Associate Research Fellow Academia Sinica Taipei, Taipei, Taiwan (Republic of China)
Body of Abstract: Plants have developed various mechanisms to defend against pathogens, including the maturation of cytokines by proteolytic cleavage. However, the role of proteases with caspase-like activity in regulating fundamental functions of innate immunity in plants has been unclear. Through a sensitive and confident approach to global analysis of the plant peptidome, we discovered CAPE9, a novel peptide cytokine derived from pathogenesis-related protein 1 (PR1) induced by salicylic acid (SA) in Arabidopsis. Production of CAPE9 requires an Enzyme Specific for CAPE (ESCAPE), a caspase-like protease that specifically targets the CNYD domain, a conserved caspase-like substrate adjacent to the CAPE domain of PR1. We showed that the production of CAPE is essential for plant immunity by treating with CAPE9 or locally expressing PR1 without the CAPE domain in the pr1 mutant. We further characterized the CNYD domain and identified a gene encode protein for ESCAPE, which exhibits caspase-like biochemistry and requires cysteine protease activity to hydrolyze aspartic acid in the CNYD substrate. ESCAPE can process PR1-eYFP to generate CAPE9-eYFP in vitro. Genetic studies demonstrated that ESCAPE is involved in the specific recognition and processing of CNYD in plants and is essential for the systemic eliciting of plant systemic acquired resistance (SAR) by the pathogen molecular pattern flg22. As the role of PR1 in plant immunity has been elusive for many decades, our study sheds light on the fundamental function of PR1 in regulating SAR and highlights the importance of caspase-like proteases in the production of CAPE9 from PR1 in this mechanism.