Production of Salicylic Acid Precursors Is a Major Function of Phenylalanine Ammonia-Lyase in the Resistance of Arabidopsis to <i>Peronospora parasitica</i>

Arabidopsis ecotype Columbia (Col-0) seedlings, transformed with a phenylalanine ammonia-lyase 1 promoter (<i>PAL1</i>)--glucuronidase (<i>GUS</i>) reporter construct, were inoculated with virulent and avirulent isolates of <i>Peronospora parasitica</i>. The <i>PAL1</i> promoter was constitutively active in the light in vascular tissue but was induced only in the vicinity of fungal structures in the incompatible interaction. A double-staining procedure was developed to distinguish between GUS activity and fungal structures. The <i>PAL1</i> promoter was activated in cells undergoing lignification in the incompatible interaction in response to the pathogen. Pretreatment of the seedlings with 2-aminoindan-2-phosphonic acid (AIP), a highly specific PAL inhibitor, made the plants completely susceptible. Lignification was suppressed after AIP treatment, and surprisingly, pathogen-induced <i>PAL1</i> promoter activity could not be detected. Treatment of the seedlings with 2-hydroxyphenylaminosulphinyl acetic acid (1,1-dimethyl ester) (OH-PAS), a cinnamyl alcohol dehydrogenase inhibitor specific for the lignification pathway, also caused a shift toward susceptibility, but the effect was not as pronounced as it was with AIP. Significantly, although OH-PAS suppressed pathogen-induced lignification, it did not suppress pathogen-induced <i>PAL1</i> promoter activation. Salicylic acid (SA), supplied to AIP-treated plants, restored resistance and both pathogen-induced lignification and activation of the <i>PAL1</i> promoter. Endogenous SA levels increased significantly in the incompatible but not in the compatible combination, and this increase was suppressed by AIP but not by OH-PAS. These results provide evidence of the central role of SA in genetically determined plant disease resistance and show that lignification per se, although providing a component of the resistance mechanism, is not the deciding factor between resistance and susceptibility.