Trade-offs in plant defense against pathogens and herbivores: A field demonstration of chemical elicitors of induced resistance

Two signaling pathways, one involving salicylic acid and another involving jasmonic acid, participate in the expression of plant resistance to pathogens and insect herbivores. In this study, we report that stimulation of systemic acquired resistance in field-grown tomato plants with the salicylate mimic, benzothiadiazole: (1) attenuates the jasmonate-induced expression of the antiherbivore defense-related enzyme polyphenol oxidase, and (2) compromises host-plant resistance to larvae of the beet armyworm, Spodoptera exigua. Conversely, treatment of plants with jasmonic acid at concentrations that induce resistance to insects reduces pathogenesis-related protein gene expression induced by benzothiadiazole, and partially reverses the protective effect of benzothiadiazole against bacterial speck disease caused by Pseudomonas syringae pv. tomato. We conclude that effective utilization of induced plant resistance to the multiple pests typically encountered in agriculture will require understanding potential signaling conflicts in plant defense responses.