Complementing the intrinsic repertoire of Ustilago maydis for degradation of the pectin backbone polygalacturonic acid

Microbial valorization of plant biomass is a key target in bioeconomy. A promising candidate for consolidated bioprocessing is the dimorphic fungus Ustilago maydis. It harbors hydrolytic enzymes to degrade biomass components and naturally produces valuable secondary metabolites like itaconic acid, malic acid or glycolipids. However, hydrolytic enzymes are mainly expressed in the hyphal form. This type of morphology should be prevented in industrial fermentation processes. Genetic activation of these enzymes can enable growth on cognate substrates also in the yeast form. Here, strains were engineered for growth on polygalacturonic acid as major component of pectin. Besides activation of intrinsic enzymes, supplementation with heterologous genes for potent enzymes was tested. The presence of an unconventional secretion pathway allowed exploiting fungal and bacterial enzymes. Growth of the engineered strains was evaluated by a recently developed method for online determination of residual substrates based on the respiration activity. This enabled the quantification of the overall consumed substrate as a key asset for the assessment of the enzyme degradation potential even on polymeric substrates. Co-fermentation of endo- and exo-polygalacturonase overexpression strains resulted in efficient growth on polygalacturonic acid. In the future, the approach will be extended to establish efficient degradation and valorization of pectin.