Abstract
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 discovered in Nicotiana tabacum (tobacco), which manifests an especially strong and rapid harpin-activated HR that becomes evident within 12-24 h after infiltration. HrpN is the major harpin of the fire blight pathogen Erwinia amylovora. We discovered natural variation 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 Xanthomonas citri pv. glycines. WAK2 silencing also compromised the avirulence activity of harpin HrpZ in the tobacco wildfire pathogen Pseudomonas syringae pv. tabaci. A natural, disruptive mutation in WAK2 correlated with the inability of tobacco accessions to mount the harpin-mediated HR. We conclude that the predicted receptor-like kinase WAK2 is required for the strong HR induced in tobacco leaves by harpin protein infiltration and can potentially mediate resistance to bacterial pathogens based on harpin recognition.