Monolignole

Monolignol, more commonly referred to in the plural as monolignols or monolignole, are hydroxycinnamyl alcohols that serve as the primary monomers of lignin, the complex phenolic polymer that reinforces plant secondary cell walls. The three principal monolignols are p-coumaryl alcohol (the H unit), coniferyl alcohol (the G unit), and sinapyl alcohol (the S unit). Their ring substitution differs in methoxy groups: H units lack methoxy groups, G units have one methoxy, and S units have two methoxy groups.

Biosynthesis occurs in the cytosol via the phenylpropanoid pathway, beginning with phenylalanine. Key enzymes include phenylalanine ammonia-lyase (PAL), cinnamate 4-hydroxylase (C4H), and 4-coumarate:CoA ligase (4CL), followed by reduction steps mediated by cinnamoyl-CoA reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD) to yield the corresponding alcohols. The monolignols are then transported to the cell wall, where oxidative coupling catalyzed by peroxidases or laccases forms lignin with a heterogeneous architecture that incorporates H, G, and S units in various linkages.

The composition of monolignols influences lignin properties, including rigidity, hydrophobicity, and recalcitrance to chemical and enzymatic degradation. In plants, lignification is developmentally regulated and responds to environmental cues. Gymnosperms typically accumulate predominantly G and H units, whereas many angiosperms include substantial S units, yielding lignins with distinct structural and physical characteristics. Understanding monolignol pathways informs plant biology, wood science, and strategies for biomass processing and bioenergy.