Overproduction of salicylic acid in plants by bacterial transgenes enhances pathogen resistance

After a hypersensitive response to invading pathogens, plants show elevated accumulation of salicylic acid (SA), induced expression of plant defense g enes, and systemic acquired resistance (SAR) to further infection by a broa d range of pathogens. There is compelling evidence that SA plays a crucial role in triggering SAR. We have transformed tobacco with two bacterial gene s coding for enzymes that convert chorismate into SA by a two-step process. When the two enzymes were targeted to the chloroplasts, the transgenic (CS A, constitutive SA biosynthesis) plants showed a 500- to 1,000-fold increas ed accumulation of SA and SA glucoside compared to control plants. Defense genes, particularly those encoding acidic pathogenesis-related (PR) protein s, were constitutively expressed in CSA plants, This expression did not aff ect the plant phenotype, but the CSA plants showed a resistance to viral an d fungal infection resembling SAR in nontransgenic plants.