Oxidative stress impacts carbon metabolism and induces upregulation of redox enzymes andsmall secreted proteins in the wood-decay fungus

Abstract

Fungi live in diverse environments requiring tolerance against abiotic and biotic stress and changing atmospheric conditions. Studies with white rot and brown rot species of Polyporales Basidiomycota have demonstrated that aerobic wood decay fungi may adapt to low oxygen and even anoxic conditions, which they undoubtedly encounter in their deadwood habitat. In the white rot fungus Phlebia radiata, oxygen depletion on lignocellulose substrates leads to hypoxia and fermentative metabolism. In this study, we elaborated the atmospheric effect further by subjecting the fungus to oxidative stress on wood substrate under aerobic and low oxygen conditions, with the aim to examine changes in gene expression and metabolic pathways as consequences of the oxidative treatment. Overall, 762 genes were significantly differentially expressed (DEGs with absolute Log2FoldChange > 1) 18 h after treatment with hydrogen peroxide according to RNA-Seq data. Half of these (348 genes) were downregulated in low oxygen (< 10% O2) cultures, with 185 genes unique to the condition but one third (121 genes) of unknown function. In aerobic cultures, a different response was observed with less DEGs showing downregulation (236 genes) while a higher number were upregulated (267 genes) and 153 of the upregulated genes were unique to the condition. Among DEGs upregulated under both conditions, small secreted proteins (SSPs) were the most abundant, followed by short-chain dehydrogenase/reductases, aldo-keto reductases, GNAT family acetyltransferases and CAZy carbohydrate active enzymes. Among downregulated DEGs, numerous SSP and CAZy classes, and genes encoding heat-shock proteins and MFS transporters were identified. Closer examination of carbon metabolism genes showed that under both conditions, oxidative treatment led to suppression of pentose catabolic pathway, glycerol metabolism, and formation of ethanol and acetate. Oxidative stress caused a substantial change in fungal gene expression, but with different responses depending on the culture atmosphere. This may be explained by contrasting fungal metabolic states before the shock as was observed in extracellular enzyme, aromatic metabolite and redox activity profiles. Surprisingly, oxidative shock caused downregulation of a few heat-shock proteins whereas small secreted proteins were either up- or downregulated, suggesting both sensing and regulative roles for these, functionally yet unknown, diverse fungal proteins.