Activation of an EDS1-mediated R-gene pathway in the snc1 mutant leads to constitutive, NPR1-independent pathogen resistance

The Arabidopsis NPR1 protein is an essential regulatory component of system ic acquired resistance (SAR). Mutations in the NPR1 gene completely block t he induction of SAR by signals such as salicylic acid (SA). An Arabidopsis mutant, size] (suppressor of npr1-1, constitutive 1), was isolated in a scr een for suppressors of npr1-1. In the npr1-1 background, the snc1 mutation resulted in constitutive resistance to Pseudomonas syringae maculicola ES43 26 and Peronospora parasitica Noco2. High levels of SA were detected in the mutant and shown to be required for manifestation of the site] phenotype. The snc1 mutation was mapped to the RPP5 resistance (R) gene cluster and th e eds1 mutation that blocks RPP5-mediated resistance suppressed snc1. These data suggest that a RPP5-related resistance pathway is activated constitut ively in snc1. This pathway does not employ NPR1. but requires the signal m olecule SA and the function of EDS1. Moreover, in snc1, constitutive resist ance is conferred in the absence of cell death, which is often associated w ith R-gene mediated resistance.