Gpa2 detects the potato cyst nematode effector GpRBP-1 in the cytoplasm but requires a balanced nucleocytoplasmic distribution to trigger 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 not yet known. Moreover, the cellular mechanisms underlying detection of the nematode effector by Gpa2 and the subsequent activation of cell death also remain unknown. Here, we co-expressed Solanum tuberosum Gpa2 fused to nuclear localisation signals and its matching effector GpRBP-1 in N. benthamiana as a model to address these questions. The results indicated that both the nuclear and cytoplasmic pools of Gpa2 contribute to effector-triggered cell death and this depends on the formation of a complex with RanGAP2, which acts as a cytoplasmic retention and stabilising factor. However, using nuclear and cytoplasmic targeting signals, we found that detection of GpRBP-1 by Gpa2 occurs specifically in the cytoplasm. Based on these results, we propose that RanGAP2 retains Gpa2 in the cytoplasm to form a pre-activation complex that aids in the detection of GpRBP-1 and the activation of immune responses in a compartment-specific manner.