Recognition-dependent activation of the Arabidopsis RRS1-R/RPS4 TIR-NLR immune receptor pair

Plant TIR domain-containing proteins exhibit NADase activity upon pathogen effector recognition, creating small molecules that activate defense via EDS1 and its associated proteins. Arabidopsis RPS4 and RRS1 comprise a TIR-NLR immune receptor pair that recognizes multiple effector proteins including AvrRps4 and PopP2. We show here that RPS4 and RRS1 acquire NADase activity upon recognition of a cognate effector. However, both in the presence and absence of effector, RPS4 and RRS1 form an oligomer that does not change in size upon effector provision. Effector-dependent defense activation by RPS4 and RRS1 requires the catalytic glutamate residue and the BB-loop motif of RPS4, which is absent in RRS1 and other RRS1-like TIR domains in Arabidopsis. Oligomerization of RPS4 and RRS1 occurs through multiple domains, but oligomerization does not require enzymatic activity. We propose that conformational changes or reconfiguration within the RPS4/RRS1 oligomer upon effector recognition derepresses the complex for immune activation.