Processing and release of the maize phytocytokine Zip1

Phytocytokines are peptide signaling molecules in plant immunity. Extracellular phytocytokines and their cognate membrane-receptors have been described in various species; however, processing and release of these signals remains largely unknown. The Zea mays immune peptide 1 (Zip1) is a maize-specific phytocytokine, which is associated to the salicylic acid (SA) signaling pathway. Zip1 resides central in its precursor PROZIP1, thus C- and N-terminal cleavage is required to release bioactive Zip1 peptide. Two apoplastic maize PLCPs, CP1 and CP2, were previously shown to cleave PROZIP1. We investigated the localization and processing steps of PROZIP1 leading to Zip1 release and found that PROZIP1 undergoes processing in the endoplasmic reticulum (ER) and cytoplasm, where the N-terminal PROZIP1 is cleaved. C-terminal PROZIP1 translocates to the apoplast via an unconventional secretion pathway, likely involving exocyst-positive organelles (EXPO). The combination of antibody detection, protease cleavage assays and mass spectrometry provided evidence for a proteolytic cascade, in which intracellular processing of PROZIP1 is executed by the calcium-dependent metacaspases ZmMC9 through arginine-dependent cleavage. After secretion, C-terminal PROZIP1 is processed by apoplastic PLCPs, which release, but also degrade the Zip1 peptide. Together, these findings reveal a two-step mechanism of phytocytokine processing, translocation, activation and clearance.