Important Role In Photoprotection Research Articles

The light-harvesting chlorophyll a/b-binding proteins of photosystem II family members are responsible for temperature sensitivity and leaf color phenotype in albino tea plant

IntroductionLight-harvesting chlorophyll a/b-binding (LHCB) protein complexes of photosystem II are integral to the formation of thylakoid structure and the photosynthetic process. They play an important role in photoprotection, a crucial process in leaf development under low-temperature stress. Nonetheless, potential key genes directly related to low-temperature response and albino phenotype have not been precisely identified in tea plant. Moreover, there are no studies simultaneously investigating multiple albino tea cultivars with different temperature sensitivity. ObjectivesThe study aimed to clarify the basic characteristics of CsLHCB gene family members, and identify critical CsLHCB genes potentially influential in leaf color phenotypic variation and low-temperature stress response by contrasting green and albino tea cultivars. Concurrently, exploring the differential expression of the CsLHCB gene family across diverse temperature-sensitive albino tea cultivars. MethodsWe identified 20 putative CsLHCB genes according to phylogenetic analysis. Evolutionary relationships, gene duplication, chromosomal localization, and structures were analyzed by TBtools; the physiological and biochemical characteristics were analyzed by protein analysis websites; the differences in coding sequences and protein accumulation in green and albino cultivars, gene expression with maturity were tested by molecular biology technology; and protein interaction was analyzed in the STRING database. ResultsAll genes were categorized into seven groups, mapping onto 7 chromosomes, including three tandem and one segmental duplication. They all own a conserved chlorophyll A-B binding protein domain. The expression of CsLHCB genes was tissue-specific, predominantly in leaves. CsLHCB5 may play a key role in the process of leaf maturation and senescence. In contrast to CsLHCB5, CsLHCB1.1, CsLHCB2, and CsLHCB3.2 were highly conserved in amino acid sequence between green and albino cultivars. In albino tea cultivars, unlike in green cultivars, the expression of CsLHCB1.1, CsLHCB1.2, and CsLHCB2 was down-regulated under low-temperature stress. The accumulation of CsLHCB1 and CsLHCB5 proteins was lower in albino tea cultivars. Greater accumulation of CsLHCB2 protein was detected in RX1 and RX2 compared to other albino cultivars. ConclusionsCsLHCB1.1, CsLHCB1.2, and CsLHCB2 play a role in the response to low-temperature stress. The amino acid sequence site mutation of CsLHCB5 would distinguish the green and albino tea cultivars. The less accumulation of CsLHCB1 and CsLHCB5 had a potential influence on albino leaves. Albino cultivars more sensitive to temperature exhibited lower CsLHCB gene expression. CsLHCB2 may serve as an indicator of temperature sensitivity differences in albino tea cultivars. This study could provide a reference for further studies of the functions of the CsLHCB family and contribute to research on the mechanism of the albino in tea plant.

Photorespiration plays an important role in H2 production by marine Chlorella pyrenoidosa

Marine Chlorella pyrenoidosa was previously reported to produce H2 under nitrogen-deprived condition. In this study, the role of photorespiration in H2 production by C. pyrenoidosa was investigated. It was found that the expression level of genes encoding three enzymes involved in photorespiration decreased, but the expression level of gene encoding glutamate synthase increased in nitrogen-deprived culture compared with control, indicating a complex adjustment of photorespiration activity during H2 production by C. pyrenoidosa. The use of a photorespiration inhibitor led not only to a decrease in H2 production by 15 % under nitrogen deprivation condition, but also to a 16 % decrease in O2 evolution in the complete medium. Moreover, it was demonstrated that the total photosynthetic oxygen evolution and Fv/Fm declined when photorespiration was blocked. This suggested that, the photorespiration pathway does not directly participate in the production of photosynthetic electrons and the transfer of hydrogen producing electrons. However, it played an important role in photoprotection of PSII during H2 production by C. pyrenoidosa, which enabled PSII to provide more photosynthetic electrons to hydrogenase.

Exogenous Arachidonic Acid Affects Fucoxanthin Biosynthesis and Photoprotection in Phaeodactylum tricornutum.

Fucoxanthin is an oxygenated carotenoid component that has been reported to play important roles in anti-oxidation, anti-obesity and anti-cancer in the human body. Fucoxanthin-chlorophyll protein (FCP) complexes participate in light harvesting and photoprotection in diatom. In order to better understand the change of fucoxanthin content and its role in photoprotection, the growth, fucoxanthin biosynthesis and photosynthetic phenotypes were studied in the diatom Phaeodactylum tricornutum under the treatment of exogenous arachidonic acid (AA). Our results demonstrated that even low concentration of AA at 0.1 mg/L strongly induced fucoxanthin accumulation in algal cells to a maximum of 1.1 mg/g dry weight (DW), which was 36.6% higher than that in the untreated ones. By principal component analysis (PCA), we also identified a close correlation between fucoxanthin accumulation and the expression of genes involved in fucoxanthin biosynthesis, especially phytoene synthase (PSY), suggesting that AA change the metabolism of fucoxanthin by inducing carotenoid metabolic enzymes at the transcriptional level. Furthermore, we found that the exogenous application of AA affected non-photochemical quenching (NPQ) and photoinhibition, which resulted from the changed diadinoxanthin (DD) and diatoxanthin (DT) cycle, and thus played an important role in photoprotection.

PRP4 Promotes Skin Cancer by Inhibiting Production of Melanin, Blocking Influx of Extracellular Calcium, and Remodeling Cell Actin Cytoskeleton.

Pre-mRNA processing factor 4B (PRP4) has previously been shown to induce epithelial-mesenchymal transition (EMT) and drug resistance in cancer cell lines. As melanin plays an important photoprotective role in the risk of sun-induced skin cancers, we have investigated whether PRP4 can induce drug resistance and regulate melanin biosynthesis in a murine melanoma (B16F10) cell line. Cells were incubated with a crucial melanogenesis stimulator, alpha-melanocyte-stimulating hormone, followed by transfection with PRP4. This resulted in the inhibition of the production of melanin via the downregulation of adenylyl cyclase-cyclic adenosine 3′,5′-monophosphate (AC)–(cAMP)–tyrosinase synthesis signaling pathway. Inhibition of melanin production by PRP4 leads to the promotion of carcinogenesis and induced drug resistance in B16F10 cells. Additionally, PRP4 overexpression upregulated the expression of β-arrestin 1 and desensitized the extracellular calcium-sensing receptor (CaSR), which in turn, inhibited the influx of extracellular Ca2+ ions. The decreased influx of Ca2+ was confirmed by a decreased expression level of calmodulin. We have demonstrated that transient receptor potential cation channel subfamily C member 1 was involved in the influx of CaSR-induced Ca2+ via a decreasing level of its expression. Furthermore, PRP4 overexpression downregulated the expression of AC, decreased the synthesis of cAMP, and modulated the actin cytoskeleton by inhibiting the expression of Ras homolog family member A (RhoA). Our investigation suggests that PRP4 inhibits the production of melanin in B16F10 cells, blocks the influx of Ca2+ through desensitization of CaSR, and modulates the actin cytoskeleton through downregulating the AC–cAMP pathway; taken together, these observations collectively lead to the promotion of skin carcinogenesis.

Effect of drought and heat stresses on photosynthesis, pigments, and xanthophyll cycle in alfalfa (Medicago sativa L.)

Climate changes severely influence photosynthetic activity of plants. Studies on the combined effect of drought and heat stress on photosynthesis, pigments, and xanthophyll cycle particularly in alfalfa (Medicago sativa L.) are limited. Thus, this study investigated the combined effects of drought and heat stress in two alfalfa varieties ('Deqin' and 'Algonguin'). Results revealed that the values of the net photosynthetic rate demonstrated a significantly decreasing tendency under stresses. The effect of stresses on ribulose 1,5-bisphosphate carboxylase activity in both varieties showed a trend of first declining and then rising. Meanwhile, phosphoenolpyruvate carboxylase activity showed an increasing tendency. The intensification of stresses led to reduced photochemical efficiency and increased capability of thermal dissipation. A linear correlation was found between nonphotochemical quenching and xanthophyll cycle components. Thus, the change in carotenoids demonstrated an important role in photoprotection, the xanthophyll cycle was the main way of dissipation of excess energy under stress conditions in alfalfa.

A Novel Method for Visualizing Melanosome and Melanin Distribution in Human Skin Tissues.

Melanin incorporated into keratinocytes plays an important role in photoprotection; however, abnormal melanin accumulation causes hyperpigmentary disorders. To understand the mechanism behind the accumulation of excess melanin in the skin, it is essential to clarify the spatial distribution of melanosomes or melanin in the epidermis. Although several markers have been used to detect melanosomes or melanin, no suitable markers to determine the precise localization of melanin in the epidermis have been reported. In this study, we showed that melanocore-interacting Kif1c-tail (M-INK), a recently developed fluorescent probe for visualizing mature melanosomes, binds to purified melanin in vitro, and applied it for detecting melanin in human skin tissues. Frozen skin sections from different phototypes were co-stained for the hemagglutinin (HA)-tagged M-INK probe and markers of melanocytes or keratinocytes, and a wide distribution of melanin was observed in the epidermis. Analysis of the different skin phototypes indicated that the fluorescent signals of HA-M-INK correlated well with skin color. The reconstruction of three-dimensional images of epidermal sheets enabled us to observe the spatial distribution of melanin in the epidermis. Thus, the HA-M-INK probe is an ideal tool to individually visualize melanin (or melanosome) distribution in melanocytes and in keratinocytes in skin tissues.

Neoxanthin affects the stability of the C2 S2 M2 -type photosystem II supercomplexes and the kinetics of state transition in Arabidopsis.

Neoxanthin (Neo), which is only bound to the peripheral antenna proteins of photosystem (PS) II, is a conserved carotenoid in all green plants. It has been demonstrated that Neo plays an important role in photoprotection and its deficiency fails to impact LHCII stability in vitro and indoor plant growth in vivo. Whether Neo is involved in maintaining the PSII complex structure or adaptive mechanisms for the everchanging environment has not yet been elucidated. In this study, the role of Neo in maintaining the structure and function of the PSII-LHCII supercomplexes was studied using Neo deficient Arabidopsis mutants. Our results show that Neo deficiency had little effect on the electron transport capacity and the plant fitness, but the PSII-LHCII supercomplexes were significantly impacted by the lack of Neo. In the absence of Neo, the M-type LHCII trimer cannot effectively associate with the C2 S2 -type PSII-LHCII supercomplexes even in moderate light conditions. Interestingly, Neo deficiency also leads to decreased PSII protein phosphorylation but rapid transition from state 1 to state 2. We suggest that Neo might enforce the interactions between LHCII and the minor antennas and that the absence of Neo makes M-type LHCII disassociate from the PSII complex, leading to the disassembly of the PSII-LHCII C2 S2 M2 supercomplexes, which results in alterations in the phosphorylation patterns of the thylakoid photosynthetic proteins and the kinetics of state transition.

Computation-Driven Analysis of Model Polyextremo-tolerant Fungus Exophiala dermatitidis: Defensive Pigment Metabolic Costs and Human Applications.

SummaryThe polyextremotolerant black yeast Exophiala dermatitidis is a tractable model system for investigation of adaptations that support growth under extreme conditions. Foremost among these adaptations are melanogenesis and carotenogenesis. A particularly important question is their metabolic production cost. However, investigation of this issue has been hindered by a relatively poor systems-level understanding of E. dermatitidis metabolism. To address this challenge, a genome-scale model (iEde2091) was developed. Using iEde2091, carotenoids were found to be more expensive to produce than melanins. Given their overlapping protective functions, this suggests that carotenoids have an underexplored yet important role in photo-protection. Furthermore, multiple defensive pigments with overlapping functions might allow E. dermatitidis to minimize cost. Because iEde2091 revealed that E. dermatitidis synthesizes the same melanins as humans and the active sites of the key tyrosinase enzyme are highly conserved this model may enable a broader understanding of melanin production across kingdoms.

Linking lipid accumulation and photosynthetic efficiency in Nannochloropsis sp. under nutrient limitation and replenishment

Oleaginous microalgae can accumulate large amounts of storage lipids that have many potential applications such as in the production of biodiesel and health products. All of the energy for the synthesis of lipids in microalgae is derived from photosynthesis. To date, however, the quantitative relationship between photosynthesis and lipid accumulation rate in microalgae is still unclear. In this study, Nannochloropsis sp. was selected to explore this relationship by investigating changes in lipid accumulation, photosynthetic efficiency, and the electron transport chain under nutrients limitation and replenishment. The results of the study demonstrated that lipids were the main form of storage of carbon and energy for this microalga. The alternative electron transport chain played an important role in photo-protection and lipid accumulation. The photosynthetic efficiency of this microalga showed a strong correlation with lipid accumulation rate (R2 = 0.959), with higher photosynthetic efficiency translating to higher lipid accumulation rates during the lipid accumulation stage of the growth cycle. The results of the present study indicated that nutrient depletion might not be the best signal to understand when microalgae begin to accumulate oil, as it appears that photosynthetic efficiency also has a large role in the rate of oil accumulation. Additionally, the study shows that excessive light might not be needed in later stages of growth for lipid accumulation, which in turn would alleviate the damage to microalgae caused by strong light making large-scale cultivation more cost-effective.

Microsphere packages of carotenoids: intact sea urchin eggs tracked by Raman spectroscopy tools.

Although natural exposure to ambient UV radiation in oligotrophic seawater at small depths can reach the levels responsible for cellular damage, the sea urchin Paracentrotus lividus is frequently in such sites, particularly on the southern Adriatic Sea shore. Spawning their eggs and spending their early life stage in rocky shores at depths of 0.5-2 m are the results of their successful adaptation strategies, although adults may dwell at greater depths. Surprisingly, there is a paucity of reports regarding the carotenoid content in sea urchin eggs. Beyond their important role in photoprotection against high UV exposure, cell division and early development, the content and distribution of carotenoids contribute to the successful survival of sea urchins and also determine the color of their gonads (roe), which is of commercial importance as a delicacy. Herein, for the first time, we have described the carotenoid content and distribution in intact, freshly released eggs of P. lividus species, non-destructively employing resonance Raman spectroscopy and imaging; near-infrared Raman spectroscopy revealed additional molecular carotenoid content. Echinenone and β-carotene resonance Raman signals were the most intense, and they were identified as the principal carotenoids that are preferentially accumulated in eggs rather than in gonads. Raman imaging in confocal mode revealed the uniform distribution of the carotenoid signal over the whole eggs, while the distribution of proteins appeared spotted. Egg carotenoids generally maintained their identity after 2 months of dry storage, with slight signs of C[double bond, length as m-dash]C bond oxidation. The potential utilization of P. lividus sea urchin eggs as valuable microsphere packages of native carotenoids is discussed.

Distinct characteristics of DNA field effect transistors embedded with marine-derived porphyra-334 under UV illumination

DNA extracted from salmon has recently attracted the attention of researchers, resulting in applications of DNA in photonic and electronic devices. Porphyra-334, a type of mycosporine-like amino acids (MAAs), also plays an important role in photoprotection for a variety of marine organisms including bacteria and algae. Although MAA and DNA molecules have been intensively studied, fabrication methodology and applicability of MAA-embedded DNA complexes for physical applications have been seldom discussed due to incompatibility between biological samples and physical platform. Here, Porphyra-334 embedded DNA was investigated to understand its electrical transport property with the aid of silicon nanowire/nanoribbon field effect transistors (NW/NR FETs). Its chemical stability was determined by cyclic voltammetry upon illumination of UV light. The current of DNA-SiNW FET was enhanced by the addition of Porphyra-334 and upon illumination of UV light. Conductivities of PDNA-SiNW FET compared to SiNW FET were increased up to ∼70% at dark and ∼40% under UV light due to the presence of Porphyra-334 and excess injection of charge carriers in Porphyra-334 embedded DNA generated by absorbing UV light, respectively. The addition of Porphyra-334 in DNA-SiNR FET lowered its energy level and resulted in large threshold voltage shift towards the negative scale. In addition, its electrochemical property was studied by cyclic voltammetry and impedance spectroscopy. Porphyra-334 in DNA solution which inhibited oxidation of DNA showed relatively lower current indicating high electrochemical stability and decrease of resistance compared to pristine DNA solution based on results of impedance spectroscopy.

The Plastid Lipocalin LCNP Is Required for Sustained Photoprotective Energy Dissipation in Arabidopsis

Light utilization is finely tuned in photosynthetic organisms to prevent cellular damage. The dissipation of excess absorbed light energy, a process termed nonphotochemical quenching (NPQ), plays an important role in photoprotection. Little is known about the sustained or slowly reversible form(s) of NPQ and whether they are photoprotective, in part due to the lack of mutants. The Arabidopsis thaliana suppressor of quenching1 (soq1) mutant exhibits enhanced sustained NPQ, which we term qH. To identify molecular players involved in qH, we screened for suppressors of soq1 and isolated mutants affecting either chlorophyllide a oxygenase or the chloroplastic lipocalin, now renamed plastid lipocalin (LCNP). Analysis of the mutants confirmed that qH is localized to the peripheral antenna (LHCII) of photosystem II and demonstrated that LCNP is required for qH, either directly (by forming NPQ sites) or indirectly (by modifying the LHCII membrane environment). qH operates under stress conditions such as cold and high light and is photoprotective, as it reduces lipid peroxidation levels. We propose that, under stress conditions, LCNP protects the thylakoid membrane by enabling sustained NPQ in LHCII, thereby preventing singlet oxygen stress.

Raman Optical Activity Reveals Carotenoid Photoactivation Events in the Orange Carotenoid Protein in Solution.

The orange carotenoid protein (OCP) plays an important role in photoprotection in cyanobacteria, which is achieved by the photoconversion from the orange dark state (OCPO) to the red active state (OCPR). Using Raman optical activity (ROA), we studied the conformations of the carotenoid chromophore in the active sites of OCPO and OCPR. This ROA measurement directly observed the chromophore conformation of native OCP in solution, and the measurement of OCPR first demonstrated the ROA spectroscopy for the transient species. For OCPO, the spectral features of ROA were mostly reproduced by the quantum chemical calculation based on the crystal structure of the OCP. Within the spatial resolution (∼2 Å), a slight modification of the polyene-chain distortion improved the agreement between the observed and calculated ROA spectra. While the crystal structure of OCPR is not available, the ROA spectrum of OCPR was reproduced by using the crystal structure of red carotenoid protein (RCP), an OCPR proxy. The present results showed that the chromophore conformations in the crystal structures of OCP and RCP hold true for OCPO and OCPR in solution. Particularly, ROA spectroscopy of the native OCPR provides a direct support for the 12 Å translocation of chromophore in the photoactivation, which was proposed by X-ray crystallography using RCP [R. L. Leverenz, M. Sutter, et al. Science 2015, 348, 1463-1466].

Fluctuation of photosynthetic pigment of water-stressed cowpea Vigna unguiculata (L.)Walp. varieties

Photosynthetic pigments are responsible for conversion of light energy into a form of chemical energy in the plants. The most important pigment in the light harvesting machinery of the plant is chlorophyll and Carotenoids play an important role in photo-protection of chlorophyll molecules. Cowpea responds to survive under water-deficit conditions via a series of physiological, cellular and molecular processes culminating in stress tolerance. Most cowpea is produced in arid and semi-arid zone. This experiment was conducted in the research field of Department of Biological Sciences, Sam Higginbottom Institute of Agricultural Technology and Sciences, Allahabad. To study the chlorophyll a, chlorophyll b and carotenoid content of three cowpea varieties (UU-0, VU-89 and KK-6) subjected to the different level of watering (daily watering, 2 days interval, 4 days interval and 6 days interval of watering). The experimental materials were arranged inRandomize Complete BlockDesign. The results indicate that the all photosynthetic pigments such as chlorophyll a, chlorophyll b and carotenoid content maximum in daily watering whereas at 6 days interval of watering observed minimum.

Evidence for the role of cyclic electron flow in photoprotection for oxygen-evolving complex

Cyclic electron flow (CEF) alleviates PSII photo-inhibition under high light by at least two different mechanisms: one is liked to thermal energy dissipation (qE) and the other one is independent of qE. However, the latter mechanism is unclear. Because the photodamage to PSII primarily occurred at the oxygen-evolving complex (OEC), and the stability of OEC is dependent on proton gradient across thylakoid membrane (ΔpH), we hypothesize that the CEF-dependent generation of ΔpH can alleviate photodamage to OEC. To test this hypothesis, we determined the effects of antimycin A (AA), methyl viologen (MV), chloramphenicol (CM), nigericin (Nig) on PSII activity and the stability of OEC for leaves of a light-demanding tropical tree species Erythrophleum guineense by the analysis of OKJIP chlorophyll a fluorescence transient. After high light treatment, the stronger decrease in Fv/Fm in the AA-, CM-, MV-, and Nig-treated samples was accompanied with larger photo damage of OEC. The AA-treated samples significantly showed lower CEF activity than the H2O-treated samples. Although the AA-treated leaves significantly showed stronger PSII photo-inhibition and photo-damage of OEC compared to the H2O-treated leaves, the value of non-photochemical quenching did not differ between them. Therefore, CEF activity was partly inhibited in the AA-treated samples, and the stronger PSII photo-inhibition in the AA-treated leaves was independent of qE. Taking together, we propose a hypothesis that CEF-dependent generation of ΔpH under high light plays an important role in photoprotection for the OEC activity.

Differences in the Responses of Photosystems I and II in Cymbidium sinense and C. tracyanum to Long-Term Chilling Stress.

The susceptibility of photosystem I (PSI) and photosystem II (PSII) to chilling stress depends on plant species, and cyclic electron flow (CEF) plays an important role in photoprotection for some species under short stress periods. However, little is known about the responses of PSI and PSII to long-term chilling stress. We studied two orchid species—Cymbidium sinense and C. tracyanum— that differ in their capacity to adapt to low temperature, and exposed plants for 19 d to stress conditions that included 4°C and a light intensity of 250 to 350 μmol photons m-2 s-1. Meanwhile, we investigated their dynamic variations in Chl fluorescence and P700 parameters. After exposure to 4°C and 250 μmol photons m-2 s-1 for 6 h, PSI activity was maintained stable in both species, but stronger PSII photoinhibition was observed in C. sinense. During the long-term treatment, the maximum quantum yield of PSII was significantly reduced, with that decrease being greater in C. sinense. After 19 d of chilling treatment, the maximum photo-oxidizable P700 declined only slightly in C. tracyanum but dropped significantly in C. sinense. Linear electron flow was largely depressed during the long-term chilling treatment, especially in C. sinense. Meanwhile, C. tracyanum showed higher CEF activity than C. sinense. These results indicate that PSII is more sensitive to chilling-light stress than PSI in both species. The rate of PSII photodamage at chilling-light stress is higher in C. sinense than C. tracyanum, and CEF contributes to photoprotection for PSI and PSII under long-term chilling stress in C. tracyanum.

Differences in the photosynthetic efficiency and photorespiration of co-occurring Euphorbiaceae liana and tree in a Chinese savanna

Lianas perform better than co-occurring trees in secondary forests or disturbed areas. Lianas and trees differ strikingly in water use strategy, which may result in a significant difference in photosynthetic light use between both growth forms. However, the difference in the photosynthetic efficiency and light energy dissipation between these two growth forms is poorly understood. Moreover, photorespiration is an important mechanism of photoprotection under conditions of high light. In this study, we used Bridelia stipularis (Linn.) Bl. (liana) and Strophioblachia fimbricalyx Boerl. (tree) in order to measure the response curves of the gas exchange and photosynthetic electron flow to the incident light gradients and intercellular CO2 concentration, as well as the hydraulic conductivity. We tested whether the photochemical efficiency and photorespiration differed between both growth forms. Our results clearly demonstrated that B. stipularis possessed a significantly higher stem and leaf specific hydraulic conductivity, total electron flow, and maximum rate of ribulose-1,5-bisphosphate regeneration compared to the sympatric tree S. fimbricalyx. Correspondingly, B. stipularis exhibited a significantly higher photochemical quenching coefficient and electron flow to photorespiration relative to S. fimbricalyx under saturating light levels. We suggested that photorespiration might play an important role in photoprotection for both species under high light, but particularly for B. stipularis. These findings could enrich our knowledge of the superior photosynthetic and growth performance of lianas over the co-occurring trees.

The role of photorespiration during H2 photoproduction in Chlorella protothecoides under nitrogen limitation.

Photorespiration in Chlorella protothecoides plays an important role in photoprotection of photosystem (PS) II in the late phase of H(2) photoproduction, allowing PSII to supply more electrons to hydrogenase.

Insusceptibility of oxygen-evolving complex to high light in Betula platyphylla.

High mountain plants growing at high altitude have to regularly cope with high light and high UV radiation that can lead to photodamage of oxygen-evolving complex (OEC). However, the underlying mechanism of photoprotection for OEC in high mountain plants is unclear. Sun leaves of Betula platyphylla were used to examine whether cyclic electron flow (CEF) around photosystem I (PSI) plays an important role in photoprotection for OEC. Our results indicated that the value of ETRI/ETRII ratio significantly increased under high light. With increasing light intensity, non-photochemical quenching (NPQ) gradually increased, and the fraction of P700 that is oxidized in a given state gradually increased. These results indicated that CEF was significantly activated under high light. After treatment with a high light of 1600 μmol photons m(-2) s(-1) for 8 h, the OEC activity did not decline, but the maximum quantum yield of PSII (F v /F m ) ratio significantly decreased. These results suggested that CEF-dependent generation of proton gradient across thylakoid membrane protected OEC activity against high light. Furthermore, the stability of PSI activity during exposure to high light suggested that the high CEF activity in B. platyphylla played an important role in photoprotection for PSI activity.

An irradiation density dependent energy relaxation in plant photosystem II antenna assembly

Plant photosystem II (PSII) is a multicomponent pigment-protein complex that harvests sunlight via pigments photoexcitation, and converts light energy into chemical energy. Against high light induced photodamage, excess light absorption of antenna pigments triggers the operation of photoprotection mechanism in plant PSII. Non-photochemical energy relaxation as a major photoprotection way is essentially correlated to the excess light absorption. Here we investigate the energy relaxation of plant PSII complexes with varying incident light density, by performing steady-state and transient chlorophyll fluorescence measurements of the grana membranes (called as BBY), functional moiety PSII reaction center and isolated light-harvesting complex LHCII under excess light irradiation. Based on the chlorophyll fluorescence decays of these samples, it is found that an irradiation density dependent energy relaxation occurs in the LHCII assemblies, especially in the antenna assembly of PSII supercomplexes in grana membrane, when irradiation increases to somewhat higher density levels. Correspondingly, the average chlorophyll fluorescence lifetime of the highly isolated BBY fragments gradually decreases from ~1680 to ~1360ps with increasing the irradiation density from 6.1×109 to 5.5×1010 photon cm−2 pulse−1. Analysis of the relation of fluorescence decay change to the aggregation extent of LHCIIs suggests that a dense arrangement of trimeric LHCIIs is likely the structural base for the occurrence of this irradiation density dependent energy relaxation. Once altering the irradiation density, this energy relaxation is quickly reversible, implying that it may play an important role in photoprotection of plant PSII.