lignin is a complex polymer derived from polymerization of coniferyl alcohol (Whetten and Sederoff 1995). Lignin is formed mainly from three monolignols: p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol (Figure 1) (Whetten and Sederoff 1991, Whetten et al. 1998). These monolignols interconnect with each other and with other cell wall polymers via different linkages (ether and carbon-carbon linkages) forming a three dimensional network polymer (Amthor 2003). Softwood lignin is based mainly on guaiacyl subunits, also known as G units or G lignin, polymerized from coniferyl
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alcohol, and low level of p-hydroxyphenyl units (H units) polymerized from p-coumaryl alcohol (Goldstein 1991, Whetten et al. 1998). On the other hand, hardwood lignin is a copolymer of guaiacyl subunits, syringyl subunits (S units or S lignin) and a low level of p-hydroxyphenyl units. These are polymerized from coniferyl, sinapyl and p-coumaryl alcohol, respectively.