PA treatment induces a localized ROS burst
To investigate whether upregulation of peroxidase genes in PA-treated shoots (see e.g. POX51 induction in Figure 4 ) is indicative of altered ROS metabolism, histological staining with 3,3-diaminobenzidine (DAB) was executed on tomato leaflets pre-treated with droplets of a PA or mock solution. Sites of PA droplets showed faint staining one hour PA application, intense staining at three and six hours and faint staining after 24 hours (Figure 5a and5c ). The size of the stained area did not exceed that of the PA droplet, which suggests that intense ROS accumulation after PA treatment is a local phenomenon.
To further test the hypothesis that ROS metabolism is affected by PA treatment, guaiacol peroxidase activity was measured in shoots and roots sampled 24 hours after the second foliar PA treatment. PA treatment increased guaiacol peroxidase activity in shoots (+58%, P < 0.001; Figure 5b ), but not in roots (P = 0.91), which further suggests that PA treatment locally affects ROS metabolism.