Perception of SA by its receptors is required for activation of NHP biosynthesis during SAR, plays crucial roles in PTI and ETI, and is involved in regulating S

Arabidopsis (Arabidopsis thaliana) MAP KINASE (MPK) proteins can function in multiple MAP kinase cascades and physiological processes. For instance, MPK4 functions in regulating development as well as in plant defense by participating in two independent MAP kinase cascades: the MEKK1-MKK1/MKK2-MPK4 cascade promotes basal resistance against pathogens and is guarded by the NB-LRR protein SUMM2, whereas the ANPs-MKK6-MPK4 cascade plays an essential role in cytokinesis. Here, we report a novel role for MKK6 in regulating plant immune responses. We found that MKK6 functions similarly to MKK1/MKK2 and works together with MEKK1 and MPK4 to prevent autoactivation of SUMM2-mediated defense responses. Interestingly, loss of MKK6 or ANP2/ANP3 results in constitutive activation of plant defense responses. The autoimmune phenotypes of mkk6 and anp2 anp3 mutant plants can be largely suppressed by a constitutively active mpk4 mutant. Further analysis showed that the constitutive defense response in anp2 anp3 is dependent on the defense regulators PAD4 and EDS1, but not on SUMM2, suggesting that the ANP2/ANP3-MKK6-MPK4 cascade may be guarded by a TIR-NB-LRR protein. Our study shows that MKK6 has multiple functions in plant defense responses in addition to cytokinesis.

Salicylic acid (SA) serves as a critical signaling molecule in plant defense. Two transcription factors, SARD1 and CBP60g, control SA biosynthesis through regulating pathogen-induced expression of Isochorismate Synthase1, which encodes a key enzyme for SA biosynthesis. Here, we report that Pattern-Triggered Immunity Compromised Receptor-like Cytoplasmic Kinase1 (PCRK1) and PCRK2 function as key regulators of SA biosynthesis. In the pcrk1 pcrk2 double mutant, pathogen-induced expression of SARD1, CBP60g, and ICS1 is greatly reduced. The pcrk1 pcrk2 double mutant, but neither of the single mutants, exhibits reduced accumulation of SA and enhanced disease susceptibility to bacterial pathogens. Both PCRK1 and PCRK2 interact with the pattern recognition receptor FLS2, and treatment with pathogen-associated molecular patterns leads to rapid phosphorylation of PCRK2. Our data suggest that PCRK1 and PCRK2 function downstream of pattern recognition receptor in a signal relay leading to the activation of SA biosynthesis.