Monolignol Biosynthesis and its Genetic Manipulation: The Good, the Bad, and the Ugly

Economic and environmental factors favor the adoption of lignocellulosic bioenergy crops for production of liquid transportation fuels. However, lignocellulosic biomass is recalcitrant to saccharification (sugar release from cell walls), and this is, at least in part, due to the presence of the phenylpropanoid-derived cell wall polymer lignin. A large body of evidence exists documenting the impacts of lignin modification in plants. This technology can lead to improved forage quality and enhanced processing properties for trees (paper pulping) and lignocellulosic energy crops. We here provide a comprehensive review of the literature on lignin modification in plants. The pathway has been targeted through down-regulating expression of the enzymes of the monolignol pathway, as well as through down-regulation or over-expression of transcription factors that control lignin biosynthesis and/or programs of secondary cell wall development. Targeting lignin modification at some steps in the monolignol pathway can result in impairment of plant growth and development, often associated with the triggering of endogenous host defense mechanisms. Recent studies suggest that it may be possible to decouple negative growth impacts from lignin reduction.