A Strobilurin Fungicide Relieves Bipolaris oryzae‐Induced Oxidative Stress in Rice

Although strobilurins are one of the most effective and broad spectrum classes of systemic fungicides, they may also increase plant stress tolerance by modulating the activity of antioxidant enzymes. To address this issue, the effect of azoxystrobin (Az) on the activity of antioxidant enzymes and on the concentrations of antioxidant metabolites and oxidative stress‐related compounds was studied in rice plants (cv. Metica‐1) either inoculated or not with Bipolaris oryzae, the causal agent of brown spot (BS). The Az minimally affected the enzyme activities, but consistently increased the glutathione reduced (GSH) concentrations in the noninoculated plants. The activities of superoxide dismutase, peroxidase, ascorbate peroxidase, glutathione peroxidase, glutathione reductase and glutathione‐S‐transferase were increased upon B. oryzae infection, but such increases were greatly limited in the Az‐sprayed plants. Catalase activity decreased in the inoculated plants compared to the noninoculated plants regardless of fungicide treatment. The GSH concentration increased in response to the B. oryzae infection, and the Az‐sprayed plants sustained higher levels of GSH at advanced stages of fungal infection than did the nonsprayed plants. The inoculated plants exhibited an extensive oxidative stress as evidenced by higher concentrations of hydrogen peroxide and malondialdehyde compared to the noninoculated plants, but lower and later increases were recorded in the Az‐sprayed plants than in the nonsprayed plants. Therefore, Az greatly reduces B. oryzae‐induced oxidative stress by limiting BS development rather than by activating antioxidant enzymes. The GSH, however, seems to be Az‐modulated, and this may partially explain the constrained oxidative stress observed in the Az‐sprayed plants.