Abstract
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.
Highlights
The fungal cell wall is essential to maintain cellular structure and protect the cells from environmental stresses; its integrity is very important for fungal survival and adaption under adverse condition (Levdansky et al, 2010)
The cell wall integrity (CWI) pathway controls cell wall synthesis and cell cycle progression through a conserved MAP kinase cascade (Bck1, Mkk1/2, and Slt2) in response to cell surface stress; its signal transmission depends on the cascade phosphorylation reaction, where protein kinase C first activates the phosphorylation of Bck1, and Bck1 activates the phosphorylation of MKK1/2 which triggers the phosphorylation of Slt2, thereby transmitting signals to downstream effectors (Levin, 2005, 2011; Jin et al, 2015)
The wild-type (WT) A. oligospora strain and the mutant strains ΔAoBck1 and ΔAoMkk1 were cultured on potato dextrose agar (PDA) plates at 28°C for 6 days, following which 7 mm diameter hyphal discs of each strain were inoculated separately in PDA, tryptone yeast-extract glucose agar (TYGA), and TG media at 28°C for 3–7 days, and the mycelial growth rate and colony morphology were observed at specific time intervals (Xie et al, 2019)
Summary
The fungal cell wall is essential to maintain cellular structure and protect the cells from environmental stresses; its integrity is very important for fungal survival and adaption under adverse condition (Levdansky et al, 2010). The CWI pathway has been characterized in several filamentous fungi, such as Aspergillus fumigatus (Valiante et al, 2009; Levdansky et al, 2010), Magnaporthe grisea (Zhao et al, 2005; Jeon et al, 2008), and Beauveria bassiana (Luo et al, 2012; Chen et al, 2014) These studies indicated that after the cascade components of the CWI pathway were destroyed in fungi, the mutant strains showed varying degrees of cell wall defects and increased their sensitivity to cell wall interfering agents, which affected their pathogenicity. Orthologous A. oligospora Bck (AoBck1) and Mkk (AoMkk1) were characterized via multi-phenotypic analyses, and their regulatory mechanisms in conidiation and stress resistance were analyzed by real-time PCR (RT-PCR)
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