Random mutagenesis analysis and identification of a novel C2H2-type transcription factor from the nematode-trapping fungus Arthrobotrys oligospora

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.

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.

Tools and basic procedures of gene manipulation in nematode-trapping fungi

Nematode-trapping fungi (NTF) are the majority of carnivorous microbes to capture nematodes through diverse and sophisticated trapping organs derived from hyphae. They can adopt carnivorous lifestyles in addition to saprophytism to obtain extra-nutrition from nematodes. As a special group of fungi, the NTF are not only excellent model organism for studying lifestyle transition of fungi but also natural resources of exploring biological control of nematodes. However, the carnivorous mechanism of NTF remains poorly understood. Nowadays, the omics studies of NTF have provided numerous genes and pathways that are associated with the phenotypes of carnivorous traits, which need molecular tools to verify. Here, we review the development and progress of gene manipulation tools in NTF, including methodology and strategy of transformation, random gene mutagenesis methods and target gene mutagenesis methods. The principle and practical approach for each method was summarized and discussed, and the basic operational flow for each tool was described. This paper offers a clear reference and instruction for researchers who work on NTF as well as other group of fungi.

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.

Methylglyoxal Has Different Impacts on the Fungistatic Roles of Ammonia and Benzaldehyde, and Lactoylglutathione Lyase Is Necessary for the Resistance of Arthrobotrys oligospora to Soil Fungistasis

Biocontrol of root-knot nematode has attracted increasing attention over the past two decades. The inconsistent field performance of biocontrol agents, which is caused by soil fungistasis, often restricts their commercial application. There is still a lack of research on the genes involved in biocontrol fungi response to soil fungistasis, which is important for optimizing practical applications of biocontrol fungi. In this study, the lactoylglutathione lyase-encoding AOL_s00004g335 in the nematophagous fungi Arthrobotrys oligospora was knocked out, and three mutant strains were obtained. The hyphal growth of mutants on the three media was almost the same as that of the wild-type strain, but mutants had slightly higher resistance to NaCl, SDS, and H₂O₂. Methylglyoxal (MG) significantly increased the resistance of A. oligospora to ammonia, but decreased the resistance to benzaldehyde. Furthermore, the resistance of the mutants to soil fungistasis was largely weakened and MG could not increase the resistance of A. oligospora to soil fungistasis. Our results revealed that MG has different effects on the fungistatic roles of ammonia and benzaldehyde and that lactoylglutathione lyase is very important for A. oligospora to resist soil fungistasis.

Transcriptome profiling reveals insertional mutagenesis suppressed the expression of candidate pathogenicity genes in honeybee fungal pathogen, Ascosphaera apis

Chalkbrood disease is caused by Ascosphaera apis which severely affects honeybee brood. Spore inoculation experiments shown pathogenicity varies among different strains and mutants, however, the molecular mechanism of pathogenicity is unclear. We sequenced, assembled and annotated the transcriptomes of wild type (SPE1) and three mutants (SPE2, SPE3 and SPE4) with reduced pathogenicity that were constructed in our previous study. Illumina sequencing generated a total of 394,910,604 clean reads and de novo Trinity-based assembled into 12,989 unigenes, among these, 9,598 genes were successfully annotated to known proteins in UniProt database. A total of 172, 3,996, and 650 genes were up-regulated and 4,403, 2,845, and 3,016 genes were down-regulated between SPE2-SPE1, SPE3-SPE1, and SPE4-SPE1, respectively. Overall, several genes with a potential role in fungal pathogenicity were detected down-regulated in mutants including 100 hydrolytic enzymes, 117 transcriptional factors, and 47 cell wall related genes. KEGG pathway enrichment analysis reveals 216 genes involved in nine pathways were down-regulated in mutants compared to wild type. The down-regulation of more pathways involved in pathogenicity in SPE2 and SPE4 than SPE3 supports their lower pathogenicity during in-vitro bioassay experiment. Expression of 12 down-regulated genes in mutants was validated by quantitative real time PCR. This study provides valuable information on transcriptome variation caused by mutation for further functional validation of candidate pathogenicity genes in A. apis.

Characterization and functional analysis of calcium/calmodulin-dependent protein kinases (CaMKs) in the nematode-trapping fungus Arthrobotrys oligospora

Ca²⁺/calmodulin-dependent protein kinases (CaMKs) are unique second-messenger molecules that impact almost all cellular processes in eukaryotes. In this study, five genes encoding different CaMKs were characterized in the nematode-trapping fungus Arthrobotrys oligospora. These CaMKs, which were retrieved from the A. oligospora genome according to their orthologs in fungi such as Aspergillus nidulans and Neurospora crassa, were expressed at a low level in vitro during mycelial growth stages. Five deletion mutants corresponding to these CaMKs led to growth defects in different media and increased sensitivity to several environmental stresses, including H₂O₂, menadione, SDS, and Congo red; they also reduced the ability to produce conidia and traps, thus causing a deficiency in nematicidal ability as well. In addition, the transcriptional levels of several typical sporulation-related genes, such as MedA, VelB, and VeA, were down-regulated in all ΔCaMK mutants compared with the wild-type (WT) strain. Moreover, these mutants exhibited hypersensitivity to heat shock and ultraviolet-radiation stresses compared with the WT strain. These results suggest that the five CaMKs in A. oligospora are involved in regulating multiple cellular processes, such as growth, environmental stress tolerance, conidiation, trap formation, and virulence.

MAP kinase Slt2 orthologs play similar roles in conidiation, trap formation, and pathogenicity in two nematode-trapping fungi

Mitogen-activated protein (MAP) kinase Slt2 is a key player in the cell-wall integrity pathway of budding yeast. In this study, we functionally characterized Slt2 orthologs AoSlt2 and MhSlt2 from the nematode-trapping fungi Arthrobotrys oligospora and Monacrosporium haptotylum, respectively. We found that disruption of AoSlt2 and MhSlt2 led to reduced mycelial growth, increased sensitivity to environmental stresses such as sodium dodecyl sulfate, Congo red, and H₂O₂, and an inability to produce conidia and nematode-trapping structures. Real-time polymerase chain reaction-based analyses showed that the transcription of sporulation-related (AbaA, Sep2, and MedA) and cell wall synthesis-related (Chs, Glu, and Gfpa) genes was down-regulated in the mutants compared with the wild-type strains. Moreover, the mutant strains showed reduced extracellular proteolytic activity and decreased transcription of three homologous serine protease-encoding genes. These results show for the first time that MAP kinase Slt2 orthologs play similar roles in regulating mycelial growth, conidiation, trap formation, stress resistance, and pathogenicity in the divergent nematode-trapping fungal species A. oligospora and M. haptotylum.

Two Rab GTPases play different roles in conidiation, trap formation, stress resistance, and virulence in the nematode-trapping fungus Arthrobotrys oligospora

Rab GTPases are the largest group of the small GTPases family, which play a pivotal role in the secretion of proteins. Arthrobotrys oligospora is a representative nematode-trapping fungus that can produce adhesive networks to capture nematodes. In this study, the roles of two Rab GTPases AoRab-7A and AoRab-2 were characterized by gene knockout in the fungus A. oligospora. The disruption of AoRab-7A hindered the mycelial growth in different media, the conidiation of ΔAoRab-7A transformants was almost abolished, and the transcription of four sporulation-related genes (AbaA, FluG, Hyp1, and VosA) was downregulated compared to the wild-type strain (WT). Furthermore, the tolerance of the ΔAoRab-7A mutants to sodium dodecyl sulfate (SDS) and H₂O₂ was also significantly reduced compared to the WT, and the transcription of several genes related to environmental resistance, such as genes for catalase and trehalose synthase, was downregulated. Similarly, the extracellular proteolytic activity was decreased. Importantly, the ΔAoRab-7A mutants were unable to produce traps and capture nematodes. However, the disruption of gene AoRab-2 only affected the conidiation slightly but non-significantly, while other phenotypic traits were unaffected. Moreover, the gene AoRab-7A was also involved in the autophagy induced by nitrogen deprivation in A. oligospora. Our results revealed for the first time that the Rab GTPases are involved in the regulation of mycelial growth, conidiation, trap formation, stress resistance, and pathogenicity in the nematode-trapping fungus A. oligospora.

Predator-prey interactions of nematode-trapping fungi and nematodes: both sides of the coin

Nematode-trapping fungi develop complex trapping devices to capture and consume nematodes. The dynamics of these organisms is especially important given the pathogenicity of nematodes and, consequently, the potential application of nematode-trapping fungi as biocontrol agents. Furthermore, both the nematodes and nematode-trapping fungi can be easily grown in laboratories, making them a unique manipulatable predator-prey system to study their coevolution. Several different aspects of these fungi have been studied, such as their genetics and the different factors triggering trap formation. In this review, we use the nematode-trapping fungus Arthrobotrys oligospora (which forms adhesive nets) as a model to describe the trapping process. We divide this process into several stages; namely attraction, recognition, trap formation, adhesion, penetration, and digestion. We summarize the latest findings in the field and current knowledge on the interactions between nematodes and nematode-trapping fungi, representing both sides of the predator-prey interaction.