Cleavage of a pathogen apoplastic protein by plant subtilases activates immunity

The plant apoplast is a harsh environment in which hydrolytic enzymes, especially proteases, accumulate during pathogen infection. However, the defense functions of most apoplastic proteases remains largely elusive. Here, we show that a newly identified small cysteine-rich secreted protein PC2 from the potato late blight pathogen Phytophthora infestans induces immunity in Solanum plant species only after cleavage by plant apoplastic subtilisin-like proteases, such as tomato P69B. A minimal 61-amino-acid core peptide carrying two key cysteines and widely conserved among most oomycete species is sufficient for PC2 activity. Kazal-like protease inhibitors, such as EPI1 produced by P. infestans can prevent PC2 cleavage and dampen PC2 elicited host immunity. This study reveals that cleavage of pathogen proteins to release immunogenic peptides is an important function of apoplastic proteases but that pathogens interfere with these functions using protease inhibitor effectors.