Fungal Strategies for Lignin Degradation

Document Type : Book Chapter; A number of microorganisms, bacteria and filamentous fungi, are able to degrade lignocellulosic components to various extents. However, only a few ones can degrade lignins, among which wood-rotting fungi. White-rot fungi, the most frequent ones, mineralize cell wall components (cellulose, hemicelluloses and lignins) and extensively degrade lignins, which results in a bleached aspect of decayed wood. Fungal lignin degradation involves secreted heme-peroxidases and laccases that use oxidants as electron acceptors (H2O2 and O-2). The three main heme-peroxidases are lignin peroxidases, manganese peroxidases and versatile peroxidases. Lignin and versatile peroxidases are able to oxidize non-phenolic lignin units. By contrast, manganese peroxidase needs diffusible Mn-chelated ions and mainly degrades lignin units with free phenolic groups. Other peroxidases have been recently found, such as dye peroxidases with potential applications in bioremediation and other industrial processes. Peroxidases need the cooperation of other oxidases such as glyoxal or aryl-alcohol oxidases to produce hydrogen peroxide. The recent availability of complete fungal genomes opens innovative opportunities to annotate lignocellulolytic gene families. Phylogenetic approaches are unique tools to comparatively analyse these data and predict or infer gene function. The increasing number of putative lignin-degrading enzymes emerging from genome analyses requires the development of high-throughput expression systems to meet the demands of industries. Several expression systems have been developed to produce more efficient recombinant 'ligninases' and auxiliary enzymes. Fungi and yeasts are useful hosts for the production of recombinant enzymes, but only a few ones are devoted to ligninase production. In this review, we give a survey of the main fungal and enzymatic actors of lignin degradation before discussing the phylogenetic inference strategy used to predict lignocellulolytic enzymes and reviewing the recombinant production of these enzymes in fungal hosts.