Plant-polysaccharide-degrading enzymes from Basidiomycetes

Abstract

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.

FIG 1

Simplified model of plant cell wall structure. (A) The structure consists of three main layers: the middle lamella and the primary and secondary walls. (A and B) The main polysaccharides and lignin which form the surrounding structure for the plasma membrane are presented in the primary (B) and secondary wall (C). The lignin content in the primary cell wall (not illustrated) varies considerably depending on the plant species (Table 1). The illustrations are not to scale.

FIG 2

Schematic representation of plant cell wall polysaccharides and selected corresponding polysaccharide-degrading enzymes. (A) Cellulose; (B) xyloglucan; (C) β-glucan; (D) heteroxylan; (E) heteromannan; (F) pectin. Enzyme abbreviations are presented in Table 2. Polysaccharide structures were drawn by using data reported previously by Mohnen (203) and Doblin et al. (204).

FIG 3

Average molecular masses (kDa for monomers) (A), isoelectric points (pI) (B), pH optima (C), and temperature optima (D) of selected CAZymes from basidiomycetous (first columns, in dark colors) and Aspergillus species (second columns, in light colors). EG, endoglucanase; CBH, cellobiohydrolase; BGL, β-glucosidase; XLN, endoxylanase; MAN, endomannanase; MND, β-mannosidase; AGL, α-galactosidase; AGU, α-glucuronidase; CDH, cellobiose dehydrogenase. The number of characterized enzymes used for calculation of mean values is marked at the root of each column. Error bars show the minimum and maximum values reported for each biochemical characteristic. −, no mean value was available.