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 catalytic cysteine residue in MC4 or the conserved arginine residue preceding the Pep1 sequence blocks PROPEP1 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.

植物可以产生动物细胞因子样的免疫肽，其中植物激发子肽（Peps）来源于其前体（PROPEP）的C末端。最近，Peps的功能已扩展到超出植物免疫力的范围。但是，将PROPEPs加工成Peps的过程由来已久。在这里，我们报告说，Ca ²⁺依赖型II型metacaspases（MCs）构成介导拟南芥中PROPEP加工的蛋白水解酶。。在原生质体中，PROPEP1与II型MC（包括MC4至MC9）的共表达可促进加工的Pep1的生成。在Pep1序列之前破坏MC4中的催化半胱氨酸残基或保守的精氨酸残基会阻止PROPEP1的裂解，而细菌激发子flg22会增强MC4介导的PROPEP1的加工。MC4在体外切割PROPEP1 ，也将PROPEP2切割为PROPEP8，但令人惊讶的是，在原生质体中却不切割PROPEP6。PROPEP1和PROPEP6之间的域交换表明，对于PROPEP处理，Pep序列上游的序列上下文具有隐藏的作用。flg22诱导PROPEP1处理和灰葡萄孢抗性是严重损害在MC4 / 5 / 6 /7个四突变植物。综上所述，我们的研究确定了II型MCs是Pep信号传导的新参与者，并为理解Peps在植物免疫力及其他方面的多方面功能的调控奠定了基础。