The growth rates of Manduca sexta (tobacco hornworm) larvae feeding on tomato plants constitutively expressing a prosystemin antisense gene were approximately 3 times higher than growth rates of larvae feeding on nontransformed control plants. The levels of proteinase inhibitor I and inhibitor II proteins in leaves of tomato plants expressing the antisense prosystemin gene remained at undetectable levels until the sixth day of larval feeding and then increased throughout the plants to 100-125 microg/g of leaf tissue after 14 days. In control plants, levels of proteinase inhibitor I and II proteins increased rapidly from the second day of larval feeding and by the eighth day contained levels of 225 microg/g of leaf tissue and 275 microg/g of leaf tissue, respectively, and then increased slowly thereafter. Prosystemin mRNA levels in antisense and control plants after 6 days and 12 days of larval feeding correlated with levels of inhibitor I and II protein levels. These experiments demonstrate that resistance of plants toward an insect pest can be modulated by genetically engineering a gene encoding a component of the inducible systemic signaling system regulating a plant defensive response.