Plant resistance to insects is governed in large part by chemical and morphological traits that negatively affect the feeding, growth, and reproduction of the attacking herbivore. Relatively little is known about the identity of specific genes involved in host resistance, or the underlying signal transduction pathways that regulate the expression of defense attributes. We are using cultivated tomato (Lycopersicon esculentum) as a molecular genetic system to address these questions. We isolated a collection of mutants that are defective in wound-induced expression of two anti-nutritive defensive proteins, namely protease inhibitor II and polyphenol oxidase. Positional cloning and biochemical studies indicate that several genes defined by this mutational analysis are involved in the biosynthesis or perception of jasmonic acid (JA), a fatty acid-derived plant stress signal. Insect bioassays showed that JA-deficient mutants were compromised in resistance to both chewing insects (e.g., Manduca sexta) and cell-content feeders (e.g., Frankliniella occidentalis). Consistent with this finding, transgenic plants with enhanced capacity for JA biosynthesis exhibited increased resistance to herbivores. Studies conducted with the two-spotted mite (Tetranychus urticae) showed that herbivore behavior and reproductive fitness were strongly influenced by the host JA signaling pathway. Interestingly, mutants abrogated in JA perception were defective in the development of glandular trichomes that have been implicated in defense against T. urticae. These findings support a central role for JA in regulating both chemical and morphological defense traits. Gene expression profiling experiments are currently underway to identify JA-regulated biochemical pathways that contribute to host resistance in tomato.