Proteasome-Dependent Degradation of RPM1 Desensitizes the RPM1-Mediated Hypersensitive Response

Abstract

The intracellular plant resistance (R) proteins, nucleotide-binding and leucine-rich repeat (NLR) proteins, mediate resistance to pathogens by enabling recognition and rapid response. The response consists of the induction of a defensive suite that typically culminates in the hypersensitive response (HR), death of the plant cells at and around an infection site. The Arabidopsis intracellular innate immune receptor protein RESISTANCE TO PSEUDOMONAS MACULICOLA1 (RPM1) is a coiled-coil (CC) type of NLR protein that specifies resistance to strains of the bacterial pathogen Pseudomonas syringae expressing the type III effector proteins AvrRpm1 and AvrB. We previously demonstrated that RPM1-myc (an epitope-tagged version of RPM1) disappears coincident with the onset of HR induced by AvrRpm1. Infection with P. syringae expressing two other type III effector proteins, AvrRpt2 and AvrRps4, also initiated RPM1-myc disappearance at time points coincident with the HR they initiate through the NLR proteins RESISTANCE TO P. SYRINGAE2 (RPS2) and RESISTANCE TO P. SYRINGAE 4 (RPS4), respectively. Here, we use mutants impaired in NLR gene dependent signaling to demonstrate that disappearance of RPM1-myc requires normal NLR gene dependent signaling steps, but does not require HR. Inhibitors of the 26S proteasome block the disappearance of RPM1-myc and enhance RPM1-myc-dependent cell death. Our data are consistent with a model in which RPM1 is degraded by the 26S proteasome to limit the extent of RPM1-dependent signaling and/or cell death. Furthermore, AvrRpt2 induces disappearance of RPM1-myc in rps2 mutant plants without HR, suggesting that RPM1 is part of the host target of the virulence activity of AvrRpt2.