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 WR4 significantly suppressed the symptoms of citrus canker in an in vivo bioassay, whereas neither the S-configuration WS4 nor the racemic mixture WRS4 showed inhibitory effects. Mechanistically, WR4 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, WR4 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.