Specific Threonine Phosphorylation of a Host Target by Two Unrelated Type III Effectors Activates a Host Innate Immune Receptor in Plants

SummaryThe Arabidopsis NB-LRR immune receptor RPM1 recognizes the Pseudomonas syringae type III effectors AvrB or AvrRpm1 to mount an immune response. Although neither effector is itself a kinase, AvrRpm1 and AvrB are known to target Arabidopsis RIN4, a negative regulator of basal plant defense, for phosphorylation. We show that RIN4 phosphorylation activates RPM1. RIN4142-176 is necessary and, with appropriate localization sequences, sufficient to support effector-triggered RPM1 activation, with the threonine residue at position 166 being critical. Phosphomimic substitutions at T166 cause effector-independent RPM1 activation. RIN4 T166 is phosphorylated in vivo in the presence of AvrB or AvrRpm1. RIN4 mutants that lose interaction with AvrB cannot be coimmunoprecipitated with RPM1. This defines a common interaction platform required for RPM1 activation by phosphorylated RIN4 in response to pathogenic effectors. Conservation of an analogous threonine across all RIN4-like proteins suggests a key function for this residue beyond the regulation of RPM1.

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► RIN4142-176 is necessary and sufficient to support effector-triggered RPM1 activation
► RIN4 T166 is phosphorylated in response to AvrB and AvrRpm1 and essential for RPM1 activation
► RIN4 T166 phosphomimic can activate RPM1 independent of bacterial effectors
► RIN4 and RPM1 interaction is required for RPM1 activation by RIN4 phosphomimic