Manipulation of salicylate content in Arabidopsis thaliana by the expression of an engineered bacterial salicylate synthase

Salicylic acid (SA) plays a central role as a signalling molecule involved in plant defense against microbial attack. Genetic manipulation of SA biosy nthesis may therefore help to generate plants that are more disease-resista nt. By fusing the two bacterial genes pchA and pchB from Pseudomonas aerugi nosa, which encode isochorismate synthase and isochorismate pyruvate-lyase, respectively, we have engineered a novel hybrid enzyme with salicylate syn thase (SAS) activity. The pchB-A fusion was expressed in Arabidopsis thalia na under the control of the constitutive cauliflower mosaic virus (CaMV) 35 S promoter, with targeting of the gene product either to the cytosol (c-SAS plants) or to the chloroplast (p-SAS plants). In p-SAS plants, the amount of free and conjugated SA was increased more than 20-fold above wild type ( WT) level, indicating that SAS is functional in Arabidopsis. P-SAS plants s howed a strongly dwarfed phenotype and produced very few seeds. Dwarfism co uld be caused by the high SA levels per se or, perhaps more likely, by a de pletion of the chorismate or isochorismate pools of the chloroplast. Target ing of SAS to the cytosol caused a slight increase in free SA and a signifi cant threefold increase in conjugated SA, probably reflecting limited chori smate availability in this compartment. Although this modest increase in to tal SA content did not strongly induce the resistance marker PR-1, it resul ted nevertheless in enhanced disease resistance towards a virulent isolate of Peronospora parasitica. Increased resistance of c-SAS lines was parallel ed with reduced seed production. Taken together, these results illustrate t hat SAS is a potent tool for the manipulation of SA levels in plants.