Characterization of the Hog1 MAPK pathway in the entomopathogenic fungus Beauveria bassiana

VdP5CDH is involved in melanin formation, stress resistance and play a regulatory role in virulence of Verticillium dahliae

Introduction

Verticillium dahliae, a soil-borne fungal pathogen, can cause cotton Verticillium wilt. In this study, VdP5CDH, the member of the ALDH_F4-17 family of carboxylate dehydrogenases, was identified in the genome of V. dahliae and investigated function in regulating virulence by generating gene deletion mutants and complementary mutants.

Methods

Homologous recombination method was used to construct mutants, transcriptome sequencing revealed gene-related metabolic pathways, and disease degree of cotton was observed through pathogen infection experiments.

Results

The conidial surface of VdP5CDH deletion strains was dented and shriveled, and the number of conidial spores increased. Compared with the wild-type (WT), the mycelial diameter of deletion mutants increased by 10.59%-11.16%, the mycelial growth showed irregular branching patterns, and misaligned arrangement. Although capable of penetrating cellophane, deletion mutants were unable to produce melanin. VdP5CDH was mainly associated with glucose metabolism, nitrogen metabolism, ABC transporter activity as well as various amino acid metabolic processes. After gene knockout, raffinose and pectin were used as the main carbon sources to promote the growth of strains and the growth rate of deletion strains in the medium containing raffinose was higher than that of WT. Consequently, the deletion mutant strains decreased utilization efficiency with which they utilized various nitrogen sources. The deletion mutants maintain responsiveness to osmotic stress and oxidative stress stimuli. Additionally, compared to WT strains, the deletion mutant strains exhibited differences in culture temperature tolerance, UV exposure response, and fungicide sensitivity. After cotton was infected with deletion strains conidial suspension, its disease index increased dramatically, while it gradually decreased after spraying with 2 mM glutamate in batches. With the increase of spraying times, the effect was more significant, and the disease index decreased by 18.95%-19.66% at 26 dpi.

Discussion

These results indicated that VdP5CDH regulates the pathogenicity of fungi and controls mycelia growth, melanin formation, conidia morphology, abiotic stress resistance, and the expression of infecting structure-related genes.

Deleting an xylosidase-encoding gene VdxyL3 increases growth and pathogenicity of Verticillium dahlia

Introduction

Verticillium dahliae causes a devastating Verticillium wilt disease on hundreds of plant species worldwide, including cotton. Understanding the interaction mechanism between V. dahliae and its hosts is the prerequisite for developing effective strategies for disease prevention.

Methods

Here, based on the previous observation of an xylosidase-encoding gene (VdxyL3) in V. dahliae being obviously up-regulated after sensing root exudates from a cotton variety susceptible to this pathogen, we investigated the function of VdxyL3 in the growth and pathogenesis of V. dahliae by generating its deletion-mutant strains (ΔVdxyL3).

Results

Deleting VdxyL3 led to increased colony expansion rate, conidial production, mycelial growth, carbon and nitrogen utilization capacities, and enhanced stress tolerance and pathogenicity of V. dahliae. VdxyL3 is a secretory protein; however, VdxyL3 failed to induce cell death in N. benthamiana based on transient expression experiment. Transcriptomic analysis identified 1300 genes differentially expressed (DEGs) between wild-type (Vd952) and ΔVdxyL3 during infection, including 348 DEGs encoding secretory proteins, among which contained 122 classical secreted proteins and 226 non-classical secreted proteins. It was notable that of the 122 classical secretory proteins, 50 were carbohydrate-active enzymes (CAZymes) and 58 were small cysteine rich proteins (SCRPs), which were required for the pathogenicity of V. dahliae.

Conclusion

The RNA-seq data thus potentially connected the genes encoding these proteins to the pathogenesis of V. dahliae. This study provides an experimental basis for further studies on the interaction between V. dahliae and cotton and the pathogenic mechanism of the fungus.

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.

Procymidone Application Contributes to Multidrug Resistance of Botrytis cinerea

The necrotrophic pathogen Botrytis cinerea infects a broad range of plant hosts and causes substantial economic losses to many crops. Although resistance to procymidone has been observed in the field, it remains uncertain why procymidone is usually involved in multidrug resistance (MDR) together with other fungicides. Nine mutants derived from the B. cinerea strain B05.10 through procymidone domestication exhibited high resistance factors (RFs) against both procymidone and fludioxonil. However, the fitness of the mutants was reduced compared to their parental strain, showing non-sporulation and moderate virulence. Furthermore, the RFs of these mutants to other fungicides, such as azoxystrobin, fluazinam, difenoconazole, and pyrimethanil, ranged from 10 to 151, indicating the occurrence of MDR. Transcriptive expression analysis using the quantitative polymerase chain reaction (qPCR) revealed that the mutants overexpressed ABC transporter genes, ranging from 2 to 93.7-fold. These mutants carried single-point mutations W647X, R96X, and Q751X within BcBos1 by DNA sequencing. These alterations in BcBos1 conferred resistance to procymidone and other fungicides in the mutants. Molecular docking analysis suggested distinct interactions between procymidone and Bos1 in the B. cinerea standard strain B05.10 or the resistant mutants, suggesting a higher affinity of the former towards binding with the fungicide. This study provides a comprehensive understanding of the biological characteristics of the resistant mutants and conducts an initial investigation into its fungicide resistance traits, providing a reference for understanding the causes of multidrug resistance of B. cinerea in the field.

Discovery of N-Benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine as a Potential Antifungal Agent against Sporidia Growth and Teliospore Germination of Sporisorium scitamineum

The cultivation of sugar cane using perennial roots is the primary planting method, which is one of the reasons for the serious occurrence of sugar cane smut disease caused by the basidiomycetous fungus Sporisorium scitamineum in the sugar cane perennial root planting area. Consequently, it is crucial to eliminate pathogens from perennial sugar cane buds. In this study, we found that MAP kinase Hog1 is necessary for heat stress resistance. Subsequent investigations revealed a significant reduction in the expression of the heat shock protein 104-encoding gene, SsHSP104, in the ss1hog1Δ mutant. Additionally, the overexpression of SsHSP104 partially restored colony growth in the ss1hog1Δ strain following heat stress treatment, demonstrating the crucial role of SsHsp104 in SsHog1-mediated heat stress tolerance. Hence, we constructed the ss1hsp104:eGFP fusion strain in the wild type of S. scitamineum to identify small-molecule compounds that could inhibit the heat stress response, leading to the discovery of N-benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine as a potential compound that targets the SsHog1 mediation SsHsp104 pathway during heat treatment. Furthermore, the combination of N-benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine and warm water treatment (45 °C for 15 min) inhibits the growth of S. scitamineum and teliospore germination, thereby reducing the occurrence of sugar cane smut diseases and indicating its potential for eliminating pathogens from perennial sugar cane buds. In conclusion, these findings suggest that N-benzyl-4-(1-bromonaphthalen-2-yl)oxybutan-1-amine is promising as a targeted compound for the SsHog1-mediated SsHsp104 pathway and may enable the reduction of hot water treatment duration and/or temperature, thereby limiting the occurrence of sugar cane smut diseases caused by S. scitamineum.

MAPK signaling pathway orchestrates and fine-tunes the pathogenicity of Colletotrichum falcatum

Colletotrichum falcatum is the causal organism of red rot, the most devastating disease of sugarcane. Mitogen-activated protein kinase (MAPK) signaling pathway plays pivotal role in coordinating the process of pathogenesis. We identified eighteen proteins implicated in MAPK signaling pathway in C. falcatum, through nanoLCMS/MS based proteomics approach. Twelve of these proteins were the part of core MAPK signaling pathway, whereas remaining proteins were indirectly implicated in MAPK signaling. Majority of these proteins had enhanced abundance in C. falcatum samples cultured with host sugarcane stalks. To validate the findings, core MAPK pathway genes (MAPKKK-NSY1, MAPK 17-MAPK17, MAPKKK 5-MAPKKK5, MAPK-HOG1B, MAPKKK-MCK1/STE11, MAPK-MST50/STE50, MAPKK-SEK1, MAPKK-MEK1/MST7/STE7, MAPKK-MKK2/STE7, MAPKKK-MST11/STE11, MAPK 5-MPK5, and MAPK-MPK-C) were analyzed by qPCR to confirm the real-time expression in C. falcatum samples cultured with host sugarcane stalks. The results of qPCR-based expression of genes were largely in agreement with the findings of proteomics. String association networks of MAPKK- MEK1/MST7/STE7, and MAPK- MPK-C revealed strong association with plenty of assorted proteins implicated in the process of pathogenesis/virulence. This is the novel and first large scale study of MAPK proteins in C. falcatum, responsible for red rot epidemics of sugarcane various countries. KEY MESSAGE: Our findings demonstrate the pivotal role of MAPK proteins in orchestrating the pathogenicity of Colletotrichum falcatum, responsible devastating red rot disease of sugarcane. SIGNIFICANCE: Our findings are novel and the first large scale study demonstrating the pivotal role of MAPK proteins in C. falcatum, responsible devastating red rot disease of sugarcane. The study will be useful for future researchers in terms of manipulating the fungal pathogenicity through genome editing.

AoMedA has a complex regulatory relationship with AoBrlA, AoAbaA, and AoWetA in conidiation, trap formation, and secondary metabolism in the nematode-trapping fungus Arthrobotrys oligospora

The asexual sporulation of filamentous fungi is an important mechanism for their reproduction, survival, and pathogenicity. In Aspergillus and several filamentous fungi, BrlA, AbaA, and WetA are the key elements of a central regulatory pathway controlling conidiation, and MedA is a developmental modifier that regulates temporal expression of central regulatory genes; however, their roles are largely unknown in nematode-trapping (NT) fungi. Arthrobotrys oligospora is a representative NT fungus, which can capture nematodes by producing adhesive networks (traps). Here, we characterized the function of AoMedA and three central developmental regulators (AoBrlA, AoAbaA, and AoWetA) in A. oligospora by gene disruption, phenotypic comparison, and multi-omics analyses, as these regulators are required for conidiation and play divergent roles in mycelial development, trap formation, lipid droplet accumulation, vacuole assembly, and secondary metabolism. A combined analysis of phenotypic traits and transcriptome showed that AoMedA and AoWetA are involved in the regulation of peroxisome, endocytosis, and autophagy. Moreover, yeast one-hybrid analysis showed that AoBrlA can regulate AoMedA, AoAbaA, and AoWetA, whereas AoMedA and AoAbaA can regulate AoWetA. Our results highlight the important roles of AoMedA, AoBrlA, AoAbaA, and AoWetA in conidiation, mycelia development, trap formation, and pathogenicity of A. oligospora and provide a basis for elucidating the relationship between conidiation and trap formation of NT fungi. IMPORTANCE Conidiation is the most common reproductive mode for many filamentous fungi and plays an essential role in the pathogenicity of fungal pathogens. Nematode-trapping (NT) fungi are a special group of filamentous fungi owing to their innate abilities to capture and digest nematodes by producing traps (trapping devices). Sporulation plays an important role in the growth and reproduction of NT fungi, and conidia are the basic components of biocontrol reagents for controlling diseases caused by plant-parasitic nematodes. Arthrobotrys oligospora is a well-known NT fungus and is a routinely used model fungus for probing the interaction between fungi and nematodes. In this study, the functions of four key regulators (AoMedA, AoBrlA, AoAbaA, and AoWetA) involved in conidiation were characterized in A. oligospora. A complex interaction between AoMedA and three central regulators was noted; these regulators are required for conidiation and trap formation and play a pleiotropic role in multiple intracellular activities. Our study first revealed the role of AoMedA and three central regulators in conidiation, trap formation, and pathogenicity of A. oligospora, which contributed to elucidating the regulatory mechanism of conidiation in NT fungi and helped in developing effective reagents for biocontrol of nematodes.

Fus3 regulates asexual development and trap morphogenesis in the nematode-trapping fungus Arthrobotrys oligospora

Mitogen-activated protein kinase (MAPK) Fus3 is an essential regulator of cell differentiation and virulence in fungal pathogens of plants and animals. However, the function and regulatory mechanism of MAPK signaling in nematode-trapping (NT) fungi remain largely unknown. NT fungi can specialize in the formation of "traps", an important indicator of transition from a saprophytic to a predatory lifestyle. Here, we characterized an orthologous Fus3 in a typical NT fungus Arthrobotrys oligospora using multi-phenotypic analysis and multi-omics approaches. Our results showed that Fus3 plays an important role in asexual growth and development, conidiation, stress response, DNA damage, autophagy, and secondary metabolism. Importantly, Fus3 plays an indispensable role in hyphal fusion, trap morphogenesis, and nematode predation. Moreover, we constructed the regulatory networks of Fus3 by means of transcriptomic and yeast two-hybrid techniques. This study provides insights into the mechanism of MAPK signaling in asexual development and pathogenicity of NT fungi.

Involvement of AoMdr1 in the Regulation of the Fluconazole Resistance, Mycelial Fusion, Conidiation, and Trap Formation of Arthrobotrys oligospora

Multidrug resistance (Mdr) proteins are critical proteins for maintenance of drug resistance in fungi. Mdr1 has been extensively studied in Candida albicans; its role in other fungi is largely unknown. In this study, we identified a homologous protein of Mdr (AoMdr1) in the nematode-trapping (NT) fungus Arthrobotrys oligospora. It was found that the deletion of Aomdr1 resulted in a significant reduction in the number of hyphal septa and nuclei as well as increased sensitivity to fluconazole and resistance to hyperosmotic stress and SDS. The deletion of Aomdr1 also led to a remarkable increase in the numbers of traps and mycelial loops in the traps. Notably, AoMdr1 was able to regulate mycelial fusion under low-nutrient conditions, but not under nutrient-rich conditions. AoMdr1 was also involved in secondary metabolism, and its deletion caused an increase in arthrobotrisins (specific compounds produced by NT fungi). These results suggest that AoMdr1 plays a crucial role in the fluconazole resistance, mycelial fusion, conidiation, trap formation, and secondary metabolism of A. oligospora. Our study contributes to the understanding of the critical role of Mdr proteins in mycelial growth and the development of NT fungi.

Host- and virus-induced gene silencing of HOG1-MAPK cascade genes in Rhizophagus irregularis inhibit arbuscule development and reduce resistance of plants to drought stress

Arbuscular mycorrhizal (AM) fungi can form beneficial associations with the most terrestrial vascular plant species. AM fungi not only facilitate plant nutrient acquisition but also enhance plant tolerance to various environmental stresses such as drought stress. However, the molecular mechanisms by which AM fungal mitogen-activated protein kinase (MAPK) cascades mediate the host adaptation to drought stimulus remains to be investigated. Recently, many studies have shown that virus-induced gene silencing (VIGS) and host-induced gene silencing (HIGS) strategies are used for functional studies of AM fungi. Here, we identify the three HOG1 (High Osmolarity Glycerol 1)-MAPK cascade genes RiSte11, RiPbs2 and RiHog1 from Rhizophagus irregularis. The expression levels of the three HOG1-MAPK genes are significantly increased in mycorrhizal roots of the plant Astragalus sinicus under severe drought stress. RiHog1 protein was predominantly localized in the nucleus of yeast in response to 1 M sorbitol treatment, and RiPbs2 interacts with RiSte11 or RiHog1 directly by pull-down assay. Importantly, VIGS or HIGS of RiSte11, RiPbs2 or RiHog1 hampers arbuscule development and decreases relative water content in plants during AM symbiosis. Moreover, silencing of HOG1-MAPK cascade genes led to the decreased expression of drought-resistant genes (RiAQPs, RiTPSs, RiNTH1 and Ri14-3-3) in the AM fungal symbiont in response to drought stress. Taken together, this study demonstrates that VIGS or HIGS of AM fungal HOG1-MAPK cascade inhibits arbuscule development and expression of AM fungal drought-resistant genes under drought stress.

Peroxin Pex14/17 Is Required for Trap Formation, and Plays Pleiotropic Roles in Mycelial Development, Stress Response, and Secondary Metabolism in Arthrobotrys oligospora

The peroxins encoded by PEX genes involved in peroxisome biogenesis play a crucial role in cellular metabolism and pathogenicity in fungi. Herein, we characterized a filamentous fungus-specific peroxin Pex14/17 in the Arthrobotrys oligospora, a representative species of nematode-trapping fungi. The deletion of AoPEX14/17 resulted in a remarkable reduction in mycelial growth, conidia yield, trap formation, and pathogenicity. Compared with the wild-type strain, the ΔAopex14/17 mutant exhibited more lipid droplet and reactive oxygen species accumulation accompanied with a significant decrease in fatty acid utilization and tolerance to oxidative stress. Transcriptomic analysis indicated that AoPEX14/17 was involved in the regulation of metabolism, genetic information processing, environmental information processing, and cellular processes. In subcellular morphology, the deletion of AoPEX14/17 resulted in a decrease in the number of cell nuclei, autophagosomes, and Woronin bodies. Metabolic profile analysis showed that AoPex14/17 affects the biosynthesis of secondary metabolites. Yeast two-hybrid assay revealed that AoPex14/17 interacted with AoPex14 but not with AoPex13. Taken together, our results suggest that Pex14/17 is the main factor for modulating growth, development, and pathogenicity in A. oligospora. IMPORTANCE Peroxisome biogenesis genes (PEX) play an important role in growth, development, and pathogenicity in pathogenic fungi. However, the roles of PEX genes remain largely unknown in nematode-trapping (NT) fungi. Here, we provide direct evidence that AoPex14/17 regulates mycelial growth, conidiation, trap formation, autophagy, endocytosis, catalase activity, stress response to oxidants, lipid metabolism, and reactive oxygen species production. Transcriptome analysis and metabolic profile suggested that AoPex14/17 is involved in multiple cellular processes and the regulation of secondary metabolism. Therefore, our study extends the functions of PEX genes, which helps to elucidate the mechanism of organelle development and trap formation in NT fungi and lays the foundation for the development of efficient nematode biocontrol agents.

VdGAL4 Modulates Microsclerotium Formation, Conidial Morphology, and Germination To Promote Virulence in Verticillium dahliae

Verticillium dahliae Kleb is a typical soilborne pathogen that can cause vascular wilt disease on more than 400 plants. Functional analysis of genes related to the growth and virulence is crucial to revealing the molecular mechanism of the pathogenicity of V. dahliae. Glycosidase hydrolases can hydrolyze the glycosidic bond, and some can cause host plant immune response to V. dahliae. Here, we reported a functional validation of VdGAL4 as an α-galactosidase that belongs to glycoside hydrolase family 27. VdGAL4 could cause plant cell death, and its signal peptide plays an important role in cellular immune response. VdGAL4-triggered cell death depends on BAK1 and SOBIR1 in Nicotiana benthamiana. In V. dahliae, the function of VdGAL4 in mycelial growth, conidia, microsclerotium, and pathogenicity was studied by constructing VdGAL4 deletion and complementation mutants. Results showed that the deletion of VdGAL4 reduced the conidial yield and conidial germination rate of V. dahliae and changed the microscopic morphology of conidia; the mycelia were arranged more disorderly and were unable to produce microsclerotium. The VdGAL4 deletion mutants exhibited reduced utilization of different carbon sources, such as raffinose and sucrose. The VdGAL4 deletion mutants were also more sensitive to abiotic stress agents of SDS, sorbitol, low-temperature stress of 16°C, and high-temperature stress of 45°C. In addition, the VdGAL4 deletion mutants lost the ability to penetrate cellophane and its mycelium were disorderly arranged. Remarkably, VdGAL4 deletion mutants exhibited reduced pathogenicity of V. dahliae. These results showed that VdGAL4 played a critical role in the pathogenicity of V. dahliae by regulating mycelial growth, conidial morphology, and the formation of microsclerotium. IMPORTANCE This study showed that α-galactosidase VdGAL4 of V. dahliae could activate plant immune response and plays an important role in conidial morphology and yield, formation of microsclerotia, and mycelial penetration. VdGAL4 deletion mutants significantly reduced the pathogenicity of V. dahliae. These findings deepened the understanding of pathogenic virulence factors and how the mechanism of pathogenic fungi infected the host, which may help to seek new strategies for effective control of plant diseases caused by pathogenic fungi.

Deletion of a Putative GPI-Anchored Protein-Encoding Gene Aog185 Impedes the Growth and Nematode-Trapping Efficiency of Arthrobotrys oligospora by Disrupting Transmembrane Transport Homeostasis

Nematode-trapping fungus (NTF) is a crucial predator of nematodes, which can capture nematodes by developing specific trapping devices. However, there is limited understanding of the role and mechanism of cell surface proteins attached to the surface of mycelia or trapping cells. Here, the effects of a putative GPI-anchored protein-encoding gene Aog185 on the growth and nematode-trapping efficiency of A. oligospora were investigated. Compared to the wild-type (WT) strain, the ΔAog185 mutant grew more slowly, exhibited a 20% decrease in conidiation, delayed conidial germination, generated fewer traps, attenuated nematode trapping efficiency, and was more sensitive to chemical stressors. Transcriptomic analysis indicated that a large number of transmembrane transport-related genes were differentially expressed between the WT and ΔAog185 mutant strains. Aog185 deletion could damage the intrinsic components of the membrane and cytoskeleton. Specifically, knockout of Aog185 disrupted transmembrane transport homeostasis during the phagocytosis, cell autophagy, and oxidative phosphorylation processes, which were associated with the fusion of cells and organelle membranes, transport of ions and substrates, and energy metabolism. Hence, the putative GPI-anchored protein-encoding gene Aog185 may contribute to the lifestyle switch of NTF and nematode capture, and the effect of Aog185 gene on cell transmembrane transport is considered key to this process. Our findings provide new insights into the mechanism of Aog185 gene during the process of nematode trapping by NTF.

Characterization of BbKlf1 as a novel transcription factor vital for asexual and infection cycles of Beauveria bassiana

The large family of C2H2-type zinc finger transcription factors (TFs) comprise the Kruppel-like factors (KLFs) that evolved relatively late in eukaryotes but remain unexplored in filamentous fungi. Here, we report that an orthologue (BbKlf1) of yeast Klf1 mediating cell wall integrity (CWI) is a wide-spectrum TF evidently localized in nucleus and cytoplasm in Beauveria bassiana. BbKlf1 features conserved domains and multiple DNA-binding motifs predicted to bind multiple promoter DNA fragments of target genes across asexual developmental and stress-responsive pathways. Despite limited impact on normal colony growth, deletion of Bbklf1 resulted in impaired CWI and hypersensitivity to Congo red-induced cell wall stress. Also, the deletion mutant was severely compromised in tolerance to oxidative and osmotic stresses, hyphal septation and differentiation, conidiation capacity (reduced by 95%), conidial quality (viability and hydrocarbon epitope pattern) and virulence. Importantly, these phenotypes correlated well with sharply repressed or nearly abolished expressions of those genes required for or involved in chitin biosynthesis, antioxidant activity, cell division and differentiation, aerial conidiation and conidial maturation. These findings indicate an essentiality of BbKlf1 for the asexual and insect-pathogenic lifecycles of B. bassiana and a novel scenario much beyond the yeast orthologue-mediated CWI, suggesting important roles of its orthologues in filamentous fungi.

The Elongator Subunit Elp3 Regulates Development, Stress Tolerance, Cell Cycle, and Virulence in the Entomopathogenic Fungus Beauveria bassiana

Transcriptional activity is mediated by chromatin remodeling, which in turn is affected by post-translational modifications, including histone acetylation. Histone acetyltransferases (HATs) are capable of promoting euchromatin formation and then activating gene transcription. Here, we characterize the Elp3 GNAT family HAT, which is also a subunit of Elongator complex, in the environmentally and economically important fungal insect pathogen, Beauveria bassiana. BbElp3 showed high localization levels to mitochondria, with some nuclear and cytoplasmic localization also apparent. Targeted gene knockout of BbElp3 resulted in impaired asexual development and morphogenesis, reduced tolerances to multiple stress conditions, reduced the ability of the fungus to utilize various carbon/nitrogen sources, increased susceptibility to rapamycin, and attenuated virulence in bioassays using the greater wax moth, Galleria mellonella. The ΔBbElp3 mutant also showed disrupted cell cycle, abnormal hyphal septation patterns, and enlarged autophagosomes in vegetative hyphae. Transcriptome analyses revealed differential expression of 775 genes (DEGs), including 336 downregulated and 438 upregulated genes in the ΔBbElp3 strain as compared to the wild type. Downregulated genes were mainly enriched in pathways involved in DNA processing and transcription, cell cycle control, cellular transportation, cell defense, and virulence, including hydrophobins, cellular transporters (ABC and MFS multidrug transporters), and insect cuticular degrading enzymes, while upregulated genes were mainly enriched in carbohydrate metabolism and amino acid metabolism. These data indicate pleiotropic effects of BbElp3 in impacting specific cellular processes related to asexual development, cell cycle, autophagy, and virulence.

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.

Histidine kinase two-component response regulators Ssk1, Skn7 and Rim15 differentially control growth, developmental and volatile organic compounds emissions as stress responses in Trichoderma atroviride

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Ssk1 may function as a hub-like RR in T. atroviride, as growth, and differentiation (conidiation).

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Skn7 has a significant role in the production of volatile organic compounds.

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Ssk1 and Rim15 have a role in mycoparasitism and both are negative regulators of volatile organic compounds. Rim15 seems to be a negative regulator of either the accumulation or biosynthesis of terpenes.

The Skn7, Ssk1 and Rim15 proteins are response regulators involved in osmotic, oxidative and nutritional stress in fungi. In order to verify the involvement of these genes in Trichoderma atroviride IMI206040’s growth, conidiation, direct antagonism against plant pathogens (Rhizoctonia solani and Sclerotinia sclerotiorum), production of volatile organic compounds (VOCs) with fungistatic effect, and interaction with plants (growth promotion), single mutants were generated, and the phenotypic patterns were analysed in comparison to the wild-type (wt) strain. The mutants were submitted to osmotic, oxidative, membrane and cell wall stress conditions in vitro. The Δskn7 and Δrim15 mutants did not show either significant differences at morphological level, or marked decreases in mycelial growth and conidiation in relation to wt, whereas Δssk1 had altered phenotypes in most conditions tested. The plant-growth promotion of Arabidopsis thaliana seedlings induced by VOCs was not quantitatively modified by any of the mutants in relation to the wt strain, although possible differences in secondary root hairs was noticed for Δrim15. The fungistatic activity was significantly altered for Δssk1 and Δrim15. Overall, the Δssk1 strain showed remarkable morphological differences, with decrease in mycelial growth and conidiation, being also affected in the antagonistic capacity against plant pathogens. The impacts demonstrated by the deletion of ssk1 suggest this gene has a relevant participation in the signalling response to different stresses in T. atroviride and in the interactive metabolism with phytopathogens and plants. On the other hand, unlike other fungal models, Skn7 did not appear to have a critical participation in the above-mentioned processes; Rim15 seemed to confirm its involvement in modulating cellular responses to nutritional status, although with a possible cross-talk with other cellular processes. Our results suggest that Ssk1 likely plays a key regulatory role, not only in basic metabolisms of T. atroviride, but also in biocontrol-related characteristics.

The Arf-GAP Proteins AoGcs1 and AoGts1 Regulate Mycelial Development, Endocytosis, and Pathogenicity in Arthrobotrys oligospora

Small GTPases from the ADP-ribosylation factor (Arf) family and their activating proteins (Arf-GAPs) regulate mycelial development, endocytosis, and virulence in fungi. Here, we identified two orthologous Arf-GAP proteins, AoGcs1 and AoGts1, in a typical nematode-trapping fungus Arthrobotrys oligospora. The transcription of Aogcs1 and Aogts1 was highly expressed in the sporulation stage. The deletion of Aogcs1 and Aogts1 caused defects in DNA damage, endocytosis, scavenging of reactive oxygen species, lipid droplet storage, mitochondrial activity, autophagy, serine protease activity, and the response to endoplasmic reticulum stress. The combined effects resulted in slow growth, decreased sporulation capacity, increased susceptibility to chemical stressors and heat shock, and decreased pathogenicity of the mutants compared with the wild-type (WT) strain. Although deletion of Aogcs1 and Aogts1 produced similar phenotfypic traits, their roles varied in conidiation and proteolytic activity. The ΔAogts1 mutant showed a remarkable reduction in conidial yield compared with the WT strain but not in proteolytic activity; in contrast, the ΔAogcs1 mutant showed an increase in proteolytic activity but not in sporulation. In addition, the growth of ΔAogcs1 and ΔAogts1 mutants was promoted by rapamycin, and the ΔAogts1 mutant was sensitive to H-89. Collectively, the ΔAogts1 mutant showed a more remarkable difference compared with the WT strain than the ΔAogcs1 mutant. Our study further illustrates the importance of Arf-GAPs in the growth, development, and pathogenicity of nematode-trapping fungi.

A Kinesin Vdkin2 Required for Vacuole Formation, Mycelium Growth, and Penetration Structure Formation of Verticillium dahliae

The soil-borne vascular fungus Verticillium dahliae infects hundreds of dicotyledonous plants, causing severe wilt diseases. During the initial colonization, V. dahliae develops a penetration peg to enable infection of cotton roots. In some phytopathogenic fungi, vacuoles play a critical role in normal formation of the infection structure. Kinesin 2 protein is associated with vacuole formation in Ustilago maydis. To identify the function of vacuoles in the V. dahliae infection structure, we identified VdKin2, an ortholog of kinesin 2, in V. dahliae and investigated its function through gene knockout. VdKin2 mutants showed severe defects in virulence and were suppressed during initial infection and root colonization based on observation of green fluorescent protein-labeled V. dahliae. We also found that deletion of VdKin2 compromised penetration peg formation and the derived septin neck. Disruption strains were viable and showed normal microsclerotia formation, whereas mycelium growth and conidial production were reduced, with shorter and more branched hyphae. Furthermore, the VdKin2 mutant, unlike wild-type V. dahliae, lacked a large basal vacuole, accompanied by a failure to generate concentrated lipid droplets. Taken together, VdKin2 regulates vacuole formation by V. dahliae, which is required for conidiation, mycelium growth, and penetration structure formation during initial plant root infection.

Identification and Functional Analysis of a Novel Hydrophobic Protein VdHP1 from Verticillium dahliae

Verticillium dahliae could cause destructive vascular wilt disease on hundreds of plant species around the world, including cotton. In this study, we characterized the function of a hydrophobin gene VdHP1 in pathogen development and pathogenicity. Results showed that VdHP1 could induce cell death and activate plant immune responses. The VdHP1 deletion mutants (ΔVdHP1) and the complement mutants (C-ΔVdHP1) were obtained by the homologous recombination method. The VdHP1 deletion mutants exhibited increased hydrophilicity, inhibited microsclerotial formation, and reduced spore smoothness. In addition, the deletion mutants were more sensitive to NaCl, while relatively insensitive to KCl and sorbitol. Mutants also had greater resistance to Congo red, UV radiation, and high temperature, which suggested that ΔVdHP1 strains have stronger resistance to abiotic stress in general. Different carbon source assays showed that the utilization ability of skim milk, cellulose, and starch was greatly enhanced in ΔVdHP1, compared with that of WT and complemented strains. Furthermore, VdHP1 did not affect mycelium penetration on cellophane but contributed to mycelium growth on surface of the living plant cells. The pathogenicity test found that the crude toxin content, colonization, and dispersal of ΔVdHP1 was significantly increased compared with the WT and complementary strains. In addition, cotton seedlings showed more severe wilting symptoms after inoculation with ΔVdHP1 strains. These results suggested that the hydrophobin VdHP1 negatively regulated the virulence of V. dahliae, and played an important role in development, adaptability, and pathogenicity in V. dahliae, which maybe provide a new viewpoint to further understand the molecular mechanisms of pathogen virulence. IMPORTANCE Verticillium dahliae is a soilborne fungal pathogen that causes a destructive vascular disease on a large number of plant hosts, resulting in great threat to agricultural production. In this study, it was illustrated that the hydrophobin VdHP1 could induce cell death and activate plant immune responses. VdHP1 affected the hydrophobicity of V. dahliae, and negatively regulated the strains resistant to stress, and the utilization ability of different carbon sources. In addition, VdHP1 did not affect mycelium penetration on cellophane but contributed to mycelium growth on surface of the living plant cells. The VdHP1 gene negatively regulated the total virulence, colonization, and dispersal of V. dahliae, with enhanced pathogenicity of mutant strains in this gene. These results suggested that the hydrophobin VdHP1 played an importance in development, adaptability, and pathogenicity in V. dahliae, and would provide a new viewpoint to further understand the molecular mechanisms of pathogen virulence.

AoPEX1 and AoPEX6 Are Required for Mycelial Growth, Conidiation, Stress Response, Fatty Acid Utilization, and Trap Formation in Arthrobotrys oligospora

Arthrobotrys oligospora (A. oligospora) is a typical nematode-trapping (NT) fungus that can capture nematodes by producing adhesive networks. Peroxisomes are single membrane-bound organelles that perform multiple physiological functions in filamentous fungi. Peroxisome biogenesis proteins are encoded by PEX genes, and the functions of PEX genes in A. oligospora and other NT fungi remain largely unknown. Here, our results demonstrated that two PEX genes (AoPEX1 and AoPEX6) are essential for mycelial growth, conidiation, fatty acid utilization, stress tolerance, and pathogenicity in A. oligospora. AoPEX1 and AoPEX6 knockout resulted in a failure to produce traps, conidia, peroxisomes, and Woronin bodies and damaged cell walls, reduced autophagosome levels, and increased lipid droplet size. Transcriptome data analysis showed that AoPEX1 and AoPEX6 deletion resulted in the upregulation of the proteasome, membranes, ribosomes, DNA replication, and cell cycle functions, and the downregulation of MAPK signaling and nitrogen metabolism. In summary, our results provide novel insights into the functions of PEX genes in the growth, development, and pathogenicity of A. oligospora and contribute to the elucidation of the regulatory mechanism of peroxisomes in trap formation and lifestyle switching in NT fungi.

IMPORTANCE Nematode-trapping (NT) fungi are important resources for the biological control of plant-parasitic nematodes. They are widely distributed in various ecological environments and capture nematodes by producing unique predatory organs (traps). However, the molecular mechanisms of trap formation and lifestyle switching in NT fungi are still unclear. Here, we provided experimental evidence that the AoPEX1 and AoPEX6 genes could regulate mycelial growth and development, trap formation, and nematode predation of A. oligospora. We further analyzed the global transcription level changes of wild-type and mutant strains using RNA-seq. This study highlights the important role of peroxisome biogenesis genes in vegetative growth, conidiation, trap formation, and pathogenicity, which contribute to probing the mechanism of organelle development and trap formation of NT fungi and lays a foundation for developing high-efficiency nematode biocontrol agents.

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.

Functional analysis of seven regulators of G protein signaling (RGSs) in the nematode-trapping fungus Arthrobotrys oligospora

Regulators of G protein signaling (RGSs) are proteins that negatively regulate G protein signal transduction. In this study, seven putative RGSs were characterized in the nematode-trapping (NT) fungus, Arthrobotrys oligospora. Deleting Rgs genes significantly increased intracellular cAMP levels, and caused defects in mycelia growth, stress resistance, conidiation, trap formation, and nematocidal activity. In particular, the ΔAoFlbA mutant was unable to produce conidia and traps. Transcriptomic analysis showed that amino acid metabolic and biosynthetic processes were significantly enriched in the ΔAoFlbA mutant compared to WT. Interestingly, Gas1 family genes are significantly expanded in A. oligospora and other NT fungi that produce adhesive traps, and are differentially expressed during trap formation in A. oligospora. Disruption of two Gas1 genes resulted in defective conidiation, trap formation, and pathogenicity. Our results indicate that RGSs play pleiotropic roles in regulating A. oligospora mycelial growth, development, and pathogenicity. Further, AoFlbA is a prominent member and required for conidiation and trap formation, possibly by regulating amino acid metabolism and biosynthesis. Our results provide a basis for elucidating the signaling mechanism of vegetative growth, lifestyle transition, and pathogenicity in NT fungi.

A putative F-box-domain-encoding gene AOL_s00076g207 regulates the development and pathogenicity of Arthrobotrys oligospora

F-box protein is a key component of the Skp1-cullin-F-box-type ubiquitin ligase complex (SCF-ULC) that marks its target proteins with ubiquitin for proteasomal degradation. In this study, we explored the potential role of AOL_s00076g207 (Aog207) in Arthrobotrys oligospora, a model fungus for studying nematodes-fungi interactions. The Aog207 gene encodes a putative F-box protein of the SCF-ULC. Deletion of Aog207 could inhibit mycelial growth in TYGA and PDA media. More importantly, the conidial germination rate of ΔAog207 mutants was remarkably declined compared to that of wild-type (WT) strain, and the mutant strains were more sensitive toward chemical stressors than the WT strain. In addition, ΔAog207 mutants generated fewer traps and captured fewer nematodes than WT strain. In summary, Aog207 disruption significantly affected the pathogenicity, mycelial growth, conidial germination, environmental adaptation and trap formation of A. oligospora. These findings may facilitate a better understanding of the nematode predation mechanism of A. oligospora and provide an experimental basis for developing biological control agents against nematodes.

Phospholipase C (AoPLC2) regulates mycelial development, trap morphogenesis, and pathogenicity of the nematode-trapping fungus Arthrobotrys oligospora

AIMS

Phospholipase C (PLC) is a hydrolase involved in signal transduction in eukaryotic cells. This study aimed to understand the function of PLC in the nematode-trapping fungus Arthrobotrys oligospora.

METHODS AND RESULTS

Orthologous PLC (AoPLC2) of A. oligospora was functionally analysed using gene disruption and multi-phenotypic analysis. Disrupting Aoplc2 caused a deformation of partial hyphal cells (about 10%) and conidia (about 50%), decreased the number of nuclei in both conidia and hyphal cells, and increased the accumulation of lipid droplets. Meanwhile, the sporulation-related genes fluG and abaA were downregulated in ΔAoplc2 mutants than in the wild-type strain. Moreover, ΔAoplc2 mutants were more sensitive to osmotic stressors. Importantly, the number of traps, electron-dense bodies in traps, and nematicidal activity of ΔAoplc2 mutants were reduced, and the shape of the traps was deformed. In addition, AoPLC2 was involved in the biosynthesis of secondary metabolites in A. oligospora.

CONCLUSIONS

AoPLC2 plays an important role in the development of hyphae, spores, and cell nuclei, responses to stress, formation of traps, and predation of nematodes in A. oligospora.

SIGNIFICANCE AND IMPACT OF STUDY

This study reveals the various functions of phospholipase C and elucidates the regulation of trap morphogenesis in nematode-trapping fungi.

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.

Molecular Mechanisms Underlying Fungicide Resistance in Citrus Postharvest Green Mold

The necrotrophic fungus Penicillium digitatum (Pd) is responsible for the green mold disease that occurs during postharvest of citrus and causes enormous economic losses around the world. Fungicides remain the main method used to control postharvest green mold in citrus fruit storage despite numerous occurrences of resistance to them. Hence, it is necessary to find new and more effective strategies to control this type of disease. This involves delving into the molecular mechanisms underlying the appearance of resistance to fungicides during the plant–pathogen interaction. Although mechanisms involved in resistance to fungicides have been studied for many years, there have now been great advances in the molecular aspects that drive fungicide resistance, which facilitates the design of new means to control green mold. A wide review allows the mechanisms underlying fungicide resistance in Pd to be unveiled, taking into account not only the chemical nature of the compounds and their target of action but also the general mechanism that could contribute to resistance to others compounds to generate what we call multidrug resistance (MDR) phenotypes. In this context, fungal transporters seem to play a relevant role, and their mode of action may be controlled along with other processes of interest, such as oxidative stress and fungal pathogenicity. Thus, the mechanisms for acquisition of resistance to fungicides seem to be part of a complex framework involving aspects of response to stress and processes of fungal virulence.

Ric8 acts as a regulator of G-protein signalling required for nematode-trapping lifecycle of Arthrobotrys oligospora

Resistance to inhibitors of cholinesterase 8 (Ric8) is a conserved guanine nucleotide exchange factor that is involved in the regulation of G-protein signalling in filamentous fungi. Here, we characterized an orthologous Ric8 (AoRic8) in Arthrobotrys oligospora by multi-omics analyses. The Aoric8 deletion (ΔAoric8) mutants lost an ability to produce traps essential for nematode predation, accompanied by a marked reduction in cAMP level. Yeast two-hybrid assay revealed that AoRic8 interacted with G-protein subunit Gα1. Moreover, the mutants were compromised in mycelia growth, conidiation, stress resistance, endocytosis, cellular components and intrahyphal hyphae. Revealed by transcriptomic analysis differentially upregulated genes in the absence of Aoric8 were involved in cell cycle, DNA replication and recombination during trap formation while downregulated genes were primarily involved in organelles, carbohydrate metabolism and amino acid metabolism. Metabolomic analysis showed that many compounds were markedly downregulated in ΔAoric8 mutants versus the wild-type strain. Our results demonstrated a crucial role for AoRic8 in the fungal growth, environmental adaption and nematode predation through control of cell cycle, organelle and secondary metabolism by G-protein signalling.

AoBck1 and AoMkk1 Are Necessary to Maintain Cell Wall Integrity, Vegetative Growth, Conidiation, Stress Resistance, and Pathogenicity in the Nematode-Trapping Fungus Arthrobotrys oligospora

The cell wall integrity (CWI) pathway is composed of three mitogen-activated protein kinases (MAPKs), Bck1, Mkk1/2, and Slt2, and is one of the main signaling pathways for fungal pathogenesis, cell wall synthesis, and integrity maintenance. In this study, we characterized orthologs of Saccharomyces cerevisiae Bck1 and Mkk1 in the nematode-trapping (NT) fungus Arthrobotrys oligospora by multiple phenotypic comparison, and the regulation of conidiation and cell wall synthesis was analyzed using real-time PCR (RT-PCR). Both ΔAoBck1 and ΔAoMkk1 mutants showed severe defects in vegetative growth, cell nucleus number, and stress resistance. Both the mutants were unable to produce spores, and the transcription of several genes associated with sporulation and cell wall biosynthesis was markedly downregulated during the conidiation stage. Further, cell walls of the ΔAoBck1 and ΔAoMkk1 mutants were severely damaged, and the Woronin body failed to respond to cellular damage. In particular, the mutants lost the ability to produce mycelial traps for nematode predation. Taken together, AoBck1 and AoMkk1 play a conserved role in mycelial growth and development, CWI, conidiation, multi-stress tolerance, trap formation, and pathogenicity. We highlighted the role of AoBck1 and AoMkk1 in regulating the Woronin body response to cellular damage and cell nucleus development in A. oligospora.

Dsk2 involves in conidiation, multi-stress tolerance and thermal adaptation in Beauveria bassiana

Dsk2 is a nuclear-enriched ubiquitin-like polyubiquitin-binding protein that regulates protein degradation in yeast but has not been explored yet in filamentous fungi, such as Beauveria bassiana. Here, we report Beauveria bassiana Dsk2 located both in the nucleus and in cytoplasm of hyphal cells. Deletion of Dsk2 resulted in mild growth defect on scant media with various carbon/nitrogen sources and dramatic attenuation in conidiation capability at optimal condition. Compared to the wild-type, ΔDsk2 strains are much more sensitive to high osmotic and oxidative pressure during vegetative growth. Meanwhile, the mutant strains showed an increased chemical tolerance to Congo red and calcofluor white, two cell wall perturbing agents. The transcriptional changes of genes involved in central development, superoxide dismutase and chitin synthesis pathway indicate that Dsk2 acts as a multi-functional regulator in adapting to environmental changes. Importantly, Dsk2 negatively regulated the ability of thermal resistance in B. bassiana, which makes it a potential target gene for constructing engineering anti-thermal strains in the circumstance of global warming. Altogether, our finding highlights novel roles of Dsk2 involved in the asexual cycle, multi-stress tolerance and pest control potential of B. bassiana.

Enhancement of carotenoid production and its regulation in edible mushroom Cordyceps militaris by abiotic stresses

Cordyceps militaris carotenoids are widely used as food additives, animal feed supplements, and so on. However, the biosynthetic pathway of carotenoids in C. militaris is still obscure. In this paper, changes of mycelial morphology and carotenoid accumulation of C. militaris were investigated under oxidative (KMnO₄) and osmotic stress (NaCl). Subsequently, qRT-PCR was employed to detect the expression levels of genes related to carotenogenesis to explore the mechanism of adaptation to abiotic stress. When the concentrations of KMnO₄ and NaCl were respectively 0.4 g/L and 2 g/L, carotenoid accumulation reached a maximum of 6616.82 ± 666.43 μg/g and 6416.77 ± 537.02 μg/g. Under the oxidative stress condition of KMnO₄, the expressions of psy and hsp70 increased significantly compared with control. Besides, the genes fus3 and hog1 were significantly enriched in the MAPK signal pathway. Compared with the control group, there was no significant difference in expression of psy in the NaCl group. Moreover, the accumulation of triacylglycerols may contribute significantly to the increase in carotenoid accumulation. The increased accumulation of antioxidant carotenoids induced under environmental stress is to resist oxidative conditions. Fus3 and Hog1 signaling in the MAPK pathway was activated and subsequently take effects on the resistance of oxidative condition by regulating related metabolic processes. C. militaris resist the stress of high oxygen by producing a large amount of glycerol and carotenoids when this fungus is cultured in a saline environment for a long time.

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.

Transcriptional Responses of Beauveria bassiana Blastospores Cultured Under Varying Glucose Concentrations

Culturing the entomopathogenic fungus, Beauveria bassiana, under high glucose concentrations coupled with high aeration results in a fungal developmental shift from hyphal growth to mostly blastospores (yeast-like cells). The underlying molecular mechanisms involved in this shift remain elusive. A systematic transcriptome analysis of the differential gene expression was preformed to uncover the fungal transcriptomic response to osmotic and oxidative stresses associated with the resulting high blastospore yield. Differential gene expression was compared under moderate (10% w/v) and high (20% w/v) glucose concentrations daily for three days. The RNAseq-based transcriptomic results depicted a higher proportion of downregulated genes when the fungus was grown under 20% glucose than 10%. Additional experiments explored a broader glucose range (4, 8, 12, 16, 20% w/v) with phenotype assessment and qRT-PCR transcript abundance measurements of selected genes. Antioxidant, calcium transport, conidiation, and osmosensor-related genes were highly upregulated in higher glucose titers (16-20%) compared to growth in lower glucose (4-6%) concentrations. The class 1 hydrophobin gene (Hyd1) was highly expressed throughout the culturing. Hyd1 is known to be involved in spore coat rodlet layer assembly, and indicates that blastospores or another cell type containing hydrophobin 1 is expressed in the haemocoel during the infection process. Furthermore, we found implications of the HOG signaling pathway with upregulation of homologous genes Ssk2 and Hog1 for all fermentation time points under hyperosmotic medium (20% glucose). These findings expand our knowledge of the molecular mechanisms behind blastospore development and may help facilitate large-scale industrial production of B. bassiana blastospores for pest control applications.

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.

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.

VdNop12, containing two tandem RNA recognition motif domains, is a crucial factor for pathogenicity and cold adaption in Verticillium dahliae

Previous studies have reported the ability of fungi to overwinter in soil or on crop debris under different environmental conditions, but how fungi adapt to chilling is still largely unknown. In this study, we have identified and characterized the RNA binding protein (RBP) (VdNop12) by screening an Agrobacterium tumefaciens-mediated transformation-mediated insertional mutational library of Verticillium dahliae. We determined that this protein was essential to the pathogen for virulence on cotton plants. VdNop12 contains two tandem RNA recognition motif domains, and its orthologs are widely distributed in filamentous fungi. Mutants produced by disruption of VdNop12 showed defects in vegetative growth, conidiation and cell wall integrity. The mutant also showed an increase in sensitivity to low temperature, as compared to the wildtype and complementation strains. Yeast complementation assay showed that VdNop12 could functionally restore the growth phenotype of ΔScNop12 mutant of Saccharomyces cerevisiae at 15°C. We demonstrated that the VdNop12 is localized in the nucleus, and its loss resulted in the downregulated expression of several genes related to cAMP-PKA and MAPK pathways in V. dahliae. Our results demonstrated a crucial role of RBPs in the regulation of morphology, cold adaption, and pathogenic development in V. dahliae.

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.

Opposite Nuclear Dynamics of Two FRH-Dominated Frequency Proteins Orchestrate Non-Rhythmic Conidiation in Beauveria bassiana

Non-rhythmic conidiation favors large-scale production of conidia serving as active ingredients of fungal insecticides, but its regulatory mechanism is unknown. Here, we report that two FREQUENCY (FRQ) proteins (Frq1/2) governed by a unique FRQ-interacting RNA helicase (FRH) orchestrate this valuable trait in Beauveria bassiana, an asexual insect-pathogenic fungus. Frq1 (964 aa) and Frq2 (583 aa) exhibited opposite expression dynamics (rhythms) in nucleus and steadily high expression levels in cytoplasm under light or in darkness no matter whether one of them was present or absent. Such opposite nuclear dynamics presented a total FRQ (pooled Frq1/2) level sufficient to persistently activate central developmental pathway in daytime and nighttime and supports continuous (non-rhythmic) conidiation for rapid maximization of conidial production in a fashion independent of photoperiod change. Importantly, both nuclear dynamics and cytoplasmic stability of Frq1 and Frq2 were abolished in the absence of the FRH-coding gene nonessential for the fungal viability, highlighting an indispensability of FRH for the behaviors of Frq1 and Frq2 in both nucleus and cytoplasm. These findings uncover a novel circadian system more complicated than the well-known Neurospora model that controls rhythmic conidiation, and provide a novel insight into molecular control of non-rhythmic conidiation in B. bassiana.

The Arf-GAP AoGlo3 regulates conidiation, endocytosis, and pathogenicity in the nematode-trapping fungus Arthrobotrys oligospora

Small GTPases of the ADP-ribosylation factor (Arf) family and their activating proteins (Arf-GAPs) regulate mycelial development and pathogenicity in yeast and filamentous fungi; however, little is known about their roles in nematode-trapping (NT) fungi. In this study, an ortholog of Arf-GAP Glo3 (AoGlo3) in Saccharomyces cerevisiae was characterized in the NT fungus Arthrobotrys oligospora. Deletion of the Aoglo3 gene resulted in growth defects and an increase in hyphal septum. Meanwhile, the sporulation capacity of the ΔAoglo3 mutant was decreased by 98%, and 67.1-71.2% spores became gourd or claviform in shape (from obovoid), which was accompanied by a significant decrease in the spore germination rate. This reduced sporulation capacity correlated with the transcriptional repression of several sporulation-related genes including fluG, rodA, abaA, medA, and lreA. The ΔAoglo3 mutant was also sensitive to several chemical stressors such as Congo red, NaCl, and sorbitol. Additionally, AoGlo3 was found to be involved in endocytosis, and more myelin figures were observed in the ΔAoglo3 mutant than in the wild-type strain, which was consistent with the presence of more autophagosomes observed in the mutant. Importantly, AoGlo3 affected the production of mycelial traps and serine proteases for nematode predation. In summary, AoGlo3 is involved in the regulation of multiple cellular processes such as mycelial growth, conidiation, environmental adaption, endocytosis, and pathogenicity in A. oligospora.

Protein Kinase Ime2 Is Required for Mycelial Growth, Conidiation, Osmoregulation, and Pathogenicity in Nematode-Trapping Fungus Arthrobotrys oligospora

Inducer of meiosis 2 (Ime2), a protein kinase that has been identified in diverse fungal species, functions in the regulation of various cellular processes, such as ascospore formation, pseudohyphal growth, and sexual reproduction. In this study, AoIme2, an ortholog of Saccharomyces cerevisiae Ime2, was characterized in the nematode-trapping fungus Arthrobotrys oligospora. Disruption of the gene Aoime2 caused defective growth, with slower mycelial growth in ΔAoime2 mutants than the wild type (WT) strain, and in the mutants, the number of hyphal septa in mycelia was higher and the number of cell nuclei in mycelia and conidia was considerably lower than in the WT strain. The conidial yields of the ΔAoime2 mutants were decreased by ∼33% relative to the WT strain, and the transcription of several sporulation-related genes, including abaA, fluG, rodA, aspB, velB, and vosA, was markedly downregulated during the conidiation stage. The ΔAoime2 mutants were highly sensitive to the osmotic stressors NaCl and sorbitol, and the cell wall of partial hyphae in the mutants was deformed. Further examination revealed that the cell wall of the traps produced by ΔAoime2 mutants became loose, and that the electron-dense bodies in trap cells were also few than in the WT strain. Moreover, Aoime2 disruption caused a reduction in trap formation and serine-protease production, and most hyphal traps produced by ΔAoime2 mutants did not form an intact hyphal loop; consequently, substantially fewer nematodes were captured by the mutants than by the WT strain. In summary, an Ime2-MAPK is identified here for the first time from a nematode-trapping fungus, and the kinase is shown to be involved in the regulation of mycelial growth and development, conidiation, osmolarity, and pathogenicity in A. oligospora.

AoStuA, an APSES transcription factor, regulates the conidiation, trap formation, stress resistance and pathogenicity of the nematode-trapping fungus Arthrobotrys oligospora

The APSES protein family comprises a conserved class of fungus-specific transcriptional regulators. Some members have been identified in partial ascomycetes. In this study, the APSES protein StuA (AoStuA) of the nematode-trapping fungus Arthrobotrys oligospora was characterized. Compared with the wild-type (WT) strain, three ΔAoStuA mutants grew relatively slowly, displayed a 96% reduction in sporulation capacity and a delay in conidial germination. The reduced sporulation capacity correlated with transcriptional repression of several sporulation-related genes. The mutants were also more sensitive to chemical stressors than the WT strain. Importantly, the mutants were unable to produce mycelial traps for nematode predation. Moreover, peroxisomes and Woronin bodies were abundant in the WT cells but hardly found in the cells of those mutants. The lack of such organelles correlated with transcriptional repression of some genes involved in the biogenesis of peroxisomes and Woronin bodies. The transcript levels of several genes involved in the cAMP/PKA signalling pathway were also significantly reduced in the mutants versus the WT strain, implicating a regulatory role of AoStuA in the transcription of genes involved in the cAMP/PKA signalling pathway that regulates an array of cellular processes and events. In particular, AoStuA is indispensable for A. oligospora trap formation and virulence.

The DUF1996 and WSC domain-containing protein Wsc1I acts as a novel sensor of multiple stress cues in Beauveria bassiana

Wsc1I homologues featuring both an N-terminal DUF1996 (domain of unknown function 1996) and a C-terminal WSC (cell wall stress-responsive component) domain exist in filamentous fungi but have never been functionally characterized. Here, Wsc1I is shown to localize in the vacuoles and cell wall/membrane of the insect mycopathogen Beauveria bassiana and hence linked to cell membrane- and vacuole-related cellular events. In B. bassiana, deletion of Wsc1I resulted in marked increases of hyphal and conidial sensitivities to hyperosmotic agents, oxidants, cell wall perturbing chemicals, and metal cations (Cu²⁺ , Zn²⁺ , Fe²⁺ , and Mg²⁺ ) despite slight impact on normal growth and conidiation. Conidia produced by the deletion mutant showed not only reduced tolerance to both 45°C heat and UVB irradiation but also attenuated virulence to a susceptible insect through normal cuticle infection or cuticle-bypassing infection. Importantly, phosphorylation of the mitogen-activated protein kinase Hog1 was largely attenuated or nearly abolished in the Wsc1I-free cells triggered with hyperosmotic, oxidative, or cell wall perturbing stress. All changes were well restored by targeted gene complementation. Our findings highlight a novel role of Wsc1I in sensing multiple stress cues upstream of the Hog1 signalling pathway and its pleiotropic effects in B. bassiana.

Two Verticillium dahliae MAPKKKs, VdSsk2 and VdSte11, Have Distinct Roles in Pathogenicity, Microsclerotial Formation, and Stress Adaptation

These data provide insights into the distinctive functions of VdSsk2 and VdSte11 in pathogenicity, stress adaptation, and microsclerotial formation in V. dahliae.

Verticillium dahliae causes destructive vascular wilt diseases on more than 200 plant species, including economically important crops and ornamental trees worldwide. The melanized microsclerotia enable the fungus to survive for years in soil and are crucial for its disease cycle. Previously, we found that the VdPbs2-VdHog1 (V. dahliae Pbs2-V. dahliae Hog1) module plays key roles in microsclerotial formation, stress responses, and virulence in V. dahliae. In this study, two mitogen-activated protein kinase kinase kinases (MAPKKKs) homologous to Ssk2p and Ste11p, which activate the Pbs2p-Hog1p module by phosphorylation in budding yeast, were identified in the genome of V. dahliae. Both ΔVdSsk2 (V. dahliaeSsk2) and ΔVdSte11 strains showed severe defects in microsclerotial formation and melanin biosynthesis, but the relative importance of these two genes in microsclerotial development was different. Deletion of VdSsk2, but not VdSte11, affected responses to osmotic stress, fungicidal response, and cell wall stressors. The ΔVdSsk2 strain exhibited a significant reduction in virulence, while the ΔVdSte11 strain was nonpathogenic due to failure to penetrate and form hyphopodia. Phosphorylation assays demonstrated that VdSsk2, but not VdSte11, can phosphorylate VdHog1 in V. dahliae. Moreover, VdCrz1, encoding a calcineurin-responsive zinc finger transcription factor and a key regulator of calcium signaling in fungi, was misregulated in the ΔVdSsk2, ΔVdPbs2, and ΔVdHog1 mutants.

IMPORTANCE These data provide insights into the distinctive functions of VdSsk2 and VdSte11 in pathogenicity, stress adaptation, and microsclerotial formation in V. dahliae.