Putative APSES family transcription factor mbp1 plays an essential role in regulating cell wall synthesis in the agaricomycete Pleurotus ostreatus

The clade A APSES family transcription factors (Mbp1, Swi4, and Swi6) contribute to cell wall synthesis regulation in fungi. Herein, evolutionary relationships among these proteins were clarified by phylogenetic analysis using various ascomycetes and basidiomycetes, and then the detailed function of Mbp1 in cell wall synthesis regulation was analyzed in Pleurotus ostreatus. Our phylogenetic analysis revealed that Mbp1 and Swi6 are widely conserved among various fungi, whereas Swi4 is a protein specific for Saccharomycotina. In P. ostreatus, two putative clade A APSES family transcription factors, protein ID 83192 and 134090, were found and identified as Mbp1 and Swi6, respectively. The mbp1 gene was then disrupted through homologous recombination using P. ostreatus 20b strain (Δku80) as a host to obtain mbp1 disruption strains (Δmbp1). Disruption of mbp1 significantly decreased the growth rate and shortened aerial hyphae, suggesting that Mbp1 is involved in mycelial growth, especially aerial hyphal growth. Furthermore, thinner cell walls, decreased relative percentage of β-glucan, and downregulation of all β-glucan synthase genes were observed in Δmbp1 strains. Therefore, Mbp1 plays an essential role in β-glucan synthesis regulation in P. ostreatus. Disruption of mbp1 also impacted the expression profiles of chitin synthase genes, septum formation, and sensitivity to a chitin synthesis inhibitor, suggesting that Mbp1 also regulates chitin synthesis. In conclusion, Mbp1 is responsible for normal mycelial growth and regulates β-glucan and chitin synthesis in P. ostreatus. To the best of our knowledge, this is the first report on the detailed function of Mbp1 in cell wall synthesis regulation in fungi.