Lignin Biosynthesis and Structure

Abstract

Lignin is the generic term for a large group of aromatic polymers resulting from the oxidative combinatorial coupling of 4-hydroxyphenylpropanoids (Boerjan et al., 2003; Ralph et al., 2004). These polymers are deposited predominantly in the walls of secondarily thickened cells, making them rigid and impervious. In addition to developmentally programmed deposition of lignin, its biosynthesis can also be induced upon various biotic and abiotic stress conditions, such as wounding, pathogen infection, metabolic stress, and perturbations in cell wall structure (Caño-Delgado et al., 2003; Tronchet et al., 2010). Because lignin protects cell wall polysaccharides from microbial degradation, thus imparting decay resistance, it is also one of the most important limiting factors in the conversion of plant biomass to pulp or biofuels. The removal of lignin from plant biomass is a costly process; hence, research efforts are now aimed at designing plants that either deposit less lignin or produce lignins that are more amenable to chemical degradation (Sticklen, 2008; Weng et al., 2008a; Mansfield, 2009). The main building blocks of lignin are the hydroxycinnamyl alcohols (or monolignols) coniferyl alcohol and sinapyl alcohol, with typically minor amounts of p-coumaryl alcohol (Fig. 1). The monolignols are synthesized from Phe through the general phenylpropanoid and monolignol-specific pathways. Phe is derived from the shikimate biosynthetic pathway in the plastid (Rippert et al., 2009). Certain enzymes of the lignin biosynthetic pathway, namely the cytochrome P450 enzymes CINNAMATE 4-HYDROXYLASE (C4H), p-COUMARATE 3-HYDROXYLASE (C3H), and FERULATE 5-HYDROXYLASE (F5H), are membrane proteins thought to be active at the cytosolic side of the endoplasmic reticulum (Chapple, 1998; Ro et al., 2001). Although metabolic channeling has been shown between PHENYLALANINE AMMONIA-LYASE (PAL) and C4H (Rasmussen and Dixon, 1999; Achnine et al., 2004), it remains unknown whether the other pathway enzymes are also part of metabolic complexes at the endoplasmic reticulum. The main biosynthetic route toward the monolignols p-coumaryl, coniferyl, and sinapyl alcohol (Boerjan et al., 2003). PAL, PHENYLALANINE AMMONIA-LYASE; C4H, CINNAMATE 4-HYDROXYLASE; 4CL, 4-COUMARATE:CoA LIGASE; C3H, p-COUMARATE 3-HYDROXYLASE; HCT, p-HYDROXYCINNAMOYL-CoA:QUINATE/SHIKIMATE p-HYDROXYCINNAMOYLTRANSFERASE; CCoAOMT, CAFFEOYL-CoA O-METHYLTRANSFERASE; CCR, CINNAMOYL-CoA REDUCTASE; F5H, FERULATE 5-HYDROXYLASE; COMT, CAFFEIC ACID O-METHYLTRANSFERASE; CAD, CINNAMYL ALCOHOL DEHYDROGENASE. The units resulting from the monolignols, when incorporated into the lignin polymer, are called guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units (Figs. 1 and 2). With some notable exceptions (Novo Uzal et al., 2009), lignins from gymnosperms are composed of G-units only (with minor amounts of H-units), whereas angiosperm dicot lignins are composed of G- and S-units. H-units are elevated in softwood compression wood and may be slightly higher in grasses (Boerjan et al., 2003). A variety of less abundant units have been identified from diverse species, and these may be incorporated into the polymer at varying levels (Ralph et al., 2004). Some units, such as those derived from the monomer sinapyl acetate, can make up to 85% of all S-units in the polymer (Lu and Ralph, 2008; Martínez et al., 2008). In addition to differences in lignin composition among taxa and species, lignin composition can also differ among cell types, as readily visualized by the histochemical Mäule staining, which is indicative of S-units, or when chemically analyzed by laser capture microdissection followed by microanalysis of lignin (Nakashima et al., 2008; Ruel et al., 2009). Lignins can even be dissimilar at the level of individual cell wall layers, as revealed by Raman, IR, and UV microspectroscopy, or by immunolabeling of the secondary wall with antibodies cross-reacting with specific lignin substructures (Shi et al., 2006; Gierlinger and Schwanninger, 2007; Gou et al., 2008; Ruel et al., 2009). Representation of a lignin polymer from poplar, as predicted from NMR-based lignin analysis (adapted from Stewart et al., 2009). It is commonly accepted that lignin evolved together with the adaptation of plants to a terrestrial life to provide them with the structural support needed for an erect growth habit (Fig. 3). Comparative genomics studies based on available plant genomes indicate that the complete biosynthetic pathway, except for F5H, first appeared in moss (based on the Physcomitrella genome sequence; Xu et al., 2009) but was absent from green algae. However, recent studies have also detected secondary walls and apparent lignin in the marine red alga Calliarthron, which diverged from vascular plants more than 1 billion years ago (Martone et al., 2009), indicating either a very strongly conserved evolutionary history of the relevant pathways or convergent evolution. In support of the latter hypothes