This chapter discusses briefly the occurrence and structure of the lignins found in higher plants. The pathways leading to lignin can be divided into several minor sequences. The first of these, the so-called “shikimic acid pathway” (which should properly be designated as the “chorismic acid pathway” according to its central intermediate) leads to the formation of the aromatic amino acids phenylalanine, tyrosine, and tryptophan. This pathway, which was originally identified in bacteria, characterizes both microorganisms and plants but does not operate in animals, with the consequence that such aromatic amino acids represent essential nutrient factors for these organisms. Among the above mentioned amino acids, phenylalanine is of outstanding importance as it is from this compound that the majority of phenolic natural products including lignin are derived. The three different types of reaction involved in the route (that is deamination of aromatic amino acids, hydroxylation of the aromatic nucleus of cinnamic acids, and methylation of these phenolic hydroxyls yielding methoxy groups) are discussed in the chapter. After feeding wheat plants with radioactive ferulic acid, labelled coniferyl alcohol and traces of the corresponding aldehyde were isolated. The specific radioactivities of these products suggested that the acid had been reduced via the aldehyde to the alcohol as the immediate lignin precursor. The results from recent enzymatic studies that could clarify the mechanisms involved in this essential conversion are reported in the chapter. The final step in lignin biosynthesis is the polymerization of thep coumaryl, coniferyl, and sinapyl alcohol units, whose biosynthesis has been reported in the chapter. Studies on the polymerization of these alcohols, in particular those in artificial systems yielding dehydrogenation polymers, suggested that this reaction represents an oxidative process.
