The role of photoprotection in disease resistance to Septoria tritici blotch in wheat

Septoria tritici blotch (STB) caused by the fungus Zymoseptoria tritici is an important foliar disease of wheat in the UK. Symptoms of chlorotic and necrotic lesions with pycnidia appear on leaves 7-14 days after infection when the disease reduces plant photosynthesis. Plants have evolved photoprotective mechanisms to avoid photo-oxidative damage and dissipate excess energy that cannot be utilised by the photosynthetic machinery. Non-Photochemical Quenching (NPQ) is the process of heat dissipation to protect the integrity of photosystem (PS) II and it typically modulates plant responses to environmental stress. Dynamic changes of NPQ reflect tight regulation of energy required to drive photochemistry and to prevent permanent damage to PSII caused by reactive oxygen and free radicals. A recently cloned STB-resistance gene, Stb6, was found to encode a conserved wall-associated receptor kinase (WAK)-like protein that confers pathogen resistance without a hypersensitive response (HR). We hypothesized that upon infection with Z. tritici, wheat genotypes containing Stb6 will exhibit NPQ dynamics associated with reduced oxidative stress in the absence of HR. Time series of chlorophyll fluorescence imaging of healthy and artificially inoculated wheat leaves as well as visual rating of disease severity were performed on wheat germplasm that either contained or lacked Stb6. NPQ induction and relaxation curves as well as a range of related physiological parameters were measured and correlated with disease severity. Significant down-regulation of NPQ in the absence of symptoms by the pathogen was observed in the resistant genotypes, suggesting that chloroplastic defence plays an important role in disease resistance. Further work will investigate the relationship between NPQ and oxidative stress in STB resistance.