Glc8, a regulator of protein phosphatase type 1, mediates oxidation tolerance, asexual development and virulence in Beauveria bassiana, a filamentous entomopathogenic fungus

Reconstruction of Gut Bacteria in Spodoptera frugiperda Infected by Beauveria bassiana Affects the Survival of Host Pest

Spodoptera frugiperda (Lepidoptera: Noctuidae) is a migratory agricultural pest that is devastating on a global scale. Beauveria bassiana is a filamentous entomopathogenic fungus that has a strong pathogenic effect on Lepidoptera pests but little is known about the microbial community in the host gut and the dominant populations in fungus-infected insects. B. bassiana AJS91881 was isolated and identified from the infected larvae of Spodoptera litura. The virulence of AJS91881 to the eggs, larvae, pupae and adults of S. frugiperda was measured. Moreover, the gut microbial community diversity of healthy and fungus-infected insects was analyzed. Our results showed that after treatment with B. bassiana AJS91881, the egg hatching rate, larval survival rate and adult lifespan of the insects were significantly reduced, and the pupae rigor rate was significantly increased compared to that of the control group. Additionally, the gut microbial community was reconstructed after B. bassiana infection. At the phylum and genus level, the relative abundance of the Proteobacteria and Serratia increased significantly in the B. bassiana treatment group. The KEGG function prediction results showed that fungal infection affected insect gut metabolism, environmental information processing, genetic information processing, organism systems and cellular processes. Fungal infection was closely related to the metabolism of various substances in the insect gut. Serratia marcescens was the bacterium with the highest relative abundance after infection by B. bassiana; intestinal bacteria S. marcescens inhibited the infection of insect fungi B. bassiana against the S. frugiperda. The presence of gut bacteria also significantly reduced the virulence of the fungi against the insects when compared to the group with the larvae fed antibiotics that were infected with fungal suspension (Germfree, GF) and healthy larvae that were infected with fungal suspension prepared with an antibiotic solution (+antibiotic). In conclusion, the reconstruction of the insect intestinal bacterial community is an indispensable link for understanding the pathogenicity of B. bassiana against S. frugiperda. Most importantly, in the later stage of fungal infection, the increased abundance of S. marcescens in the insect intestine inhibited the virulence of B. bassiana to some extent. The findings aid in understanding changes in the gut microbiota during the early stages of entomopathogenic fungal infection of insects and the involvement of insect gut microbes in host defense mediated by pathogenic fungal infection. This study is also conducive to understanding the interaction between entomopathogenic fungi, hosts and gut microbes, and provides a new idea for the joint use of entomopathogenic fungi and gut bacteria to control pests.

Succinate Dehydrogenase Subunit C Contributes to Mycelial Growth and Development, Stress Response, and Virulence in the Insect Parasitic Fungus Beauveria bassiana

Succinate dehydrogenase (SDH), also known as respiratory chain complex II, plays a crucial role in energy production in which SdhC functions as an anchored subunit in the inner membrane of mitochondria. In this study, domain annotation analyses revealed that two SdhC domain-containing proteins were present in the filamentous insect-pathogenic fungus Beauveria bassiana, and they were named BbSdhC1 and BbSdhC2, respectively. Only BbSdhC1 localized to mitochondria; hence, this protein is considered the ortholog of SdhC in B. bassiana. Ablation of BbSdhC1 led to significantly reduced vegetative growth on various nutrients. The ΔBbsdhc1 mutant displayed the significantly reduced ATP synthesis and abnormal differentiation under aerial and submerged conditions. Notably, the BbSdhC1 loss resulted in enhanced intracellular levels of reactive oxygen species (ROS) and impaired growth of mycelia under oxidative stress. Finally, insect bioassays (via cuticle and intrahemocoel injection infection) revealed that disruption of BbSdhC1 significantly attenuated fungal virulence against the insect hosts. These findings indicate that BbSdhC1 contributes to vegetative growth, resistance to oxidative stress, differentiation, and virulence of B. bassiana due to its roles in energy generation and maintaining the homeostasis of the intracellular ROS levels. IMPORTANCE The electron transport chain (ETC) is critical for energy supply by mediating the electron flow along the mitochondrial membrane. Succinate dehydrogenase (SDH) is also known as complex II in the ETC, in which SdhC is a subunit anchored in mitochondrial membrane. However, the physiological roles of SdhC remain enigmatic in filamentous fungi. In filamentous insect-pathogenic fungus B. bassiana, SdhC is required for maintaining mitochondrial functionality, which is critical for fungal stress response, development, and pathogenicity. These findings improve our understanding of physiological mechanisms of ETC components involved in pathogenicity of the entomopathogenic fungi.

Ohr - OhrR, a neglected and highly efficient antioxidant system: Structure, catalysis, phylogeny, regulation, and physiological roles

Ohrs (organic hydroperoxide resistance proteins) are antioxidant enzymes that play central roles in the response of microorganisms to organic peroxides. Here, we describe recent advances in the structure, catalysis, phylogeny, regulation, and physiological roles of Ohr proteins and of its transcriptional regulator, OhrR, highlighting their unique features. Ohr is extremely efficient in reducing fatty acid peroxides and peroxynitrite, two oxidants relevant in host-pathogen interactions. The highly reactive Cys residue of Ohr, named peroxidatic Cys (C_p), composes together with an arginine and a glutamate the catalytic triad. The catalytic cycle of Ohrs involves a condensation between a sulfenic acid (C_p-SOH) and the thiol of the second conserved Cys, leading to the formation of an intra-subunit disulfide bond, which is then reduced by dihydrolipoamide or lipoylated proteins. A structural switch takes place during catalysis, with the opening and closure of the active site by the so-called Arg-loop. Ohr is part of the Ohr/OsmC super-family that also comprises OsmC and Ohr-like proteins. Members of the Ohr, OsmC and Ohr-like subgroups present low sequence similarities among themselves, but share a high structural conservation, presenting two Cys residues in their active site. The pattern of gene expression is also distinct among members of the Ohr/OsmC subfamilies. The expression of ohr genes increases upon organic hydroperoxides treatment, whereas the signals for the upregulation of osmC are entry into the stationary phase and/or osmotic stress. For many ohr genes, the upregulation by organic hydroperoxides is mediated by OhrR, a Cys-based transcriptional regulator that only binds to its target DNAs in its reduced state. Since Ohrs and OhrRs are involved in virulence of some microorganisms and are absent in vertebrate and vascular plants, they may represent targets for novel therapeutic approaches based on the disruption of this key bacterial organic peroxide defense system.

The Tudor Domain-Containing Protein BbTdp1 Contributes to Fungal Cell Development, the Cell Cycle, Virulence, and Transcriptional Regulation in the Insect Pathogenic Fungus Beauveria bassiana

Beauveria bassiana is an insect pathogenic fungus that serves as a model system for exploring the mechanisms of fungal development and host-pathogen interactions. Clinical and experimental studies have indicated that SND1 is closely correlated with the progression and invasiveness of common cancers as a potential oncogene, but this gene has rarely been studied in fungi. Here, we characterized the contributions of an SND1 ortholog (Tdp1) by constructing a BbTdp1 deletion strain and a complemented strain of B. bassiana. Compared with the wild-type (WT) strain, the ΔBbTdp1 mutant lost conidiation capacity (∼87.7%) and blastospore (∼96.3%) yields, increased sensitivity to chemical stress (4.4 to 54.3%) and heat shock (∼44.2%), and decreased virulence following topical application (∼24.7%) and hemocoel injection (∼40.0%). Flow cytometry readings showed smaller sizes of both conidia and blastospores for ΔBbTdp1 mutants. Transcriptomic data revealed 4,094 differentially expressed genes (|log2 ratio| > 2 and a q value of <0.05) between ΔBbTdp1 mutants and the WT strain, which accounted for 41.6% of the total genes, indicating that extreme fluctuation in the global gene expression pattern had occurred. Moreover, deletion of BbTdp1 led to an abnormal cell cycle with a longer S phase and shorter G2/M and G0/G1 phases of blastospores, and enzyme-linked immunosorbent assay confirmed that the level of phosphorylated cyclin-dependent kinase 1 (Cdk1) in the ΔBbTdp1 strain was ∼31.5% lower than in the WT strain. In summary, our study is the first to report that BbTdp1 plays a vital role in regulating conidia and blastospore yields, fungal morphological changes, and pathogenicity in entomopathogenic fungi. IMPORTANCE In this study, we used Beauveria bassiana as a biological model to report the role of BbTdp1 in entomopathogenic fungi. Our findings indicated that BbTdp1 contributed significantly to cell development, the cell cycle, and virulence in B. bassiana. In addition, deletion of BbTdp1 led to drastic fluctuations in the transcriptional profile. BbTdp1 can be developed as a novel target for B. bassiana development and pathogenicity, which also provides a framework for the study of Tdp1 in other fungi.

Vital roles of Pks11, a highly reducing polyketide synthase, in fungal conidiation, antioxidant activity, conidial cell wall integrity, and UV tolerance of Beauveria bassiana

Fungal polyketide synthases play important and differential roles in synthesizing secondary metabolites and regulating several cell events, including asexual development, environmental adaptation, and pathogenicity. This study shows the important functions of a highly reducing polyketide synthase, Pks11, in Beauveria bassiana, a filamentous fungal insect pathogen used worldwide for pest biocontrol. The deletion of pks11 led to severe defects in conidial yields on different media and a decrease of 36.27% in the mean thickness of conidial cell wall under normal conditions. Compared with the wild-type, Δpks11 showed higher tolerance to oxidation and increased sensitivity to high temperature during colony growth. Moreover, the lack of pks11 caused a decrease in conidial germination after exposure to UV radiation but did not affect the virulence of B. bassiana against Galleria mellonella larvae via typical cuticle infection. These findings concurred with the alteration in the transcript levels of some phenotype-related genes. These data suggested that pks11 played vital roles in the asexual development, cell wall integrity, and fungal responses to oxidation, high temperature, and UV irradiation of B. bassiana.

Analyses of transcriptomics and metabolomics reveal pathway of vacuolar Sur7 contributed to biocontrol potential of entomopathogenic Beauveria bassiana

Beauveria bassiana is a critical entomopathogenic fungus for pest biocontrol, whose efficiency depends on fungal development and stress resistance. Unlike its revealed location in plasma membrane patches in other organisms, B. bassiana Sur7 specifically localized in vacuoles. This vacuolar Sur7 was previously demonstrated to affect stress tolerance, hyphal development and virulence. There, however, remain more mechanistic details to be explored. In this study, transcriptomics and metabolomics were applied to investigate the mechanism of vacuolar Sur7. Analyses of transcriptomics and metabolomics displayed many differentially expressed genes and abundant metabolites in response to Sur7 loss, respectively. Together with genes associated with vacuolar biofunction (including transportation and hydrolysis), the altered metabolites contributed to cell wall construction and stress resistance. Particularly, an N-acetylglucosamine-associated Brg1/Nrg1 pathway was enriched and partially affected by Sur7. Absence of Sur7 changed the expression level of Brg1/Nrg1 pathway-related transcript factors, which interfered with downstream phenotype of sporulation. In addition, Sur7 was involved in the accumulation of sphingoid bases, which may affect sphingolipid-related signaling pathway. Although experimental evidence is further required, our studies provide a preliminary framework for future exploring the regulatory mechanism of Sur7, and give a new version of metabolic agency connecting Sur7 and downstream signaling pathway.

Transcriptomic analysis of Sur7-mediated response of Beauveria bassiana to different nutritional conditions

Integrity of the cell wall is requisite for fungal growth and function. Sur7 governs cell wall composition, and affects conidial sporulation and germination in Beauveria bassiana, a filamentous entomopathogenic fungus. The role of Sur7 in fungal growth on various nutrients remains unclear. We have previously reported that Sur7 deletion results in the attenuation of B. bassiana growth on supplemented Sabouraud dextrose agar (SDAY) and minimal Czapek-Dox agar (CDA) compared to wild type (WT). Here, we used transcriptomic analysis to compare WT and Sur7 mutant (ΔSur7) responses to CDA and SDAY. Growth on CDA, compared with that on SDAY, affected the expression of more genes in the WT than in the mutant. Differentially expressed genes were enriched for transportation process terms in the ΔSur7 mutant and metabolic process terms in the WT. Different processes were repressed in the ΔSur7 (metabolic process) and WT (ribosome synthesis) cells. Despite the shared enrichment of nitrogen metabolism genes, differentially expressed genes were enriched in distinct saccharide-energy metabolism terms in each strain. We conclude that Sur7 ensures the growth of B. bassiana in a minimal medium by influencing the expression of genes involved in the consumption of sucrose via specific energy metabolism pathways.

Screening of Beauveria bassiana with high biocontrol potential based on ARTP mutagenesis and high-throughput FACS

Beauveria bassiana is a promising biocontrol agent due to its entomopathogenic activities and residue-free characteristics. However, its susceptibility to abiotic stresses and naturally low virulence limit the effective application of this fungus. To effectively obtain fungal strains with high biocontrol potential, fluorescence-activated cell sorting (FACS) was used to screen mutant libraries generated by atmospheric and room temperature plasma (ARTP). Among about 8000 mutants obtained by ARTP mutagenesis, six candidate mutants were selected according to the forward scatter (FSC) signal readings of FACS. B6, with a 37.4% higher FSC reading than wild-type (WT), showed a 32.6% increase in virulence. It also presented a 13.5% decrease in median germinating time (GT₅₀) and a 12.1% increase in blastospore production. Comparative analysis between insect transcriptional responses to B6 and WT infection showed that the immune response coupled with protein digestion and absorption progress was highly activated in B6-infected Galleria mellonella larvae, while fatty acid synthesis was suppressed after 3 days of infection. Our results confirmed the feasibility of sorting B. bassiana with high biocontrol potential via the combination of ARTP and FACS and facilitated the understanding of insect-pathogen interactions, highlighting a new strategy for modifying entomopathogenic fungi to improve the efficiency of biological control.

Involvement of BbTpc1, an important Zn(II)2Cys6 transcriptional regulator, in chitin biosynthesis, fungal development and virulence of an insect mycopathogen

Chitin is one of the major components of the fungal cell wall and contributes to the mechanical strength and shape of the fungal cell. Zn(II)₂Cys₆ transcription factors are unique to the fungal kingdom and have a variety of functions in some fungi. However, the mechanisms by which Zn(II)₂Cys₆ proteins affect entomopathogenic fungi are largely unknown. Here, we characterized the Zn(II)₂Cys₆ transcription factor BbTpc1 in the insect pathogenic fungus Beauveria bassiana. Disruption of BbTpc1 resulted in a distinct changes in vegetative growth and septation patterns, and a significant decrease in conidia and blastospore yield. The ΔBbTpc1 mutant displayed impaired resistance to chemical stresses and heat shock and attenuated virulence in topical and intrahemocoel injection assays. Importantly, the ΔBbTpc1 mutant had an abnormal cell wall with altered wall thickness and chitin synthesis, which were accompanied by transcriptional repression of the chitin synthetase family genes. In addition, comparative transcriptomics revealed that deletion of BbTpc1 altered fungal asexual reproduction via different genetic pathways. These data revealed that BbTpc1 regulates fungal development, chitin synthesis and biological control potential in B. bassiana.

Smt3, a homologue of yeast SUMO, contributes to asexual development, environmental adaptation, and host infection of a filamentous entomopathogen

Small ubiquitin-like modifiers (SUMOs) act as the modifiers that regulate several important eukaryotic cell events during sumoylation, but little is known about the functions of SUMO or sumoylation in filamentous entomopathogens. Here, we report the important roles of a single SUMO-encoding gene, smt3, in Beauveria bassiana, a filamentous fungal insect pathogen that serves as a main source of wide-spectrum fungal insecticides. The deletion of smt3 led to significant growth defects on the minimal media with different carbon and nitrogen sources, an obvious reduction (45.7 %) in aerial conidiation during optimal cultivation, and increasing sensitivities to metal ions, oxidation, cell wall perturbation, and the fungicide carbendazim during conidial germination and/or colony growth. Compared with the wild-type, the percentage of germination of conidia stored at 4 °C decreased by 83.9 %, and virulence to Galleria mellonella via normal infection was delayed by 24.6 %. However, conidial thermotolerance increased slightly by 11.4 % in Δsmt3. These findings concurred with the repressed transcripts of some phenotype-related genes and decreased activities of antioxidant enzymes. Taken together, smt3 or sumoylation plays vital roles in the asexual development, environmental adaptation, and pathogenicity of B. bassiana.

Mitochondrial fission is necessary for mitophagy, development and virulence of the insect pathogenic fungus Beauveria bassiana

AIMS

Mitochondrial fission is an essential dynamics that maintains mitochondrial morphology and function. This study seeks to determine the roles of mitochondrial fission in the filamentous entomopathogenic fungus Beauveria bassiana.

MATERIAL AND METHODS

Three fission-related genes (BbFis1, BbMdv1 and BbDnm1) were functionally characterized via protein intracellular localization and construction of gene disruption mutants.

RESULTS

Mitochondrial localization was only observed for BbFis1 which interacts with BbMdv1, but BbMdv1 did not have interaction with BbDnm1. Single disruption mutant of three genes generated the elongated and enlarged mitochondria which could not be eliminated via the mitophagy. Three mutant strains displayed the reduced ATP synthesis and vegetative growth compared with the wild type. Three genes were involved in the early stage of conidiation and unnecessary for the late stage. However, all three genes significantly contribute to blastospore development under submerged condition, and the loss of BbMdv1 had the greatest effects compared with the losses of BbFis1 or BbDnm1. Finally, disruption of three genes significantly attenuated fungal virulence, but their mutations had different influences.

CONCLUSIONS

In addition to their consistent roles in mitochondrial division and mitophagy, three fission-related genes perform divergent roles in the development and virulence of the entomopathogenic fungus B. bassiana.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that mitochondrial fission is associated with lifecycle of B. bassiana. These findings provide information for the manipulation of fungal physiology and facilitate the application of entomopathogenic fungi.

MrArk1, an actin-regulating kinase gene, is required for endocytosis and involved in sustaining conidiation capacity and virulence in Metarhizium robertsii

Actin-regulating kinase (Ark) plays an important role in controlling endocytosis, which has been shown to be involved in the development and virulence of several fungal pathogens. However, it remains unclear whether Ark1 is required for the development and pathogenicity of an entomopathogenic fungus. Here, MrArk1 (MAA_03415), a homologue of yeast Ark1, was characterized in the insect pathogenic fungus, Metarhizium robertsii. Disruption of MrArk1 led to defects in endocytosis and a marked reduction (58%) in conidiation capacity. The reduced conidiation level was accompanied by repression of several key conidiation-related genes, including brlA, abaA, and wetA. Additionally, the deletion mutant showed a significant decrease in its tolerance to heat shock, but not to UV-B irradiation. Bioassays demonstrated attenuated virulence for the deletion mutant against Galleria mellonella via normal cuticle infection, accompanied by suppressed appressorium formation and reduced transcript levels of several genes involved in cuticle penetration. Taken together, our results indicate that MrArk1 is involved in the heat tolerance, sporulation, and virulence of M. robertsii, and thus is an important factor for sustaining the fungal potential against insect pests.

Autophagy-related gene ATG7 participates in the asexual development, stress response and virulence of filamentous insect pathogenic fungus Beauveria bassiana

Autophagy is a sophisticated mechanism for maintaining cellular homeostasis, in which E1-like enzyme (ATG7) controls the activation of ubiquitin-like conjugation system in the autophagy pathway. In the insect pathogenic fungus Beauveria bassiana, a yeast ortholog of ATG7 was identified and functionally analyzed. Ablation of BbATG7 gene blocks the autophagic process under starvation stress. The mutant ΔBbATG7 exhibited impaired growth on the media with chitin as single nitrogen source. On rich media, gene loss did not cause notable effect on vegetative growth, but resulted in a considerable reduction in conidiation (71.6%) and blastospore yield (61.1%) in the mutant. In addition, the ΔBbATG7 mutant displayed increased sensitivity to stress caused by menadione and Congo red. The virulence of ΔBbATG7 mutant was significantly attenuated as indicated in topical and intrahemocoel injection assays. Our study indicates that BbATG7 contributes to B. bassiana virulence via regulating autophagy pathway and playing non-autophagic functions in the infection cycle.