Damage to functional components and partial degradation of Photosystem II reaction center proteins upon chloroplast exposure to ultraviolet-B radiation

UV-B-induced molecular mechanisms of stress physiology responses in the major northern Chinese conifer Pinus tabuliformis Carr

During their lifetimes, plants are exposed to different abiotic stress factors eliciting various physiological responses and triggering important defense processes. For UV-B radiation responses in forest trees, the genetics and molecular regulation remain to be elucidated. Here, we exposed Pinus tabuliformis Carr., a major conifer from northern China, to short-term high-intensity UV-B and employed a systems biology approach to characterize the early physiological processes and the hierarchical gene regulation, which revealed a temporal transition from primary to secondary metabolism, the buildup of enhanced antioxidant capacity and stress-signaling activation. Our findings showed that photosynthesis and biosynthesis of photosynthetic pigments were inhibited, while flavonoids and their related derivates biosynthesis, as well as glutathione and glutathione S-transferase mediated antioxidant processes, were enhanced. Likewise, stress-related phytohormones (jasmonic acid, salicylic acid and ethylene), kinase and reactive oxygen species signal transduction pathways were activated. Biological processes regulated by auxin and karrikin were, for the first time, found to be involved in plant defense against UV-B by promoting the biosynthesis of flavonoids and the improvement of antioxidant capacity in our research system. Our work evaluated the physiological and transcriptome perturbations in a conifer's response to UV-B, and generally, highlighted the necessity of a systems biology approach in addressing plant stress biology.

Coordinated downregulation of the photosynthetic apparatus as a protective mechanism against UV exposure in the diatom Corethron hystrix

The effect of ultraviolet radiation (UVR) on photosynthetic efficiency and the resulting mechanisms against UV exposure employed by phytoplankton are not completely understood. To address this knowledge gap, we developed a novel close-coupled, wavelength-configurable platform designed to produce precise and repeatable in vitro irradiation of Corethron hystrix, a member of a genera found abundantly in the Southern Ocean where UV exposure is high. We aimed to determine its metabolic, protective, and repair mechanisms as a function of varying levels of specific electromagnetic energy. Our results show that the physiological responses to each energy level of UV have a negative linear decrease in the photosynthetic efficiency of photosystem II proportional to UV intensity, corresponding to a large increase in the turnover time of quinone reoxidation. Gene expression changes of photosystem II-related reaction center proteins D1, CP43, and CP47 showed coordinated downregulation whereas the central metabolic pathway demonstrated mixed expression of up and downregulated transcripts after UVR exposure. These results suggest that while UVR may damage photosynthetic machinery, oxidative damage may limit production of new photosynthetic and electron transport complexes as a result of UVR exposure.

Exclusion of solar UV radiation improves photosynthetic performance and yield of wheat varieties

Field studies were conducted to determine the potential for alterations in photosynthetic performance and grain yield of four wheat (Triticum aestivum) varieties of India- Vidisha, Purna, Swarna and Naveen Chandausi by ambient ultraviolet radiation (UV). The plants were grown in specially designed UV exclusion chambers, wrapped with filters that excluded UV-B (<315 nm), UV-A/B (<400 nm) or transmitted ambient UV or lacked filters. The results indicated that solar UV exclusion increased the leaf mass per area ratio, leaf weight ratio and chlorophylls per unit area of flag leaves in all the four varieties of wheat. Polyphasic chlorophyll a fluorescence transients from the flag leaves of UV excluded wheat plants gave a higher fluorescence yield. Exclusion of solar UV significantly enhanced photosynthetic performance as a consequence of increased efficiency of PS II, performance index (PIABS) and rate of photosynthesis in the flag leaves of wheat varieties along with a remarkable increase in carbonic anhydrase, Rubisco and nitrate reductase activities. This additional fixation of carbon and nitrogen by exclusion of UV was channelized towards the improvement in grain yield of wheat varieties as there was a decrease in the UV-B absorbing substances and an increase in soluble protein content in flag leaves of all the four varieties of wheat. The magnitude of response for UV exclusion for all the measured parameters was higher in two varieties of wheat Vidisha and Purna as compared to Swarna and Naveen Chandausi. Cumulative stress response index (CSRI) for each variety was developed from the cumulative sum of physiological and yield parameters such as leaf mass area ratio of flag leaf, total chlorophyll content, performance index at absorption basis, rate of photosynthesis and grain yield. All the varieties had a negative CSRI, demonstrating a negative impact of ambient UV radiation. Naveen Chandausi and Swarna are less sensitive to ambient UV radiation; Vidisha is more sensitive to both UV-A and UV-B and Purna is more sensitive to ambient UV-B radiation.

Isoprene emission aids recovery of photosynthetic performance in transgenic Nicotiana tabacum following high intensity acute UV-B exposure

Isoprene emission by terrestrial plants is believed to play a role in mitigating the effects of abiotic stress on photosynthesis. Ultraviolet-B light (UV-B) induces damage to the photosynthetic apparatus of plants, but the role of isoprene in UV-B tolerance is poorly understood. To investigate this putative protective role, we exposed non-emitting (NE) control and transgenic isoprene emitting (IE) Nicotiana tabacum (tobacco) plants to high intensity UV-B exposure. Methanol emissions increased with UV-B intensity, indicating oxidative damage. However, isoprene emission was unaffected during exposure to UV-B radiation, but declined in the 48 h following UV-B treatment at the highest UV-B intensities of 9 and 15 Wm(-2). Photosynthesis and the performance of photosystem II (PSII) declined to similar extents in IE and NE plants following UV-B exposure, suggesting that isoprene emission did not ameliorate the immediate impact of UV-B on photosynthesis. However, after the stress, photosynthesis and PSII recovered in IE plants, which maintained isoprene formation, but not in NE plants. Recovery of IE plants was also associated with elevated antioxidant levels and cycling; suggesting that both isoprene formation and antioxidant systems contributed to reinstating the integrity and functionality of cellular membranes and photosynthesis following exposure to excessive levels of UV-B radiation.

A study of the phase transition of reheated diphenyl carbazide (DPC) by using UV spectroscopy

Phase transition phenomenon in reheated diphenyl carbazide (DPC) is studied here using UV spectroscopy. The optical band gap for reheated DPC is obtained by measuring the optical diffused reflectance (DR) and equals to 3.55 eV. Also, the optical band gap is calculated using UV technique and equals to 3.548 eV. The absorbance of reheated DPC is studied at some selected temperatures in order to check the presence of phase transitions at 90°C and 125°C. According to the present work, the band gaps are calculated at 80°C, 110°C and 130°C and equal to 3.548 eV. But at 100°C, the optical band gap has changed to 4.139 eV. It was found that each phase of reheated DPC belongs to a certain definite crystal structure. The presence of the phase transitions are checked and confirmed by scanning electron microscopy (SEM). The structural properties and morphology of reheated diphenyl carbazide are investigated by SEM. The SEM images are taken at some selected temperatures to confirm the presence of phase transitions.

Hydrogen peroxide contributes to the ultraviolet-B (280-315 nm) induced oxidative stress of plant leaves through multiple pathways

Solar 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.

Impact of increasing Ultraviolet-B (UV-B) radiation on photosynthetic processes

Increased UV-B radiation on the earth's surface due to depletion of stratospheric ozone layer is one of the changes of current climate-change pattern. The deleterious effects of UV-B radiation on photosynthesis and photosynthetic productivity of plants are reviewed. Perusal of relevant literature reveals that UV-B radiation inflicts damage to the photosynthetic apparatus of green plants at multiple sites. The sites of damage include oxygen evolving complex, D1/D2 reaction center proteins and other components on the donor and acceptor sides of PS II. The radiation inactivates light harvesting complex II and alters gene expression for synthesis of PS II reaction center proteins. Mn cluster of water oxidation complex is the most important primary target of UV-B stress whereas D1 and D2 proteins, quinone molecules and cytochrome b are the subsequent targets of UV-B. In addition, photosynthetic carbon reduction is also sensitive to UV-B radiation which has a direct effect on the activity and content of Rubisco. Some indirect effects of UV-B radiation include changes in photosynthetic pigments, stomatal conductance and leaf and canopy morphology. The failure of protective mechanisms makes PS II further vulnerable to the UV-B radiation. Reactive oxygen species are involved in UV-B induced responses in plants, both as signaling and damaging agents. Exclusion of ambient UV components under field conditions results in the enhancement of the rate of photosynthesis, PS II efficiency and subsequently increases the biomass accumulation and crop yield. It is concluded that predicted future increase in UV-B irradiation will have significant impact on the photosynthetic efficiency and the productivity of higher plants.

Responses of photosynthetic properties and chloroplast ultrastructure of Bryum argenteum from a desert biological soil crust to elevated ultraviolet-B radiation

Our understanding of plant responses to enhanced ultraviolet-B (UV-B) radiation has improved over recent decades. However, research on cryptogams is scarce and it remains controversial whether UV-B radiation causes changes in physiology related to photosynthesis. To investigate the effects of supplementary UV-B radiation on photosynthesis and chloroplast ultrastructure in Bryum argenteum Hedw., specimens were cultured for 10 days under four UV-B treatments (2.75, 3.08, 3.25 and 3.41 W m(-2) ), simulating depletion of 0% (control), 6%, 9% and 12% of stratospheric ozone at the latitude of Shapotou, a temperate desert area of northwest China. Analyses showed malondialdehyde content significantly increased, whereas chlorophyll (Chl) fluorescence parameters and Chl contents decreased with increased UV-B intensity. These results corresponded with changes in thylakoid protein complexes and chloroplast ultrastructure. Overall, enhanced UV-B radiation leads to significant decreases in photosynthetic function and serious destruction of the chloroplast ultrastructure of B. argenteum. The degree of negative influences increased with the intensity of UV-B radiation. These results may not only provide a potential mechanism for supplemental UV-B effects on photosynthesis of moss crust, but also establish a theoretical basis for further studies of adaptation and response mechanisms of desert ecosystems under future ozone depletion.

From ozone depletion to agriculture: understanding the role of UV radiation in sustainable crop production

Largely because of concerns regarding global climate change, there is a burgeoning interest in the application of fundamental scientific knowledge in order to better exploit environmental cues in the achievement of desirable endpoints in crop production. Ultraviolet (UV) radiation is an energetic driver of a diverse range of plant responses and, despite historical concerns regarding the damaging consequences of UV-B radiation for global plant productivity as related to stratospheric ozone depletion, current developments representative of a range of organizational scales suggest that key plant responses to UV-B radiation may be exploitable in the context of a sustainable contribution towards the strengthening of global crop production, including alterations in secondary metabolism, enhanced photoprotection, up-regulation of the antioxidative response and modified resistance to pest and disease attack. Here, we discuss the prospect of this paradigm shift in photobiology, and consider the linkages between fundamental plant biology and crop-level outcomes that can be applied to the plant UV-B response, in addition to the consequences for related biota and many other facets of agro-ecosystem processes.

Damage and protection of the photosynthetic apparatus from UV-B radiation. I. Effect of ascorbate

In this work, the effect of the exogenously added ascorbate (Asc) against the UV-B inhibition of the photosystem II (PSII) functions in isolated pea thylakoid membranes was studied. The results reveal that Asc decreases the UV-B induced damage of the donor and the acceptor side of PSII during short treatment up to 60 min. The exogenous Asc exhibits a different UV-protective effect on PSII centers in grana and stroma lamellae, as the effect is more pronounced on the PSIIβ centers in comparison to PSIIα centers. Data also suggest that one of the possible protective roles of the Asc in photosynthetic membranes is the modification of the oxygen-evolving complex by influence on the initial S(0)-S(1) state distribution in the dark.

The ability of cyanobacterial cells to restore UV-B radiation induced damage to Photosystem II is influenced by photolyase dependent DNA repair

Damage of DNA and Photosystem-II are among the most significant effects of UV-B irradiation in photosynthetic organisms. Both damaged DNA and Photosystem-II can be repaired, which represent important defense mechanisms against detrimental UV-B effects. Correlation of Photosystem-II damage and repair with the concurrent DNA damage and repair was investigated in the cyanobacterium Synechocystis PCC6803 using its wild type and a photolyase deficient mutant, which is unable to repair UV-B induced DNA damages. A significant amount of damaged DNA accumulated during UV-B exposure in the photolyase mutant concomitant with decreased Photosystem-II activity and D1 protein amount. The transcript level of psbA3, which is a UV-responsive copy of the psbA gene family encoding the D1 subunit of the Photosystem-II reaction center, is also decreased in the photolyase mutant. The wild-type cells, however, did not accumulate damaged DNA during UV-B exposure, suffered smaller losses of Photosystem-II activity and D1 protein, and maintained higher level of psbA3 transcripts than the photolyase mutant. It is concluded that the repair capacity of Photosystem-II depends on the ability of cells to repair UV-B-damaged DNA through maintaining the transcription of genes, which are essential for protein synthesis-dependent repair of the Photosystem-II reaction center.

Cell survival after UV radiation stress in the unicellular chlorophyte Dunaliella tertiolecta is mediated by DNA repair and MAPK phosphorylation

Ultraviolet radiation (UVR) induces damage in a variety of organisms, and cells may adapt by developing repair or tolerance mechanisms to counteract such damage; otherwise, the cellular fate is cell death. Here, the effect of UVR-induced cell damage and the associated signalling and repair mechanisms by which cells are able to survive was studied in Dunaliella tertiolecta. UVR did not cause cell death, as shown by the absence of SYTOX Green-positive labelling cells. Ultrastructure analysis by transmission electron microscopy demonstrated that the cells were alive but were subjected to morphological changes such as starch accumulation, chromatin disaggregation, and chloroplast degradation. This behaviour paralleled a decrease in F(v)/F(m) and the formation of cyclobutane-pyrimidine dimers, showing a 10-fold increase at the end of the time course. There was a high accumulation of the repressor of transcriptional gene silencing (ROS1), as well as the cell proliferation nuclear antigen (PCNA) in UVR-treated cells, revealing activation of DNA repair mechanisms. The degree of phosphorylation of c-Jun N-terminal kinase (JNK) and p38-like mitogen-activated protein kinases was higher in UVR-exposed cells; however, the opposite occurred with the phosphorylated extracellular signal-regulated kinase (ERK). This confirmed that both JNK and p38 need to be phosphorylated to trigger the stress response, as well as the fact that cell division is arrested when an ERK is dephosphorylated. In parallel, both DEVDase and WEHDase caspase-like enzymatic activities were active even though the cells were not dead, suggesting that these proteases must be considered within a wider frame of stress proteins, rather than specifically being involved in cell death in these organisms.

Understanding UV-driven metabolism in the hypersaline ciliate Fabrea salina

By using NMR spectroscopy, a non-invasive investigation technique, we performed in vivo experiments aimed at uncovering the metabolic pathways involved in the early response of Fabrea salina cells to ultraviolet (UV) radiation. This hypersaline ciliate was chosen as a model organism because of its well-known high resistance to UV radiation. Identical cell samples were exposed to visible radiation only (control samples, CS) and to UV-B + UV-A + visible radiation (treated samples, TS), and NMR spectra of in vivo cells were collected at different exposure times. Resonances were identified through one- and two-dimensional experiments. To compare experiments performed at variable irradiation times on different culture batches, metabolite signals affected by the UV exposure were normalized to corresponding intensity at τ = 0, the zero exposure time. The most affected metabolites are all osmoprotectants, namely, choline, glycine-betaine, betaines, ectoine, proline, α-trehalose and sucrose. The time course of these signals presents qualitative differences between CS and TS, and most of these osmoprotectants tend to accumulate significantly in TS in a UV dose-dependent manner. A picture of the immediate stress response of F. salina against UV radiation in terms of osmoprotection, water retention and salting-out prevention is described.

Photosynthetic response of clusterbean chloroplasts to UV-B radiation: energy imbalance and loss in redox homeostasis between Q(A) and Q(B) of photosystem II

The effects of ultraviolet-B (UV-B: 280-320 nm) radiation on the photosynthetic pigments, primary photochemical reactions of thylakoids and the rate of carbon assimilation (P(n)) in the cotyledons of clusterbean (Cyamopsis tetragonoloba) seedlings have been examined. The radiation induces an imbalance between the energy absorbed through the photophysical process of photosystem (PS) II and the energy consumed for carbon assimilation. Decline in the primary photochemistry of PS II induced by UV-B in the background of relatively stable P(n), has been implicated in the creation of the energy imbalance(.) The radiation induced damage of PS II hinders the flow of electron from Q(A) to Q(B) resulting in a loss in the redox homeostasis between the Q(A) to Q(B) leading to an accumulation of Q(A)(-). The accumulation of Q(A)(-) generates an excitation pressure that diminishes the PS II-mediated O(2) evolution, maximal photochemical potential (F(v)/F(m)) and PS II quantum yield (Φ(PS II)). While UV-B radiation inactivates the carotenoid-mediated protective mechanisms, the accumulation of flavonoids seems to have a small role in protecting the photosynthetic apparatus from UV-B onslaught. The failure of protective mechanisms makes PS II further vulnerable to the radiation and facilitates the accumulation of malondialdehyde (MDA) indicating the involvement of reactive oxygen species (ROS) metabolism in UV-B-induced damage of photosynthetic apparatus of clusterbean cotyledons.

In vivo NMR metabolic profiling of Fabrea salina reveals sequential defense mechanisms against ultraviolet radiation

Fabrea salina is a hypersaline ciliate that is known to be among the strongest ultraviolet (UV)-resistant microorganisms; however, the molecular mechanisms of this resistance are almost unknown. By means of in vivo NMR spectroscopy, we determined the metabolic profile of living F. salina cells exposed to visible light and to polychromatic UV-B + UV-A + Vis radiation for several different exposure times. We used unsupervised pattern-recognition analysis to compare these profiles and discovered some metabolites whose concentration changed specifically upon UV exposure and in a dose-dependent manner. This variation was interpreted in terms of a two-phase cell reaction involving at least two different pathways: an early response consisting of degradation processes, followed by a late response activating osmoprotection mechanisms. The first step alters the concentration of formate, acetate, and saturated fatty-acid metabolites, whereas the osmoprotection modifies the activity of betaine moieties and other functionally related metabolites. In the latter pathway, alanine, proline, and sugars suggest a possible incipient protein synthesis as defense and/or degeneration mechanisms. We conclude that NMR spectroscopy on in vivo cells is an optimal approach for investigating the effect of UV-induced stress on the whole metabolome of F. salina because it minimizes the invasiveness of the measurement.

IR spectroscopic analysis of polymorphism in C13H14N4O

IR analysis is used here to investigate the changes in N-N, N-H, CO modes of thermally treated diphenyl carbazide (DPC) during the variation of temperature from room temperature up to ≈160°C. Polymorphism in DPC compound has been studied here by detecting the changes in some IR spectroscopic parameters (e.g., mode shift, band contour) during the elevation of temperature. Also, DSC, X-ray, NMR and atomic mass spectra are used as confirming tools for what is obtained by IR. All of the vibrations of DPC were found to be due to ionic fundamentals 3311 cm(-1), 3097 cm(-1), 3052 cm(-1), 1677 cm(-1), 1602 cm(-1), 1492 cm(-1), 1306 cm(-1), 1252 cm(-1), 887 cm(-1) and 755 cm(-1). The results revealed for the first time that the thermally treated DPC traverse four different phase transformations at 50°C, 90°C, 125°C and 140°C. The crystal structure was found to be amorphous, monoclinic, tetragonal, orthorhombic and amorphous within a temperature range (30°C-160°C). X-ray diffraction patterns support the results obtained by IR and DSC.

Effects of continuous UV-irradiation on the antioxidant activities of quercetin and rutin in solution in the presence of lecithin as the protective target

The stabilities and antioxidant action of two selected flavonoids, quercetin and rutin, dissolved in methanol and water, toward continuous UV-irradiation from three different sub-ranges (UV-A, UV-B and UV-C) were studied. The flavonoids underwent degradation (bleaching) following first-order kinetics. The bleaching rates were highly dependent on the energy input of the involved UV-photons. The antioxidant activities of the two flavonoids on UV-induced lecithin lipid peroxidation were studied by the TBA-MDA test, and appeared to be also affected by the continuous UV irradiation. The energy input of the incident UV-photons again played a major governing role, but an impact of the flavonoids structures cannot be neglected.

Improved UV-B screening capacity does not prevent negative effects of ambient UV irradiance on PSII performance in High Arctic plants. Results from a six year UV exclusion study

Long-term responses of ambient solar ultraviolet (UV) radiation were investigated on Salix arctica and Vaccinium uliginosum in a High Arctic heath ecosystem in Zackenberg, northeast Greenland. Over a period of six years, UV exclusion was conducted in the growing season by means of filters: 60% UV-B reduction, 90% UV-B+UV-A reduction, UV transparent filter control, and an open control without filter. Plant responses were evaluated using specific leaf area, leaf content of UV-B absorbing compounds and PSII performance parameters derived from chlorophyll-a fluorescence induction curves. Based on the JIP-test, we calculated the total performance index PI(total), which includes the integrating antennae, the PSII reaction center, intersystem electron transport and reduction of PSI end acceptors-dependent parameters. In both species, UV exclusion significantly decreased the content of UV-B-absorbing compounds. Salix increased its specific leaf area, while Vaccinium decreased it. UV exclusion increased the PI(total) in both species during all six years of experimentation. This response was governed by a significantly decreased RC/ABS, a marginally non-significant increased ET(o)/TR(o) and a significantly increased TR(o)/ABS=F(V)/F(M) and RE(o)/ET(o). These results demonstrate the current level of ambient UV-B to decrease PSII performance significantly in these High Arctic plants. It appears that the two plant species both have improved their UV-screening capacity, but through different strategies, although this did not sufficiently prevent negative effects of the ambient UV radiation. We argue the decreased PSII performance to be part of a response decreasing plant carbon uptake. We speculate the negative effects on PSII performance mediated by ambient UV irradiance to be present in years where warming induces early snowmelt, exposing the vegetation to high spring UV-B, and to be present in the future to the degree the ozone layer is not fully recovered.

Intra- and interspecific differences of 10 barley and 10 tomato cultivars in response to short-time UV-B radiation: a study analysing thermoluminescence, fluorescence, gas-exchange and biochemical parameters

The impact of UV-B radiation on 10 genotypically different barley and tomato cultivars was tested in a predictive study to screen for potentially UV-tolerant accessions and to analyze underlying mechanisms for UV-B sensitivity. Plant response was analyzed by measuring thermoluminescence, fluorescence, gas exchange and antioxidant status. Generally, barley cultivars proved to be much more sensitive against UV-B radiation than tomato cultivars. Statistical cluster analysis could resolve two barley groups with distinct differences in reaction patterns. The UV-B sensitive group showed a stronger loss in PSII photochemistry and a lower gas-exchange performance and regulation after UV-B radiation compared to the more tolerant group. The results indicate that photosynthetic light and dark reactions have to play optimally in concert to render plants more tolerant against UV-B radiation. Hence, measuring thermoluminescence/fluorescence and gas exchange in parallel will have much higher potential in identifying tolerant cultivars and will help to understand the underlying mechanisms.

Ultraviolet-B and water stress effects on growth, gas exchange and oxidative stress in sunflower plants

The effects and interaction of drought and UV-B radiation were studied in sunflower plants (Helianthus annuus L. var. Catissol-01), growing in a greenhouse under natural photoperiod conditions. The plants received approximately 1.7 W m(-2) (controls) or 8.6 W m(-2) (+UV-B) of UV-B radiation for 7 h per day. The UV-B and water stress treatments started 18 days after sowing. After a period of 12 days of stress, half of the water-stressed plants (including both UV-B irradiated or non-irradiated) were rehydrated. Both drought and UV-B radiation treatments resulted in lower shoot dry matter per plant, but there was no significant interaction between the two treatments. Water stress and UV-B radiation reduced photosynthesis, stomatal conductance and transpiration. However, the amplitude of the effects of both stressors was dependent on the interactions. This resulted in alleviation of the negative effect of drought on photosynthesis and transpiration by UV-B radiation as the water stress intensified. Intercelluar CO(2) concentration was initially reduced in all treatments compared to control plants but it increased with time. Photosynthetic pigments were not affected by UV-B radiation. Water stress reduced photosynthetic pigments only under high UV-B radiation. The decrease was more accentuated for chlorophyll a than for chlorophyll b. As a measure for the maximum efficiency of photosystem II in darkness F (v)/F (m) was used, which was not affected by drought stress but initially reduced by UV-B radiation. Independent of water supply, UV-B radiation increased the activity of pirogalol peroxidase and did not increase the level of malondialdehyde. On the other hand, water stress did not alter the activity of pirogalol peroxidase and caused membrane damage as assessed by lipid peroxidation. The application of UV-B radiation together with drought seemed to have a protective effect by lowering the intensity of lipid peroxidation caused by water stress. The content of proline was not affected by UV-B radiation but was increased by water stress under both low and high UV-B radiation. After 24 h of rehydration, most of the parameters analyzed recovered to the same level as the unstressed plants.

Fast and reversible response of thylakoid-associated polyamines during and after UV-B stress: a comparative study of the wild type and a mutant lacking chlorophyll b of unicellular green alga Scenedesmus obliquus

The functional and biochemical aspects of the photosynthetic apparatus in response to UV-B radiation were examined in unicellular oxygenic algae Scenedesmus obliquus. The wild type (Wt) and a chlorophyll b-less mutant (Wt-lhc) were used as a specific tool for the understanding of antenna role. Photosynthesis was monitored during and after UV-B stress by time resolved fluorescence spectroscopy and polarography. Carotenoids, such as neoxanthin, loroxanthin, lutein, violaxanthin, antheraxanthin, zeaxanthin, alpha- and beta-carotene, cellular and thylakoid-associated putrescine, spermidine, spermine and subcomplexes of light-harvesting complex (LHCII) of photosystem II (PSII) were investigated to assess their possible involvement in response to UV-B. Oxygen evolution depression by UV-B was higher in the Wt-lhc mutant than in the Wt. Photosynthesis recovery occurred in the Wt, but not in the mutant. The dissipation of excess excitation energy during UV-B stress was accompanied by changes in the thylakoid-associated polyamines which were much higher than changes in xanthophylls. We conclude that, at least in the unicellular green alga S. obliquus, mutants lacking chlorophyll b have significant lower capacity for recovery after UV-B stress. In addition, the comparison of xanthophylls and thylakoid-associated polyamines reveals that the latter are more responsive to UV-B stress and in a reversible manner.

Ambient UV-B radiation decreases photosynthesis in high arctic Vaccinium uliginosum

An UV-B-exclusion experiment was established in high arctic Zackenberg, Northeast Greenland, to investigate the possible effects of ambient UV-B on plant performance. During almost a whole growing season, canopy gas exchange and Chl fluorescence were measured on Vaccinium uliginosum (bog blueberry). Leaf area, biomass, carbon, nitrogen and UV-B-absorbing compounds were determined from a late season harvest. Compared with the reduced UV-B treatment, the plants in ambient UV-B were found to have a higher content of UV-B-absorbing compounds, and canopy net photosynthesis was as an average 23% lower during the season. By means of the JIP-test, it was found that the potential of processing light energy through the photosynthetic machinery was slightly reduced in ambient UV-B. This indicates that not only the UV-B effects on PSII may be responsible for some of the observed reduction of photosynthesis but also the effects on other parts of the photosynthetic machinery, e.g. the Calvin cycle, might be important. The 60% reduction of the UV-B irradiance used in this study implies a higher relative change in the UV-B load than many of the supplemental experiments do, but the substantial effect on photosynthesis clearly indicates that V. uliginosum is negatively affected by the current level of UV-B.

Sensitivity of the photosynthetic apparatus to UV-A radiation: role of light-harvesting complex II-photosystem II supercomplex organization

In this work we study the effect of UV-A radiation on the function of the photosynthetic apparatus in thylakoid membranes with different organization of the light-harvesting complex II-photosystem II (LHCII-PSII) supercomplex. Leaves and isolated thylakoid membranes from a number of previously characterized pea species with different LHCII size and organization were subjected to UV-A treatment. A relationship was found between the molecular organization of the LHCII (ratio of the oligomeric to monomeric forms of LHCII) and UV-A-induced changes both in the energy transfer from PSII to PSI and between the chlorophyll-protein complexes within the LHCII-PSII supercomplex. Dependence on the organization of the LHCII was also found with regard to the degree of inhibition of the photosynthetic oxygen evolution. The susceptibility of energy transfer and oxygen evolution to UV-A radiation decreased with increasing LHCII oligomerization when the UV-A treatment was performed on isolated thylakoid membranes, in contrast to the effect observed in thylakoid membranes isolated from pre-irradiated pea leaves. The data suggest that UV-A radiation leads mainly to damage of the PSIIalpha centers. Comparison of membranes with different organization of their LHCII-PSII supercomplex shows that the oligomeric forms of LHCII play a key role for sensitivity to UV-A radiation of the photosynthetic apparatus.

The effects of UV-B radiation on photosynthesis in relation to Photosystem II photochemistry, thermal dissipation and antioxidant defenses in winter wheat (Triticum aestivum L.) seedlings at different growth temperatures

To study the UV-B effect on photosynthesis in winter wheat at different day/night temperatures, biologically effective UV-B radiation at 4.2 (L_UVB) and 10.3 (H_UVB) kJ m^-2 d^-1 was provided on the seedlings at 25/20°C or 10/5°C. UV-B radiation inhibited net photosynthesis rate (P_n) by enhanced intensity and decreased temperature without change of intercellular CO₂ concentrations (C_i). Decreased maximal quantum yield of Photosystem II (F_v/F_m) and increased minimum fluorescence (F_o) were observed in H_UVB at both temperatures and L_UVB at 10/5°C. H_UVB increased total pool size (VAZ) of xanthophyll cycle pigments, but decreased the de-epoxidation state (DEPS) of these pigments at both temperatures, while L_UVB only decreased DEPS at 10/5°C. The activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) and the redox states of ascorbate and glutathione (AsA/DAsA and GSH/GSSG) were enhanced at 25/20°C, while there were increased SOD and CAT, unaltered APX activities and AsA/DHA, as well as decreased GR activity and GSH/GSSG in L_UVB and H_UVB at 10/5°C. UV-B radiation resulted in higher H₂O₂ and thiobarbituric acid reactive substance (TBARS) concentrations at 10/5°C than 25/20°C. It appears that low temperature alone did not influence photosynthesis but aggravated UV-B induced photoinhibition, which was associated with PSII photochemistry rather than stomatal limitation. Xanthophyll cycle pigments failed to provide photoprotection through thermal dissipation. The antioxidant system was up-regulated in L_UVB and H_UVB at 25/20°C, but was impaired at 10/5°C. Low temperature intensified UV-B induced photoinhibition and damage by weakening the antioxidant system.

Solar ultraviolet-B radiation increases phenolic content and ferric reducing antioxidant power in Avena sativa

We examined the influence of solar ultraviolet-B radiation (UV-B; 280-320 nm) on the maximum photochemical efficiency of photosystem II (F(v)/F(m)), bulk-soluble phenolic concentrations, ferric-reducing antioxidant power (FRAP) and growth of Avena sativa. Treatments involved placing filters on frames over potted plants that reduced levels of biologically effective UV-B by either 71% (reduced UV-B) or by 19% (near-ambient UV-B) over the 52 day experiment (04 July-25 August 2002). Plants growing under near-ambient UV-B had 38% less total biomass than those under reduced UV-B. The reduction in biomass was mainly the result of a 24% lower leaf elongation rate, resulting in shorter leaves and less total leaf area than plants under reduced UV-B. In addition, plants growing under near-ambient UV-B had up to 17% lower F(v)/F(m) values early in the experiment, and this effect declined with plant age. Concentrations of bulk-soluble phenolics and FRAP values were 17 and 24% higher under near-ambient UV-B than under reduced UV-B, respectively. There was a positive relationship between bulk-soluble phenolic concentrations and FRAP values. There were no UV-B effects on concentrations of carotenoids (carotenes + xanthophylls).

The sensitivity of Photosystem II to damage by UV-B radiation depends on the oxidation state of the water-splitting complex

The water-oxidizing complex of Photosystem II is an important target of ultraviolet-B (280-320 nm) radiation, but the mechanistic background of the UV-B induced damage is not well understood. Here we studied the UV-B sensitivity of Photosystem II in different oxidation states, called S-states of the water-oxidizing complex. Photosystem II centers of isolated spinach thylakoids were synchronized to different distributions of the S(0), S(1), S(2) and S(3) states by using packages of visible light flashes and were exposed to UV-B flashes from an excimer laser (lambda=308 nm). The loss of oxygen evolving activity showed that the extent of UV-B damage is S-state-dependent. Analysis of the data obtained from different synchronizing flash protocols indicated that the UV-sensitivity of Photosystem II is significantly higher in the S(3) and S(2) states than in the S(1) and S(0) states. The data are discussed in terms of a model where UV-B-induced inhibition of water oxidation is caused either by direct absorption within the catalytic manganese cluster or by damaging intermediates of the water oxidation process.

The role of the FtsH and Deg proteases in the repair of UV-B radiation-damaged Photosystem II in the cyanobacterium Synechocystis PCC 6803

The photosystem two (PSII) complex found in oxygenic photosynthetic organisms is susceptible to damage by UV-B irradiation and undergoes repair in vivo to maintain activity. Until now there has been little information on the identity of the enzymes involved in repair. In the present study we have investigated the involvement of the FtsH and Deg protease families in the degradation of UV-B-damaged PSII reaction center subunits, D1 and D2, in the cyanobacterium Synechocystis 6803. PSII activity in a DeltaFtsH (slr0228) strain, with an inactivated slr0228 gene, showed increased sensitivity to UV-B radiation and impaired recovery of activity in visible light after UV-B exposure. In contrast, in DeltaDeg-G cells, in which all the three deg genes were inactivated, the damage and recovery kinetics were the same as in the WT. Immunoblotting showed that the loss of both the D1 and D2 proteins was retarded in DeltaFtsH (slr0228) during UV-B exposure, and the extent of their restoration during the recovery period was decreased relative to the WT. However, in the DeltaDeg-G cells the damage and recovery kinetics of D1 and D2 were the same as in the WT. These data demonstrate a key role of FtsH (slr0228), but not the Deg proteases, for the repair of PS II during and following UV-B radiation at the step of degrading both of the UV-B damaged D1 and D2 reaction center subunits.

Impact of elevated UV-B radiation on photosynthetic electron transport, primary productivity and carbon allocation in estuarine epipelic diatoms

Epipelic diatoms are important components of microphytobenthic biofilms. Cultures of four diatom species (Amphora coffeaeformis, Cylindrotheca closterium, Navicula perminuta and Nitzschia epithemioides) and assemblages of mixed diatom species collected from an estuary were exposed to elevated levels of ultraviolet-B (UV-B) radiation. Short exposures to UV-B resulted in decreases in photosystem II (PSII) photochemistry, photosynthetic electron transport, photosynthetic carbon assimilation and changes in the pattern of allocation of assimilated carbon into soluble colloidal, extracellular polysaccharides (EPS) and glucan pools. The magnitude of the effects of the UV-B treatments varied between species and was also dependent upon the photosynthetically active photon flux density (PPFD) to which the cells were also exposed, with effects being greater at lower light levels. Both increases in nonphotochemical quenching of excitation energy in the pigment antennae and photodamage to the D1 reaction centres contributed to decreases in PSII photochemistry. All species demonstrated a rapid ability to recover from perturbations of PSII photochemistry, with some species recovering during the UV-B exposure period. Some of the perturbations induced in carbon metabolism were independent of effects on PSII photochemistry and photosynthetic electron transport. Elevated UV-B can significantly inhibit photosynthetic performance, and modify carbon metabolism in epipelic diatoms. However, the ecological effects of UV-B at the community level are difficult to predict as large variations occur between species.

UVB Effects on the Photosystem II-D1 Protein of Phytoplankton and Natural Phytoplankton Communities

The reaction center of photosystem II is susceptible to photodamage. In particular the D1 protein located in the photosystem II core has a rapid, light-dependent turnover termed the photosystem II repair cycle that, under illumination, degrades and resynthesizes D1 protein to limit accumulation of photodamaged photosystem II. Most studies concerning the effects of UVB (280-320 nm) on this cycle have been on cyanobacteria or specific phytoplankton species rather than on natural communities of phytoplankton. During a 5-year multidisciplinary project on the effects of UV radiation (200-400 nm) on natural systems, the effects of UVB on the D1 protein of natural phytoplankton communities were assessed. This review provides an overview of photoinhibitory effects of light on cultured and natural phytoplankton, with an emphasis on the interrelation of UVB exposure, D1 protein degradation and the repair of photosystem II through D1 resynthesis. Although the UVB component of the solar spectrum contributes to the primary photoinactivation of photosystem II, we conclude that, in natural communities, inhibition of the rate of the photosystem II repair cycle is a more important influence of UVB on primary productivity. Indeed, exposing tropical and temperate phytoplankton communities to supplemented UVB had more inhibitory effect on D1 synthesis than on the D1 degradation process itself. However, the rate of net D1 damage was faster for the tropical communities, likely because of the effects of high ambient light and water temperature on mechanisms of protein degradation and synthesis.

Photoacclimation to Long-Term Ultraviolet Radiation Exposure of Natural Sub-Antarctic Phytoplankton Communities: Fixed Surface Incubations Versus Mixed Mesocosms

Solar UVB radiation (280-320 nm) is known to have detrimental effects on marine phytoplankton. Associated with the seasonal ozone hole in Antarctica, stratospheric ozone depletion occasionally influences the sub-Antarctic (Beagle Channel, Argentina) region, enhancing levels of UVB. The primary objective of this work was to study the effects of several (i.e. 6-10) days of exposure to UVB on the taxonomic composition and photosynthetic inhibition of local phytoplankton communities. For different light treatments, fixed-depth incubations placed in an outdoors water tank were compared with incubations in 1900 L mesocosms, where vertical mixing was present. Phytoplankton growth was inhibited by UV radiation (UVR) in fixed-depth experiments but not in the mixed mesocosms. Under fixed and mixed conditions alike, photosynthesis was significantly inhibited by UVB at the beginning of the experiment but no longer after several days of exposure, suggesting that cells had acclimated to radiation conditions. There was a change in species composition in response to UVR exposure in both experiments, which likely explained acclimation. In the community exposed to fixed conditions this change was from a phytoflagellate-dominated assemblage to a community with high relative abundance of diatoms after 6 days of exposure. UVA was responsible for most of the observed growth inhibition; however, the reduction in photosynthesis was produced by UVB. The reasons behind this variability in responses to UVR are associated with species-specific sensitivity and acclimation, and the previous light history of cells. In the community exposed in mesocosms, an assemblage codominated by phytoflagellates and diatoms was observed at the beginning of the experiments. After 10 days of exposure, green algae (Eutreptiella sp.) had increased, and phytoflagellates were the dominant group. The synthesis of mycosporine-like amino acids (MAAs), antioxidant enzymes and photosynthetic antenna pigments, in relation to repair and protection processes, may explain the reduced inhibition of both growth and photosynthesis that was observed in the phytoplankton community after several days of exposure. For environments such as the Beagle Channel seasonally exposed to the ozone hole, the results obtained from the fixed-depth experiments show that species can cope with UVR by means of MAA synthesis, while mixing would primarily promote a change in species composition and defense strategies.

Protective effect of supplemental low intensity white light on ultraviolet-B exposure-induced impairment in cyanobacterium Spirulina platensis: formation of air vacuoles as a possible protective measure

Intact trichomes of Spirulina platensis were exposed to 1-5 h of low (0.2 mW cm(-2)) or high (0.6 mW cm(-2)) intensity UV-B (280-320 nm) radiation, alone or with photosynthetically active radiation (PAR) of supplemental 50 muE m(-2) s(-1) white light (WL). The mitigating effect of supplemental WL on UV-B induced alterations in Spirulina were investigated by monitoring time-dependent change in photosystem (PS) II mediated O(2) evolution, absorption, circular dichroism (CD) spectra, and ultrastructure. At low intensity, UV-B induced loss in PS II-catalyzed O(2) evolution, but caused no change in the absorption spectrum. At high intensity, UV-B caused a decrease in absorption by phycobilisomes (PBsomes), which was only partly prevented by the presence of low-intensity supplemental WL. The CD spectral analysis revealed that UV-B exposure caused time-dependent enhancement of the negative psi-type bands at 452 and 689 nm, reflecting alterations in the macroaggregation of chlorophyll-protein complexes. This enhancement of negative PS II-type bands was substantially arrested by the presence of supplemental WL exposure, even when UV-B exposure was continued for 5 h. These changes in UV-B-induced CD spectrum suggest alterations in the antenna structure of Spirulina involving both PBsomes and Chlorophyll a. Thus, supplemental low intensity WL arrests, to large extent, the macroaggregation of pigment-protein complexes. Furthermore, the electron micrographs of Spirulina revealed that UV-B exposure caused disorganization of the cellular ultrastructure, while the inclusion of supplemental WL enhanced the formation of air vacuoles in Spirulina. We suggest that the formation of vacuoles by supplemental WL is a protective feature against UV-B.

Effects of UV irradiation on barley and tomato leaves: thermoluminescence as a method to screen the impact of UV radiation on crop plants

The effect of different UV intensities and irradiation times on barley and tomato leaves was investigated by analysis of thermoluminescence (TL) and chlorophyll (chl) fluorescence measurements. Epifluorescence microscopy was used to estimate the epidermal UV transmittance of leaves. In barley a strong supression of TL emission from the S₂Q_B^- (B-band) and the S₂Q_A^- (Q-band) charge recombination was observed increasing with prolonged UV exposure. Primary barley leaves were more sensitive to UV than secondary leaves. In tomato plants a decrease in the B-band only takes place at very high UV intensities and after prolonged exposure times (4 h). The impact of UV in cotyledons was more pronounced than in pinnate leaves of tomato plants. The strong differences in sensitivity to UV in the investigated barley and tomato variety may be due to different concentrations of UV screening pigments in the epidermal layer as demonstrated by epifluorescence measurements. The results show that TL has the same potential to analyse the sensitivity or tolerance of crop plants to UV irradiation as routine fluorescence techniques. Furthermore, TL is directly monitoring the radical pair states of PSII and can distinguish between UV-induced donor and acceptor site-related damage.

Thylakoid-associated Polyamines Adjust the UV-B Sensitivity of the Photosynthetic Apparatus by Means of Light-harvesting Complex II Changes¶

The sensitivity of the photosynthetic apparatus to ultraviolet-B (UV-B) irradiation was studied in cultures of unicellular green alga Scenedesmus obliquus incubated in low light (low photosynthetically active radiation intensity [LL]) and high light (high photosynthetically active radiation intensity [HL]) conditions, treated or not with exogenous polyamines. Biochemical and physicochemical measurements showed that UV-B radiation induces a decrease in the thylakoid-associated putrescine (Put) and an increase in spermine (Spm), so that the reduction of Put/Spm ratio leads to the increase of light-harvesting complex II (LHCII) size per active reaction center and, consequently, the amplification of UV-B effects on the photosynthetic apparatus. The separation of oligomeric and monomeric forms of LHCII from isolated thylakoids showed that UV-B induces an increase in the oligomeric forms of LHCII, which was more intense in LL than in HL. By manipulating the LHCII size with exogenous polyamines, the sensitivity degree of the photosynthetic apparatus to UV-B changed significantly. Specifically, the addition of Put decreased highly the sensitivity of LL culture to UV-B because of the inhibitory effect of Put on the LHCII size increasing, whereas the addition of Spm enhanced the UV-B injury induced in HL culture because of the increasing of LHCII size. The ability of the photosynthetic apparatus to recover the UV-B induced changes was also investigated.

Similar stress responses are elicited by copper and ultraviolet radiation in the aquatic plant Lemna gibba: implication of reactive oxygen species as common signals

Metals and ultraviolet (UV) radiation are two environmental stressors that can cause damage to plants. These two types of stressors often impact simultaneously on plants and both are known to promote reactive oxygen species (ROS) production. However, little information is available on the potential parallel stress responses elicited by metals and UV radiation. Using the aquatic plant Lemna gibba, we found that copper and simulated solar radiation (SSR, a light source containing photosynthetically active radiation (PAR) and UV radiation) induced similar responses in the plants. Both copper and SSR caused ROS formation. The ROS levels were higher when copper was combined with SSR than when applied with PAR. Higher concentrations of copper plus PAR caused toxicity as monitored by diminished growth and chlorophyll content. This toxicity was more pronounced when copper was combined with SSR. Because the generation of ROS was also higher when copper was combined with SSR, we attributed this enhanced toxicity to elevated levels of ROS. In comparison to PAR-grown plants, SSR treated plants exhibited elevated levels of superoxide dismutase (SOD) and glutathione reductase (GR). These enzyme levels were further elevated under both PAR and SSR when copper was added at concentrations that generated ROS. Interestingly, copper treatment in the absence of SSR (i.e. copper plus PAR) induced synthesis of the same flavonoids as those observed in SSR without copper. Finally, addition of either dimethyl thiourea or GSH (two common ROS scavengers) lowered in vivo ROS production, alleviated toxicity and diminished induction of GR as well as accumulation of UV absorbing compounds. Thus, the potential of ROS being a common signal for acclimation to stress by both copper and UV can be considered.

Effects of ultraviolet radiation on plant cells

Recent measurements of ozone levels have led to concern that the stratospheric ozone layer is being depleted as a result of contamination with man-made chlorofluorocarbons. Concomitantly, the amounts of solar UV-B radiation reaching the Earth's surface is increasing. UV-B radiation has been shown to be harmful to living organisms, damaging DNA, proteins, lipids and membranes. Plants, which use sunlight for photosynthesis and are unable to avoid exposure to enhanced levels of UV-B radiation, are at risk. Thus, mechanisms by which plants may protect themselves from UV radiation are of particular interest. This review will summarizes the main aspects of ultraviolet radiation on plants at physiological and biochemical level, with particular emphasis on protective structures and mechanisms.

Effect of solar ultraviolet-B radiation during springtime ozone depletion on photosynthesis and biomass production of Antarctic vascular plants

We assessed the influence of springtime solar UV-B radiation that was naturally enhanced during several days due to ozone depletion on biomass production and photosynthesis of vascular plants along the Antarctic Peninsula. Naturally growing plants of Colobanthus quitensis (Kunth) Bartl. and Deschampsia antarctica Desv. were potted and grown under filters that absorbed or transmitted most solar UV-B. Plants exposed to solar UV-B from mid-October to early January produced 11% to 22% less total, as well as above ground biomass, and 24% to 31% less total leaf area. These growth reductions did not appear to be associated with reductions in photosynthesis per se: Although rates of photosynthetic O(2) evolution were reduced on a chlorophyll and a dry-mass basis, on a leaf area basis they were not affected by UV-B exposure. Leaves on plants exposed to UV-B were denser, probably thicker, and had higher concentrations of photosynthetic and UV-B absorbing pigments. We suspect that the development of thicker leaves containing more photosynthetic and screening pigments allowed these plants to maintain their photosynthetic rates per unit leaf area. Exposure to UV-B led to reductions in quantum yield of photosystem II, based on fluorescence measurements of adaxial leaf surfaces, and we suspect that UV-B impaired photosynthesis in the upper mesophyll of leaves. Because the ratio of variable to maximal fluorescence, as well as the initial slope of the photosynthetic light response, were unaffected by UV-B exposure, we suggest that impairments in photosynthesis in the upper mesophyll were associated with light-independent enzymatic, rather than photosystem II, limitations.

Oxygen evolution loss and structural transitions in photosystem II induced by low intensity UV-B radiation of 280 nm wavelength

UV-B radiation of 280 nm wavelength (UV280) and low intensity (2.0 W/m2) gives rise to an important oxygen evolution (OE) loss in photosystem II (PSII) particles isolated from the thylakoid membrane of plant chloroplasts on the one hand, and to structural changes, or transitions, in the proteins of the PSII complex on the other hand. The latter UV280 effect was studied in this work by Fourier transform infrared (FT-IR) spectroscopy. First, irradiation of the PSII particles with UV280 for about 40 min causes an almost complete loss of OE activity. The remaining OE after 15, 20, 30 and 40 min is respectively 52, 44, 27 and 12% of the OE activity in control PSH particles kept in darkness. Secondly, difference FT-IR spectra of PSII particles irradiated for 30 min, i.e., [PSII irradiated with UV280]-minus-[PSII non-irradiated], show that the UV280 light is at the origin of significant IR absorbance changes in several spectral regions: (i) amide I (1696-1620 cm(-1)) and amide II (1580-1520 cm(-1)), (ii) tyrosine side chain (1620-1580 cm(-1) and 1520-1500 cm(-1), i.e., the v8a, v8b and v19a vibrational modes, respectively), and (iii) chlorophylls (1750-1696 cm(-1)). Thirdly, comparison of the UV-B effect reported here with structural changes induced by heat-stress in PSII proteins [M. Joshi, M. Fragata, Z. Naturforsch. 54c (1999) 35-43] clearly indicates that the stability of the functional centers in the PSII complex is dependent on a dynamic equilibrium between a-helix conformers and extended chain (beta-strand) structures. In this framework, transient 'alpha-helix-to-beta-strand transitions' are susceptible of giving rise in vivo to recurrent changes in the activity of the PSII complex, and as such act as a control mechanism of the photosynthetic function in the thylakoid membrane.

UV-B radiation induced exchange of the D1 reaction centre subunits produced from the psbA2 and psbA3 genes in the Cyanobacterium synechocystis sp. PCC 6803

UV-B irradiation of Synechocystis 6803 cells inhibits photosystem II activity, which can be restored via de novo synthesis of the D1 (and D2) reaction center subunits. Recently we have shown that of the two psbA genes that encode identical D1 proteins in Synechocystis 6803, UV-B preferentially enhances the transcription of psbA3 compared to that of psbA2 [Máté, Z., Sass, L., Szekeres, M., Vass, I. and Nagy, F. (1998) J. Biol. Chem. 273, 17439-17444]. Here we studied the effect of UV-B on the synthesis of the D1 protein from the psbA2 and psbA3 genes in the P7 mutant of Synechocystis 6803. In this mutant, psbA2 carries the Ala251-->Val point mutation, which confers resistance to the photosystem II electron transport inhibitor metribuzin, but psbA3 is the same as in the wild-type. By applying variable chlorophyll fluorescence measurements to distinguish between metribuzin-sensitive and metribuzin-resistant photosystem II centers we quantified the amount of the D1 protein produced from each of the psbA3 and psbA2 genes. When the cells were exposed to UV-B light, the fraction of D1 protein produced from the psbA3 gene was increased from 15-20 to 32-40% of the total D1. This effect was reversible by transferring the cells to visible light. The rate of D1 production from psbA3 increased with increasing UV-B intensities, and was a transient phenomenon at low UV-B levels (0.1 microE x m-2 x s-1). It is concluded that the enhancement of psbA3 gene transcription by UV-B light leads to enhanced D1 protein synthesis from this gene. Our findings demonstrate that the main role of psbA3 transcription activated by UV-B is to increase the size of the psbA mRNA pool available for translation when a rapid repair of the D1 protein is needed under UV-B stress.