Melatonin promotes stomatal immunity by mediating phytohormone crosstalk in Arabidopsis thaliana

Melatonin, a phytohormone, has drawn growing interest for its important role in improving plant tolerance to biotic and abiotic stresses. However, its specific function and underlying mechanisms in stomatal immunity remain to be fully elucidated. In this study, we investigated the role of melatonin in mitigating bacterial invasion of Arabidopsis thaliana by modulating stomatal immune responses. To achieve this, melatonin biosynthesis and receptor mutants were employed to monitor guard cell responses to Pseudomonas syringae . Furthermore, single-cell type metabolomics and multi-omics analyses were integrated to dissect the signaling pathways involved. The results demonstrated that melatonin is essential for maintaining stomatal closure during bacterial infection. Single-cell metabolomics revealed that melatonin decreases jasmonic acid (JA) biosynthesis, thereby alleviating the antagonistic interaction with salicylic acid (SA) and positively regulating resistance to bacterial infections. Moreover, multi-omics data support melatonin-mediated crosstalk among JA/coronatine, SA-Non-Pathogenesis-Related 1 (NPR1), and reactive oxygen species/nitric oxide (ROS/NO) in stomatal immunity. This study reveals a sophisticated melatonin-regulated phytohormone crosstalk that orchestrates stomatal immunity against bacterial pathogens. These findings provide novel insights into the integration of melatonin signaling with systemic defense responses in plants.