C14-HSL derivatives induce resistance against P. syringae. A, Proliferation of Pst on 5-week-old Arabidopsis was analyzed at hpi as indicated. Plants were untreated (control) or pretreated with acetone or 6 μm oxo-C14-HSL, 3 d prior to infection with Pst bacteria. **P ≤ 0.005; ***P ≤ 0.0005. B, Colony forming units (cfu) of Pst harvested from 5-week-old Arabidopsis plants pretreated for 3 d with acetone, 6 μm oxo-C14-HSL, or 6 μm OH-C14-HSL, and subsequently inoculated with Pst bacteria for 96 h. a = P < 5.2E-6, b = P < 9.7E-11 (Student’s t test).