From: Treseder KK, Lennon JT. 2015. Fungal traits that drive ecosystem dynamics on land. Microbiology and Molecular Biology Reviews 79:243-262.
Cellulose is a major component of plant cell walls, and, accordingly, the most abundant biopolymer on land (Klemm et al. 2005). It is essentially a chain of glucose units that can be used by fungi for energy. A portion of this consumed glucose is used for anabolic processes (biomass formation) while the remainder is used for catabolic processes (energy generation), which releases CO2 to the environment. First, though, fungi use extracellular cellulases to degrade cellulose into smaller compounds, such as cellobiose or glucose, which they can then take up across cell walls and metabolize (Lynd et al. 2002, Edwards et al. 2008). Cellulases vary in their kinetics and mechanisms of catalysis. For example, endoglucanases are one type of cellulase that breaks cellulose into oligosaccharides that vary in length. Another type, cellobiohydrolases, release cellobiose or glucose from cellulose. Moreover, β-glucosidases hydrolyze cellobiose to glucose. In addition, the more recently described lytic polysaccharide monooxygenase (i.e., auxiliary redox enzyme AA9, ref Levasseur et al. 2013) can degrade relatively recalcitrant forms of cellulose, such as cellulose that is highly crystalline (Langston et al. 2011) or cross-linked with lignin or other cell wall constituents (Harris et al. 2010).
Many—but not all—fungi possess some capacity to break down cellulose (e.g., Floudas et al. 2012, Sasikala and Gopal 2014). Cellulose degraders are well-represented among the Ascomycota and Basidiomycota (Edwards et al. 2008), and capacity to break down cellulose is especially strong in the class Agaricomycetes (Riley et al. 2014). In contrast, cellulose degraders are less common in the other phyla, with the exception of certain species of the genus Mucor in the Mucoromycotina (Lynd et al. 2002), and gut symbionts in the Neocallimastigomycota (Lee et al. 1997).
Fungal genes related to cellulose breakdown
The genetic capacity for cellobiohydrolase (GH7) and crystalline cellulase (AA9) production varies most at the subphylum level:
Phylogenetic distribution of cellulase production:
Free-living filamentous fungi have the greatest capacity for cellobiohydrolase (GH7) and crystalline cellulase (AA9) production:
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