Disruption of PAMP-Induced MAP Kinase Cascade by a Pseudomonas syringae Effector Activates Plant Immunity Mediated by the NB-LRR Protein SUMM2

Abstract

Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) serves as a primary plant defense response against microbial pathogens, with MEKK1, MKK1/MKK2, and MPK4 functioning as a MAP kinase cascade downstream of PAMP receptors. Plant Resistance (R) proteins sense specific pathogen effectors to initiate a second defense mechanism, termed effector-triggered immunity (ETI). In a screen for suppressors of the mkk1 mkk2 autoimmune phenotype, we identify the nucleotide-binding leucine-rich repeat (NB-LRR) protein SUMM2 and find that the MEKK1-MKK1/MKK2-MPK4 cascade negatively regulates SUMM2-mediated immunity. Further, the MEKK1-MKK1/MKK2-MPK4 cascade positively regulates basal defense targeted by the Pseudomonas syringae pathogenic effector HopAI1, which inhibits MPK4 kinase activity. Inactivation of MPK4 by HopAI1 results in activation of SUMM2-mediated defense responses. Our data suggest that SUMM2 is an R protein that becomes active when the MEKK1-MKK1/MKK2-MPK4 cascade is disrupted by pathogens, supporting the hypothesis that R proteins evolved to protect plants when microbial effectors suppress basal resistance.

Changes in PUB22 Ubiquitination Modes Triggered by MITOGEN-ACTIVATED PROTEIN KINASE3 Dampen the Immune Response

“…In line with this, mpk3 phenocopies the pub22 pub23 pub24 triple mutant, both of which display increased responses (Trujillo et al, 2008;Frei dit Frey et al, 2014). However, MPK4 function during immunity needs to be reassessed, as phenotypes in mpk4 may originate from the activation of the NODlike receptor SUMM2 (Zhang et al, 2012). (C) Electrostatic surface potentials of human E4B (PDB ID code 3L1X) and the structural model of PUB22 Thr62Glu.…”

“…While MPK3 and MPK6 are believed to function as partially redundant positive regulators of immunity, MPK4 has been attributed a negative regulatory role (Rodriguez et al, 2010). However, recent studies have helped to draw a more precise picture, revealing more complex interactions (Popescu et al, 2009;Zhang et al, 2012;Frei dit Frey et al, 2014;Lassowskat et al, 2014).…”

MYB75 Phosphorylation by MPK4 Is Required for Light-Induced Anthocyanin Accumulation in Arabidopsis

“…The mpk4 mutants exhibit autoimmunity because the MPK4 pathway is guarded by the immune receptor SUMM2, which is regulated by SUMM1 Zhang et al, 2012). To test whether mpk4 autoimmunity affects anthocyanin accumulation, we analyzed their accumulation in mpk4-3 summ1 and mpk4-3 summ2 double mutants.…”

Subversion of MAPK signaling by pathogenic bacteria

“…In Arabidopsis, HopAI1 targets MPK3, MPK4 and MPK6 and inactivates them to suppress plant defense responses. 24,25 An additional interesting P. syringae effector is HopF2, an ADP-ribosyltransferase that suppresses two branches of the MAPK cascade. Firstly, it catalyzes ADP-ribosylation of MKK5, thereby suppressing downstream MPK3 and MPK6 activation.…”