Biosynthesis of Isoflavonoid Phytoalexins in Pea

植物经常合成抗生素化合物(植物抗毒素)以应对感染，目前人们正致力于研究其生物合成的调控。这项研究考察了豌豆植物保卫素、(+)pisatin和(-)maackiain生物合成途径中的一些末端的植物保卫素特异性反应。产生该途径的第一个手性中间体的异黄酮还原酶已经得到纯化，并正在进行表征。其他将被研究的酶是执行Maackiain合成的最后一步的蝶形酮合成酶和6‘-脱氧查尔酮合成酶，它是胁迫诱导的类黄酮途径分支的第一个酶。之前人们认为pisatin合成的倒数第二步是加氧酶反应，但这一假设与最近的发现不一致。为了解决这个问题，这个项目需要做的工作包括活体标记实验和酶研究。针对纯化酶的抗体正被用于获得相应的cdna克隆。这种方法提供了有关途径酶的特异性如何决定豆科植物抗毒素之间存在的化学多样性的信息。这些酶的克隆基因最终可以通过改变转基因植物的植物保卫素补体来进一步评估植物保卫素在抗病中的作用。众所周知，许多植物在受到病原体侵染时会合成植物防御素。人们经常提出，这些抗菌化合物的产生代表了一种有效的防御潜在病原体的机制。然而，支持这一假设的直接证据一直很难获得。最近对豌豆植物和一种导致豌豆病害的真菌进行的研究表明，植物保卫素在确定植物抗性方面发挥着重要作用。本研究的目的是检测豌豆中植物抗毒素的生物合成。这些发现有助于解决有关生物合成途径的调节和植物保卫素在抗病中的作用等有趣的问题。这项工作的未来意义是提高植物抗毒素防御反应有效性的新策略。

Plants often synthesize antibiotic compounds (phytoalexins) in response to infection, and considerable effort is currently being devoted to research on regulation of their biosynthesis. This research examines some of the terminal, phytoalexin-specific reactions in the biosynthetic pathway for the pea phytoalexins, (+)pisatin and (-)maackiain. The isoflavone reductase which produces the first chiral intermediate of the pathway has been purified and is being characterized. Other enzymes to be studied are pterocarpan synthase, which performs the terminal step of maackiain synthesis, and 6'-deoxychalcone synthase, the first enzyme of the stress-induced branch of the flavonoid pathway. The penultimate step in pisatin synthesis was previously thought to be an oxygenase reaction, but this hypothesis is not consistent with recent findings. Work to be done on this project to resolve this question includes labeling experiments in vivo and enzyme studies. Antibodies against the purified enzymes are being used to obtain corresponding cDNA clones. This approach provides information about how the specificities of the pathway enzymes determine and chemical diversity that exists among legume phytoalexins. Cloned genes for these enzymes can eventually allow further evalation of the role of phytoalexins in disease resistance by modifying the phytoalexin complement of transgenic plants. It is well-established that many plant species synthesize phytoalexins in response to infection by pathogens. It is often proposed that the production of these antimicrobial compounds respresents an active mechanism of defense against potential pathogens. However, direct evidence supporting this hypothesis has been difficult to obtain. Recent work carried out with pea plants and a fungus that causes disease in pea argues that phytoalexins can play an important role in the determination of plant resistance. The goal of this reaearch is to examine phytoalexin biosynthesis in pea. Such findings are useful in addressing interesting questions concerning the regulation of the biosynthetic pathway and the role of phytoalexins in disease resistance. Future implications of this work are novel strategies for enhancing the effectiveness of the phytoalexin defense response.