Important Photoprotective Mechanism Research Articles

Exploring Mycosporine-like Amino Acid UV-Absorbing Natural Products for a New Generation of Environmentally Friendly Sunscreens.

Human skin needs additional protection from damaging ultraviolet radiation (UVR: 280-400 nm). Harmful UVR exposure leads to DNA damage and the development of skin cancer. Available sunscreens offer chemical protection from detrimental sun radiation to a certain extent. However, many synthetic sunscreens do not provide sufficient UVR protection due to the lack of photostability of their UV-absorbing active ingredients and/or the lack of ability to prevent the formation of free radicals, inevitably leading to skin damage. In addition, synthetic sunscreens may negatively affect human skin, causing irritation, accelerating skin aging and even resulting in allergic reactions. Beyond the potential negative effect on human health, some synthetic sunscreens have been shown to have a harmful impact on the environment. Consequently, identifying photostable, biodegradable, non-toxic, and renewable natural UV filters is imperative to address human health needs and provide a sustainable environmental solution. In nature, marine, freshwater, and terrestrial organisms are protected from harmful UVR through several important photoprotective mechanisms, including the synthesis of UV-absorbing compounds such as mycosporine-like amino acids (MAAs). Beyond MAAs, several other promising, natural UV-absorbing products could be considered for the future development of natural sunscreens. This review investigates the damaging impact of UVR on human health and the necessity of using sunscreens for UV protection, specifically UV-absorbing natural products that are more environmentally friendly than synthetic UV filters. Critical challenges and limitations related to using MAAs in sunscreen formulations are also evaluated. Furthermore, we explain how the genetic diversity of MAA biosynthetic pathways may be linked to their bioactivities and assess MAAs' potential for applications in human health.

HHL1 and SOQ1 synergistically regulate nonphotochemical quenching in Arabidopsis

Nonphotochemical quenching (NPQ) is an important photoprotective mechanism that quickly dissipates excess light energy as heat. NPQ can be induced in a few seconds to several hours; most studies of this process have focused on the rapid induction of NPQ. Recently, a new, slowly induced form of NPQ, called qH, was found during the discovery of the quenching inhibitor suppressor of quenching 1 (SOQ1). However, the specific mechanism of qH remains unclear. Here, we found that hypersensitive to high light 1 (HHL1)-a damage repair factor of photosystem II-interacts with SOQ1. The enhanced NPQ phenotype of the hhl1 mutant is similar to that of the soq1 mutant, which is not related to energy-dependent quenching or other known NPQ components. Furthermore, the hhl1 soq1 double mutant showed higher NPQ than the single mutants, but its pigment content and composition were similar to those of the wildtype. Overexpressing HHL1 decreased NPQ in hhl1 to below wildtype levels, whereas NPQ in hhl1 plants overexpressing SOQ1 was lower than that in hhl1 but higher than that in the wildtype. Moreover, we found that HHL1 promotes the SOQ1-mediated inhibition of plastidial lipoprotein through its von Willebrand factor type A domain. We propose that HHL1 and SOQ1 synergistically regulate NPQ.

PROTECTIVE ROLES OF D1 PROTEIN TURNOVER AND THE XANTHOPHYLL CYCLE IN TOMATO (SOLANUM LYCOPERSICUM) UNDER SUB-HIGH TEMPERATURE AND HIGH LIGHT STRESS

<List> <ListItem><ItemContent> • D1 turnover plays a more important role than xanthophyll cycle in photoprotection under sub-high temperature and high light (HH) conditions in tomato. </ItemContent></ListItem> <ListItem><ItemContent> • D1 protein turnover and xanthophyll cycle avoided the accumulation of excess energy and photooxidative damage of photosystem through maintaining photosynthetic pigments content and inducing changes in NPQ core components. </ItemContent></ListItem> <ListItem><ItemContent> • D1 protein turnover and xanthophyll cycle can also avoided accumulation of ROS and membrane lipid peroxidation by maintaining the activity of the ROS scavenging system. </ItemContent></ListItem></List> D1 protein turnover and the xanthophyll cycle (XC) are important photoprotective mechanisms in plants that operate under adverse conditions. Here, streptomycin sulfate (SM) and dithiothreitol (DTT) were used in tomato plants as inhibitors of D1 protein turnover and XC to elucidate their photoprotective impacts under sub-high temperature and high light conditions (HH, 35°C, 1000 µmol·m^-²·s^-¹). SM and DTT treatments significantly reduced the net photosynthetic rate, apparent quantum efficiency, maximum photochemical efficiency, and potential activity of photosystem II, leading to photoinhibition and a decline in plant biomass under HH. The increase in reactive oxygen species levels resulted in thylakoid membrane lipid peroxidation. In addition, there were increased non-photochemical quenching and decreased chlorophyll pigments in SM and DTT application, causing an inhibition of D1 protein production at both transcriptional and translational levels. Overall, inhibition of D1 turnover caused greater photoinhibition than XC inhibition. Additionally, the recovery levels of most photosynthesis indicators in DTT-treated plants were higher than in SM-treated plants. These findings support the view that D1 turnover has a more important role than XC in photoprotection in tomato under HH conditions.

Increased photosystem II translation efficiency as an important photoprotective mechanism in an Arabidopsis thaliana ecotype (Tibet-0) adapted to high light environments

The Qinghai-Tibet Plateau (QTP) is the highest and largest young plateau in the world, with a wide variety of extreme environments. Due to the low density of air and high transparency of the atmosphere, QTP has abundant sunshine and high light intensity. It is important for plants in QTP to evolve mechanism of photoprotection to adapt the high-light conditions. Arabidopsis thaliana is an important model organism in plant science and molecular biology. Tibet-0 was the first Arabidopsis thaliana ecotype collected in the QTP, at >4000 m above sea level. Here, we systematically analyzed the photosynthetic responses of Tibet-0 to high light. Our results show that Tibet-0 has more efficient PSII photosynthetic activity under continuous high-light conditions (1000 μmol photons m−2 s−1) compared to Columbia (Col-0). Chlorophyll fluorescence analysis showed that Tibet-0 exhibited enhanced PSII quantum yield (ФPSII), photochemical quenching (qP), and electron transport rate (ETR) under high-light conditions. However, anthocyanin accumulation was lower in Tibet-0. Blue Native-PAGE and immunoblot analyses showed that the abundances of PSII complexes and subunits were significantly higher in Tibet-0 vs. Col-0 under standard and high-light conditions. Notably, the associations of PSII core subunit mRNAs psbA, psbB, psbC and psbD with chloroplast ribosomal protein were significantly higher in Tibet-0, based on ribosome-nascent chain complex (RNC)-quantitative PCR. Moreover, the expression of amounts of chloroplast ribosomal protein-related genes was activated in Tibet-0 after high-light treatment. These results suggest that the enhanced photoprotection capability of Tibet-0 arises via constitutively higher translation efficiency for PSII proteins due to activated ribosomal protein expression. This study thus provides insight into the mechanism underlying high-light adaptation in plants at high altitudes.

Thylakoid membrane reorganizations revealed by small-angle neutron scattering of Monstera deliciosa leaves associated with non-photochemical quenching.

Non-photochemical quenching (NPQ) is an important photoprotective mechanism in plants and algae. Although the process is extensively studied, little is known about its relationship with ultrastructural changes of the thylakoid membranes. In order to better understand this relationship, we studied the effects of illumination on the organization of thylakoid membranes in Monstera deliciosa leaves. This evergreen species is known to exhibit very large NPQ and to possess giant grana with dozens of stacked thylakoids. It is thus ideally suited for small-angle neutron scattering measurements (SANS)—a non-invasive technique, which is capable of providing spatially and statistically averaged information on the periodicity of the thylakoid membranes and their rapid reorganizations in vivo. We show that NPQ-inducing illumination causes a strong decrease in the periodic order of granum thylakoid membranes. Development of NPQ and light-induced ultrastructural changes, as well as the relaxation processes, follow similar kinetic patterns. Surprisingly, whereas NPQ is suppressed by diuron, it impedes only the relaxation of the structural changes and not its formation, suggesting that structural changes do not cause but enable NPQ. We also demonstrate that the diminishment of SANS peak does not originate from light-induced redistribution and reorientation of chloroplasts inside the cells.

Photoprotection by mitochondrial alternative pathway is enhanced at heat but disabled at chilling.

The mitochondrial alternative pathway (AP) represents an important photoprotective mechanism for the chloroplast, but the temperature sensitivity of its photoprotective role is unknown. In this study, using the aox1a Arabidopsis mutant, the photoprotective role of the AP was verified under various temperatures, and the mechanism underlying the temperature sensitivity of the AP's photoprotective role was clarified. It was observed that the photoprotective role of the AP increased with rising temperature but was absent at low temperature. The photoprotective role of the AP was severely reduced under non-photorespiratory conditions. Disturbance of the AP inhibited the conversion of glycine to serine in mitochondria, which may restrain upstream photorespiratory metabolism and aggravate photoinhibition. With rising temperatures, photorespiration accelerated and the restraint of photorespiration caused by disturbance of the AP also increased, determining the temperature sensitivity of the AP's photoprotective role. We also verified that not only the AP but also the cytochrome pathway in mitochondria contributes to photoprotection by maintaining photorespiration.

Cyclic electron flow protects photosystem I donor side under low night temperature in tomato

Cyclic electron flow (CEF) around photosystem I (PSI) is an important photoprotective mechanism, mediated mainly by PGR5/PGRL1 proteins and the chloroplast NAD(P)H dehydrogenase (NDH) complex. Here, chemical inhibition of PGR5/PGRL1- or NDH-dependent CEF was performed to determine their roles in preventing photoinhibition in tomato leaves under low night temperature (LNT) stress. Experimental treatments were with either antimycin A (A) or rotenone (R) to inhibit PGR5/PGRL1- or NDH-dependent CEF, respectively. The results showed that net photosynthetic rate (Pn), antioxidant enzyme activity, maximal photochemistry efficiency (Fv/Fm), effective photochemical quantum yield of PSII [Y(II)], non-photochemical quenching (NPQ), coefficient of photochemical fluorescence quenching (qP), and effective photochemical quantum yield of PSI [Y(I)] decreased more under LNT following A treatment than following R treatment. In A-treated plants, analysis of the normalized relative variable fluorescence at the K step (Vk) and the relative variable fluorescence at the J step (VJ) showed that both the donor and acceptor sides of PSII were severely damaged under LNT. Moreover, the donor side limitation of PSI [Y(ND)] increased whereas the acceptor side limitation of PSI [Y(NA)] decreased and the expression of genes encoding photosystem core subunits was significantly reduced. A treatment also resulted in increased ROS content and inhibition of SOD and APX activities under LNT. These data suggest that PGR5/PGRL1-dependent CEF plays a more important role than NDH-dependent CEF regarding resistance to LNT stress in tomato.

Slow zeaxanthin accumulation and the enhancement of CP26 collectively contribute to an atypical non‐photochemical quenching in macroalga Ulva prolifera under high light

Non-photochemical quenching (NPQ) is an important photoprotective mechanism in plants, which dissipates excess energy and further protects the photosynthetic apparatus under high light stress. NPQ can be dissected into a number of components: qE, qZ, and qI. In general, NPQ is catalyzed by two independent mechanisms, with the faster-activated quenching catalyzed by the monomeric light-harvesting complex (LHCII) proteins and the slowly activated quenching catalyzed by LHCII trimers, both processes depending on zeaxanthin but to different extent. Here, we studied the NPQ of the intertidal green macroalga, Ulva prolifera, and found that the NPQ of U.prolifera lack the faster-activated quenching, and showed much greater sensitivity to dithiothreitol (DTT) than to dicyclohexylcarbodiimide (DCCD). Further results suggested that the monomeric LHC proteins in U.prolifera included only CP29 and CP26, but lacked CP24, unlike Arabidopsis thaliana and the moss Physcomitrella patens. Moreover, the expression levels of CP26 increased significantly following exposure to high light, but the concentrations of the two important photoprotective proteins (PsbS and light-harvesting complex stress-related [LhcSR]) did not change upon the same conditions. Analysis of the xanthophyll cycle pigments showed that, upon exposure to high light, zeaxanthin synthesis in U.prolifera was gradual and much slower than that in P.patens, and could effectively be inhibited by DTT. Based on these results, we speculate the enhancement of CP26 and slow zeaxanthin accumulation provide an atypical NPQ, making this green macroalga well adapted to the intertidal environments.

Photorespiration is crucial for salinity acclimation in castor bean

The last few decades have seen sustained research interest in the relationship between photoprotection and photoinhibition, but it is still unclear whether photoinhibition is a lasting form of photoprotection or consequence of undesirable photodamage under stress conditions such as salinity. Additionally, while photorespiration (PR) is known to alter cellular redox balance by consuming reducing equivalents, the photoprotective role of PR is controversial. Using the wild crop species (Ricinus communis) this study investigated the role of two key enzymes of the photorespiratory pathway (viz. catalase [CAT], and glycolate oxidase [GO]) in plant photoprotection and oxidative metabolism responses to salinity. The data show that in R. communis, PR is an important photoprotective mechanism avoiding photoinhibition in plants under salinity. Suppressing the activity of GO disrupted the fine balance between the photosynthetic electron transport and the Calvin-Benson cycle reactions. Additionally, inhibiting GO, and by consequence PR, led to decreases in the activation state and activity of Rubisco. Although the inhibition of GO in the absence of salinity decreased photosynthesis, other photoprotective mechanisms and the enzymatic and non-enzymatic antioxidant system remained functional. In contrast, inhibiting CAT activity triggered increasing H2O2 and O.−2 in R. communis plants, irrespective of whether they were exposed to salinity or not. These findings are discussed in terms of the physiological importance of PR in the acclimation of this wild crop species to salinity and its consequences on photosynthetic efficiency.

Modeling Photoprotection at Global Scale: The Relative Role of Nonphotosynthetic Pigments, Physiological State, and Species Composition

AbstractMicroalgae are capable of acclimating to dynamic light environments, as they have developed mechanisms to optimize light harvesting and photosynthetic electron transport. When absorption of light exceeds photosynthetic capacity, various physiological protective mechanisms prevent damage of the photosynthetic apparatus. Xanthophyll pigments provide one of the most important photoprotective mechanisms to dissipate the excess light energy and prevent photoinhibition. In this study, we coupled a mechanistic model for phytoplankton photoinhibition with the global biogeochemical model Regulated Ecosystem Model version 2. The assumption that photoinhibition is small in phytoplankton communities acclimated to ambient light allowed us to predict the photoprotective needs of phytoplankton. When comparing the predicted photoprotective needs to observations of pigment content determined by high‐performance liquid chromatography, our results showed that photoprotective response seems to be mediated in most parts of the ocean by a variable ratio of xanthophyll pigments to chlorophyll. The variability in the ratio appeared to be mainly driven by changes in phytoplankton community composition. Exceptions appeared at high latitudes where other energy dissipating mechanisms seem to play a role in photoprotection and both taxonomic changes and physiological acclimation determine community pigment signature. Understanding the variability of community pigment signature is crucial for modeling the coupling of light absorption to carbon fixation in the ocean. Insights about how much of this variability is attributable to changes in community composition may allow us to improve the match between remotely sensed optical data and the underlying phytoplankton community.

The regulation of P700 is an important photoprotective mechanism to NaCl-salinity in Jatropha curcas.

Salinity commonly affects photosynthesis and crop production worldwide. Salt stress disrupts the fine balance between photosynthetic electron transport and the Calvin cycle reactions, leading to over-reduction and excess energy within the thylakoids. The excess energy triggers reactive oxygen species (ROS) overproduction that causes photoinhibition in both photosystems (PS) I and II. However, the role of PSI photoinhibition and its physiological mechanisms for photoprotection have not yet been fully elucidated. In the present study, we analyzed the effects of 15 consecutive days of 100 mM NaCl in Jatropha curcas plants, primarily focusing on the photosynthetic electron flow at PSI level. We found that J. curcas plants have important photoprotective mechanisms to cope with the harmful effects of salinity. We show that maintaining P700 in an oxidized state is an important photoprotector mechanism, avoiding ROS burst in J. curcas exposed to salinity. In addition, upon photoinhibition of PSI, the highly reduced electron transport chain triggers a significant increase in H2 O2 content which can lead to the production of hydroxyl radical by Mehler reactions in chloroplast, thereby increasing PSI photoinhibition.

Differential physiological responses of Tunisian wild grapevines (Vitis vinifera L. subsp. sylvestris) to NaCl salt stress

The effects of salt stress on growth, organic and inorganic solute accumulation and chlorophyll florescence were studied on 3-month-old plants of six Tunisian wild grapevine accessions in order to identify salt tolerance mechanisms and select tolerant genotypes. Potted plants were grown under controlled conditions and irrigated for 14 days with 0, 100 and 150 mM NaCl Long Ashton nutrient solution. Salt begins to adversely affect plant growth and plant nutrition at 100 mM NaCl. Compared to control, shoot growth rates were 21.5% less for Khedhayria, Tebaba and Ouchteta, 33% for Djebba and Zouarâa and 49% for Houamdia. They were assigned to stomatal and non-stomatal factors. Stomatal conductance decreased after 1 day at 150 mM NaCl in all accessions in response to reduced leaf water potential. Leaves in tolerant accessions were well hydrated through efficient osmotic adjustment, sufficient potassium flux and selectivity of K+ versus Na+. In addition, salt tolerance of wild grapes was related to limiting Na+ transport to lamina and to compartmentalization of Cl− on root and leaf vacuoles, improved in Tebaba and Khedhayria by the uptake of K+. At the same time, disturbances of the PSII have been noted as non-stomatal factors, and the most important photoprotective mechanism against photosynthetic damages was non-photochemical energy dissipation. However, chlorophyll fluorescence parameters were more stable in Tebaba compared to Khedhayria, thus showing better salt tolerance. Based on our results, wild grapevine accessions could be classified from most tolerant to most sensitive as follows: Tebaba > Khedhayria > Ouchteta > Zouarâa > Djebba > Houamdia.

Light Energy Partitioning and Photoprotection in an Exotic Species (Salix Psammophila) Grown in a Semi-Arid Area of Northwestern China

Thermal dissipation of excess excitation energy is an important photoprotective mechanism that plants have evolved to cope with surplus illumination. However, light-energy-partitioning dynamics in an exotic sand-dune willow (Salix psammophila) commonly used in restoring and/or stabilizing sand lands in northwestern China is largely unknown. In this study, chlorophyll fluorescence (ChlF) of photosystem II (PSII) was continuously monitored in situ in Salix psammophila to investigate plant acclimation processes driven by excessive solar radiation and extreme air temperatures (Ta). As part of a heat-regulation mechanism, energy partitioning is shown to vary with prevailing environmental conditions. In this investigation, energy absorbed during periods of moderate photosynthetically active radiation (PAR &lt; 1200 μmoL·m−2·s−1) was largely allocated towards photochemistry (ΦPSII) with nominal amounts to thermal dissipation through reversible thermal dissipation (ΦNPQr). In extremely high solar radiation (PAR &gt; 1500 μmoL·m−2·s−1) or in a cold temperature (Ta &lt; 0 °C), more energy was dissipated by way of non-regulated thermal energy (Φf,D) and sustained thermal dissipation (ΦNPQs), leading to non-reversible photoinhibition or photodamage. This was mainly as a result of the low utilization and high absorption of light energy by PSII under cold conditions and physiologically-induced vulnerability. It was concluded that Salix psammophila had a clear tolerance to high temperatures and moderate solar radiation, but tended to be more vulnerable to high solar radiation and cold temperature. Based on species sensitivity to extreme environmental conditions, practical application and extension of Salix psammophila for land-restoration purposes should be approached cautiously, especially in high-latitude or high-altitude desert ecosystems commonly affected by events of high solar radiation and cold temperature.

Non-photochemical Quenching Plays a Key Role in Light Acclimation of Rice Plants Differing in Leaf Color

Non-photochemical quenching (NPQ) is an important photoprotective mechanism in rice; however, little is known regarding its role in the photosynthetic response of rice plants with differing in leaf color to different irradiances. In this study, two rice genotypes containing different chlorophyll contents, namely Zhefu802 (high chlorophyll) and Chl-8 (low chlorophyll), were subjected to moderate or high levels of light intensity at the 6-leaf stage. Chl-8 possessed a lower chlorophyll content and higher chlorophyll a:b ratio compared with Zhefu802, while Pn, Fv/Fm, and ΦPSII contents were higher in Chl-8. Further results indicated that no significant differences were observed in the activities of Rubisco, Mg2+-ATPase, and Ca2+-ATPase between these genotypes. This suggested that no significant difference in the capacity for CO2 assimilation exists between Zhe802 and Chl-8. Additionally, no significant differences in stomatal limitation were observed between the genotypes. Interestingly, higher NPQ and energy quenching (qE), as well as lower photoinhibitory quenching (qI) and production of reactive oxygen species (ROS) was observed in Chl-8 compared with Zhefu802 under both moderate and high light treatments. This indicated that NPQ could improve photosynthesis in rice under both moderate and high light intensities, particularly the latter, whereby NPQ alleviates photodamage by reducing ROS production. Both zeaxanthin content and the expression of PsbS1 were associated with the induction of NPQ under moderate light, while only zeaxanthin was associated with NPQ induction under high light. In summary, NPQ could improve photosynthesis in rice under moderate light and alleviate photodamage under high light via a decrease in ROS generation.

Dynamic summer solar radiation in Antarctic coastal ecosystems and its effects on photosyn thesis of the endemic Antarctic brown macroalga Desmarestia menziesii (Phaeophyceae)

Ultraviolet (UV) radiation is a significant stress factor that harms life in both terrestrial and aquatic ecosystems in Antarctica. In summer (January–February), ground-level solar radiation regimes at the Antarctic Carlini Station (62°14'S, 58°40'W) on King George Island (South Shetland Islands) can be highly variable, depending on the presence of clouds. Spectrally-resolved underwater radiation regimes were measured at three study sites in the inner and outer Potter Cove nearby Carlini Station. The clear waters at Penon de Pesca allowed PAR, UV-A and UV-B radiation to penetrate deeply into the water column, expressed by z1% (i. e. 1%-depths) at 23–25 m, 20–22 m and 13–16 m, respectively, as well as by low attenuation coefficients of downward radiation (Kd). In contrast, turbid waters in the inner Potter Cove and at Penon Uno reduced the penetration of these three wavebands significantly. The photo-physiological mechanisms allowing macroalgae to acclimate to the incident gradients of PAR and UV radiation at Penon de Pesca were further elaborated by assessing photo-physiological data on the brown macroalga Desmarestia menziesii (Phaeophyceae), exposed to a PAR range between 15 and 130 μ mol photons m-2 s-1, either in the presence or absence of UV radiation (10.3 W m -2 UV-A and 0.73 W m -2 UV-B). PAM-fluorometry-based measurements revealed a similar decrease of the optimum quantum yield of photosystem II (PSII) under both PAR and UV radiation and a stronger effect of PAR over UV radiation in the regulation of maximum photosynthetic electron transport rate (ETRmax) as well as the photosynthetic light saturation (Ek). The down-regulation of the photochemistry of PSII by PAR as well as the reduction in the photosynthetic electron transport capacity (i.e.ETRmax) indicate important photoprotective mechanisms allowing D. menziesii to response effectively to a combination of PAR and UV stress in their upper subtidal habitat.

Photosystem II Subunit PsbS Is Involved in the Induction of LHCSR Protein-dependent Energy Dissipation in Chlamydomonas reinhardtii

Non-photochemical quenching of excess excitation energy is an important photoprotective mechanism in photosynthetic organisms. In Arabidopsis thaliana, a high quenching capacity is constitutively present and depends on the PsbS protein. In the green alga Chlamydomonas reinhardtii, non-photochemical quenching becomes activated upon high light acclimation and requires the accumulation of light harvesting complex stress-related (LHCSR) proteins. Expression of the PsbS protein in C. reinhardtii has not been reported yet. Here, we show that PsbS is a light-induced protein in C. reinhardtii, whose accumulation under high light is further controlled by CO2 availability. PsbS accumulated after several hours of high light illumination at low CO2 At high CO2, however, PsbS was only transiently expressed under high light and was degraded after 1 h of high light exposure. PsbS accumulation correlated with an enhanced non-photochemical quenching capacity in high light-acclimated cells grown at low CO2 However, PsbS could not compensate for the function of LHCSR in an LHCSR-deficient mutant. Knockdown of PsbS accumulation led to reduction of both non-photochemical quenching capacity and LHCSR3 accumulation. Our data suggest that PsbS is essential for the activation of non-photochemical quenching in C. reinhardtii, possibly by promoting conformational changes required for activation of LHCSR3-dependent quenching in the antenna of photosystem II.

PsbS interactions involved in the activation of energy dissipation in Arabidopsis.

The non-photochemical quenching of light energy as heat (NPQ) is an important photoprotective mechanism that is activated in plants when light absorption exceeds the capacity of light utilization in photosynthesis. The PsbS protein plays a central role in this process and is supposed to activate NPQ through specific, light-regulated interactions with photosystem (PS) II antenna proteins. However, NPQ-specific interaction partners of PsbS in the thylakoid membrane are still unknown. Here, we have determined the localization and protein interactions of PsbS in thylakoid membranes in the NPQ-inactive (dark) and NPQ-active (light) states. Our results corroborate a localization of PsbS in PSII supercomplexes and support the model that the light activation of NPQ is based on the monomerization of dimeric PsbS and a light-induced enhanced interaction of PsbS with Lhcb1, the major component of trimeric light-harvesting complexes in PSII.

In vivo epidermal UV-A absorbance is induced by sunlight and protects Soldanella alpina leaves from photoinhibition.

Alpine plants like Soldanella alpina L. are subjected to high PAR and high UV radiation. Among the important photoprotective mechanisms that prevent photoinhibition under such conditions, passive optical barriers such as UV-absorbing compounds were investigated. In this study, temporal and spatial patterns of epidermal UV-A absorbance for S. alpina leaves were investigated with a combination of absorbance measurements at 375nm and imaging methods. UV-A absorbance was highest in plants acclimated to full sunlight and was markedly stable during the leaves' lifetime. UV-A absorbance was correlated with leaf structure (leaf mass per area ratio, density of epidermal cells and stomata) and biochemical features such as chlorophyll and carotenoid content and ratio, which are characteristics of light acclimation. UV-A-absorbing compounds were mainly localised in the epidermal vacuoles and trichomes. Leaves with low UV-A absorbance were significantly more photosensitive than leaves with high UV-A absorbance. However, the epidermal UV-A absorbance increased in low-absorbance leaves under full sunlight even in the absence of UV radiation. Results suggest that high epidermal UV-A absorbance protects S. alpina leaves from photoinactivation, which is especially important after snowmelt, when plants are suddenly exposed to full sunlight.

Alternative electron sinks are crucial for conferring photoprotection in field-grown cotton under water deficit during flowering and boll setting stages.

To clarify the photoprotective mechanisms of cotton leaves under water deficit in the field, leaf gas exchange, chlorophyll a fluorescence as well as the corresponding physiological responses were examined in cotton (Gossypium hirsutum L.) to evaluate electron flux distribution. With increasing water deficit, net photosynthetic rate (Pn) significantly decreased, the total electron flux through PSII [Je(PSII)] gradually decreased and the fraction of electron flux required to sustain CO2 assimilation [Je(PCR)] markedly declined. Simultaneously, the ratio of quantum efficiency of PSII [Φ(PSII)] to the quantum efficiency of CO2 fixation [Φ(CO2)] increased, accompanied by an increase in the alternative electron flux (Ja). The enhanced alternative electron flux of O2-dependent Ja(O2-dependent) indicated that electrons had been transported to O2 in the Mehler-peroxide reaction (MPR) and that the remaining alternative electron flux Ja(O2-independent) had been used for nitrate reduction, as indicated by an increase in nitrate reductase (NR) and glutathinone reductase (GR) activities. In addition, mild water deficit increased the proportion of electron flux for the photorespiratory carbon oxidation [Je(PCO)]. Water deficit significantly increased surperoxide radical production rate (O2-•) and hydrogen peroxide content (H2O2), and the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POD) and catalase (CAT) in cotton leaves also increased under water deficit. Therefore, the Mehler-peroxidation reaction, photorespiration and nitrate reduction helped to dissipated excess light energy, being important photoprotective mechanisms for adapting the photosynthetic apparatus to mild and moderate water deficit in cotton.

Insights into the region responding to ΔpH change in major light harvesting complex

Nonradioactive dissipation of excitation energy is the most important photoprotective mechanism in plants. In high light, the trans-thylakoid ΔpH can activate the xanthophyll cycle and protonate the PsbS protein, which are important in regulating the process of non-photochemical quenching. Despite it has been proven that the ΔpH can directly switch the major light harvesting complex (LHCIIb) from the light harvesting state to the energy dissipating state both in vitro and in vivo, the region responding to ΔpH change in LHCIIb remains obscure. In the present study, we proposed that Asp211 and Asp215 should be the amino acids that respond to the change of the trans-thylakoid ΔpH on the basis of pKa calculations of LHCIIb. Furthermore, Asp215 is the outmost acidic amino acid in trimeric LHCIIb, and its pKa value is sensitive to the protein environment, suggesting that the regulation of the protonation state of Asp215 may be important for the aggregation of LHCIIb occurred in the process of qE in vivo and in vitro.