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Some aspects of phenolic metabolism in healthy and rust infected flax cotyledons Lam, Tung Hoi

Abstract

Phytochemical and enzymatic experiments were conducted to study the metabolism of phenolic compounds in the cotyledons of flax (Linum usitatissimum L. ‘Koto’) infected with strains #3. and #210 of flax rust (Melampsora lini Pers. Lév.). The primary objective was to further the understanding of the role of phenolic compounds in the host-parasite relationship with respect to resistance and susceptibility. The phenolic constituents of flax include about 14 esters and glycosides of cinnamic acids, viz., p-coumaric, caffeic, ferulic and sinapic acids, and 8 glycosides of flavones, 4 of which are of the apigenin-type and 4 of the luteolin-type. Most of the cinnamic acid derivatives have a free hydroxyl group and would therefore be good substrates for oxidation. Except for an initial drop, the total soluble phenolic content in infected resistant tissue was always higher than in the healthy control or in infected susceptible tissue. This quantitative change in phenolic content after infection supports the involvement of phenolics in resistance. Tracer studies showed that the metabolism of phenylalanine in flax follows the order cinnamic → p-coumaric → caffeic → ferulic acids. There was no qualitative change in the pathway of phenylalanine metabolism after infection. The incorporation of phenylalanine-U-(14)C into phenolic compounds was higher in the resistant combination than in the healthy control or the susceptible combination. The resistant reacting tissue also showed the highest conversion of monohydric phenols into dihydric phenols. On the other hand, incorporation of phenylalanine-U-(14)C into protein was highest in the susceptible combination. There was a higher accumulation of radioactivity from phenylalanine-U-(14)C into ethanol-insoluble, non-proteinaceous material around the lesions in the resistant than in the susceptible combination. These findings are in agreement with the hypothesis that, after infection, there is an enhanced flow of aromatic amino acids into protein synthesis in the susceptible tissue whereas in the resistant reacting tissues there is a shift in favour of phenolic metabolism. The enhancement of phenylalanine ammonia-lyase by as much as 5-fold in the resistant tissue at 2 days after inoculation also supports the above hypothesis. The activities of peroxidase, polyphenol oxidase and β-glucosidase were also enhanced in the resistant combination, whereas in the susceptible combination polyphenol oxidase and β-glucosidase activities were lower than in the healthy control. There was a sequential enhancement of phenylalanine ammonia-lyase, total soluble phenolic content and polyphenol oxidase in the resistant reacting tissue. These results suggest that oxidation of phenolic compounds is important for resistance and that the suppression of the oxidative enzyme, polyphenol, oxidase, may be essential for the survival of the pathogen in this biotroph-host combination. The evidence suggests that phenolic metabolism plays an important role in resistance and susceptibility in host-parasite relations. It is very likely that phenolic compounds and their oxidative products only execute the job of resistance. The triggering mechanism for the enhancement of phenolic metabolism, which remains unknown, and the mechanisms by which phenolic metabolites act against the pathogen are discussed.

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