SilencingFolylpolyglutamate Synthetase1(FPGS1) in Switchgrass (Panicum virgatumL.) Improves Lignocellulosic Biofuel Production

Switchgrass (Panicum virgatumL.) is a lignocellulosic perennial grass with great potential in bioenergy field. Lignocellulosic bioenergy crops are mostly resistant to cell wall deconstruction, and therefore yield suboptimal levels of biofuel. The one-carbon pathway (also known as C1 metabolism) is critical for polymer methylation, including that of lignin and hemicelluloses in cell walls. Folylpolyglutamate synthetase (FPGS) catalyzes a biochemical reaction that leads to the formation of folylpolyglutamate, an important cofactor for many enzymes in the C1 pathway. In this study, the putatively novel switchgrassPvFPGS1gene was identified and its functional role in cell wall composition and biofuel production was examined by RNAi knockdown analysis. ThePvFPGS1-downregulated plants were analyzed in the field over three growing seasons. Transgenic plants with the highest reduction inPvFPGS1expression grew slower and produced lower end-of-season biomass. Transgenic plants with low-to-moderate reduction inPvFPGS1transcript levels produced equivalent biomass as controls. There were no significant differences observed for lignin content and syringyl/guaiacyl lignin monomer ratio in the low-to-moderately reducedPvFPGS1transgenic lines compared with the controls. Similarly, sugar release efficiency was also not significantly different in these transgenic lines compared with the control lines. However, transgenic plants produced up to 18% more ethanol while maintaining congruent growth and biomass as non-transgenic controls. Severity of rust disease among transgenic and control lines were not different during the time course of the field experiments. Altogether, the unchanged lignin content and composition in the low-to-moderatePvFPGS1-downregulated lines may suggest that partial downregulation ofPvFPGS1expression did not impact lignin biosynthesis in switchgrass. In conclusion, the manipulation ofPvFPGS1expression in bioenergy crops may be useful to increase biofuel potential with no growth penalty or increased susceptibility to rust in feedstock.