Transcriptome profile of acibenzolar-S-methyl-induced genes in tomato suggests a complex polygenic effect on resistance to Phytophthora infestans

Induced resistance by chemicals such as acibenzolar-S-methyl -ASM (commercialized as Actigard by Syngenta Inc) mimics the biological activation of systemic acquired resistance (SAR). ASM takes the place of salicylic acid (SA) in the SAR signal pathway inducing the same molecular markers and range of resistance. The goal of our work was to understand the downstream molecular events by which ASM confers resistance to Phytophthora infestans in tomatoes. To accomplish this goal we assayed gene expression in ASM-treated plants using a microarray with more than 12,000 tomato ESTs. As many as 300 genes were responsive to ASM. Of these, 117 were detected in most of the biological replications. Basal defense associated genes as well as SAR and disease resistance genes (R-like) involved in induced resistance and effector-triggered immunity were highly expressed. We attempted to determine the phenotype of 13 of these genes by virus induced gene silencing (VIGS). These 13 genes were selected on the basis of previous implication in plant defense response and by reliability of induction by ASM. VIGS was partially successful for three of the 13 genes, but this partial silencing did not lead to a significant reduction in the effect of ASM. The ethylene pathway was also activated in response to ASM, but a tomato mutant not responsive to ethylene remained responsive to ASM. It seems most likely that the ASM effect is complex and polygenic, depending on the effect of several genes.

► ASM – a salicylic acid analog – confers resistance to tomato against P. infestans.
► Using a microarray we found that as many as 300 genes are responsive to ASM in tomato.
► Biotic, abiotic, redox and ethylene pathways are induced in ASM-treated tomatoes.
► Likely, ASM effect is complex and polygenic, depending on the effect of several genes.