Foliar Herbivory Suppresses Arbuscular Mycorrhizal Colonisation by Weakening Symbiosis Signalling in Root Exudates

Foliar insect herbivory could affect arbuscular mycorrhizal fungi (AMF), yet the underlying mechanisms remain understudied. Here, we examined the response of AMF symbiosis signals to foliar herbivory, using six herbaceous plant species and a generalist herbivorous insect. We found AMF colonisation was suppressed by foliar herbivory. After insect attack, plants allocated more biomass to belowground parts and the attack induced defence responses in aboveground parts. Notably, foliar herbivory increased shoot flavonoid concentrations but decreased root flavonoid concentrations. Moreover, quercetin and strigol concentrations in the root exudates were reduced by foliar herbivory. We further tested effect of the root exudates on the in-vitro germination of spores of two common AMF species. Spore germination was lower in treatments with herbivore-induced root exudates than in treatments with no-herbivore root exudates. Moreover, addition of herbivory-modified root exudates reduced AMF colonisation of healthy plants when compared to addition of root exudates from non-herbivory plants. Our results suggest that foliar herbivory weakened symbiosis signalling in root exudates, which could have contributed to the observed lower AMF colonisation following herbivory. Therefore, herbivore-induced symbiosis signalling needs to be considered when studying plant-mediated interactions between foliar herbivores and root microbes.