The <i>psyI</i> and <i>psyR</i> Genes of <i>Pseudomonas syringae</i> pv. <i>tomato</i> DC3000 Contribute to Bacterial Virulence on Tomato

The LuxI/LuxR system, which produces and perceives <i>N</i>-acyl homoserine lactones (AHLs), plays a significant role in regulating pathogenesis and communication in gram-negative bacteria. A homologous system exists in <i>Pseudomonas syringae</i> pv. <i>tomato</i> (<i>Pst</i>) DC3000, which is encoded by the single-copy genes <i>psyI/psyR</i>. We created a double-knockout mutant of the AHL synthase (<i>psyI</i>) and the AHL receptor (<i>psyR</i>) genes in <i>Pst</i> DC3000 and a corresponding complemented strain to gain insights into their role in plant-pathogen interactions and the metabolic processes associated with this system in vitro. We observed that the mutant strain <i>psyI^-R^-</i> can overcome stomatal immunity as the parental strain <i>Pst</i> DC3000. However, the <i>psyI^-R^-</i> apoplastic population is significantly smaller than that of <i>Pst</i> DC3000 at 3 days postinoculation. Furthermore, PsyI and PsyR are required for necrosis but not chlorosis in infected tomato leaves, evidenced by the observed pharmacological and genetic complementation of the <i>psyI^-R^-</i> strain. By comparison, the coronatine-deficient mutant DB29 is unable to cause either necrosis or chlorosis on leaves. The reduced virulence of <i>psyI^-R^-</i> is associated with an intermediate induction level of the tomato <i>PR2b</i> marker gene for salicylic acid signaling and reduced expression of bacterial virulence genes in infected leaf tissue. Transcriptomic analysis of the <i>psyI^-R^-</i> and DB29 mutants showed misregulation of genes related to the bacterial secretion system, chemotaxis, flagellar motility, ABC transporters, and iron transporters and a partial overlap between metabolic processes regulated by PsyI/PsyR and coronatine. Altogether, these findings indicate that PsyI and PsyR are required for full bacterium virulence at the later stages of infection in tomato leaves.