Role of Intracellular Hydrogen Peroxide as Signalling Molecule for Plant Senescence

All aerobic organisms use molecular oxygen as terminal oxidant during respiration. Oxygen is neither very reactive nor harmful, but it has the potential to be only partially reduced, leading to the formation of very reactive and therefore toxic intermediates, like singlet oxygen (1O2), superoxide radical (O2·-), hydroperoxylradical (HO2·-), hydrogen peroxide (H2O2) and hydroxylradical (·OH). These forms are called “reactive oxygen species” (ROS). All ROS are extremely reactive and may oxidize biological molecules, such as DNA, proteins and lipids. However, these reactive molecules are unavoidable by-products of an aerobic metabolism. It is known that reactive oxygen species may have a dual role in plant stress response (Dat et al. 2000). Whereas high concentrations of hydrogen peroxide are toxic for the cell, low concentrations may act as signal which triggers the plant response upon a variety of biotic and abiotic stresses (Dat et al., 2000; Grant & Loake, 2000). It has been known for many years that common signal transduction molecules like MAPKs and calmodulin play an important role in some of these ROS signal transduction pathways.