Modifying plant cell walls for bioenergy production.

Predictions about future supply of crude oil, increase in greenhouse gases and burgeoning human population have recently necessitated a search for sustainable and environmentally responsible sources of alternative energy. Bioconversion of organic biomass, largely derived from plant residues, to biofuels has been viewed as one of the most attractive avenues of research. More than half of the plant biomass produced on earth consists of cell walls that are enriched in carbohydrate polymers such as cellulose and hemicellulose. Breakdown of these carbohydrate polymers by pre-treatments and cellulolytic enzymes to monomer sugars (saccharification) which is followed by fermentation of sugars to bioethanol is one of the favoured ways of producing biofuels. Presence of phenolic compounds in form of lignin hinders access of cell wall degrading enzymes to carbohydrates, and genetic engineering of genes and metabolic pathways involved in cell wall formation could dramatically alter the outcome for biofuel production. In recent years, several reports have described such approaches that improve saccharification efficiency and this review discusses some of those advances. Another approach is to use quantitative trait locus (QTL) mapping and/or association mapping to identify genes/molecular markers that are associated with cell wall polymer synthesis. These candidate genes can be used for marker-assisted breeding and/or could be manipulated using transgenic approaches. A combination of these techniques is likely to provide novel tools in producing feedstocks for efficient use of selected plant biomass in biofuel production.