Caffeoyl coenzyme A O-methyltransferase and lignin biosynthesis

Lignin, a complex phenylpropanoid compound, is polymerized from the monolig nols p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. These three monolignols differ only by the 3- and 5-methoxyl groups. Therefore, enzyma tic reactions controlling the methylations of the 3- and 5-hydroxyls of mon olignol precursors are critical to determine the lignin composition. Recent biochemical and transgenic studies have indicated that the methylation pat hways in monolignol biosynthesis are much more complicated than we have pre viously envisioned. It has been demonstrated that caffeoyl CoA O-methyltran sferase plays an essential role in the synthesis of guaiacyl lignin units a s well as in the supply of substrates for the synthesis of syringyl lignin units. Caffeic acid O-methyltransferase has been found to essentially contr ol the biosynthesis of syringyl lignin units. These new findings have great ly enriched our knowledge on the methylation pathways in monolignol biosynt hesis. (C) 2001 Elsevier Science Ltd. All rights reserved.