Phytoalexin properties

This protocol could be extended to a range of different ,/i-unsaturated carbonyl compounds and either activated or deactivated aryl iodides [22], An application of related Heck chemistry to the synthesis of methylated resveratrol (3,4, 5-trihydroxy-( )-stilbene) is shown in Scheme 6.4 [23]. The phytoalexin resveratrol exhibits a variety of interesting biological and therapeutic properties, among them activity against several human cancer cell lines. Botella and Najera have shown that the trimethyl ether of resveratrol (Scheme 6.4) can be rapidly prepared by microwave-assisted Heck reaction of the appropriate aryl iodide and styrene derivatives, using the same oxime-derived palladacycle as indicated in Scheme 6.3. 

Welle, R. and Grisebach, H., Properties and solubilization of the prenyltransferase of isoflavonoid phytoalexin biosynthesis in soybean. Phytochemistry, 30, 479, 1991. 

Specific Control of Phytoalexin Accumulation by "Metabolite Shunting" of Biosynthetic Pathways. Graham and coworkers (personal communication), at the Monsanto Laboratories, St. Louis, have developed techniques to selectively shunt defensive metabolites, particularly of the shikimic acid cycle. Through various techniques, certain compounds are applied to plant aerial or root parts, and these compounds have the property of inducing specific accumulations of secondary metabolites. The directions of these accumulations are under known enzymic control (48), and the regulation of these enzymes is achieved by selecting appropriate inducers. Such inducers seem to provide a novel approach to the control of insects by magnifying the ability of plants to produce and concentrate antiherbivory compounds. 

Plants have envolved a wide array of chemical defences against pathogens. These include secondary metabolites with anti-microbial properties [57]. Some secondary metabolites are constitutively present in normally developed healthy plants, whereas others are induced by pathogen invasion [58]. The latter include phytoalexins, which are synthesized de novo after a pathogen attack [43], and phytoanticipins [59]. 

Cruickshank, I. A.M., Perrin, D.R. Studies on phytoalexins. Ill The isolation, assay, and general properties of a phytoalexin from Pisum sativum L. Aus J Biol Sci 1961 14 336 348. 

Microsomal preparations from yeast-elicited cell cultures of chickpea catalyzed the hydroxylation of formononetin and biochanin A to their 2 - and 3 -hydroxy derivatives.43 44 Neither daidzein nor genistein were accepted as substrates. Both hydroxylation reactions seem to be catalyzed by two distinct enzymes, since they exhibited different physicochemical properties and induction kinetics in cell cultures and roots of chickpeas.45 Both 2 - and 3 -hydroxylations of formononetin are prerequisite reactions in the pathway for biosynthesis of the phytoalexins, medicarpin and maackiain in alfalfa and chickpea, respectively8. 

Aliette (aluminum tris-0 ethylphosphonate) has been reported to enhance defense reactions and phytoalexin accumulation in grapes and tomatoes in response to infection by Plasmopara viticola and Phytophthora spp., respectively, and to trigger phenolic accumulation and hypersensitive cell death in tomatoes, peppers, and beans in response to infection while possessing little direct fungitoxicity (116,117). However, recent data cast doubt on the earlier reports of the low activity of Aliette as an inhibitor of Phytophthora sporulation mi vitro (118), and have attributed the protective properties of the compound to phosphorous acid which is formed in plant tissues or in certain buffer solutions of Aliette (119,120). Toxicity of phosphorous acid to Oomycetes is reversible by phosphate ion, and this may explain Aliette s lack of fungitoxicity in certain growth media. 

We can distinguish between secondary metabolites that are already present prior to an attack or wounding, so-called constitutive compounds, and others that are induced by these processes and made de novo. Inducing agents, which have been termed elicitors by phytopathologists, can be cell wall fragments of microbes, the plant itself, or many other chemical constituents (4,17,22-24). The induced compounds are called phytoalexins, which is merely a functional term, since these compounds often do not differ in structure from constitutive natural products. In another way this term is misleading, since it implies that the induced compound is only active in plant-microbe interactions, whereas in reality it often has multiple functions that include antimicrobial and antiherbivoral properties (see below). 

Although fungitoxlclty is the most characteristic biological property, phytoalexins may be antibacterial as well. Additionally, phytoalexins of legumes have been shown to be toxic to insects, while those from the sweet potato are poisonous to vertebrates (5J. 

Even if the phytoalexins so far isolated have little commercial utility, it is still possible that useful substances may yet appear. But the more exciting possibility is that consideration of the chemical structures of natural phytoalexins and of their modes of action against fungal infections may provide clues for the development of synthetic pesticides. The complexity of the chemical structures of the natural phytoalexins may make them uneconomical to manufacture but a comparison should be made with the synthetic pyrethroid insecticides. The natural pyrethrums have complex chemical structures but simpler compounds, economical to manufacture, have been developed on the basis of the structures of natural pyrethrums and many of these have much more desirable properties for use in agriculture than the natural substances. There would seem to be no reason why simpler compounds based on the structures of natural phytoalexins should not provide synthetic fungicides as important and useful as the synthetic pyrethroids. This is a future challenge for the synthetic organic chemists in this area. 

Butylisocyanate formed is a very reactive and unstable compound, so that its action is local and of short duration. Butyl isocyanate, as well as butylamine, may contribute to the phytoalexin-inducing properties of benomyl preparations observed by Reilly and Klarman (1972). 

Coevolutionary biochemical interactions of plants with their natural predators could contribute to uncover medicinally useful chemicals such as antioxidants [15]. For example, many flavonoids possess allelochemical properties by warding off microbial or animal pathogens as phytoalexins or as antifeedants, and the phytoalexin response is frequently accompanied by an oxidative burst in the cell [16]. Moreover, it has been demonstrated that root flavonoids, mainly flavones, isoflavones and some... 

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