associated with diverse cellular activities) and required for biogenesis of the complex III in mitochondrial electron transfer chain. However, the presence and biological roles of bcs1 remain largely unknown in the filamentous fungi. In present study, genome-wide identification revealed that there were six BCS1-domain containing proteins (Bbbcs1a through f) in the filamentous insect pathogenic fungus Beauveria bassiana, five of which (except for Bbbcs1f) were functionally analyzed. Phenotypic evaluation revealed that only Bbbcs1b and Bbbcs1c contributed to fungal physiologies, and they localized to nuclei and mitochondria, respectively. Hence, Bbbcs1c is considered as the ortholog of yeast bcs1 in B. bassiana. Ablation of Bbbcs1c did not affect biogenesis of mitochondria, but its loss significantly attenuated mitochondrial functionality (e.g., ATP synthesis and mitochondrial targeting of proteins) significantly. ΔBbbcs1c mutant displayed the impaired phenotypes in vegetative growth, stress response, development, and virulence. Notably, ΔBbbcs1c mutant displayed the increased sensitivity to linoleic acid (LA) stress and lost the intracellular fatty acid homeostasis. The Bbbcs1c loss compromised the mitochondrial membrane potential, and LA stress exacerbated this damage. These findings indicate that Bbbcs1c is a functional homolog of yeast bcs1 in B. bassiana and links mitochondrial functionality to unique lifestyle in the entomopathogenic fungi.

Sensing of a spore surface protein by a Drosophila chemosensory protein induces behavioral defense against fungal parasitic infections

In addition to innate immunity in a physiological context, insects have evolved behavioral defenses against parasite attacks. Here, we report that Drosophila can sense the CFEM (common in fungal extracellular membrane) protein Mcdc9, which acts as a negative virulence factor of the entomopathogenic fungus Metarhizium robertsii. The individual deletions of 18 CFEM genes in Metarhizium followed by fly infection identified three null mutants that could kill the flies more quickly than the wild-type strain, among which Mcdc9 can coat fungal spores and interact with the fly chemosensory protein CheA75a. The deletion of Mcdc9 in the fungus or the knockdown of CheA75a in flies had a similar effect, in which a greater number of fungal spores were left on flies than on the respective controls after topical infection. Thus, similar to the accelerated death of the wild-type flies treated with ΔMcdc9, the CheA75a^RNAi flies succumbed more quickly than the control insects topically challenged with the wild-type strain. The CheA75a gene is highly transcribed in fly legs and wings, and positive electrophysiological responses were evidenced in tarsal sensilla after stimulation with the Mcdc9 protein. The results imply that this CFEM protein could be sensed as a contact elicitor inducing the hygienic behavior of flies against fungal parasitic infection, which reveals a previously unsuspected mechanism of fungus-insect interactions.