Kitaoka Naoki, Wu Yisheng, Zi Jiachen, Peters Reuben J
Roy J. Carver Department of Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, IA, 50011, USA.
Plant J. 2016 Oct;88(2):271-279. doi: 10.1111/tpj.13249. Epub 2016 Sep 1.
Rice (Oryza sativa) produces a variety of labdane-related diterpenoids as phytoalexins and allelochemicals. The production of these important natural products has been partially elucidated. However, the oxidases responsible for production of the keto groups found in many of these diterpenoids have largely remained unknown. Only one short-chain alcohol dehydrogenase/reductases (SDRs), which has been proposed to catalyze the last step in such a pathway, has been characterized to date. While rice contains >220 SDRs, only the transcription of five has been shown to be induced by the fungal cell wall elicitor chitin. This includes the momilactone A synthase (OsMAS/SDR110C-MS1), with the other four all falling in the same SDR110C family, further suggesting roles in diterpenoid biosynthesis. Here, biochemical characterization with simplified substrate analogs was first used to indicate potential functions, which were then supported by further analyses with key biosynthetic intermediates. Kinetic studies were then employed to further clarify these roles. Surprisingly, OsSDR110C-MS2 more efficiently catalyzes the final oxidation to produce momilactone A that was previously assigned to OsMAS/SDR110C-MS1, and we speculate that this latter SDR may have an alternative function instead. Conversely, two of these SDRs clearly appear to act in oryzalexin biosynthesis, with OsSDR110C-MI3 readily oxidizing the 3α-hydroxyl of oryzalexin D, while OsSDR110C-MS3 can also oxidize the accompanying 7β-hydroxyl. Together, these SDRs then serve to produce oryzalexins A-C from oryzalexin D, essentially completing elucidation of the biosynthesis of this family of rice phytoalexins.
水稻（Oryza sativa）会产生多种与半日花烷相关的二萜类化合物，作为植保素和化感物质。这些重要天然产物的合成过程已部分阐明。然而，许多此类二萜类化合物中酮基生成所涉及的氧化酶在很大程度上仍不为人知。迄今为止，仅鉴定出一种短链醇脱氢酶/还原酶（SDR），有人提出它催化该途径的最后一步反应。虽然水稻含有超过220种SDR，但只有5种的转录已被证明受真菌细胞壁激发子几丁质诱导。这其中包括稻瘟菌素A合酶（OsMAS/SDR110C-MS1），其他4种都属于同一个SDR110C家族，这进一步表明它们在二萜类生物合成中发挥作用。本文首先使用简化的底物类似物进行生化特性分析以表明潜在功能，随后通过对关键生物合成中间体的进一步分析加以佐证。接着利用动力学研究进一步阐明这些作用。令人惊讶的是，OsSDR110C-MS2能更有效地催化最终氧化反应生成稻瘟菌素A，而此前认为该反应由OsMAS/SDR110C-MS1催化，我们推测后一种SDR可能具有其他功能。相反，其中两种SDR显然在稻抗霉素生物合成中起作用，OsSDR110C-MI3能轻易氧化稻抗霉素D的3α-羟基，而OsSDR110C-MS3也能氧化其伴随的7β-羟基。这些SDR共同作用，从稻抗霉素D生成稻抗霉素A-C，基本完成了对这一类水稻植保素生物合成的阐明。
