LAUSR

Plants can produce animal cytokine-like immune peptides, among which the plant elicitor peptides (Peps) derive from the C-termini of their precursors (PROPEPs). Recently, the functions of Peps have been expanded beyond 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 catalytic cysteine residue in MC4 or the conserved arginine residue preceding the Pep1 sequence blocks PROPEP1 cleavage, whereas the bacterial elicitor flg22 can enhance the MC4-mediated PROPEP1 processing. MC4 can cleave PROPEP1 in vitro and also cleave 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 findings reveal the type-II MCs as new players in Pep signaling, and lay the foundation for understanding the regulation of multifaceted functions of Peps in plant immunity and beyond.