A disease resistance gene in Arabidopsis with specificity for two different pathogen avirulence genes

The RPS3 and RPM1 disease resistance loci of Arabidopsis confer resistance to Pseudomonas syringae strains that carry the avirulence genes avrB and avrRpm1, respectively. We have previously shown that RPS3 and RPM1 are closely linked genetically. Here, we show that RPS3 and RPM1 are in fact the same gene. We screened a mutagenized Arabidopsis population with a P. syringae strain carrying avrB and found 12 susceptible mutants. All 12 mutants were also susceptible to an isogenic strain carrying avrRpm1, indicating a loss of both RPS3 and RPM1 functions. No mutants were recovered that lost only RPS3 function. Genetic analysis of four independent mutants revealed that the lesions were in RPS3. Thus, a single gene in Arabidopsis confers resistance that is specific to two distinct pathogen avirulence genes--a gene-for-genes interaction. This observation suggests that the RPS3/RPM1 gene product can bind multiple pathogen ligands, or alternatively, that it does not function as a receptor.