ATR2 C ala2 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 ^Cala2 effector was unknown. To reveal ATR2 ^Cala2 , an F ₂ population was generated from a cross between Hpa-Cala2 and Hpa-Noks1. We identified ATR2 ^Cala2 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 ^Cala2 and its effector function were verified by biolistic bombardment, ectopic expression and Hpa infection. ATR2 ^Cala2 is recognized in accession Col-0 but not in Ler-0 in which RPP2A and RPP2B are absent. In ATR2 ^Emoy2 and ATR2 ^Noks1 alleles, a frameshift results in an early stop codon. RPP2A and RPP2B are essential for the recognition of ATR2 ^Cala2 . Stable and transient expression of ATR2 ^Cala2 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 ^Cala2 -recognizing accessions.
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