<i>Xylella fastidiosa</i> (<i>Xf</i>) is a gram-negative, xylem-limited plant pathogenic bacterium that causes disease in a variety of economically important agricultural crops including Pierce’s disease of grapevine. <i>Xf</i> biofilms formed in xylem vessels play a key role in early colonization and pathogenicity by providing a protected niche and enhanced cell survival. Like many other bacteria, <i>Xf</i> possesses homologs to the PhoP/Q two-component regulatory system, which differentially regulates genes in responses to divalent periplasmic cation concentration, antimicrobial peptides and other environmental stimuli. <i>Xf</i> knockout mutants deficient in the production of PhoP/Q exhibit phenotypic differences in cell dispersal and clumping when grown in liquid culture. Grapevine pathogenicity assays showed PhoP/Q mutants are non-pathogenic and are unable to successfully colonize or move within the xylem vessels. These results are likely due to an inability of <i>Xf </i>to successfully sense, respond and adapt to the nutrient-limited environment of the xylem. PhoQ, responsible for sensing the environment and essential for <i>Xf</i> survival in grapevines, is being evaluated as a novel disease control target through the use of inhibitory peptides. The PhoP regulon is also being explored to further our understanding of PhoP/Q induced gene regulation in <i>Xf.</i>
