Exogenous Melatonin Enhances Rice Plant Resistance Against Xanthomonas oryzae pv. oryzae

Rice bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae, is one of the most serious diseases of rice. In this study we found that exogenous melatonin can increase rice resistance to BB. Treatment of rice plants with exogenous melatonin (20 µg/ml) increased nitrate reductase, nitric oxide synthase, and peroxidase activity, enabling high intracellular concentrations of melatonin, nitric oxide, and H₂O₂. The expression of NPR1, a key regulator in the salicylic acid signaling pathway, was upregulated more than 10-fold when the plants were challenged with melatonin. Similarly, the messenger RNA level of PDF1.2, a jasmonic acid-induced defense marker, was 15 times higher in the treated plants than in the control plants. Moreover, three pathogenesis-related proteins, PR1b, PR8a, and PR9, were upregulated 20-fold in the presence of melatonin. The application of melatonin (100 µg/ml) to soil-grown rice reduced the incidence of BB by 86.21%. Taken together, these results not only provide a better understanding of melatonin-mediated innate immunity to X. oryzae pv. oryzae in rice but also represent a promising cultivation strategy to protect rice against X. oryzae pv. oryzae infection.