開催日: 2017/09/20 - 2017/09/22
Rice produces an array of labdane-related diterpenoids that function as phytoalexins or/and allelochemicals. The biochemical function of the rice diterpene synthases has been elucidated. The downstream oxygenation reactions have been proposed to be catalyzed by cytochromes P450 (CYPs), with subsequent further oxidation catalyzed by short-chain alcohol dehydrogenases/reductases (SDRs). In this study, we report the CYP and SDR activities which propose the biosynthetic pathway of Oryzalexins and Momilactone A (13). SDRs induced by the elicitor chitin were assayed with the rice diterpenoid biosynthetic intermediates. Two of these SDRs clearly appear to act in oryzalexin biosynthesis, with OsSDR110C-MI3 readily oxidizing the 3α-hydroxyl of Oryzalexin D (8), while OsSDR110C-MS3 can also oxidize the accompanying 7β-hydroxyl. Momilactone A (13) is a rice diterpenoid which is produced from syn-pimaradiene (14). CYP76M8, CYP99A2, CYP99A3, and CYP701A8 have been reported to react with syn-pimaradiene. In addition to these CYPs, an SDR catalyzing the final step in production of Momilactone A (13) has been identified. However, the order in which the CYPs react has been unclear. This is evaluated here using our Escherichia coli metabolic engineering system and in vitro enzyme assays. Various pairs of CYPs were co-expressed with the requisite reductase, in E. coli also engineered to produce syn-pimaradiene (14). syn-Pimaradiene-19,6β-hemiacetal (20) was detected in E. coli culture co-expressing CYP99A3 and CYP76M8, and syn-pimaradiene-3β,6β-diol (16) was detected in E. coli culture co-expressing CYP701A8 and CYP76M8. Moreover, syn-pimaradiene-19,6β-hemiacetal (20) could be further oxidized to the corresponding syn-pimaradien-19,6β-olide (21) by OsSDR110C-MS1. It also was found that CYP701A8 was able to oxidize syn-pimaradien-19,6β-olide (21) to Momilactone A (13).