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

Abstract

Cowpea [ Vigna unguiculata(L.) Walp.] seedlings, raised from seeds treated with acibenzolar-S-methyl [benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester; BTH], were inoculated at 7 days old withColletotrichum destructivum . Tissue penetration was reduced markedly and intracellular infection vesicles were invariably 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 phenylpropanoid/flavonoid pathway, phenylalanine ammonia-lyase (PAL) and chalcone isomerase (CHI). Subsequently, there was an early, accelerated accumulation of the isoflavonoid phytoalexins kievitone and phaseollidin in treated hypocotyls. In addition, several protein bands, in the low-molecular weight range, developed in these treated challenged tissues. These responses occurred 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 early defence response rather than by altering the constitutive resistance of tissues.