A conserved Phytophthora apoplastic trypsin-like serine protease targets the receptor-like kinase BAK1 to dampen plant immunity.

Perception of microbial pathogens by plant cell-surface pattern recognition receptors (PRRs) activates pattern-triggered immunity (PTI) in plants. The receptor-like kinase BAK1 functions as co-receptor of many PRRs and is a central immune regulator in PTI signal transduction. However, the molecular mechanism by which microbial pathogens manipulate BAK1 in the apoplast to overcome this layer of immunity remains largely unknown. In this study, we performed a large-scale screening of Phytophthora apoplastic effectors suppressing cell death triggered by Phytophthora elicitin INF1 and identified an apoplastic trypsin-like serine protease PsTry1. PsTry1 associates with BAK1 in soybean and N. benthamiana, and widely suppresses immune response triggered by different MAMPs. Further study revealed that PsTry1 cleaves the extracellular domain of soybean GmBAK1 and the ability of PsTry1 to suppress plant immunity depends on its proteolysis activity. An extensive Ala substitution mutagenesis screen revealed that Leu163 of GmBAK1 is a key residue essential for PsTry1 cleavage. Furthermore, PsTry1 is highly conserved among Phytophthora pathogens and multiple homologues are capable of suppressing PTI through cleavage of BAK1. Collectively, this study reveals a novel strategy exploited by phytopathogens to suppress plant apoplastic immunity.