Plants use a range of sophisticated strategies to protect themselves against herbivores and pathogens, such as the production of jasmonates, a group of plant hormones that prime the plant's defense system upon attack. However, defense-related mechanisms, such as the jasmonate response, play a more diverse role than previously appreciated. Jasmonates also regulate key mutualist relationships, leading to a suit of potential conflicting selection pressures as a single response is employed to mediate multiple species interactions. Here, we experimentally manipulate the host's jasmonate response and document the impact on two key plant mutualisms: (i) changes to arbuscular mycorrhizal symbionts belowground (ii) modifications to floral traits affecting pollinator mutualists aboveground. By exogenously applying a range of methyl jasmonate solutions to cucumber plant roots grown with and without mycorrhizal fungi, we are able to examine the potential costs of the jasmonate response to both above and belowground mutualists. We demonstrate that the negative effect of jasmonates on floral traits depends on whether the plant is mycorrhizal or nonmycorrhizal. Mycorrhization had a positive effect on floral traits, but benefits were lost with jasmonate application. While low levels of jasmonate decreased floral traits, these same jasmonate levels increased colonization by the mycorrhizal symbiont three-fold, but only under high phosphorus conditions. Our results highlight potential conflicts for the host in the regulation of their mutualists under different conditions and suggest that the overall impact of the jasmonate response depends on the plant mycorrhizal status and its nutrient context. These findings suggest that increasing the jasmonate response may lead to differential costs and benefits for plants and their mutualists, and highlight potential conflict in planta, with mycorrhizal symbionts benefiting from intermediate levels of jasmonates while certain floral traits can be depressed at this same level. © 2009 The Authors. Journal compilation © 2009 British Ecological Society.