A host KH RNA-binding protein is a susceptibility factor targeted by an RXLR effector to promote late blight disease

Plant pathogens deliver effector proteins that alter host processes to create an environment conducive to colonization. Attention has focused on identifying the targets of effectors and how their manipulation facilitates disease. RXLR effector Pi04089 from the potato blight pathogen Phytophthora infestans accumulates in the host nucleus and enhances colonization when transiently expressed in planta. Its nuclear localization is required for enhanced P.infestans colonization. Pi04089 interacts in yeast and in planta with a putative potato K-homology (KH) RNA-binding protein, StKRBP1. Co-localization of Pi04089 and StKRBP1, and bimolecular fluorescence complementation between them, indicate they associate at nuclear speckles. StKRBP1 protein levels increased when it was co-expressed with Pi04089. Indeed, such accumulation of StKRBP1 was observed also on the first day of leaf colonization by the pathogen. Remarkably, overexpression of StKRBP1 significantly enhances P.infestans infection. Mutation of the nucleotide-binding motif GxxG to GDDG in all three KH domains of StKRBP1 abolishes its interaction with Pi04089, its localization to nuclear speckles, and its increased accumulation when co-expressed with the effector. Moreover, the mutant StKRBP1 protein no longer enhances leaf colonization by P.infestans, implying that nucleotide binding is likely required for this activity. We thus argue that StKRBP1 can be regarded as a susceptibility factor, as its activity is beneficial to the pathogen. One of the key questions in plant pathology is how the effector proteins from microbial pathogens alter host processes to promote susceptibility. We show that an effector from the potato late blight pathogen targets a host RNA-binding protein, increasing its stability. Remarkably, the RNA-binding protein acts as a susceptibility factor, enhancing disease development.