Grain number per panicle critically determines rice yield.GNP3 encodes a MITOGEN-ACTIVATED PROTEIN KINASE KINASE KINASE 22 (OsMKKK22) that phosphorylates S-adenosyl-L-methionine synthetase 1 (SAMS1), triggering its degradation to suppress ethylene biosynthesis.To identify the residue(s) being phosphorylated by GNP3, we performed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of recombinant SAMS1 incubated with GNP3.