Lignin-degrading enzymes produced by Pleurotus species during solid state fermentation of wheat straw

Abstract

Three enzymes, i.e. manganese peroxidase (MnP), aryl-alcohol oxidase (AAO) and laccase, presumptively involved in lignin degradation, were detected and quantified during the solid state fermentation (SSF) of wheat straw with five Pleurotus species. The highest levels were produced by P. pulmonarius. In the course of the study, it was realized that several aspects of the physiology of ligninolysis during SSF differed from those reported in liquid culture. In particular, MnP activity appeared during a more extended period of time, and it was not dependent on the presence of peptone in the medium or inhibited by Mn2+, as in liquid cultures of Pleurotus species. It is suggested that these differences are the consequence of the artificial growth conditions prevailing in liquid cultures of basidiomycetes. In particular, the separation between growth phase and secondary metabolism can be observed only in liquid culture. Under SSF conditions, a third type of metabolism probably occurs, characterized by close relationships between hyphal growth and secondary metabolism events, including production and release of ligninolytic enzymes for enabling substrate colonization. Finally, the use of 14C-lignin straw provide indirect evidence for MnP involvement in lignin degradation under SSF conditions (probably via Mn3+ formation), since the addition of Mn2+, which did not increase MnP levels, strongly stimulated lignin mineralization by P. pulmonarius.