*7.5. Further Evolution of Brown rot Fungi from the Precursors of White Rot Species*

Brown rot fungi evolved relatively recently from white rot fungi starting about 280 million years ago in the Permian period [31]. Only 20 years ago, brown rot fungi were looked upon by most mycologists as less evolutionarily advanced because many of the CAZymes and all of the lignin-degrading peroxidase enzymes in the precursor white rot fungi were lost as the brown rot fungi evolved. The brown rot fungi now dominate wood decay in the coniferous forests of the northern hemisphere, and there are chemical wood-processing efficiencies that can be learned from studying their evolutionary divergence from the white rots. Enzymes from both white and brown rot fungi are too large to penetrate the intact wood cell wall, and some form of chemical "pretreatment" is typically needed before enzymes can function efficiently in deconstructing the wood cell wall. The evolution of the bio-catalytic CMF mechanism by brown rot fungi is generally considered a key step that allowed these fungi to more efficiently deconstruct wood without the production of nutritionally costly enzymes. The CMF mechanism has considerable promise for use in the bioprocessing of lignocellulosic materials [39]. Enzymatic systems in industrial processes are widely used for a variety of applications to reduce energy inputs required for catalysis but enzymes can be fragile and often require highly specific reaction conditions including specific temperatures and buffering systems. Mimicking bio-catalytic processes without the use of enzymes may be a fruitful pathway for industrial processing for bio-based coatings.

#### *7.6. Soft Rot Fungi*

Although soft rot damage of wood was first observed in the 1860s, soft rot fungi were not classified as a decay type until the 1950s [40]. Most soft rot fungi are Ascomycota species. There are two types of soft rot attack. Type 1 soft rot involves formation of diamond-shaped cavities aligned with the cellulose microfibril angle within the S-2 cell wall layer, while Type 2 is a more generalized erosion of the S-2 cell wall layer from the lumen outward [20]. Type 2 attack is more prevalent, but some species can produce both types of damage depending on the timber as well as environmental conditions [41]. Soft rot fungi are often found in more extreme, and wetter conditions that are less suitable for traditional white and brown rot fungi [39]. Their damage tends to be confined to the external few mm of wood that is exposed to the environment, possibly because oxygen levels are too low in interior wood below ground to support more aggressive Basidiomycota fungal species. However, particularly in Scandinavian reports, soft rot has been observed to extend more deeply in some products such as utility poles. Soft rot damage presents an interesting mixture of white and brown rot characteristics in that these fungi utilize both cellulose and hemicellulose, but they can clearly degrade lignin as evidenced by the cavities and erosion they cause. Several soft rot fungi are known to produce laccase which is also involved in lignin degradation by white rot fungi [39].

Soft rot fungi tend to have very large, but localized effects on wood properties and these effects are magnified because the damage tends to be on the exterior of the timber where flexural properties for products such as utility poles become more important. In other products, such as boards that will be used for paneling, soft rot fungi can impart an appearance that some people consider as desirable for rustic interiors, and because only the surface wood is degraded, these fungi can sometimes be considered as enhancing the properties of certain wood products.

#### **8. Specific Fungal Chemistries that Impact Polymeric Coatings and How These Can Be Harnessed for Biotechnological Applications**

The characteristic ability of all three decay types to attack the three primary wood polymers highlights the effect of wood cell wall chemistry on convergence of fungal strategies for accessing these resources, but differences in processes creates potential opportunities for the using these fungi in industrial biomodification processes. The most heavily researched applications have been delignification for pulping and biodetoxification of xenobiotic pollutants, but fungi could also be used to modify various polymeric materials including those used for wood coatings.

#### *8.1. White Rot Fungi*

White rot fungi have been used in biotechnological and bio-processing applications for more than 40 years [42–44]. White rot fungi have been studied since the 1970s to free cellulose from lignin and release individual fibers in bio-based pulping systems. Wood composites have also developed by using isolated lignin from pulp liquors, or lignin residues that migrate to the surface of wood fibers during pulping. In both cases the lignin is modified to produce a "sticky" lignin radical by white rot fungal enzyme systems including peroxidase and laccase-mediator systems [45,46]. Similarly, laccases have also been used to create bioactive polymer coatings using soft plasma jet processing [47]. The ability of white rot fungi to depolymerize lignin has also been extensively explored for bioremediation of structurally similar pollutants and xenobiotics but the applications have been limited because many sites are anaerobic [48,49]

The emerging bio-economy, the development of biorefineries and the production of cellulose-derived sugars for fermentation and direct conversion to biofuels and platform chemicals have all created renewed interest in application of white rot fungi [50,51]. A number of white rot species, including *Pycnoporus cinnarbarinus*, *Phlebia subserialis*, *Dichomitus squalens* and *Ceriporiopsis subvermispora*, have been assessed for use in the bioprocessing of wood to make bio-based products and energy, but there is considerable opportunity to expand the suite of organisms to take advantage of the range of enzymatic capabilities [52]. Early researchers who explored bio-based deconstruction of wood for pulp fiber focused primarily on complete lignin removal, but subsequent studies showed that some white rot fungi had more subtle effects that resulted in reduced energy requirements in mechanical pulp production while improving other paper properties. Bio-bleaching of pulp using *Phanerochaete crassa*, *P. chrysosporium*, and *Pleurotus pulmonarius* previously has been studied for replacement of chlorine in conventional pulp bleaching processes [53,54], but bio-bleaching using these organisms has not been commercialized, illustrating the difficulty in scaling up laboratory results.

The need for oxygen and the filamentous nature of white rot fungi has largely limited most applications, but several of the CAZymes as well as the lignin degrading enzymes have been cloned into yeast and bacterial vectors for use in biorefinery applications. Many peroxidases have broad capabilities to oxidize phenolic substrates and could be used to activate phenolic-based coatings. Additional research in this area using enzyme cocktails, including the use of relatively newly discovered lytic polysaccharide monooxygenase (LPMO) enzymes, is needed to enhance platform chemical yield from biomass [55].

The desire for more sustainable processes for producing polymers and coatings using fungal peroxidases should encourage a re-evaluation of previous biomass conversion research, which was often discarded because the economics could not compete with less sustainable processes [56]. Fungal derived peroxidases could be used to provide platform chemicals for further synthesis. While lignin remains a puzzling and complex polymer that has largely defied technological advances geared to utilization, development of bio-processes that depolymerize lignin and utilize the broad array of monomeric lignin breakdown products created during industrial pulping could create an array of feedstocks similar to those derived from petroleum sources. Adapting microbial systems to utilize raw lignin, while producing platform chemicals useful for polymer development, is an important step in this process [57].
