Dual role of benzoxazinoids in plant response to combined drought stress and aphid feeding

Plants produce specialized metabolites like benzoxazinoids (BXDs) to survive adverse environmental conditions and deter herbivory, while insects have evolved mechanisms to overcome plant defences. BXDs, produced by cereals, are best known for their role in defence against herbivores. Recent studies have shown that BXDs also accumulate under abiotic stresses, raising questions about their function in combined stress tolerance and herbivore resistance. We investigated potential roles of BXDs in wheat (Triticum aestivum) seedlings under combined drought stress and bird cherry-oat aphid (Rhopalosiphum padi) feeding using physiological, biochemical, metabolic, and gene expression analyses. Aphids feeding on drought-stressed plants showed reduced fitness and altered expression of stress-related genes compared to those on well-watered plants, consistent with the observation that drought-stressed plants have higher levels of defensive BXDs and callose in drought-stressed leaves. Gene expression analysis revealed that BXD biosynthetic (BXs), callose (GSL: Glucan synthase-like), and MYB transcription factor genes were upregulated in response to both drought and aphid herbivory in wheat leaves, with an additive effect under combined stress. Since BXDs are known to trigger callose deposition and MYBs regulate BXD biosynthesis and callose accumulation, we suggest that drought-induced callose accumulation is caused by elevated BXD levels controlled by MYBs. This callose accumulation potentially supports plant vigour and protects plants against aphids by hindering their feeding. Furthermore, MYBs, particularly TaMYB31, play a regulatory role in wheat stress response by regulating expression of BXs and GSL genes.