1Department of Plant Pathology, University of California, Davis, CA 95616, USA; 2Department of Entomology, University of California, Davis, CA 95616, USA

Background and objectives
Plants are often simultaneously challenged by insects and pathogens capable of triggering an array of signals to induce responses that may be beneficial or detrimental to plant health and productivity. The tomato plant, Lycopersicon esculentum, provides a model for the study of inducible defences against insect herbivores and pathogens, defences that can be strongly influenced by the mix of signals generated by biotic stresses as well as by abiotic stresses such as drought, nutrient limitation or high soil salinity. The ability to utilize these defences in pest control depends, in part, upon a fundamental knowledge of their biochemical nature and specificity, the signalling systems that regulate their expression, and the compatibility of these signalling systems. Our research points to a potential increased vulnerability to insect herbivory when plants are triggered to express systemic acquired resistance (SAR) to pathogens [1]. A negative interaction between two signalling pathways, one involving salicylic acid, which is involved in SAR to pathogens, and another involving jasmonic acid (JA), known to induce resistance (IR) to insects, is indicated at the levels of biochemical response and response to biotic challenge in greenhouse and field experiments. Unresolved is the degree of reciprocity for this interaction, as well as the interplay of other stress-induced regulatory signals on the inducibility of host resistance to different insects and pathogens.

Results and conclusions
The extent to which positive or negative interactions occur during expression of resistance to arthropods of different feeding guilds and to different types of pathogens in tomato foliage is being assessed in greenhouse and field experiments with combinations of a synthetic salicylate mimic, benzothiadiazole (BTH), and JA. Analyses of hallmark gene expression combined with bioassays of insect and pathogen performance gauge the degree of signal-response coupling and the efficacy of the induced defences. To determine if a trade-off in SAR and IR is apparent at the level of gene expression, the mRNA levels of a tomato pathogenesis-related protein P4 and the proteinase inhibitor PINII were determined since they provide good markers for chemical inducers of resistance to pathogens (BTH) and to insects (JA), respectively. Also, polyphenol oxidase (PPO) activity was evaluated as a strong marker for IR [2]. Tomato leaves were analysed following challenge with a variety of agents, including the noctuids Spodoptera exigua and Helicoverpa zea, and the pathogens Pseudomonas syringae pv. tomato and Phytophthora infestans.

Expression studies of PINII, P4 and PPO indicate that their induction in tomato does not follow a strict pattern based on the type of biological inducer (insect versus pathogen) or chemical treatment, with each specific treatment inducing a distinct pattern of gene expression. However, plants induced to express disease resistance with BTH displayed a diminished expression of jasmonate-activated PINII and PPO, consistent with an observed increase in susceptibility to herbivory by noctuid insects. Our experiments with different insects and pathogens suggest that the suite of induced responses and their corresponding impact on resistance depend on the feeding habit and pathogen type of the inducing and challenge organisms. Importantly, these studies suggest that salicylate-based protection against pathogens may make plants more vulnerable to certain insects.

1. Fidantsef AL, Stout MJ, Duffey SS, Bostock RM, 1997. Phytopathology 87, S30 (Abstr.).
2. Stout MJ, Workman KV, Bostock RM, Duffey SS, 1998. Entomologia Experimentalis et Applicata (in press).