Fig. 2

Lignin biosynthesis. A In the plant cell (cytosol) the amino acid phenylalanine is converted to the three canonical monolignols (p-coumaroyl alcohol, coniferyl alcohol and sinapyl alcohol, indicated in blue) in the “grid-like” phenylpropanoid pathway, where phenylalanine goes through a series of ring (horizontal reactions on the biosynthetic grid) and sidechain (vertical reactions on the biosynthetic grid) modification to produce the monolignols for export to the plant secondary wall for lignification. Some non-canonical monolignols can also be synthesized via the phenylpropanoid pathway, or are intermediates in this pathway (indicated in orange). B Following biosynthesis, the monolignols are transported outside the cell, into the apoplastic space, where they are radicalized via laccase and peroxidase enzymes, and these radicals are then incorporated in the lignin polymer via radical coupling. The polymer shown here depicts a poplar lignin, mainly composed of β-aryl-ether (β-O-4) bonds (red) and syringyl monomers. 4CL: 4-coumarate:CoA ligase; C3H: p-coumarate 3-hydroxylase; C4H: cinnamate 4-hydroxylase; CAD: cinnamyl alcohol dehydrogenase; CCoAOMT: caffeoyl-CoA O-methyltransferase; CCR: cinnamoyl-CoA reductase; COMT: caffeic acid O-methyltransferase; CSE: caffeoyl shikimate esterase; F5H: ferulate 5-hydroxylase; HCT: hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyl transferase; PAL: phenylalanine ammonia-lysase