LAUSR

Summary Phytophthora infestans causes severe losses in potato production. The MAPK kinase StMKK1 was previously found to negatively regulate potato immunity to P. infestans. Our results showed that StMKK1 interacts with a protein tyrosine phosphatase, referred to as StPTP1a, and StMKK1 directly phosphorylates StPTP1a at residues Ser‐99, Tyr‐223 and Thr‐290. StPTP1a is a functional phosphatase and the phosphorylation of StPTP1a at these three residues enhances its stability and catalytic activity. StPTP1a negatively regulates potato immunity and represses SA‐related gene expression. Furthermore, StPTP1a interacts with, and dephosphorylates, the StMKK1 downstream signalling targets StMPK4 and −7 at their Tyr‐203 residue resulting in the repression of salicylic acid (SA)‐related immunity. Silencing of NbPTP1a + NbMPK4 or NbPTP1a + NbMPK7 abolished the plant immunity to P. infestans caused by NbPTP1a silencing, indicating that PTP1a functions upstream of NbMPK4 and NbMPK7. StMKK1 requires StPTP1a to negatively regulate SA‐related immunity and StPTP1a is phosphorylated and stabilized during immune activation to promote the de‐phosphorylation of StMPK4 and −7. Our results reveal that potato StMKK1 activates and stabilizes the tyrosine phosphatase StPTP1a that in its turn de‐phosphorylates StMPK4 and −7, thereby repressing plant SA‐related immunity.