Differences in 2,6-Dimethoxy-1,4-benzoquinone Reduction Systems between Brown-rot and White-rot Fungi

Brown-rot fungi decompose polysaccharides such as cellulose and hemicellulose through mechanisms that do not involve extracellular polysaccharide-degrading enzymes. Instead, oxidative degradation occurs via the Fenton reaction, which requires hydroquinone as an extracellular electron donor. However, the mechanisms by which brown-rot fungi generate and transport hydroquinones remains unclear. This study aimed to investigate the quinone reduction systems in the brown-rot fungus Tyromyces palustris and compare it with that in the white-rot fungus Pleurotus ostreatus. Using fungal protoplasts, we found that T. palustris rapidly reduced 2,6-dimethoxy-1,4-benzoquinone (2,6-DMBQ) to 1,4-dihydroxy-2,6-dimethoxybenzene (2,6-DMHQ), and 2,6-DMHQ, which accumulated exclusively in the extracellular fraction. In contrast, P. ostreatus exhibited lower reduction activity, with 2,6-DMBQ and 2,6-DMHQ retained within the intracellular compartments. The addition of the uncoupling reagent carbonyl cyanide m-chlorophenylhydrazone (CCCP), which disrupts the pH gradient across the plasma membrane, had little effect on extracellular 2,6-DMHQ formation in T. palustris but significantly inhibited 2,6-DMBQ reduction in P. ostreatus. These findings suggest that brown-rot fungi employ a plasma membrane-bound enzyme with an outward-facing quinone-reducing site, enabling the extracellular supply of hydroquinone to facilitate the Fenton reaction. Understanding this mechanism enhances our knowledge of fungal biodegradation and may have applications in biotechnological processes involving lignocellulose degradation.