| Summary: | <p>Abstract</p> <p>Background</p> <p>The Gram-negative bacterium <it>Xanthomonas citri </it>subsp. <it>citri </it>(<it>Xac</it>) causes citrus canker, one of the most destructive diseases of citrus worldwide. In our previous work, a transposon mutant of <it>Xac </it>strain 306 with an insertion in the <it>XAC3110 </it>locus was isolated in a screening that aimed at identifying genes related to biofilm formation. The <it>XAC3110 </it>locus was named as <it>bdp24 </it>for biofilm-defective phenotype and the mutant was observed to be affected in extracellular polysaccharide (EPS) and lipopolysaccharide (LPS) biosynthesis and cell motility. In this study, we further characterized the <it>bdp24 </it>(XAC3110) gene (designated as <it>gpsX</it>) using genetic complementation assays and expanded the knowledge about the function of the <it>gpsX </it>gene in <it>Xac </it>pathogenesis by investigating the roles of <it>gpsX </it>in EPS and LPS production, cell motility, biofilm formation on host leaves, stress tolerance, growth <it>in planta</it>, and host virulence of the citrus canker bacterium.</p> <p>Results</p> <p>The <it>gpsX </it>gene encodes a putative glycosyltransferase, which is highly conserved in the sequenced strains of <it>Xanthomonas</it>. Mutation of <it>gpsX </it>resulted in a significant reduction of the amount of EPS and loss of two LPS bands visualized on sodium dodecylsulphate- polyacrylamide gels. Biofilm assays revealed that the <it>gpsX </it>mutation affected biofilm formation by <it>Xac </it>on abiotic and biotic surfaces. The <it>gpsX </it>mutant showed delayed bacterial growth and caused reduced development of disease symptoms in susceptible citrus leaves. The <it>gpsX </it>mutant was more sensitive than the wild-type strain to various stresses, including the H<sub>2</sub>O<sub>2 </sub>oxidative stress. The mutant also showed attenuated ability in cell motility but not in flagellar formation. Quantitative reverse transcription-PCR assays indicated that mutation of <it>gpsX </it>did not affect the expression of virulence genes such as <it>pthA </it>in <it>Xac </it>strain 306. The affected phenotypes of the <it>gpsX </it>mutant could be complemented to wild-type levels by the intact <it>gpsX </it>gene.</p> <p>Conclusions</p> <p>Taken together, our data confirm that the <it>gpsX </it>gene is involved in EPS and LPS synthesis and biofilm formation in <it>Xac </it>and suggest that the <it>gpsX </it>gene contributes to the adaptation of <it>Xac </it>to the host microenvironments at early stage of infection and thus is required for full virulence on host plants.</p>
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