Chiral Mandelic Acid Derivatives as Inhibitors Targeting the Type III Secretion System of Xanthomonas citri Subsp. citri

Xanthomonas citri subsp. citri ( Xcc ), the pathogen responsible for citrus canker, causes major economic damage worldwide. The type III secretion system (T3SS) is essential for Xcc pathogenicity. Here, 39 chiral 1,3,4-oxadiazole sulfide derivatives of mandelic acid were synthesized, with 17 compounds exhibiting significant inhibitory activity (>70% inhibition) against the T3SS of Xcc jx-6 at 40 μg/mL. Among these, the R -configuration W R 4 significantly suppressed the symptoms of citrus canker in an in vivo bioassay, whereas neither the S -configuration W S 4 nor the racemic mixture W RS 4 showed inhibitory effects. Mechanistically, W R 4 suppressed T3SS gene transcription and Hpa1 protein secretion, and counteracted pathogen-induced suppression of host immunity, without affecting other virulence factors. From an environmental safety perspective, W R 4 exhibited low toxicity against nontarget organisms, including earthworms, silkworms, and zebrafish. These findings reveal the potential of the R -configuration derivatives as sustainable T3SS inhibitors.