Pretreatment of lignocellulosic biomass with the white-rot fungi increases biodegradability and leads to high-quality ruminant feed. For example, white-rot fungi-treated cedar wood shows significant improve­ment for rumen digestibility (Okano et al., 2005). When high-lignin forages such as grass, oat straw and alfalfa stems were treated with various white-rot fungi, substantial improvements in digestibilities have also been obtained (Akin et al., 1995,1993; Jung et al., 1992).

White-Rot Fungus Pretreatment in Biological Pulping

White-rot fungi have also been used in biological pulp­ing (biopulping) to reduce the utilization of chemicals in the pulping industry and decrease the environmental hazard caused by the traditional pulping process (Singh et al., 2010). Biopulping process removes not only lignin and hemicellulose but also some of the wood extractives. It can also improve paper quality and significantly reduce the electrical energy and cooking time required for pulp­ing wood chips (Ali and Sreekrishnan, 2001; Hunt et al., 2004; Singh et al., 2010). When C. subvermispora was used for biopulping of agricultural residues including rice, wheat and barley straw samples, the tensile strength and burst factor of hand sheets produced from the bio­pulping process improved significantly compared to the chemical process (Yaghoubi et al., 2008). Blanchette et al. (Blanchette et al., 1992) evaluated the potential application in biopulping of 19 strains of P. chrysosporium and 9 strains of C. subvermispora. For the P. chrysosporium isolates, only a few strains preferentially removed large amounts of lignin from wood while the majority of the isolates removed all cell wall components nonselectively. In contrast, all nine isolates of C. subvermispora led to moderate weight losses and preferential degradation of lignin in aspen, birch and loblolly pine wood.

White-Rot Fungus Pretreatment of Biomass for Biofiber

Microbial pretreatment can also improve the feature of the fiber in biomass for biocomposite production. For example, corn stalk pretreated with the white-rot fungus Trametes hirsuta has been used to produce fiberboard by hot pressing without adhesive. The corn stalk-based fiberboard made of the pretreated biomass has an in­crease of 3.40- and 8.87-fold in moduli of rupture and elasticity, respectively, over the fiberboard made from untreated corn stalk. Further analyses showed that the increase in the mechanical properties of the fiberboard resulted from the pretreated biomass possessing more than twice the number of hydroxyl groups, an 18% higher crystallinity, and twice the polysaccharide content of untreated corn stalk (Wu et al., 2011).

Brown-Rot Fungi

Brown-rot fungi are Basidiomycete fungi that, unlike white-rot fungi, selectively modify and then completely hydrolyze lignocellulose polysaccharides, typically without secreting an exoacting glucanase and without removing lignin (Schilling et al., 2009; Tewalt and Schil­ling, 2010). The wood decay resulting from the action of brown-rot fungi leads to an increased volume of pores in the wood cell wall and decreased degree of polymeriza­tion of holocellulose along with a dramatic weight loss (Flournoy et al., 1991). Depolymerization of holocellu — lose occurs rapidly during the early decay process lead­ing to an extensive degradation of holocellulose in wood (Blanchette, 1995; Irbe et al., 2011; Kumar et al., 2009) and as high as 75% wood strength loss even when only 1% weight loss has occurred (Green and Highley, 1997; Richards, 1954; Wilcox, 1978).

The exact mechanism for brown-rot decay is still un­clear. For the selective removal of polysaccharides, a two-step procedure has been proposed: a nonenzymatic radical-based modification of the wood cell wall through small molecules, followed by secretion of enzymes to catalyze the breakdown of polysaccharides into their sugar monomers (Green and Highley, 1997; Tewalt and Schilling, 2010). However, cellulose and hemicellulose removal by brown-rot fungi does not open up cell walls to facilitate enzyme penetration (Flournoy et al., 1991). Primarily because enzymes are too large to penetrate the decayed wood, attack by cellulolytic enzymes may only be limited to a localized, superficial area (Baldrian and Valaskova, 2008; Flournoy et al., 1991). It has been proposed that Fenton’s reagents and not enzymes are responsible for rapid wood decomposition early in brown-rot decay (Green and Highley, 1997; Jensen et al., 2001; Ray et al., 2010). Other study results also support that hydroxyl radicals (HO’) generated through Fenton chemistry (H2O2—Fe(II)) initiate lignocellulose breakdown (Arantes et al., 2012; Contreras et al., 2007; Hammel et al., 2002; Kaneko et al., 2005; Kramer et al., 2004; Suzuki et al., 2006). Consequently, this suggests that reactive oxygen species play an important role in the early stages of wood degradation by brown-rot fungi (Irbe et al., 2011). In brown-rot wood decay, hemi — cellulose is removed considerably faster than cellulose (Curling et al., 2002; Highley, 1987; Monrroy et al., 2011). Consistently, the total secretome hemicellulase expression and activity for brown-rot fungi peak prior to cellulase activity (Lyr, 1960; Martinez et al., 2009).

Hemicellulose is embedded in cellulose microfibrils and its prior removal may facilitate cellulose degrada­tion and removal (Green and Highley, 1997). Continual degradation of holocellulose by brown-rot fungi leads to gradually increased weight loss but the percent crys­tallinity in decayed wood increases apparently at an early stage, peaks between 2 and 4 weeks and then de­creases implying structural changes of cellulose chains during fungal attack (Howell et al., 2009). Towards the end of brown-rot decay, nearly 100% of carbohydrates can be removed; however, most of the lignin remains (Eriksson et al., 1990). Only a small fraction of the lignin is oxidized, demethylated and depolymerized, often leading to lignin-derived volatile components (Ewen et al., 2004; Irbe et al., 2011; Schilling et al., 2012).

Recently, the potential application of brown-rot fungi for the pretreatment of biomass to increase downstream enzymatic hydrolysis has been explored. When spruce and pine woods were treated with one of two brown — rot fungi, Gloeophyllum trabeum or Fomitopsis pinicola, saccharification efficiency was increased significantly even though total sugar yield was low, probably due to low enzyme loading (Schilling et al., 2009). In another effort, G. trabeum-treated pine wood block only led to a maximum 22% glucose release even though 60 FPU Cel — luclast was loaded, suggesting brown-rot fungus G. tra — beum modification of pine wood may not be sufficient to increase cellulose accessibility (Tewalt and Schilling,

2010) . Similarly, when the brown-rot fungi G. trabeum and Laetoporeus sulphureus were used for the pretreat­ment of the wood Pinus radiate and Eucalyptus globules, the highest glucose yield was 14% after 8 weeks of biodegradation (Monrroy et al., 2011). On the other hand, when G. trabeum was used to pretreat different biomass including aspen, spruce, or corn stover, sugar yield was significantly increased up to threefold. In the best case, a 2-week pretreatment of aspen by G. trabeum led to a 72% cellulose-to-glucose yield corresponding to 51% yield relative to original glucan. For corn stover, a weak colonization with minor degradation by another tested brown-rot fungus, Postia placenta, resulted in more than a twofold increase in sugar yield (Schilling et al., 2012). Similar to wood biomass, when corn stover is pretreated with the brown-rot fungus Fomitopsis sp. IMER2, the amorphous regions of the cellulose are pref­erentially degraded in contrast to the significant lignin degradation by the white-rot fungus I. lacteus CD2 (Zeng et al., 2011). In another successful case, simple pre­treatment of Scots pine (Pinus sylvestris) with the brown rot fungus Coniophora puteana for 15 days permitted re­covery of greater than 70% of the glucose present in the biomass, with a total wood mass loss of 9%, suggest­ing great potential for use of this specific group of fungi in lignocellulosic biomass pretreatment (Ray et al., 2010). Brown-rot fungi therefore hold significant potential for practical application in biological pretreatment.