Phytoalexins families

Recently, a new polyketide biosynthetic pathway in bacteria that parallels the well studied plant PKSs has been discovered that can assemble small aromatic metabolites.8,9 These type III PKSs10 are members of the chalcone synthase (CHS) and stilbene synthase (STS) family of PKSs previously thought to be restricted to plants.11 The best studied type III PKS is CHS. Physiologically, CHS catalyzes the biosynthesis of 4,2, 4, 6 -tetrahydroxychalcone (chalcone). Moreover, in some organisms CHS works in concert with chalcone reductase (CHR) to produce 4,2 ,4 -trihydroxychalcone (deoxychalcone) (Fig. 12.1). Both natural products constitute plant secondary metabolites that are used as precursors for the biosynthesis of anthocyanin pigments, anti-microbial phytoalexins, and chemical inducers of Rhizobium nodulation genes.12... 

Indole-3-acetaldoxime (30) and 4-hydroxyphenylacetaldoxime were shown to be metabolized by plant and pest fungi to 32 and to other related indole- and 4-hydroxyphenyl carboxylic acids, however, the biochemical transformation differed between the two fungi. These biochemical transformations may be relevant to the ability of certain fungi to cause plant diseases . Compound 30 has been shown to be a key intermediate in the biosynthesis of camalexin, 3-thiazolyl-2 -yl-indole, a member of the family of phytoalexins that are produced in response to pathogen attack. It was demonstrated that CYP71A13 catalyzes the conversion of 30 to indole-3-acetonitrile, which is essential for the biosynthesis of camalexin. Thus, the literature supplies a large body of evidence for the existence of a set of enzymes dedicated to the production of the auxin, 32, from tryptophan via indole-3-acetaldoxime (30). 

In the family of the Gramineae, which includes some of man s most important crops, active lignification seems to be of special importance for induced resistance mechanisms (19,20). This may be correlated with the nearly complete absence of phytoalexins in this family (21). In spite of an intensive search for such infection-induced fungitoxic substances, no phytoalexins have been found in wheat to date (22). Nevertheless, induced lignification has been shown to play an important role in disease resistance of wheat against a variety of fungal pathogens (4) ... 

The phytoalexin elicitor (PE) family of /1-D-glucans (Fig. 5) was selected as a target because these oligosaccharides had been synthesized previously in solution48,49 and on the solid support.50,51 It was our intention to investigate not only the application of glycosyl phosphates to automated solid-phase synthesis, but also to compare our method to the previously described reports of PE syntheses. 

The pterocarpan pisatin was first isolated and characterized as a phytoalexin in Pi sum sativum in the early 1960s.234 235 Since then, structurally diverse phytoalexins have been reported from more than 30 families of higher plants. Much progress is also being made in studies on phytoalexin biosynthesis based on genomic information. 

The acid fragmentation of suitably substituted bicyclo[2.2.2]octanes to cyclohexenones and subsequent ring closure to spirovetivanes via ir-cation cyclization has been used by Murai for a highly stereoselective isolubimin (56) synthesis. (56) has been considered to play an important role in the bio-genetic pathway of various spirovetivane phytoalexins in the Solanaceae family. Treatment of methyl... 

As shown in the following scheme, a SEM-ether derived phenyl acetylene was treated with PtClj in the presence of CO to give a benzo[4>]furan, which was first subjected to desilylation, followed by Pd-catalyzed intramolecular etherification, affording the tetracyclic skeleton of pterocarpane family of phytoalexins <05JA15024>. A similar type of synthetic transformation was so applied to make a key intermediate in the total synthesis of vibsanol <05JA15022>. 2,3-Disubstituted benzo[h]furans were prepared under very mild reaction conditions by the Pd/Cu-catalyzed cross-coupling of various o-iodoanisoles and terminal... 

The known phytoalexins now number at least 200 chemical structures, over half of which have been characterized from a single family, the Leguminosae. A significant proportion of these structures are uniquely produced in this way and are not otherwise known as natural products. 

Most phytoalexins can be classified as either masked pheno-lics (i8oflavonoids, stllbenes, phenanthrenes, benzfurans) or as terpenoids. Fatty acid-derived polyacetylenes have been recorded in at least four plant families and nitrogen-containing phytoalexins have been described recently from the Caryophyllaceae and Gramlneae. 

Phytoalexins have been characterized from about 20 plant families, ranging from monocots to dlcots and from trees to herbs (Table 1). 

The family most intensively investigated for phytoalexins is the Legumlnosae. Well over 500 species have now been screened at Reading, using the drop-diffusate technique (9 10). This is convenient in that it provides a phytoalexin solution, free from other plant constituents. The disadvantage is that only milligram amounts are available for Identification. However, modern methods of separation and spectral analysis usually operate successfully on this scale. 

The main point to observe is that virtually all species tested gave a positive response. There is thus little doubt that phytoalexin synthesis is a universal attribute in this family. Even in the genus Luplnus, which has been shown to have considerable numbers of antifungal isopentenylisoflavones on the plant surface (11, 12), phytoalexin Induction could be detected (10). 

Yet another family producing polyacetylenes is the Composltae two acetylenics safynol and dehydrosafynol are formed in the diseased safflower (24). Our own experiments at Reading Indicate that acetylenes are formed In other members. This is however a very large family and other responses have been detected as well. TVo sesquiterpene lactones have been encountered as phytoalexins In Lac-tuca (25) while coumarins are reported as such In the sunflower Hellanthus annuus (26) ... 

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