Fig. 1

The monolignol biosynthesis pathway, showing all known available routes to H, C, G and S monolignols in dicots. C-lignin is made exclusively of caffeyl alcohol (C) units, biosynthesis of which requires prevention of the O-methylation of the 3-OH group by CCoAOMT and/or COMT. The reactions which are blocked during the phase of C-lignin biosynthesis in the Cleome seed coat are shown with a sign. The 3-OH group in caffeyl alcohol can be introduced by the direct action of C3H on coumarate, or by the combined activities of HCT, C3′H and CSE in the so-called “shikimate shunt”. S lignin is not made in the Cleome seed coat due to lack of expression of F5H. Enzymes are L-phenylalanine ammonia-lyase (PAL), cinnamic acid 4-hydroxylase (C4H), hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase (HCT), coumaroyl shikimate 3′-hydroxylase (C3′H), caffeoyl shikimate esterase (CSE), coumarate 3-hydroxylase (C3H), caffeic acid/5-hydroxyconiferaldehyde 3/5-O-methyltransferase (COMT), 4-hydroxycinnamate CoA ligase (4CL), caffeoyl CoA 3-O-methyltransferase (CCoAOMT), cinnamoyl CoA reductase (CCR), cinnamyl alcohol dehydrogenase (CAD), ferulic acid/coniferaldehyde 5-hydroxylase (F5H), laccase (LAC)