As the steady decline in the Earth's stratospheric ozone layer and parallel increase in solar ultraviolet-B (UV-B: 280-315 nm) has come to an end, the focus of plant UV research has been shifted from regarding UV-B as threatening plant life to recognizing it as a regulatory factor. While UV-B photoreceptor mediated signaling is increasingly understood, the role of UV-B inducible reactive oxygen species is still to be explored. Earlier experiments with high UV-B irradiation doses and isolated thylakoid membranes demonstrated the potential of UV-B to trigger oxidative stress.However, under realistic UV conditions pro-oxidants cannot be reliably traced in more complex biological samples possessing an array of antioxidant defenses. In the absence of direct experimental evidence we must rely on indications and propose hypotheses on how and whether pro-oxidants, such as reactive oxygen species contribute to acclimative responses. Here we briefly review how a balance between pro-oxidants and antioxidants is affected by UV-B in whole plant experiments performed in controlled environments. A working hypothesis is proposed in which the extents of UVinduced peroxidase and superoxide dismutase activations affect the success of acclimation to UV-B.

a b s t r a c tSolar UV-B (280-315 nm) radiation is a developmental signal in plants but may also cause oxidative stress when combined with other environmental factors. Using computer modeling and in solution experiments we show that UV-B is capable of photosensitizing hydroxyl radical production from hydrogen peroxide. We present evidence that the oxidative effect of UV-B in leaves is at least twofold: (i) it increases cellular hydrogen peroxide concentrations, to a larger extent in pyridoxine antioxidant mutant pdx1.3-1 Arabidopsis and; (ii) is capable of a partial photo-conversion of both 'natural' and 'extra' hydrogen peroxide to hydroxyl radicals. As stress conditions other than UV can increase cellular hydrogen peroxide levels, synergistic deleterious effects of various stresses may be expected already under ambient solar UV-B.

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