Molecular Cloning of ATR5 Emoy2 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 ATR5 Emoy2 , an avirulence protein from the downy mildew pathogen Hyaloperonospora arabidopsidis isolate Emoy2. ATR5 Emoy2 triggers defense response in host lines expressing the functional RPP5 allele from Landsberg erecta (Ler-0). ATR5 Emoy2 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 ATR5L genes varies among H. arabidopsidis isolates. ATR5 Emoy2 and ATR5L proteins 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 ATR5 Emoy2 are not required to trigger an RPP5-dependent immune response. Bioinformatics screen of H. arabidopsidis Emoy2 genome revealed the presence of 173 open reading frames that potentially encode for secreted proteins similar to ATR5 Emoy2 , in which they share some motifs such as EER but there is no canonical RXLR motif.