Characterization of Beauveria bassiana isolates from Kyrgyzstan

We report on the enzootic foci of the insect pathogenic fungus, Beauveria bassiana, found in high meadows in the middle mountain steppes of Kyrgyzstan, at elevations from 1000 m to 2200 m. The growth characteristics of various B. bassiana isolates on different media and as a function of temperature were studied. In addition, the ability of the fungal isolates to produce enzymes with amylase, protease and lipase activities was investigated. Dense biomass production on inexpensive solid media (oatmeal and bean oil meal) produced conidia used for insect bioassays targeting white grub larvae (Phyllophaga fullo, Coleoptera, Scarabaeidae) and nymphal and adult populations of whiteflies (Trialeurodes vaporariorum, Hemiptera, Aleyrodidae). The efficacies of the tested B. bassiana strains for third instar white grub larvae varied, with only two strains showing high entomopathogenic activity. At 25 °C, mortality reached 73% for Bav.5-Gal and 74% for Bav.1-Lep at 55 d post-infection, but was lower, 27% and 29%, respectively, at 12 °C. These two strains produced significantly higher mortality in adult and whitefly nymphs, with 65-75% mortality 6 d post-infection. Based on morphological characters, including production of ellipsoidal conidia, and molecular characters (ITS, partial 18S (SSU rDNA) and EF1-α sequences), the isolates were identified as Beauveria bassiana belonging to Clade E from Asia. Our results add to data on the diversity of ecosystems inhabited by B. bassiana and provide a local resource for pest control efforts.

Microbial Pyrrolnitrin: Natural Metabolite with Immense Practical Utility

Pyrrolnitrin (PRN) is a microbial pyrrole halometabolite of immense antimicrobial significance for agricultural, pharmaceutical and industrial implications. The compound and its derivatives have been isolated from rhizospheric fluorescent or non-fluorescent pseudomonads, Serratia and Burkholderia. They are known to confer biological control against a wide range of phytopathogenic fungi, and thus offer strong plant protection prospects against soil and seed-borne phytopathogenic diseases. Although chemical synthesis of PRN has been obtained using different steps, microbial production is still the most useful option for producing this metabolite. In many of the plant-associated isolates of Serratia and Burkholderia, production of PRN is dependent on the quorum-sensing regulation that usually involves N-acylhomoserine lactone (AHL) autoinducer signals. When applied on the organisms as antimicrobial agent, the molecule impedes synthesis of key biomolecules (DNA, RNA and protein), uncouples with oxidative phosphorylation, inhibits mitotic division and hampers several biological mechanisms. With its potential broad-spectrum activities, low phototoxicity, non-toxic nature and specificity for impacts on non-target organisms, the metabolite has emerged as a lead molecule of industrial importance, which has led to developing cost-effective methods for the biosynthesis of PRN using microbial fermentation. Quantum of work narrating focused research efforts in the emergence of this potential microbial metabolite is summarized here to present a consolidated, sequential and updated insight into the chemistry, biology and applicability of this natural molecule.

Meta-analysis of the role of entomopathogenic and unspecialized fungal endophytes as plant bodyguards

Herbaceous plants harbour species-rich communities of asymptomatic endophytic fungi. Although some of these endophytes are entomopathogenic, many are not, and remarkably little is known about how the presence of these fungi in plant tissues affects phytophagous insects. Here we show through a meta-analysis that both entomopathogenic and nonentomopathogenic endophytes have a negative effect on insect herbivores. Growth and performance of polyphagous and sucking insects are reduced by nonentomopathogenic endophytes, but monophages are unaffected, likely because the latter are better adapted to secondary metabolites produced or induced by the fungi. Furthermore, studies using excised leaves report weaker effects than those with intact plants, likely caused by chemical changes being masked by leaf excision. Most surprisingly, endophyte infection of seeds produces the greatest effect on insect herbivores in subsequent mature plants, even though the usual mode of fungal transmission is infection of leaves by airborne spores. We conclude that these ubiquitous hidden fungi may be important bodyguards of plants. However, in order to fully understand their roles in plant protection, we must be aware that minor differences in experimental design can lead to contradictory results.

Genome sequence of Isaria javanica and comparative genome analysis insights into family S53 peptidase evolution in fungal entomopathogens

The fungus Isaria javanica is an important entomopathogen that parasitizes various insects and is effective for pest control. In this study, we sequenced and assembled the genomes (IJ1G and IJ2G) of two I. javanica strains isolated from different insects. The genomes were approximately 35 Mb in size with 11,441 and 11,143 protein-coding genes, respectively. Using a phylogenomic approach, we evaluated genome evolution across five entomopathogenic fungi in Cordycipitaceae. By comparative genome analysis, it was found that family S53 serine peptidases were expanded in Cordycipitaceae entomopathogens, particularly in I. javanica. Gene duplication events were identified based on phylogenetic relationships inferred from 82 S53 peptidases within six entomopathogenic fungal genomes. Moreover, we found that carbohydrate-active enzymes and proteinases were the largest secretory protein groups encoded in the I. javanica genome, especially chitinases (GH18), serine and aspartic peptidases (S53, S08, S10, A01). Pathogenesis-related genes and genes for bacterial-like toxins and secondary metabolites were also identified. By comparative transcriptome analysis, differentially expressed genes in response to insect nutrients (in vitro) were identified. Moreover, most S53 peptidases were detected to be significantly upregulated during the initial fungal infection process in insects (in vivo) by RT-qPCR. Our results provide new clues about understanding evolution of pathogenic proteases and may suggest that abundant S53 peptidases in the I. javanica genome may contribute to its effective parasitism on various insects.

In vivo transcriptomic analysis of Beauveria bassiana reveals differences in infection strategies in Galleria mellonella and Plutella xylostella

BACKGROUND

Insect pests have evolved various defense mechanisms to combat fungal infection, and fungi have developed multiple strategies to overcome the immune defense responses of insects. However, transcriptomic analysis of fungal strategies for infecting different pests has not been reported.

RESULTS

Transcriptomic profiling of Beauveria bassiana was performed at 12, 24 and 48 h after infecting Galleria mellonella and Plutella xylostella, and 540, 847 and 932 differentially expressed genes were detected, respectively. Functional categorization showed that most of these genes are involved in the ribosome, nitrogen metabolism and oxidative phosphorylation pathways. Thirty-one differentially expressed virulence genes (including genes involved in adhesion, degradation, host colonization and killing, and secondary metabolism) were found, suggesting that different molecular mechanisms were used by the fungus during the infection of different pests, which was further confirmed by disrupting creA and fkh2. Virulence assay results showed that ΔcreA and Δfkh2 strains of B. bassiana had distinct fold changes in their 50% lethal time (LT₅₀ ) values (compared with the control stains) during infection of G. mellonella (ΔcreA: 1.38-fold > Δfkh2: 1.18-fold) and P. xylostella (ΔcreA: 1.44-fold < Δfkh2: 2.25-fold). creA was expressed at higher levels during the infection of G. mellonella compared with P. xylostella, whereas fkh2 showed the opposite expression pattern, demonstrating that creA and Fkh2 have different roles in B. bassiana during the infection of G. mellonella and P. xylostella.

CONCLUSION

These findings demonstrate that B. bassiana regulates different genes to infect different insects, advancing knowledge of the molecular mechanisms of Beauveria-pest interactions. © 2018 Society of Chemical Industry.

Distribution and Evolution of Nonribosomal Peptide Synthetase Gene Clusters in the Ceratocystidaceae

In filamentous fungi, genes in secondary metabolite biosynthetic pathways are generally clustered. In the case of those pathways involved in nonribosomal peptide production, a nonribosomal peptide synthetase (NRPS) gene is commonly found as a main element of the cluster. Large multifunctional enzymes are encoded by members of this gene family that produce a broad spectrum of bioactive compounds. In this research, we applied genome-based identification of nonribosomal peptide biosynthetic gene clusters in the family Ceratocystidaceae. For this purpose, we used the whole genome sequences of species from the genera Ceratocystis,Davidsoniella,Thielaviopsis, Endoconidiophora,Bretziella, Huntiella, and Ambrosiella. To identify and characterize the clusters, different bioinformatics and phylogenetic approaches, as well as PCR-based methods were used. In all genomes studied, two highly conserved NRPS genes (one monomodular and one multimodular) were identified and their potential products were predicted to be siderophores. Expression analysis of two Huntiella species (H. moniliformis and H. omanensis) confirmed the accuracy of the annotations and proved that the genes in both clusters are expressed. Furthermore, a phylogenetic analysis showed that both NRPS genes of the Ceratocystidaceae formed distinct and well supported clades in their respective phylograms, where they grouped with other known NRPSs involved in siderophore production. Overall, these findings improve our understanding of the diversity and evolution of NRPS biosynthetic pathways in the family Ceratocystidaceae.

Evaluation of genotoxic and cytotoxic effects of ethyl acetate extract of Aspergillus flavus on Spodoptera litura

AIM

Recent concerns about the possible adverse effects of agricultural chemicals on health and environment have generated a considerable interest in biological alternatives. This study aimed to test the insecticidal potential of fungus Aspergillus flavus and revealed its genotoxic and cytotoxic effects using Spodoptera litura (Fabricius) as a model.

METHODS AND RESULTS

The fungus was isolated from the surface of the dead insect and investigated for its insecticidal potential against S. litura by bioassay studies. Significant increase in mortality, prolonged development period and reduced adult emergence in S. litura were observed in larva fed on diet supplemented with fungal extract. In addition, fungus was also found to cause oxidative stress, DNA damage and cell death. Significantly higher percentages of necrotic cells and DNA damage were observed in larvae treated with fungal extract. Furthermore, DNA repair studies predicted the longevity of toxic effects induced by fungus. Phytochemical and ultra-high performance liquid chromatography studies revealed the presence of phenolic compounds in the extract and liquid chromatography-mass spectrometry indicated it to be a non-aflatoxin strain of A. flavus. Fungal extract was less toxic to mammalian cell lines as compared to cytotoxic drug doxorubicin (DOX) in the MTT assay.

CONCLUSION

The study highlights the insecticidal potential of A. flavus by revealing its genotoxicity and cytotoxicity causing potential. This is the first report showing the genotoxic and cytotoxic effects of the fungus A. flavus on S. litura.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study provides a useful insight to explore microbial agents as biopesticides in order to reduce various environmental as well as human health problems due to synthetic pesticides.

Beauvetetraones A–C, phomaligadione-derived polyketide dimers from the entomopathogenic fungus, Beauveria bassiana

Beauvetetraones A–C, phomaligadione-derived polyketide dimers, were isolated from the entomopathogenic fungus Beauveria bassiana using LC/MS-guided isolation and their putative biosynthetic pathway is proposed.

Dual Biocontrol Potential of the Entomopathogenic Fungus, Isaria javanica, for Both Aphids and Plant Fungal Pathogens

Dual biocontrol of both insects and plant pathogens has been reported for certain fungal entomopathogens, including Beauveria bassiana and Lecanicillum spp. In this study, we demonstrate, for the first time, the dual biocontrol potential of two fungal isolates identified by morphological and phylogenetic analyses as Isaria javanica. Both these isolates caused mortality in the greater wax moth, and hence can be considered entomopathogens. Spores of the isolates were also pathogenic to nymphs of the green peach aphid (Myzus persicae), with an LC50 value of 107 spores/mL 4 days after inoculation and an LT50 of 4.2 days with a dose of 108 spores/mL. In vitro antifungal assays also demonstrated a strong inhibitory effect on the growth of two fungi that are pathogenic to peppers, Colletotrichum gloeosporioides and Phytophthora capsici. These results indicate that I. javanica isolates could be used as novel biocontrol agents for the simultaneous control of aphids and fungal diseases, such as anthracnose and Phytophthora blight, in an integrated pest management framework for red pepper.

The Inhibitory Effects of Cyclodepsipeptides from the Entomopathogenic Fungus Beauveria bassiana on Myofibroblast Differentiation in A549 Alveolar Epithelial Cells

Pulmonary fibrosis (PF) is a chronic and fatal lung disease with few treatment options. Although the pathogenesis of PF is not clear, a chronic inflammatory response to continuous damage is considered the cause of pulmonary fibrosis. PF is characterized by excessive accumulation of extracellular matrix (ECM), therefore, inhibition of myofibroblast differentiation is a good therapeutic target for PF. As part of our continuing endeavor to explore biologically active metabolites from insect-associated microbes, we found that the MeOH extract of the culture broth from the entomopathogenic fungus Beauveria bassiana inhibited collagen induction and E-cadherin down-regulation. In order to identify active compounds, we carried out chemical analysis of the MeOH extract with the assistance of LC/MS-guided isolation approach, which led to the successful identification of four cyclodepsipeptides 1–4. Among the isolates, compound 2 showed inhibitory effects on myofibroblast differentiation induced by TGF-β1. Compound 2 inhibited induction of α-SMA and N-cadherin, which are myofibroblast markers, and blocked the accumulation of ECM proteins such as collagen and fibronectin. Overall these findings demonstrate that compound 2 can be used to attenuate pulmonary fibrosis by targeting myo- fibroblast differentiation.

Consortium of five fungal isolates conditioning root growth and arbuscular mycorrhiza in soybean, corn, and sugarcane

Plant growth and arbuscular mycorrhizal colonization were studied in sugarcane, corn and soybean by applying five plant growth promoting fungi: Beauveria bassiana, Metarhizium anisopliae, Pochonia chlamydosporia, Purpureocillium lilacinum, and Trichoderma asperella. Sugarcane, corn and soybean were grown in pots under two treatments: (1) inoculation with the fungal consortium and (2) control without inoculation. In the inoculated treatment, fungal spore suspension were applied to the seeds and shoots were sprayed every 28 days. Means were analyzed by analysis of variance and Tukey's test at 5% probability level. The experiment was arranged in a completely randomized design, with six replications. Fungi consortium mediate root growth in soybean and corn, and arbuscular mycorrhizal colonization in soybean and sugarcane. These findings are probably caused by the fungi producing phytohormones and inducing the plants to synthesize phytohormones: auxins for root growth; and jasmonic, abscisic, and salicylic acids with a role in the regulation of mycorrhizal colonization. These effects are important when seeking conservation strategies in agriculture and livestock production, since Fungi consortium can better mediate soil resource acquisition, promoting greater absorption of nutrients and water.

Interaction of entomopathogenic fungi with the host immune system

Entomopathogenic fungi can invade wide range of insect hosts in the natural world and have been used as environmentally friendly alternatives to chemical insecticides for pest control. Studies of host-pathogen interactions provide valuable insights into the coevolutionay arms race between fungal pathogens and their hosts. Entomopathogenic fungi have evolved a series of sophisticated strategies to counter insect immune defenses. In response to fungal infection, insect hosts rely on behavior avoidance, physical barrier and innate immune defenses in the fight against invading pathogens. The insect cuticle acts as the first physical barrier against pathogens. It is an inhospitable physiological environment that contains chemicals (e.g., antimicrobial peptides and reactive oxygen species), which inhibit fungal growth. In addition, innate immune responses, including cellular immunity and humoral immunity, play critical roles in preventing fungal infection. In this review, we outline the current state of our knowledge of insect defenses to fungal infection and discuss the strategies by which entomopathogenic fungi counter the host immune system. Increased knowledge regarding the molecular interactions between entomopathogenic fungi and the insect host could provide new strategies for pest management.

Microbiota in insect fungal pathology

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

Genetically engineering better fungal biopesticides

Microbial insect pathogens offer an alternative means of pest control with the potential to wean us off our heavy reliance on chemical pesticides. Insect pathogenic fungi play an important natural role in controlling disease vectors and agricultural pests. Most commercial products employ Ascomycetes in the genera Metarhizium and Beauveria. However, their utilization has been limited by inconsistent field results as a consequence of sensitivity to abiotic stresses and naturally low virulence. Other naturally occurring biocontrol agents also face these hurdles to successful application, but the availability of complete genomes and recombinant DNA technologies have facilitated design of multiple fungal pathogens with enhanced virulence and stress resistance. Many natural and synthetic genes have been inserted into entomopathogen genomes. Some of the biggest gains in virulence have been obtained using genes encoding neurotoxic peptides, peptides that manipulate host physiology and proteases and chitinases that degrade the insect cuticle. Prokaryotes, particularly extremophiles, are useful sources of genes for improving entomopathogen resistance to ultraviolet (UV) radiation. These biological insecticides are environmentally friendly and cost-effective insect pest control options. © 2017 Society of Chemical Industry.

Hydrophobins contribute to root colonization and stress responses in the rhizosphere-competent insect pathogenic fungus Beauveria bassiana

The hyd1/hyd2 hydrophobins are important constituents of the conidial cell wall of the insect pathogenic fungus Beauveria bassiana. This fungus can also form intimate associations with several plant species. Here, we show that inactivation of two Class I hydrophobin genes, hyd1 or hyd2, significantly decreases the interaction of B. bassiana with bean roots. Curiously, the ∆hyd1/∆hyd2 double mutant was less impaired in root association than Δhyd1 or Δhyd2. Loss of hyd genes affected growth rate, conidiation ability and oosporein production. Expression patterns for genes involved in conidiation, cell wall integrity, insect virulence, signal transduction, adhesion, hydrophobicity and oosporein production were screened in the deletion mutants grown in different conditions. Repression of the major MAP-Kinase signal transduction pathways (Slt2 MAPK pathway) was observed that was more pronounced in the single versus double hyd mutants under certain conditions. The ∆hyd1/∆hyd2 double mutant showed up-regulation of the Hog1 MAPK and the Msn2 transcription factor under certain conditions when compared to the wild-type or single hyd mutants. The expression of the bad2 adhesin and the oosporein polyketide synthase 9 gene was severely reduced in all of the mutants. On the other hand, fewer changes were observed in the expression of key conidiation and cell wall integrity genes in hyd mutants compared to wild-type. Taken together, the data from this study indicated pleiotropic consequences of deletion of hyd1 and hyd2 on signalling and stress pathways as well as the ability of the fungus to form stable associations with plant roots.

Strategies to diversify natural products for drug discovery

Natural product libraries contain specialized metabolites derived from plants, animals, and microorganisms that play a pivotal role in drug discovery due to their immense structural diversity and wide variety of biological activities. The strategies to greatly extend natural product scaffolds through available biological and chemical approaches offer unique opportunities to access a new series of natural product analogues, enabling the construction of diverse natural product-like libraries. The affordability of these structurally diverse molecules has been a crucial step in accelerating drug discovery. This review provides an overview of various approaches to exploit the diversity of compounds for natural product-based drug development, drawing upon a series of examples to illustrate each strategy.

Isaria fumosorosea KCh J2 Entomopathogenic Strain as an Effective Biocatalyst for Steroid Compound Transformations

The catalytic activity of enzymes produced by an entomopathogenic filamentous fungus (Isaria fumosorosea KCh J2) towards selected steroid compounds (androstenedione, adrenosterone, progesterone, 17α-methyltestosterone and dehydroepiandrosterone) was investigated. All tested substrates were efficiently transformed. The structure of the substrate has a crucial impact on regio- and stereoselectivity of hydroxylation since it affects binding to the active site of the enzyme. Androstenedione was hydroxylated in the 7α-position to give a key intermediate in the synthesis of the diuretic-7α-hydroxyandrost-4-ene-3,17-dione with 82% conversion. Adrenosterone and 17α-methyltestosterone were hydroxylated in the 6β-position. Hydroxylated derivatives such as 15β-hydroxy-17α-methyltestosterone and 6β,12β-dihydroxy-17α-methyltestosterone were also observed. In the culture of Isaria fumosorosea KCh J2, DHEA was effectively hydroxylated in the C-7 position and then oxidized to give 7-oxo-DHEA, 3β,7α- and 3β,7β-dihydroxy-17a-oxa-d-homo-androst-5-ene-17-one. We obtained 7β-OH-DHEA lactone with 82% yield during 3 days transformation of highly concentrated (5 g/L) DHEA.

Genome-Wide Analysis of Secondary Metabolite Gene Clusters in Ophiostoma ulmi and Ophiostoma novo-ulmi Reveals a Fujikurin-Like Gene Cluster with a Putative Role in Infection

The emergence of new microbial pathogens can result in destructive outbreaks, since their hosts have limited resistance and pathogens may be excessively aggressive. Described as the major ecological incident of the twentieth century, Dutch elm disease, caused by ascomycete fungi from the Ophiostoma genus, has caused a significant decline in elm tree populations (Ulmus sp.) in North America and Europe. Genome sequencing of the two main causative agents of Dutch elm disease (Ophiostoma ulmi and Ophiostoma novo-ulmi), along with closely related species with different lifestyles, allows for unique comparisons to be made to identify how pathogens and virulence determinants have emerged. Among several established virulence determinants, secondary metabolites (SMs) have been suggested to play significant roles during phytopathogen infection. Interestingly, the secondary metabolism of Dutch elm pathogens remains almost unexplored, and little is known about how SM biosynthetic genes are organized in these species. To better understand the metabolic potential of O. ulmi and O. novo-ulmi, we performed a deep survey and description of SM biosynthetic gene clusters (BGCs) in these species and assessed their conservation among eight species from the Ophiostomataceae family. Among 19 identified BGCs, a fujikurin-like gene cluster (OpPKS8) was unique to Dutch elm pathogens. Phylogenetic analysis revealed that orthologs for this gene cluster are widespread among phytopathogens and plant-associated fungi, suggesting that OpPKS8 may have been horizontally acquired by the Ophiostoma genus. Moreover, the detailed identification of several BGCs paves the way for future in-depth research and supports the potential impact of secondary metabolism on Ophiostoma genus’ lifestyle.

Lack of resistance development in Bemisia tabaci to Isaria fumosorosea after multiple generations of selection

The emergence of insecticide resistant insect pests is of significant concern worldwide. The whitefly, Bemisia tabaci, is an important agricultural pest and has shown incredible resilience developing resistance to a number of chemical pesticides. Entomopathogenic fungi such as Isaria fumosorosea offer an attractive alternative to chemical pesticides for insect control, and this fungus has been shown to be an effective pathogen of B. tabaci. Little is known concerning the potential for the development of resistance to I. fumosorosea by B. tabaci. Five generations of successive survivors of B. tabaci infected by I. fumosorosea were assayed with I. fumosorosea. No significant differences in susceptibility to I. fumosorosea, number of ovarioles, or ovipostioning were seen between any of the generations tested. Effects of I. fumosorosea and cell-free ethyl acetate fractions derived from the fungus on the B. tabaci fat body, ovary, and vitellogenin were also investigated. These data revealed significant deformation and degradation of ovary tissues and associated vitellogenin by the fungal mycelium as well as by cell-free ethyl acetate fungal extracts. These data indicate the lack of the emergence of resistance to I. fumosorosea under the conditions tested and demonstrate invasion of the insect reproductive tissues during fungal infection.

Regulatory cascade and biological activity of Beauveria bassiana oosporein that limits bacterial growth after host death

The regulatory network and biological functions of the fungal secondary metabolite oosporein have remained obscure. Beauveria bassiana has evolved the ability to parasitize insects and outcompete microbial challengers for assimilation of host nutrients. A novel zinc finger transcription factor, BbSmr1 (B. bassiana secondary metabolite regulator 1), was identified in a screen for oosporein overproduction. Deletion of Bbsmr1 resulted in up-regulation of the oosporein biosynthetic gene cluster (OpS genes) and constitutive oosporein production. Oosporein production was abolished in double mutants of Bbsmr1 and a second transcription factor, OpS3, within the oosporein gene cluster (ΔBbsmr1ΔOpS3), indicating that BbSmr1 acts as a negative regulator of OpS3 expression. Real-time quantitative PCR and a GFP promoter fusion construct of OpS1, the oosporein polyketide synthase, indicated that OpS1 is expressed mainly in insect cadavers at 24-48 h after death. Bacterial colony analysis in B. bassiana-infected insect hosts revealed increasing counts until host death, with a dramatic decrease (∼90%) after death that correlated with oosporein production. In vitro studies verified the inhibitory activity of oosporein against bacteria derived from insect cadavers. These results suggest that oosporein acts as an antimicrobial compound to limit microbial competition on B. bassiana-killed hosts, allowing the fungus to maximally use host nutrients to grow and sporulate on infected cadavers.

The PacC transcription factor regulates secondary metabolite production and stress response, but has only minor effects on virulence in the insect pathogenic fungus Beauveria bassiana

The PacC transcription factor is an important component of the fungal ambient pH-responsive regulatory system. Loss of pacC in the insect pathogenic fungus Beauveria bassiana resulted in an alkaline pH-dependent decrease in growth and pH-dependent increased susceptibility to osmotic (salt, sorbitol) stress and SDS. Extreme susceptibility to Congo Red was noted irrespective of pH, and ΔBbpacC conidia showed subtle increases in UV susceptibility. The ΔBbPacC mutant showed a reduced ability to acidify media during growth due to failure to produce oxalic acid. The ΔBbPacC mutant also did not produce the insecticidal compound dipicolinic acid, however, production of a yellow-colored compound was noted. The compound, named bassianolone B, was purified and its structure determined. Despite defects in growth, stress resistance, and oxalate/insecticidal compound production, only a small decrease in virulence was seen for the ΔBbpacC strain in topical insect bioassays using larvae from the greater waxmoth, Galleria mellonella or adults of the beetle, Tenebrio molitor. However, slightly more pronounced decreases were seen in virulence via intrahemcoel injection assays (G. mellonella) and in assays using T. molitor larvae. These data suggest important roles for BbpacC in mediating growth at alkaline pH, regulating secondary metabolite production, and in targeting specific insect stages.

Metabolic Conservation and Diversification of Metarhizium Species Correlate with Fungal Host-Specificity

The ascomycete genus Metarhizium contains several species of insect pathogenic fungi ranging from specialists with narrow host ranges to generalists that can infect diverse invertebrates. Genetic and metabolic conservations and diversifications of Metarhizium species are not well understood. In this study, using the genome information of seven Metarhizium species, we performed a comparative analysis of gene clusters involved in secondary metabolisms (SMs) in these species. The results revealed that the generalist species contain more SM gene clusters than the specialists, and that both conserved and divergent evolutions may have occurred in SM genes during fungal speciation. In particular, the loss/gain events, as well as gene mutagenesis, are evident for the gene cluster responsible for the biosynthesis of non-ribosomal cyclopeptide destruxins. The presence of conserved SM gene clusters in Metarhizium and other divergently evolved insect pathogenic fungi implies their link to fungal entomopathogenicity. Mass spectrometry based metabolomic analyses were also conducted to investigate the chemical diversities of seven Metarhizium species. Consistent with the evolutionary relationships of SM genes among the seven species, significant differences are observed in fungal metabolic profiles, whether the same or different metabolites are produced in different species. Clustering analysis based on the metabolome data revealed that Metarhizium species could be grouped based on their association to fungal host specificity. Our metabolomics-based methods also facilitate the identification of bioactive metabolites that have not been reported previously in Metarhizium. The results of this study will benefit future investigations of the chemical biology of insect-fungal interactions.

Disruptions of the genes involved in lysine biosynthesis, iron acquisition, and secondary metabolisms affect virulence and fitness in Metarhizium robertsii

Based on genomic analysis, polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) pathways account for biosynthesis of the majority of the secondary metabolites produced by the entomopathogenic fungus Metarhizium robertsii. To evaluate the contribution of these pathways to M. robertsii fitness and/or virulence, mutants deleted for mrpptA, the Sfp-type 4' phosphopantetheinyl transferase gene required for their activation were generated. ΔmrpptA strains were deficient in PKS and NRPS activity resulting in colonies that lacked the typical green pigment and failed to produce the nonribosomal peptides (destruxins, serinocylins, and the siderophores ferricrocin and metachelins) as well as the hybrid polyketide-peptides (NG-39x) that are all produced by the wild type (WT) M. robertsii. The ΔmrpptA colonies were also auxotrophic for lysine. Two other mutant strains were generated: ΔmraarA, in which the α-aminoadipate reductase gene critical for lysine biosynthesis was disrupted, and ΔmrsidA, in which the L-ornithine N⁵-oxygenase gene that is critical for hydroxamate siderophore biosynthesis was disrupted. The phenotypes of these mutants were compared to those of ΔmrpptA to separate effects of the loss of lysine or siderophore production from the overall effect of losing all polyketide and non-ribosomal peptide production. Loss of lysine biosynthesis marginally increased resistance to H₂O₂ while it had little effect on the sensitivity to the cell wall disruptor sodium dodecyl sulfate (SDS) and no effect on sensitivity to iron deprivation. In contrast, combined loss of metachelin and ferricrocin through the inactivation of mrsidA resulted in mutants that were as hypersensitive or slightly more sensitive to H₂O₂, iron deprivation, and SDS, and were either identical or marginally higher in ΔmrpptA strains. In contrast to ΔmrpptA, loss of mrsidA did not completely abolish siderophore activity, which suggests the production of one or more non-hydroxamate iron-chelating compounds. Deletion of mrpptA, mrsidA, and mraarA reduced conidium production and conidia of a GFP-tagged ΔmrpptA strain displayed a longer germination delay than WT on insect cuticles, a deficiency that was rescued by lysine supplementation. Compared with WT, ΔmrpptA strains displayed ∼19-fold reduction in virulence against Drosophila suzukii. In contrast, lysine auxotrophy and loss of siderophores accounted for ∼2 and ∼6-fold decreases in virulence, respectively. Deletion of mrpptA had no significant effect on growth inhibition of Bacillus cereus. Our results suggest that PKS and NRPS metabolism plays a significant role in M. robertsii virulence, depresses conidium production, and contributes marginally to resistance to oxidative stress and iron homeostasis, but has no significant antibacterial effect.

Secondary metabolite gene clusters in the entomopathogen fungus Metarhizium anisopliae: genome identification and patterns of expression in a cuticle infection model

Background

The described species from the Metarhizium genus are cosmopolitan fungi that infect arthropod hosts. Interestingly, while some species infect a wide range of hosts (host-generalists), other species infect only a few arthropods (host-specialists). This singular evolutionary trait permits unique comparisons to determine how pathogens and virulence determinants emerge. Among the several virulence determinants that have been described, secondary metabolites (SMs) are suggested to play essential roles during fungal infection. Despite progress in the study of pathogen-host relationships, the majority of genes related to SM production in Metarhizium spp. are uncharacterized, and little is known about their genomic organization, expression and regulation. To better understand how infection conditions may affect SM production in Metarhizium anisopliae, we have performed a deep survey and description of SM biosynthetic gene clusters (BGCs) in M. anisopliae, analyzed RNA-seq data from fungi grown on cattle-tick cuticles, evaluated the differential expression of BGCs, and assessed conservation among the Metarhizium genus. Furthermore, our analysis extended to the construction of a phylogeny for the following three BGCs: a tropolone/citrinin-related compound (MaPKS1), a pseurotin-related compound (MaNRPS-PKS2), and a putative helvolic acid (MaTERP1).

Results

Among 73 BGCs identified in M. anisopliae, 20 % were up-regulated during initial tick cuticle infection and presumably possess virulence-related roles. These up-regulated BGCs include known clusters, such as destruxin, NG39x and ferricrocin, together with putative helvolic acid and, pseurotin and tropolone/citrinin-related compound clusters as well as uncharacterized clusters. Furthermore, several previously characterized and putative BGCs were silent or down-regulated in initial infection conditions, indicating minor participation over the course of infection.

Interestingly, several up-regulated BGCs were not conserved in host-specialist species from the Metarhizium genus, indicating differences in the metabolic strategies employed by generalist and specialist species to overcome and kill their host. These differences in metabolic potential may have been partially shaped by horizontal gene transfer (HGT) events, as our phylogenetic analysis provided evidence that the putative helvolic acid cluster in Metarhizium spp. originated from an HGT event.

Conclusions

Several unknown BGCs are described, and aspects of their organization, regulation and origin are discussed, providing further support for the impact of SM on the Metarhizium genus lifestyle and infection process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3067-6) contains supplementary material, which is available to authorized users.

The production and uses of Beauveria bassiana as a microbial insecticide

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

Fungi with multifunctional lifestyles: endophytic insect pathogenic fungi

This review examines the symbiotic, evolutionary, proteomic and genetic basis for a group of fungi that occupy a specialized niche as insect pathogens as well as endophytes. We focus primarily on species in the genera Metarhizium and Beauveria, traditionally recognized as insect pathogenic fungi but are also found as plant symbionts. Phylogenetic evidence suggests that these fungi are more closely related to grass endophytes and diverged from that lineage ca. 100 MYA. We explore how the dual life cycles of these fungi as insect pathogens and endophytes are coupled. We discuss the evolution of insect pathogenesis while maintaining an endophytic lifestyle and provide examples of genes that may be involved in the transition toward insect pathogenicity. That is, some genes for insect pathogenesis may have been co-opted from genes involved in endophytic colonization. Other genes may be multifunctional and serve in both lifestyle capacities. We suggest that their evolution as insect pathogens allowed them to effectively barter a specialized nitrogen source (i.e. insects) with host plants for photosynthate. These ubiquitous fungi may play an important role as plant growth promoters and have a potential reservoir of secondary metabolites.

Insights into Adaptations to a Near-Obligate Nematode Endoparasitic Lifestyle from the Finished Genome of Drechmeria coniospora

Nematophagous fungi employ three distinct predatory strategies: nematode trapping, parasitism of females and eggs, and endoparasitism. While endoparasites play key roles in controlling nematode populations in nature, their application for integrated pest management is hindered by the limited understanding of their biology. We present a comparative analysis of a high quality finished genome assembly of Drechmeria coniospora, a model endoparasitic nematophagous fungus, integrated with a transcriptomic study. Adaptation of D. coniospora to its almost completely obligate endoparasitic lifestyle led to the simplification of many orthologous gene families involved in the saprophytic trophic mode, while maintaining orthologs of most known fungal pathogen-host interaction proteins, stress response circuits and putative effectors of the small secreted protein type. The need to adhere to and penetrate the host cuticle led to a selective radiation of surface proteins and hydrolytic enzymes. Although the endoparasite has a simplified secondary metabolome, it produces a novel peptaibiotic family that shows antibacterial, antifungal and nematicidal activities. Our analyses emphasize the basic malleability of the D. coniospora genome: loss of genes advantageous for the saprophytic lifestyle; modulation of elements that its cohort species utilize for entomopathogenesis; and expansion of protein families necessary for the nematode endoparasitic lifestyle.

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.

Natural products from microbes associated with insects

Here we review discoveries of secondary metabolites from microbes associated with insects. We mainly focus on natural products, where the ecological role has been at least partially elucidated, and/or the pharmaceutical properties evaluated, and on compounds with unique structural features. We demonstrate that the exploration of specific microbial–host interactions, in combination with multidisciplinary dereplication processes, has emerged as a successful strategy to identify novel chemical entities and to shed light on the ecology and evolution of defensive associations.

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.

Exploitation of Fungal Biodiversity for Discovery of Novel Antibiotics

Fungi were among the first sources for antibiotics. The discovery and development of the penicillin-type and cephalosporin-type β-lactams and their synthetic versions were transformative in emergence of the modern pharmaceutical industry. They remain some of the most important antibiotics, even 70 years after their discovery. Meanwhile, thousands of fungal metabolites have been discovered, yet these metabolites have only contributed a few additional compounds that have entered clinical development. Substantial expansion in fungal biodiversity assessment along with the availability of modern "-OMICS" technology and revolutionary developments in fungal biotechnology have been made in the last 15 years subsequent to the exit of most of the big Pharma companies from the field of novel antibiotics discovery. Therefore, the timing seems opportune to revisit these fascinating chemically rich organisms as a reservoir of small-molecule templates for lead discovery. This review will describe ongoing interdisciplinary scenarios in which specialists in fungal biology collaborate with chemists, pharmacologists and biochemical and process engineers in order to reveal and make new antibiotics. The utility of a pre-selection process based on phylogenetic data and distribution of secondary metabolite encoding gene cluster will be highlighted. Examples of novel bioactive metabolites from fungi derived from special ecological groups and new phylogenetic lineages will also be discussed.

Evolution of Chemical Diversity in a Group of Non-Reduced Polyketide Gene Clusters: Using Phylogenetics to Inform the Search for Novel Fungal Natural Products

Fungal polyketides are a diverse class of natural products, or secondary metabolites (SMs), with a wide range of bioactivities often associated with toxicity. Here, we focus on a group of non-reducing polyketide synthases (NR-PKSs) in the fungal phylum Ascomycota that lack a thioesterase domain for product release, group V. Although widespread in ascomycete taxa, this group of NR-PKSs is notably absent in the mycotoxigenic genus Fusarium and, surprisingly, found in genera not known for their secondary metabolite production (e.g., the mycorrhizal genus Oidiodendron, the powdery mildew genus Blumeria, and the causative agent of white-nose syndrome in bats, Pseudogymnoascus destructans). This group of NR-PKSs, in association with the other enzymes encoded by their gene clusters, produces a variety of different chemical classes including naphthacenediones, anthraquinones, benzophenones, grisandienes, and diphenyl ethers. We discuss the modification of and transitions between these chemical classes, the requisite enzymes, and the evolution of the SM gene clusters that encode them. Integrating this information, we predict the likely products of related but uncharacterized SM clusters, and we speculate upon the utility of these classes of SMs as virulence factors or chemical defenses to various plant, animal, and insect pathogens, as well as mutualistic fungi.

Epigenetic genome mining of an endophytic fungus leads to the pleiotropic biosynthesis of natural products

The small-molecule biosynthetic potential of most filamentous fungi has remained largely unexplored and represents an attractive source for the discovery of new compounds. Genome sequencing of Calcarisporium arbuscula, a mushroom-endophytic fungus, revealed 68 core genes that are involved in natural product biosynthesis. This is in sharp contrast to the predominant production of the ATPase inhibitors aurovertin B and D in the wild-type fungus. Inactivation of a histone H3 deacetylase led to pleiotropic activation and overexpression of more than 75 % of the biosynthetic genes. Sampling of the overproduced compounds led to the isolation of ten compounds of which four contained new structures, including the cyclic peptides arbumycin and arbumelin, the diterpenoid arbuscullic acid A, and the meroterpenoid arbuscullic acid B. Such epigenetic modifications therefore provide a rapid and global approach to mine the chemical diversity of endophytic fungi.

Assessing gene expression during pathogenesis: Use of qRT-PCR to follow toxin production in the entomopathogenic fungus Beauveria bassiana during infection and immune response of the insect host Triatoma infestans

Entomopathogenic fungi secrete toxic secondary metabolites during the invasion of the insect hemocoel as part of the infection process. Although these compounds have been frequently mentioned as virulence factors, the roles of many of them remain poorly understood, including the question of whether they are expressed during the infection process. A major hurdle to this issue remains the low sensitivity of biochemical detection techniques (e.g., HPLC) within the complex samples that may contain trace quantities of fungal molecules inside the insect. In this study, quantitative reverse transcription real-time PCR (qRT-PCR) was used to measure the transcript levels within the insect fungal pathogen Beauveria bassiana, that encode for the synthetase enzymes of the secondary metabolites tenellin (BbtenS), beauvericin (BbbeaS) and bassianolide (BbbslS) during the infection of Triatoma infestans, a Chagas disease insect vector. Absolute quantification was performed at different time periods after insect treatment with various concentrations of propagules, either by immersing the insects in conidial suspensions or by injecting them with blastospores. Both BbtenS and BbbeaS were highly expressed in conidia-treated insects at days 3 and 12 post-treatment. In blastospore-injected insects, BbtenS and BbbeaS expression peaked at 24h post-injection and were also highly expressed in insect cadavers. The levels of BbbslS transcripts were much lower in all conditions tested. The expression patterns of insect genes encoding proteins that belong to the T. infestans humoral immune system were also evaluated with the same technique. This qPCR-based methodology can contribute to decifering the dynamics of entomopathogenic fungal infection at the molecular level.

Differential metabolic responses of Beauveria bassiana cultured in pupae extracts, root exudates and its interactions with insect and plant

Beauveria bassiana is a kind of world-wide entomopathogenic fungus and can also colonize plant rhizosphere. Previous researches showed differential expression of genes when entomopathogenic fungi are cultured in insect or plant materials. However, so far there is no report on metabolic alterations of B. bassiana in the environments of insect or plant. The purpose of this paper is to address this problem. Herein, we first provide the metabolomic analysis of B. bassiana cultured in insect pupae extracts (derived from Euproctis pseudoconspersa and Bombyx mori, EPP and BMP), plant root exudates (derived from asparagus and carrot, ARE and CRE), distilled water and minimal media (MM), respectively. Principal components analysis (PCA) shows that mycelia cultured in pupae extracts and root exudates are evidently separated and individually separated from MM, which indicates that fungus accommodates to insect and plant environments by different metabolic regulation mechanisms. Subsequently, orthogonal projection on latent structure-discriminant analysis (OPLS-DA) identifies differential metabolites in fungus under three environments relative to MM. Hierarchical clustering analysis (HCA) is performed to cluster compounds based on biochemical relationships, showing that sphingolipids are increased in BMP but are decreased in EPP. This observation further implies that sphingolipid metabolism may be involved in the adaptation of fungus to different hosts. In the meantime, sphingolipids are significantly decreased in root exudates but they are not decreased in distilled water, suggesting that some components of the root exudates can suppress sphingolipid to down-regulate sphingolipid metabolism. Pathway analysis finds that fatty acid metabolism is maintained at high level but non-ribosomal peptides (NRP) synthesis is unaffected in mycelia cultured in pupae extracts. In contrast, fatty acid metabolism is not changed but NRP synthesis is high in mycelia cultured in root exudates and distilled water. This indicates that fungal fatty acid metabolism is enhanced when contacting insect, but when in the absence of insect hosts NRP synthesis is increased. Ornithine, arginine and GABA are decreased in mycelia cultured in pupae extracts and root exudates but remain unchanged in distilled water, which suggests that they may be associated with fungal cross-talk with insects and plants. Trehalose and mannitol are decreased while adenine is increased in three conditions, signifying carbon shortage in cells. Together, these results unveil that B. bassiana has differential metabolic responses in pupae extracts and root exudates, and metabolic similarity in root exudates and distilled water is possibly due to the lack of insect components.

New insights into the echinocandins and other fungal non-ribosomal peptides and peptaibiotics

Non-ribosomal peptide synthetases (NRPSs) are a primary modality for fungal peptidic natural product assembly and are responsible for some of the best known, most useful, and most destructive fungal metabolites. Through genome sequencing and computer-assisted recognition of modular motifs of catalytic domains, one can now confidently identify most NRPS biosynthetic genes of a fungal strain. The biosynthetic gene clusters responsible for two of the most important classes of NRP fungal derived drugs, cyclosporine and the echinocandins, have been recently characterized by genomic sequencing and annotation. Complete biosynthetic gene clusters for the pneumocandins and echinocandins have been mapped at the genetic level and functionally characterized to some extent. Genomic sequencing of representative strains of most of the variants in the echinocandin family, including the wild-type of the three fungal strains employed for industrial-scale production of caspofungin, micafungin and anidulofungin, has enabled characterization of the basic architecture of the echinocandin NRPS pathways. A comparative analysis of how pathway genes cause variations in lipoinitiation, biosynthesis of the non-proteinogenic amino acids, amino acid substitutions, and hydroxylations and sulfonations of the core peptide and contribute to the molecular diversity of the family is presented. We also review new information on the natural functions of NRPs, the differences between fungal and bacterial NRPSs, and functional characterization of selected NRPS gene clusters. Continuing discovery of the new fungal nonribosomal peptides has contributed new structural diversity and potential insights into their biological functions among other natural peptides and peptaibiotics. We therefore provide an update on new peptides, depsipeptides and peptaibols discovered in the Fungi since 2009.