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. / Land and Food Systems, Faculty of / Graduate

Typescript (photocopy) / iv, 123 leaves, [3] leaves of plates : ill. (part col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Genetics, 1978

The possible biochemical mechanisms of varietal and species specificity of obligate fungal parasites are considered in relation to the established genetics and biochemistry of host-parasite systems. The two general theories of specificity which have been put forward are identified. One invokes the induction of successful pathogenesis as a primary determinant in specificity while the other predicts that recognition of avirulent pathogens by the host leads to induced resistance which acts as the determinant in specificity. The former theory is supported by evidence for the appearance of novel host enzymes during disease development. The latter is supported by observations of host gene derepression and phytoalexin accumulation which occur in resistance responses at times prior to any significant response in susceptible combinations. These theories are examined experimentally in the flax/flax rust system. The molecular origin of ribonuclease with altered catalytic properties, which arises during disease development, is examined. Rigorous purification reveals that the altered catalytic properties can be accounted for by altered proportions of ribonuclease I isozymes. These changes are similar during resistant and susceptible reactions until sporulation occurs. Induced resistance is characterized and compared to primary changes during susceptibility through a detailed study of RNA and protein synthesis. Enhanced RNA synthesis occurs in the resistant reaction at times prior to any measurable response in the susceptible combination, although both combinations exhibit higher rates of RNA synthesis at later times. RNA was fractionated by electrophoresis and affinity chromatography and messenger activity assessed by in vitro translation. Enhanced RNA synthesis is characterized by decreased messenger polyadenylation. However, polyadenylated and non-polyadenylated messenger RNA were shown to encode many common polypeptides; this provides an explanation for the fact that few changes in iri vivo protein synthesis can be detected by one or two dimensional electrophoresis. The only marked changes in protein synthesis occur in the susceptible combination and include a dramatic decline in the synthesis of the large subunit of ribulose-1,5-bisphosphate carboxylase. It is concluded that incompatibility of an avirulent race of rust with the flax variety Bombay is determined by induced resistance resulting from recognition of the avirulent pathogen prior to the initiation of pathogenesis. However, specific biochemical events in the host, particularly those involving chloroplast function, may be necessary for successful pathogen development. / Land and Food Systems, Faculty of / Graduate

Bibliography: 136-146 / v, 146 leaves, [9] leaves of plates : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--Dept. of Agronomy, University of Adelaide, 1987