A Gain-of-Function Mutation in a Plant Disease Resistance Gene Leads to Constitutive Activation of Downstream Signal Transduction

Plants have evolved sophisticated defense mechanisms against pathogen infections, during which resistance ( R ) genes play central roles in recognizing pathogens and initiating defense cascades. Most of the cloned R genes share two common domains: the central domain, which encodes a nucleotide binding adaptor shared by APAF-1, certain R proteins, and CED-4 (NB-ARC), plus a C-terminal region that encodes Leu-rich repeats (LRR). In Arabidopsis, a dominant mutant, suppressor of npr1-1 , constitutive 1 ( snc1 ), was identified previously that constitutively expresses pathogenesis-related ( PR ) genes and resistance against both Pseudomonas syringae pv maculicola ES4326 and Peronospora parasitica Noco2. The snc1 mutation was mapped to the RPP4 cluster. In snc1 , one of the TIR-NB-LRR–type R genes contains a point mutation that results in a single amino acid change from Glu to Lys in the region between NB-ARC and LRR. Deletions of this R gene in snc1 reverted the plants to wild-type morphology and completely abolished constitutive PR gene expression and disease resistance. The constitutive activation of the defense responses was not the result of the overexpression of the R gene, because its expression level was not altered in snc1 . Our data suggest that the point mutation in snc1 renders the R gene constitutively active without interaction with pathogens. To analyze signal transduction pathways downstream of snc1 , epistasis analyses between snc1 and pad4-1 or eds5-3 were performed. Although the resistance signaling in snc1 was fully dependent on PAD4 , it was only partially affected by blocking salicylic acid (SA) synthesis, suggesting that snc1 activates both SA-dependent and SA-independent resistance pathways.