*7.4. White Rot Fungi*

White rot fungi can depolymerize all three cell wall polymers using enzymes secreted by the hyphae, often leaving a whitened mass of fibers at the advanced stages of decay (Figure 2). White rot fungi metabolize all three cell wall polymers and can cause weight losses approaching 97% of the original wood. Most white rot fungi are known as "simultaneous" white rots and tend to digest and utilize the wood polymeric components as they are depolymerized. However, some "selective" white rot fungi can preferentially digest the hemicellulose and lignin components leaving much of the crystalline cellulose relatively undegraded. In either type of white rot, structural losses occur more slowly than in brown rots, and cell wall depolymerization is more directly linked to enzymatic erosion of the cell wall layers and also to the metabolic processes of the fungus. This coupling of degradation with utilization results in declines in properties such as tension or bending that parallel mass losses caused by decay. White rot fungi tend to attack hardwoods, but can also attack softwoods [35]. Carbohydrate active enzymes (CAZymes) and lignin degrading enzymes from white rot fungi have been previously explored for use in pulp and paper processing, and other industrial processes to free lignin from wood fiber with reduced use of harsher chemicals.

**Figure 2.** Advanced stage of white rot simultaneous decay. The wood in this advanced state of decay can be quite spongy and lightweight when dried. However, when decay is active, the wood will be moist and stands of wood can often be peeled from the surface along the grain. The flecks of white in the image are mycelial mats of the fungus.

Enzymes of specific importance in lignin degradation by white rot fungi are primarily oxidative and include: laccase, lignin peroxidase, manganese peroxidase, versatile peroxidase and the dye-decolorizing peroxidases [39]. Many of these enzymes are unique to white rot fungi with only a relatively few dye-decolorizing peroxidases, for example, found in select lignin-degrading bacteria. Relative to CAZymes, white rot fungi have a much more complete suite of glycoside hydrolases and also lytic polysaccharide monooxygenase enzymes that can attack cellulose both from the ends of the long-chain polymer (exo-glucanases) as well as centrally along the polymer chain (endo-glucanases). This dual action, combined with a suite of enzymes that can act on oligosaccharide depolymerization products, allows white rots to deconstruct a variety of holocellulose polymers.
