The NLR protein SUMM 2 senses the disruption of an immune signaling MAP kinase cascade via CRCK 3

MAP kinase signaling is an integral part of plant immunity. Disruption of the MEKK 1‐ MKK 1/2‐ MPK 4 kinase cascade results in constitutive immune responses mediated by the NLR protein SUMM 2, but the molecular mechanism is so far poorly characterized. Here, we report that SUMM 2 monitors a substrate protein of MPK 4, CALMODULIN ‐ BINDING RECEPTOR ‐ LIKE CYTOPLASMIC KINASE 3 ( CRCK 3). Similar to SUMM 2, CRCK 3 was isolated from a suppressor screen of mkk1 mkk2 and is required for the autoimmunity phenotypes in mekk1 , mkk1 mkk2, and mpk4 mutants. In wild‐type plants, CRCK 3 is mostly phosphorylated. MPK 4 interacts with CRCK 3 and can phosphorylate CRCK 3 in vitro . In mpk4 mutant plants, phosphorylation of CRCK 3 is substantially reduced, suggesting that MPK 4 phosphorylates CRCK 3 in vivo . Further, CRCK 3 associates with SUMM 2 in planta , suggesting SUMM 2 senses the disruption of the MEKK 1‐ MKK 1/2‐ MPK 4 kinase cascade through CRCK 3. Our study suggests that a MAP kinase substrate is used as a guardee or decoy for monitoring the integrity of MAP kinase signaling.