National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Ibaraki 305-8604, Japan
This study reports the mode of action of acibenzolar-S-methyl (ASM) against Japanese pear scab, caused by Venturia nashicola. Pretreatment of potted Japanese pear trees with ASM reduced scab symptoms and potentiated several lines of plant defense response. This included transcripts encoding polygalacturonase-inhibiting protein (PGIP) that were highly and transiently promoted after scab inoculation of plants pretreated with ASM, suggesting a possible role for defenses involved in direct interaction with the pathogen. The activity of the key enzyme of phenylpropanoid pathway, phenylalanine ammonia lyase (PAL), was enhanced in scab-inoculated leaves pretreated with ASM only 7 days after inoculation, suggesting that it may play a minor role in induced resistance. In this work, salicylic acid (SA) accumulation was enhanced in ASM-treated leaves for the first time, according to an equivalent time course to that of PAL activity. However, a delayed induction of SA accumulation in ASM-treated leaves compared with kinetics of induction of several pathogenesis- related (PR) proteins or their encoding genes suggested that resistance triggered by ASM may be SA-independent. Among these PR proteins, PR-1, chitinase and PR-10 were promoted early by ASM after scab inoculation. Peroxidase, as well as enzymes involved in the oxidative burst such as superoxide dismutase, catalase, and ascorbate peroxidase were weakly activated with ASM treatment alone or pathogen inoculation alone and highly enhanced in ASM pretreated plants upon challenge inoculation, suggesting the occurrence of priming phenomenon during the interaction of Japanese pear-ASM-V. nashicola. An early potentiation of the activity of these enzymes after scab inoculation of leaves pretreated with ASM suggested that active oxygen species may be involved as a signal for the activation of PR proteins or genes.