proline iminopeptidase gene upregulated in planta by a LuxR homologue is essential for pathogenicity of Xanthomonas campestris pv. campestris

Expression of bacterial genes is often regulated by complex mechanisms, some of which involve host cues. Analysis of the Xanthomonas campestris pv. campestris (Xcc) genome sequence revealed the presence of an xccR/pip locus. The upstream gene xccR is a luxR homologue, while pip codes for a proline iminopeptidase. A lux box-like element, named luxXc box, locates in the pip promoter region. In this work, we show that disruption of either xccR or pip resulted in significantly attenuated virulence of Xcc. Under medium culture conditions, the pip expression was significantly enhanced by overexpression of XccR and the luxXc box is necessary for this enhancement. We further show that expression of a pip promoter-gusA fusion either inserted in the bacterial chromosome or resided in a plasmid was markedly induced when the bacteria grew in planta. Disruption of either xccR or the luxXc box abolished the in planta induction, while disruption of pip enhanced the induction. Taken together, these data demonstrate that pip is indispensable for Xcc virulence and suggest a model for Xcc-host interaction in which the pathogen senses some host factor(s) to activate XccR that subsequently interacts with the luxXc box to induce the expression of pip for facilitating Xcc infection.