Building the wall: recent advances in understanding lignin metabolism in grasses

Secondary cell walls account for the majority of total plant biomass and, as mostly composed of polysaccharides, constitute a promising source of fermentable sugars for the production of biofuels and biomaterials. However, the presence of the aromatic polymer lignin largely precludes the release of monosaccharides during enzymatic hydrolysis of cell wall polysaccharides in the biorefinery. Therefore, it is essential to unraveling the molecular mechanisms underlying lignin metabolism in order to better exploit the potential of lignocellulosic biomass. In the context of the bioeconomy, grasses emerge as a prominent lignocellulosic feedstock due to their high yield potential for biomass production. Still, many aspects of lignin metabolism in grasses, including transcriptional regulation, biosynthesis and polymerization, remain poorly understood, in contrast to eudicots species. Moreover, grasses differ considerably from eudicots in vascular patterning and cell wall composition, suggesting the presence of many grass- specific molecular and biochemical mechanisms that are not found in eudicots and whose knowledge cannot be extrapolated from data obtained with eudicot model plants. Here, we summarize the most recent advances on structural features of grass lignin and on functional characterization of genes directly involved in diverse aspects of lignin metabolism in grasses.