Functional characterization of triterpene synthases in Cibotium barometz

Cibotium barometz (Linn.) J. Sm., a tree fern in the Dicksoniaceae family, is an economically important industrial exported plant in China and widely used in Traditional Chinese Medicine. C. barometz produces a range of bioactive triterpenes and their metabolites. However, the biosynthetic pathway of triterpenes in C. barometz remains unknown. To clarify the origin of diverse triterpenes in C. barometz, we conducted de novo transcriptome sequencing and analysis of C. barometz rhizomes and leaves to identify the candidate genes involved in C. barometz triterpene biosynthesis. Three C. barometz triterpene synthases (CbTSs) candidate genes were obtained. All of them were highly expressed in C. barometz rhizomes, consisting of the accumulation pattern of triterpenes in C. barometz. To characterize the function of these CbTSs, we constructed a squalene- and oxidosqualene-overproducing yeast chassis by overexpressing all the enzymes in the MVA pathway under the control of GAL-regulated promoter and disrupted the GAL80 gene in Saccharomyces cerevisiae simultaneously. Heterologous expressing CbTS1, CbTS2, and CbTS3 in engineering yeast strain produced cycloartenol, dammaradiene, and diploptene, respectively. Phylogenetic analysis revealed that CbTS1 belongs to oxidosqualene cyclase, while CbTS2 and CbTS3 belong to squalene cyclase. These results decipher enzymatic mechanisms underlying the origin of diverse triterpene in C. barometz.

Identification of triterpenes and functional characterization of oxidosqualene cyclases involved in triterpene biosynthesis in lettuce (Lactuca sativa)

Lettuce (Lactuca sativa) is a member of the family Asteraceae and is most often used for green salads. Triterpenes are the largest class of natural compounds in plants and have beneficial health effects. Here, we identified various triterpene esters (taraxasterol acetates, ψ-taraxasterol acetates, taraxerol acetates, lupeol acetates, α-amyrin acetates, β-amyrin acetates, and germanicol acetate) and free triterpenes (α-amyrin, β-amyrin, taraxerol, and taraxasterol) in both the leaves and roots of lettuce. Triterpene biosynthesis occurs through the action of oxidosqualene cyclase (OSC), which generates various types of triterpenes from 2,3-oxidosqualene. None of the OSC genes involved in triterpene biosynthesis in lettuce have been characterized. Five putative lettuce OSC genes (LsOSC1, LsOSC2, LsOSC3, LsOSC4, and LsOSC5) were selected from a transcriptome database. These five genes were functionally characterized via heterologous expression in yeast. The first two enzymes were multifunctional triterpene synthase and the last three genes were monofunctional. Transgenic yeast expressing LsOSC1 produced five triterpenes, namely, taraxasterol, Ψ-taraxasterol, α-amyrin, β-amyrin, and dammarenediol-II. Yeast expressing LsOSC2 produced baurenol and Ψ-taraxasterol. LsOSC3, LsOSC4, and LsOSC5 expression led to β-amyrin, taraxerol, and lupeol production, respectively. Transcriptional activity assessment of the five genes revealed that all the OSC genes were more actively transcribed in roots than in leaves, and LsOSC5 among the five OSC genes showed the highest expression in both the leaves and the roots. In conclusion, we identified structurally diverse free triterpenes and triterpene esters in lettuce plants and characterized five OSC genes, which are key enzymes involved in the biosynthesis of diverse triterpenes in lettuce.

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 Taraxacum officinale, has been performed. Taraxacum 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.