Signal interactions in pathogen and insect attack: systemic plant-mediatedinteractions between pathogens and herbivores of the tomato, Lycopersicon esculentum

plant-mediated interactions (i.e., induced resistance) between plant pathog ens and insect herbivores were investigated using several pests of the cult ivated tomato, Lycopersicon esculentum. Single leaflets of tomato leaves we re injured by allowing a third-instar Helicoverpa zea larva to feed on the leaflets or by inoculating the leaflets with Pseudomonas syringae pv. tomat o (the causal agent of bacterial speck in tomato; Pst) or with Phytophthora infestans (che causal agent of late blight). Leaflets on separate plants w ere sprayed with benzothiadiazole, a chemical inducer of resistance to Pst. The effects of these treatments on the resistance of uninoculated or undam aged leaflets to both Pst and H. zea were then assessed after appropriate p eriods of time. The levels or activities of several defense-related protein s were determined in parallel. Infection of leaflets by Pst decreased the s uitability of uninoculated leaflets of the same leaf for both H. tea and fo r Pst. Similarly, feeding by H, tea caused leaf-systemic increases in resis tance to both H. tea and Pst. Infection of leaflets by P, infestans, in con trast, had no effect on resistance of leaflets to H. tea. Treatment of leav es with benzothiadiazole induced resistance to Pst but improved suitability of leaflets for H. tea. Feeding by H, eea caused the systemic accumulation of proteinase inhibitor mRNA and the systemic induction of polyphenol oxid ase activity; in contrast, treatment with benzothiadiazole and inoculation with P, infestans caused the systemic accumulation of pathogenesis-related protein mRNA and the systemic induction of peroxidase activity. Inoculation of leaflets with Pst caused the leaf-systemic accumulation of both pathoge nesis-related protein and proteinase inhibitor mRNA and the systemic induct ion of both peroxidase and polyphenol oxidase activity. These results provi de clear evidence for reciprocal induced resistance involving certain patho gens and arthropod herbivores of tomato. In addition, these results provide several insights into the integration and coordination of the induced defe nses of tomato against multiple pests and suggest that the expression of re sistance against some pests may compromise resistance to others. (C) 1999 A cademic Press.