Puzzling out the role of plant and fungal laccases with auxiliary enzymes during polymerization and depolymerization of lignin in a polysaccharide environment – DEOX

Lignin is a complex phenolic polymer embedded into the polysaccharide matrix of the plant cell wall. The lignin deposition makes the plant biomass highly recalcitrant to human utilization. Interestingly, the polymerization of lignin in the plant cell wall and the depolymerization of lignin by microorganisms are both achieved by oxidases named laccases. Plant laccases oxidize small phenols synthesized in the cytosol prior to incorporation into the polymer, while fungal laccases are capable of breaking down the polymer into smaller oligomers using natural mediators. In both cases, laccase-mediated oxidation generates oligolignols phenoxy radicals that are naturally reactive towards polymerization. However in nature, when lignin is oxidized by fungi, re-polymerization from small oligomers is not predominant. This is mainly due to the production of fungal dehydrogenases which are able to reduce the phenoxy radicals. These enzymes named auxiliary enzymes have a strong affinities for oligosaccharides that are used as electron acceptors. The DEOX project will compare the activities of plant and fungal laccases and laccase/dehydrogenase couples to unveil the mechanism of lignin biosynthesis and fungal lignin depolymerization in an environment enriched in oligosaccharides. These studies will be carried out in different reaction matrices of poly/oligo-saccharides-lignols and of hydroxycinnamic acid-grafted plant cell walls. During this project, an innovative plant material will be produced: we will express fungal laccases and dehydrogenases in the cell wall of genetically modified plants, aiming at modifying the lignin structure and content in planta. Ultimately, to understand the differences between fungal and plant laccases, high yield production of plant laccases in heterologous expression systems will allow detailed functional characterization, and paves the way to structural studies by protein crystallography.
DEOX is based on a novel consortium of four partners sharing common interests and complementary skills.