Presence/absence polymorphism and differential expression of two diverged forms of PiAVR2 in Phytophthora infestans determine virulence on R2 plants

A detailed molecular understanding of how oomycete plant pathogens evade disease resistance is essential to inform the deployment of resistance (R) genes that will be durable. Map-based cloning, transient expression in planta, pathogen transformation and DNA sequence diversity across diverse isolates were used to identify and characterize PiAVR2 from the potato late blight pathogen Phytophthora infestans. PiA VR2 is an RXLR-EER effector that is up-regulated during biotrophy, accumulates at the site of haustoria formation, and is recognized inside host cells by the potato R2 protein during infection. Transformation of a virulent P. infestans isolate to express PiAVR2 conveys a gain-of-avirulence phenotype, indicating this is a dominant gene triggering R2-dependent disease resistance. Both presence/absence polymorphism and transcriptional differences explain virulence on R2 plants. Isolates that infect R2 plants express Piavr2, which evades recognition by R2. PiAVR2 and Piavr2 encode proteins that differ in 13 amino acids, 8 of which reside in the C-terminal effector domain; one or more of these specifies recognition by R2. Nevertheless, few polymorphisms were observed within each gene in pathogen isolates, perhaps indicating that PiAVR2 and Piavr2 are fixed within the population. Our results direct a search for R genes that recognize Piavr2 which, deployed in combination with R2, may exert a strong selection pressure against the P. infestans population