Protective ant-plant interactions, important in both temperate and tropical communities, are increasingly used to study a wide range of phenomena of general interest. As antiherbivore defenses "worn on the outside," they pose fewer barriers to experimentation than do direct (e.g., chemical) plant defenses. This makes them tractable models to study resource allocation to defense and mechanisms regulating it. As multi-trophic level interactions varying in species specificity and impact on fitness of participants, ant-plant-herbivore associations figure prominently in studies of food-web structure and functioning. As horizontally transmitted mutualisms that are vulnerable to parasites and "cheaters," ant-plant symbioses are studied to probe the evolutionary dynamics of interspecies interactions. These symbioses, products of coevolution between plants and insect societies, offer rich material for studying ant social evolution in novel contexts, in settings where colony limits, resource supply, and nest-site availability are all more easily quantifiable than in the ground-nesting ants hitherto used as models. PROTECTIVE ANT-PLANT MUTUALISMS 427 Other plants lack obvious ant-specialized traits but frequently harbor ant-tended hemipteran trophobionts. Recent reviews of ant-hemipteran interactions are provided by Delabie (2001), Gullan (1997), and Gullan & Kosztarab (1997). Most ant-tended hemipterans are phloem-feeders and excrete excess liquid as sugarrich honeydew. An important resource for ants, honeydew-producing hemipterans are often monopolized by territorial, ecologically dominant ants (Blüthgen et al. 2000). These generalist predators can strongly reduce densities of phytophagous insects.

Plants respond to herbivore attack with the release of volatile organic compounds (VOCs), which can attract predatory arthropods and/or repel herbivores and thus serve as a means of defense against herbivores. Such VOCs might also be perceived by neighboring plants to adjust their defensive phenotype according to the present risk of attack. We exposed lima bean plants at their natural growing site to volatiles of beetle-damaged conspecific shoots. This reduced herbivore damage and increased the growth rate of the exposed plants. To investigate whether VOCs also can serve in signaling processes within the same individual plant we focused on undamaged ''receiver'' leaves that were either exposed or not exposed to VOCs released by induced ''emitter'' leaves. Extrafloral nectar secretion by receiver leaves increased when they were exposed to VOCs of induced emitters of neighboring plants or of the same shoot, yet not when VOCs were removed from the system. Extrafloral nectar attracts predatory arthropods and represents an induced defense mechanism. The volatiles also primed extrafloral nectar secretion to show an augmented response to subsequent damage. Herbivore-induced VOCs elicit a defensive response in undamaged plants (or parts of plants) under natural conditions, and they function as external signal for within-plant communication, thus serving also a physiological role in the systemic response of a plant to local damage.ant-plant interaction ͉ extrafloral nectar ͉ indirect defense ͉ lima bean ͉ plant-plant communication

Review SummaryMany plants interact with carnivores as an indirect defence against herbivores. The release of volatile organic compounds (VOCs) and the secretion of extrafloral nectar (EFN) are induced by insect feeding, a response that is mediated by the plant hormone, jasmonic acid. Although VOCs mainly attract predatory mites and parasitic wasps, while EFN mainly attracts ants, many more animal-plant interactions are influenced by these two traits. Other traits involved in defensive tritrophic interactions are cellular food bodies and domatia, which serve the nutrition and housing of predators. They are not known to respond to herbivory, while food body production can be induced by the presence of the mutualists. Interactions among the different defensive traits, and between them and other biotic and abiotic factors exist on the genetic, physiological, and ecological levels, but so far remain understudied. Indirect defences are increasingly being discussed as an environmentally-friendly crop protection strategy, but much more knowledge on their fitness effects under certain environmental conditions is required before we can understand their ecological and evolutionary relevance, and before tritrophic interactions can serve as a reliable tool in agronomy.New Phytologist (2008) 178:

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