Accumulation of Autofluorogenic Compounds at the Penetration Site of Blumeria graminis f. sp. tritici is Associated with Both Benzothiadiazole-induced and Quantitative Resistance of Wheat

The effect of benzothiadiazole-7-carbothioic acid S-methyl ester (BTH; Bion®) on the autofluorescence responses of adaxial epidermal cells, activity of phenylalanine ammonia-lyase (PAL) as well as fungal penetration efficiency were investigated after inoculation of the wheat cultivars Monopol (susceptible) and Zentos (resistant) with Blumeria graminis f. sp. tritici (Bgt). The frequency and intensity of autofluorescence at the Bgt-attempted penetration site were higher in epidermal cells of cv. Zentos than in those of the cv. Monopol. In both cultivars, foliar application of the resistance inducer BTH caused an intensification of the autofluorescent responses localized below the Bgt-appressoria (in papilla and halo). The frequency of Bgt-attacked epidermal cells showing whole-cell autofluorescence (hypersensitive cell death) was low in both cultivars and slightly enhanced by BTH only in cv. Monopol. Two peaks of PAL-activity were detected. The first occurred 4 hai coinciding with primary germ tube formation and the second at 12 hai during appressorium formation. BTH caused a significant increase of the PAL-activity at 12 and 18 hai in cv. Monopol. Increase in PAL-activity was closely associated with enhanced localized autofluorescence, suggesting that the phenomena are correlated. BTH-treatment markedly decreased the penetration efficiency of the powdery mildew fungus in cv. Monopol, but not in cv. Zentos which has high penetration resistance. The results suggest that enhanced PAL-activity and synthesis of autofluorogenic compounds, probably of phenolic nature, are involved in quantitative resistance and in BTH-induced defence mechanisms of wheat plants where they act to inhibit penetration of attacked cells.