Transcriptional response of the white-rot fungus Dichomitus squalens to polysaccharides reveals a co-expression network of plant biomass conversion related genes

Wood-degrading white-rot fungi can efficiently degrade all plant biomass components, but the molecular mechanisms behind the degradation of plant polysaccharides remain poorly understood. For example, the gene sets and expression levels induced by the plant polysaccharide-derived monosaccharides in white-rot fungi do not reflect those induced by crude plant biomass substrates. To explore the molecular response of the white-rot fungus Dichomitus squalens to plant-derived oligo- and polysaccharides, we investigated the transcriptomes from mono- and dikaryotic strains of the fungus on 10 substrates and compared the expression of carbohydrate-active enzyme-encoding genes to that previously reported for different monosaccharides and cellobiose. Our results revealed that in D. squalens, a robust response to cellulose leads to its effective depolymerization, with an orthologue of the ascomycete Trichoderma reesei ACE3 likely acting as a central transcriptional regulator. The conserved response between cellulose and cellobiose further confirms cellobiose as the main cellulase inducer in D. squalens. Surprisingly, despite low abundance of pectin in the natural wood substrate of D. squalens, we identified polygalacturonic acid as a major inducer of a broad-targeted pectinolytic response including pectinase, pectin-related sugar transporter and catabolism genes, and four fungal specific transcription factors. This indicates that D. squalens has not only maintained its ability to degrade minor polysaccharide components in its biotope, but also a regulatory system spanning from extracellular degradation to metabolic conversion. Our study contributes to a deeper understanding of the molecular mechanisms behind white-rot fungal plant polysaccharide degradation and provides leads for functional studies of potential transcriptional regulators in basidiomycetes.