Fungal species play a critical role in plant biomass decay and can be divided into the fol­lowing three categories: (a) saprophytic fungi, which prefer dead and decaying material; (b) parasitic fungi, which prefer colonizing the biomass of the living host; and (c) mycorrhizal fungi, which form partnerships with specific plant species, mostly with living trees. Most fungi are saprophytic and are found in nature growing on dead organic matter. They are effective at secreting enzymes that degrade large polymers such as cellulose, hemicellulose, lignin, pectin, starch, and protein found in the organic matter to release nutrients that can be taken up and used by the fungi. The filamentous fungi are widespread in nature and for most part are obligate aerobes. Typically, fungi colonize substrates with tubular, branching hyphae that collectively form the mycelium. The hyphae cell walls composed of p-glucans and chitin embedded in a matrix that includes p-glucans and glycoproteins. As shown in Figure 15.1, growth is by extension of the hyphal tips that enables the mycelium to spread and penetrate throughout the substrate allowing new areas to be colonized as the nutrients are used from the earlier colonized substrate. Extensive communication and nutrient trans­fer is possible through the hyphae and between different regions of the mycelium. Older parts of the mycelium may die after nutrients in the substrate are exhausted and material from these areas can be transported to younger regions of the mycelium. Many ascomycetes and basidiomycetes produce a group of small (70-120 amino acids) amphiphilic proteins that promote attachment of hyphae to hydrophobic surfaces (9, 10). Hydrophobins are

self-assembling proteins that have been demonstrated to function at hydrophilic — hydrophobic interfaces to form films and to function as surfactants (10). Hydrophobins have been proven to lower the surface energy of water allowing the fungal hyphae to pene­trate the air-water interface and grow up into the air (11,12). Because of their adhesive and surface activity, hydrophobins are an interesting feature of biomass colonization.

Producing plant cell wall-degrading enzymes is widespread in fungi and has been de­scribed in the anaerobic fungi found in ruminants (13) and all other subdivisions of aerobic fungi including members of the Zygomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes. A major ecological activity of many fungi, especially members of the Ba- sidiomycetes, is the decomposition oflignocellulosic materials such as wood and other plant material. Fungal breakdown of biomass occurs by succession where the species of one fun­gal community alters the substrate enough to allow other species to become established and colonize (14). Two types of wood rot are known: brown rot, in which the cellulose is preferentially used and the lignin is not metabolized and white rot where both cellulose and lignin is used. White rot, brown rot, and soft rot fungi are recognized among those that colonize dead wood. Soft rot fungi degrade cellulose and hemicellulose in the wood under conditions of high moisture content leaving the wood soft or spongy without being fully degraded. Soft rot has also been reported to occur in dry environments and under other extreme conditions (15). Soft rot decay of wood reportedly occurs at historic sites in Antarctica where extreme environmental conditions inhibit wood decay from other fungi

(15) . Soft rot is caused primarily by fungi classified in the phylum Ascomycota with the decay characterized by cavities that form within the biomass structure. Soft rot fungi can also cause a progressive degradation of secondary cell walls that can be completely degraded except for the middle lamella between the cells (15). The white rot fungi exhibit a large amount of diversity, but are generally members of the Basidiomycetes or other higher fungi, and produce enzymes that are degraded lignin in addition to cellulases and hemicellulases

(16) . The hyphae of white rot fungi rapidly colonize wood by growing within the lumen of the cells and degrading the cell walls. Although they can degrade the substrate, white areas of the wood remain where the lignin and hemicellulose have been removed ahead of the cellulose. Under conditions where the biomass is moist members of the Basidiomycota will rapidly degrade cellulose and lignin by generating oxidants such as hydroxyl radicals.

Parasitic fungi are the second largest group, of whose members do considerable dam­age to growing plants. Indeed, root diseases and mycorrhizal systems have a similarity regarding parasitism. Root pathogens, such as Rhizoctonia, Fusarium, Verticillium, Sclero — tinia, and Pythium are stimulated by roots. These fungi progressively invade and colonize the meristematic root tissues and ultimately cause necrosis.

The rhizosphere is the region immediately outside the root that generally has more mi­crobial activity than the surrounding soil. Mycorrhiza refers to the symbiotic association between plant roots and fungi classified as ectomycorrhizae where the fungi form an extensive sheath around the root, and arbuscular mycorrhizae where the fungal mycelium becomes embedded with root tissue. Mycorrhizal associations are widespread and are found on most plants in diverse environments. Although the production of hydrolytic enzymes has been described in many mycorrhizal fungi, most do not use cellulose for metabolism but obtain carbon from root secretions (17). Mycorrhizal fungi have been demonstrated to have weak cellulase and endopolygalacturonase activities (17). Hohnjec etal. described the presence of three different endoglucanases with different preferences for sugar bonds and four different pectinolytic or polygalacturonate-degrading enzymes in G. mosseae and G. intraradices — colonized Medicago roots (18). Cellulase, pectinase, and xyloglucanase activities have been found in the external mycelium of arbuscular mycorrhizas (19). The production of these hydrolytic enzymes is thought to allow one for the modification of the extracellular matrix of the root and allow fungal colonization (18).