The Cyst Nematode SPRYSEC Protein RBP-1 Elicits Gpa2- and RanGAP2-Dependent Plant Cell Death

Plant NB-LRR proteins confer robust protection against microbes and metazoanparasites by recognizing pathogen-derived avirulence (Avr) proteins that aredelivered to the host cytoplasm. Microbial Avr proteins usually function asvirulence factors in compatible interactions; however, little is known about thetypes of metazoan proteins recognized by NB-LRR proteins and their relationshipwith virulence. In this report, we demonstrate that the secreted protein RBP-1from the potato cyst nematode Globodera pallida elicits defenseresponses, including cell death typical of a hypersensitive response (HR),through the NB-LRR protein Gpa2. Gp-Rbp-1 variants fromG. pallida populations both virulent and avirulent toGpa2 demonstrated a high degree of polymorphism, withpositive selection detected at numerous sites. All Gp-RBP-1protein variants from an avirulent population were recognized by Gpa2, whereasvirulent populations possessed Gp-RBP-1 protein variants bothrecognized and non-recognized by Gpa2. Recognition of Gp-RBP-1by Gpa2 correlated to a single amino acid polymorphism at position 187 in theGp-RBP-1 SPRY domain. Gp-RBP-1 expressedfrom Potato virus X elicited Gpa2-mediated defenses that required RanGTPase-activating protein 2 (RanGAP2), a protein known to interact with the Gpa2N terminus. Tethering RanGAP2 and Gp-RBP-1 variants via fusionproteins resulted in an enhancement of Gpa2-mediated responses. However,activation of Gpa2 was still dependent on the recognition specificity conferredby amino acid 187 and the Gpa2 LRR domain. These results suggest a two-tieredprocess wherein RanGAP2 mediates an initial interaction with pathogen-deliveredGp-RBP-1 proteins but where the Gpa2 LRR determines whichof these interactions will be productive.