Lignin is a phenolic biopolymer with several important functions in plants. It is an essential cell wall component of woody plants, representing approximately 15-35% of the dry weight of the trees. It provides stiffness and strength to the stem and enables water and solute transport in the vascular xylem. It has also been considered to have a protective role against pathogens and herbivores (Denton 1998). Lignin is synthesized from aromatic heteropolymers that mainly originate from the oxidative polymerization of monolignols 4-hydroxycinnamyl alcohol, coniferyl alcohol and sinapyl alcohol. These components produce hydroxylphenyl (H), guaiacyl (G) and syringyl (S) lignin units, respectively. In conifers, lignin is mainly composed of G units, while in deciduous tree species both G and S units are involved. Lignin biosynthesis and lignin properties have been the targets of intensive research for more than a century (Sarkanen 1971; Higuchi 1985; Sederoff and Chang 1991; Chiang et al. 1994), mostly because extraction of lignin from wood fibre is an important cost factor for the pulp and paper industry. In general, the efficiency of wood pulping is directly proportional to the amount of S units in lignin. The G units of gymnosperms have a 5-aro-matic position leading to very strong carbon-carbon bonds, which make them fairly resistant to depolymerization required during pulping (Boudet and Grima-Pettenati 1996).
Lignin precursor biosynthesis is certainly the best known metabolic pathway of the phenolic metabolome of plants. During the last decade, a lot of new information has been gathered on this highly complex biosynthetic process (Osakabe et al. 1999; Li et al. 2000, 2001). It has been demonstrated that many enzymatic reactions occur at the level of hydroxycinnamic esters, aldehydes and alcohols (e.g. Osakabe et al. 1999; Hoffman et al. 2004). The current simplified model of lignin biosynthesis in angiosperms is presented, for instance, by Li et al. (2001), Boudet et al. (2003) and Hoffman et al. (2004). Initially, phenylalanine ammonia-lyase (PAL) catalyzes the deamination of phenylalanine to cinnamate, followed by cinnamate-4-hydroxylase (C4H) which produces 4-coumarate. Subsequently, in trees the role of 4-coumarate:coenzyme A ligase (4CL) that catalyzes the reaction from acids to esters (i.e. from 4-coumarate to p-coumaroyl CoA) has recently been emphasized (Hu et al. 1998, 1999; Harding et al. 2002). p-Coumarate 3-hydroxylase (C3H) with the novel HCT, i.e. an acyltransferase controlling shikimate and quinate ester intermediates accomplish after a series of reactions the synthesis of caffeoyl CoA from p-coumaroyl CoA.
In the new concept of lignin precursor biosynthesis in trees, the role of caf-feoyl coenzyme A O-methyltransferase (CCoAOMT) (Meyermans et al. 2000; Zhong et al. 2000) that catalyzes the reaction from caffeoyl CoA to feruloyl CoA, has also been demonstrated. This suggests that the route from caffeate to sinapate is not used for lignin biosynthesis (Osakabe et al. 1999; Li et al. 2001). Subsequently, cinnamoyl-CoA reductase (CCR) catalyzes the reaction from esters to aldehydes (i.e. from feruloyl CoA to coniferaldehyde), followed by cinnamyl-alcohol dehydrogenase (CAD) catalyzing the reaction from aldehydes to alcohols (i.e. from coniferaldehyde to coniferyl alcohol). The conversion of guaiacyl monolignol intermediates into syringyl types may take place at coniferaldehyde (Osakabe et al. 1999; Li et al. 2000). Coniferaldehyde 5-hydroxylase, which is also known as ferulate 5-hydroxylase (Cald5H and F5H, respectively), and 5-hydroxyconiferaldehyde O-methyltransferase also known as caffeate/5-hydroxyferulate O-methyl-transferase (AldOMT and COMT, respectively), catalyze the reactions from coniferylaldehyde to produce sinapylaldehyde. The last step in the syringyl monolignol pathway is catalysed by sinapyl-alcohol dehydrogenase SAD (Li et al. 2001) leading to sinapyl alcohol. However, the syringyl lignin biosynthesis in angiosperms has been found to operate via multiple pathways, depending on the phylogenetic group of the tree species (Yamauchi et al. 2003). The last step in lignin biosynthesis, the oxidation of monolignols, is presumably catalyzed by peroxidases or laccases. Peroxidases can generate phenoxy radicals, and these radicals are coupled into lignin polymers. What is more, the complex issue of linkage specificity during monolignol polymerization in the cell wall has evoked active discussions between the dirigent protein (Davin and Lewis 2000) and random coupling models (Sederoff et al. 1999; Hatfield and Vermerris 2001; Boerjan et al. 2003).
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