Gpa2 detects potato cyst nematode effector RBP-1 in the cytoplasm yet requires a nucleocytoplasmic balance for cell death

The potato immune receptor Gpa2 confers host-specific resistance to the cyst nematode Globodera pallida. When transiently expressed in Nicotiana benthamiana it triggers cell death upon recognition of the matching effector GpRBP-1. Effector-triggered immunity by Gpa2 depends on the host factor RanGAP2, which is known to regulate the nucleocytoplasmic distribution and functioning of the highly homologous potato immune receptor Rx1. However, the subcellular localisation of Gpa2 and the role of RanGAP2 in determining the subcellular localisation of Gpa2 is yet unknown. Moreover, cellular mechanisms underlying nematode effector detection by Gpa2 and subsequent cell death activation remain unknown. Here, we co-expressed Gpa2 fused to nuclear localisation signals and its matching effector GpRBP-1 as a model to address these questions. We show that both the nuclear and cytoplasmic pool of Gpa2 contribute to effector-triggered cell death which depends on complex formation with RanGAP2 as a cytoplasmic retention and stabilising factor. However, using nuclear and cytoplasmic targeting signals, we demonstrate that GpRBP-1 detection by Gpa2 occurs specifically in the cytoplasm. From these data, a picture emerges in which RanGAP2 retains Gpa2 in the cytoplasm to form a pre-activation complex, which aids in GpRBP-1 detection and the activation of immune responses in a compartment-specific manner.