Xanthomonas citri subsp. citri type III effector PthA4 directs the dynamical expression of a putative citrus carbohydrate-binding protein gene for canker formation

Xanthomonas citrisubsp.citri(Xcc), the causal agent of citrus canker, elicits canker symptoms in citrus plants because of the transcriptional activator-like (TAL) effector PthA4, which activates the expression of the citrus susceptibility geneCsLOB1. This study reports the regulation of the putative carbohydrate-binding protein geneCs9g12620by PthA4-mediated induction of CsLOB1 duringXccinfection. We found that the transcription ofCs9g12620was induced by infection withXccin a PthA4-dependent manner. Even though it specifically bound to a putative TAL effector-binding element in theCs9g12620promoter, PthA4 exerted a suppressive effect on the promoter activity. In contrast, CsLOB1 bound to theCs9g12620promoter to activate its expression. The silencing ofCsLOB1significantly reduced the level of expression ofCs9g12620, which demonstrated thatCs9g12620was directly regulated by CsLOB1. Intriguingly, PhtA4 interacted with CsLOB1 and exerted feedback control that suppressed the induction of expression ofCs9g12620by CsLOB1. Transient overexpression and gene silencing revealed thatCs9g12620was required for the optimal development of canker symptoms. These results support the hypothesis that the expression ofCs9g12620is dynamically directed by PthA4 for canker formation through the PthA4-mediated induction of CsLOB1.