Molecular Targets for the Genetic Modification of Feedstock Quality

Lignocellulosic biomass is recalcitrant to saccharification (sugar release from cell walls), and this is, at least in part, due to the presence of the phenylpropanoid-derived cell wall polymer lignin. Analysis of alfalfa (Medicago sativa) plants in which lignin content and composition had been modified through independently down-regulating each of eight enzymes in the lignin pathway revealed that reduction in lignin content progressively increases sugar release from dried biomass by both acid pre-treatment (hot sulfuric acid) and digestion by a cellulase mixture. However, the gains in fermentable sugar production are partially offset by reductions in plant yield in some, but not all, of these transgenic lines. Genetic experiments in Arabidopsis point to salicylic acid production as being responsible for most of  the reduced growth phenotypes of lignin down-regulated plants. Targeting transcriptional regulators of the lignin or whole secondary cell wall pathways provides an alternative strategy for lignin down-regulation. We have identified both positive and negative regulators of lignification. Loss of function of WRKY12 in Arabidopsis leads to a 50% increase in stem biomass density, and overexpression of the PvMYB4 repressor in switchgrass improves ethanol yields more effectively than down-regulation of pathway enzymes. Strategies that combine manipulation of lignin biosynthetic and regulatory genes have potential to deliver bioenergy feedstocks or forages combining increased cell wall density with improved digestibility.