Silk tree, Albizia julibrissin Duraz, is an old ornamental plant and extensively cultivated in Asia. Previous works have discovered that the terpenoids were the dominating compounds in the floral VOC of A. julibrissin, however the biosynthesis of these terpenoids was poorly understood so far. Here, 11 terpene synthase genes (TPSs) were identified by transcriptome sequencing that fell into TPS-a, TPS-b and TPS-g subfamilies. The enzymatic activity tests showed that five genes were functional: AjTPS2 was a sesquiterpene synthase and produced α-farnesene and (Z, E)-β-farnesene; AjTPS5 was able to catalyze the formation of five monoterpenes and nine sesquiterpenes; AjTPS7, AjTPS9 and AjTPS10 were dedicated monoterpene synthases, as AjTPS7 and AjTPS10 formed the single product β-ocimene and linalool, respectively, and AjTPS9 produced γ-terpinene with other three monoterpenes. More importantly, the main catalytic products of the characterized AjTPSs were consistent with the terpenoids observed in A. julibrissin volatiles. Combining terpene chemistry, TPSs biochemical activities and gene expression analysis, we demonstrate that AjTPS2, AjTPS5, AjTPS7, AjTPS9 and AjTPS10 are responsible for the volatile terpenoids biosynthesis in A. julibrissin.

中文翻译：

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五种 TPS 负责合欢中挥发性萜类生物合成

合欢树是一种古老的观赏植物，在亚洲广泛种植。以前的工作已经发现萜类化合物是 A. julibrissin 花 VOC 中的主要化合物，但是迄今为止对这些萜类化合物的生物合成知之甚少。在这里，通过转录组测序鉴定了 11 个萜烯合酶基因 (TPS)，它们属于 TPS-a、TPS-b 和 TPS-g 亚家族。酶活性测试表明，5 个基因具有功能：AjTPS2 是倍半萜烯合酶，产生 α-法呢烯和 (Z, E)-β-法呢烯；AjTPS5 能够催化五种单萜和九种倍半萜的形成；AjTPS7、AjTPS9 和 AjTPS10 是专用的单萜合酶，因为 AjTPS7 和 AjTPS10 分别形成单产物 β-罗勒烯和芳樟醇，AjTPS9 与其他三种单萜一起产生 γ-萜品烯。更重要的是，所表征的 AjTPS 的主要催化产物与在 A. julibrissin 挥发物中观察到的萜类化合物一致。结合萜烯化学、TPS 生化活性和基因表达分析，我们证明 AjTPS2、AjTPS5、AjTPS7、AjTPS9 和 AjTPS10 负责 A. julibrissin 中挥发性萜类化合物的生物合成。