Characterization of the lignification response in the wheat-Pyrenophora tritici-repentis system
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Lignin, a product of the phenylpropanoid pathway, is produced as part of a defence response in many plants. In the wheat-'Pyrenophora tritici-repentis ' system (a) the time and magnitude of indu tion of Phenylalanine Ammonia Lyase, Caffeic acid 'O'-Methyltransferase, Caffeoyl CoA 'O'-Methyltransferase, Cinnamyl Alcohol Dehydrogenase and Peroxidase enzymes and (b) the accumulation of 'PAL' and ' pox' genes transcripts were measured. The enzymes were assayed in the infected and control (water sprayed) leaf tissues of a resistant (Erik) and a susceptible (Glenlea) cultivar at 0, 24, 48 and 72 h post-inoculation. No difference in the time and magnitude of induction of PAL, COMT, CCOMT, CAD and POX enzymes were observed between Erik and Glenlea, when the activities were expressed on a tissue weight basis and on lesion area. 'PAL' northern analysis indicated the presence of higher levels of a 2.4 Kb ' PAL' gene transcript in the infected leaf tissue of Erik compared to Glenlea at 48 host-inoculation. 'pox' RT-PCR analysis indicated (a) no expression of 'pox1, pox4' genes; (b) a prominent 677 bp 'pox2' PCR product in the infected tissue of both resistant and susceptible cultivars and (c) a 734 bp ' pox3' product only in the infected leaf tissue of Salamouni pox northern analysis indicated a lack of difference in the induction of 1.2 Kb transcript in the infected tissue of Salamouni and Glenlea and a weaker induction of the same transcript in the infected tissue of Erik. These results indicate a flow of the phenylpropanoid and lignin synthesis pathway, leading towards the synthesis of lignin in the infected tissue of both Erik and Glenlea. Hence, in the wheat-'Pyrenophora tritici-repentis' system, lignin synthesised in response to infection may not play a dominant role in providing resistance.