Development of a transformation system for Hirsutella spp. and visualization of the mode of nematode infection by GFP-labeled H. minnesotensis

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

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

In vivo infection of Bursaphelenchus xylophilus by the fungus Esteya vermicola

BACKGROUND

As the causal agent of pine wilt disease, Bursaphelenchus xylophilus, is a serious pathogen of forest pine trees. Esteya vermicola is a nematophagous fungus of B. xylophilus and exhibits great potential as a biological control agent. However, the in vivo infection mechanism of E. vermicola on B. xylophilus is unclear. Experiments were conducted to study the colonization of host plant and infection of B. xylophilus by E. vermicola inside pine tree xylem.

RESULTS

A green fluorescent protein (GFP)-tagged E. vermicola transformant was constructed as a biomarker to study the in vivo colonization and infection of B. xylophilus in pine trees. The in vitro infection of B. xylophilus by E. vermicola was observed through GFP expression. The bacilloid conidia produced by trophic hyphae in the body of the nematode are described. Additionally, the monitoring of in vivo colonization by GFP-tagged E. vermicola showed the germination and hyphal extension of this fungus after inoculation. Moreover, B. xylophilus infected by this biocontrol agent were extracted from healthy seedlings and observed in the xylem of trees that were wilting due to pine wilt disease.

CONCLUSION

Evidence of fungal colonization and infection of B. xylophilus by E. vermicola is provided to improve our understanding of the in vivo infection mechanisms used by this nematophagous fungus against B. xylophilus. The infection of B. xylophilus by E. vermicola was inferred to begin with the implantation of propagules, and this inference will require future investigation. The colonization of Esteya vermicola in host pine tree xylem and the in vivo infection of pinewood nematode by E. vermicola were investigated using the green fluorescence protein transformant. © 2020 Society of Chemical Industry.

The transcription factor SKN7 regulates conidiation, thermotolerance, apoptotic-like cell death and parasitism in the nematode endoparasitic fungus Hirsutella minnesotensis

The transcription factor SKN7 is a highly conserved protein among fungi and was initially recognized as a response regulator that protects cells from oxidative stress and maintains cell wall integrity in yeast. Orthologs of SKN7 are extensively present in biocontrol agents of plant pathogens, but they had not been functionally characterized. Here, we identified and characterized the transcription factor SKN7 in the nematode endoparasitic fungus Hirsutella minnesotensis. Null mutant lacking HIM-SKN7 (HIM_03620), which was generated by a gene disruption strategy, demonstrated reduced conidiation, increased sensitivity to high temperature, hydrogen peroxide, mannitol and ethanol, and reduced fungal resistance to farnesol. However, over-expression mutant showed increased conidial production, thermotolerance and resistance to farnesol, suggesting that HIM-SKN7 regulates antiapoptotic-like cell death in H. minnesotensis. Moreover, the results showed that in null mutant, H. minnesotensis had decreased endoparasitic ability as compared to wild type and over-expression strain. During the infection process, the relative expression of the HIM-SKN7 gene was significantly induced in the wild type and over-expression strain. The results of the present study advance our understanding of the functions of the SKN7 gene in biocontrol agents, in particular, nematode endoparasitic fungi.

Comparative Genomic Analysis of Drechmeria coniospora Reveals Core and Specific Genetic Requirements for Fungal Endoparasitism of Nematodes

Drechmeria coniospora is an obligate fungal pathogen that infects nematodes via the adhesion of specialized spores to the host cuticle. D. coniospora is frequently found associated with Caenorhabditis elegans in environmental samples. It is used in the study of the nematode's response to fungal infection. Full understanding of this bi-partite interaction requires knowledge of the pathogen's genome, analysis of its gene expression program and a capacity for genetic engineering. The acquisition of all three is reported here. A phylogenetic analysis placed D. coniospora close to the truffle parasite Tolypocladium ophioglossoides, and Hirsutella minnesotensis, another nematophagous fungus. Ascomycete nematopathogenicity is polyphyletic; D. coniospora represents a branch that has not been molecularly characterized. A detailed in silico functional analysis, comparing D. coniospora to 11 fungal species, revealed genes and gene families potentially involved in virulence and showed it to be a highly specialized pathogen. A targeted comparison with nematophagous fungi highlighted D. coniospora-specific genes and a core set of genes associated with nematode parasitism. A comparative gene expression analysis of samples from fungal spores and mycelia, and infected C. elegans, gave a molecular view of the different stages of the D. coniospora lifecycle. Transformation of D. coniospora allowed targeted gene knock-out and the production of fungus that expresses fluorescent reporter genes. It also permitted the initial characterisation of a potential fungal counter-defensive strategy, involving interference with a host antimicrobial mechanism. This high-quality annotated genome for D. coniospora gives insights into the evolution and virulence of nematode-destroying fungi. Coupled with genetic transformation, it opens the way for molecular dissection of D. coniospora physiology, and will allow both sides of the interaction between D. coniospora and C. elegans, as well as the evolutionary arms race that exists between pathogen and host, to be studied.

The mitochondrial genome of the nematode endoparasitic fungus Hirsutella rhossiliensis

In this study, we report the complete mitochondrial genome of Hirsutella rhossiliensis (Ophiocordycipitaceae, Hypocreales, Ascomycota). We construct the mitochondrial DNA genome organization of 62 483 bp in length of H. rhossiliensis by using the whole-genome resequencing method. Conserved genes including the large and small rRNA subunits, 26 tRNA and 14 protein-coding genes are identified. These protein-coding genes utilize ATG, GTG or TTG as initiation codons and TAA or TAG as termination codons. Moreover, we detect 10 group I introns and one unclassified intron in six genes (rnl, cob, cox1, cox3, nad1 and nad5) encoding ORFs of ribosomal protein S3 and GIY-YIG/LAGLIDADG endonucleases or hypothetical proteins. This mitochondrial genome will be useful in understanding the distribution and genetic diversity of this species.