Elevated atmospheric CO2 suppresses jasmonate and silicon‐based defences without affecting herbivores

Elevated atmospheric CO2 (eCO2) not only increases plant growth but can also interfere with defence against insect herbivory through the disruption of the jasmonic acid (JA) pathway. Silicon (Si) plays an important role in plant stress tolerance and resistance to herbivory, particularly in grasses, many of which accumulate high amounts of Si. Activation of the JA pathway has been reported to stimulate Si uptake, while Si supplementation can alter both constitutive and induced phytohormone levels. A reduction in JA concentration under eCO2 has the potential to reduce Si uptake in plants. Using both Si supplemented (Si+) and control (Si‐) plants (Brachypodium distachyon) grown under ambient (400 ppm) and elevated (640 ppm) CO2 concentrations, we tested how plant growth, foliar Si concentration and endogenous JA responded to methyl jasmonate (MeJA) application and the subsequent effects on insect herbivore performance (Helicoverpa armigera). Elevated CO2 reduced Si concentration by 19% and endogenous JA by 70% on average. MeJA significantly increased Si concentration in Si+ plants. Si+ plants had higher baseline JA levels compared to Si‐ plants under control conditions (i.e. no stress), however, when plants were chemically induced with MeJA, the JA response was on average 84% lower in Si+ plants compared to Si‐ plants. Plants without MeJA treatment showed the opposite response, i.e. Si+ plants had higher baseline JA levels compared to Si‐ plants. Si significantly reduced herbivore consumption and growth rate. Despite eCO2 significantly reducing both Si and endogenous JA, no effect was seen on herbivores feeding on eCO2 plants. Collectively our results suggest that Si alters the JA response of plants. We show that JA induces Si uptake, however Si then reduces the JA response of plants under induced stress conditions. However, predicted increases in CO2 levels within this century may significantly reduce Si‐based mechanical defences against herbivory via a reduction in endogenous JA