Heat shock-induced enhanced susceptibility of barley to Bipolaris sorokiniana is associated with elevated ROS production and plant defence-related gene expression.

Heat stress alters plant defence responses to pathogens. Short-term heat shock promotes infections by biotrophic pathogens. However, little is known about how heat shock affects infection by hemibiotrophic pathogens like Bipolaris sorokiniana (teleomorph: Cochliobolus sativus). We assessed the effect of heat shock in B. sorokiniana-susceptible barley (Hordeum vulgare cv. Ingrid) by monitoring leaf spot symptoms, B. sorokiniana biomass, ROS and plant defence-related gene expression following pre-exposure to heat shock. For heat shock, barley plants were kept at 49 °C for 20 s. B. sorokiniana biomass was assessed by qPCR, ROS levels determined by histochemical staining, while gene expression was assayed by RT-qPCR. Heat shock suppressed defence responses of barley to B. sorokiniana, resulting in more severe necrotic symptoms and increased fungal biomass, as compared to untreated plants. Heat shock-induced increased susceptibility was accompanied by significant increases in ROS (superoxide, H2 O2 ). Transient expression of plant defence-related antioxidant genes and a barley programmed cell death inhibitor (HvBI-1) were induced in response to heat shock. However, heat shock followed by B. sorokiniana infection caused further transient increases in expression of HvSOD and HvBI-1 correlated with enhanced susceptibility. Expression of the HvPR-1b gene encoding pathogenesis-related protein-1b increased several fold 24 h after B. sorokiniana infection, however, heat shock further increased transcript levels along with enhanced susceptibility. Heat shock induces enhanced susceptibility of barley to B. sorokiniana, associated with elevated ROS levels and expression of plant defence-related genes encoding antioxidants, a cell death inhibitor, and PR-1b. Our results may contribute to elucidating the influence of heat shock on barley defence responses to hemibiotrophic pathogens.

Artificial elevation of glutathione contents in salicylic acid-deficient tobacco (Nicotiana tabacum cv. Xanthi NahG) reduces susceptibility to the powdery mildew pathogen Euoidium longipes.

The effects of elevated glutathione levels on defence responses to powdery mildew (Euoidium longipes) were investigated in a salicylic acid-deficient tobacco (Nicotiana tabacum cv. Xanthi NahG) and wild-type cv. Xanthi plants, where salicylic acid (SA) contents are normal. Aqueous solutions of reduced glutathione (GSH) and its synthetic precursor R-2-oxothiazolidine-4-carboxylic acid (OTC) were injected into leaves of tobacco plants 3 h before powdery mildew inoculation. SA-deficient NahG tobacco was hyper-susceptible to E. longipes, as judged by significantly more severe powdery mildew symptoms and enhanced pathogen accumulation. Strikingly, elevation of GSH levels in SA-deficient NahG tobacco restored susceptibility to E. longipes to the extent seen in wild-type plants (i.e. enhanced basal resistance). However, expression of the SA-mediated pathogenesis-related gene (NtPR-1a) did not increase significantly in GSH or OTC-pretreated and powdery mildew-inoculated NahG tobacco, suggesting that the induction of this PR gene may not be directly involved in the defence responses induced by GSH. Our results demonstrate that artificial elevation of glutathione content can significantly reduce susceptibility to powdery mildew in SA-deficient tobacco.