LAUSR

Abstract Phytophthora species are destructive plant pathogens that cause significant crop losses worldwide. To understand plant susceptibility to oomycete pathogens and to explore novel disease resistance strategies, we employed the Arabidopsis thaliana–Phytophthora parasitica model pathosystem and screened for A. thaliana T‐DNA insertion mutant lines resistant to P. parasitica. This led to the identification of the resistant mutant 267‐31, which carries two T‐DNA insertion sites in the promoter region of the ethylene‐responsive factor 19 gene (ERF019). Quantitative reverse transcription PCR (RT‐qPCR) assays showed that the expression of ERF019 was induced during P. parasitica infection in the wild type, which was suppressed in the 267‐31 mutant. Additional erf019 mutants were generated using CRISPR/Cas9 technology and were confirmed to have increased resistance to P. parasitica. In contrast, ERF019 overexpression lines were more susceptible. Transient overexpression assays in Nicotiana benthamiana showed that the nuclear localization of ERF019 is crucial for its susceptible function. RT‐qPCR analyses showed that the expression of marker genes for multiple defence pathways was significantly up‐regulated in the mutant compared with the wild type during infection. Flg22‐induced hydrogen peroxide accumulation and reactive oxygen species burst were impaired in ERF019 overexpression lines, and flg22‐induced MAPK activation was enhanced in erf019 mutants. Moreover, transient overexpression of ERF019 strongly suppressed INF‐triggered cell death in N. benthamiana. These results reveal the importance of ERF019 in mediating plant susceptibility to P. parasitica through suppression of pathogen‐associated molecular pattern‐triggered immunity.