Late Blight Effector Pi05910 Suppresses Plant Immunity by Promoting Destabilisation of Host Scaffold Protein RACK1

Late blight, caused by oomycete pathogen Phytophthora infestans, is one of the most devastating diseases of potato and tomato. Plant resistance to pathogens relies on a sophisticated innate immune system, which is often targeted by pathogen-derived effectors. P. infestans secretes numerous RXLR effectors to suppress host immunity. In this study, we show that the P. infestans core effector Pi05910 functions as a virulence factor and targets the host scaffold protein Receptor for Activated C Kinase 1 (RACK1), which is identified to positively regulate potato resistance to late blight. Functional analyses showed that StRACK1-silencing in potato, and tobacco rattle virus (TRV)-based virus-induced NbRACK1-silencing in Nicotiana benthamiana, consistently increased host susceptibility to P. infestans colonisation, whereas transient expression of NbRACK1 in N. benthamiana or stable overexpression of StRACK1 in potato enhanced late blight resistance. Pi05910 physically interacts with RACK1 and promotes its destabilisation via the host 26S proteasome pathway. The destabilisation of RACK1 led to the inhibition of PAMP elicitor INF1-triggered plant immune responses, including reactive oxygen species burst and expression of defence-related genes such as StWRKY7 and StWRKY8, and suppression of potato and N. benthamiana growth and global protein translation. Our findings reveal a strategy whereby a pathogen effector sabotages a key potato immune scaffold StRACK1 to promote plant susceptibility to P. infestans infection, providing new insight into mechanisms of effector-triggered plant susceptibility.