A novel elicitor protein from P hytophthora parasitica induces plant basal immunity and systemic acquired resistance

Summary The interaction between P hytophthora pathogens and host plants involves the exchange of complex molecular signals from both sides. Recent studies of P hytophthora have led to the identification of various apoplastic elicitors known to trigger plant immunity. Here, we provide evidence that the protein encoded by OPEL of P hytophthora parasitica is a novel elicitor. Homologues of OPEL were identified only in oomycetes, but not in fungi and other organisms. Quantitative reverse transcription‐polymerase chain reaction ( RT‐PCR ) revealed that OPEL is expressed throughout the development of P . parasitica and is especially highly induced after plant infection. Infiltration of OPEL recombinant protein from E scherichia coli into leaves of N icotiana tabacum (cv. S amsun NN ) resulted in cell death, callose deposition, the production of reactive oxygen species and induced expression of pathogen‐associated molecular pattern ( PAMP )‐triggered immunity markers and salicylic acid‐responsive defence genes. Moreover, the infiltration conferred systemic resistance against a broad spectrum of pathogens, including T obacco mosaic virus , the bacteria wilt pathogen R alstonia solanacearum and P . parasitica . In addition to the signal peptide, OPEL contains three conserved domains: a thaumatin‐like domain, a glycine‐rich protein domain and a glycosyl hydrolase ( GH ) domain. Intriguingly, mutation of a putative laminarinase active site motif in the predicted GH domain abolished its elicitor activity, which suggests enzymatic activity of OPEL in triggering the defence response.