Molecular Cloning of ATR5Emoy2 From Hyaloperonospora arabidopsidis, an Avirulence Determinant That Triggers RPP5-mediated Defense in Arabidopsis

RPP5 is the seminal example of a cytoplasmic NB-LRR receptor-like protein that confers downy mildew resistance in Arabidopsis thaliana. In this study, we describe the cloning and molecular characterization of the gene encoding ATR5Emoy2, an avirulence protein from the downy mildew pathogen Hyaloperonospora arabidopsidis isolate Emoy2. ATR5Emoy2 triggers defence response in host lines expressing the functional RPP5 allele from Landsberg erecta (Ler-0). ATR5Emoy2 is embedded in a cluster with two additional ATR5-like (ATR5L) genes, most likely resulting from gene duplications. ATR5L proteins do not trigger RPP5-mediated resistance and the copy number of ATR5Ls varies among H. arabidopsidis isolates. ATR5Emoy2 and ATR5Ls contain a signal peptide, canonical EER motif, and an RGD motif. However, they lack the canonical translocation motif RXLR, which characterizes most oomycete effectors identified so far. The signal peptide and the N-terminal regions including the EER motif of ATR5Emoy2 are not required to trigger an RPP5-dependent immune response. Bioinformatics screen of H. arabidopsidis Emoy2 genome revealed the presence of 173 ORFs that potentially encode for secreted proteins similar to ATR5Emoy2, in which they share some motifs such as EER but there is no canonical RXLR motif.