Lignin monomers from outside the canonical monolignol biosynthetic pathway

Abstract

Lignin is a phenylpropanoid polymer derived from the oxidative radical coupling of three phydroxycinnamyl alcohols, the so-called monolignols: p-coumaryl, coniferyl, and sinapyl alcohols. Several other phenolic compounds, all deriving from the shikimate-derived monolignol biosynthetic pathway, have been found to behave as lignin monomers in many plants, including the monolignol ester conjugates (with acetate, p-coumarate, ferulate and p-hydroxybenzoate) or the incompletely methylated monomers caffeyl and 5-hydroxyconiferyl alcohols, among others. However, recent studies have demonstrated the occurrence of polyphenolic compounds derived from other biosynthetic pathways in the lignins of several plants. This is the case for the flavone tricin recently established as a monomer in the lignins from grasses and other monocots or the newly discovered hydroxystilbenes (resveratrol, isorhapontigenin and piceatannol) that are implicated as monomers in the lignification of palm fruit endocarps. The discovery of ‘non-conventional’ phenolic precursors arising from other biosynthetic pathways expands the traditional definition of lignin and reveals that any phenolic compound that are delivered to the cell wall may be oxidized and incorporated into the lignin polymer during lignification.