Cellulose is the main polymeric component of the plant cell wall, the most abundant polysaccharide on Earth, and an important renewable resource. Basidiomycetous fungi belong to its most potent degraders because many species grow on dead wood or litter, in environment rich in cellulose. Fungal cellulolytic systems differ from the complex cellulolytic systems of bacteria. For the degradation of cellulose, basidiomycetes utilize a set of hydrolytic enzymes typically composed of endoglucanase, cellobiohydrolase and β-glucosidase. In some species, the absence of cellobiohydrolase is substituted by the production of processive endoglucanases combining the properties of both of these enzymes. In addition, systems producing hydroxyl radicals based on cellobiose dehydrogenase, quinone redox cycling or glycopeptide-based Fenton reaction are involved in the degradation of several plant cell wall components, including cellulose. The complete cellulolytic complex used by a single fungal species is typically composed of more than one of the above mechanisms that contribute to the utilization of cellulose as a source of carbon or energy or degrade it to ensure fast substrate colonization. The efficiency and regulation of cellulose degradation differs among wood-rotting, litter-decomposing, mycorrhizal or plant pathogenic fungi and yeasts due to the different roles of cellulose degradation in the physiology and ecology of the individual groups.

Degradation of cellulose by basidiomycetous fungi.

Microorganisms are efficient degraders of starch, chitin, and the polysaccharides in plant cell walls. Attempts to purify hydrolases led to the realization that a microorganism may produce a multiplicity of enzymes, referred to as a system, for the efficient utilization of a polysaccharide. In order to fully characterize a particular enzyme, it must be obtained free of the other components of a system. Quite often, this proves to be very difficult because of the complexity of a system. This realization led to the cloning of the genes encoding them as an approach to eliminating other components. More than 400 such genes have been cloned and sequenced, and the enzymes they encode have been grouped into more than 50 families of related amino acid sequences. The enzyme systems revealed in this manner are complex on two quite different levels. First, many of the individual enzymes are complex, as they are modular proteins comprising one or more catalytic domains linked to ancillary domains that often include one or more substrate-binding domains. Second, the systems are complex, comprising from a few to 20 or more enzymes, all of which hydrolyze a particular substrate. Systems for the hydrolysis of plant cell walls usually contain more components than systems for the hydrolysis of starch and chitin because the cell walls contain several polysaccharides. In general, the systems produced by different microorganisms for the hydrolysis of a particular polysaccharide comprise similar enzymes from the same families.

Microbial hydrolysis of polysaccharides.

SUMMARY Basidiomycete fungi subsist on various types of plant material in diverse environments, from living and dead trees and forest litter to crops and grasses and to decaying plant matter in soils. Due to the variation in their natural carbon sources, basidiomycetes have highly varied plant-polysaccharide-degrading capabilities. This topic is not as well studied for basidiomycetes as for ascomycete fungi, which are the main sources of knowledge on fungal plant polysaccharide degradation. Research on plant-biomass-decaying fungi has focused on isolating enzymes for current and future applications, such as for the production of fuels, the food industry, and waste treatment. More recently, genomic studies of basidiomycete fungi have provided a profound view of the plant-biomass-degrading potential of wood-rotting, litter-decomposing, plant-pathogenic, and ectomycorrhizal (ECM) basidiomycetes. This review summarizes the current knowledge on plant polysaccharide depolymerization by basidiomycete species from diverse habitats. In addition, these data are compared to those for the most broadly studied ascomycete genus, Aspergillus, to provide insight into specific features of basidiomycetes with respect to plant polysaccharide degradation.

Plant-Polysaccharide-Degrading Enzymes from Basidiomycetes

Cellulose degradation by brown rot fungi, such as Postia placenta, is poorly understood relative to the phylogenetically related white rot basidiomycete, Phanerochaete chrysosporium To elucidate the number, structure, and regulation of genes involved in lignocellulosic cell wall attack, secretome and transcriptome analyses were performed on both wood decay fungi cultured for 5 days in media containing ball-milled aspen or glucose as the sole carbon source Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), a total of 67 and 79 proteins were identified in the extracellular fluids of P placenta and P chrysosporium cultures, respectively Viewed together with transcript profiles, P chrysosporium employs an array of extracellular glycosyl hydrolases to simultaneously attack cellulose and hemicelluloses In contrast, under these same conditions, P placenta secretes an array of hemicellulases but few potential cellulases The two species display distinct expression patterns for oxidoreductase-encoding genes In P placenta, these patterns are consistent with an extracellular Fenton system and include the upregulation of genes involved in iron acquisition, in the synthesis of low-molecular-weight quinones, and possibly in redox cycling reactions

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Comparative Transcriptome and Secretome Analysis of Wood Decay Fungi Postia placenta and Phanerochaete chrysosporium

Many fungi degrade cellulose and hemicelluloses using extracellular hydrolytic enzymes, but fungi that degrade woody biomass are the only ones to efficiently degrade polysaccharides encased in lignin. White-rot basidiomycetes begin by mineralizing the lignin, using extracellular oxidative enzymes to cleave this recalcitrant biopolymer. Enzymes with likely roles include lignin peroxidases, manganese peroxidases, versatile peroxidases and laccases. In some cases the enzyme may attack the lignin polymer directly; in others the ligninolytic agent is likely a small molecule that one of the enzymes has oxidized to a reactive form. So far, all white rot fungi appear to secrete manganese peroxidases, and most produce laccases, whereas the other two enzymes are less common. After ligninolysis, white-rot fungi assimilate the remaining polysaccharides using conventional glycosylhydrolase systems that contain both endo- and exo-acting enzymes. Brown rot basidiomycetes also degrade lignocellulose efficiently, but their biodegradative systems are less comprehensive. These fungi generally lack ligninolytic enzymes, initiating decay instead with reactive oxygen species generated from the reaction between Fe2+ and H2O2. The limited disruption caused by these oxidants apparently allows a limited set of endo-acting glycosylhydrolases to depolymerize the remaining polysaccharides. Biological wood pulping is one promising application of ligninolytic fungi.

https://www.fpl.fs.fed.us/documnts/pdf2010/fpl_2010_hatakka001.pdf

Bo Ek

Papers

The 1,4-β-D-glucan glucanohydrolases from Phanerochaete chrysosporium. Re-assessment of their significance in cellulose degradation mechanisms