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Oxidative stress tolerance is critical for xylem colonization and virulence of xylem-limited pathogens Xanthomonas albilineans and Xylella fastidiosa

Mukesh Jain: Institute of Food and Agricultural Sciences, University of Florida

<div><em>Xanthomonas albilineans </em>(Xal) and <em>Xylella fastidiosa </em>(Xf)<em> are</em> xylem-limited bacteria causing leaf scald of sugarcane and Pierce’s disease in grapevines, respectively. Hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) 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 (<em>E. coli</em> <em>katG </em>homologs) are annotated in both Xal and Xf genomes<span>. The Xal <em>katG</em> (VB87_RS05200) was replaced by an enhanced Green florescent protein gene (<em>gfp</em>) </span>via marker exchange. Xal Δ<em>katG/gfp mutant strains exhibited significantly reduced pathogenicity on </em><em>newly emerged </em><em>sugarcane </em><em>leaves </em><em>and compromised viability in planta as compared to wild type Xal. Similarly, </em><span>Xf <em>katG</em> (PD_RS06770) was replaced by <em>nptII</em> via marker exchange. Challenge inoculations of <em>Vitis vinifera</em> grapevines with Xf </span>Δ<em>katG/nptII mutant strains exhibited only limited disease symptom</em><em>s</em><em> near the inoculation sites. The pathogenicity of the </em>Xal Δ<em>katG/gfp and </em>Xf <em>ΔkatG/nptII mutant strains were completely restored by complementation with full-length Xal katG (</em>cloned in pUFR047) and Xf <em>katG</em> <em>(cloned in pBBR1MCS-5). Since Xf has only a single peroxidase gene, it is being evaluated as a target for chemical control.</em></div>