Plastic defence expression in plants

Phenotypic plasticity is the ability of an organism to express different phenotypes in response to changing environments and becomes particularly obvious when plants alter their transcriptome after enemy attack. The resulting alterations affect the metabolic, chemical and morphological phenotype and cause resistance or tolerance phenomena, which allow plants to main high fitness in the presence of enemies. Volatiles released from damaged plants can be received by their neighbours or undamaged parts of the same plant to mount an adequate level of resistance and thereby add a further level of phenotypic plasticity. The induced defence responses also include attraction of the third trophic level and, thus, dramatic changes of the ‘extended phenotype’ of the plant, that is, its surrounding fauna. The underlying interactions are, at least partly, under the control of the plant and, thus, subject to co-evolutionary processes. Fitness costs are a common explanation for the evolution of inducible resistance expression. However, variability in the resistance phenotype can per se be beneficial, because it makes counter-adaptations by the plants’ enemies more difficult. In the case of indirect defences, which are mediated by plant-carnivore mutualisms, signal reliability and reciprocal responses among phenotypically plastic partners appear necessary prerequisites for their evolutionary stabilisation. The expression of resistance and tolerance is induced by enemy attack but is also under control by abiotic factors, such as resource supply, and by biotic parameters, such as current and anticipated competition, efficiency of the expressed resistance and ontogenetic stage. All these levels of plasticity help plants to survive as sessile organisms in a rapidly changing environment and in the presence of mobile enemies.