Another way to manipulate induced defenses of plants, is by genetic modification of signal-transduction or biosynthetic pathways. Carefully modified plants that differ in a single gene can be compared with wild-type plants to gain more insight into mechanisms of induced defense and the resulting ecological effects (for review, see Snoeren et al. 2007). Otherwise identical plants can be induced by herbivory and the differences between the wild type and mutant can be studied at the levels of gene expression, volatile production, and response of the plant-associated community. The approach is limited by the availability of modified plants. For some species like A. thaliana however, a wide variety of mutant and transgenic plants is available. Genetic modification is an excellent tool to study the ecological relevance of individual compounds that are difficult to obtain synthetically. For example, undamaged Arabidopsis plants transformed with a terpene synthase from strawberry, produce the terpenoids 4,8-dimethyl-1,3(E),7-nonatriene and (E)-nerolidol which results in the attraction of predatory mites (Kappers et al. 2005). However, A. thaliana has its limitations, such as its very early phenology and consequently limited interaction with potentially associated organisms, as a model for community ecology studies.
Transgenic N. attenuata plants that have been modified in the octadecanoid signal-transduction pathway received more herbivory and by more herbivore species than wild type control plants (Kessler et al. 2004). Similarly, the tomato mutant def-1 is deficient in JA accumulation through a mutation early in the JA pathway. As a result, the plants produce an incomplete volatile blend in response to herbivore damage, and the natural enemies of the herbivores do not discriminate between volatile blends from induced and uninduced plants (Thaler et al. 2002; Ament et al. 2004). However, for many ecological model species mutants or genetically modified plants are not (yet) available.
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