A TonB-dependent transducer is responsible for regulation of pathogenicity-related genes in Xanthomonas axonopodis pv. citri

The TonB-dependent transducer is required for plant pathogenicity in several plant pathogenic bacteria. In this study, we investigated the role of a putative TonB-dependent transducer, XAC4131, in Xanthomonas axonopodis pv. citri, the causal agent of citrus canker disease. A mutation in the XAC4131 gene caused a delay in the elicitation of the hypersensitive reaction in tobacco leaves. However, the pathogenicity in citrus leaves was similar to that of wild type. In hrp-inducing medium, XAC4131 controlled the expression of the hrp regulatory gene hrpG. Also, XAC4131 was involved in expression of the adjacent genes rpoE_XAC4129 and XAC4130, which encode RpoE(XAC4129) and FecR-like protein, respectively. The results suggest that XAC4131 controls the expression of hrpG indirectly, probably via rpoE_XAC4129 and XAC4130. Furthermore, we also demonstrated that transcription of rpoE_XAC4129, XAC4130 and XAC4131 seems to be regulated by the Fur protein.