Understanding Lignification: Challenges Beyond Monolignol Biosynthesis

Lignin, a major component of vascular plant cell wall, provides mechanical support for plants to stand upright and enables xylems to withstand the negative pressure generated during water transport. Although important for plant growth, the presence of lignin limits access to cell wall polysaccharides and thereby negatively affects human utilization of biomass such as its use as livestock feed, in paper manufacturing, and in lignocellulosic biofuel production. Because of its significant economic impact, lignin has been one of the most intensively studied subjects in plant biochemistry. Lignin is a heterogeneous phenolic polymer largely composed of three major types of monomers (monolignols), p-coumaryl, coniferyl, and sinapyl alcohols. During lignin deposition, monolignols are synthesized in the cytoplasm, translocated to the apoplast, and polymerized into lignin. Over the last two decades, the biosynthesis of monolignols has been a major focus of research on lignification. We now believe that the monolignol biosynthetic pathway has been relatively well elucidated, at least in angiosperms. Based upon this knowledge, genetic engineering approaches have been used to successfully modify lignin content and/or composition in a variety of plant species (for review, see Boerjan et al., 2003; Li et al., 2008).