Two O-Methyltransferases Mediate Multiple Methylation Steps in the Biosynthesis of Coumarins in Cnidium monnieri

Coumarins with methoxy groups such as osthole (1), xanthotoxin (2), bergapten (3), and isopimpinellin (4) are typical bioactive ingredients of many medicinal plants. The methylation steps remain widely unknown. Herein, we report the discovery of two methyltransferases in the biosynthesis of O-methyl coumarins in Cnidium monnieri by transcriptome mining, heterologous expression, and in vitro enzymatic assays. The results reveal that (i) CmOMT1 catalyzes the methylation of osthenol (8) as the final step in the biosynthesis of 1, (ii) CmOMT2 shows the highest efficiency and preference for methylating xanthotoxol (11) to form 2, and (iii) CmOMT1 and CmOMT2 also efficiently transform bergaptol (10) and 8-hydroxybergapten (13) into 3 or 4, suggesting the CmOMTs mediate multistep methylations in the biosynthesis of linear furanocoumarins in C. monnieri.