Production and diffusion of chloroplastic H2O2 and its implication to signalling

Hydrogen peroxide (H2O2) is recognized as an important signalling molecule. There are two important aspects to this function: H2O2 production and its diffusion to its sites of action. The production of H2O2 by photosynthetic electron transport and its ability to diffuse through the chloroplast envelope membranes has been investigated using spin trapping electron paramagnetic resonance spectroscopy and H2O2-sensitive fluorescence dyes. It was found that, even at low light intensity, a portion of H2O2 produced inside the chloroplasts can leave the chloroplasts thus escaping the effective antioxidant systems located inside the chloroplast. The production of H2O2 by chloroplasts and the appearance of H2O2 outside chloroplasts increased with increasing light intensity and time of illumination. The amount of H2O2 that can be detected outside the chloroplasts has been shown to be up to 5% of the total H2O2 produced inside the chloroplasts at high light intensities. The fact that H2O2 produced by chloroplasts can be detected outside these organelles is an important finding in terms of understanding how chloroplastic H2O2 can serve as a signal molecule.