Insights into regulatory roles of MAPK-cascaded pathways in multiple stress responses and life cycles of insect and nematode mycopathogens

Essential roles of Rad6 in conidial property, stress tolerance, and pathogenicity of Beauveria bassiana

Rad6 functions as a ubiquitin-conjugating protein that regulates cellular processes in many fungal species. However, its role in filamentous entomopathogenic fungi remains poorly understood. This study characterizes Rad6 in Beauveria bassiana, a filamentous fungus widely employed as a critical fungicide globally. The results demonstrate a significant association between Rad6 and conidial properties, heat shock response, and UV-B tolerance. Concurrently, the mutant strain exhibited heightened sensitivity to oxidative stress, cell wall interfering agents, DNA damage stress, and prolonged heat shock. Furthermore, the absence of Rad6 significantly extended the median lethal time (LT50) of Galleria mellonella infected by B. bassiana. This delay could be attributed to reduced Pr1 proteases and extracellular cuticle-degrading enzymes, diminished dimorphic transition rates, and dysregulated antioxidant enzymes. Additionally, the absence of Rad6 had a more pronounced effect on genetic information processing, metabolism, and cellular processes under normal conditions. However, its impact was limited to metabolism in oxidative stress. This study offers a comprehensive understanding of the pivotal roles of Rad6 in conidial and hyphal stress tolerance, environmental adaptation, and the pathogenesis of Beauveria bassiana.

Cla4A, a Novel Regulator of Gene Expression Networks Required for Asexual and Insect-Pathogenic Lifecycles of Beauveria bassiana

Cla4, an orthologous p21-activated kinase crucial for non-entomopathogenic fungal lifestyles, has two paralogs (Cla4A/B) functionally unknown in hypocrealean entomopathogens. Here, we report a regulatory role of Cla4A in gene expression networks of Beauveria bassiana required for asexual and entomopathogenic lifecycles while Cla4B is functionally redundant. The deletion of cla4A resulted in severe growth defects, reduced stress tolerance, delayed conidiation, altered conidiation mode, impaired conidial quality, and abolished pathogenicity through cuticular penetration, contrasting with no phenotype affected by cla4B deletion. In ∆cla4A, 5288 dysregulated genes were associated with phenotypic defects, which were restored by targeted gene complementation. Among those, 3699 genes were downregulated, including more than 1300 abolished at the transcriptomic level. Hundreds of those downregulated genes were involved in the regulation of transcription, translation, and post-translational modifications and the organization and function of the nuclear chromosome, chromatin, and protein-DNA complex. DNA-binding elements in promoter regions of 130 dysregulated genes were predicted to be targeted by Cla4A domains. Samples of purified Cla4A extract were proven to bind promoter DNAs of 12 predicted genes involved in multiple stress-responsive pathways. Therefore, Cla4A acts as a novel regulator of genomic expression and stability and mediates gene expression networks required for insect-pathogenic fungal adaptations to the host and environment.

Cross-talk between low temperature and other environmental factors

Low temperatures are rarely experienced in isolation. The impacts of low temperatures on insects can be exacerbated or alleviated by the addition of other environmental factors, including, for example, desiccation, hypoxia, or infection. One way in which environmental factors can interact is through cross-talk where different factors enact common signaling pathways. In this review, I highlight the breadth of abiotic and biotic factors that can interact with low-temperature tolerance in both natural and artificial environments; and discuss some of the candidate pathways that are possibly responsible for cross-talk between several factors. Specifically, I discuss three interesting candidates: the neurohormone octopamine, circadian clock gene vrille, and microbes. Finally, I discuss applications of cross-talk studies, and provide recommendations for researchers.

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.

Blue Light Exposure Caused Large-Scale Transcriptional Changes in the Abdomen and Reduced the Reproductive Fitness of the Fall Armyworm Spodoptera frugiperda

In the present study, we found that blue light stress negatively affected the development periods, body weight, survival and reproduction of Spodoptera frugiperda, and it showed a dose-dependent reaction, as longer irradiation caused severer effects. Further transcriptome analysis found blue light stress induced fast and large-scale transcriptional changes in the head, thorax and, particularly, the abdomen of female S. frugiperda adults. A functional enrichment analysis indicated that shorter durations of blue light irradiation induced the upregulation of more stress response- and defense-related genes or pathways, such as abiotic stimuli detection and response, oxidative stress, ion channels and protein-kinase-based signal pathways. In the abdomen, however, different durations of blue-light-exposure treatments all induced the downregulation of a large number genes and pathways related to cellular processes, metabolism, catalysis and reproduction, which may be a trade-off between antistress defense and other processes or a strategy to escape stressful conditions. These results indicate irradiation duration- and tissue-specific blue light stress responses and consequences, as well as suggest that the stress that results in transcriptional alterations is associated with the stress that causes a fitness reduction in S. frugiperda females.

Serine protease CrKP43 interacts with MAPK and regulates fungal development and mycoparasitism in Clonostachys chloroleuca

IMPORTANCE

Mycoparasites play important roles in the biocontrol of plant fungal diseases, during which they secret multiple hydrolases such as serine proteases to degrade their fungal hosts. In this study, we demonstrated that the serine protease CrKP43 was involved in C. chloroleuca development and mycoparasitism with the regulation of Crmapk. To the best of our knowledge, it is the first report on the functions and regulatory mechanisms of serine proteases in C. chloroleuca. Our findings will provide new insight into the regulatory mechanisms of serine proteases in mycoparasites and contribute to clarifying the mechanisms underlying mycoparasitism of C. chloroleuca, which will facilitate the development of highly efficient fungal biocontrol agents as well.

Glucose-6-phosphate 1-Epimerase CrGlu6 Contributes to Development and Biocontrol Efficiency in Clonostachys chloroleuca

Clonostachys chloroleuca (formerly classified as C. rosea) is an important mycoparasite active against various plant fungal pathogens. Mitogen-activated protein kinase (MAPK) signaling pathways are vital in mycoparasitic interactions; they participate in responses to diverse stresses and mediate fungal development. In previous studies, the MAPK-encoding gene Crmapk has been proven to be involved in mycoparasitism and the biocontrol processes of C. chloroleuca, but its regulatory mechanisms remain unclear. Aldose 1-epimerases are key enzymes in filamentous fungi that generate energy for fungal growth and development. By protein-protein interaction assays, the glucose-6-phosphate 1-epimerase CrGlu6 was found to interact with Crmapk, and expression of the CrGlu6 gene was significantly upregulated when C. chloroleuca colonized Sclerotinia sclerotiorum sclerotia. Gene deletion and complementation analyses showed that CrGlu6 deficiency caused abnormal morphology of hyphae and cells, and greatly reduced conidiation. Moreover, deletion mutants presented much lower antifungal activities and mycoparasitic ability, and control efficiency against sclerotinia stem rot was markedly decreased. When the CrGlu6 gene was reinserted, all biological characteristics and biocontrol activities were recovered. These findings provide new insight into the mechanisms of glucose-6-phosphate 1-epimerase in mycoparasitism and help to further reveal the regulation of MAPK and its interacting proteins in the biocontrol of C. chloroleuca.

Protein Kinase PoxMKK1 Regulates Plant-Polysaccharide-Degrading Enzyme Biosynthesis, Mycelial Growth and Conidiation in Penicillium oxalicum

The ability to adapt to changing environmental conditions is crucial for living organisms, as it enables them to successfully compete in natural niches, a process which generally depends upon protein phosphorylation-mediated signaling transduction. In the present study, protein kinase PoxMKK1, an ortholog of mitogen-activated protein kinase kinase Ste7 in Saccharomyces cerevisiae, was identified and characterized in the filamentous fungus Penicillium oxalicum. Deletion of PoxMKK1 in P. oxalicum ΔPoxKu70 led the fungus to lose 64.4-88.6% and 38.0-86.1% of its plant-polysaccharide-degrading enzyme (PPDE) production on day 4 after a shift under submerged- and solid-state fermentation, respectively, compared with the control strain ΔPoxKu70. In addition, PoxMKK1 affected hypha growth and sporulation, though this was dependent on culture formats and carbon sources. Comparative transcriptomics and real-time quantitative reverse transcription PCR assay revealed that PoxMKK1 activated the expression of genes encoding major PPDEs, known regulatory genes (i.e., PoxClrB and PoxCxrB) and cellodextrin transporter genes (i.e., PoxCdtD and PoxCdtC), while it inhibited the essential conidiation-regulating genes, including PoxBrlA, PoxAbaA and PoxFlbD. Notably, regulons modulated by PoxMKK1 and its downstream mitogen-activated protein kinase PoxMK1 co-shared 611 differential expression genes, including 29 PPDE genes, 23 regulatory genes, and 16 sugar-transporter genes. Collectively, these data broaden our insights into the diverse functions of Ste7-like protein kinase, especially regulation of PPDE biosynthesis, in filamentous fungi.

Essential Role of WetA, but No Role of VosA, in Asexual Development, Conidial Maturation and Insect Pathogenicity of Metarhizium robertsii

Conidial maturation, which is crucial for conidial quality, is controlled by the asexual development activator WetA and the downstream, velvety protein VosA in Aspergillus. Their orthologs have proved functional in conidial quality control of Beauveria bassiana, as seen in Aspergillus, but are functionally unexplored, in Metarhizium robertsii, another hypocrealean insect pathogen. Here, WetA and VosA prove essential and nonessential for M. robertsii's life cycle, respectively. Disruption of wetA increased hyphal sensitivity to oxidative stress and Congo red-induced cell wall stress, but had little impact on radial growth. The ΔwetA mutant was severely compromised in conidiation capacity and conidial quality, which was featured by slower germination, decreased UV resistance, reduced hydrophobicity, and deformed hydrophobin rodlet bundles that were assembled onto conidial coat. The mutant's virulence was greatly attenuated via normal infection due to a blockage of infection-required cellular processes. All examined phenotypes were unaffected for the ΔvosA mutant. Intriguingly, mannitol was much less accumulated in the 7- and 15-day-old cultures of ΔwetA and ΔvosA than of control strains, while accumulated trehalose was not detectable at all, revealing little a link of intracellular polyol accumulation to conidial maturation. Transcriptomic analysis revealed differential regulation of 160 genes (up/down ratio: 72:88) in ΔwetA. These genes were mostly involved in cellular component, biological process, and molecular function but rarely associated with asexual development. Conclusively, WetA plays a relatively conserved role in M. robertsii's spore surface structure, and also a differentiated role in some other cellular processes associated with conidial maturation. VosA is functionally redundant in M. robertsii unlike its ortholog in B. bassiana. IMPORTANCE WetA and VosA regulate conidiation and conidial maturation required for the life cycle of Beauveria bassiana, like they do in Aspergillus, but remain functionally unexplored in Metarhizium robertsii, another hypocrealean pathogen considered to have evolved insect pathogenicity ~130 million years later than B. bassiana. This study reveals a similar role of WetA ortholog in asexual development, conidial maturation, and insect pathogenicity, and also its distinctive role in mediating some other conidial maturation-related cellular events, but has functional redundancy of VosA in M. robertsii. The maturation process vital for conidial quality proves dependent on a role of WetA in spore wall assembly but is independent of its role in intracellular polyol accumulation. Transcriptomic analysis reveals a link of WetA to 160 genes involved in cellular component, biological process, and molecular function. Our study unveils that M. robertsii WetA or VosA is functionally differential or different from those learned in B. bassiana and other ascomycetes.

Role of c-Jun NH2 -terminal kinase-mediated mitogen-activated protein kinase pathway in response to pesticides in Apis cerana cerana

The mitogen-activated protein kinase (MAPK) cascade pathway plays an important role in regulating stress responses. The function of the c-Jun NH₂ -terminal kinase (JNK), a component of the MAPK cascade pathway, in Apis cerana cerana (Acc) remains unclear. Here, JNK was isolated and identified from Acc. Bioinformatics analyses revealed there is a typical serine/threonine protein kinase catalytic domain in the AccJNK protein. An expression profile analysis showed that AccJNK was significantly induced by pesticide treatments. To further explore the functional mechanisms of AccJNK, a yeast 2-hybrid screen was performed, activator protein-1 (AP-1) was screened as the interaction partner of AccJNK, and the interaction relationship was further verified by pull-down assay. Quantitative real-time polymerase chain reaction showed the expression pattern of AccAP-1 was similar to that of AccJNK. After a knockdown of AccJNK or AccAP-1 by RNA interference, the survival rate of Acc after pesticide treatments increased. Additionally, the expression levels of antioxidant-related genes and the activities of antioxidant enzymes increased, suggesting that the knockdown of AccJNK or AccAP-1 increased the antioxidant capacity of bees. Our study revealed that the JNK-mediated MAPK pathway responds to pesticide stress by altering the antioxidant capacity of Acc.

Roles of BrlA and AbaA in Mediating Asexual and Insect Pathogenic Lifecycles of Metarhizium robertsii

BrlA and AbaA are key activators of the central developmental pathway (CDP) that controls asexual development in Aspergillus but their roles remain insufficiently understood in hypocerealean insect pathogens. Here, regulatory roles of BrlA and AbaA orthologs in Metarhizium robertsii (Clavicipitaceae) were characterized for comparison to those elucidated previously in Beauveria bassiana (Cordycipitaceae) at phenotypic and transcriptomic levels. Time-course transcription profiles of brlA, abaA, and the other CDP activator gene wetA revealed that they were not so sequentially activated in M. robertsii as learned in Aspergillus. Aerial conidiation essential for fungal infection and dispersal, submerged blastospore production mimicking yeast-like budding proliferation in insect hemocoel, and insect pathogenicity via cuticular penetration were all abolished as a consequence of brlA or abaA disruption, which had little impact on normal hyphal growth. The disruptants were severely compromised in virulence via cuticle-bypassing infection (intrahemocoel injection) and differentially impaired in cellular tolerance to oxidative and cell wall-perturbing stresses. The ΔbrlA and ΔabaA mutant shad 255 and 233 dysregulated genes (up/down ratios: 52:203 and 101:122) respectively, including 108 genes co-dysregulated. These counts were small compared with 1513 and 2869 dysregulated genes (up/down ratios: 707:806 and 1513:1356) identified in ΔbrlA and ΔabaA mutants of B. bassiana. Results revealed not only conserved roles for BrlA and AbaA in asexual developmental control but also their indispensable roles in fungal adaptation to the insect-pathogenic lifecycle and host habitats. Intriguingly, BrlA- or AbaA-controlled gene expression networks are largely different between the two insect pathogens, in which similar phenotypes were compromised in the absence of either brlA or abaA.

FluG and FluG-like FlrA Coregulate Manifold Gene Sets Vital for Fungal Insect-Pathogenic Lifestyle but Not Involved in Asexual Development

The central developmental pathway (CDP) activator gene brlA is activated by the upstream genes fluG and flbA–flbE in Aspergillus nidulans. Increasing evidences of fungal genome divergence make it necessary to clarify whether such genetic principles fit Pezizomycotina. Previously, fluG disruption resulted in limited conidiation defect and little effect on the expression of brlA and flbA–flbE in Beauveria bassiana possessing the other FluG-like regulator FlrA. Here, single-disruption (SD) mutants of flrA and double-disruption (DD) mutants of flrA and fluG were analyzed to clarify whether FlrA and FluG are upstream regulators of key CDP genes. Despite similar subcellular localization, no protein-protein interaction was detected between FlrA and FluG, suggesting mutual independence. Three flrA SD mutants showed phenotypes similar to those previously described for ΔfluG, including limited conidiation defect, facilitated blastospore production, impaired spore quality, blocked host infection, delayed proliferation in vivo, attenuated virulence, and increased sensitivities to multiple stresses. Three DD mutants resembled the SD mutants in all phenotypes except more compromised pathogenicity and tolerance to heat shock- or calcofluor white-induced stress. No CDP gene appeared in 1,622 and 2,234 genes dysregulated in the ΔflrA and ΔfluG mutants, respectively. The majority (up/down ratio: 540:875) of those dysregulated genes were co-upregulated or co-downregulated at similar levels in the two mutants. These findings unravel novel roles for flrA and fluG in coregulating manifold gene sets vital for fungal adaptation to insect-pathogenic lifestyle and environment but not involved in CDP activation.

IMPORTANCE FluG is a core regulator upstream of central developmental pathway (CDP) in Aspergillus nidulans but multiple FluG-like regulators (FLRs) remain functionally uncharacterized in ascomycetes. Our previous study revealed no role for FluG in the CDP activation and an existence of sole FLR (FlrA) in an insect-pathogenic fungus. This study reveals a similarity of FlrA to FluG in domain architecture and subcellular localization. Experimental data from analyses of targeted single- and double-gene knockout mutants demonstrate similar roles of FrlA and FluG in stress tolerance and infection cycle but no role of either in CDP activation. Transcriptomic analyses reveal that FlrA and FluG coregulate a large number of same genes at similar levels. However, the regulated genes include no key CDP gene. These findings uncover that FlrA and FluG play similar roles in the fungal adaptation to insect-pathogenic lifestyle and environment but no role in the activation of CDP.

PKC-SWI6 signaling regulates asexual development, cell wall integrity, stress response, and lifestyle transition in the nematode-trapping fungus Arthrobotrys oligospora

Predatory fungi possess intricate signal transduction systems that regulate their development and support successful infection of the host. Herein, we characterized three components of the cell wall integrity-controlling pathway, namely protein kinase C (AoPKC), SLT2-MAPK (AoSLT2), and SWI6 (AoSWI6), in a representative nematode-trapping fungus Arthrobotrys oligospora, using gene disruption and multi-omics approaches. The phenotypic traits (such as mycelia development, conidiation, stress response, and trap morphogenesis) and metabolic profiles of ΔAopkc and ΔAoswi6 mutants were similar but differed from those of the ΔAoslt2 mutants. Transcriptomic analysis indicated that the genes differentially expressed in the absence of Aoswi6 were involved in DNA replication, repair, and recombination during trap formation. Moreover, the yeast two-hybrid assay showed that AoPKC interacted with AoSWI6, suggesting that in A. oligospora, PKC can directly regulate SWI6, bypassing the SLT2 signaling cascade. Conclusively, our findings deepen our understanding of the regulatory mechanism of asexual development and lifestyle switching in nematode-trapping fungi.

Three Small Cysteine-Free Proteins (CFP1–3) Are Required for Insect-Pathogenic Lifestyle of Metarhizium robertsii

Unique CFP (cysteine-free protein; 120 aa) has been identified as an extraordinary virulence factor in Beauveria bassiana (Cordycipitaceae), a main source of wide-spectrum fungal insecticides. Its homologs exclusively exist in wide-spectrum insect pathogens of Hypocreales, suggesting their importance for a fungal insect-pathogenic lifestyle. In this study, all three CFP homologs (CFP1–3, 128–145 aa) were proven essential virulence factors in Metarhizium robertsii (Clavicipitaceae). Despite limited effects on asexual cycles in vitro, knockout mutants of cfp1,cfp2 and cfp3 were severely compromised in their capability for normal cuticle infection, in which most tested Galleria mellonella larvae survived. The blocked cuticle infection concurred with reduced secretion of extracellular enzymes, including Pr1 proteases required cuticle penetration. Cuticle-bypassing infection by intrahemocoel injection of ~250 conidia per larva resulted in a greater reduction in virulence in the mutant of cfp1 (82%) than of cfp2 (21%) or cfp3 (25%) versus the parental wild-type. Transcriptomic analysis revealed dysregulation of 604 genes (up/down ratio: 251:353) in the Δcfp1 mutant. Many of them were involved in virulence-related cellular processes and events aside from 154 functionally unknown genes (up/down ratio: 56:98). These results reinforce the essential roles of small CFP homologs in hypocrealean fungal adaptation to insect-pathogenic lifestyle and their exploitability for the genetic improvement of fungal insecticidal activity.

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

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

Unveiling a Novel Role of Cdc42 in Pyruvate Metabolism Pathway to Mediate Insecticidal Activity of Beauveria bassiana

The small GTPase Cdc42 acts as a molecular switch essential for cell cycles and polar growth in model yeast, but has not been explored in Beaurveria bassiana, an insect-pathogenic fungus serving as a main source of fungal formulations against arthropod pests. Here, we show the indispensability of Cdc42 for fungal insecticidal activity. Deletion of cdc42 in B. bassiana resulted in a great loss of virulence to Galleria mellonella, a model insect, via normal cuticle infection as well as defects in conidial germination, radial growth, aerial conidiation, and conidial tolerance to heat and UVB irradiation. The deleted mutant’s hyphae formed fewer or more septa and produced unicellular blastospores with disturbed cell cycles under submerged-culture conditions. Transcriptomic analysis revealed differential expression of 746 genes and dysregulation of pyruvate metabolism and related pathways, which were validated by marked changes in intracellular pyruvate content, ATP content, related enzyme activities, and in extracellular beauvericin content and Pr1 protease activity vital for fungal virulence. These findings uncover a novel role for Cdc42 in the pathways of pyruvate metabolism and the pyruvate-involved tricarboxylic acid cycle (TCA cycle) and a linkage of the novel role with its indispensability for the biological control potential of B. bassiana against arthropod pests.

AoSsk1, a Response Regulator Required for Mycelial Growth and Development, Stress Responses, Trap Formation, and the Secondary Metabolism in Arthrobotrys oligospora

Ssk1, a response regulator of the two-component signaling system, plays an important role in the cellular response to hyperosmotic stress in fungi. Herein, an ortholog of ssk1 (Aossk1) was characterized in the nematode-trapping fungus Arthrobotrys oligospora using gene disruption and multi-phenotypic comparison. The deletion of Aossk1 resulted in defective growth, deformed and swollen hyphal cells, an increased hyphal septum, and a shrunken nucleus. Compared to the wild-type (WT) strain, the number of autophagosomes and lipid droplets in the hyphal cells of the ΔAossk1 mutant decreased, whereas their volumes considerably increased. Aossk1 disruption caused a 95% reduction in conidial yield and remarkable defects in tolerance to osmotic and oxidative stress. Meanwhile, the transcript levels of several sporulation-related genes were significantly decreased in the ΔAossk1 mutant compared to the WT strain, including abaA, brlA, flbC, fluG, and rodA. Moreover, the loss of Aossk1 resulted in a remarkable increase in trap formation and predation efficiency. In addition, many metabolites were markedly downregulated in the ΔAossk1 mutant compared to the WT strain. Our results highlight that AoSsk1 is a crucial regulator of asexual development, stress responses, the secondary metabolism, and pathogenicity, and can be useful in probing the regulatory mechanism underlying the trap formation and lifestyle switching of nematode-trapping fungi.

Molecular basis and regulatory mechanisms underlying fungal insecticides' resistance to solar ultraviolet irradiation

Resistance to solar ultraviolet (UV) irradiation is crucial for field-persistent control efficacies of fungal formulations against arthropod pests, because their active ingredients are formulated conidia very sensitive to solar UV wavelengths. This review seeks to summarize advances in studies aiming to quantify, understand and improve conidial UV resistance. One focus of studies has been on the many sets of genes that have been revealed in the postgenomic era to contribute to or mediate UV resistance in the insect pathogens serving as main sources of fungal insecticides. Such genetic studies have unveiled the broad basis of UV-resistant molecules including cytosolic solutes, cell wall components, various antioxidant enzymes, and numerous effectors and signaling proteins, that function in developmental, biosynthetic and stress-responsive pathways. Another focus has been on the molecular basis and regulatory mechanisms underlying photorepair of UV-induced DNA lesions and photoreactivation of UV-impaired conidia. Studies have shed light upon a photoprotective mechanism depending on not only one or two photorepair-required photolyases, but also two white collar proteins and other partners that play similar or more important roles in photorepair via interactions with photolyases. Research hotspots are suggested to explore a regulatory network of fungal photoprotection and to improve the development and application strategies of UV-resistant fungal insecticides. © 2021 Society of Chemical Industry.

Conserved and Noncanonical Activities of Two Histone H3K36 Methyltransferases Required for Insect-Pathogenic Lifestyle of Beauveria bassiana

Set2 and Ash1 are histone methyltransferases (KMTs) in the KMT3 family normally used to catalyze methylation of histone H3K36 (H3K36me) but remain unexplored in fungal insect pathogens. Here, we report broader/greater roles of Set2 and Ash1 in mono-/di-/trimethylation (me1/me2/me3) of H3K4 than of H3K36 in Beauveria bassiana and function similarly to Set1/KMT2, which has been reported to catalyze H3K4me3 as an epigenetic mark of cre1 (carbon catabolite repressor) to upregulate the classes I and II hydrophobin genes hyd1 and hyd2 required for conidial hydrophobicity and adherence to insect cuticle. H3K4me3 was more attenuated than H3K36me3 in the absence of set2 (72% versus 67%) or ash1 (92% versus 12%), leading to sharply repressed or nearly abolished expression of cre1, hyd1 and hyd2, as well as reduced hydrophobicity. Consequently, the delta-set2 and delta-ash1 mutants were differentially compromised in radial growth on various media or under different stresses, aerial conidiation under normal culture conditions, virulence, and cellular events crucial for normal cuticle infection and hemocoel colonization, accompanied by transcriptional repression of subsets of genes involved in or required for asexual development and multiple stress responses. These findings unravel novel roles of Set2 and Ash1 in the co-catalysis of usually Set1-reliant H3K4me3 required for fungal insect-pathogenic lifestyle.

Genome-Wide Insight into Profound Effect of Carbon Catabolite Repressor (Cre1) on the Insect-Pathogenic Lifecycle of Beauveriabassiana

Carbon catabolite repression (CCR) is critical for the preferential utilization of glucose derived from environmental carbon sources and regulated by carbon catabolite repressor A (Cre1/CreA) in filamentous fungi. However, a role of Cre1-mediated CCR in insect-pathogenic fungal utilization of host nutrients during normal cuticle infection (NCI) and hemocoel colonization remains explored insufficiently. Here, we report an indispensability of Cre1 for Beauveriabassiana's utilization of nutrients in insect integument and hemocoel. Deletion of cre1 resulted in severe defects in radial growth on various media, hypersensitivity to oxidative stress, abolished pathogenicity via NCI or intrahemocoel injection (cuticle-bypassing infection) but no change in conidial hydrophobicity and adherence to insect cuticle. Markedly reduced biomass accumulation in the Δcre1 cultures was directly causative of severe defect in aerial conidiation and reduced secretion of various cuticle-degrading enzymes. The majority (1117) of 1881 dysregulated genes identified from the Δcre1 versus wild-type cultures were significantly downregulated, leading to substantial repression of many enriched function terms and pathways, particularly those involved in carbon and nitrogen metabolisms, cuticle degradation, antioxidant response, cellular transport and homeostasis, and direct/indirect gene mediation. These findings offer a novel insight into profound effect of Cre1 on the insect-pathogenic lifestyle of B. bassiana.

Prey sensing and response in a nematode-trapping fungus is governed by the MAPK pheromone response pathway

Detection of surrounding organisms in the environment plays a major role in the evolution of interspecies interactions, such as predator-prey relationships. Nematode-trapping fungi (NTF) are predators that develop specialized trap structures to capture, kill, and consume nematodes when food sources are limited. Despite the identification of various factors that induce trap morphogenesis, the mechanisms underlying the differentiation process have remained largely unclear. Here, we demonstrate that the highly conserved pheromone-response MAPK pathway is essential for sensing ascarosides, a conserved molecular signature of nemaotdes, and is required for the predatory lifestyle switch in the NTF Arthrobotrys oligospora. Gene deletion of STE7 (MAPKK) and FUS3 (MAPK) abolished nematode-induced trap morphogenesis and conidiation and impaired the growth of hyphae. The conserved transcription factor Ste12 acting downstream of the pheromone-response pathway also plays a vital role in the predation of A. oligospora. Transcriptional profiling of a ste12 mutant identified a small subset of genes with diverse functions that are Ste12 dependent and could trigger trap differentiation. Our work has revealed that A. oligospora perceives and interprets the ascarosides produced by nematodes via the conserved pheromone signaling pathway in fungi, providing molecular insights into the mechanisms of communication between a fungal predator and its nematode prey.

SET1/KMT2-governed histone H3K4 methylation coordinates the lifecycle in vivo and in vitro of the fungal insect pathogen Beauveria bassiana

Biological control potential of insect-pathogenic fungi against pests is an overall output of various cellular processes regulated by signalling and epigenetic networks. In Beauveria bassiana, mono/di/trimethylation of histone H3 Lys 4 (H3K4me1/me2/m3) was abolished by inactivation of the histone lysine methyltransferase SET1/KMT2, leading to marked virulence loss, reductions in conidial hydrophobicity and adherence to insect cuticle, impeded proliferation in vivo, severe defects in growth and conidiation, and increased sensitivities to cell wall perturbation, H₂ O₂ and heat shock. Such compromised phenotypes correlated well with transcriptional abolishment or repression of carbon catabolite-repressing transcription factor Cre1, classes I and II hydrophobins Hyd1 and Hyd2 required for cell hydrophobicity, key developmental regulators, and stress-responsive enzymes/proteins. Particularly, expression of cre1, which upregulates hyd4 upon activation by KMT2-mediated H3K4me3 in Metarhizium robertsii, was nearly abolished in the Δset1 mutant, leading to abolished expression of hyd1 and hyd2 as homologues of hyd4. These data suggest that the SET1-Cre1-Hyd1/2 pathway function in B. bassiana like the KMT2-Cre1-Hyd4 pathway elucidated to mediate pathogenicity in M. robertsii. Our findings unveil not only a regulatory role for the SET1-cored pathway in fungal virulence but also its novel role in mediating asexual cycle in vitro and stress responses in B. bassiana.

Essential Role of COP9 Signalosome Subunit 5 (Csn5) in Insect Pathogenicity and Asexual Development of Beauveria bassiana

Csn5 is a subunit ofthe COP9/signalosome complex in model fungi. Here, we report heavier accumulation of orthologous Csn5 in the nucleus than in the cytoplasm and its indispensability to insect pathogenicity and virulence-related cellular events of Beauveria bassiana. Deletion of csn5 led to a 68% increase in intracellular ubiquitin accumulation and the dysregulation of 18 genes encoding ubiquitin-activating (E1), -conjugating (E2), and -ligating (E3) enzymes and ubiquitin-specific proteases, suggesting the role of Csn5 in balanced ubiquitination/deubiquitination. Consequently, the deletion mutant displayed abolished insect pathogenicity, marked reductions in conidial hydrophobicity and adherence to the insect cuticle, the abolished secretion of cuticle penetration-required enzymes, blocked haemocoel colonisation, and reduced conidiation capacity despite unaffected biomass accumulation. These phenotypes correlated well with sharply repressed or abolished expressions of key hydrophobin genes required for hydrophobin biosynthesis/assembly and of developmental activator genes essential for aerial conidiation and submerged blastospore production. In the mutant, increased sensitivities to heat shock and oxidative stress also correlated with reduced expression levels of several heat-responsive genes and decreased activities of antioxidant enzymes. Altogether, Csn5-reliant ubiquitination/deubiquitination balance coordinates the expression of those crucial genes and the quality control of functionally important enzymes, which are collectively essential for fungal pathogenicity, virulence-related cellular events, and asexual development.

Comparative roles of three adhesin genes (adh1-3) in insect-pathogenic lifecycle of Beauveria bassiana

Adherence of conidia to insect integument is crucial for initiation of fungal infection through cuticular penetration and was previously reported to rely upon the Metarhizium-type adhesin Mad1 rather than Mad2, another adhesin crucial for conidial adherence of Metarhizium anisopliae to plant root surface. Mad1 and Mad2 have since been considered to function in fungal insect pathogenesis and plant root colonization respectively. Here, three adhesins were characterized in Beauveria bassiana, including Adh1/Mad1, Adh2/Mad2, and Adh3 known as filamentous hemagglutinin/adhesin and virulence factor in animal-pathogenic bacteria. Among those, only Adh2 was found to play a substantial role in sustaining the fungal virulence and some phenotypes associated with biological control potential. Disruption of adh2 resulted in decreased conidial adherence to insect wing cuticle, attenuated virulence via normal cuticle infection or cuticle-bypassing infection (injection), reduced blastospore production in an insect hemolymph-mimicking broth, largely reduced conidiation capacity, impaired conidial quality indicative of lowered viability, hydrophobicity, and UV resistance, but no growth defects on rich and scant media under normal or stressful culture conditions. The main phenotypic changes correlated well with repressed expression of developmental activator genes required for aerial conidiation and submerged blastospore production and of key hydrophobin genes essential for hydrophobin synthesis and assembly into rodlet bundles of conidial coat crucial for conidial adherence. In contrast, either adh1 or adh3 disruption caused insignificant changes in all phenotypes examined. These findings offer novel insight into a significance of Adh2, but a dispensability of Adh1 or Adh3, for insect-pathogenic lifecycle of B. bassiana. KEY POINTS: • Three adhesins (Adh1-3) of Beauvera bassiana are functionally characterized. • Adh2 plays a role in sustaining virulence and lifecycle-related cellular events. • Either Adh1 or Adh3 is dispensable for insect-pathogenic lifecycle of B. bassiana.

Long noncoding RNAs are potentially involved in the degeneration of virulence in an aphid-obligate pathogen, Conidiobolus obscurus (Entomophthoromycotina)

Virulence attenuation frequently occurs in in vitro culturing of pathogenic microbes. In this study, we investigated the total putative long noncoding RNAs (lncRNAs) in an aphid-obligate pathogen, Conidiobolus obscurus, and screened the differentially expressed (DE) lncRNAs and protein-coding genes involved in the virulence decline. The virulence was significantly attenuated after eight subculturing events, in which the median lethal concentration of the conidia ejected from mycelial mats relative to the bamboo aphid, Takecallis taiwanus, increased from 36.1 to 126.1 conidia mm^–2, four days after inoculation. In total, 1,252 lncRNAs were identified based on the genome-wide transcriptional analysis. By characterizing their molecular structures and expression patterns, we found that the lncRNAs possessed shorter transcripts, lower expression, and fewer exons than did protein-coding genes in C. obscurus. A total of 410 DE genes of 329 protein-coding genes and 81 lncRNAs were identified. The functional enrichment analysis showed the DE genes were enriched in peptidase activity, protein folding, autophagy, and metabolism. Moreover, target prediction analysis of the 81 lncRNAs revealed 3,111 cis-regulated and 23 trans-regulated mRNAs, while 121 DE lncRNA-mRNA pairs were possibly involved in virulence decline. Moreover, the DE lncRNA-regulated target genes mainly encoded small heat shock proteins, secretory proteins, transporters, autophagy proteins, and other stress response-related proteins. This implies that the decline in virulence regulated by lncRNAs was likely associated with the environmental stress response of C. obscurus. Hence, these findings can provide insights into the lncRNA molecules of Entomophthoromycotina, with regards to virulence regulators of entomopathogens.

Transcriptomic insights into the effects of CytCo, a novel nematotoxic protein, on the pine wood nematode Bursaphelenchus xylophilus

Background

The pine wood nematode Bursaphelenchus xylophilus is a destructive pest of Pinus trees worldwide and lacks effective control measures. Screening for nematotoxic proteins has been undertaken to develop new strategies for nematode control.

Results

The results of the present study provided initial insights into the responses of B. xylophilus exposed to a nematotoxic cytolytic-like protein (CytCo) based on transcriptome profiling. A large set of differentially expressed genes (DEGs = 1265) was found to be related to nematode development, reproduction, metabolism, motion, and immune system. In response to the toxic protein, B. xylophilus upregulated DEGs encoding cuticle collagens, transporters, and cytochrome P450. In addition, many DEGs related to cell death, lipid metabolism, major sperm proteins, proteinases/peptidases, phosphatases, kinases, virulence factors, and transthyretin-like proteins were downregulated. Gene Ontology enrichment analysis showed that the CytCo treatment substantially affected DEGs involved in muscle contraction, lipid localization, and the mitogen-activated protein kinase cascade. The pathway richness of the Kyoto Encyclopedia of Genes and Genomes showed that the DEGs were concentrated in lysosomes and involved in fatty acid degradation. Weighted co-expression network analysis indicated that the hub genes affected by CytCo were associated with the nematode cuticular collagen.

Conclusions

These results showed that CytCo toxin interferes with gene expression to exert multiple nematotoxic effects, thereby providing insights into its potential use in pine wood nematode control.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07714-y.

DIM5/KMT1 controls fungal insect pathogenicity and genome stability by methylation of histone H3K4, H3K9 and H3K36

Mono-, di- and tri-methylation of histone H3 Lys 9, Lys 4, and Lys 36 (H3K_me1/me2/me3) required for mediation of DNA-based cellular events in eukaryotes usually rely upon the activities of histone lysine methyltransferases (KMTs) classified to the KMT1, KMT2, and KMT3 families, respectively. Here, an H3K9-specific DIM5/KMT1 orthologue, which lacks a C-terminal post-SET domain and localizes mainly in nucleus, is reported to have both conserved and noncanonical roles in methylating the H3 core lysines in Beauveria bassiana, an insect-pathogenic fungus serving as a main source of wide-spectrum fungal insecticides. Disruption of dim5 led to abolishment of H3K9me3 and marked attenuation of H3K4me1/me2, H3K9me1/me2 and H3K36me2. Consequently, the Δdim5 mutant lost the whole insect pathogenicity through normal cuticle infection, and was compromised severely in virulence through cuticle-bypassing infection (hemocoel injection) and also in a series of cellular events critical for the fungal virulence and lifecycle in vivo and in vitro, including reduced hyphal growth, blocked conidiation, impeded proliferation in vivo, altered carbohydrate epitopes, disturbed cell cycle, reduced biosynthesis and secretion of cuticle-degrading enzymes, and increased sensitivities to various stresses. Among 1,201 dysregulated genes (up/down ratio: 712:489) associated with those phenotypic changes, 92 (up/down ratio: 59:33) encode transcription factors and proteins or enzymes involved in posttranslational modifications, implying that the DIM5-methylated H3 core lysines could act as preferential marks of those transcription-active genes crucial for global gene regulation. These findings uncover a novel scenario of DIM5 and its indispensability for insect-pathogenic lifestyle and genome stability of B. bassiana.

Distinctive role of fluG in the adaptation of Beauveria bassiana to insect-pathogenic lifecycle and environmental stresses

The upstream developmental activation (UDA) pathway comprises three fluG-cored cascades (fluG-flbA, fluG-flbE/B/D and fluG-flbC) that activate the key gene brlA of central developmental pathway (CDP) to initiate conidiation in aspergilli. However, the core role of fluG remains poorly understood in other fungi. Here, we report distinctive role of fluG in the insect-pathogenic lifecycle of Beauveria bassiana. Disruption of fluG resulted in limited conidiation defect, which was mitigated with incubation time and associated with time-course up-regulation/down-regulation of all flb and CDP genes and another fluG-like gene (BBA_06309). In ΔfluG, increased sensitivities to various stresses correlated with repression of corresponding stress-responsive genes. Its virulence through normal cuticle infection was attenuated greatly due to blocked secretion of cuticle-degrading enzymes and delayed formation of hyphal bodies (blastospores) to accelerate proliferation in vivo and host death. In submerged ΔfluG cultures mimicking insect haemolymph, largely increased blastospore production concurred with drastic up-regulation of the CDP genes brlA and abaA, which was associated with earlier up-regulation of most flb genes in the cultures. Our results unveil an essentiality of fluG for fungal adaptation to insect-pathogenic lifecycle and suggest the other fluG-like gene to act as an alternative player in the UDA pathway of B. bassiana.

Calcofluor white hypersensitive proteins contribute to stress tolerance and pathogenicity in entomopathogenic fungus, Metarhizium acridum

BACKGROUND

Fungal cell wall integrity is vital for fungal pathogenesis and stress tolerance. Calcofluor white (CFW), a cell wall perturbing agent, inhibits fungal growth by binding chitin in the cell wall. The roles of CFW sensitive proteins remain insufficiently understood in pathogenic fungi.

RESULTS

We investigated two calcofluor white hypersensitive proteins, MaCwh1 and MaCwh43, in the entomopathogenic fungus Metarhizium acridum. Both Green fluorescent protein (GFP)-tagged MaCwh1 and MaCwh43 localized at the endoplasmic reticulum. Our results showed that the ΔMacwh1 and ΔMacwh43 mutants were more sensitive to CFW and ultraviolet irradiation stress compared to wild-type and complement strains. ΔMacwh1 had a stronger sensitivity to these stresses than ΔMacwh43. Both ΔMacwh1 and ΔMacwh43 mutants showed smoother cell wall surface, and drastically reduced chitin and mannose glycoprotein level in the cell wall and glycerol level in conidia compared to wild type. Insect bioassay showed significantly attenuated virulence for both ΔMacwh1 and ΔMacwh43 mutants with impaired ability in penetrating the host cuticle. RNA-Seq analysis revealed that a large number of genes presumably involved in cell wall construction and modification, pathogenicity and stress response were down-regulated in both ΔMacwh1 and ΔMacwh43 mutants.

CONCLUSIONS

These findings demonstrate that both Macwh1 and Macwh43 affect the fungal cell wall ultrastructure and contribute to the stress tolerance and pest control potential in M. acrdium. © 2020 Society of Chemical Industry.

A mixture containing the herbicides Mesotrione and Atrazine imposes toxicological risks on workers of Partamona helleri

A mixture of Mesotrione and Atrazine (Calaris®) has been reported as an improvement of the atrazine herbicides, which are agrochemicals used for weed control. However, its possible harmful effects on non-target organisms, including pollinators, needs to be better understood. In this work, the effects of the mix of herbicides on food consumption, behaviour (walking distance, and meandering), and the morphology of the midgut of the stingless bee Partamona helleri were studied. Foragers were orally exposed to different concentrations of the mix. The concentrations leading to 10% and 50% mortality (LC₁₀ and LC₅₀, respectively) were estimated and used in the analysis of behaviour and morphology. The ingestion of contaminated diets (50% aqueous sucrose solution + mix) led to a reduction in food consumption by the bees when compared to the control, bees fed a non-contaminated diet (sucrose solution). Ingestion of the LC₅₀ diet reduced locomotor activity, increased meandering, induced the degradation of the epithelium and peritrophic matrix, and also changed the number of cells positive for signalling-pathway proteins in the midgut. These results show the potential toxicological effects and environmental impacts of the mix of herbicides in beneficial insects, including a native bee.

A Small Cysteine-Free Protein Acts as a Novel Regulator of Fungal Insect-Pathogenic Lifecycle and Genomic Expression

Small secreted proteins (SSPs), particularly cysteine-rich proteins secreted during fungal infection, comprise virulence effectors in plant-pathogenic fungi but remain unknown in insect-pathogenic fungi. We report here that only a small cysteine-free protein (CFP) is indispensable for insect pathogenicity of Beauveria bassiana among 10 studied SSPs (99 to 274 amino acids [aa]), including seven hypothetical proteins containing 0 to 12 Cys residues. CFP (120 aa) features an N-terminal signal peptide (residues 1 to 17), a nuclear localization signal motif (residues 24 to 57), and no predictable domain. Its homologs exist exclusively in insect-pathogenic Cordycipitaceae and Clavicipitaceae. Fluorescence-tagged CFP fusion protein was localized in the nucleus but extracellularly undetectable, suggesting an inability for CFP to be secreted out. Disruption of cfp resulted in abolished pathogenicity via normal cuticle infection, attenuated virulence via hemocoel injection, compromised conidiation capacity versus little growth defect, impaired conidial coat, blocked secretion of cuticle-degrading enzymes, impeded proliferation in vivo, disturbed cell cycle, reduced stress tolerance, and 1,818 dysregulated genes (genomic 17.54%). Hundreds of those genes correlated with phenotypic changes observed in the disruption mutant. Intriguingly, nearly 40% of those dysregulated genes encode hypothetical or unknown proteins, and another 13% encode transcription factors and enzymes or proteins collectively involved in genome-wide gene regulation. However, purified CFP showed no DNA-binding activity in an electrophoretic mobility shift assay. These findings unveil that CFP is a novel regulator of fungal insect-pathogenic life cycle and genomic expression and that cysteine richness is dispensable for distinguishing virulence effectors from putative SSPs in B. bassiana IMPORTANCE Small cysteine-rich proteins secreted during plant-pathogenic fungal infection comprise virulence effectors. Our study confirms that only a cysteine-free protein (CFP) is determinant to insect-pathogenic fungal virulence among 10 small putatively secreted proteins containing 0 to 12 Cys residues. Disruption of cfp abolished insect pathogenicity and caused not only a series of compromised cellular events associated with host infection and disease development but also dysregulation of 1,818 genes, although no DNA-binding activity was detected in purified CFP samples. Nearly 13% of those genes encode transcription factors and enzymes or proteins collectively involved in transcriptional regulation. Altogether, CFP serves as a novel regulator of the fungal insect-pathogenic life cycle and genomic expression. Cysteine richness is dispensable for distinguishing virulence effectors from the fungal SSPs.

Ubr1-mediated ubiquitylation orchestrates asexual development, polar growth, and virulence-related cellular events in Beauveria bassiana

The E3 ubiquitin ligase Ubr1 is a core player in yeast ubiquitylation and protein quality control required for cellular events including proteasomal degradation and gene activity but has been rarely explored in filamentous fungi. We show here an essentiality of orthologous Ubr1-mediated ubiquitylation for the activation of central developmental pathway (CPD) and the CPD-controlled cellular events in Beauveria bassiana, a filamentous fungal insect pathogen that undergoes an asexual cycle in vitro or in vivo. As a result of ubr1 disruption, intracellular free ubiquitin accumulation increased by 1.4-fold, indicating an impaired ability for the disruptant to transfer ubiquitin to target proteins. Consequently, the disruptant was compromised in polar growth featured with curved or hook-like germ tubes and abnormally branched hyphae, leading to impeded propagation of aberrant hyphal bodies in infected insect hemocoel and attenuated virulence. In the mutant, sharply repressed expression of three CDP activator genes (brlA, abaA, and wetA) correlated well with severe defects in aerial conidiation and submerged blastospore (hyphal body) production in insect hemolymph or a mimicking medium. Moreover, the disruptant was sensitive to cell wall perturbation or lysing and showed increased catalase activity and resistance to hydrogen peroxide despite null response to high osmolarity or heat shock. Most of the examined genes involved in polar growth and cell wall integrity were down-regulated in the disruptant. These findings uncover that the Ubr1-mediated ubiquitylation orchestrates polar growth and the CDP-regulated asexual cycle in vitro and in vivo in B. bassiana. KEY POINTS: • Ubr1 is an E3 ubiquitin ligase essential for ubiquitylation in Beauveria bassiana. • Ubr1-mediated ubiquitylation is required for activation of central development pathway. • Ubr1 orchestrates polar growth and asexual cycle in vitro and in vivo.

Essential Roles of Two FRQ Proteins (Frq1 and Frq2) in Beauveria bassiana's Virulence, Infection Cycle, and Calcofluor-Specific Signaling

Two FRQ proteins (Frq1 and Frq2) distinct in molecular mass and structure coexist in Beauveria bassiana, an asexual insect-pathogenic fungus. Frq1 and Frq2 have been proven to have opposite nuclear rhythms that can persistently activate developmental activator genes and hence orchestrate nonrhythmic conidiation in vitro under light or in darkness. Here, we report the essentiality of either FRQ, but Frq2 being more important than Frq1, for the fungal virulence and infection cycle. The fungal virulence was attenuated significantly more in the absence of frq2 than in the absence of frq1 through either normal cuticle infection or cuticle-bypassing infection by intrahemocoel injection, accompanied by differentially reduced secretion of Pr1 proteases required for the cuticle infection and delayed development of hyphal bodies in vivo, which usually propagate by yeast-like budding in the host hemocoel to accelerate insect death from mycosis. Despite insignificant changes in radial growth under normal, oxidative, and hyperosmotic culture conditions, conidial yields of the Δfrq1 and Δfrq2 mutants on insect cadavers were sharply reduced, and the reduction increased with shortening daylight length on day 9 or 12 after death, indicating that both Frq1 and Frq2 are required for the fungal infection cycle in host habitats. Intriguingly, the Δfrq1 and Δfrq2 mutants showed hypersensitivity and high resistance to cell wall-perturbing calcofluor white, coinciding respectively with the calcofluor-triggered cells' hypo- and hyperphosphorylated signals of Slt2, a mitogen-activated protein kinase (MAPK) required for mediation of cell wall integrity. This finding offers a novel insight into opposite roles of Frq1 and Frq2 in calcofluor-specific signal transduction via the fungal Slt2 cascade.IMPORTANCE Opposite nuclear rhythms of two distinct FRQ proteins (Frq1 and Frq2) coexisting in an asexual fungal insect pathogen have been shown to orchestrate the fungal nonrhythmic conidiation in vitro in a circadian day independent of photoperiod change. This paper reports essential roles of both Frq1 and Frq2, but a greater role for Frq2, in sustaining the fungal virulence and infection cycle since either frq1 or frq2 deletion led to marked delay of lethal action against a model insect and drastic reduction of conidial yield on insect cadavers. Moreover, the frq1 and frq2 mutants display hypersensitivity and high resistance to cell wall perturbation and have hypo- and hyperphosphorylated MAPK/Slt2 in calcofluor white-triggered cells, respectively. These findings uncover a requirement of Frq1 and Frq2 for the fungal infection cycle in host habitats and provide a novel insight into their opposite roles in calcofluor-specific signal transduction through the MAPK/Slt2 cascade.

Subtilisin-like Pr1 proteases marking the evolution of pathogenicity in a wide-spectrum insect-pathogenic fungus

Subtilisin-like Pr1 proteases of insect-pathogenic fungi are a large family of extracellular cuticle-degrading enzymes that presumably determine a capability of hyphal invasion into insect hemocoel through normal cuticle infection, but remain poorly understood although often considered as virulence factors for genetic improvement of fungal potential against pests. Here, we report that not all of 11 Pr1 family members necessarily function in Beauveria bassiana, an ancient wide-spectrum pathogen evolved insect pathogenicity ~200 million years ago. These Pr1 proteases are phylogenetically similar to or distinct from 11 homologues (Pr1A–K) early named in Metarhizium anisopliae complex, a young entomopathogen lineage undergoing molecular evolution toward Pr1 diversification, and hence renamed Pr1A1/A2, Pr1B1–B3, Pr1 C, Pr1F1–F4,4 and Pr1 G, respectively. Multiple analyses of all single gene-deleted and rescued mutants led to the recognition of five conserved members (Pr1C, Pr1G, Pr1A2, Pr1B1, and Pr1B2) contributing significantly to the fungal pathogenicity to insect. The conserved Pr1 proteases were proven to function only in cuticle degradation, individually contribute 19–29% to virulence, but play no role in post-infection cellular events critical for fungal killing action. Six other Pr1 proteases were not functional at all in either cuticle degradation during host infection or virulence-related cellular events post-infection. Therefore, only the five conserved proteases are collectively required for, and hence mark evolution of, insect pathogenicity in B. bassiana. These findings provide the first referable base for insight into the evolution of Pr1 family members in different lineages of fungal insect pathogens.

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

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

A mitogen-activated protein kinase PoxMK1 mediates regulation of the production of plant-biomass-degrading enzymes, vegetative growth, and pigment biosynthesis in Penicillium oxalicum

Mitogen-activated protein kinase (MAPK) cascades are broadly conserved and play essential roles in multiple cellular processes, including fungal development, pathogenicity, and secondary metabolism. Their function, however, also exhibits species and strain specificity. Penicillium oxalicum secretes plant-biomass-degrading enzymes (PBDEs) that contribute to the carbon cycle in the natural environment and to utilization of lignocellulose in industrial processes. However, knowledge of the MAPK pathway in P. oxalicum has been relatively limited. In this study, comparative transcriptomic analysis of P. oxalicum, cultured on different carbon sources, found ten putative kinase genes with significantly modified transcriptional levels. Six of these putative kinase genes were knocked out in the parental strain ∆PoxKu70, and deletion of the gene, Fus3/Kss1-like PoxMK1 (POX00158), resulted in the largest reduction (91.1%) in filter paper cellulase production. Further tests revealed that the mutant ∆PoxMK1 lost 37.1 to 92.2% of PBDE production, under both submerged- and solid-state fermentation conditions, compared with ∆PoxKu70. In addition, the mutant ∆PoxMK1 had reduced vegetative growth and increased pigment biosynthesis. Comparative transcriptomic analysis showed that PoxMK1 deletion from P. oxalicum downregulated the expression of major PBDE genes and known regulatory genes such as PoxClrB and PoxCxrB, whereas the transcription of pigment biosynthesis-related genes was upregulated. Comparative phosphoproteomic analysis revealed that PoxMK1 deletion considerably modified phosphorylation of key transcription- and signal transduction-associated proteins, including transcription factors Mcm1 and Atf1, RNA polymerase II subunits Rpb1 and Rpb9, MAPK-associated Hog1 and Ste7, and cyclin-dependent kinase Kin28. These findings provide novel insights into understanding signal transduction and regulation of PBDE gene expression in fungi.Key points• PoxMK1 is involved in expression of PBDE- and pigment synthesis-related genes.• PoxMK1 is required for vegetative growth of P. oxalicum.• PoxMK1 is involved in phosphorylation of key TFs, kinases, and RNA polymerase II.

P-type Na+/K+ ATPases essential and nonessential for cellular homeostasis and insect pathogenicity of Beauveria bassiana

ENA1 and ENA2 are P-type IID/ENA Na+/K+-ATPases required for cellular homeostasis in yeasts but remain poorly understood in filamentous fungal insect pathogens. Here, we characterized seven genes encoding five ENA1/2 homologues (ENA1a-c and ENA2a/b) and two P-type IIC/NK Na+/K+-ATPases (NK1/2) in Beauveria bassiana, an insect-pathogenic fungus serving as a main source of fungal insecticides worldwide. Most of these genes were highly responsive to alkaline pH and Na+/K+ cues at transcription level. Cellular Na+, K+ and H+ homeostasis was disturbed only in the absence of ena1a or ena2b. The disturbed homeostasis featured acceleration of vacuolar acidification, elevation of cytosolic Na+/K+ level at pH 5.0 to 9.0, and stabilization of extracellular H+ level to initial pH 7.5 during a 5-day period of submerged incubation. Despite little defect in hyphal growth and asexual development, the Δena1a and Δena2b mutants were less tolerant to metal cations (Na+, K+, Li+, Zn2+, Mn2+ and Fe3+), cell wall perturbation, oxidation, non-cation hyperosmolarity and UVB irradiation, severely compromised in insect pathogenicity via normal cuticle infection, and attenuated in virulence via hemocoel injection. The deletion mutants of five other ENA and NK genes showed little change in vacuolar pH and all examined phenotypes. Therefore, only ENA1a and ENA2b evidently involved in both transmembrane and vacuolar activities are essential for cellular cation homeostasis, insect pathogenicity and multiple stress tolerance in B. bassiana. These findings provide a novel insight into ENA1a- and ENA2b-dependent vacuolar pH stability, cation-homeostatic process and fungal fitness to host insect and environment.

The disruption of the MAPKK gene triggering the synthesis of flavonoids in endophytic fungus Phomopsis liquidambaris

Flavonoids, which are mainly extracted from plants, are important antioxidants and play an important role in human diseases. However, the growing market demand is limited by low productivity and complex production processes. Herein, the flavonoids biosynthesis pathway of the endophytic fungus Phomopsis liquidambaris was revealed. The mitogen-activated protein kinase kinase (MAPKK) of the strain was disrupted using a newly constructed CRISPR-Cas9 system mediated by two gRNAs which was conducive to cause plasmid loss. The disruption of the MAPKK gene triggered the biosynthesis of flavonoids against stress and resulted in the precipitation of flavonoids from fermentation broth. Naringenin, kaempferol and quercetin were detected in fed-batch fermentation with yields of 5.65 mg/L, 1.96 mg/L and 2.37 mg/L from P. liquidambaris for dry cell weigh using the mixture of glucose and xylose and corn steep powder as carbon source and nitrogen source for 72 h, respectively. The biosynthesis of flavonoids was triggered by disruption of MAPKK gene in P. liquidambaris and the mutant could utilize xylose.

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

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

Colony heating protects honey bee populations from a risk of contact with wide-spectrum Beauveria bassiana insecticides applied in the field

BACKGROUND

The safety of fungal insecticides to apiculture is a public concern but remains poorly understood. This study seeks to evaluate whether, how and why wide-spectrum Beauveria bassiana insecticides are safe to honey bees in a novel assessment system.

RESULTS

Mesonotum dipping with a 10⁸ conidia ml^-1 suspension and body contact with conidial suspension in sucrose solution caused high mortalities of adult forager bees at 25 °C optimal for conidial germination and hyphal invasion. Intriguingly, colony sizes in the hives contaminated by the forager bees contacting viable and inactivated conidia at two sites (1.2 km in distance), respectively, showed similar increase percentages (31.7% versus 29.2%) during a 4-week summer period of exposure to environment. No sign of fungal infection was found within each of the monitored colonies. Neither was fungal outgrowth observed on surfaces of bee cadavers cleaned from each hive at either site. Hourly counts of cleaned cadavers from videotapes presented no significant difference in colony-cleaning behavior between the two sites. During the period, in-hive temperatures at both sites were persistently stabilized at approximately 35 °C, which abolished conidial germination and were far above the out-hive temperature range.

CONCLUSION

It is colony heating that protects honey bee populations from a risk of forager bees' contact with formulated conidia applied for arthropod pest control. No role was detected for colony self-cleaning behavior in protecting the bee colonies from the risk. © 2020 Society of Chemical Industry.

Three proline rotamases involved in calcium homeostasis play differential roles in stress tolerance, virulence and calcineurin regulation of Beauveria bassiana

FK506-sensitive proline rotamases (FPRs), also known as FK506-binding proteins (FKBPs), can mediate immunosuppressive drug resistance in budding yeast but their physiological roles in filamentous fungi remain opaque. Here, we report that three FPRs (cytosolic/nuclear 12.15-kD Fpr1, membrane-associated 14.78-kD Fpr2 and nuclear 50.43-kD Fpr3) are all equally essential for cellular Ca²⁺ homeostasis and contribute significantly to calcineurin activity at different levels in the insect-pathogenic fungus Beauveria bassiana although the deletion of fpr1 alone conferred resistance to FK506. Radial growth, conidiation, conidial viability and virulence were less compromised in the absence of fpr1 or fpr2 than in the absence of fpr3, which abolished almost all growth on scant media and reduced growth moderately on rich media. The Δfpr3 mutant was more sensitive to Na⁺ , K⁺ , Mn²⁺ , Ca²⁺ , Cu²⁺ , metal chelate, heat shock and UVB irradiation than was Δfpr2 while both mutants were equally sensitive to Zn²⁺ , Mg²⁺ , Fe²⁺ , H₂ O₂ and cell wall-perturbing agents. In contrast, the Δfpr1 mutant was less sensitive to fewer stress cues. Most of 32 examined genes involved in DNA damage repair, Na⁺ /K⁺ detoxification or osmotolerance and Ca²⁺ homeostasis were downregulated sharply in Δfpr2 and Δfpr3 but rarely so affected in Δfpr1, coinciding well with their phenotypic changes. These findings uncover important, but differential, roles of three FPRs in the fungal adaptation to insect host and environment and provide novel insight into their essential roles in calcium signalling pathway.

Photoprotective Role of Photolyase-Interacting RAD23 and Its Pleiotropic Effect on the Insect-Pathogenic Fungus Beauveria bassiana

RAD23 can repair yeast DNA lesions through nucleotide excision repair (NER), a mechanism that is dependent on proteasome activity and ubiquitin chains but different from photolyase-depending photorepair of UV-induced DNA damages. However, this accessory NER protein remains functionally unknown in filamentous fungi. In this study, orthologous RAD23 in Beauveria bassiana, an insect-pathogenic fungus that is a main source of fungal insecticides, was found to interact with the photolyase PHR2, enabling repair of DNA lesions by degradation of UVB-induced cytotoxic (6-4)-pyrimidine-pyrimidine photoproducts under visible light, and it hence plays an essential role in the photoreactivation of UVB-inactivated conidia but no role in reactivation of such conidia through NER in dark conditions. Fluorescence-labeled RAD23 was shown to normally localize in the cytoplasm, to migrate to vacuoles in the absence of carbon, nitrogen, or both, and to enter nuclei under various stresses, which include UVB, a harmful wavelength of sunlight. Deletion of the rad23 gene resulted in an 84% decrease in conidial UVB resistance, a 95% reduction in photoreactivation rate of UVB-inactivated conidia, and a drastic repression of phr2 A yeast two-hybrid assay revealed a positive RAD23-PHR2 interaction. Overexpression of phr2 in the Δrad23 mutant largely mitigated the severe defect of the Δrad23 mutant in photoreactivation. Also, the deletion mutant was severely compromised in radial growth, conidiation, conidial quality, virulence, multiple stress tolerance, and transcriptional expression of many phenotype-related genes. These findings unveil not only the pleiotropic effects of RAD23 in B. bassiana but also a novel RAD23-PHR2 interaction that is essential for the photoprotection of filamentous fungal cells from UVB damage.IMPORTANCE RAD23 is able to repair yeast DNA lesions through nucleotide excision in full darkness, a mechanism distinct from photolyase-dependent photorepair of UV-induced DNA damage but functionally unknown in filamentous fungi. Our study unveils that the RAD23 ortholog in a filamentous fungal insect pathogen varies in subcellular localization according to external cues, interacts with a photolyase required for photorepair of cytotoxic (6-4)-pyrimidine-pyrimidine photoproducts in UV-induced DNA lesions, and plays an essential role in conidial UVB resistance and reactivation of UVB-inactivated conidia under visible light rather than in the dark, as required for nucleotide excision repair. Loss-of-function mutations of RAD23 exert pleiotropic effects on radial growth, aerial conidiation, multiple stress responses, virulence, virulence-related cellular events, and phenotype-related gene expression. These findings highlight a novel mechanism underlying the photoreactivation of UVB-impaired fungal cells by RAD23 interacting with the photolyase, as well as its essentiality for filamentous fungal life.

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

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

Nuclear Ssr4 Is Required for the In Vitro and In Vivo Asexual Cycles and Global Gene Activity of Beauveria bassiana

Ssr4 is known to serve as a cosubunit of chromatin-remodeling SWI/SNF and RSC complexes in yeasts but has not been functionally characterized in fungi. This study unveils for the first time the pleiotropic effects caused by deletion of ssr4 and its role in mediating global gene expression in a fungal insect pathogen. Our findings confirm an essential role of Ssr4 in hydrophobin biosynthesis and assembly required for growth, differentiation, and development of aerial hyphae for conidiation and conidial adhesion to insect surface and its essentiality for insect pathogenicity and virulence-related cellular events. Importantly, Ssr4 can regulate nearly one-fourth of all genes in the fungal genome in direct and indirect manners, including dozens involved in gene activity and hundreds involved in metabolism and/or transport of carbohydrates, amino acids, lipids, and/or inorganic ions. These findings highlight a significance of Ssr4 for filamentous fungal lifestyle.

Ssr4 serves as a cosubunit of chromatin-remodeling SWI/SNF and RSC complexes in yeasts but remains functionally uncharacterized due to its essentiality for yeast viability. Here, we report pleiotropic effects of the deletion of the ssr4 ortholog nonessential for cell viability in Beauveria bassiana, an asexual insect mycopathogen. The deletion of ssr4 resulted in severe growth defects on different carbon/nitrogen sources, increased hyphal hydrophilicity, blocked hyphal differentiation, and 98% reduced conidiation capacity compared to a wild-type standard. The limited Δssr4 conidia featured an impaired coat with disordered or obscure hydrophobin rodlet bundles, decreased hydrophobicity, increased size, and lost insect pathogenicity via normal cuticle infection and 90% of virulence via intrahemocoel injection. The expression of genes required for hydrophobin biosynthesis and assembly of the rodlet layer was drastically repressed in more hydrophilic Δssr4 cells. Transcriptomic analysis revealed 2,517 genes differentially expressed in the Δssr4 mutant, including 1,505 downregulated genes and 1,012 upregulated genes. The proteins encoded by hundreds of repressed genes were involved in metabolism and/or transport of carbohydrates, amino acids, and lipids, inorganic ion transport and energy production or conversion, including dozens involved in DNA replication, transcription, translation, and posttranslational modifications. However, purified Ssr4 samples showed no DNA-binding activity, implying that the role of Ssr4 in genome-wide gene regulation could rely upon its acting as a cosubunit of the two complexes. These findings provide the first insight into an essential role of Ssr4 in the asexual cycle in vitro and in vivo of B. bassiana and highlights its importance for the filamentous fungal lifestyle.

IMPORTANCE Ssr4 is known to serve as a cosubunit of chromatin-remodeling SWI/SNF and RSC complexes in yeasts but has not been functionally characterized in fungi. This study unveils for the first time the pleiotropic effects caused by deletion of ssr4 and its role in mediating global gene expression in a fungal insect pathogen. Our findings confirm an essential role of Ssr4 in hydrophobin biosynthesis and assembly required for growth, differentiation, and development of aerial hyphae for conidiation and conidial adhesion to insect surface and its essentiality for insect pathogenicity and virulence-related cellular events. Importantly, Ssr4 can regulate nearly one-fourth of all genes in the fungal genome in direct and indirect manners, including dozens involved in gene activity and hundreds involved in metabolism and/or transport of carbohydrates, amino acids, lipids, and/or inorganic ions. These findings highlight a significance of Ssr4 for filamentous fungal lifestyle.

Characterization of a cytolytic-like gene from the aphid-obligate fungal pathogen Conidiobolus obscurus

Cytolytic (Cyt)-like genes are known by omics analyses to exist widely in bacterial and fungal pathogens, but their insecticidal activities in fungi remains unknown. A full-length coding sequence of a Cyt-like gene was first amplified from Conidiobolus obscurus (an obligate aphid-pathogenic fungus) through RACE (rapid-amplification of cDNA ends). The deduced protein structure was structurally characterized by a single Cyt-typical α/β domain. The expression level of the Cyt-like gene in conidia correlated well with the fungal virulence against aphids (r² = 0.97). The results demonstrate the Cyt-like gene acts as an important virulence factor of C. obscurus against aphids, and has potential for exploitation in aphid control.

The transmembrane protein MaSho1 negatively regulates conidial yield by shifting the conidiation pattern in Metarhizium acridum

Sho1 is an important membrane sensor upstream of the HOG-MAPK signaling pathway, which plays critical roles in osmotic pressure response, growth, and virulence in fungi. Here, a Sho1 homolog (MaSho1), containing four transmembrane domains and one Src homology (SH3) domain, was characterized in Metarhizium acridum, a fungal pathogen of locusts. Targeted gene disruption of MaSho1 impaired cell wall integrity, virulence, and tolerances to UV-B and oxidative stresses, while none of them was affected when the SH3 domain was deleted. Intriguingly, disruption of MaSho1 significantly increased conidial yield, which was not affected in the SH3 domain mutant. Furthermore, it was found that deletion of MaSho1 led to microcycle conidiation of M. acridum on the normal conidiation medium. Deletion of MaSho1 significantly shortened the hyphal cells but had no effect on conidial germination. Digital gene expression profiling during conidiation indicated that differential expression of genes was associated with mycelial development, cell division, and differentiation between the wild type and the MaSho1 mutant. These data suggested that disruption of MaSho1 shifted the conidiation pattern by altering the transcription of genes to inhibit mycelial growth, thereby promoting the conidiation of M. acridum.

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

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