MAP kinase signaling is an integral part of plant immunity. Disruption of the MEKK1‐MKK1/2‐MPK4 kinase cascade results in constitutive immune responses mediated by the NLR protein SUMM2, but the molecular mechanism is so far poorly characterized. Here, we report that SUMM2 monitors a substrate protein of MPK4, CALMODULIN‐BINDING RECEPTOR‐LIKE CYTOPLASMIC KINASE 3 (CRCK3). Similar to SUMM2, CRCK3 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, CRCK3 is mostly phosphorylated. MPK4 interacts with CRCK3 and can phosphorylate CRCK3 in vitro. In mpk4 mutant plants, phosphorylation of CRCK3 is substantially reduced, suggesting that MPK4 phosphorylates CRCK3 in vivo. Further, CRCK3 associates with SUMM2 in planta, suggesting SUMM2 senses the disruption of the MEKK1‐MKK1/2‐MPK4 kinase cascade through CRCK3. Our study suggests that a MAP kinase substrate is used as a guardee or decoy for monitoring the integrity of MAP kinase signaling.
Pathogen‐dependent disruption of the MEKK1‐MKK1/2‐MPK4 kinase cascade results in defence responses mediated by the NLR protein SUMM2. This study shows that SUMM2 senses this disruption via the MPK4 substrate CRCK3, which is also required for the SUMM2‐dependent autoimmune phenotypes in kinase cascade mutants.
CALMODULIN‐BINDING RECEPTOR‐LIKE CYTOPLASMIC KINASE 3 (CRCK3) is required for the autoimmune phenotypes in mekk1, mkk1 mkk2, and mpk4 mutants.
CRCK3 is a substrate of MPK4 monitored by the nucleotide binding and leucine‐rich repeat receptor (NLR) protein SUMM2.
Inactivation of MPK4 leads to reduced phosphorylation of CRCK3 and activation of SUMM2‐mediated immune responses.
EMBO Reports (2017) 18: 292–302
- Received May 10, 2016.
- Revision received October 25, 2016.
- Accepted November 4, 2016.
- © 2016 The Authors
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