Lignin primary structures and dirigent sites.

Although lignin is the second most abundant plant substance in vascular plants, its mode of synthesis is still the subject of much debate. However, recent progress has provided crucial evidence to support the theory that lignin primary structure is controlled at the proteinaceous level. Evidence for control over lignin assembly has been demonstrated with the discovery of monomer-invariant aryl-O-ether linkages in lignins that upon alkaline cleavage release the corresponding monomers in equimolar amounts, regardless of monolignol composition. Current evidence would indicate that there are only a few native lignin primary structures, the entire sequences of which now need to be fully determined. A provisional mechanistic model is proposed to account for macromolecular lignin assembly through the participation of proteins harboring arrays of dirigent (monolignol radical binding) sites.