Melatonin treatments reduce the pathogenicity and inhibit the growth of Xanthomonas oryzae pv. oryzicola

Rice bacterial leaf streak ( BLS ), caused by Xanthomonas oryzae pv. oryzicola (Xoc), is a prevalent disease worldwide. Melatonin, an indoleamine with the chemical name N ‐acetyl‐5‐methoxytryptamine, is known to inhibit the growth of human pathogenic bacteria. However, the impact of melatonin on agricultural pathogenic bacteria remains poorly understood. In this study, the effect of melatonin on Xoc was investigated. Exogenous melatonin (200 μg mL −1 ) significantly inhibited the growth of Xoc and reduced the mRNA expression level of seven cell division‐related genes. Melatonin reduced the pathogenicity of Xoc towards the susceptible rice IR24. The lesion length in IR24 infected by Xoc pretreated with melatonin (200 μg mL −1 ) was more than 23% smaller than the lesion length of the control group. Similarly, infection of IR24 leaves by Xoc pretreated with melatonin (200 μg mL −1 ) reduced the bacterial population by 45%. The biofilm formation, swimming motility and extracellular protease activities of Xoc also decreased under melatonin treatment. Melatonin only slightly changed the morphology of Xoc but significantly reduced the mRNA expression of toxin genes. Furthermore, treating leaves with exogenous melatonin reduced the incidence of BLS by 17%. It is reported for the first time that melatonin reduces pathogenicity and may serve as a bactericide for controlling BLS disease.