Ecological and phytohormonal aspects of plant volatile emission in response to single and dual infestations with herbivores and phytopathogens

In their natural environment, plants are faced with a multitude of attackers, of which insect herbivores and plant pathogens are an important component. In response to these attacks, plants release volatile organic compounds (VOCs), which play an important role in the communication between plants and the associated community members, such as other herbivores, phytopathogens and the natural enemies of herbivores. While numerous studies have focused on either plantpathogen or plantinsect interactions, less is known when these two sets of interactions co-occur. Depending on the mode of attack of the pathogen (necrotroph vs. biotroph) or herbivore (chewing vs. piercing-sucking) they will activate different defence pathways in the plant in which the phytohormones salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) play key roles. As these pathways can crosstalk, a pathogen infection can interfere in a plant's defence response to herbivory, and vice versa. Infestation of a plant with organisms inducing SA signalling prior to or simultaneously with attack by organisms that induce the JA pathway often suppresses JA signalling. However, the impact of this signalling pathway crosstalk on VOC induction is not clear cut, as there is high variability in the effects on volatile emissions, ranging from suppression to enhanced emission. The effects of the modified volatile blends on the foraging success of carnivorous natural enemies of herbivorous insects have started to be investigated. Foraging success of natural enemies generally withstands this modification of the host-induced VOC blend, but the presence or absence of key compounds is an important determinant of the response of certain carnivores. Further studies incorporating plantinsect and plantpathogen interactions at different levels of biological integration will provide valuable insight in how plants integrate signals from different suites of attacking organisms into an adaptive defence response.