Elevated atmospheric CO2 and silicon antagonistically regulate anti-herbivore phytohormone and defence gene expression levels in wheat

Silicon (Si) accumulation by grasses is a key mechanism for alleviating biotic and abiotic stresses, including insect herbivory. In addition to conferring physical resistance, tissue silicification may enhance anti-herbivore phytohormone production, such as the jasmonic and salicylic (JA and SA) acid pathways, and downstream regulation of defence genes, although this is poorly understood. Elevated atmospheric carbon dioxide (eCO2) concentrations predicted by climate models are reported to reduce Si accumulation in several plant taxa and may therefore compromise Si-augmented resistance. We investigated how Si enrichment and eCO2 regulate the JA and SA pathways and expression of defence genes in wheat (Triticum aestivum) challenged by a global insect pest (Helicoverpa armigera). Si treatments increased JA production and expression of β-1,3-ENDOGLUCANASE (GNS), and MITOGEN-ACTIVATED PROTEIN KINASE (MAPK; WCK-1) defence genes, while suppressing SA production, resulting in reduced feeding and growth of H. armigera. In contrast, under eCO2 conditions, Si accumulation was reduced, GNS downregulated, but SA production was upregulated. Despite compromised plant defences, H. armigera growth rates were reduced under eCO2. We conclude that eCO2 and Si supplementation contrastingly regulate anti-herbivore defences in wheat; these important drivers operate independently and may influence future patterns of pest resistance in wheat under projected rises in atmospheric CO2. •Silicon accumulation has been shown to enhance plant defence responses to biotic stress such as herbivores.•Silicon accumulation can increases defence gene expression following herbivores attack.•eCO2 concentrations often reduce Si accumulation some grasses and may therefore compromise Si-augmented resistance.•eCO2 and Si supplementation contrastingly regulate anti-herbivore defences in wheat.