Field Performance of Transgenic Switchgrass for Cell Wall Traits

North America. High biomass production and wide adaptation has made switchgrass a leading lignocellulosic feedstock.  One of its major limitations is the recalcitrance of complex carbohydrates to hydrolysis for conversion of lignocellulosic biomass into biofuels. Lignin is a primary contributor to recalcitrance as it creates a physical and chemical barrier to enzymatic access of cell wall polysaccharides. Therefore, genetic manipulation of the lignin biosynthetic pathway in an effort to reduce lignin content is a promising approach for overcoming this inherent cell wall recalcitrance. Low-lignin transgenic switchgrass plants were produced  via down-regulation of caffeic acid O-methyltransferase (COMT), or by up-regulation of the MYB4 transcription factor, an R2R3 type MYB repressor of the lignin biosynthetic pathway. Resulting COMT transgenic plants exhibited increased ethanol yields by up to 38% compared to the control and required less severe pretreatment and 300-400% less cellulase enzyme loading. Resulting MYB4 overexpression transgenic plants demonstrated 3-fold increase in sugar release efficiency and 2.5-fold increase in ethanol production without pretreatment compared to control; all analyses used greenhouse-grown plants. Field trials of COMT and MYB transgenic switchgrass are underway in Knoxville, Tennessee where plants are assessed during a period of three growing seasons for 1) agronomic biomass performance (tiller height, plant width, tiller number) and biomass yield, 2) cell wall characterization 3) biorefinery performance including pretreatment response, sugar release efficiency, and ethanol yield, and 4) rust disease susceptibility of transgenic plants compared to controls.