Oxidative stress tolerance is critical for xylem colonization and virulence of xylem-limited pathogens Xanthomonas albilineans and Xylella fastidiosa
Xanthomonas albilineans (Xal) and Xylella fastidiosa (Xf) are xylem-limited bacteria causing leaf scald of sugarcane and Pierce’s disease in grapevines, respectively. Hydrogen peroxide (H2O2) is essential for protoxylem differentiation and cell wall lignification during xylem maturation, resulting in an oxidative environment in differentiated (dead) xylem vessels. We hypothesize that xylem-limited pathogens such as Xal and Xf must therefore breach an intrinsic oxidative burden for successful colonization. Genes encoding a bifunctional heme-dependent catalase and broad spectrum peroxidase (E. coli katG homologs) are annotated in both Xal and Xf genomes. The Xal katG (VB87_RS05200) was replaced by an enhanced Green florescent protein gene (gfp) via marker exchange. Xal ΔkatG/gfp mutant strains exhibited significantly reduced pathogenicity on newly emerged sugarcane leaves and compromised viability in planta as compared to wild type Xal. Similarly, Xf katG (PD_RS06770) was replaced by nptII via marker exchange. Challenge inoculations of Vitis vinifera grapevines with Xf ΔkatG/nptII mutant strains exhibited only limited disease symptoms near the inoculation sites. The pathogenicity of the Xal ΔkatG/gfp and Xf ΔkatG/nptII mutant strains were completely restored by complementation with full-length Xal katG (cloned in pUFR047) and Xf katG (cloned in pBBR1MCS-5). Since Xf has only a single peroxidase gene, it is being evaluated as a target for chemical control.