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.