Proteomic Characterization of Lignocellulolytic Enzymes Secreted by the Insect-Associated Fungus Daldinia decipiens oita, Isolated from a Forest in Northern Japan

Abstract

Wood-devastating insects utilize their symbiotic microbes with lignocellulose-degrading abilities to extract energy from recalcitrant woods. It is well known that free-living lignocellulose-degrading fungi secrete various carbohydrate-active enzymes (CAZymes) to degrade plant cell wall components, mainly cellulose, hemicellulose, and lignin. However, CAZymes from insect-symbiotic fungi have not been well documented except for a few examples. In this study, an insect-associated fungus, Daldinia decipiens oita, was isolated as a potential symbiotic fungus of female Xiphydria albopicta captured from Hokkaido forest. This fungus was grown in seven different media containing a single carbon source, glucose, cellulose, xylan, mannan, pectin, poplar, or larch, and the secreted proteins were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). A total of 128 CAZymes, including domains of 92 glycoside hydrolases, 15 carbohydrate esterases, 5 polysaccharide lyases, 17 auxiliary activities, and 11 carbohydrate-binding modules, were identified, and these are involved in degradation of cellulose and hemicellulose but not lignin. Together with the results of polysaccharide-degrading activity measurements, we concluded that D. decipiens oita tightly regulates the expression of these CAZymes in response to the tested plant cell wall materials. Overall, this study described the detailed proteomic approach of a woodwasp-associated fungus and revealed that the new isolate, D. decipiens oita, secretes diverse CAZymes to efficiently degrade lignocellulose in the symbiotic environment.IMPORTANCE Recent studies show the potential impacts of insect symbiont microbes on biofuel application with regard to their degradation capability of a recalcitrant plant cell wall. In this study, we describe a novel fungal isolate, D. decipiens oita, as a single symbiotic fungus from the Xiphydria woodwasp found in the northern forests of Japan. Our detailed secretome analyses of D. decipiens oita, together with activity measurements, reveal that this insect-associated fungus exhibits high and broad activities for plant cell wall material degradation, suggesting potential applications within the biomass conversion industry for plant mass degradation.

Keywords: cellulolytic fungus; cellulose; glycoside hydrolases; hemicellulose; proteomics.