Requirement of the galU Gene for Polysaccharide Production by and Pathogenicity and Growth In Planta of Xanthomonas citri subsp. citri

Xanthomonas citri subsp.citri is the causal agent of citrus canker, which has a significant impact on citrus production. In this study, we characterized thegalU gene ofX. citri subsp.citri . TwogalU mutants (F6 and D12) were identified in anX. citri subsp.citri EZ-Tn5 Tnp transposon library. Rescue cloning, sequence analysis, and Southern blot analysis indicated that both of these mutants had a single copy of the EZ-Tn5 transposon inserted ingalU in the chromosome. Further study showed thatgalU was required for biosynthesis of extracellular polysaccharides (EPS; xanthan gum) and capsular polysaccharide (CPS) and biofilm formation. Mutation ofgalU resulted in a loss of pathogenicity for grapefruit. The loss of pathogenicity of agalU mutant resulted from its inability to growin planta rather than from the effect on virulence genes. Quantitative reverse transcription-PCR assays indicated that mutation ofgalU did not impair the expression of key virulence genes, such aspthA ofX. citri subsp.citri . Although D12 had a growth rate similar to that of the wild-type strain in nutrient broth, no D12 population became established in the intercellular spaces of citrus leaves. Coinoculation of agalU mutant with the wild-type strain did not promote growth of thegalU mutantin planta . Defects in EPS and CPS production, pathogenicity, and growthin planta of thegalU mutant were complemented to the wild-type level using plasmid pCGU2.1 containing an intactgalU gene. These data indicate that thegalU gene contributes toX. citri subsp.citri growth in intercellular spaces and is involved in EPS and CPS synthesis and biofilm formation.