<Original>O-Methyltransferase as a Tool to Evaluate the Lignin Evolution

Abstract

The ratios of sinapate (SA)- to ferulate (FA)- formation (SA/FA ratio) by O-methyltransferases (OMTs) were surveyed over fifty plant species of 43 families. The OMTs are roughly classified into three groups by using the SA/FA ratio, i.e. gymnosperm-, angiosperm- and grass-types. OMTs in gymnosperm only catalyze FA-formation whereas angiosperm ones catalyze both FA- and SA-formation. Monocotyledons and herbaceous plants showed the apparent ratios which lay between typical gymnosperm- and angiosperm-ones. These substrate specificities well explain why gymnosperm lignin contains almost entirely guaiacyl lignin whereas angiosperm one contains both guaiacyl and syringyl lignins. A few exceptional cases were found in Cupressaceae-, Trochodendron-, and grass-OMTs, which were discussed in relation to lignin biosynthesis. The ratios of the OMT activities are discussed with respect to the usefulness as an evolutional marker for analyzing lignin evolution.