Metabolic Analysis Reveals Alkaloid-Mediated Resistance of Tobacco against Ralstonia solanacearum

Tobacco (Nicotiana tabacum L.) is a globally important industrial crop, yet its productivity is severely threatened by bacterial wilt caused by R. solanacearum. In this study, integrative metabolomics and transcriptomics profiling of tobacco plants infected with R. solanacearum identified 571 differentially accumulated metabolites (DAMs), peaking at 96 h postinoculation (hpi). Among 88 alkaloid DAMs, six plumerane alkaloids (indole, indole-3-acetic acid (IAA), 3-indoleacetonitrile (IAN), methoxyindoleacetic acid (MIAA), N-acetyl-5-hydroxytryptamine, and N-acetylisatin) showed the most pronounced accumulation. Functional assays demonstrated that indole and IAN directly inhibited bacterial growth by disrupting membrane integrity. In contrast, MIAA displayed no direct antibacterial activity but significantly reduced the disease index by inducing a reactive oxygen species (ROS) burst and upregulating the expression of defense-related genes. These findings establish plumerane alkaloids as critical contributors to plant chemical defense against bacterial wilt and identify MIAA as a promising plant-derived elicitor for disease control.