Hydrogen peroxide (H2O2) is produced predominantly in plant cells during photosynthesis and
photorespiration, and to a lesser extent, in respiration processes. It is the most stable of the so–
called reactive oxygen species (ROS), and therefore plays a crucial role as a signalling molecule
in various physiological processes. Intra- and intercellular levels of H2O2 increase during environmental
stresses. Hydrogen peroxide interacts with thiol-containing proteins and activates different
signalling pathways as well as transcription factors, which in turn regulate gene expression
and cell-cycle processes. Genetic systems controlling cellular redox homeostasis and H2O2
signalling are discussed. In addition to photosynthetic and respiratory metabolism, the extracellular
matrix (ECM) plays an important role in the generation of H2O2, which regulates plant
growth, development, acclimatory and defence responses. During various environmental stresses
the highest levels of H2O2 are observed in the leaf veins. Most of our knowledge about H2O2 in
plants has been obtained from obligate C3 plants. The potential role of H2O2 in the photosynthetic
mode of carbon assimilation, such as C4 metabolism and CAM (Crassulacean acid metabolism)
is discussed. We speculate that early in the evolution of oxygenic photosynthesis on Earth, H2O2
could have been involved in the evolution of modern photosystem II.