Figure 1.

Monolignol biosynthetic pathway. PAL, Phenylalanine ammonia-lyase; C4H, cinnamate 4-hydroxylase; 4CL, 4-coumarate:CoA ligase; HCT, p-hydroxycinnamoyl-CoA:shikimate/quinate p-hydroxycinnamoyltransferase; C3H, p-coumaroyl-shikimic acid/quinic acid 3-hydroxylase; SAD; sinapyl alcohol dehydrogenase; H, p-hydroxyphenyl; G, guaiacyl; 5H, 5-hydroxyguaiacyl; S, syringyl; ??, direct conversion not convincingly demonstrated; ?, conversion demonstrated; 4CL??; some species have 4CL activity toward sinapic acid; CCR? and F5H?, substrate not tested; others, enzymatic activity shown in vitro. Bold arrows indicate the presumed flux throughout the metabolic grid (for further discussion, see Boerjan et al., 2003).