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Wheat (Triticum aestivumL.) resistance to Hessian fly (HF,Mayetiola destructor) conferred by HF resistance (R) genes, is often vulnerable to heat stress. This study aims to investigate the molecular basis of heat-induced susceptibility in wheat to HF. We compared the resistant cultivar ‘Molly’, which carriesH13, with its susceptible near-isogenic line ‘Newton’ to determine how elevated temperature alters phenotypic and molecular responses to HF infestation. Our phenotyping results showed that a single 24 h heat treatment at 30 °C was sufficient to compromise Molly’s resistance, resulting in more than 70% of plants becoming susceptible. Transcriptomic profiling revealed that resistant Molly exhibited stronger and broader defenses than Newton under normal temperature, whereas heat-stressed Molly displayed extensive transcriptional reprogramming resembling the naturally susceptible Newton wheat. Comparative analysis of transcriptomic profiles identified 74 genes that are consistently regulated across all susceptible states at 24 h after initial HF infestation, including heat-stressed Molly and Newton under both high and normal temperatures, but not in the resistant Molly under normal temperature. Functional annotation of these susceptibility-related genes in combination with previous findings suggest that susceptibility is likely associated with increased auxin-related activity, reduced salicylic acid (SA) and OPDA-associated defense signaling, and altered coordination of defense pathways that favor feeding-site establishment and redirecting host resource to developing HF larvae. The identification of susceptibility-associated candidate genes provides molecular targets for breeding wheat cultivars with more durable resistance under rising temperatures. 

The Hessian fly (HF,Mayetiola destructor) is one of the most destructive pests of wheat and wheat-related cereals. Wheat resistance and/or susceptibility to HF are often affected by the levels of phytohormones in plants. In this study, we tested the impact of phytohormones on Molly wheat resistance to HF biotype GP by externally applying phytohormones, including salicylic acid (SA), jasmonic acid (JA), 12-oxophytodienoic acid (OPDA), and auxin (indole acetic acid, IAA) to wheat seedlings under heat conditions. Our results indicated that the impact of externally applied phytohormones on wheat resistance to HF depends on the timing of phytohormone application and/or HF larval density at HF feeding sites in the plants. The early application of SA, OPDA, and IAA enhanced wheat resistance to HF under heat stress at low larval density, while the delayed application of SA, OPDA, and IAA did not affect wheat resistance to HF at high larval density. 

Abstract Heat stress compromises wheat resistance to Hessian fly (HF, Mayetiola destructor (Say)) (Diptera: Cecidomyiidae) infestation. The objective of this research is to analyze the molecular basis of heat-induced loss of wheat resistance to HF infestation using RNA Sequencing (RNA-seq). To this end, two resistant wheat cultivars ‘Molly’ and ‘Caldwell’ containing the resistance genes H13 and H6, respectively, were infested with an avirulent HF biotype GP and treated with different temperatures to examine the impact of heat stress on their resistance phenotypes. Tissue samples collected from HF feeding sites in Molly plants were subjected to RNA-seq analysis to determine the effect of heat stress on transcript expression of genes in wheat plants. Our results indicate that resistance to HF infestation in Caldwell is more sensitive to heat stress than that in Molly, and that heat stress down-regulates most genes involved in primary metabolism and biosynthesis of lignin and cuticular wax, but up-regulate most or all genes involved in auxin and 12-oxo-phytodienoic acid (OPDA) signaling pathways. Our results and previous reports suggest that heat stress may impair the processes in wheat plants that produce and mobilize chemical resources needed for synthesizing defensive compounds, weaken cell wall and cuticle defense, decrease OPDA signaling, but increase auxin signaling, leading to the suppressed resistance and activation of susceptibility.