Advances in modifying lignin for enhanced biofuel production

Renewable and sustainable liquid transportation biofuels based on lignocellulosics conversion face several obstacles that must be overcome in order for them to become commercially viable and cost-competitive. The presence of lignin is one of the most significant contributors to biomass recalcitrance and consequently increases the costs associated with conversion. Lignins are complex aromatic biopolymers, derived from hydroxyphenylpropanoids, that vary in composition and structure as a function of genotype, phenotype, and environment, as well as with the cell type and maturity of the plant tissue. Lignins consist of (mainly) syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) units, derived from sinapyl, coniferyl, and p-coumaryl alcohols. These units are not discrete within either the cell or a given lignin molecule, and the compositional ratios of these three moieties can vary significantly. This inherent complexity and heterogeneity of lignin, both in structure and composition, make it extremely difficult to develop a conversion technology that can efficiently process a wide range of sustainable feedstocks cost-effectively. There is a growing body of work that has demonstrated several genetic engineering strategies that, when coupled with an integrated approach to conversion, hold significant promise for the development of tailored feedstocks designed for biofuel production. The knowledgebase is at the point where researchers are also able to contemplate strategies to 'design' the lignin polymer for easier processing. The realization of advanced analytical techniques and an increasing number of plant genomes are enabling researchers to take a systems approach towards understanding and engineering lignin to develop these optimal feedstocks.