Lignin Formation in Plants. The Dilemma of Linkage Specificity

Lignification is the process of forming the collective of phenylpropanoid macromolecules termed lignin. There are two ways to define lignin: 1) from a chemical point of view (i.e. its chemical composition and structure), and 2) from a functional view that stresses what lignin does within the plant. It has been recognized for 50 years now that lignin is a polymeric material composed of phenylpropanoid units derived from three cinnamyl alcohols (monolignols): p-coumaryl, coniferyl, and sinapyl alcohols (Fig. It was suspected that this view might be too simplistic (Sarkanen and Ludwig, 1971), and there are now many examples showing that other phenolics can be incorporated into lignins (for review, see Sederoff et al., 1999). From a functional point of view, lignins impart strength to cell walls, facilitate water transport, and impede the degradation of wall polysaccharides, thus acting as a major line of defense against pathogens, insects, and other herbivores.