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. 1). 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. The lignification process encompasses the biosynthesis of monolignols, their transport to the cell wall, and polymerization into the final molecule. This discussion will focus on the final phase—the formation of the lignin macromolecule. Bond formation is thought to result from oxidative (radical-mediated) coupling between a monolignol and the growing oligomer/polymer. The oxidative coupling between monolignols can result in the formation of several different interunit linkages (Fig. 2). In native lignins, 8-O-4-linkages are the most abundant, whereas for lignins formed in vitro by mixing coniferyl alcohol, hydrogen peroxide, and peroxidase, higher percentages of 8–8and 8–5-linkages are found (Nimz and Ludemann, 1976; Terashima et al., 1996; Chen, 1998). How is this apparent specificity in chemical bonds between lignin subunits controlled? Currently, there are two models for coupling radicals to produce a functional lignin molecule. One, the random coupling model, which emerged during early studies on the structure of lignin, centers on the hypothesis that lignin formation proceeds through coupling of individual monolignols to the growing lignin polymer in a near-random fashion (Harkin, 1967; Freudenberg and Neish, 1968; Adler, 1977). In this view, the amount and type of individual phenolics available at the lignification site and normal chemical coupling properties (Syrjanen and Brunow, 1998) regulate lignin formation. The second model, the dirigent protein model, is more recent and suggests that lignification must be under strict regulation of specialized proteins that control the formation of individual bonds (Lewis and Davin, 1998; Davin and Lewis, 2000). This new model for lignin formation stems from the definition of dirigent proteins (Davin et al., 1997). Dirigent proteins direct the coupling of two monolignol radicals, producing a dimer with a single regioand stereoconfiguration. These dimers are known as lignans and are commonly found in many plants. The rationale for this new model is the belief that nature would not leave the formation of such an important molecule as lignin “to chance” (Davin and Lewis, 2000). It is argued that the only way to explain the high proportion of 8-O-4 linkages in lignin would be through regulation by specific dirigent proteins (Davin and Lewis, 2000). We will evaluate both models to determine how well each fits with the current state of knowledge based on experimental evidence.