A Review on Delineating the Distribution of p-Coumarate vs. p-Hydroxybenzoate as Acylating Groups in Lignin across Monocot Families

Lignin is a major component of plant cell walls that is essential to their function. However, the strong bonds that bind the various subunits of lignin, and its cross-linking with other plant cell wall polymers, make it one of the most important factors in the recalcitrance of plant cell walls against polysaccharide utilization. Lignin restricts the degradation of structural polysaccharides by hydrolytic enzymes, thereby limiting the bioconversion of forages and fibrous crops into liquid fuels and other industrial products. Over the past decade it has become apparent that the pretreatment of lignocellulosic materials to remove or modify the lignin, hence altering the metabolic malleability of lignification, provides potential for engineering the troublesome polymer to be more amenable to bioprocessing. The knowledge of the structure of the lignin polymer, including the strong binding between its various subunits, is thus important to develop appropriate pretreatment methods for lignin modification and/or removal. Acylation in lignin subunits provides a valuable distinguishing feature between monocot and dicot plants. A range of acylated monolignols, notably monolignol γ-acetates, γ-hydroxybenzoates, and γ-p-coumarates are known to be lignin precursor ‘monomers’ (monomer conjugates). Generally, in monocots, the monolignols are preacylated by p-coumaric acid (pCA) whereas in dicots the lignins are acylated by p-hydroxybenzoate. However, the Aracacae family shows similarity with many of the dicots in containing p-hydroxy benzoate instead of p-coumarate as the units acylating the lignin. Hence there is differential distribution of lignin acylating groups across monocot families, which is the subject of present study.