Resistance to the Pseudomonas syringae Effector HopA1 Is Governed by the TIR-NBS-LRR Protein RPS6 and Is Enhanced by Mutations in SRFR1   

The Pseudomonas syringae-Arabidopsis (Arabidopsis thaliana) interaction is an extensively studied plant-pathogen system. Arabidopsis possesses approximately 150 putative resistance genes encoding nucleotide binding site (NBS) and leucine-rich repeat (LRR) domain-containing proteins. The majority of these belong to the Toll/Interleukin-1 receptor (TIR)-NBS-LRR (TNL) class. Comparative studies with the coiled-coil-NBS-LRR genes RPS2, RPM1, and RPS5 and isogenic P. syringae strains expressing single corresponding avirulence genes have been particularly fruitful in dissecting specific and common resistance signaling components. However, the major TNL class is represented by a single known P. syringae resistance gene, RPS4. We previously identified hopA1 from P. syringae pv syringae strain 61 as an avirulence gene that signals through ENHANCED DISEASE SUSCEPTIBILITY1, indicating that the corresponding resistance gene RPS6 belongs to the TNL class. Here we report the identification of RPS6 based on a forward-genetic screen and map-based cloning. Among resistance proteins of known function, the deduced amino acid sequence of RPS6 shows highest similarity to the TNL resistance protein RAC1 that determines resistance to the oomycete pathogen Albugo candida. Similar to RPS4 and other TNL genes, RPS6 generates alternatively spliced transcripts, although the alternative transcript structures are RPS6 specific. We previously characterized SRFR1 as a negative regulator of avrRps4-triggered immunity. Interestingly, mutations in SRFR1 also enhanced HopA1-triggered immunity in rps6 mutants. In conclusion, the cloning of RPS6 and comparisons with RPS4 will contribute to a closer dissection of the TNL resistance pathway in Arabidopsis.