The Harpin-Induced Hypersensitive Reaction in Nicotianatabacum Requires Wall-Associated Kinase 2

Harpins are proteins secreted by many gram-negative, plant-pathogenic bacteria that stimulate the hypersensitive reaction (HR),a host cell death defence response, when infiltrated into plant leaves as purified proteins. This activity of harpins was first discov-ered in Nicotiana tabacum (tobacco), which manifests an especially strong and rapid harpin-activated HR that becomes evidentwithin 12–24 h after infiltration. HrpN is the major harpin of the fire blight pathogen Erwinia amylovora. We discovered naturalvariation in the HrpN-induced HR among tobacco accessions and identified candidate genes using genetic mapping and bulked-segregant analysis with whole genome sequencing. Virus-induced gene silencing of candidate gene Wall-Associated Kinase 2(WAK2) abrogated the HR in response to HrpN and HpaG, a harpin from the soybean bacterial pustule pathogen Xanthomonascitri pv. glycines. WAK2 silencing also compromised the avirulence activity of harpin HrpZ in the tobacco wildfire pathogenPseudomonas syringae pv. tabaci. A natural, disruptive mutation in WAK2 correlated with the inability of tobacco accessionsto mount the harpin-mediated HR. We conclude that the predicted receptor-like kinase WAK2 is required for the strong HRinduced in tobacco leaves by harpin protein infiltration and can potentially mediate resistance to bacterial pathogens based onharpin recognition.