Lignin Biosynthesis

Ligninis typically polymerized from three phenylpropanoid monomers (see Figure), 4-coumaryl, coniferyl, and sinapyl alcohols, also known as the H, G and S monolignols. In dicotyledons, lignin is polymerized from S and G monolignols and low levels of H and other intermediates (Harkin, 1967; Freudenberg and Neish, 1968; Sarkanen, 1971; Ralph et al., 2008).
 
In wood of Populus, the S/G ratio is approximately 2.2 (Sarkanen and Hergert, 1971; Ralph et al., 2004, 2008). Knowledge of the enzymes and metabolites of the lignin pathway stems from tracer studies by Brown, Neish and Higuchi in the 1950’s (Brown, 1961; Brown and Neish, 1955a, 1955b, 1956; Weight et al., 1958; Brown and Neish, 1959; Higuchi, 1959, Higuchi and Brown, 1963a, 1963b). Ten enzyme families convert phenylalanine to monolignols, with a principal path through a metabolic grid (see Figure; Ye et al., 1994; Higuchi, 1997, Zhong et al., 1998; Osakabe et al., 1999; Humphreys et al., 1999; Li et al., 2000, 2001; Gui et all., 2001; Schoch et al., 2001; Dixon et al., 2001; Hoffmann et al., 2003; Higuchi, 2003; Ralph et al., 2004, 2008; Vanholme et al., 2008).
 
Monolignols may be exported to the cell wall via Golgi-mediated vesicle fusion with the plasma membrane (Takabe et al., 1985; Fukushima et al., 1997; Kaneda et al., 2008) or partition-based membrane diffusion (Boija et al., 2006, 2007). In the wall, monolignols are irreversibly oxidized to phenoxy radicals (see Figure) by H2O2-dependent class III peroxidases (POs) through an electron oxidoreduction cycle (Freudenberg, 1956; Brown, 1961; Harkin, 1967; Freudenberg and Neish, 1968; Sarkanen, 1971; Adler, 1977, Hosel, 1981; Dunford, 1991; Higuchi, 1997; Tsutsumi et al., 1997; Matsui et al., 1997; Morales and Barcelo, 1997; Aoyama et al., 2002; Ralph et al., 2004; Sasaki et al., 2006; Cosio and Dunand, 2008). Phenoxy radicals in monolignols are delocalized to form mesomeric structures capable of coupling with each other or with resonance-stabilized radicals in a growing lignin polymer (Harkin, 1967; Freudenberg and Neish, 1968; Ralph et al., 2004, 2008; Holmgren et al., 2006).
 
The biosynthesis of lignin occurs primarily through the addition of a monomer to a polymer. The propensity for cross-coupling by radicals of both coupling partners at specific positions is dictated by region-chemistry, forming specific chemical linkages (Adler, 1977; Ralph et al., 2004, 2008). The process is “combinatorial” (Adler, 1977; Ralph et al., 2004, 2008). β-O-4, β-5 and β-β are typical G and S interunit linkages (see Figure; Erdtman, 1950; Freudenberg, 1956; Brown, 1961; Harkin, 1967; Freudenberg and Neish, 1968; Sarkanen, 1971; Higuchi, 1997; Ralph et al., 2004, 2008; Holmgren et al., 2006).
 
 
 
 
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