Reducing cell wall recalcitrance and increasing biomass for enhanced ethanol production in switchgrass (Panicum virgatum L.)

Lignocellulosic biomass is a potential large-scale biofuel feedstock for conversion to ethanol through saccharification and fermentation. The presence of lignin in lignocellulosic biomass impedes its degradation and subsequent fermentation. The removal of lignin by pretreatment is the most expensive step in the production of lignocellulosic biofuels. Manipulation of monolignol pathway is needed to reduce lignin and for the rational design of engineered cell walls of lignocellulosic feedstocks.

PvMYB4, a transcriptional repressor of lignin gene expression was identified and evaluated for its potential for improving switchgrass as a lignocellulosic feedstock. Ectopic overexpression of PvMYB4 in transgenic switchgrass reduced lignin content and increased sugar release efficiency up to three-fold over that of the non-transgenic control. The transgenic plants yielded up to 2.6-fold more ethanol than controls. Detailed biomass characterization revealed alteration of lignin content, xylan/pectin and lignin linkages, lignin polymer size and internal linkages in lignin leading to reduced recalcitrance and increased ethanol yield. Genetically engineered PvMYB4 switchgrass therefore can be used as potential germplasm for improvement of lignocellulosic feedstocks. It is currently being grown in field experiments.

Increasing biomass production is also important for biofuel crops. Sucrose synthase catalyzes the conversion of sucrose and uridine di-phosphate (UDP) into UDP-glucose which is used by cellulose synthase for cell wall biosynthesis. Bioinformatic and cluster analysis was used to identify four sucrose synthase genes in switchgrass. Transient subcellular localization revealed that PvSUS1 localizes to the plasma membrane. Transgenic switchgrass plants overexpressing PvSUS1 had increases in biomass and cellulose content. For switchgrass and other bioenergy feedstocks, the overexpression of SUS1 genes might be a realistic strategy to increase both plant biomass and cellulose content to maximize biofuel production per land area cultivated.

Taken together PvMYB4 and PvSUS1 would be two excellent candidate genes to stack in switchgrass to provide improvements in two important aspects of feedstock improvement: greater amounts of less recalcitrant biomass. In addition, these genes could also be overexpressed in other crops such as maize. It could be envisage that a US corn crop overexpressing both these genes together could give greater corn grain yield and less recalcitrant stover for lignocellulosic biofuel production.