Recognition of Phytophthora infestans RXLR-dEER effectors by resistance proteins is triggered by C-terminal domains comprising W motifs

The Phytophthora infestans avirulence genes PiAvr1 and PiAvr4 encode RXLR-dEER effector proteins and belong to a family of oomycete avirulence homologs (Avh). Avh proteins are rapidly evolving but nevertheless, the majority has recognizable C-terminal motifs (Jiang et al. 2008 PNAS). PiAvr4 was isolated by positional cloning. Loss of avirulence on R4 potato is caused by frame shift mutations resulting in truncated PiAvr4 proteins (van Poppel et al. 2008 MPMI). The genomic region harboring PiAvr4 shows conserved synteny with Phytophthora sojae and P. ramorum but PiAvr4 itself is located on a 100 kb indel that breaks the conserved synteny, and is surrounded by transposons. In the C-terminus PiAvr4 has three W motifs and one Y motif. W2 in combination with either W1 or W3 triggers necrosis in potato plants carrying resistance gene R4. PiAvr1 was isolated by anchoring Avr1-associated markers on the genome sequence. This lead to a 800 kb region with seven Avh genes, one of which is PiAvr1, the counterpart of resistance gene R1. Also PiAvr1 has W and Y motifs. Domain swapping revealed which motifs determine avirulence on R1 potato. Analysis of the role of PiAvr1 and PiAvr4 in virulence is in progress.