The chemical ecology of interactions between aboveground and belowground induced responses: a multitrophic perspective

Plants are exposed to a wide variety of aboveground and belowground herbivores and pathogens. In response to these phytophages, many plant species defend themselves with induced responses. Local induction often results in systemic induced responses, because induction signals are transported throughout the plant and elicit induced responses in undamaged organs. Root-feeding phytophages thus may affect the performance of shoot phytophages and their natural enemies, and vice versa. So far, studies of induced defences have focussed exclusively on aboveground interactions. Therefore, the ecological and evolutionary consequences of interactions between aboveground and belowground induced responses for plants and the multitrophic community associated with them, are still poorly understood. Because such interactions may alter the net costs and benefits of induced responses, they may be of seminal importance for understanding the evolution of induced responses in plants. Here I propose to study the ecological and evolutionary consequences of interactions between aboveground and belowground induced responses for plants, phytophages and their natural enemies. I will use naturally occurring Brassicaceae , including plant species with life-histories ranging from annual to perennial. I will first characterise the physiological interactions between aboveground and belowground inducible responses in a few selected plant species, using both naturally occurring phytophages and signalling hormones (jasmonic acid and salicylic acid). Second, I will assess how these induced responses affect naturally occurring aboveground and belowground phytophages and their natural enemies, alone and in combination. Specifically, I will analyse whether plants are able to prioritise their induced response to the most damaging phytophage. Third, I will evaluate how these interactions between aboveground and belowground phytophage-induced responses affect plant fitness, in absence and presence of a multitrophic community. The results of these experiments will show the importance of aboveground and belowground multitrophic interactions as a selection pressure for induced responses in plants.