Regulation and diversity of plant polysaccharide utilisation in fungi

Abstract

Filamentous fungi obtain their nutrients by degrading dead or living plant material. Plant material consists of different cell wall and storage polysaccharides. Due to the complex structure and the variety of plant polysaccharides, filamentous fungi secrete a wide range of plant polysaccharide degrading enzymes into the environment to degrade these polysaccharides into monosaccharides that can be taken up into the fungal cell and be used as carbon source. The aim of this Thesis was to study how fungi degrade plant polysaccharides and how this process is regulated. This study focused in particular on pentose-related regulators in different fungi and the enzymatic spectrum fungi produce to degrade polysaccharides. Firstly, a novel transcriptional regulator involved in L-arabinose release and catabolism was identified in the industrially important fungus Aspergillus niger. Functional analysis of AraR and XlnR confirmed the previously suggested antagonistic relation between the two regulators involved in D-xylose and L-arabinose catabolism. The presence of the AraR/XlnR-regulator system was shown to be restricted to A. niger and other species belonging to the order of Eurotiales. Secondly, comparative analysis of the function of the AraR/XlnR-regulator system in A. niger and the model fungus Aspergillus nidulans revealed that homologous regulatory systems can differ significantly in related fungi. The regulatory role of AraR in pentose catabolism was different between these two Aspergilli, while this is not the case for XlnR. Thirdly, a comparison of the function of the XlnR ortholog (Xlr1) from the plant pathogenic fungus M. oryzae to A. niger XlnR showed that Xlr1 regulates D-xylose catabolism, but had no major role in L-arabinose catabolism. Since no ortholog for AraR is present in the M. oryzae, it suggests that another regulator must be involved in the regulation of L-arabinose release and catabolism. In addition, it was shown that Xlr1 has a role in plant pathogenicity. Finally, this Thesis describes the first analysis of CAZy (Carbohydrate Active enzymes) families of a zygomycete fungus (Rhizopus orzyae). Comparative analysis of the CAZy repertoires and growth preferences of R. orzyae and a selection of ascomycete and basidiomycete species demonstrated highly specialized plant and fungal cell wall degrading abilities for R. oryzae, different from ascomycetes and basidiomycetes. Knowledge of the regulatory mechanisms involved in polysaccharide degradation and the CAZyme repertoire in the genome contributes not only to the understanding of the role of filamentous fungi in nature, but is also highly relevant to industry. The data will aid in defining new strategies to engineer fungi to improve the production of commercial enzyme mixtures.