Eucalyptus Trees with Reduced Lignin Content Display Reduced Recalcitrance
Lignocellulosic materials offer an attractive replacement for food-based crops used to produce ethanol, but understanding the interactions within the cell wall is vital to overcome the highly recalcitrant nature of lignocellulosic biomass. One factor imparting plant cell wall recalcitrance is lignin, which can be manipulated by making changes in the lignin biosynthetic pathway. Changes to lignin gene expression in switchgrass and Populus have shown increased sugar release and reduced recalcitrance. Researchers at the Department of Energy’s BioEnergy Science Center have sought to transfer these results to eucalyptus, a fast-growing, warm climate, woody biofeedstock also suitable for cellulosic biofuel production. The researchers genetically engineered reduced gene expression of two key lignin biosynthesis enzymes, cinnamate 4-hydroxylase (C4H) and p-coumaroyl quinate/shikimate 3′-hydroxylase (C3’H), in eucalyptus. The engineered plants were evaluated for cell wall composition and reduced recalcitrance. Eucalyptus trees with down-regulated C4H or C3’H expression displayed lowered overall lignin content than the control samples. The C3’H and C4H down-regulated lines also had different lignin compositions when compared to the control eucalyptus trees. Both the C4H and C3’H down-regulated lines had reduced recalcitrance as indicated by increased sugar release, which was determined using enzymatic conversion assays utilizing both no pretreatment and a hot water pretreatment. Lowering lignin content rather than altering lignin content was found to have the largest impact on reducing recalcitrance of the transgenic eucalyptus variants. The development of lower recalcitrance trees opens up the possibility of using alternative pretreatment strategies in biomass conversion processes that can reduce processing costs.
Sykes, R. W., E. L. Gjersing, K. Foutz, W. H. Rottmann, S. A. Kuhn, C. E. Foster, A. Ziebell, G. B. Turner, S. R. Decker, M. A. W. Hinchee, and M. F. Davis. 2015. “Down Regulation of P-Coumaroyl Quinate/Shikimate 3′-Hydroxylase (C3’H) and Cinnamate 4-Hydroxylase (C4H) Genes in the Lignin Biosynthetic Pathway of Eucalyptus urophylla x E. grandis Leads to Improved Sugar Release,” Biotechnology for Biofuels 8,128. DOI: 10.1186/s13068-015-0316-x.