Molecular Breeding to Enhance Ethanol Production from Corn and Sorghum Stover

Political and environmental concerns have resulted in a growing interest in renewable energy, especially transportation fuels. In the United States the majority of fuel ethanol is currently produced from corn (Zea mays L.) starch, but grain supplies will be insufficient to meet anticipated demands. Enzymatic hydrolysis of lignocellulosic biomass such as corn and sorghum [Sorghum bicolor (L.) Moench] stover can provide an abundant alternative source of fermentable sugars. While production of cellulosic ethanol from stover is feasible from an energy-balance perspective, its production is currently not economically competitive. Along with improvements in bioprocessing, enhancing the yield and composition of the biomass has the potential to make ethanol production considerably more cost effective. This requires (i) a better understanding of how cell wall composition and structure affect the efficiency of enzymatic hydrolysis, (ii) the development of traits that enhance biomass conversion efficiency and increase biomass yield, and (iii) the development of rapid screening protocols to evaluate biomass conversion efficiency. Several genetic resources are available to improve maize and sorghum as sources of lignocellulosic biomass. This includes the use of existing mutants, forward and reverse genetics to obtain novel mutants, and transgenic approaches in which the expression of genes of interest is modified. Plant breeding can be implemented to improve biomass yield, biomass quality, and biomass conversion efficiency, either through selection among progeny obtained by crossing parents with desirable traits, or as a way to enhance the agronomic performance of promising mutants and transgenics. Examples from current research will be used to illustrate progress in these different areas.