PS 10-121 - Elevated atmospheric CO2 and Silicon antagonistically impacted phytohormones and defence-related genes in wheat

Abstract: Silicon (Si) accumulation by grasses is a key mechanism for alleviating biotic and abiotic stresses, including insect herbivory. In addition to physical resistance, tissue silicification may enhance anti-herbivore phytohormone production, including 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 wheat (Triticum aestivum) regulates the JA and SA pathways and expression of defence genes in response to Si enrichment and eCO₂ when challenged by a global pest (Helicoverpa armigera). Si increased JA production and expression of GNS and WCK-1 defence genes, while suppressing SA production, resulting in reduced feeding and growth of H. armigera. In contrast, wheat accumulated less Si, upregulated SA production and downregulated expression of LOX and PAL genes under eCO₂ conditions. Despite compromised plant defences, H. armigera growth rates were reduced under eCO₂. We conclude that wheat regulates anti-herbivore defences in opposing ways in response to eCO₂ and Si supplementation; these important drivers operate independently but may influence future patterns of pest resistance in wheat.