CHARACTERIZATION OF AN ARABIDOPSIS MUTANT THAT IS NONRESPONSIVE TO INDUCERS OF SYSTEMIC ACQUIRED-RESISTANCE

Systemic acquired resistance (SAR) is a general defense response in pl ants that is characterized by the expression of pathogenesis-related ( PR) genes. SAR can be induced after a hypersensitive response to an av irulent pathogen or by treatment with either salicylic acid (SA) or 2, 6-dichloroisonicotinic acid (INA). To dissect the signal transduction pathway of SAR, we isolated an Arabidopsis mutant that lacks the expre ssion of an SA-, INA-, and pathogen-responsive chimeric reporter gene composed of the 5' untranslated region of an Arabidopsis PR gene, beta -1,3-glucanase (BGL2), and the coding region of beta-glucuronidase (GU S). This mutant, npr1 (nonexpresser of PR genes), carries a single rec essive mutation that abolishes the SAR-responsive expression of other PR genes as well. While SA-, INA-, or avirulent pathogen-induced SAR p rotects wild-type plants from Pseudomonas syringae infection, the muta nt cannot be protected by pretreatment with these inducers. The insens itivity of npr1 to SA, INA, and avirulent pathogens in SAR induction i ndicates that these inducers share a common signal transduction pathwa y. Moreover, in npr1, the localized expression of PR genes induced by a virulent Pseudomonas pathogen is disrupted, and the lesion formed is less confined. These results suggest a role for PR genes in preventin g the proximal spread of pathogens in addition to their suggested role in SAR.