Normal pollen growth requires tip-localised ROS production from plasma membrane NADPH oxidase and ROS are also implicated in pollen-stigma interactions. The self incompatibility (SI) response, which eventually results in pollen programmed cell death of incompatible pollen, involves instant arrest of pollen tube growth associated with NADPH oxidase inactivation and a very rapid perturbation of respiration associated with increases in mitochondrially-sourced ROS. These processes have been investigated in Arabidopsis expressing the poppy SI system along with the hydrogen peroxide biosensor roGFP2-Orp1 targeted to pollen organelles.