Plastic defence expression in plants
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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.
KeywordsCompensation Induced resistance Indirect defence Phenotypic plasticity Priming Tolerance
The expression levels of constitutive resistance are not affected by encounters with plant enemies. They, can, however, be subject to phenotypic plasticity with respect to other factors such as, for example, abiotic conditions.
All strategies that increase plant fitness in the presence of enemies. The term as used here comprises both resistance and tolerance.
Direct resistance traits are those that directly interact with the plant enemy in order to reduce feeding activity or infection level.
Indirect defence traits do not directly interact with the plant enemy but rather enhance the presence of ‘enemies of the enemy of the plant’.
Induced resistance traits change their expression level in response to an encounter with an enemy of the plant.
The capacity of a certain genotype to express different phenotypes in response to changing environmental conditions.
Plant traits that reduce the degree or probability at which enemies of the plants exert damage.
Tolerance traits minimize the fitness loss that is caused by a certain level of damage.
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