Different contributions of the peroxisomal import protein Pex5 and Pex7 to development, stress response and virulence of insect fungal pathogen Beauveria bassiana

The Bbotf1 Zn(Ⅱ)2Cys6 transcription factor contributes to antioxidant response, fatty acid assimilation, peroxisome proliferation and infection cycles in insect pathogenic fungus Beauveria bassiana

The abilities to withstand oxidation and assimilate fatty acids are critical for successful infection by many pathogenic fungi. Here, we characterized a Zn(II)2Cys6 transcription factor Bbotf1 in the insect pathogenic fungus Beauveria bassiana, which links oxidative response and fatty acid assimilation via regulating peroxisome proliferation. The null mutant ΔBbotf1 showed impaired resistance to oxidants, accompanied by decreased activities of antioxidant enzymes including CATs, PODs and SODs, and down-regulated expression of many antioxidation-associated genes under oxidative stress condition. Meanwhile, Bbotf1 acts as an activator to regulate fatty acid assimilation, lipid and iron homeostasis as well as peroxisome proliferation and localization, and the expressions of some critical genes related to glyoxylate cycle and peroxins were down-regulated in ΔBbotf1 in presence of oleic acid. In addition, ΔBbotf1 was more sensitive to osmotic stressors, CFW, SDS and LDS. Insect bioassays revealed that insignificant changes in virulence were seen between the null mutant and parent strain when conidia produced on CZP plates were used for topical application. However, propagules recovered from cadavers killed by ΔBbotf1 exhibited impaired virulence as compared with counterparts of the parent strain. These data offer a novel insight into fine-tuned aspects of Bbotf1 concerning multi-stress responses, lipid catabolism and infection cycles.

Functional insights of three RING-finger peroxins in the life cycle of the insect pathogenic fungus Beauveria bassiana

Peroxisomes play important roles in fungal physiological processes. The RING-finger complex consists of peroxins Pex2, Pex10, and Pex12 and is essential for recycling of receptors responsible for peroxisomal targeting of matrix proteins. In this study, these three peroxins were functionally characterized in the entomopathogenic fungus Beauveria bassiana (Bb). These three peroxins are associated with peroxisomes, in which BbPex2 interacted with BbPex10 and BbPex12. Ablation of these peroxins did not completely block the peroxisome biogenesis, but abolish peroxisomal targeting of matrix proteins via both PTS1 and PTS2 pathways. Three disruptants displayed different phenotypic defects in growth on nutrients and under stress conditions, but have similar defects in acetyl-CoA biosynthesis, development, and virulence. Strikingly, BbPex10 played a less important role in fungal growth on tested nutrients than other two peroxins; whereas, BbPex2 performed a less important contribution to fungal growth under stresses. This investigation reinforces the peroxisomal roles in the lifecycle of entomopathogenic fungi and highlights the unequal functions of different peroxins in peroxisomal biology.

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.

Peroxins in Peroxisomal Receptor Export System Contribute to Development, Stress Response, and Virulence of Insect Pathogenic Fungus Beauveria bassiana

In filamentous fungi, recycling of receptors responsible for protein targeting to peroxisomes depends on the receptor export system (RES), which consists of peroxins Pex1, Pex6, and Pex26. This study seeks to functionally characterize these peroxins in the entomopathogenic fungus Beauveria bassiana. BbPex1, BbPex6, and BbPex26 are associated with peroxisomes and interact with each other. The loss of these peroxins did not completely abolish the peroxisome biogenesis. Three peroxins were all absolutely required for PTS1 pathway; however, only BbPex6 and BbPex26 were required for protein translocation via PTS2 pathway. Three gene disruption mutants displayed the similar phenotypic defects in assimilation of nutrients (e.g., fatty acid, protein, and chitin), stress response (e.g., oxidative and osmotic stress), and virulence. Notably, all disruptant displayed significantly enhanced sensitivity to linoleic acid, a polyunsaturated fatty acid. This study reinforces the essential roles of the peroxisome in the lifecycle of entomopathogenic fungi and highlights peroxisomal roles in combating the host defense system.

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.