Characterization of virulence traits and the underlying regulatory mechanisms of Xanthomonas citri subsp. citri

Xanthomonas citri subsp. citri (XCC) is the causal agent of citrus canker which is one of the most serious diseases of citrus. Citrus canker has a significant impact on national and international citrus markets and trade. An understanding of virulence mechanism of XCC would assist the development of effective control measures against citrus canker. The goals of this study are to identify potential virulence traits of XCC and to characterize the underlying regulatory mechanisms coordinating gene expression in XCC during citrus canker infection. Transposon insertion mutagenesis showed that galU was required for biosynthesis of extracellular polysaccharides, capsular polysaccharide, biofilm formation, and virulence on host. Further study revealed that galU is critical for bacterial fitness in planta. To understand the regulatory mechanisms coordinating the expression of virulence traits in XCC, we designed and conducted genome-wide microarray analyses to identify genes under control of HrpG and HrpX, which are critical regulators for the pathogenicity of XCC. It showed that HrpG and HrpX not only control diverse virulence traits, but also regulate multiple cellular activities responding to the host environment, such as amino acid biosynthesis, oxidative phosphorylation, pentose-phosphate pathway, transport of sugar, iron and potassium, and the phenolic catabolism. To study the regulatory mechanism of quorum sensing on virulence traits of XCC, the mutants of the core genes of quorum sensing, rpfF, rpfC and rpfG genes, were constructed. Comparison of the transcriptomes of QS mutants with that of wild type stain revealed that QS temporally regulates the expression of a large set of genes, including genes involved in chemotaxis and flagellar biosynthesis, genes related to metabolism, and genes encoding virulence traits such as type II secretion system substrates, type III secretion system and effectors. Cross talk between the QS regulon and HrpG regulon has also been identified, suggesting that the interplay of global signaling network by HrpG and QS assists XCC to coordinate the expression of multiple virulence traits for modification and adaptation to the host environment during infection. Altogether, this study demonstrated the complexity of signaling pathways underlying the regulation of XCC virulence traits and the interplay between the regulatory cascades.