Recent discoveries of novel lignin monomers derived from the flavonoid and stilbene biosynthetic pathways

Abstract

Lignin is a phenylpropanoid polymer derived essentially from the oxidative radical coupling of three p-hydroxycinnamyl alcohols (so-called “monolignols”) differing in their degree of methoxylation, 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 revealed that some phenolic compounds derived from other biosynthetic pathways also act as true lignin monomers participating in coupling and cross-coupling reactions during lignification in several plants. This is the case for the flavone tricin recently established as a monomer in the lignins from grasses and other monocots (and the flavanone naringenin found in the lignin in an fnsII rice mutant) or the newly discovered hydroxystilbenes (resveratrol, isorhapontigenin and piceatannol) that are implicated as monomers in the lignin of palm fruit endocarps. The discovery in lignins of ‘non-conventional’ phenolic precursors arising from other biosynthetic pathways expands the traditional definition of lignin, illustrates yet again the high metabolic plasticity of lignification, and reveals that any phenolic compound that is delivered to the cell wall may be oxidized and incorporated into the lignin polymer during lignification, subject exclusively to its chemical compatibility. On the other hand, these discoveries reveal that valuable flavonoids and hydroxystilbenes could also be obtained from agricultural resides (cereal straws or palm fruit shells) or from low-value lignin side-streams in lignocellulose processing mills, which will open new opportunities for the valorization of these residues that are considered as wastes.