| Abstract |
The depletion of stratospheric ozone over the Antarctic and Arctic has been observed since 1974 and 1990, respectively. The principal consequence of stratospheric ozone depletion is the increase in ultraviolet B (UVB: 280-320 nm) radiation reaching the Earth's surface. The potential impact of the enhanced solar UVB radiation predicted by atmospheric models has been the subject of investigation for the last two decades. The data collected from various reports involves roughly 300 species and varieties of plants. Around one-third to one-half of these plants showed physiological damage and/or growth reductions in response to UVB. Many studies have identified PSII as the most labile component of the photosynthetic apparatus to elevated UVB radiation. Still now the underlying mechanisms are a controversial subject that makes difficult to evaluate the environmental relevance of UVB effects on photosynthesis. Several different target sites have been proposed. These include the reaction center of PSII, the light harvesting complex (LHCII) and the acceptor/donor side of PSII. In spite of the great amount of research devoted to the effects of UVB radiation on plants during the past decades, efforts are still needed to clarify the molecular background of the UVB damage, as well as the protective and repair mechanisms. The primary target of UVB radiation in the photosynthetic apparatus is not clearly established. In addition, there are discrepancies between laboratory and field studies that make it difficult to estimate how much the projected increase in UVB radiation at the Earth's surface will affect photosynthesis. In this context, this work was focused on the factors determining the sensitivity/tolerance of the photosynthetic apparatus to UVB and their regulation. The investigations were carried out in cultures of wt and wt-lhc mutant (similar to wt but without LHCII) of Scenedesmus obliquus. The results demonstrate that there is a fine mechanism that regulates the photosynthetic behavior to UVB radiation. This mechanism adjusts the molecular structure, conformation and function of the photosynthetic apparatus to UVB through regulation of the LHCII antenna. When exposed to UVB irradiation, which increases the over-excitation of PSII reaction centers, the photosynthetic apparatus adopts a behavior that simulates photoadaptation to low PAR (photosynthetically active radiation: 400-700 nm) intensities. Moreover, this UVBinduced alteration is strongly affected by the PAR background used during UVB treatment. Low PAR (LL) intensities increase the susceptibility of the photosynthetic apparatus to UVB damage, whilst high PAR (HL) intensities confer certain degree of protection, making the photosynthetic apparatus more tolerant to UVB stress. Furthermore, the synergistic action of LL or the antagonistic action of HL with UVB radiation is related to the changes in the thylakoidal Put/Spm ratio that adjust the oligomerization status of LHCII. The overall conclusion is that polyamine changes in the thylakoid membranes act as a primary mechanism which adjusts the degree of sensitivity of the photosynthetic apparatus to UVB radiation by regulating the size of the functional antenna and therefore the photochemical and non-photochemical quenching of absorbed energy. UVB simulates the same molecular and bioenergetic changes that characterize the adaptive response of the photosynthetic apparatus to low light intensities. This means a low Put/Spm ratio in thylakoids which leads to increase in the LHCII size, inactivation of reaction centers and, therefore, enhanced nonphotochemical quenching. Photoadaptation to high light conditions induces exactly the opposite changes (high Put/Spm ratio in thylakoids leads to a LHCII size decrease, activation of reaction centers and, subsequently, to increased photochemistry rates). Therefore, HL adaptation acts antagonistically to the UVB effect and enhances the tolerance against UVB radiation. In contrast, LL adaptation amplifies the UVB effect and minimizes the tolerance and enhances the sensitivity to UVB. In fact, PAR intensity influences the excitation pressure of PSII, which adjusts the balance of Put and Spm levels in thylakoids and especially in LHCII forms. Comparative experiments with wt and wt-lhc mutant (similar to wt but without LHCII) cultures confirmed that the sensitivity/tolerance of a photosynthetic organism depends on the LHCII characteristics. Series of action spectra and the difference of action spectra between wt and wt-lhc cultures (x(wtwt-lhc)) showed clearly that three primary photoreceptors (active and inactive PChlide (620-640/442 nm), an unknown carotenoid absorbing at 535 nm and the reaction center of PSI (690-730 nm) increase the tolerance of the photosynthetic apparatus to UVB by inducing responses that simulate HL adaptation and subsequently reduce the over-excitation exerted on PSII by UVB. To the contrary, chlorophylls (Chl a and b) are the primary photoreceptors responsible for the enhanced sensitivity of the photosynthetic apparatus against UVB radiation by increasing the excitation pressure exerted on PSII. Another important finding of the present study is that a photosynthetic apparatus without LHCII has no potential to recover the UVB-induced damage. Moreover, even when there is LHCII, recovery is strictly expressed under light conditions. Cultures incubated in darkness during UVB treatment have no potential to restore the PSII activity affected by UVB. The practical importance of the present study consists in the fact that through artificial changes of the Put/Spm ratio (exogenous supplied polyamines) it is possible to simulate LL-adapted or HL-adapted photosynthetic apparatus and therefore organisms are absolutely tolerant or sensitive towards UVB, independent from the ambient light conditions. In the context of projected increases in the levels of UVB radiation reaching the Earth's surface, as predicted by different scenarios of ozone depletion, data presented herein conclusively indicate that the photosynthetic apparatus possesses the tools and mechanisms to adapt to stress by adjusting the polyamine pattern and subsequently the balance between energy absorption and dissipation. Good examples of this conclusion are the plants grown in the Mediterranean region, generally or, more specifically, in Crete. Although in Crete plants receive daily a great amount of solar energy (high PAR and UVB intensities), their growth is not affected by UVB because the intense photosynthetic capacity in a high PAR environment counteracts the harmful effects of UVB radiation.
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