Plant biomass-acting enzymes produced by the ascomycete fungi Penicillium subrubescens and Aspergillus niger and their potential in biotechnological applications

Abstract

Plant biomass contains complex polysaccharides that can be divided into structural and storage polysaccharides. Plant biomass is a major carbon source utilized by chemoheterotrophic microorganisms, such as filamentous fungi to grow and produce energy. Plant biomass-degrading fungi secrete a wide range of extracellular enzymes to convert complex plant biomass into metabolizable compounds. As a result, they are able to colonize and degrade a wide range of organic materials. The industrially and biotechnologically important fungal genera Penicillium and Aspergillus are among the best-studied decomposers of plant biomass polysaccharides. The focus of this study was on two ascomycete fungi, Penicillium subrubescens (FBCC 1632) and Aspergillus niger N402 (ATCC 64947), and their potential for producing extracellular enzymes for the decomposition of plant biomass polysaccharides. Here, a new species, native Finnish isolate, P. subrubescens sp. nov. was described. It belongs to the section Lanata-Divaricata subgenus Aspergilloides. P. subrubescens was selected amongst 200 fungal strains screened for their ability to produce polyfructan inulin-degrading enzymes. Inulin is a storage polysaccharide located in the roots and tubers of flowering plants. P. subrubescens was identified based on the combined analysis of phenotype together with molecular phylogenetic analysis and extrolite data. P. subrubescens produced inulinase more efficiently than it did invertase. Only fructose, not fructo-oligosaccharides was detected as the endproduct of inulin hydrolysis indicating exo-type inulinase activity. The ability of P. subrubescens and A. niger to produce plant structural polysaccharide-degrading enzymes was studied by growth profiling and in plant biomass-containing liquid cultures. These data indicated similar (hemi-)cellulolytic capacities for these fungi. The extracellular enzyme mixtures of P. subrubescens and A. niger were used in the hydrolysis of wheat bran, sugar beet pulp and a mixture of these. Its favourable ability to hydrolyse complex plant-derived biomasses indicated that P. subrubescens has the potential to produce biotechnologically important enzyme mixtures. A new feruloyl esterase, FaeC of A. niger was cloned and heterologously produced from the plant biomass-acting enzymes. The biochemical properties of recombinant FaeC (rFaeC) were characterized, and the hydrolysis of wheat arabinoxylan and sugar beet pectin by rFaeC released both ferulic and p-coumaric acid. The synergistic activity of rFaeC and xylanase was detected in hydrolysis of plant biomass-derived substrates. The induction of faeA, faeB and faeC gene expression in the presence of various phenolic compounds and complex polysaccharides was examined. The differing expression levels of the three fae genes suggests that the corresponding enzymes can act cooperatively, leading to improvement in the efficiency of plant biomass decomposition.

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