Early Root Herbivory Impairs Arbuscular Mycorrhizal Fungal Colonization and Shifts Defence Allocation in Establishing Plantago lanceolata

Research into plant-mediated indirect interactions between arbuscular mycorrhizal (AM) fungi and insect herbivores has focussed on those between plant shoots and above-ground herbivores, despite the fact that only below-ground herbivores share the same part of the host plant as AM fungi. Using Plantago lanceolata L., we aimed to characterise how early root herbivory by the vine weevil (Otiorhynchus sulcatus F.) affected subsequent colonization by AM fungi (Glomus spp.) and determine how the two affected plant growth and defensive chemistry. We exposed four week old P. lanceolata to root herbivory and AM fungi using a 2×2 factorial design (and quantified subsequent effects on plant biomass and iridoid glycosides (IGs) concentrations. Otiorhynchus sulcatus reduced root growth by c. 64%, whereas plant growth was unaffected by AM fungi. Root herbivory reduced extent of AM fungal colonization (by c. 61%). O. sulcatus did not influence overall IG concentrations, but caused qualitative shifts in root and shoot IGs, specifically increasing the proportion of the more toxic catalpol. These changes may reflect defensive allocation in the plant against further attack. This study demonstrates that very early root herbivory during plant development can shape future patterns of AM fungal colonization and influence defensive allocation in the plant.