Disease resistance (R) protein that specifically recognizes the hopA1 type III effector avirulence protein from Pseudomonas syringae. Resistance proteins guard the plant against pathogens that contain an appropriate avirulence protein via an indirect interaction with this avirulence protein. That triggers a defense system including the hypersensitive response, which restricts the pathogen growth. H2O + NAD(+) = ADP-D-ribose + H(+) + nicotinamide Interacts with EDS1. A number of isoforms are produced. Ubiquitous. The TIR domain mediates NAD(+) hydrolase (NADase) activity. Self-association of TIR domains is required for NADase activity. Ecotype cv. Columbia does not respond with a hypersensitive response to hopA1.