Chemical treatments
Piperonylic acid (PA; Sigma-Aldrich, catalog nr. P49805) was prepared as a 100 mM stock solution in dimethyl sulfoxide (DMSO; Duchefa Biochemie), diluted with distilled water and amended with 0.1% (v/v) Tween 20 (Sigma-Aldrich). Mock treatments consisted of distilled water with the same concentrations of Tween 20 and DMSO. Plants were sprayed with an atomizer until thoroughly wetted. Unless otherwise mentioned, PA was applied twice (8 and 1 days before inoculation) at a concentration of 300 µM.