The secreted PAMP-induced peptide StPIP1_1 activates immune responses in potato

Treatment of potato plants with the pathogen-associated molecular pattern Pep-13 leads to the activation of more than 1200 genes. One of these, StPIP1_1, encodes a protein of 76 amino acids with sequence homology to PAMP-induced secreted peptides (PIPs) from Arabidopsis thaliana. Expression of StPIP1_1 is also induced in response to infection with Phytophthora infestans, the causal agent of late blight disease. Apoplastic localization of StPIP1_1-mCherry fusion proteins is dependent on the presence of the predicted signal peptide. A synthetic peptide corresponding to the last 13 amino acids of StPIP1_1 elicits the expression of the StPIP1_1 gene itself, as well as that of pathogenesis related genes. The oxidative burst induced by exogenously applied StPIP1_1 peptide in potato leaf disks is dependent on functional StSERK3A/B, suggesting that StPIP1_1 perception occurs via a receptor complex involving the co-receptor StSERK3A/B. Moreover, StPIP1_1 induces expression of FRK1 in Arabidopsis in an RLK7-dependent manner. Expression of an RLK from potato with high sequence homology to AtRLK7 is induced by StPIP1_1, by Pep-13 and in response to infection with P. infestans. These observations are consistent with the hypothesis that, upon secretion, StPIP1_1 acts as an endogenous peptide required for amplification of the defense response.