1-02: Insights into lignin in the reduced recalcitrance of transgenic switchgrass

Reducing the natural recalcitrance of lignocellulosic biomass for saccharification is required for a viable biomass to biofuel process and remains a challenge. The natural recalcitrance of biomass is believed to be largely due to the complex physical/chemical structure of plant cell walls, particularly the association between lignin and cellulose and hemicellulose. Genetic engineering of switchgrass through targeted modification of key genes in the lignin biosynthesis pathway has resulted in reduced recalcitrance and improved release of fermentable sugars during enzymatic hydrolysis. In this study, lignin samples were isolated from wild-type and two transgenic switchgrass lines which were obtained through caffeic acid 3-O-methyltransferase (COMT) down-regulation and PvMYB4 overexpressing, respectively. The detailed chemical structures of lignins including molecular weights, monolignol ratios and relative interunit linkages abundance were characterized using gel permeation chromatography and one/two-dimensional nuclear magnetic resonance (NMR) techniques (¹³C, ¹³P, HSQC and HMBC NMR). The structural characteristics of isolated lignins were compared between wild-type and the two transgenic lines (i.e., COMT down-regulated and PvMYB4 overexpressing). The results of lignin structural changes and their relationship/contribution to the reduced recalcitrance of transgenic switchgrass are discussed with a perspective to future genetic improvement of biomass for enhanced enzymatic hydrolysis.