Zoologie III, Theodor-Boveri-Institut, Biozentrum, Am Hubland, 97074 Würzburg, Germany.
Many studies have been devoted to the cost of plant defence against pathogens and herbivores. Most focused on costs resulting from genetic constraints or on allocation costs. Our studies on Macaranga (tropical pioneer trees defended by mutualistic ants against herbivores and pathogens) and on chemically induced pathogen resistance in wheat and bean have shed light on a further, indirect type of cost of defence. (a) Macaranga plants suffer from severe herbivory and pathogen attack when deprived of their mutualistic ants. Chitinase activities in different Macaranga plants are very low, and their function is fulfilled effectively by ants. Low chitinase activities may be a necessary trait for establishing obligate interactions with ants, since high activities in the plants' tissue probably would have detrimental effects on the ants, which have to gnaw entrance holes through living plant tissue. (b) Wheat (Triticum aestivum) plants treated with benzothiadiazole to chemically induce their resistance against pathogens gained less biomass and produced less seeds as compared to untreated controls. Allocation costs of induced pathogen resistance may result from the fact that nitrogen used for synthesis of defence-related proteins can not be diverted to growth-relevant processes. Similar effects occur in beans (Vicia faba). In both species, detrimental effects on symbiotic microorganisms could be observed. Treatment with benzothiadiazole led to a lower intensity of mycorrhiza (wheat) and to the development of less and smaller nodules (bean). Detrimental effects on symbiotic organisms may be a phenomenon leading indirectly to high fitness costs of plant defence against pathogens and herbivores.