Azelaic Acid-Mediated Resistance in Rice Against Infection by Bipolaris oryzae

Brown spot, caused by the fungus Bipolaris oryzae, has led to significant yield losses in rice production worldwide. This study hypothesized that azelaic acid (AzA) could reduce brown spot symptoms in rice leaves by potentiating biochemical defense reactions. A 2 × 2 factorial experiment was arranged in a completely randomized design with five replications per sampling time. The factors studied were plants sprayed with water (control) or AzA (10 mM; 7.5 mL per plant), either non-inoculated or inoculated with B. oryzae. In the in vitro assay, conidia exposed to AzA solutions at rates of 2.5, 5, 7.5, and 10 mM and to the fungicide solution did not form germ tubes compared to those in the control (water) treatment. The area of fungal colonies on oat-agar medium was reduced for the fungicide and AzA (rates increasing from 2.5 to 10 mM) treatments compared to the control (water) treatment. The EC₅₀ value was 3.8 mM AzA. Brown spot severity significantly decreased by 57, 48, 52, and 58% at 36, 60, 84, and 108 h after inoculation (hai) for AzA-sprayed plants compared to water-sprayed ones. The area under brown spot progress curve significantly decreased by 53% for AzA-sprayed plants compared to water-sprayed ones. Greatest activities of defense-related enzymes (chitinase at 108 hai, β-1,3-glucanase at 60 hai, phenylalanine ammonia-lyase at 60 and 108 hai, and lipoxygenase at 84 and 108 hai), a higher concentration of lignin at 84 and 108 hai, and a more robust antioxidative metabolism (higher activities of ascorbate peroxidase at 36 hai, catalase at 84 and 108 hai, and superoxide dismutase at 84 hai) were obtained for AzA-sprayed infected plants. The higher concentration of the superoxide anion radical in AzA-sprayed infected leaves helped to intensify the cell defense reactions against fungal infection and had a fungistatic effect against its hyphae and conidia germination. The findings of this study provide valuable insights into using AzA to potentiate foliar defense reactions in rice plants to hamper the infection by B. oryzae.