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. 2011 May 19;7(5):e1002047. doi: 10.1371/journal.pcbi.1002047

Figure 1. Successive amendments of the metabolic pathways and transport processes leading to four hydroxycinnamyl alcohols.

Figure 1

The commonly accepted pathway of monolignol biosynthesis, which produces p-hydroxyphenyl (H), guaiacyl (G), 5-hydroxyconiferyl (5H), and syringyl (S) lignin monomers, is presented in black, with solid arrows representing metabolic conversions and open arrows collectively representing all events during the transport of monolignol precursors into the cell wall. Important revisions suggested by the recent identification of two CCR isoforms—CCR1 and CCR2—are colored in red and discussed in the text. Arrows colored in blue represent additional reactions and transport processes that are probably negligible in wild-type plants but found to become significant in some transgenic strains. Abbreviations for enzymes: PAL, phenylalanine ammonia-lyase; C4H, cinnamate 4-hydroxylase; 4CL, 4-coumarate:CoA ligase; CCR, cinnamoyl-CoA reductase; CAD, cinnamyl alcohol dehydrogenase; HCT, hydroxycinnamoyl CoA:shikimate hydroxycinnamoyl transferase; C3H, p-coumarate 3-hydroxylase; CCoAOMT, caffeoyl-CoA O-methyltransferase; COMT, caffeic acid O-methyltransferase; F5H, ferulate 5-hydroxylase.