R1-mediated immunity triggered by the Phytophthora infestans RXLR effector AVR1 is activated inside the nucleus

Phytophthora infestans is a devastating plant pathogen that causes late blight on potato and tomato. To colonize host plants, P. infestans secretes effectors that can modulate host defence. Well-known are the RXLR effectors that are translocated into host cells to manipulate the cell machinery. To counteract the pathogen, potato exploits nucleotidebinding leucine-rich repeat (NLR) immune receptors that confer resistance against P. infestans upon recognition of a RXLR effector, with each NLR protein (or R protein) having its own matching RXLR effector (or AVR protein). The mechanisms underlying NLR-mediated resistance are still poorly understood. In this study we exploited fluorescent tags and nuclear localization and export signals (NLS/NES) for determining the subcellular localization of the potato NLR protein R1 and the P. infestans RXLR effector AVR1, and for targeting these proteins to nucleus or cytoplasm. Microscopic imaging revealed that both R1 and AVR1 occur in nucleus and cytoplasm, and in close proximity. Transient expression of NLS- or NES-tagged R1 and AVR1 in Nicotiana benthamiana showed that activation of R1-mediated hypersensitive response and resistance requires localization of the R1/AVR1 pair in the nucleus. However, AVR1-mediated suppression of cell death in absence of R1 is dependent on localization of AVR1 in the cytoplasm. A balanced nucleocytoplasmic partitioning of AVR1 seems to be a prerequisite.