The known unknowns in lignin biosynthesis and its engineering to improve lignocellulosic saccharification efficiency

Biofuels produced from lignocellulosic biomass provide energy security and reduce greenhouse gas emissions with positive impacts on sustainability. Lignocellulose byproducts are expected to impact the production of cellulosic ethanol. However, lignin is a major factor imposing biomass recalcitrance to saccharification. Although structural phenolics can be removed from lignocellulosic biomass, physical and chemical pretreatments are expensive. In turn, the structure and composition of lignin is very flexible. Up- and downregulation of genes and, thereby, enzymes involved in core and non-core lignin biosynthesis can reduce lignin content and improve biomass saccharification. Alternatively, insertion of heterologous genes to the phenylpropanoid pathway may lead to the incorporation of alternative monomers into lignin, creating desirable properties and preserving the biological roles of lignin with a positive effect on saccharification. Studies reporting the effect of fine regulation on lignin structures are mapping how plants can be transformed to enhance saccharification while preserving productivity. Based on these findings and events, this review updates the state-of-art changes in lignin biosynthesis to improve saccharification efficiency. The main findings reported over the last decade were summarized to permit researchers an overview of this relevant scientific subject.