Suppressing Symptomless Nonhost Resistance of Barley to Tobacco mosaic virus by Short-Term Heat Stress—Role of Superoxide in Resistance

Our previous research has demonstrated the role of optimal temperatures and reactive oxygen species (ROS) in maintaining symptomless nonhost resistance (NHR) of barley to powdery mildews. However, the exact functions of temperature and ROS in NHR of plants, including barley, to viral infections are not known. Although barley is a nonhost for Tobacco mosaic virus (TMV), this virus can replicate in barley leaves at temperatures of ca. 30 °C. Here we elucidated the influence of short-term heat stress pre-treatments (30 °C, 3 h; heat shock at 49 °C, 20 s) on the symptomless NHR of barley to TMV and the role of the ROS superoxide (O2.−) in maintaining NHR. Heat stress and antioxidant (superoxide dismutase and catalase, SOD + CAT) treatments resulted in 50–100% higher TMV levels, while combined heat shock and SOD + CAT application caused further increases in TMV and appearance of cell and tissue death resembling a hypersensitive response (HR). An early (from 2 h after inoculation) burst of O2.− was essentially absent in TMV-infected barley exposed to short-term heat stress pre-treatments. Expression of barley genes regulating ROS (O2.−) metabolism (HvRBOHF2, HvSOD1) and cell death (HvBI-1) displayed an inverse correlation with TMV levels even at later time points (1–4 days after inoculation), implying a role in symptomless NHR, while increased levels of the antioxidant glutathione marked heat stress-induced suppression of NHR. We demonstrated that short-term heat stress and antioxidant treatments result in compromised NHR of barley to TMV, pointing to the role of optimal temperatures and ROS (O2.−) in symptomless NHR to virus infections.