Abstract

The lignin polymer is essential for the development of vascular plants but is also the major cause of lignocellulosic biomass recalcitrance to efficient industrial processing. From an applications perspective, it is desirable that second-generation bioenergy crops have lignin that is readily degraded by chemical pretreatments but still fulfills its biological role in plants. Because plants can tolerate large variations in lignin composition, often without apparent adverse effects, substitution of some fraction of the traditional monolignols (i.e. coniferyl and sinapyl alcohol in dicots) by alternative monomers through genetic engineering is a promising strategy to tailor lignin in bioenergy crops. Upon incorporation into the lignin polymer, these alternative monolignols result in a lignin that is easy-to-degrade or easy-to-extract. Several plant metabolites are being proposed as good alternative monomers and they can be classified as follows: i) monomers that directly produce a readily cleavable functionality in the lignin polymer, ii) hydrophilic monomers, iii) difunctional monomers and monomer conjugates linked via a readily cleavable functionality, iv) monomers that minimize lignin-polysaccharide cross-linking and v) monomers that give rise to shorter lignin polymers. The presence of the candidate alternative monomers in plants allows the cloning of their biosynthetic pathways into the desired target organism. After synthesis of the alternative monomers in the cytoplasm, they have to be transported to the cell wall where they need to polymerize with traditional monomers, giving rise to the alternative lignin polymers. In some cases, besides the cloning of the biosynthetic pathway towards the alternative monomers, it will be necessary to alter endogenous pathways in order to avoiding substrate competition and to re-direct the metabolic flux.