Plants can produce animal cytokine-like immune peptides,among which plant elicitor peptides(Peps)derive from the C termini of their precursors(PROPEPs).Recently,the functions of Peps have been expanded beyond plant immunity.However,a long-standing enigma is how PROPEPs are processed into Peps.Here,we report that the Ca2+-dependent type-Ⅱ metacaspases(MCs)constitute the proteolytic enzymes to mediate PROPEP processing in Arabidopsis.In protoplasts,co-expression of PROPEP1 with type-Ⅱ MCs,including MC4 to MC9,can promote the generation of processed Pep1.Destruction of the cat-alytic cysteine residue in MC4 or the conserved arginine residue preceding the Pep1 sequence blocks PRO-PEP1 cleavage,whereas the bacterial elicitor flg22 enhances the MC4-mediated PROPEP1 processing.MC4 cleaves PROPEP1 in vitro and also cleaves PROPEP2 to PROPEP8,but,surprisingly,not PROPEP6 in protoplasts.Domain swapping between PROPEP1 and PROPEP6 suggests a hidden role of the sequence context upstream of the Pep sequence for PROPEP processing.flg22-induced PROPEP1 processing and Botrytis cinerea resistance are severely impaired in the mc4/5/6/7 quadruple-mutant plants.Taken together,our study identifies the type-Ⅱ MCs as new players in Pep signaling,and lays the foundation for understanding the regulation of multifaceted functions of Peps in plant immunity and beyond.