A fungal pathogen manipulates phytocytokine signaling for plant infection.

Phytocytokines, hormone-like plant peptides, play crucial roles in immune regulation and development. Phytosulfokine (PSK), known to mediate plant growth, also modulates plant pattern-triggered immunity (PTI). Here, we demonstrate that VdSCP8, a small cysteine-containing effector from Verticillium dahliae, functions as a virulence-promoting protein that suppresses PTI in Arabidopsis thaliana and Nicotiana benthamiana. Apoplastic SCP8 suppresses immune activation mediated by leucine-rich repeat ectodomain pattern recognition receptors. SCP8 virulence and immunosuppressive activities require PHYTOSULFOKINE RECEPTOR 1 (PSKR1), which binds PSK and forms a complex with co-receptor BAK1 to suppress PTI. Our findings underscore SCP8's role in PTI suppression, facilitating PSKR1-BAK1 complex formation and promoting PSK accumulation through a plant subtilase. These results highlight how a multi-host plant pathogen manipulates PTI through enhancing immunosuppressive PSK signaling.