Department of Ecology and Evolution, University of Chicago, 1101 East 57th Street, Chicago 60637, U.S.A.
Pseudomonas viridiflava is a common pathogen of Arabidopsis thaliana in wild populations, yet very little is known about mechanisms of resistance and virulence in this interaction. We examined the induced defense response of A. thaliana to several strains of P. viridiflava collected from this host by quantifying the expression of PR-1 and LOX2/PDF1.2, which serve as markers for induction of the salicylic and jasmonic acid (JA) pathways, respectively. Growth of these strains then was assessed on Col-0, the fad3/7/8 and coi1-1 mutants deficient in JA- and ethylene (ET)-induced defense responses, and the sid2-1 mutant deficient in salicylic acid-induced defense responses. All strains of P. viridiflava induced high expression of LOX2 and PDF1.2 on Col-0. In contrast, PR-1 expression was delayed and reduced relative to PDF1.2 expression. Additionally, three of four P. viridiflava strains were more virulent on fad3/7/8 relative to Col-0, whereas all strains were more virulent on coi1-1 relative to Col-0, indicating that P. viridi-flava generally may be suppressed by JA/ET-mediated defense responses. In contrast, no increase in the growth of P. viridiflava strains was observed in the sid2-1 mutant relative to Col-0. Parallel experiments were performed with the closely related P. syringae pv. tomato for comparative purposes. In addition, we assessed the role of pectate lyase and the alternative sigma factor HrpL in P. viridiflava virulence on A. thaliana and found that pectate lyase activity is correlated with virulence, whereas the removal of pectate lyase or HrpL significantly reduced virulence.