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 with acibenzolar-S-methyl [benzo(1,2,3) thiadiazole-7-carbothioic acid S-ethyl ester BTH], were inoculated at 7 days old with Colletotrichum destructivum. Tissue penetration as reduced markedly and intracellular infection vesicles were invariable, - restricted to the initially-infected epidermal cells of treated hypocotyls and leaves. The destructive necrotrophic phase of disease development was effectively blocked by a hypersensitive response in these cells, thereby protecting seedlings against damping-off. The enhanced resistance of BTH-treated tissues was associated with rapid, transient increases in the activities of two key enzymes of the pathway phenylaianine ammonia-lyase (PAL) and and chalcone isomerase (CHI). Subsequently, there was an early, accelerated accumulation of the isoflavonoid pliN-toalexins kievitone and phaseollidin in treated hypocotyls. In addition, several protein bands, in the low-molecular weight range, developed in these treated challenged tissues. These following inoculation of a normally susceptible cultivar (IT82E-60) with the pathogen, and were not observed in induced. uninoculated tissues, These results Suggest that BTH protects cowpea seedlings by potentiating an carly defence response rather than by altering the constitutive resistance of tissues. (C) 2001 Academic Press.