Importance of the rpf gene cluster in phyllosphere colonization and pathogenicity of Xanthomonas albilineans

The genome of Xanthomonas albilineans, the causal agent of sugarcane leaf scald, contains a cluster of genes called rpf for regulation of pathogenicity factors. This cluster of genes encodes a quorum sensing (also called cell-cell signaling) system that controls several cellular and biological processes in Xylella fastidiosa, Stenotrophomonas maltophilia and several species of Xanthomonas. The rpf gene cluster is especially used by these pathogenic bacteria to regulate the expression of factors contributing to virulence such as extracellular polysaccharides, extracellular enzymes and flagellum synthesis. This cell-cell signaling is mediated by a signal molecule called DSF (for Diffusible Signaling Factor), and gene rpfF has been shown to be required for production of DSF. Perception of DSF and signal transduction depends on a two-component signal transduction system which comprises the sensor kinase RpfC and the regulator RpfG. The role of these rpf genes in X albilineans is not yet clear. In this study, several mutants of the rpf gene cluster of X albilineans were assessed for their capacity i) to produce DSF and disease symptoms, ii) to colonize the phyllosphere and iii) to spread within the sugarcane stalk. Mutations of rpfF in strain XaFL07-1 of X albilineans from Florida resulted in loss of DSF production and no or only small reduction of disease severity. Single deletion mutants of rpfC and rpfG also produced symptoms that were similar in severity to those caused by the wild type strain of the pathogen. However, when the entire rpfGCF region was deleted, the capacity of the pathogen to cause disease symptoms was slightly reduced and swimming motility was severely affected. Similarly, the capacity of the pathogen to colonize the phyllosphere of the host plant was affected only when the entire rpfGCF region was deleted. Unexpectedly, this latter mutant appeared to produce another signalling molecule, as evidenced by a positive reaction of the DSF reporter strain. Furthermore, all mutations in the DSF signal transduction system resulted in the capacity of the pathogen to colonize sugarcane spatially more efficiently than the wild type strain, whereas mutations in rpfF did not. Therefore, DSF encoded by rpfF does not appear essential for symptom expression, phyllosphere and sugarcane stalk colonization by X. albilineans strain XaFL07-1. However, mutation data in the sensor/regulator system suggest that pathogenicity and cell. cell signalling of X. albilineans may involve another signalling molecule or another sensor/regulator that remain to be identified. (Résumé d'auteur