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Bretagne Biotechnologie Ve  ge  tale, BBV, Penn-ar-prat, F-29250 Saint Pol-de-Le Âon, France Acibenzolar-S-methyl (ASM), a member of the benzothiadiazoles (BTH), was tested on cauliflower (Brassica oleracea var. botrytis) seedlings for its ability to induce resistance against downy mildew (Peronospora parasitica). Seven-dayold seedlings of the susceptible cultivar Billabong, sprayed with solutions containing 0´015 or 0´075 mg a.i. mL 21 ASM and inoculated 1±8 days later with P. parasitica, were assessed for disease 7 days after inoculation. Sporulation was reduced by 50% at the highest dose of ASM in plants inoculated 1 day later, with complete protection exhibited by plants inoculated 2 days after ASM treatment. Seedlings treated with 0´05 mg a.i. mL 21 ASM solution, inoculated 4 days later and harvested 0±7 days later, were analysed for pathogenesis-related (PR) proteins, chitinase and b-1,3 glucanase. ASM significantly induced b-1,3 glucanase activity which increased with time in inoculated seedlings, as confirmed by the presence of PR-2. Chitinase activity was not significantly induced by ASM, and the treated seedlings also did not accumulate the basic PR-3C and the acidic PR-3Q which both exhibit chitinase activity. Analysis of three other acidic (PR-1C, PR-5S, PR-8) and one basic (PR-6) PR proteins in the ASM-treated seedlings showed that only PR-1 and PR-5 were slightly and slowly induced (4±5 days after treatment), but this induction was more pronounced after inoculation with P. parasitica.

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