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

Abstract

<i>Xanthomonas citri</i> subsp. <i>citri</i> (<i>Xcc</i>), 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 gene <i>CsLOB1</i>. This study reports the regulation of the putative carbohydrate-binding protein gene <i>Cs9g12620</i> by PthA4-mediated induction of CsLOB1 during <i>Xcc</i> infection. We found that the transcription of <i>Cs9g12620</i> was induced by infection with <i>Xcc</i> in a PthA4-dependent manner. Even though it specifically bound to a putative TAL effector-binding element in the <i>Cs9g12620</i> promoter, PthA4 exerted a suppressive effect on the promoter activity. In contrast, CsLOB1 bound to the <i>Cs9g12620</i> promoter to activate its expression. The silencing of <i>CsLOB1</i> significantly reduced the level of expression of <i>Cs9g12620</i>, which demonstrated that <i>Cs9g12620</i> was directly regulated by CsLOB1. Intriguingly, PhtA4 interacted with CsLOB1 and exerted feedback control that suppressed the induction of expression of <i>Cs9g12620</i> by CsLOB1. Transient overexpression and gene silencing revealed that <i>Cs9g12620</i> was required for the optimal development of canker symptoms. These results support the hypothesis that the expression of <i>Cs9g12620</i> is dynamically directed by PthA4 for canker formation through the PthA4-mediated induction of CsLOB1.