The superoxide anion (O2⋅−) is another ROS that is commonly produced during the HR. Interestingly, nitro blue tetrazolium (NBT) staining for O2⋅− revealed no significant change in O2⋅− production in N. benthamiana leaves infiltrated with YN-1 or the ΔhrcU mutant (data not shown). These results indicated that H2O2 but not O2⋅− contributes to Xoo-induced hypersensitive necrosis.