T. harzianum regulation on inactivated oomycete mycelium
The breakdown of oomycete necromass is an important source of organic matter for composting. How Trichoderma harzianum, an important composting fungus, regulates gene expression and produces exo-proteins for degradation of oomycete necromass is poorly understood, especially related to cellulose, an important component of oomycete necromass. Complementary techniques of chemical compositional analysis, transcriptomics, exo-proteomics, enzymatic assays, and fungal genetics were used to analyze the degradation of inactivated oomycete mycelial powder – a surrogate for oomycete necromass. In total, 1,556 genes were upregulated and 212 exo-proteins were produced in T. harzianum oomycete mycelial powder cultures, and about 25% of the produced proteins showed corresponding gene upregulation. The enzymes detected, such as ß-1,3-glucanases, and ß-1,4-glucanases (cellulases), matched well with the composition of oomycete mycelial powder. Linkage compositional analysis showed that the mycelial powder contained ~ 60% 1,3 linkages and ~19% 1,4 linkages. The enzyme cocktail from the submerged cultures converted approximately one-third of the mycelial powder to glucose by in vitro assays. The conversion of the mycelial powder to glucose was not substantially reduced by deletion of the cellulolytic transcriptional activator XYR1. Deletion of XYR1 did decrease cellulase activity but only ~1% of mycelial powder-induced genes appeared to be XYR1-regulated. In conclusion, T. harzianum produces suitable enzyme cocktails for oomycete mycelial powder degradation, with ß-1,3-glucanases likely playing a more important role than cellulases. T. harzianum cellulases may either be relatively unimportant for the degradation, or may not be co-activated alongside CAZymes degrading less recalcitrant parts of the mycelial powder.