Inhibition of phenylalanine ammonia-lyase suppresses the resistance induced by benzothiadiazole in wheat to Blumeria graminis f. sp tritici

Benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH) induced systemic resistance in the susceptible wheat cv. Monopol to Blumeria graminis f. sp. tritici (Bgt). Wheat leaves were inoculated with Bgt 15 days after the treatment with BTH, and the resistance responses were investigated at early infection stages. BTH enhanced the autofluorescence localized at the attempted penetration sites (halo and papilla regions) 18 h post inoculation (hpi) and clearly decreased the penetration efficiency of the fungus. While the activities of the enzyme phenylalanine ammonia-lyase (PAL) and peroxidase were higher in BTH-treated wheat than in untreated controls, the activity of cinnamyl alcohol dehydrogenase (CAD) remained unmodified. Cell wall-bound phenolic compounds (i.e. coumaric and ferulic acids) were higher in BTH treated plants 18 hpi than in the controls, whereas lignin content was not altered. Soluble phenolics were slightly increased by inoculation but were not significantly influenced by BTH treatment. Inhibition of PAL by AOPP (alpha-aminooxy-beta-phenylpropionic acid) reduced the localized accumulation of autofluorogenic compounds and suppressed the resistance induced by BTH. In contrast, the inhibition of CAD did not cause any modification in resistance as well as autofluorescence. The results suggested that phenylpropanoid compounds, possibly phenolic acids esterified to cell wall, are more rapidly synthesized in response to powdery mildew infection of BTH-treated wheat plants. Therefore, a rapid accumulation of phenolic compounds at the Bgt-attempted penetration sites may be involved in the resistance induced by BTH. (C) 2000 Academic Press.