Feedstock Crop Genetic Engineering for Alcohol Fuels

Abstract

One of the goals of the U.S. government is to have “cellulosic ethanol” produced from a variety of sources, including feedstock crop biomass (a mass of raw material used in alcohol fuels processing), because these biomass sources contain polysaccharides that can be converted into fermentable sugars. Furthermore, the feedstock biomass sources are renewable and could become available at a billion tonnes per year in the United States. There are three major steps associated with the conversion of feedstock biomass into cellulosic ethanol. The first is the production of hydrolysis enzymes such as microbial cellulases, which convert the cellulose of feedstock biomass into fermentable sugars. The second step is the pretreatment processes used to break down the recalcitrant lignocellulose complex of feedstock into more reactive intermediates and to remove the lignin residues so the cellulase enzymes can have access to cellulose. The third step is fermentation of sugars into ethanol. The first two steps are the subject of this review. Plant genetic engineering has been used to directly express heterologous versions of cellulase and hemicellulase enzymes in situ. Plants have also been genetically modified for less lignin content or for more digestible lignin. An increase in feedstock polysaccharides and an increase in overall crop biomass via crop genetic engineering have also been reported. This article reviews the advancements made in feedstock crop genetic engineering in the above areas and discusses possible near‐future perspectives.