Basic leucine zipper (bZIP) domain transcription factor MBZ1 regulates cell wall integrity, spore adherence, and virulence in Metarhizium robertsii

Transcription Factors in Biocontrol Fungi

Transcription factors are extensively found in fungi and are involved in the regulation of multiple biological processes, including growth, development, conidiation, morphology, stresses tolerance, and virulence, as well as the production of secondary metabolites. Biocontrol is a complex biological process through which several biocontrol behaviors, such as the secretion of cell wall-degrading enzymes and the production of secondary metabolites, are regulated by transcription factors. To date, biocontrol-related transcription factors have been reported in several biocontrol fungi, such as Beauveria bassiana, Clonostachys rosea, Coniothyrium minitans, and different species in the genera Metarhizium, Trichoderma, and Arthrobotrys. However, comprehensive reviews summarizing and analyzing transcription factors with biocontrol potential in these fungi are scarce. This review begins by giving a basic overview of transcription factors and their functions. Then, the role of biocontrol-related transcription factors in biocontrol fungi is discussed. Lastly, possible approaches for further work on transcription factors in biocontrol fungi are suggested. This review provides a basis for further elaborating the molecular mechanisms of transcription factors in the context of biocontrol.

The basic leucine zipper domain (bZIP) transcription factor BbYap1 promotes evasion of host humoral immunity and regulates lipid homeostasis contributing to fungal virulence in Beauveria bassiana

Basic leucine zipper domain transcription factors (TFs), of which yeast activator protein (Yap) is a significant class, are crucial for the development of sclerotia, the stress response, vegetative growth, and spore adhesion. Nevertheless, nothing is known about how Yap TFs contribute to the pathogenicity of entomopathogenic fungus. In this work, Beauveria bassiana was used to identify and knock out the yeast gene BbYap1, which is similar to Yap. The BbYap1 gene deletion has an impact on lipid homeostasis of B. bassiana; oleic acid, for example, dropped by 95.69%. The BbYap1 mutant exhibited much less virulence and vegetative development in comparison to the wild strain, while demonstrating a greater sensitivity to chemical stress. It is noteworthy that the physiological abnormalities brought on by BbYap1 deletion were largely repaired by the addition of exogenous oleic acid, as seen by the notable increase in insect survival in the blood cavity injection group. Following infection with the BbYap1 mutant, the host exhibits a considerable down-regulation of the expression of β-1,3-glucan recognition protein, gallerimycin, gloverin, and moricin-like protein genes. Likewise, the introduction of exogenous oleic acid markedly increased the host's expression of the aforementioned genes. In summary, BbYap1 regulates cellular enzyme lipid homeostasis and fungal virulence by eluding host humoral defense, which contributes to fungal chemical stress and vegetative development.

IMPORTANCE

Entomopathogenic fungi (EPF) offer an effective and eco-friendly alternative to curb insect populations in biocontrol strategy. When EPF enter the hemolymph of their host, they encounter a variety of stress reactions, such as immunological and oxidative stress. Basic leucine zipper domain transcription factors, of which yeast activator protein (Yap) is a significant class, have diverse biological functions related to metabolism, development, reproduction, conidiation, stress responses, and pathogenicity. This study demonstrates that BbYap1 of Beauveria bassiana regulates cellular enzyme lipid homeostasis and fungal virulence by eluding host humoral defense, which contributes to fungal chemical stress and vegetative development. These findings offer fresh perspectives for comprehending molecular roles of YAP in EPF.

Subcellular biochemistry and biology of filamentous entomopathogenic fungi

Filamentous entomopathogenic fungi (EPF) function as important biotic factors regulating the arthropod population in natural ecosystems and have great potential as biocontrol agents in modern agriculture. In the infection cycle, EPF undergo a plethora of physiological processes, including metabolism (e.g., cuticle hydrolysis and nutrient utilization), development (e.g., dimorphism and conidiation), stress response (e.g., oxidative and osmotic stresses), and immune evasion from the host. In-depth explorations of the mechanisms involved in the lifecycle of EPF offer excellent opportunities to increase their virulence and stability, which increases the efficacy of EPF in biocontrol programs. This review discusses the current state of knowledge relating to the biological roles and regulatory mechanisms of organelles and subcellular structures in the physiology of EPF, as well as some suggestions for future investigation.

Hypothetical protein MAA_07646 is required for stress resistance and pathogenicity in Metarhizium robertsii

Metarhizium robertsii, a vital entomopathogenic fungus for pest management, relies on various virulence-related proteins for infection. Identifying these proteins, especially those with unknown functions, can illuminate the fungus's virulence mechanisms. Through RNA-seq, we discovered that the hypothetical protein MAA_07646 was significantly upregulated during appressorium formation in M. robertsii. In this study, we characterized MAA_07646, finding its presence in both the nucleus and cytoplasm. Surprisingly, it did not affect vegetative growth, conidiation, or chemical tolerance. However, it played a role in heat and UV radiation sensitivity. Notably, ΔMAA_07646 exhibited reduced virulence in Galleria mellonella larvae due to impaired appressorium formation and decreased expression of virulence-related genes. In conclusion, MAA_07646 contributes to thermotolerance, UV resistance, and virulence in M. robertsii. Understanding its function sheds light on the insecticidal potential of M. robertsii's hypothetical proteins.

The homeobox transcription factor MrHOX7 contributes to stress tolerance and virulence in the entomopathogenic fungus Metarhizium robertsii

Entomopathogenic fungi, including Metarhizium species, represent promising environmentally friendly biopesticides. Understanding the molecular mechanisms governing their infection processes is vital for enhancing their effectiveness. Transcription factors (TFs) play critical roles in gene regulation, yet the functions of many TFs in M. robertsii remain unknown. Homeobox transcription factors, implicated in diverse cellular processes, have received limited attention in entomopathogenic fungi. Here, we identify and characterize, a homeobox TF, MrHOX7, in the model entomopathogenic fungus M. robertsii. Subcellular localization and transcriptional profiling revealed MrHOX7's nuclear localization and high expression during conidia and appressoria formation. Deletion of Mrhox7 (ΔMrhox7) enhanced conidial tolerance to heat and UV-B stress, accompanying with upregulated stress-related gene expression. Intriguingly, ΔMrhox7 exhibits inhibited virulence exclusively through topical inoculation. Further investigations unveiled reduced conidial adhesion and appressorium formation, with downregulation of the adhesion gene Mad1 and appressorium-related genes, as the underlying causes of the reduced fungal virulence. Our findings illuminate the role of MrHOX7 in stress tolerance and virulence, providing insights into the molecular basis of fungal biopesticides.

Integrated Comparative Transcriptome and Weighted Gene Co-Expression Network Analysis Provide Valuable Insights into the Mechanisms of Pinhead Initiation in Chinese Caterpillar Mushroom Ophiocordyceps sinensis (Ascomycota)

The initiation and formation of the "pinhead" is the key node in growth process of Ophiocordyceps sinensis (Chinese Cordyceps). The research on the mechanism of changes in this growth stage is the basis for realizing the industrialization of its artificial cultivation. Clarifying the mechanisms of pinhead initiation is essential for its further application. Here, we performed a comprehensive transcriptome analysis of pinhead initiation process in O. sinensis. Comparative transcriptome analysis revealed remarkable variation in gene expression and enriched pathways at different pinhead initiation stages. Gene co-expression network analysis by WGCNA identified 4 modules highly relevant to different pinhead initiation stages, and 23 hub genes. The biological function analysis and hub gene annotation of these identified modules demonstrated that transmembrane transport and nucleotide excision repair were the topmost enriched in pre-pinhead initiation stage, carbohydrate metabolism and protein glycosylation were specially enriched in pinhead initiation stage, nucleotide binding and DNA metabolic process were over-represented after pinhead stage. These key regulators are mainly involved in carbohydrate metabolism, synthesis of proteins and nucleic acids. This work excavated the candidate pathways and hub genes related to the pinhead initiation stage, which will serve as a reference for realizing the industrialization of artificial cultivation in O. sinensis.

Nematode-induced trap formation regulated by the histone H3K4 methyltransferase AoSET1 in the nematode-trapping fungus Arthrobotrys oligospora

The methylation of lysine 4 of histone H3 (H3K4), catalyzed by the histone methyltransferase KMT2/SET1, has been functionally identified in many pathogenic fungi but remains unexplored in nematode-trapping fungi (NTFs). Here, we report a regulatory mechanism of an H3K4-specific SET1 orthologue, AoSET1, in the typical nematode-trapping fungus Arthrobotrys oligospora. When the fungus is induced by the nematode, the expression of AoSET1 is up-regulated. Disruption of AoSet1 led to the abolishment of H3K4me. Consequently, the yield of traps and conidia of ΔAoSet1 was significantly lower than that of the WT strain, and the growth rate and pathogenicity were also compromised. Moreover, H3K4 trimethylation was enriched mainly in the promoter of two bZip transcription factor genes (AobZip129 and AobZip350) and ultimately up-regulated the expression level of these two transcription factor genes. In the ΔAoSet1 and AoH3K4A strains, the H3K4me modification level was significantly decreased at the promoter of transcription factor genes AobZip129 and AobZip350. These results suggest that AoSET1-mediated H3KEme serves as an epigenetic marker of the promoter region of the targeted transcription factor genes. Furthermore, we found that AobZip129 negatively regulates the formation of adhesive networks and the pathogenicity of downstream AoPABP1 and AoCPR1. Our findings confirm that the epigenetic regulatory mechanism plays a pivotal role in regulating trap formation and pathogenesis in NTFs, and provide novel insights into the mechanisms of interaction between NTFs and nematodes.

Whole genome sequencing and analysis of multiple isolates of Ceratocystis destructans, the causal agent of Ceratocystis canker of almond in California

Ceratocystis canker caused by Ceratocystis destructans is a severe disease of almond, reducing the longevity and productivity of infected trees. Once the disease has established in an individual tree, there is no cure, and management efforts are often limited to removing the infected area of cankers. In this study, we present the genome assemblies of five C. destructans isolates isolated from symptomatic almond trees. The genomes were assembled into a genome size of 27.2 ± 0.9 Mbp with an average of 6924 ± 135 protein-coding genes and an average GC content of 48.8 ± 0.02%. We concentrated our efforts on identifying putative virulence factors of canker pathogens. Analysis of the secreted carbohydrate-active enzymes showed that the genomes harbored 83.4 ± 1.8 secreted CAZymes. The secreted CAZymes covered all the known categories of CAZymes. AntiSMASH revealed that the genomes had at least 7 biosynthetic gene clusters, with one of the non-ribosomal peptide synthases encoding dimethylcoprogen, a conserved virulence determinant of plant pathogenic ascomycetes. From the predicted proteome, we also annotated cytochrome P450 monooxygenases, and transporters, these are well-established virulence determinants of canker pathogens. Moreover, we managed to identify 57.4 ± 2.1 putative effector proteins. Gene Ontology (GO) annotation was applied to compare gene content with two closely related species C. fimbriata, and C. albifundus. This study provides the first genome assemblies for C. destructans, expanding genomic resources for an important almond canker pathogen. The acquired knowledge provides a foundation for further advanced studies, such as molecular interactions with the host, which is critical for breeding for resistance.

Overview of fungal terpene synthases and their regulation

Terpenes and terpenoids are a group of isoprene-derived molecules that constitute the largest group of natural products and secondary metabolites produced by living things, with more than 25,000 compounds reported. These compounds are synthesized by enzymes called terpene synthases, which include several families of cyclases and enzymes. These are responsible for adding functional groups to cyclized structures. Fungal terpenoids are of great interest for their pharmacological properties; therefore, understanding the mechanisms that regulate their synthesis (regulation of the mevalonate pathway, regulation of gene expression, and availability of cofactors) is essential to direct their production. For this reason, this review addresses the detailed study of the biosynthesis of fungal terpenoids and their regulation by various physiological and environmental factors.

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.

The invasion process of the entomopathogenic fungus Ophiocordyceps sinensis into the larvae of ghost moths (Thitarodes xiaojinensis) using a GFP-labeled strain

Chinese cordyceps is a well-known and valuable traditional Chinese medicine that forms after Ophiocordyceps sinensis parasitizes ghost moth larvae. The low natural infection rate of O. sinensis limits large-scale artificial cultivation of Chinese cordyceps, and the invasion process is unclear. To investigate the temporal and spatial regulation when O. sinensis enters ghost moths, we constructed an O. sinensis transformant that stably expresses green fluorescent protein (GFP). Inoculating Thitarodes xiaojinensis larvae with a high concentration of GFP-labeled O. sinensis, we observed that O. sinensis conidia could adhere to the host cuticle within 2 days, germinate penetration pegs within 4 days, and produce blastospores in the host hemocoel within 6 days. The reconstructed three-dimensional (3D) structures of the invasion sites showed that penetration pegs germinated directly from O. sinensis conidia at the joining site with the larval cuticle. Differentiated appressoria or hyphae along the host epicuticle are not required for O. sinensis to invade ghost moths. Overall, the specific invasion process of O. sinensis into its host is clarified, and we provided a new perspective on the invasion process of entomopathogenic fungi.

cAMP Signalling Pathway in Biocontrol Fungi

Biocontrol is a complex process, in which a variety of physiological and biochemical characteristics are altered. The cAMP signalling pathway is an important signal transduction pathway in biocontrol fungi and consists of several key components. The G-protein system contains G-protein coupled receptors (GPCRs), heterotrimeric G-proteins, adenylate cyclase (AC), cAMP-dependent protein kinase (PKA), and downstream transcription factors (TFs). The cAMP signalling pathway can regulate fungal growth, development, differentiation, sporulation, morphology, secondary metabolite production, environmental stress tolerance, and the biocontrol of pathogens. However, few reviews of the cAMP signalling pathway in comprehensive biocontrol processes have been reported. This work reviews and discusses the functions and applications of genes encoding each component in the cAMP signalling pathway from biocontrol fungi, including the G-protein system components, AC, PKA, and TFs, in biocontrol behaviour. Finally, future suggestions are provided for constructing a complete cAMP signalling pathway in biocontrol fungi containing all the components and downstream effectors involved in biocontrol behavior. This review provides useful information for the understanding the biocontrol mechanism of biocontrol fungi by utilising the cAMP signalling pathway.

Prophenoloxidase-positive tubes derived from the hindguts may be the doorkeeper to detoxify the waste metabolites collected by Malpighian tubules in Lepidoptera insects

Prophenoloxidase (PPO), an important immunity protein in insects, is mainly produced by hemocytes and released into the hemolymph upon cell lysis. In addition, PPO can also be produced by epidermal cells in the foregut to detoxify the toxic plant secondary metabolites and in the hindgut to kill pathogens through PPO-induced melanization. Previously, we noticed a pair of tubes extended from the larval hindgut became melanized upon staining in dopamine dissolved in 30% ethanol. However, the structure and function of these tubes are largely unknown. In this study, we performed staining of the tubes and the neighboring Malpighian tubule for further confirmation. Eventually, we detected PPO inside epidermal cells of the tubes, and called them as PPO-positive tubes. We observed that the PPO-positive tubes are physically derived from the hindgut but strongly adhere to the Malpighian tubule. Inside the PPO-positive tubes, there is an acellular peritrophic membrane to protect the epidermal cells. Furthermore, the PPO-positive tubes act like a doorkeeper to firstly detoxify the metabolite wastes collected by the Malpighian tubule from the hemolymph.

The Transcriptomic Mechanism of a Novel Autolysis Induced by a Recombinant Antibacterial Peptide from Chicken Expressed in Pichia pastoris

Autolysis is a common physiological process in eukaryotic cells that is often prevented or applied, especially in yeast expression systems. In this study, an antimicrobial peptide from chicken (AMP) was recombinantly expressed in the Pichia pastoris expression system, which induced a series of cellular autolysis phenotypes after methanol treatment, such as the aggregated, lysed, irregular, and enlarged cell morphology, while the cells expressing a recombinant aflatoxin-detoxifizyme (ADTZ) were not autolyzed. A comparative transcriptomic analysis showed that the transcriptomic profiles of cells derived from the autolysis and non-autolysis groups were well discriminated, suggesting that the mechanisms of autolysis were at the transcriptional level. A further differential expression gene (DEG) analysis showed that the DEGs from the two groups were involved mainly in autophagy, the MAPK signaling pathway, transcriptional factors, the central carbon metabolism, anti-stress functions, and so on. In the autolysis group, the cell activity was significantly reduced with the MAPK signaling pathway, the central carbon metabolism was down-regulated, and components of the cytoplasm-to-vacuole targeting (CVT) and mitophagy pathways were up-regulated, suggesting that the autophagy involved in the trafficking of intracellular molecules in the vacuole and mitochondrion contributed to autolysis, which was regulated by transcriptional factors and signal pathways at the transcriptional level. This study provides a theoretical basis for genetic modifications to prevent or utilize cell autolysis in the recombinant protein expression system.

The Toxins of Beauveria bassiana and the Strategies to Improve Their Virulence to Insects

The long-term and excessive usage of pesticides is an enormous burden on the environment, which also increases pest resistance. To overcome this problem, research and application of entomopathogenic fungi, which are both environmentally friendly and cause lower resistance, have gained great momentum. Entomopathogenic fungi have a wide range of prospects. Apart from Bacillus thuringiensis, Beauveria bassiana is the most studied biopesticide. After invading insect hosts, B. bassiana produces a variety of toxins, which are secondary metabolites such as beauvericin, bassianin, bassianolide, beauverolides, tenellin, oosporein, and oxalic acid. These toxins help B. bassiana to parasitize and kill the hosts. This review unequivocally considers beauveria toxins highly promising and summarizes their attack mechanism(s) on the host insect immune system. Genetic engineering strategies to improve toxin principles, genes, or virulent molecules of B. bassiana have also been discussed. Lastly, we discuss the future perspective of Beauveria toxin research, including newly discovered toxins.

Regulation of conidiation, polarity growth, and pathogenicity by MrSte12 transcription factor in entomopathogenic fungus, Metarhizium rileyi

Metarhizium rileyi, a well-known filamentous biocontrol fungus, is the main pathogen of numerous field pests, especially noctuid pests. To explore the potential factors involved in the fungal pathogenicity, MrSte12, an important and conserved functional transcription factor in mitogen-activated protein kinase pathway was carried out by functional analysis. Homologous recombination was used to disrupt the MrSte12 gene in M. rileyi. The deletant fungal strain exhibited malformed hyphae and impaired conidiogenesis, and conidia could not be collected from △MrSte12 in vitro towards SMAY medium. Although conidia could be collected again supplemented with KCl within SMAY medium, the conidial germination, growth and stress tolerance were much weaker compared with that in WT. Additionally, △MrSte12 showed a dramatic reduction in virulence in intra-hemolymph injections and no pathogenicity in topical inoculations against noctuid pests, which is due to the failure of appressorium formation. Moreover, the content of chitin and β-1, 3-glucan in cell wall significantly reduced in mutant conidia. These results indicate that the MrSte12 gene markedly contributes to invasive growth and conidiation, as well as the major pathogenicity in M. rileyi.

Activation of microlipophagy during early infection of insect hosts by Metarhizium robertsii

The requirement of macroautophagic/autophagic machinery for filamentous fungal development and pathogenicity has been recognized, but the underlying effects and mechanisms remain elusive. The insect pathogenic fungus Metarhizium robertsii infects hosts by cuticular penetration through the formation of the infection structure appressoria. Here, we show that autophagic fluxes were highly activated during the appressorial formation of M. robertsii. Genome-wide deletion of the autophagy-related genes and insect bioassays identified 10 of 23 encoded MrATG genes with requirements for topical fungal infection of insect hosts. Besides the defect in forming appressoria on insects (two null mutants), these virulence-reduced mutants were largely impaired in penetrating cellophane membrane and insect cuticles, suggesting their failures in generating proper appressorium turgor. We found that the conidial storage of lipid droplets (LDs) had no obvious difference between strains, but autophagic LD degradation was impaired in different mutants. After induction of cell autophagy by nitrogen starvation, we found that LD entry into vacuoles was unaffected in the selected mutant cells with potential failures in forming autophagosomes. The finding therefore reveals a microlipophagy machinery employed in this fungus and that the direct engulfment of LDs occurs without inhibition by the downstream defective lipolysis. Our data first unveil the activation and contribution of microlipophagy to fungal infection biology. The obtained technique may benefit future detection of microlipophagy in different organisms by examining vacuolar or lysosomal engulfment of LDs in core autophagic gene deletion mutants.

Conservative production of galactosaminogalactan in Metarhizium is responsible for appressorium mucilage production and topical infection of insect hosts

The exopolysaccharide galactosaminogalactan (GAG) has been well characterized in Aspergilli, especially the human pathogen Aspergillus fumigatus. It has been found that a five-gene cluster is responsible for GAG biosynthesis in Aspergilli to mediate fungal adherence, biofilm formation, immunosuppression or induction of host immune defences. Herein, we report the presence of the conserved GAG biosynthetic gene cluster in the insect pathogenic fungus Metarhizium robertsii to mediate either similar or unique biological functions. Deletion of the gene cluster disabled fungal ability to produce GAG on germ tubes, mycelia and appressoria. Relative to the wild type strain, null mutant was impaired in topical infection but not injection of insect hosts. We found that GAG production by Metarhizium is partially acetylated and could mediate fungal adherence to hydrophobic insect cuticles, biofilm formation, and penetration of insect cuticles. In particular, it was first confirmed that this exopolymer is responsible for the formation of appressorium mucilage, the essential extracellular matrix formed along with the infection structure differentiation to mediate cell attachment and expression of cuticle degrading enzymes. In contrast to its production during A. fumigatus invasive growth, GAG is not produced on the Metarhizium cells harvested from insect hemocoels; however, the polymer can glue germ tubes into aggregates to form mycelium pellets in liquid culture. The results of this study unravel the biosynthesis and unique function of GAG in a fungal system apart from the aspergilli species.

MrPEX33 is involved in infection-related morphogenesis and pathogenicity of Metarhizium robertsii

Peroxisomes, being indispensable organelles, play an important role in different biological processes in eukaryotes. PEX33, a filamentous fungus-specific peroxin of the docking machinery of peroxisomes, is involved in the virulence and development of other fungal pathogens. However, it is not clear whether PEX33 is necessary for the pathogenicity and development of an insect pathogenic fungus. In the present study, we report the presence of homologs of PEX33, namely MrPEX33 (MAA_05331), in the entomopathogenic fungus, Metarhizium robertsii. An M. robertsii transgenic strain expressing the fusion protein with MrPEX33-GFP and mCherry-PTS1 showed that MrPEX33 localizes to peroxisomes. The results also demonstrated that MrPEX33 is involved in the peroxisomal import pathway by peroxisomal targeting signals. Targeted gene deletion of MrPEX33 led to a significant decline in the asexual sporulation capacity, which was accompanied by downregulation of several conidiation-associated genes, such as wetA, abaA, and brlA. More importantly, our bioassay results showed that the virulence of ∆MrPEX33 mutants, against Galleria mellonella through cuticle infection, was greatly reduced. This was further accompanied by a significant drop in appressorium formation and cuticle penetration. Additionally, ∆MrPEX33 mutants showed a significant decrease in tolerance to cell wall integrity and oxidative stress. Taken together, our results suggest that MrPEX33 is involved in the cuticle infection-related morphogenesis and pathogenicity. KEY POINTS: • MrPEX33 is a specific peroxin of the docking machinery of peroxisomes. • MrPEX33 localizes to peroxisomes and is involved in the import of matrix proteins. • MrPEX33 is involved in the pathogenicity associated with cuticle infections.

Metarhizium: jack of all trades, master of many

The genus Metarhizium and Pochonia chlamydosporia comprise a monophyletic clade of highly abundant globally distributed fungi that can transition between long-term beneficial associations with plants to transitory pathogenic associations with frequently encountered protozoans, nematodes or insects. Some very common 'specialist generalist' species are adapted to particular soil and plant ecologies, but can overpower a wide spectrum of insects with numerous enzymes and toxins that result from extensive gene duplications made possible by loss of meiosis and associated genome defence mechanisms. These species use parasexuality instead of sex to combine beneficial mutations from separate clonal individuals into one genome (Vicar of Bray dynamics). More weakly endophytic species which kill a narrow range of insects retain sexuality to facilitate host-pathogen coevolution (Red Queen dynamics). Metarhizium species can fit into numerous environments because they are very flexible at the genetic, physiological and ecological levels, providing tractable models to address how new mechanisms for econutritional heterogeneity, host switching and virulence are acquired and relate to diverse sexual life histories and speciation. Many new molecules and functions have been discovered that underpin Metarhizium associations, and have furthered our understanding of the crucial ecology of these fungi in multiple habitats.

MrHex1 is Required for Woronin Body Formation, Fungal Development and Virulence in Metarhizium robertsii

The Woronin body (WB) is a peroxisome-derived dense-core vesicle, a self-assembling hexagonal crystal of a single protein Hex1. This organelle is specific to the ascomycete fungi belonging to the Pezizomycotina subphylum by functioning in sealing septal pores in response to mycelium damage and the control of cell heterogeneity. We retrieved all available Hex1-domain containing proteins of different fungi from the GenBank database and found considerable length variations among 460 obtained Hex1 proteins. However, a highly conserved Hex1 domain containing 75 amino acid residues with a specific S/A-R/S-L consensus motif for targeting peroxisome is present at the carboxy-terminus of each protein. A homologous Hex1 gene, named MrHex1, was deleted in the entomopathogenic fungus Metarhizium robertsii. It was found that MrHex1 was responsible for WB formation in M. robertsii and involved in sealing septal pores to maintain cell integrity and heterogeneity. Different assays indicated that, relative to the wild-type (WT) strain, ∆Mrhex1 demonstrated a growth defect on a solid medium and substantial reductions of conidiation, appressorium formation and topical infectivity against insect hosts. However, there was no obvious virulence difference between WT and mutants during injection of insects. We also found that ∆MrHex1 could tolerate different stress conditions like the WT and the gene-rescued mutant of M. robertsii, which is in contrast to the reports of the stress-response defects of the Hex1 null mutants of other fungal species. In addition to revealing the phenotypic/functional alterations of the Hex1 deletion mutants between different pathotype fungi, the results of this study may benefit the understanding of the evolution and WB-control of fungal entomopathogenicity.

The bZIP Transcription Factor AflRsmA Regulates Aflatoxin B1 Biosynthesis, Oxidative Stress Response and Sclerotium Formation in Aspergillus flavus

Fungal secondary metabolites play important roles not only in fungal ecology but also in humans living as beneficial medicine or harmful toxins. In filamentous fungi, bZIP-type transcription factors (TFs) are associated with the proteins involved in oxidative stress response and secondary metabolism. In this study, a connection between a bZIP TF and oxidative stress induction of secondary metabolism is uncovered in an opportunistic pathogen Aspergillus flavus, which produces carcinogenic and mutagenic aflatoxins. The bZIP transcription factor AflRsmA was identified by a homology research of A. flavus genome with the bZIP protein RsmA, involved in secondary metabolites production in Aspergillusnidulans. The AflrsmA deletion strain (ΔAflrsmA) displayed less sensitivity to the oxidative reagents tert-Butyl hydroperoxide (tBOOH) in comparison with wild type (WT) and AflrsmA overexpression strain (AflrsmA^OE), while AflrsmA^OE strain increased sensitivity to the oxidative reagents menadione sodium bisulfite (MSB) compared to WT and ΔAflrsmA strains. Without oxidative treatment, aflatoxin B₁ (AFB₁) production of ΔAflrsmA strains was consistent with that of WT, but AflrsmA^OE strain produced more AFB₁ than WT; tBOOH and MSB treatment decreased AFB₁ production of ΔAflrsmA compared to WT. Besides, relative to WT, ΔAflrsmA strain decreased sclerotia, while AflrsmA^OE strain increased sclerotia. The decrease of AFB₁ by ΔAflrsmA but increase of AFB₁ by AflrsmA^OE was on corn. Our results suggest that AFB₁ biosynthesis is regulated by AflRsmA by oxidative stress pathways and provide insights into a possible function of AflRsmA in mediating AFB₁ biosynthesis response host defense in pathogen A. flavus.

Unveiling the function and regulation control of the DUF3129 family proteins in fungal infection of hosts

Many prokaryotic and eukaryotic proteins contain domains of unknown function (DUFs). A DUF3129 family of proteins is widely encoded in the genomes of fungal pathogens. A few studies in plant and insect pathogens indicated that the DUF3129 genes are required for fungal penetration of host cuticles with an unclear mechanism. We found that a varied number of DUF3129 proteins is present in different fungal species and the proteins are evolutionarily diverged from each other at the inter- and intra-specific levels. By using the insect pathogenic fungus Metarhizium robertsii as a model, we performed experiments and found that the seven DUF3129 proteins encoded by this fungus are localized to cellular lipid droplets (LDs). Individual deletion of these genes did not affect fungal formation of the infection structure appressoria and the accumulation of LDs in fungal conidia. When compared with the wild-type (WT) strain, insect bioassays revealed that the virulence of most null mutants were significantly impaired during topical infection but not during injection of insects. Carbon starvation and the subsequent Western blot analysis indicated that the LD-specific perilipin protein was completely degraded in the WT cells whereas varied levels of perilipin could be detected in the mutant cells, which signified that depletion of LD content was delayed in mutant cells, and DUF3129 proteins are therefore involved in LD degradation. We also provided biochemical evidence that these DUF3129 genes are transcriptionally regulated by a yeast Ste12-like transcription factor. The findings of this study not only unveil the function of DUF3129 proteins but also better understand the diverse mechanism of fungus-host interactions. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

MrSt12 implicated in the regulation of transcription factor AFTF1 by Fus3-MAPK during cuticle penetration by the entomopathogenic fungus Metarhizium robertsii

Metarhizium robertsii is a versatile fungus with multifactorial lifestyles, and it is an emerging fungal model for investigating the mechanisms of multiple lifestyle transitions that involve trans-kingdom host jumping. Penetration of the insect cuticle is the necessary step for the transition from saprophytic or symbiotic to pathogenic lifestyle. Previously, we found the transcription factor AFTF1 plays an important role in cuticle penetration, which is precisely regulated by Fus3-MAPK, Slt2-MAPK, and the membrane protein Mr-OPY2. Here, we identified a transcription factor (MrSt12) that directly regulated the transcription of Aftf1 by physically interacting with the cis-acting element (ATGAAACA) in the promoter of Aftf1. The deletion mutant of MrSt12 failed to form the infection structure appressorium and was thus nonpathogenic. We further found that the regulation of Aftf1 by MrSt12 was directly controlled by the Fus3-MAPK. In conclusion, we found a new signaling cascade containing Fus3-MAPK, MrSt12, and AFTF1, which regulates cuticle penetration by M. robertsii.

Regulation of conidiation, dimorphic transition, and microsclerotia formation by MrSwi6 transcription factor in dimorphic fungus Metarhizium rileyi

Microsclerotia (MS) produced in the liquid culture of the dimorphic insect pathogen Metarhizium rileyi can be used as a mycoinsecticide. Bioinformatics analysis demonstrated that the cell cycle signaling pathway was involved in regulating MS formation. To investigate the mechanisms by which the signaling pathway is regulated, a cell cycle box binding transcription factor MrSwi6 of M. rileyi was characterized. MrSwi6 was highly expressed during periods of yeast-hypha transition and conidia and MS formation. When compared with wild-type and complemented strains, disruption of MrSwi6 significantly reduced conidia (15-36%) and MS formation (96.2%), and exhibited decreased virulence levels. Digital expression profiling revealed that genes involved in antioxidation, pigment biosynthesis, and ion transport and storage were regulated by MrSwi6 during conidia and MS development. These results confirmed the significance of MrSwi6 in dimorphic transition, conidia and MS formation, and virulence in M. rileyi.

A transcription factor, MrMsn2, in the dimorphic fungus Metarhizium rileyi is essential for dimorphism transition, aggravated pigmentation, conidiation and microsclerotia formation

Microsclerotia (MS) are pseudoparenchymatous aggregations of hyphae of fungi that can be induced in liquid culture for biocontrol applications. Previously, we determined that the high-osmolarity glycerol (HOG) signalling pathway was involved in regulating MS development in the dimorphic insect pathogen Metarhizium rileyi. To further investigate the mechanisms by which the signalling pathway is regulated, we characterized the transcriptional factor MrMsn2, a homologue of the yeast C2 H2 transcriptional factor Msn2, which is predicted to function downstream of the HOG pathway in M. rileyi. Compared with wild-type and complemented strains, disruption of MrMsn2 increased the yeast-to-hypha transition rate, enhanced conidiation capacity and aggravated pigmentation in M. rileyi. The ▵MrMsn2 mutants were sensitive to stress, produced morphologically abnormal clones and had significantly reduced MS formation and decreased virulence levels. Digital expression profiling revealed that genes involved in antioxidation, pigment biosynthesis and ion transport and storage were regulated by MrMsn2 during conidia and MS development. Taken together, our findings confirm that MrMsn2 controlled the yeast-to-hypha transition, conidia and MS formation, and virulence.

Cyclosporine Biosynthesis in Tolypocladium inflatum Benefits Fungal Adaptation to the Environment

The cycloundecapeptide cyclosporin A (CsA) was first isolated from the insect-pathogenic fungus Tolypocladium inflatum for its antifungal activity and later developed as an immunosuppressant drug. However, the full biosynthetic mechanism of CsA remains unknown and has puzzled researchers for decades. In this study, the biosynthetic gene cluster is suggested to include 12 genes encoding enzymes, including the nonribosomal peptide synthetase (NRPS) (SimA) responsible for assembling the 11 amino acid substrates of cyclosporine and a polyketide synthase (PKS) (SimG) to mediate the production of the unusual amino acid (4R)-4-[(E)-2-butenyl]-4-methyl-l-threonine (Bmt). Individual deletion of 10 genes, isolation of intermediates, and substrate feeding experiments show that Bmt is biosynthesized by three enzymes, including SimG, SimI, and SimJ. The substrate d-alanine is catalyzed from l-alanine by alanine racemase SimB. Gene cluster transcription is regulated by a putative basic leucine zipper (bZIP)-type protein encoded by the cluster gene SimL We also found that the cluster cyclophilin (SimC) and transporter (SimD) genes contribute to the tolerance of CsA in the CsA-producing fungus. We also found that cyclosporine production could enable the fungus to outcompete other fungi during cocultivation tests. Deletion of the CsA biosynthetic genes also impaired fungal virulence against insect hosts. Taking all the data together, in addition to proposing a biosynthetic pathway of cyclosporines, the results of this study suggest that CsA produced by this fungus might play important ecological roles in fungal environment interactions.IMPORTANCE The cyclopeptide cyclosporin A was first isolated from the filamentous fungus Tolypocladium inflatum showing antifungal activity and was later developed as an immunosuppressant drug. We report the biosynthetic mechanism of cyclosporines that are mediated by a cluster of genes encoding NRPS and PKS controlled by a bZIP-type transcriptional regulator. The two unusual amino acids Bmt and d-Ala are produced by the PKS pathway and alanine racemase, respectively. The cyclophilin and transporter genes jointly contribute to fungal self-protection against cyclosporines. Cyclosporine confers on T. inflatum the abilities to outcompete other fungi in competitive interactions and to facilitate fungal infection of insect hosts, which therefore benefits fungal adaptations to different environments.

Diverse effect of phosphatidylcholine biosynthetic genes on phospholipid homeostasis, cell autophagy and fungal developments in Metarhizium robertsii

Phosphatidylcholine (PC) plays an important role in maintaining membrane integrity and functionality. In this study, two key genes (Mrpct and Mrpem) putatively involved in the cytidine diphosphate (CDP)-choline and phosphatidylethanolamine N-methyltransferase (PEMT) pathways for PC biosynthesis were characterized in the insect pathogenic fungus Metarhizium robertsii. The results indicated that disruption of Mrpct did not lead to any reduction of total PC content but impaired fungal virulence and increased cellular accumulation of triacylglycerol. Deletion of Mrpem reduced PC content and impaired fungal conidiation and infection structure differentiation but did not result in virulence defects. Lipidomic analysis revealed that deletion of Mrpct and Mrpem resulted in dissimilar effects on increase and decrease of PC moieties and other phospholipid species accumulations. Interestingly, we found that these two genes played opposite roles in activation of cell autophagy when the fungi were grown in a nutrient-rich medium. The connection between PC metabolism and autophagy was confirmed because PC content was drastically reduced in Mratg8Δ and that the addition of PC could rescue null mutant sporulation defect. The results of this study facilitate the understanding of PC metabolism on fungal physiology.

Prophenoloxidase-Mediated Ex Vivo Immunity to Delay Fungal Infection after Insect Ecdysis

Skin immunity protects animals from airborne pathogen infection. Unlike mammals, arthropods, including insects, undergo periodic ecdysis to grow and develop. Newly molted insects emerge with unsclerotized thin cuticles but successfully escape pathogenic infections during the post-molt period. Here we show that prophenoloxidases (PPOs) in molting fluids remain bioactive on the integument and impede fungal infection after ecdysis. We found that the purified plasma PPOs or recombinant PPOs could effectively bind to fungal spores (conidia) by targeting the cell wall components chitin and β-1,3-glucan. Pretreatment of the spores of the fungal pathogen Beauveria bassiana with PPOs increased spore hydrophilicity and reduced spore adhesion activity, resulting in a significant decrease in virulence as compared with mock infection. We also identified a spore-secreted protease BPS8, a member of peptidase S8 family of protease that degrade PPOs at high levels to benefit fungal infection, but which at lower doses activate PPOs to inhibit spore germination after melanization. These data indicate that insects have evolved a distinct strategy of ex vivo immunity to survive pathogen infections after ecdysis using PPOs in molting fluids retained on the underdeveloped and tender integument of newly molted insects for protection against airborne fungal infection.

Stress2TF: a manually curated database of TF regulation in plant response to stress

Considerable studies demonstrate that plant transcription factors (TFs) play key regulatory roles in abiotic/biotic stress conditions, such as drought and pathogen attack. However, there is no effort dedicated to curate experimentally validated stress-TF regulatory relationships from these individual reports into a central database, which put an obstacle in the exploration of stress-TF regulations in plants. To address this issue, we presented a literature-curated database 'Stress2TF' that currently documented 1533 regulatory relationships between 71 abiotic/biotic stresses and 558 TFs in 47 plant species. Each entry in Stress2TF contains detailed information about a stress-TF relationship such as plant name, stress name, TF and brief description of stress-TF relationship. Stress2TF provided a user-friendly interface for entry browse, search and download. In addition, a submission page and several useful tools (e.g., BLAST, network visualization) were integrated. Stress2TF may be a valuable resource for the research of stress-TF regulatory mechanisms in plants. Stress2TF is available at http://csgenomics.ahau.edu.cn/Stress2TF.

Divergent LysM effectors contribute to the virulence of Beauveria bassiana by evasion of insect immune defenses

The lysin motif (LysM) containing proteins can bind chitin and are ubiquitous in various organisms including fungi. In plant pathogenic fungi, a few LysM proteins have been characterized as effectors to suppress chitin-induced immunity in plant hosts and therefore contribute to fungal virulence. The effector mechanism is still questioned in fungus-animal interactions. In this study, we found that LysM proteins are also present in animal pathogenic fungi and have evolved divergently. The genome of the insect pathogen Beauveria bassiana encodes 12 LysM proteins, and the genes were differentially transcribed by the fungus when grown in different conditions. Deletion of six genes that were expressed by the fungus growing in insects revealed that two, Blys2 and Blys5, were required for full fungal virulence. Both proteins could bind chitin and Blys5 (containing two LysM domains) could additionally bind chitosan and cellulose. Truncation analysis of Blys2 (containing five LysM domains) indicated that the combination of LysM domains could determine protein-binding affinity and specificity for different carbohydrates. Relative to the wild-type strain, loss of Blys2 or Blys5 could impair fungal propagation in insect hemocoels and lead to the upregulation of antifungal gene in insects. Interestingly, the virulence defects of ΔBlys2 and ΔBlys5 could be fully restored by complementation with the Slp1 effector from the rice blast fungus Magnaporthe oryzae. In contrast to Slp1 and Blys2, Blys5 could potentially protect fungal hyphae against chitinase hydrolysis. The results of this study not only advance the understanding of LysM protein evolution but also establish the effector mechanism of fungus-animal interactions.

Insect pathogenic fungi are of importance for both applied and basic research. Relative to the advances in understanding fungus-plant interactions, the mechanisms of the molecular pathogenesis of entomopathogenic fungi are rather limitedly understood. In particular, the machinery of effector-mediated inhibition of host immunity has not been well established in fungus-insect interactions. LysM effectors have been characterized as virulence factors in plant pathogens to suppress chitin-triggered immunity in plants. We found that the divergent LysM proteins are also present in animal pathogens. By using the insect pathogen Beauveria bassiana as a model, we revealed that two of 12 encoded LysM protein genes Blys2 and Blys5 that were transcribed by the fungus growing in insects are required for full fungal virulence against insect hosts. Interestingly, the virulence defects of ΔBys2 and ΔBys5 could be fully restored by complementation with the divergent Slp1 effector from the plant pathogen Magnaporthe oryzae. Both Blys2 and Blys5 can deregulate insect immune responses, and the latter can additionally protect fungal cells from chitinase hydrolysis. The findings of this study establish the contribution of LysM effectors to fungal virulence against insect hosts.

The Insect Pathogens

Fungi are the most common disease-causing agents of insects; aside from playing a crucial role in natural ecosystems, insect-killing fungi are being used as alternatives to chemical insecticides and as resources for biotechnology and pharmaceuticals. Some common experimentally tractable genera, such as Metarhizium spp., exemplify genetic diversity and dispersal because they contain numerous intraspecific variants with distinct environmental and insect host ranges. The availability of tools for molecular genetics and multiple sequenced genomes has made these fungi ideal experimental models for answering basic questions on the genetic and genomic processes behind adaptive phenotypes. For example, comparative genomics of entomopathogenic fungi has shown they exhibit diverse reproductive modes that often determine rates and patterns of genome evolution and are linked as cause or effect with pathogenic strategies. Fungal-insect pathogens represent lifestyle adaptations that evolved numerous times, and there are significant differences in host range and pathogenic strategies between the major groups. However, typically, spores landing on the cuticle produce appressoria and infection pegs that breach the cuticle using mechanical pressure and cuticle-degrading enzymes. Once inside the insect body cavity, fungal pathogens face a potent and comprehensively studied immune defense by which the host attempts to eliminate or reduce an infection. The Fungal Kingdom stands alone in the range, extent, and complexity of their manipulation of arthropod behavior. In part, this is because most only sporulate on cadavers, so they must ensure the dying host positions itself to allow efficient transmission.

Insect Pathogenic Fungi: Genomics, Molecular Interactions, and Genetic Improvements

Entomopathogenic fungi play a pivotal role in the regulation of insect populations in nature, and representative species have been developed as promising environmentally friendly mycoinsecticides. Recent advances in the genome biology of insect pathogenic fungi have revealed genomic features associated with fungal adaptation to insect hosts and different host ranges, as well as the evolutionary relationships between insect and noninsect pathogens. By using species in the Beauveria and Metarhizium genera as models, molecular biology studies have revealed the genes that function in fungus-insect interactions and thereby contribute to fungal virulence. Taken together with efforts toward genetic improvement of fungal virulence and stress resistance, knowledge of entomopathogenic fungi will potentiate cost-effective applications of mycoinsecticides for pest control in the field. Relative to our advanced insights into the mechanisms of fungal pathogenesis in plants and humans, future studies will be necessary to unravel the gene-for-gene relationships in fungus-insect interactive models.

Advances in Genomics of Entomopathogenic Fungi

Fungi are the commonest pathogens of insects and crucial regulators of insect populations. The rapid advance of genome technologies has revolutionized our understanding of entomopathogenic fungi with multiple Metarhizium spp. sequenced, as well as Beauveria bassiana, Cordyceps militaris, and Ophiocordyceps sinensis among others. Phylogenomic analysis suggests that the ancestors of many of these fungi were plant endophytes or pathogens, with entomopathogenicity being an acquired characteristic. These fungi now occupy a wide range of habitats and hosts, and their genomes have provided a wealth of information on the evolution of virulence-related characteristics, as well as the protein families and genomic structure associated with ecological and econutritional heterogeneity, genome evolution, and host range diversification. In particular, their evolutionary transition from plant pathogens or endophytes to insect pathogens provides a novel perspective on how new functional mechanisms important for host switching and virulence are acquired. Importantly, genomic resources have helped make entomopathogenic fungi ideal model systems for answering basic questions in parasitology, entomology, and speciation. At the same time, identifying the selective forces that act upon entomopathogen fitness traits could underpin both the development of new mycoinsecticides and further our understanding of the natural roles of these fungi in nature. These roles frequently include mutualistic relationships with plants. Genomics has also facilitated the rapid identification of genes encoding biologically useful molecules, with implications for the development of pharmaceuticals and the use of these fungi as bioreactors.

A vision for the innovative study of fungal biology in China: Presidential address

I am proud to be elected as the sixth president of the Mycological Society of China, and highly pleased to have a chance to share my personal opinion here with my fellow mycologists and students regarding the innovative performance of fungal biology studies in China. A stepwise buildup of knowledge and sharp scientific vision is the prerequisite for innovative studies. Taken together with the most advanced techniques and elegant experimental designs, the scholars would have a better chance to acquire novel and conceptual results rather than the “me too” stories by focusing on the mechanisms related with fungal unique biology.