Biotechnology Division, Council of Scientific and Industrial Research-Central Institute of Medicinal and Aromatic Plants, Lucknow, 226015, India.
Chemical Sciences Division, Council of Scientific and Industrial Research-Central Institute of Medicinal and Aromatic Plants, Lucknow, 226015, India.
Plant Sci. 2020 Mar;292:110382. doi: 10.1016/j.plantsci.2019.110382. Epub 2019 Dec 18.
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.
诃子（Terminalia arjuna）树在印度传统医学中被广泛用于治疗心血管疾病。该树以组织偏好的方式积累生物活性三萜糖苷（皂苷）和苷元（皂苷元）。具有潜在心脏保护功能的齐墩果烷三萜类化合物/皂苷（来源于 β-香树脂醇）主要在树皮中积累。然而，诃子三萜皂苷生物合成途径的酶仍有待鉴定和生化表征。在这里，我们采用了转录组学、代谢组学和生化相结合的方法，从功能上定义了一套氧化鲨烯环化酶（OSCs），它们催化三萜骨架多样化的关键反应。从叶和茎皮样本中获得的 1.31 亿个 Illumina NextSeq500 测序读数的从头组装导致了总共 156650 个参考转录本。鉴定并功能表征了 4 种不同的 OSCs（TaOSC1-4），它们的序列同一性为 54-71%。TaOSC1、TaOSC3 和 TaOSC4 被生物化学表征为 β-香树脂醇合酶、环阿屯醇合酶和羽扇豆醇合酶，而 TaOSC2 被发现是一种多功能 OSC，既能产生 α-香树脂醇，也能产生 β-香树脂醇，但对 α-香树脂醇产物表现出偏好。TaOSC1 和 TaOSC2 都能产生 β-香树脂醇，这是齐墩果烷三萜/皂苷生物合成的直接前体；但是，TaOSC1 转录本在树皮中表达更优先，这表明 TaOSC1 在树皮中齐墩果烷三萜类化合物/皂苷的生物合成中起着主要作用。
