Fig. 1

The phenylpropanoid biosynthesis pathway. The phenylpropanoid pathway produces phenolic compounds branching from phenylalanine ammonia lyase (PAL), including monolignols that lead to the biosynthesis of lignin subunits. The disruption of some of these enzymes results in brown midrib (bmr) mutants. Sorghum plants with bmr12-ref have a null mutation in caffeic acid O-methyltransferase (COMT) and plants with bmr6-ref have a null mutation in cinnamyl alcohol dehydrogenase (CAD). Both mutations result in altered lignification, such as the reduction of S-lignin in bmr12, and altered concentrations of wall-bound and soluble phenolics. Other abbreviations: C4H (cinnamate 4-hydroxylase), 4CL (4-coumarate: CoA ligase), HCT (p-hydroxycinnamoyltransferase), C3’H (4-coumarate hydroxylase), CSE (caffeoyl shikimate esterase), CCoAOMT (caffeoyl CoA-O-methyltransferase), CCR (cinnamoyl-CoA reductase), F5H (ferulate 5-hydroxylase)