Reactive oxygen species as agents of wood decay by fungi

Although many wood decay basidiomycetes secrete oxidative and hydrolytic enzymes that participate in lignocellulose biodedgradation, it is generally recognized now that these enzymes cannot penetrate sound wood, and that fungi must employ smaller agents to initiate decay. Reactive oxygen species (ROS) are likely candidates, and evidence is accumulating that some wood decay fungi produce these oxidants. For example, we have observed that, when they grow on wood, white rot and brown rot basidiomycetes degrade polyethylene glycol, a recalcitrant polymer that resists attack by species other than nonspecific, extracellular oxidants. By contrast, xylariaceous ascomycetes do not degrade polyethylene glycol, and therefore do not produce these oxidants when they degrade wood. The ROS that basidiomycetes employ remain unidentified in most cases, but the hydroxyl radical (·OH) is a likely participant, because there is a well-established pathway for its production via the Fenton reaction (H2O2 + Fe2+ + H+ → H2O + Fe3+ + ·OH). The two best documented fungal pathways for ·OH production involve the extracellular reduction of Fe3+ and O2 by cellobiose dehydrogenase or by secreted hydroquinones. Peroxyl (ROO·) and hydroperoxyl (·OOH) radicals, although they are weaker oxidants, may also be involved in wood decay because they are produced via reactions of ·OH and by some fungal enzymes.