Combination of Pretreatment with White Rot Fungi and Modification of Primary and Secondary Cell Walls Improves Saccharification

Abstract

Plant cell walls have protective and structural functions conferring resistance to degradation. The lignin and hemicellulose network surrounding the cellulose microfibrils is insoluble unless subjected to harsh treatments. As lignin, pectin and xylan are effective barriers to cellulose extraction and hydrolysis, reducing their presence in cell walls improves saccharification. Microorganisms that can depolymerise lignin are of extreme interest to the biofuel industry. White rot fungi can be effective in pretreatment of lignocellulosic biomass prior to saccharification. Here, we show the cumulative effects of pretreating biomass with two white rot fungi, Phanerochaete chrysosporium and Trametes cingulata, on tobacco lines with reduced lignin or xylan, caused by suppression of the CINNAMOYL-CoA REDUCTASE, CINNAMATE-4-HYDROXYLASE, TOBACCO PEROXIDASE 60 or UDP-GLUCURONATE DECARBOXYLASE and on Arabidopsis thaliana with reduced de-esterified homogalacturonan content, obtained by overexpressing a pectin methyl esterase inhibitor or constitutively expressing the Aspergillus nigerPOLYGALACTURONASE II gene. Tests were extended to fresh material from an Arabidopsis mutant for a cell wall peroxidase. We demonstrate that fungal pretreatment is a reliable method of improving cellulose accessibility in biofuel feedstocks, fresh material and cell wall residues from different plants. These results contribute to the understanding of the consequences of primary and secondary cell wall perturbations on lignocellulosic biomass accessibility to white rot fungi and on saccharification yield. A comparison of the effects of P. chrysosporium and T. cingulata on tobacco saccharification also highlights the limitation of current knowledge in this research field and the necessity to systematically test culture conditions to avoid generalisations.