Cloning and characterization of oxidosqualene cyclases involved in taraxasterol, taraxerol, and bauerenol triterpene biosynthesis in Taraxacum coreanum

Triterpenes, consisting of six isoprene units, are one of the largest classes of natural compounds in plants. The genus Taraxacum is in the family Asteraceae and is widely distributed in the Northern Hemisphere. Various triterpenes, especially taraxerol and taraxasterol, are present in Taraxacum plants. Triterpene biosynthesis occurs through the action of oxidosqualene cyclase (OSC), which generates various types of triterpenes from 2,3-oxidosqualene after the rearrangement of the triterpene skeleton. However, no functional characterization of the OSC genes involved in triterpene biosynthesis, except for a lupeol synthase in T. officinale, has been performed. T. coreanum, or Korean dandelion, grows in Korea and China. Putative OSC genes in T. coreanum plants were isolated by transcriptome analysis, and four of these (TcOSC1, TcOSC2, TcOSC3, and TcOSC4) were functionally characterized by heterologous expression in yeast. Both TcOSC1 and TcOSC2 were closely related to dammarenediol-II synthases. TcOSC3 and TcOSC4 were strongly grouped with β-amyrin synthases. Functional analysis revealed that TcOSC1 produced several triterpenes, including taraxasterol; Ψ-taraxasterol; α-, β-, and δ-amyrin; and dammarenediol-II. TcOSC2 catalyzed the production of bauerenol and another unknown triterpene, TcOSC3 catalyzed the production of β-amyrin. TcOSC4 catalyzed the production of taraxerol. Moreover, we identified taraxasterol, ψ-taraxasterol, taraxerol, lupeol, δ-amyrin, α-amyrin, β-amyrin, and bauerenol in the roots and leaves of T. coreanum. Our results suggest that TcOSC1, TcOSC2, TcOSC3, and TcOSC4 are key triterpene biosynthetic enzymes in T. coreanum. These enzymes are novel triterpene synthases involved in the production of taraxasterol, bauerenol, and taraxerol.