Arbuscular mycorrhizal fungi affect plant tolerance and chemical defences to herbivory through different mechanisms

Arbuscular mycorrhizal fungi (AMF) are ubiquitous plant symbionts that affect plant nutrient status, patterns of resource allocation and rates of plant growth. In addition, AMF influence the expression of plant defence, which may affect subsequent interactions between plants and higher trophic levels. Tolerance to herbivory and chemical defence represent two distinct defence strategies of plants, and there is accumulating evidence that AMF can influence each strategy separately. However, investigations on the simultaneous effects of AMF on tolerance and chemical defence mechanisms are lacking. Using six milkweed (Asclepias) species and three levels of AMF inoculum, we show that plant tolerance to herbivory increases with foliar P concentrations, while both foliar cardenolide concentration and latex exudation (major chemical defences in milkweeds) increase with foliar N concentrations, but decrease with plant growth rate. Additionally, foliar cardenolide concentration increases with foliar P concentration, and latex exudation increases with root biomass. Because AMF have significant effects on all of these traits (foliar N and P concentrations, root biomass and growth rate), AMF can change plant tolerance and chemical defence simultaneously, albeit through different mechanisms. Synthesis. Overall, plant tolerance and chemical defence to herbivores are affected by AMF through changes in basic plant physiology such as nutrient status, allocation patterns and growth rate, and our study suggests that it may be possible to predict effects of AMF on plant defences through these simple traits.