The plant defence activator acibenzolar-S-methyl primes cowpea [Vigna unguiculata (L.) Walp.] seedlings for rapid induction of resistance

Cowpea [Vigna unguiculata L. Walp.] seedlings. raised front weds treated wi th acibenzolar-S-methyl [benzo(1,2,3) thiadiazole-7-carbothioic acid S-ethy l ester BTH], were inoculated at 7 days old with Colletotrichum destructivu m. Tissue penetration as reduced markedly and intracellular infection vesic les were invariable, - restricted to the initially-infected epidermal cells of treated hypocotyls and leaves. The destructive necrotrophic phase of di sease development was effectively blocked by a hypersensitive response in t hese cells, thereby protecting seedlings against damping-off. The enhanced resistance of BTH-treated tissues was associated with rapid, transient incr eases in the activities of two key enzymes of the pathway phenylaianine amm onia-lyase (PAL) and and chalcone isomerase (CHI). Subsequently, there was an early, accelerated accumulation of the isoflavonoid pliN-toalexins kievi tone and phaseollidin in treated hypocotyls. In addition, several protein b ands, in the low-molecular weight range, developed in these treated challen ged tissues. These following inoculation of a normally susceptible cultivar (IT82E-60) with the pathogen, and were not observed in induced. uninoculat ed tissues, These results Suggest that BTH protects cowpea seedlings by pot entiating an carly defence response rather than by altering the constitutiv e resistance of tissues. (C) 2001 Academic Press.