The Exopolysaccharide of Xylella fastidiosa Is Essential for Biofilm Formation, Plant Virulence, and Vector Transmission

Abstract

Exopolysaccharides (EPS) synthesized by plant-pathogenic bacteria are generally essential for virulence. The role of EPS produced by the vector-transmitted bacterium Xylella fastidiosa was investigated by knocking out two genes implicated in the EPS biosynthesis, gumD and gumH. Mutant strains were affected in growth characteristics in vitro, including adhesion to surfaces and biofilm formation. In addition, different assays were used to demonstrate that the mutant strains produced significantly less EPS compared with the wild type. Furthermore, gas chromatography-mass spectrometry showed that both mutant strains did not produce oligosaccharides. Biologically, the mutants were deficient in movement within plants, resulting in an avirulent phenotype. Additionally, mutant strains were affected in transmission by insects: they were very poorly transmitted by and retained within vectors. The gene expression profile indicated upregulation of genes implicated in cell-to-cell signaling and adhesins while downregulation in genes was required for within-plant movement in EPS-deficient strains. These results suggest an essential role for EPS in X. fastidiosa interactions with both plants and insects.

Genomic Insights into Xylella fastidiosa Interactions with Plant and Insect Hosts

“…Antibodies that could recognize the EPS produced by X. fastidiosa were recently used to illustrate its production both in culture and in planta (Roper et al 2007). Generation of gum mutants (Killiny et al 2013) recently helped to better characterize the role of EPS in X. fastidiosa virulence. In addition to their reduced capacity to form biofilms in culture, gum mutants, once mechanically introduced in plants, were avirulent and did not result in the development of any symptoms in grapevines (Killiny et al 2013).…”

“…Generation of gum mutants (Killiny et al 2013) recently helped to better characterize the role of EPS in X. fastidiosa virulence. In addition to their reduced capacity to form biofilms in culture, gum mutants, once mechanically introduced in plants, were avirulent and did not result in the development of any symptoms in grapevines (Killiny et al 2013). These traits were also associated with an apparent lack of plant colonization and an altered motility compared to that of the wildtype cells.…”

Response of Xylella fastidiosa to Zinc: Decreased Culturability, Increased Exopolysaccharide Production, and Formation of Resilient Biofilms under Flow Conditions

“…The same is true for EPSs, which binds cation to their negatively charged groups (71,72). The negatively charged residues in the EPS from X. fastidiosa (21,41) could chelate Zn, which was shown to accumulate Zn in biofilms (13), thus preventing accumulation of toxic levels inside the bacterial cell. A similar well-characterized case involves the response of E. amylovora to copper, where the bacterium increases EPS production upon contact with high levels of Cu (73).…”

“…Here, the expression of genes involved in EPS synthesis in X. fastidiosa (gtaB, xanB, gumD, and gumH) was analyzed to identify changes in expression induced by the presence of Zn. GtaB (UTP-glucose-1-phosphate uridylyltransferase) transfers a UDP unit to glucose-1-P. XanB (phosphomannose isomerase-GDP-mannose pyrophosphorylase) transfers a GDP unit to mannose-1-P. GumD (UMP-gluco- syltransferase) and GumH (GDP-mannosyltransferase) add the first glucose and mannose units, respectively, to the EPS repeating unit (21,41). As shown by RT-qPCR, Zn affects the initial steps of EPS synthesis in X. fastidiosa, increasing the availability of nucleotidyl precursors.…”

“…The biofilm matrix can contain EPS, proteins, and/or nucleic acids (19), and during infection, EPS protects bacteria from coming into contact with toxic molecules and can induce symptoms such as wilt in the host, as in Ralstonia solanacearum or Erwinia amylovora infections (20). In X. fastidiosa, it was demonstrated recently that EPS production is funda-mental for biofilm production, virulence, and insect transmission (21) and that X. fastidiosa biofilms accumulate high levels of Zn and other heavy metals (13). Also, in the closely related Xanthomonas spp., mutants affected in Zn homoeostasis are compromised in EPS production and virulence (22,23).…”