We reveal that a family of plant stress-induced signalling peptides, SERINE-RICH ENDOGENOUS PEPTIDES (SCOOPs), is much larger than originally thought, and identify key proteases required for their biogenesis. We find that impairing SCOOP biogenesis phenocopies a mutant of the SCOOP receptor MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2).