Trends in Plant Science
Higher plants and UV-B radiation: balancing damage, repair and acclimation
Section snippets
Effects of solar UV-B on plants
UV-B radiation has many direct and indirect effects on plants, including damage to DNA, proteins and membranes (Appendix A); alterations in transpiration and photosynthesis; and changes in growth, development and morphology[3]. UV-B exposure was found to lead to a reduction in biomass accumulation in some studies3, 4, but not in others5, 6. Contradictory results might be caused by methodological differences, including levels of UV-B5, 7, PAR[8]and interactions with other environmental factors4,
DNA as a target for UV-B
Absorbance of UV-B photons by DNA triggers the formation of cyclobutane-pyrimidine dimers (CPDs) and, to a lesser extent, pyrimidine (6–4)-pyrimidinone dimers [(6–4) photoproducts][19]. Apart from being mutagenic, DNA modifications disrupt cellular metabolism. Both RNA- and DNA-polymerase are unable to read through unrepaired dimers, leading to a blockage in gene transcription and DNA replication[19]. Repair of UV-B-damaged DNA is mainly via light-dependent photoreactivation[19]. Arabidopsis
Accumulation of secondary metabolites
UV-B induces accumulation of a range of secondary metabolites, which in turn affect numerous physiological functions. Low fluences of UV stimulate the general phenylpropanoid pathway, resulting in accumulation of flavonoids and sinapic esters28, 29. Arabidopsis mutants that do not accumulate flavonoids and/or sinapic esters are highly UV sensitive[29]. Flavonoids and sinapic esters protect by specifically absorbing in the wavelength region from 280 to 340 nm (but not in the PAR waveband, which
Whole-plant responses
UV-B induces changes in leaf and plant morphology (Fig. 3)[6]. The mechanism underlying these alterations is not clear. Leaf curling is a photomorphogenic response, observable at low fluences of UV-B, that helps diminish the leaf area exposed to UV[7]. A protective function has also been hypothesized for leaf or epidermal thickening, as this would increase the length of the UV-B screening pathway (Fig. 3). Indeed, the thick epidermis of field-grown conifers screens UV-B exceptionally efficiently
Conclusions
The biological impact of UV-B radiation on plants is a combined function of damage, repair and acclimation. Multiple targets for UV-B radiation have been identified, although little is known about their environmental relevance. Acclimation and repair mechanisms that diminish the damaging effects of UV-B radiation have evolved9, 12. This is especially true for plants from regions of naturally high levels of UV-B irradiation[9]. Since plants are exposed to continuously varying levels of UV-B,
Acknowledgements
M.A.K.J. acknowledges support by the Royal Netherlands Academy of Arts and Sciences. B.M.G. acknowledges the support of the National Science and Engineering Research Council of Canada. We thank Drs A.R.van der Krol, J.F.H. Snel and A.R.J. Jansen-Driesenaar for critical comments on the manuscript.
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