Selenium-Based Monoselenides as T3SS Inhibitors for Control of Citrus Canker

Citrus canker, caused by Xanthomonas citri subsp. citri (Xcc) remains a serious constraint on citrus production. To explore antivirulence strategies, 75 selenium-containing diselenides and monoselenides were synthesized as a bioisosteric extension of previously reported sulfur-based scaffolds and screened using an hpa1 promoter-lux reporter to evaluate T3SS inhibition. Three monoselenides (II6, II19, and II37) markedly inhibited the hpa1 promoter activity. Among them, II19 and II37 suppressed T3SS-dependent phenotypes, including the hypersensitive response in Nicotiana benthamiana and canker lesions on citrus, without affecting bacterial growth in vitro. II19 also improved disease suppression when used together with the DSF-degrading biocontrol strain Burkholderia anthina HN-8. Transcriptome analysis, supported by qRT-PCR and Western blotting, confirmed that II19 downregulates key hrp/hc genes and limits Hpa1 secretion. Toxicity assays in silkworms and earthworms revealed no adverse effects at concentrations exceeding the practical use levels. These findings highlight monoselenides as environmentally acceptable T3SS inhibitors suitable for integrated citrus canker management.