Suzuki Hideyuki, Achnine Lahoucine, Xu Ran, Matsuda Seiichi P T, Dixon Richard A
Plant Biology Division, The Samuel Roberts Noble Foundation, 2510 Sam Noble Parkway, Ardmore, OK 73401, USA.
Plant J. 2002 Dec;32(6):1033-48. doi: 10.1046/j.1365-313x.2002.01497.x.
The saponins of the model legume Medicago truncatula are glycosides of at least five different triterpene aglycones: soyasapogenol B, soyasapogenol E, medicagenic acid, hederagenin and bayogenin. These aglycones are most likely derived from beta-amyrin, a product of the cyclization of 2,3-oxidosqualene. Mining M. truncatula EST data sets led to the identification of sequences putatively encoding three early enzymes of triterpene aglycone formation: squalene synthase (SS), squalene epoxidase (SE), and beta-amyrin synthase (beta-AS). SS was functionally characterized by expression in Escherichia coli, two forms of SE by complementation of the yeast erg1 mutant, and beta-AS by expression in yeast. Beta-amyrin was the sole product of the cyclization of squalene epoxide by the recombinant M. truncatulabeta-AS, as judged by GC-MS and NMR. Transcripts encoding beta-AS, SS and one form of SE were strongly and co-ordinately induced, associated with accumulation of triterpenes, upon exposure of M. truncatula cell suspension cultures to methyl jasmonate. Sterol composition remained unaffected by jasmonate treatment. Molecular verification of induction of the triterpene pathway in a cell culture system provides a new tool for saponin pathway gene discovery by DNA array-based approaches.
大豆皂醇B、大豆皂醇E、苜蓿酸、常春藤皂苷元及羟基常春藤皂苷元。这些苷元很可能源自β-香树脂醇，即2,3-氧化角鲨烯环化的产物。对蒺藜苜蓿EST数据集的挖掘使得能够鉴定出可能编码三萜苷元形成过程中三种早期酶的序列：鲨烯合酶（SS）、鲨烯环氧酶（SE）和β-香树脂醇合酶（β-AS）。通过在大肠杆菌中表达对SS进行功能表征，通过对酵母erg1突变体进行互补对两种形式的SE进行功能表征，以及通过在酵母中表达对β-AS进行功能表征。通过气相色谱-质谱联用（GC-MS）和核磁共振（NMR）判断，重组蒺藜苜蓿β-AS催化鲨烯环氧物环化的唯一产物是β-香树脂醇。当蒺藜苜蓿细胞悬浮培养物暴露于茉莉酸甲酯时，编码β-AS、SS和一种形式SE的转录本被强烈且协同诱导，这与三萜类化合物的积累相关。甾醇组成不受茉莉酸处理的影响。在细胞培养系统中对三萜途径诱导进行分子验证，为通过基于DNA阵列的方法发现皂苷途径基因提供了一种新工具。
