Improvement of lignocellulosic biomass in planta: A review of feedstocks, biomass recalcitrance, and strategic manipulation of ideal plants designed for ethanol production and processability

Plant biomass, or lignocellulosic biomass, is evaluated worldwide as a potential feedstock for the sustainable production of bioenergy in the near future due to its abundance, availability and renewability. Promising sources of plant biomass include agricultural residues and energy crops; however, the natural recalcitrance of this material is a major bottleneck for lignocellulose-derived ethanol production. The current process requires pre-treatment with severe conditions to disrupt the plant cell wall structures and remove hemicellulose and lignin components so that cellulose is more accessible to cellulases. However, the generation of enzyme inhibitors/deactivators and toxic substances during pre-treatment may subsequently affect enzymatic saccharification and fermentation processing. The pre-treatment and saccharification processability can be simplified if the plant biomass resistance to biochemical or enzymatic treatment is reduced. While there are many developed pre-treatment technologies and formulated enzyme cocktails that match pre-treated substrates, there has been attempt to design ideal energy crops via plant genetic manipulation. Cellulose engineering is aimed at reducing the crystallinity of cellulose structures. Expression of cellulose-disrupting proteins, including carbohydrate-binding modules, expansins, and swollenins, produces irregular forms of cellulose fibrils, which change from tightly packed fibrils to splayed ribbons with a high sugar release after enzymatic treatment. In addition, modifying genes and proteins involved in cellulose synthesis resulted in an unusual secondary cell wall deposition and composition and a lower crystallinity index. Reducing lignin content though engineering lignin biosynthesis pathways improves the saccharification process; however, abnormal growth and plant fitness remain problematic when improper genes are selected for manipulation. Lignin composition can be modified by introducing phenolic derivatives or peptide cross-links upon lignification, and these approaches might minimise the interference with plant growth and development. Hemicellulose biosynthesis is a complicated process. Currently, the reduction of hemicellulose content relies mostly on enzymes involved in xyloglucan/glucoarabinoxylan synthesis and the arrangements of those polymers in developing wood. Additionally, several glycosyltransferase and glycoside hydrolases are believed to be involved in hemicellulose modification in relation to loosened cell walls. Importantly, the expression of foreign glycoside hydrolases in plants may facilitate the reduction of enzyme loadings, thus making lignocellulosic ethanol production economically viable.