Heat shock confers enhanced susceptibility of barley to a necrotrophic pathogen, <i>Pyrenophora teres</i> f. <i>teres</i> , leading to a more pronounced redox imbalance

High temperature-induced plant stress affects defence responses to pathogens. Susceptibility of plants to biotrophic and hemibiotrophic infections are mostly enhanced by short-term heat shock (HS), but little is known about HS effects on plants infected by necrotrophic pathogens. Here we evaluated the effects of HS in susceptible barley (Hordeum vulgare cvs. Ingrid and Himalaya) infected with the necrotrophic Pyrenophora teres f. teres (Ptt) by monitoring disease symptoms, Ptt biomass, plant defence components (ROS, antioxidants and defence-related gene expression). Ptt biomass was assessed by qPCR, hydrogen peroxide (H₂O₂) levels using DAB histochemical staining, enzyme activities and glutathione levels with spectrophotometric methods, and gene expression using reverse transcription qPCR. HS suppressed defence responses to Ptt in both barley cultivars, resulting in increased necrotization and fungal biomass. HS-induced enhanced susceptibility was accompanied by significant increases in H₂O₂ production. Activities of GR and GST enzymes were induced by Ptt inoculation in both cultivars, but HS further elevated GST activity only in cv. Ingrid. Glutathione concentrations increased in cv. Ingrid due to combined effects of HS and Ptt inoculation. Expression of HvPR-1b encoding pathogenesis-related (PR) protein-1b increased primarily in response to Ptt infection; however, HS-treated plants had higher transcript levels along with enhanced susceptibility. HS induces enhanced susceptibility of barley to Ptt, associated with elevated ROS levels and expression of plant defence-related genes encoding antioxidants and PR-1b. Our results contribute to elucidating the influence of HS on barley defence responses to necrotrophic pathogens.