Potato NPH3/RPT2-like protein StNRL1, targeted by a Phytophthora infestans RXLR effector, is a susceptibility factor

Plant pathogens deliver effectors to manipulate host processes. We know little about how fungal and oomycete effectors target host proteins to promote susceptibility, yet such knowledge is vital to understand crop disease. We show that either transient expression in Nicotiana benthamiana, or stable transgenic expression in potato (Solanum tuberosum), of Phytophthora infestans RXLR effector Pi02860 enhances leaf colonization by the pathogen. Expression of Pi02860 also attenuates cell death triggered by the P. infestans MAMP INF1, indicating that the effector suppresses pattern-triggered immunity (PTI). However, the effector does not attenuate cell death triggered by Cf4/Avr4 co-expression, showing that it does not suppress all cell death activated by cell surface receptors. Pi02860 interacts in yeast-2-hybrid assays with potato NPH3/RPT2-like 1 (NRL1). Interaction of Pi02860 in planta was confirmed by co-immunoprecipitation and bimolecular fluorescence complementation assays. Virus-induced gene silencing (VIGS) of NRL1 in N. benthamiana resulted in reduced P. infestans colonization and accelerated INF1-mediated cell death, indicating that this host protein acts as a negative regulator of immunity. Moreover, whereas NRL1 VIGS had no effect on the ability of P. infestans effector AVR3a to suppress INF1-mediated cell death, such suppression by Pi02860 was significantly attenuated, indicating that this activity of Pi02860 is mediated by NRL1. Transient overexpression of NRL1 resulted in suppression of INF1-mediated cell death and enhanced P. infestans leaf colonization, demonstrating that NRL1 acts as a susceptibility factor to promote late blight disease.