RNA interference suppression of lignin biosynthesis increases fermentable sugar yields for biofuel production from field-grown sugarcane

The agronomic performance, cell wall characteristics and enzymatic saccharification efficiency of transgenic sugarcane plants with modified lignin were evaluated under replicated field conditions. Caffeic acid O-methyltransferase (COMT) was stably suppressed by RNAi in the field, resulting in transcript reduction of 80%-91%. Along with COMT suppression, total lignin content was reduced by 6%-12% in different transgenic lines. Suppression of COMT also altered lignin composition by reducing syringyl units and p-coumarate incorporation into lignin. Reduction in total lignin by 6% improved saccharification efficiency by 19%-23% with no significant difference in biomass yield, plant height, stalk diameter, tiller number, total structural carbohydrates or brix value when compared with nontransgenic tissue culture-derived or transgenic control plants. Lignin reduction of 8%-12% compromised biomass yield, but increased saccharification efficiency by 28%-32% compared with control plants. Biomass from transgenic sugarcane lines that have 6%-12% less lignin requires approximately one-third of the hydrolysis time or 3- to 4-fold less enzyme to release an equal or greater amount of fermentable sugar than nontransgenic plants. Reducing the recalcitrance of lignocellulosic biomass to saccharification by modifying lignin biosynthesis is expected to greatly benefit the economic competitiveness of sugarcane as a biofuel feedstock.