Transcriptome analysis and functional characterization of oxidosqualene cyclases of the arjuna triterpene saponin pathway.

Arjuna (Terminalia arjuna) tree has been popular in Indian traditional medicine to treat cardiovascular ailments. The tree accumulates bioactive triterpene glycosides (saponins) and aglycones (sapogenins), in a tissue-preferential manner. Oleanane triterpenes/saponins (derived from β-amyrin) with potential cardioprotective function predominantly accumulate in the bark. However, arjuna triterpene saponin pathway enzymes remain to be identified and biochemically characterized. Here, we employed a combined transcriptomics, metabolomics and biochemical approach to functionally define a suite of oxidosqualene cyclases (OSCs) that catalyzed key reactions towards triterpene scaffold diversification. De novo assembly of 131 millions Illumina NextSeq500 sequencing reads obtained from leaf and stem bark samples led to a total of 156,650 reference transcripts. Four distinct OSCs (TaOSC1-4) with 54-71 % sequence identities were identified and functionally characterized. TaOSC1, TaOSC3 and TaOSC4 were biochemically characterized as β-amyrin synthase, cycloartenol synthase and lupeol synthase, respectively. However, TaOSC2 was found to be a multifunctional OSC producing both α-amyrin and β-amyrin, but showed a preference for α-amyrin product. Both TaOSC1 and TaOSC2 produced β-amyrin, the direct precursor for oleanane triterpene/saponin biosynthesis; but, TaOSC1 transcript expressed preferentially in bark, suggesting a major role of TaOSC1 in the biosynthesis of oleanane triterpenes/saponins in bark.