The WY domain of an RxLR effector drives interactions with a host target phosphatase to mimic host regulatory proteins and promote Phytophthora infestans infection.

Plant pathogens manipulate the cellular environment of the host to facilitate infection and colonization, often leading to plant diseases. To accomplish this, many specialized pathogens secrete virulence proteins called effectors into the host cell, which subvert processes such as immune signalling, gene transcription, and host metabolism. Phytophthora infestans, the causative agent of potato late blight, employs an expanded repertoire of RxLR effectors with WY domains to manipulate the host through direct interaction with protein targets. However, our understanding of the molecular mechanisms underlying the interactions between WY effectors and their host targets remains limited. In this study, we performed a structural and biophysical characterization of the P. infestans WY effector, Pi04314, in complex with the potato Protein Phosphatase 1-c (PP1c). We elucidate how Pi04314 uses a WY domain and a specialised C-terminal loop carrying a KVxF motif that interact with conserved surfaces on PP1c, known to be used by host regulatory proteins for guiding function. Through biophysical and in planta analyses, we demonstrate that Pi04314 WY or KVxF mutants lose their ability to bind PP1c. The loss of PP1c binding correlates with changes in PP1c nucleolar localisation and a decrease in lesion size in plant infection assays. This study provides insights into the manipulation of plant hosts by pathogens, revealing how effectors exploit key regulatory interfaces in host proteins to modify their function and facilitate disease.