To identify the key phosphorylation residues of SlLYK4, we performed liquid chromatography-tandem mass spectrometry (LC-MS/MS)in vitro kinase assays with liquid of GST-SlLYK4 and MBP-SlLYK13CD or MBP (mock) after in vitro kinase assays. And then SlLYK4-FLAG was transiently expressed in tomato hairy roots and samples were collected post EPS or chitin treatment, followed by immunoprecipitation-mass spectrometry analysis.