Inducible indirect defence of plants: from mechanisms to ecological functions

Inducible defences allow plants to be phenotypically plastic. Inducible indirect defence of plants by attracting carnivorous enemies of herbivorous arthropods can vary with plant species and genotype, with herbivore species or instar and potentially with other environmental conditions. So far, inducible indirect defence has mostly been studied for simple linear food chains. However, ultimately, ecologists should address inducible indirect defence in a food web context, where more than one organism (different herbivores and pathogens) may attack a plant and where a plant that emits herbivore-induced volatiles is surrounded by other plants that emit odours that can mix with the herbivore-induced volatiles from the attacked plant. Evolutionary ecologists are interested in the costs and benefits of interactions between plants and their attackers. These may be investigated by comparing different plant genotypes. The best comparison is between plant individuals that differ in only a single or restricted number of known traits. Such genotypes are difficult to obtain by conventional methods. However, rapid progress in the study of mechanisms of plant-attacker interactions and in the field of molecular genetics and genomics provides new tools that can be exploited by ecologists. For instance, genomic knowledge on Arabidopsis thaliana and the availability of characterized mutants and transgenes that are altered in one or a restricted number of genes can be exploited to address functional aspects of inducible indirect defence. In this paper we review progress in the knowledge of mechanisms of inducible indirect defence of plants and its importance for investigating the functional aspects of plant responses to herbivorous arthropods. Finally we identify some of the ecological questions that can be addressed by exploiting mechanistic aspects of inducible indirect defence.