Diterpenoids from Isodon Serra with Inhibitory Activity Against Xanthomonas Oryzae Pathovar Oryzae

Rice bacterial blight is a disease caused by the bacterium Xanthomonas oryzae pv. oryzae (Xoo), continues to threaten global rice production and food security by significantly reducing yields. The frequent application of synthetic bactericides has raised growing concerns over environmental persistence and potential health hazards. In search of safer, plant-derived alternatives, this study investigated the antibacterial potential of four diterpenoids isolated from Isodon serra (I. serra), a traditional Chinese medicinal herb. The compounds, gerardianin C (1), (+)-15-hydroxysalvinolone (2), 11,12,15-trihydroxy-8,11,13-abietatrien-7-one (3), and graciliflorin F (4), purified and structurally characterized using nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (MS) after purification. Antibacterial assays revealed that all four compounds exhibited inhibitory activity against Xoo, with IC50values of 17.2, 22.4, 31.2, and 12.5 M, respectively. Minimum inhibitory concentration (MIC75) and minimum bactericidal concentration MBC assays confirmed graciliflorin F as the most potent compound, substantiating these findings. These results highlight the potential of I. serra-derived diterpenoids as promising candidates for the development of environmentally friendly bactericides targeting phytopathogens such as Xoo. This study not only expands the known bioactivity profile of Isodon species but also contributes to the broader effort to integrate traditional medicinal plants into sustainable agricultural practices.