UV-irradiation provokes generation of superoxide on cell wall polygalacturonic acid.

We examined the redox effects of UV irradiation on cell wall isolates from Pisum sativum leaves, and polygalacturonic and galacturonic acid, in the presence of hydrogen peroxide. For this purpose, electron paramagnetic resonance spectroscopy and two spin-traps (DEPMPO and BMPO), capable of differentiating between various free radicals, were applied. Systems were exposed to UV-B (maximum emission at 312 nm) and UV-A (352 nm) for 10 min (6 J m(-2) s(-1)). Cell wall isolates exposed to UV in the presence of hydrogen peroxide, produced hydroxyl radical, carbon dioxide radical and superoxide. The production of superoxide was observed for cell wall isolates, polygalacturonic acid (in the presence and in the absence of calcium) and galacturonic acid, and it was diminished upon superoxide dismutase supplementation. The production is at least partially based on the reaction of hydroxyl radicals with (poly)galacturonic acid having carbon dioxide radicals as a products. Acting as a strong reducing agent, carbon dioxide radical reacts with molecular oxygen to produce superoxide. The results presented here shed a new light on: (1) the redox-modulating role of cell wall; (2) the production of superoxide in the extracellular compartment; (3) the mechanisms involved in translating UV stress into molecular signaling and (4) some other UV-related phenomena in plants, such as CO(2) emission.