Transcriptome analysis identifies key genes involved in the regulation of epidermal lupeol biosynthesis in Ricinus communis

Lupeol is a pentacyclic triterpene which possesses many pharmacological activities. It has been found in many plants but generally with low content. Specifically, castor bean (Ricinus communis L.) contains high amounts of lupeol deposited on the outer surface, which is the major composition of epidermal wax. However, the biochemical pathways or regulatory genes responsible for lupeol accumulation in castor are largely unknown. In this study, a comparative transcriptome study was performed between the epidermis and stele of castor hypocotyl. Thirty-nine candidate genes involved in lupeol biosynthesis were identified and their expressions were analyzed. In particular, five selected genes associated with the mevalonate (MVA) pathway and triterpene downstream biosynthesis showed high correlation between their expression levels and the lupeol contents. Furthermore, several kinds of transcription factors were found to be specifically expressed in the epidermis. Especially, four transcription factors could activate the expression of Lupeol Synthase (RcLUS) in vivo, which is a key gene involved in lupeol synthesis. Taken together, these data will not only advance our understanding of triterpenoid synthesis in plants, but also provides potential candidate genes for enhancing lupeol biosynthesis by genetic engineering.