Protein-tyrosine-phosphatase that acts as a negative regulator of MPK6 and MPK3 signaling by dephosphorylating and repressing MPK6 and MPK3. Modulates defense response by repressing salicylic acid (SA) production, camalexin biosynthesis and SNC1-mediated responses. Acts as a negative regulator of MPK6-mediated pathogen-associated molecular pattern (PAMP) responses, including MPK6 and MPK3 activation, accumulation of extracellular reactive oxygen species and inhibition of seedling growth. Involved in UV-B stress tolerance. May be involved in salt and genotoxic stress responses. H2O + O-phospho-L-tyrosyl-[protein] = L-tyrosyl-[protein] + phosphate Interacts with MPK6. May interact with MPK3 and MPK4. Phosphorylated on threonine and serine residues by MPK6. No visible phenotype under normal growth conditions in Wassilewskija (Ws) ecotype (PubMed:11274055), but Columbia (Col) ecotype show growth defects, elevated levels of salicylic acid (SA) and constitutive defense responses (PubMed:19789277). The observed mkp1 phenotype in Col ecotype is largely due to the Col-specific TIR-NB-LRR receptor-like protein SNC1, which is absent in Ws ecotype.