Richman A S, Gijzen M, Starratt A N, Yang Z, Brandle J E
Southern Crop Protection and Food Research Centre, Agriculture and Agri-Food Canada, London, Ontario, Canada.
Plant J. 1999 Aug;19(4):411-21. doi: 10.1046/j.1365-313x.1999.00531.x.
Stevia rebaudiana Bertoni leaves accumulate a mixture of at least eight different glycosides derived from the tetracyclic diterpene steviol. These natural products taste intensely sweet and have similar biosynthetic origins to those of gibberellic acid (GA). The initial steps leading to the formation of GA result from the two-step cyclization of geranylgeranyl diphosphate (GGDP) to (-)-kaurene via the action of two terpene cyclases (-)-copalyl diphosphate synthase (CPS) and (-)-kaurene synthase (KS). Steviol biosynthesis probably uses the same mechanism although the genes and enzymes from S. rebaudiana that are involved in the cyclization of GGDP have not been characterized. We have isolated both the CPS and KS genes from S. rebaudiana and found that recombinant CPS and KS were catalytically active, suggesting that the CPS and KS genes participate in steviol biosynthesis. The genes coding for CPS and KS are usually present in single copies in most plant species and their expression is normally low and limited to rapidly growing tissues. The KS gene has been duplicated in the S. rebaudiana genome and both the KS and CPS genes are highly expressed in mature leaves, a pattern opposite to that found with GA biosynthesis. This pattern may, at least in part, lead to temporal and spatial separation of GA and steviol biosynthesis and probably helps to prevent over-expression from interfering with normal GA metabolism. Our results show that CPS and KS are part of the steviol glycoside biosynthetic pathway and that Stevia rebaudiana has recruited two genes to secondary metabolism from a highly regulated pathway involved in hormone biosynthesis.
甜叶菊叶子积累了至少八种源自四环二萜甜菊醇的不同糖苷混合物。这些天然产物尝起来甜度极高，并且与赤霉素（GA）有着相似的生物合成起源。GA形成的起始步骤是通过两种萜烯环化酶（-）-柯巴基焦磷酸合酶（CPS）和（-）-贝壳杉烯合酶（KS）的作用，将香叶基香叶基二磷酸（GGDP）两步环化生成（-）-贝壳杉烯。甜菊醇的生物合成可能采用相同的机制，尽管甜叶菊中参与GGDP环化的基因和酶尚未得到表征。我们从甜叶菊中分离出了CPS和KS基因，发现重组CPS和KS具有催化活性，这表明CPS和KS基因参与了甜菊醇的生物合成。编码CPS和KS的基因在大多数植物物种中通常以单拷贝形式存在，它们的表达通常较低，且仅限于快速生长的组织。KS基因在甜叶菊基因组中发生了复制，并且KS和CPS基因在成熟叶片中均高度表达，这与GA生物合成的模式相反。这种模式可能至少部分导致了GA和甜菊醇生物合成在时间和空间上的分离，并且可能有助于防止过表达干扰正常的GA代谢。我们的结果表明，CPS和KS是甜菊糖苷生物合成途径的一部分，并且甜叶菊已经从参与激素生物合成的高度调控途径中招募了两个基因用于次生代谢。
