Entomologia Experimentalis et Applicata 86: 267–279, 1998. 267 c 1998 Kluwer Academic Publishers. Printed in Belgium. Stimulation and attenuation of induced resistance by elicitors and inhibitors of chemical induction in tomato (Lycopersicon esculentum) foliage Michael J. Stout , Kathi V. Workman, Richard M. Bostock 1 & Sean S. Duffey 2 Departments of Entomology and 1 Plant Pathology, University of California, Davis, Davis, CA 95616, USA; Current address: Department of Entomology, Louisiana State University Agricultural Center, 402 Life Sciences Building, Baton Rouge, LA 70803–1710, USA Accepted: November 11, 1997 Key words: proteinase inhibitors, oxidative enzymes, salicylic acid, jasmonic acid, induced resistance Abstract Elicitors and inhibitors of chemical induction were used to manipulate the activities of several putative defense- related proteins in leaves of the tomato, Lycopersicon esculentum Mill. The four presumptive defenses manipulated were proteinase inhibitors, polyphenol oxidase, peroxidase, and lipoxygenase. The elicitors used were jasmonic acid, methyl jasmonate, ultraviolet light, and feeding by larvae of the noctuid, Helicoverpa zea Boddie; the inhibitors used were salicylic acid and acetylsalicylic acid. These chemical manipulations were combined with short-term growth assays using larvae of the generalist noctuid, Spodoptera exigua Hubner, in order to assess the relative roles of the proteins in induced resistance to S. exigua. When activities of proteinase inhibitors and/or polyphenol oxidase in leaf tissue were high (e.g., in damaged or elicited plants), growth rates of larvae of S. exigua were low; when activities of polyphenol oxidase and proteinase inhibitors were low (e.g., in undamaged or damaged, inhibited plants), growth rates of larvae were high. In contrast, high activities of peroxidase and lipoxygenase were not associated with decreases in suitability of leaf tissue for S. exigua. The association of high levels of proteinase inhibitors and polyphenol oxidase with resistance to S. exigua – irrespective of the presence or absence of damage – strongly implicates these proteins as causal agents in induced resistance to S. exigua. Introduction Many plants respond chemically to feeding by insects or mechanical damage, with well over a hundred examples of damage- or insect-induced changes in levels or activities of primary and secondary plant metabolites present in the literature (Tallamy & Raupp, 1991; Karban & Baldwin, 1997). Most plant responses to damage appear to involve coordinate changes in the levels of multiple chemicals, as is true of plant responses to pathogen invasion (Karban & Myers, 1989; Ward et al., 1991; Stout et al., 1996). Two examples will suffice to illustrate the diversity of changes in primary and secondary metabolism which can occur following damage to a plant. Damage to tobacco foliage not only caused increases in the levels of the alkaloid nicotine but also caused a number of Deceased. alterations in primary metabolism, including increases in photosynthetic capacity and increases in foliar nitro- gen, nitrate, and protein (Baldwin & Ohnmeiss, 1994). In separate experiments, wounding of tobacco foliage was also shown to induce serine proteinase inhibitors (McManus et al., 1994). In cotton foliage, feeding by larvae of the noctuid, Helicoverpa zea Boddie, induced increases in the levels of several phenolics and in the activities of the defense-related enzymes peroxidase, lipoxygenase, ascorbate oxidase, and diamine oxidase, and caused decreases in levels of protein (Bi et al., 1997). Thus, although studies of induced phytochemic- al responses often focus on only one or a few chemicals, phytochemical responses to damage usually involve simultaneous changes in levels of primary metabol- ites, low-molecular weight organic compounds, and defense-related proteins.