N-acetylglucosamine sensing in the filamentous soil fungus Trichoderma reesei

N-acetylglucosamine (GlcNAc) is involved in diverse signaling pathways in dimorphic yeasts and bacteria and is related to morphogenetic switching, mating, stress, virulence, and cell death. Recently, GlcNAc has been shown to promote plant growth by shaping the bacterial soil community. However, the role of GlcNAc sensing in filamentous soil fungi has not been investigated. By using Trichoderma reesei as a model organism, we show here that GlcNAc impacts the expression of around 2100 genes. Carbohydrate metabolism, amino acid metabolism, and secondary metabolism were the three most strongly affected classes of eukaryotic orthologous groups (KOG classes). Two key regulators of GlcNAc catabolism, the NDT80 domain-containing transcriptional regulator RON1, and a GlcNAc sensor, NGS1, are needed for differential regulation of two-thirds of these genes. In silico structural modeling of NGS1 identified a domain with homology to the GCN5-related histone acetyltransferase from Candida albicans, which serves as a GlcNAc catabolism regulator and GlcNAc sensor. Finally, we characterized the third regulator of GlcNAc sensing in T. reesei, which is the highly specific GlcNAc transporter N-acetylglucosamine transporter (NGT1). Using a deletion mutant of ngt1, we demonstrate that GlcNAc has to enter the cell to activate the GlcNAc catabolic gene expression. Interestingly, in contrast to dimorphic yeasts, the pathways for defense and pathogenicity seem to be induced in T. reesei by external GlcNAc. Given the ancestral role of Trichoderma spp. in the fungal kingdom and the highly conserved GlcNAc catabolism cluster that includes their regulators in many species of fungi, we propose a regulatory network for GlcNAc sensing in soil fungi.