Modification of the nanostructure of lignocellulose cell walls via a non-enzymatic lignocellulose deconstruction system in brown rot wood-decay fungi

Abstract

Wood decayed by brown rot fungi and wood treated with the chelator-mediated Fenton (CMF) reaction, either alone or together with a cellulose enzyme cocktail, was analyzed by small angle neutron scattering (SANS), sum frequency generation (SFG) spectroscopy, Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Results showed that the CMF mechanism mimicked brown rot fungal attack for both holocellulose and lignin components of the wood. Crystalline cellulose and lignin were both depolymerized by the CMF reaction. Porosity of the softwood cell wall did not increase during CMF treatment, enzymes secreted by the fungi did not penetrate the decayed wood. The enzymes in the cellulose cocktail also did not appear to alter the effects of the CMF-treated wood relative to enhancing cell wall deconstruction. This suggests a rethinking of current brown rot decay models and supports a model where monomeric sugars and oligosaccharides diffuse from the softwood cell walls during non-enzymatic action. In this regard, the CMF mechanism should not be thought of as a “pretreatment” used to permit enzymatic penetration into softwood cell walls, but instead it enhances polysaccharide components diffusing to fungal enzymes located in wood cell lumen environments during decay. SANS and other data are consistent with a model for repolymerization and aggregation of at least some portion of the lignin within the cell wall, and this is supported by AFM and TEM data. The data suggest that new approaches for conversion of wood substrates to platform chemicals in biorefineries could be achieved using the CMF mechanism with >75% solubilization of lignocellulose, but that a more selective suite of enzymes and other downstream treatments may be required to work when using CMF deconstruction technology. Strategies to enhance polysaccharide release from lignocellulose substrates for enhanced enzymatic action and fermentation of the released fraction would also aid in the efficient recovery of the more uniform modified lignin fraction that the CMF reaction generates to enhance biorefinery profitability.

The Importance of Moisture for Brown Rot Degradation of Modified Wood: A Critical Discussion

“…This chelator-mediated Fenton (CMF) degradation is followed by enzymatic degradation, in which cellulases and hemicellulases hydrolyse the polysaccharides [14][15][16]. Water in the wood cell wall is crucial for the diffusion of the CMF metabolites [14,17] and fungi are sensitive to water stress. Low water content is often the limiting factor restricting the establishment of decay communities and lowering rates of decay [18].…”

Resistance of thermally modified and pressurized hot water extracted Scots pine sapwood against decay by the brown-rot fungus Rhodonia placenta

“…R. placenta causes brown-rot decay that degrades the cell wall carbohydrates and results in the accumulation of (oxidized) lignin in the wood residue (Winandy and Morrell 1993;Kirk and Highley 1973). The decay process of brown-rot fungi is best known from studies on Gloeophyllum trabeum (Goodell et al 1997;Arantes et al 2011), but certain aspects of this process have also been verified for other brown-rot fungi, including R. placenta (Hyde and Wood 1997;Cohen et al 2002;Arantes et al 2011;Zhang et al 2016;Goodell et al 2017). Brownrot fungi have developed a non-enzymatic decay system that is based on low molecular weight oxidants and chelators to overcome the limitation of enzymes to penetrate the sound wood cell wall (Zhang et al 2016;Goodell et al 2017).…”

Biochemical characterization ofSerpula lacrymansiron-reductase enzymes in lignocellulose breakdown

“…Brown rot fungi, derived from a white rot ancestry, have reduced enzymatic capacity to mineralize lignin and typically target the depolymerisation of hemicellulose and cellulose [6][7][8]. Studies into the mechanism used by different species of brown rot fungi and the subsequent effect on woody substrates have increased our understanding of the decay process [9][10][11]. Depolymerisation of lignocellulose is thought to be largely driven by a non-enzymatic chelatormediated Fenton (CMF) system [10] that in the presence of Fe 2+ generates reactive hydroxyl radicals [12][13][14].…”

“…However, transcriptomic and proteomic analysis failed to detect the expression of this gene in S. lacrymans wood cultures [7,20]. While the role of the S. lacrymans GH6 remains uncertain, a role for a chelator-mediated Fenton system in the depolymerisation of cellulose has been proposed for brown rot species [11]. Cellulose-targeted iron reduction, combined with substrate induction of iron-reducing phenolate biosynthesis, might explain the particular ability of brown rot fungi in the Boletales, such as the dry rot fungus S. lacrymans, to degrade crystalline cellulose without the presence of lignin [7].…”