Caffeoyl coenzyme A O-methyltransferase and lignin biosynthesis.

Lignin, a complex phenylpropanoid compound, is polymerized from the monolignols p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. These three monolignols differ only by the 3- and 5-methoxyl groups. Therefore, enzymatic reactions controlling the methylations of the 3- and 5-hydroxyls of monolignol precursors are critical to determine the lignin composition. Recent biochemical and transgenic studies have indicated that the methylation pathways in monolignol biosynthesis are much more complicated than we have previously envisioned. It has been demonstrated that caffeoyl CoA O-methyltransferase plays an essential role in the synthesis of guaiacyl lignin units as well as in the supply of substrates for the synthesis of syringyl lignin units. Caffeic acid O-methyltransferase has been found to essentially control the biosynthesis of syringyl lignin units. These new findings have greatly enriched our knowledge on the methylation pathways in monolignol biosynthesis.