PRX34 mediates the oxidative burst in Arabidopsis. Here we characterized two additional Arabidopsis prx34 null mutants (prx34-2, prx34-3), besides the well-studied prx34-1. Due to a decrease in corresponding peroxidase, the activity that generates reactive oxygen species (ROS) was significantly lower in cell wall extracts of prx34-2 and prx34-3 plants. Consistently, the prx34-2 and prx34-3 exhibited reduced accumulation both of ROS and callose in Flg22-elicitor-treated leaves, leading to enhanced susceptibility to bacterial and fungal pathogens. In contrast, ectopic expression of PRX34 in the wild type caused enhanced resistance. PRX34 is thus a component for disease resistance in Arabidopsis.