Biosynthesis of Isoflavonoid Phytoalexins in Pea

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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.