A MUTATION IN ARABIDOPSIS THAT LEADS TO CONSTITUTIVE EXPRESSION OF SYSTEMIC ACQUIRED-RESISTANCE

Systemic acquired resistance (SAR) is a nonspecific defense response i n plants that is associated with an increase in the endogenous level o f salicylic acid (SA) and elevated expression of pathogenesis-related (PR) genes. To identify mutants involved in the regulation of PR genes and the onset of SAR, we transformed Arabidopsis with a reporter gene containing the promoter of a beta-1,3-glucanase-encoding PR gene (BGL 2) and the coding region of beta-glucuronidase (GUS). The resulting tr ansgenic line (BGL2-GUS) was mutagenized, and the M(2) progeny were sc ored for constitutive GUS activity. We report the characterization of one mutant, cpr1 (constitutive expresser of PR genes), that was identi fied in this screen and shown by RNA gel blot analysis also to have el evated expression of the endogenous PR genes BGL2, PR-1, and PR-5. Gen etic analyses indicated that the phenotype conferred by cpr1 is caused by a single, recessive nuclear mutation and is suppressed in plants p roducing a bacterial salicylate hydroxylase, which inactivates SA. Fur thermore, biochemical analysis showed that the endogenous level of SA is elevated in the mutant. Finally, the cpr1 plants were found to be r esistant to the fungal pathogen Peronospora parasitica NOCO2 and the b acterial pathogen Pseudomonas syringae pv maculicola ES4326, which are virulent in wild-type BGL2-GUS plants. Because the cpr1 mutation is r ecessive and associated with an elevated endogenous level of SA, we pr opose that the CPR1 gene product acts upstream of SA as a negative reg ulator of SAR.