Benzothiadiazole (BTH) induces cell-death independent resistance in Phaseolus vulgaris against Uromyces appendiculatus

Benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), trade name Bion(R), was used to induce resistance in bean cultivars Borlotto Nano Lingua di Fuoco (BLF), Borlotto Taylor, Cannellino, Cannellino Montalbano, Saxa and Top Crop, against rust caused by Uromyces appendiculatus. A single 0.3 mm BTH spray 7 days before inoculation was sufficient to fully control the disease in all the examined cultivars. As regards the more susceptible BLF, either a single treatment 14 days before inoculation, or three applications on the third, fifth and seventh day before inoculation, were equally effective to prevent infection. Histochemical and cytochemical investigations showed that BTH causes hydrogen peroxide (H2O2) accumulation in the treated tissues. H2O2 deposits were localized in situ for the first time in the apoplast of the leaf epidermis. No cell death was detected at BTH concentrations below the phytotoxicity threshold, suggesting that acquired resistance against bean rust is mainly related to the enhanced activity of anionic peroxidases, promoted by H2O2 accumulation, thereby leading to cell wall strengthening. This hypothesis is also supported by the long induction phase required to establish complete resistance.