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 H ₂O² 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.