ATR2 from Arabidopsis-infecting downy mildew requires 4 TIR-NLR immune receptors for full recognition.

Arabidopsis Col-0 RPP2A and RPP2B confer recognition of Arabidopsis downy mildew (Hyaloperonospora arabidopsidis [Hpa]) isolate Cala2, but the identity of the recognized ATR2 effector was unknown. To reveal ATR2, an F population was generated from a cross between Hpa-Cala2 and Hpa-Noks1. We identified ATR2 as a non-canonical RxLR-type effector that carries a signal peptide, a dEER motif, and WY domains but no RxLR motif. Recognition of ATR2 and its effector function were verified by biolistic bombardment, ectopic expression and Hpa infection. ATR2 is recognized in accession Col-0 but not in Ler-0 in which RPP2A and RPP2B are absent. In ATR2 and ATR2 alleles, a frameshift results in an early stop codon. RPP2A and RPP2B are essential for the recognition of ATR2. Stable and transient expression of ATR2 under 35S promoter in Arabidopsis and Nicotiana benthamiana enhances disease susceptibility. Two additional Col-0 TIR-NLR (TNL) genes (RPP2C and RPP2D) adjacent to RPP2A and RPP2B are quantitatively required for full resistance to Hpa-Cala2. We compared RPP2 haplotypes in multiple Arabidopsis accessions and showed that all four genes are present in all ATR2-recognizing accessions.