PSII Reaction Center Research Articles

Physiological and thylakoid proteome analyses of Anabaena sp. PCC 7120 for monitoring the photosynthetic responses under cadmium stress

Photosynthetic organisms are highly susceptible to cadmium (Cd) as it interferes with the structural and functional aspects of the photosynthesis. As a major group of photosynthetic prokaryotes, cyanobacteria are also affected by Cd. The inhibitory effects of Cd on cyanobacterial photosynthesis have been studied from an early stage, but the mechanism of Cd toxicity is still unclear. Therefore, we investigated the photosynthetic responses of Anabaena sp. PCC 7120 under Cd stress (LC50) at physiological and thylakoid proteome levels via chlorophyll a fluorescence measurements and blue native (BN)-SDS PAGE, respectively. The findings revealed that Cd exposure triggered the intracellular ROS production and negatively affected the photosynthetic performance. With Cd exposure, effective photochemical quantum yield of PSII (Y(II)) and photochemical quenching efficiency (qP and qL) were significantly reduced, whereas the non-regulated energy dissipation (Y(NO)) was increased. Fast fluorescence kinetic measurements showed that Cd exposure resulted in the inactivation of PSII reaction centres, which interfered with the transfer of energy from antenna complexes to reaction centre, decreased the ability to convert excitation energy into electron transport, increased thermal dissipation, and thus decreased linear electron flow efficiency. Conversely, the efficiency of electron transfer from intermediate carriers to final PSI acceptors was increased, which might be related to the induction of PSI-based cyclic electron flow. This assumption was confirmed by an enhanced post-illumination fluorescence transient. Consistent with these observations, BN-SDS PAGE showed that Cd exposure significantly decreased PSII subunit proteins accumulation, but slightly increased PSI proteins accumulation. Additionally, only minor reductions in the accumulation of ATP synthase/NDH-1 and Cytochrome b6f subunit proteins were observed. The transcript levels of most of the selected subunit proteins were in accordance with BN-SDS PAGE. Collectively, our experiments show that a shift to PSI-based cyclic electron flow was a protective strategy of Anabaena sp. PCC 7120 against Cd-induced PSII photo-damage.

Inhibition of photosynthesis in Melia azedarach and Ligustrum lucidum induced by manganese toxicity using OJIP chlorophyll a fluorescence transient

Manganese (Mn) excess is a major abiotic stress for plant growth. In the present study, we investigated the effects of Mn on photosynthesis in Ligustrum lucidum and Melia azedarach leaves using chlorophyll fluorescence transients. Both plant species were exposed to two Mn concentrations (0.5 and 1 mmol) for 10 and 30 d. Results showed that excess Mn significantly inhibited photosynthesis. With the increase of Mn concentration and stress time, the inhibition was more serious. Mn stress impaired PSII at the donor and the acceptor side by damaging the oxygen-evolving complex and limiting electron transport downstream of QA in both trees. A significant decline in 820 nm reflection curve absorption was observed in M. azedarach, suggesting that the oxidization-reduction reactions in PSI were inhibited but this phenomenon was not observed in L. lucidum. Therefore, excess Mn impaired the whole electron transport chain associated with inactive PSII reaction centers in Ligustrum lucidum and inhibited oxidization-reduction reactions in PSI in M. azedarach.

Photosynthetic Gas Exchange and Chlorophyll a Fluorescence in Salicornia brachiata (Roxb.) Under Osmotic Stress

Osmotic stress negatively affects the photosynthetic efficiency and cause a significant loss of crop productivity. Salicornia brachiata (Roxb.) is a eu-halophyte. We hereby report on photosynthetic gas exchange and chlorophyll fluorescence in S. brachiata under sodium chloride (NaCl), seawater and polyethylene glycol (PEG) induced osmotic stress. It grows luxuriantly and exhibited a higher tolerance index and better accumulation of organic solutes under 100% strength of seawater (32.5 ppt) and 0.5 M NaCl salinity. It exhibited comparatively better gas exchange, stomatal conductance, PSII photochemistry and electron transfer under 100% strength of seawater salinity. Higher chlorophyll a/b ratio under stress conditions indicated a lower ratio of PSII to PSI and balanced excitation of PSI and PSII in S. brachiata resulting in efficient photosynthetic processes. The lower total chlorophyll/carotenoids ratio and higher non-photochemical quenching indicated the photo-protection and safer dissipation of heat energy in S. brachiata under stress. The 100% strength of seawater and 0.5 M NaCl salinity in S. brachiata did not cause significant changes in antenna size, connectivity between PSII reaction centres (RCs) and reduction of electrons on PSII donor side. The 20% PEG induced the inactivation of RCs and cause damage to PSII RCs in S. brachiata thus reduced the electron transfer from QA– to QB pool-sized and activity of water-splitting complex. Higher φ(P0) and FV/FM in S. brachiata under seawater salinity indicated a comparatively better quantum yield of primary photochemistry. The higher PITotal in S. brachiata under 100% strength of seawater and 0.5 M NaCl stress indicated a better energy flux reaching to PSII RCs, electron transport and performance of RCs. The higher strengths of osmotic stress cause reduction in the quantum yield of PSII electron transport and capturing efficiency of excitation energy by open PSII RCs in S. brachiata.

Chlororespiration protects the photosynthetic apparatus against photoinhibition by alleviating inhibition of photodamaged-PSII repair in Haematococcus pluvialis at the green motile stage

During outdoor mass cultivation, Haematococcus pluvialis cells, especially at the green motile stage, are frequently exposed to high light. However, the photoprotective mechanisms in H. pluvialis have not been well characterized. This study aimed to confirm the photoprotection of chlororespiration in H. pluvialis at the green motile stage. Under high light, inhibition of plastid terminal oxidase (PTOX), a key enzyme of chlororespiration, increased the relative variable fluorescence at the I-step (VI), causing a decrease in the plastoquinone (PQ) pool, which further led to photosystem II (PSII) acceptor side over-reduction. This condition disturbed the balance between light absorption and energy utilization, causing the over-excitation of PSII reaction centers, which was indicated by the fact that the inhibition of PTOX increased the relative variable fluorescence at the J-step (VJ) and the light absorption flux per active reaction center (ABS/RC), but decreased the electron transport flux per active reaction center (ETo/RC) in H. pluvialis. The over-reduction of the PSII acceptor side and the over-excitation of PSII reaction centers inevitably caused more severe photoinhibition as shown by a decrease in Fv/Fm (maximal photochemical efficiency of PSII). Furthermore, the level of photoinhibition was not changed when the PTOX was inhibited in the presence of the photodamaged-PSII repair inhibitor chloramphenicol, indicating that the inhibition of PTOX did not accelerate the photodamage to PSII directly. The above results suggest that the chlororespiration pathway protects H. pluvialis at the green motile stage against photoinhibition by decreasing the inhibition of the photodamaged-PSII repair mechanism.

Simultaneously measuring pulse-amplitude-modulated (PAM) chlorophyll fluorescence of leaves at wavelengths shorter and longer than 700nm.

PAM fluorescence of leaves of cherry laurel (Prunus laurocerasus L.) was measured simultaneously in the spectral range below 700nm (sw) and above 700nm (lw). A high-sensitivity photodiode was employed to measure the low intensities of sw fluorescence. Photosystem II (PSII) performance was analyzed by the saturation pulse method during a light response curve with subsequent dark phase. The sw fluorescence was more variable, resulting in higher PSII photochemical yields compared to lw fluorescence. The variations between sw and lw data were explained by different levels of photosystem I (PSI) fluorescence: the contribution of PSI fluorescence to minimum fluorescence (F0) was calculated to be 14% at sw wavelengths and 45% at lw wavelengths. With the results obtained, the validity of an earlier method for the quantification of PSI fluorescence (Genty et al. in Photosynth Res 26:133-139, 1990, https://doi.org/10.1007/BF00047085 ) was reconsidered. After subtracting PSI fluorescence from all fluorescence levels, the maximum PSII photochemical yield (FV/FM) in the sw range was 0.862 and it was 0.883 in the lw range. The lower FV/FM at sw wavelengths was suggested to arise from inactive PSII reaction centers in the outermost leaf layers. Polyphasic fluorescence transients (OJIP or OI1I2P kinetics) were recorded simultaneously at sw and lw wavelengths: the slowest phase of the kinetics (IP or I2P) corresponded to 11% and 13% of total variable sw and lw fluorescence, respectively. The idea that this difference is due to variable PSI fluorescence is critically discussed. Potential future applications of simultaneously recording fluorescence in two spectral windows include studies of PSI non-photochemical quenching and state I-state II transitions, as well as measuring the fluorescence from pH-sensitive dyes simultaneously with chlorophyll fluorescence.

Effects of metamitron on fruit set and photosynthetic biological characteristics of apples.

With dwarfing interstock Fuji apples as the test materials and water treatment as the control (CK), we examined the fruit thinning effect and its influences on leaves' photosynthesis by spraying 200, 300, and 400 mg·L-1 metamitron during the young fruit period to solve artificial fruit thinning problems (time-consuming, much labor, and low efficiency). The results showed that metamitron application could significantly reduce the inflorescence and flowers' fruit-setting rate by 16.5%-22.8% and 50.9%-53.9%, respectively. The treatment of 300 mg·L-1 metamitron had the strongest fruit thinning effect, with a single fruit rate of 46.6% and a double fruit rate of 18.3%. As a photosynthesis inhibitor, metamitron application reduced the chlorophyll content of leaves and strongly affected photosynthesis. The inhibitory effect on chlorophyll content disappeared after 15 days of the treatment, while that on the net photosynthetic rate disappeared gradually after 11 days of the treatment. The application of metamitron significantly reduced the maximum quantum yield of PSⅡ reaction center (Fv/Fm), actual photochemical efficiency (ΦPSⅡ), photochemical quenching coefficient (qP) and non-photochemical quenching coefficient (NPQ), with such inhibitory effect having been lasted for 15 days. OJIP analysis showed that metamitron caused damage to the apple leaves' oxygen-evolving complex, especially limiting the transfer of electrons in the PSⅡ reaction center from QA to QB. Metamitron treatment increased Wk, and significantly decreased ψo, RC/CSm, and PIabs. Besides, 300 mg·L-1 metamitron had the most significant effect. Our results showed that metamitron destroyed the structure of the PSⅡ reaction center of apple leaves and hindered the transfer of electrons from the donor to the receptor of PSⅡ. Consequently, the photosynthetic rate was affected, and the young fruits fell off due to the lack of accumulation of photosynthetic products.

The Leafless Orchid Cymbidium macrorhizon Performs Photosynthesis in the Pericarp during the Fruiting Season.

Photosynthesis with highly photoreactive chlorophyll (Chl) provides energy for plant growth but with simultaneous risk of photooxidative damage and photoprotection costs. Although the leafless orchid Cymbidium macrorhizon mostly depends on mycorrhizal fungi for carbon, it accumulates Chl particularly during fruiting and may not be fully mycoheterotrophic. In fact, stable isotopic analysis suggested that the fruiting C. macrorhizon specimens obtain a significant proportion of its carbon demands through photosynthesis. However, actual photosynthetic characteristics of this leafless orchid are unknown. To reveal the functionality of photosynthetic electron transport in C. macrorhizon, we compared its photosynthetic properties with those of its relative mixotrophic orchid Cymbidium goeringii and the model plant Arabidopsis thaliana. Compared with C. goeringii and A. thaliana, maximum photochemical efficiency of PSII was substantially low in C. macrorhizon. Chl fluorescence induction kinetics revealed that the electron transport capacity of PSII was limited in C. macrorhizon. Chl fluorescence analysis at 77 K suggested partial energetic disconnection of the light-harvesting antenna from the PSII reaction center in C. macrorhizon. Despite its low PSII photochemical efficiency, C. macrorhizon showed photosynthetic electron transport activity both in the field and under laboratory conditions. Cymbidium macrorhizon developed strong nonphotochemical quenching in response to increased light intensity as did C. goeringii, suggesting the functionality of photoprotective systems in this orchid. Moreover, C. macrorhizon fruit developed stomata on the pericarp and showed net O2-evolving activity. Our data demonstrate that C. macrorhizon can perform photosynthetic electron transport in the pericarp, although its contribution to net carbon acquisition may be limited.

Light-adapted charge-separated state of photosystem II: structural and functional dynamics of the closed reaction center.

Photosystem II (PSII) uses solar energy to oxidize water and delivers electrons for life on Earth. The photochemical reaction center of PSII is known to possess two stationary states. In the open state (PSIIO), the absorption of a single photon triggers electron-transfer steps, which convert PSII into the charge-separated closed state (PSIIC). Here, by using steady-state and time-resolved spectroscopic techniques on Spinacia oleracea and Thermosynechococcus vulcanus preparations, we show that additional illumination gradually transforms PSIIC into a light-adapted charge-separated state (PSIIL). The PSIIC-to-PSIIL transition, observed at all temperatures between 80 and 308 K, is responsible for a large part of the variable chlorophyll-a fluorescence (Fv) and is associated with subtle, dark-reversible reorganizations in the core complexes, protein conformational changes at noncryogenic temperatures, and marked variations in the rates of photochemical and photophysical reactions. The build-up of PSIIL requires a series of light-induced events generating rapidly recombining primary radical pairs, spaced by sufficient waiting times between these events-pointing to the roles of local electric-field transients and dielectric relaxation processes. We show that the maximum fluorescence level, Fm, is associated with PSIIL rather than with PSIIC, and thus the Fv/Fm parameter cannot be equated with the quantum efficiency of PSII photochemistry. Our findings resolve the controversies and explain the peculiar features of chlorophyll-a fluorescence kinetics, a tool to monitor the functional activity and the structural-functional plasticity of PSII in different wild-types and mutant organisms and under stress conditions.

Differential response of photosynthetic apparatus towards alkaline pH treatment in NIES-39 and PCC 7345 strains of Arthrospira platensis.

Alkaline stress is one of the severe abiotic stresses, which is not well studied so far, especially among cyanobacteria. To affirm the characteristics of alkaline stress and the subsequent adaptive responses in Arthrospira platensis NIES-39 and Arthrospira platensis PCC 7345, photosynthetic pigments, spectral properties of thylakoids, PSII and PSI activities, and pigment-protein profiles of thylakoids under different pH regimes were examined. The accessory pigments showed a pH-mediated sensitivity. The pigment-protein complexes of thylakoids are also affected, resulting in the altered fluorescence emission profile. At pH 11, a possible shift of the PBsome antenna complex from PSII to PSI is observed. PSII reaction center is found to be more susceptible to alkaline stress in comparison to the PSI. In Arthrospira platensis NIES-39 at pH 11, a drop of 68% in the oxygen evolution with a significant increase of PSI activity by 114% is recorded within 24 h of pH treatment. Alterations in the cellular ultrastructure of Arthrospira platensis NIES-39 at pH 11 were observed, along with the increased number of plastoglobules attached with the thylakoid membranes. Arthrospira platensis NIES-39 is more adaptable to pH variation than Arthrospira platensis PCC 7345.

Effects of iron limitation on carbon balance and photophysiology of the Antarctic diatom Chaetoceros cf. simplex

In the Southern Ocean (SO), iron (Fe) limitation strongly inhibits phytoplankton growth and generally decreases their primary productivity. Diatoms are a key component in the carbon (C) cycle, by taking up large amounts of anthropogenic CO2 through the biological carbon pump. In this study, we investigated the effects of Fe availability (no Fe and 4 nM FeCl3 addition) on the physiology of Chaetoceros cf. simplex, an ecologically relevant SO diatom. Our results are the first combining oxygen evolution and uptake rates with particulate organic carbon (POC) build up, pigments, photophysiological parameters and intracellular trace metal (TM) quotas in an Fe-deficient Antarctic diatom. Decreases in both oxygen evolution (through photosynthesis, P) and uptake (respiration, R) coincided with a lowered growth rate of Fe-deficient cells. In addition, cells displayed reduced electron transport rates (ETR) and chlorophyll a (Chla) content, resulting in reduced cellular POC formation. Interestingly, no differences were observed in non-photochemical quenching (NPQ) or in the ratio of gross photosynthesis to respiration (GP:R). Furthermore, TM quotas were measured, which represent an important and rarely quantified parameter in previous studies. Cellular quotas of manganese, zinc, cobalt and copper remained unchanged while Fe quotas of Fe-deficient cells were reduced by 60% compared with High Fe cells. Based on our data, Fe-deficient Chaetoceros cf. simplex cells were able to efficiently acclimate to low Fe conditions, reducing their intracellular Fe concentrations, the number of functional reaction centers of photosystem II (RCII) and photosynthetic rates, thus avoiding light absorption rather than dissipating the energy through NPQ. Our results demonstrate how Chaetoceros cf. simplex can adapt their physiology to lowered assimilatory metabolism by decreasing respiratory losses.

Effects of Root Zone Temperature Increase on Physiological Indexes and Photosynthesis of Different Genotype Maize Seedlings

This experiment investigated the effects of different degrees of warming of the root zone on the antioxidant enzyme activity and malondialdehyde (MDA) content, chlorophyll fluorescence parameters, and photosynthetic physiological parameters of maize (Zea mays L.) seedling roots. Different genotypes of maize types SD609 (Stay-green Maize type) and SD902 (senescent type) were used as test materials in the test and the test set three root zone temperatures (24, 30, and 36°С). The results showed that properly increasing the root zone temperature could significantly promote the growth of maize seedlings, increase the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and actual photochemical efficiency (ΦPS) electron transfer rate (ETR), photochemical quenching coefficient (qP), increase leaf photosynthesis and transpiration rate. However, when the root zone temperature was too high, the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in the roots of maize seedlings would be significantly reduced, and the content of malondialdehyde (MDA) would be increased. This lead to excessive accumulation of reactive oxygen, which affects the functions of root system absorption, synthesis, transportation, etc., leading to reduced leaf enzyme activity, damaged to the PSII reaction center, and reduced photosynthetic rate. Among them, the Stay-green Maize type SD609 had a significantly smaller change than the aging type SD902. Under high temperature stress in the root zone, the relatively high antioxidant enzyme activity, better stability of the PSII reaction center, and photoprotection mechanisms of SD609 were the physiological basis for maintaining a stable photosynthetic rate and resisting high temperature in the root zone. PSII reaction center damage and relatively low non-photochemical quenching coefficient may be an important factor for non-stomatal limitation of SD902.

Effects of polystyrene microplastics (PS-MPs) on the growth, physiology, and biochemical characteristics of Hydrilla verticillata

In order to evaluate the effects of polystyrene microplastics (PS-MPs) on the growth, physiology, and biochemical characteristics of submerged plants, we exposed a typical submerged plant, Hydrilla verticillata, to a series of concentrations (i.e. 0, 5, 10, 30, 50, 100 mg·L-1) of 3 μm polystyrene microplastics (PS-MPs) and measured parameters including height, biomass, chlorophyll content, antioxidant enzyme activity, photosynthetic fluorescence. The results showed that the height of H. Verticillata significantly decreased at the high PS-MP concentrations (50 to 100 mg·L-1), while the fresh weight significantly increased at the low PS-MP concentration (5 mg·L-1). The fresh weight of H. verticillata gradually decreased with the increasing PS-MP concentration but the dry weight did not change. The total amount of chlorophyll, chlorophyll a, and chlorophyll a/b significantly decreased with the increases of the PS-MP concentrations, while the chlorophyll b did not change. PS-MPs affected the antioxidant enzyme activities of H. verticillata. The activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) were first increased and then decreased with the increasing PS-MP concentration. The chlorophyll fluorescence parameters (Fo, Fm, Fv/Fm) decreased with the increasing concentration of PS-MP and the 1-Qp-Lss value (reflective of the closing of PSⅡ reaction center) was increased under the stable state, probably due to the inhibited PSⅡ reaction center. The overall intensity of fluorescence imaging of H. verticillata decreased with the increasing concentration of PS-MPs. When the PS-MP concentration was lower than 10 mg·L-1, the photosynthetic activity of the leaves was normal. In contrast, when the PS-MP concentration was higher than 30 mg·L-1, it caused significant adverse effects on leaves, including weaker photosynthetic intensity and the presence of yellow or withered leaves. Our results suggested that H. verticillata could tolerate PS-MP pollution but its growth and photosynthesis would be inhibited at high concentrations (>30 mg·L-1). Our results provided basic information to better understand the eco-physiological effects of PS-MPs in the freshwater environment.

Inhibition of non-photochemical quenching increases functional absorption cross-section of photosystem II as excitation from closed reaction centres is transferred to open centres, facilitating earlier light saturation of photosynthetic electron transport.

Induction of non-photochemical quenching (NPQ) of chlorophyll fluorescence in leaves affords photoprotection to the photosynthetic apparatus when, for whatever reason, photon capture in the antennae of photosystems exceeds their capacity to utilise this excitation in photochemistry and ultimately in CO2 assimilation. Here we augment traditional monitoring of NPQ using the fast time resolution, remote and relatively non-intrusive light induced fluorescence transient (LIFT) technique (Kolber et al . 2005 ; Osmond et al . 2017 ) that allows direct measurement of functional (σ'PSII ) and optical cross-sections (a 'PSII ) of PSII in situ , and calculates the half saturation light intensity for ETR (E k ). These parameters are obtained from the saturation and relaxation phases of fluorescence transients elicited by a sequence of 270, high intensity 1 μs flashlets at controlled time intervals over a period of 30 ms in the QA flash at intervals of a few seconds. We report that although σ'PSII undergoes large transient increases after transfer from dark to strong white light (WL) it declines little in steady-state as NPQ is induced in shade- and sun-grown spinach and Arabidopsis genotypes Col , OEpsbs , pgr 5bkg , stn 7 and stn 7/8. In contrast, σ'PSII increases by ~30% when induction of NPQ in spinach is inhibited by dithiothreitol and by inhibition of NPQ in Arabidopsis npq 1, npq 4 and pgr 5. We propose this increase in σ'PSII arises as some excitation from closed PSII reaction centres is transferred to open centres when excitation partitioning to photochemistry (Y II ) and NPQ (Y NP ) declines, and is indicated by an increased excitation dissipation from closed PSII centres (Y NO , including fluorescence emission). Although E k increases following dissipation of excitation as heat when NPQ is engaged, it declines when NPQ is inhibited. Evidently photochemistry becomes more easily light saturated when excitation is transferred from closed RCIIs to open centres with larger σ'PSII . The NPQ mutant pgr 5 is an exception; E k increases markedly in strong light as electron transport QA → PQ and PQ → PSI accelerate and the PQ pool becomes strongly reduced. These novel in situ observations are discussed in the context of contemporary evidence for functional and structural changes in the photosynthetic apparatus during induction of NPQ.

Photosynthetic response of some Triticum cultivars to the combined influence of nanofertilizers and water deficit

The effects of water deficit modelling by polyethylene glycol (PEG), metal nanoparticles (NPs) and their combined action on water status, chlorophyll fluorescence and pigments composition in the leaves of three drought tolerant wheat genotypes – Triticum aestivum L. (Trypilska and Favorytka cultivars) and Triticum dicoccum Schrank. (cv. Holikovska), have been studied. Water deficit, induced by PEG, as well as the treatment with metal NPs (Cu, Fe, Mn, Zn) and their combined action, showed no effects on the maximal quantum yield of PSII photochemistry (Fv/Fm) in dark-adapted leaves of studied plants. However, water deficit decreased the efficiency of excitation energy capture by open PSII reaction centers (Fv′/Fm′) in light-adapted leaves of the Favorytka cultivar. The increase of chlorophyll fluorescence photochemical quenching (qP) and actual quantum yield of PSII electron transport (φPSII) levels, the decrease in non-photochemical quenching coefficient (qN) were observed in Favorytka and Holikovska cultivars under the combined action of water stress and NPs treatment. According to photosynthetic pigments composition, the NPs treatment of wheat plants, grown on medium with PEG, did not bear an additional pressure on the seedlings. However, the NPs treatment did not decrease the negative effect of the simulated water deficit of the cv. Favorytka seedlings.

The Mechanism of Non-Photochemical Quenching in Plants: Localization and Driving Forces.

Non-photochemical chlorophyll fluorescence quenching (NPQ) remains one of the most studied topics of the 21st century in photosynthesis research. Over the past 30 years, profound knowledge has been obtained on the molecular mechanism of NPQ in higher plants. First, the largely overlooked significance of NPQ in protecting the reaction center of photosystem II (RCII) against damage, and the ways to assess its effectiveness are highlighted. Then, the key in vivo signals that can monitor the life of the major NPQ component, qE, are presented. Finally, recent knowledge on the site of qE and the possible molecular events that transmit ΔpH into the conformational change in the major LHCII [the major trimeric light harvesting complex of photosystem II (PSII)] antenna complex are discussed. Recently, number of reports on Arabidopsis mutants lacking various antenna components of PSII confirmed that the in vivo site of qE rests within the major trimeric LHCII complex. Experiments on biochemistry, spectroscopy, microscopy and molecular modeling suggest an interplay between thylakoid membrane geometry and the dynamics of LHCII, the PsbS (PSII subunit S) protein and thylakoid lipids. The molecular basis for the qE-related conformational change in the thylakoid membrane, including the possible onset of a hydrophobic mismatch between LHCII and lipids, potentiated by PsbS protein, begins to unfold.

Photoacclimation to high-light stress in Chlamydomonas reinhardtii during conditional senescence relies on generating pH-dependent, high-quenching centres

Microalgae can respond to long-term increases in light intensity by altering the concentration of photosynthetic complexes. Under active growth, the ability of Chlamydomonas reinhardtii to acclimate to excess light is dependent on cell division to reduce the concentration of photosynthetic complexes. But, in batch culture, cells eventually reach stationary phase where their ability to divide is limited; this should impact their capacity to undergo photoacclimation. Our goal is to dissect excess-light responses as cells approach stationary phase and to determine how the strategies of photoacclimation differ compared to cells in the exponential-growth phase. In this study, cultures exited exponential growth and transitioned into a declining growth phase (DGP), where cells continued a slow rate of growth for the next seven days in both low (LL) and high-light (HL). During this period, both cultures experience a conditional senescence-related decline in chlorophyll levels. Under HL, however, the senescing cultures have a rapid decline in PSII reaction centres, maintain a stable concentration of LHCII antenna, rapidly increase LHCSR levels, and have a sustained increase in Fo/Fm. Collectively this implies that the remaining antenna act as pH-dependent, quenching centres, presumably to protect the senescing chloroplast against HL. We discovered that acclimating to HL post-exponential phase involves active degradation that is intertwined with the normal senescence process that allowed for a limited rate of cell division.

A functional molecular marker for detecting blister blight disease resistance in tea (Camellia sinensis L.).

Identification of an EST-SSR molecular marker associated with Blister blight, a common fungal disease of tea, facilitating marker-assisted selection, marking a milestone in tea molecular breeding. lister blight (BB) leaf disease of tea, caused by the fungus Exobasidium vexans, results in 25-30% crop loss annually. BB is presently controlled by Cu based fungicides, but genetic resistance is the most viable option in disease management. Tea is a naturally out-crossing, woody perennial necessitating a long time for completion of a breeding programme. Marker-assisted selection (MAS) is vital to expedite breeding programmes and also for better accuracy in gene identification. The aim of the current research was to derive marker-trait associations using an F1 population segregating for BB. The population was genotyped at 11 expressed sequence tag simple sequence repeat loci followed by detecting the alleles by fragment analysis. The genotypic and phenotypic data were subjected to single-marker analysis resulting in the identification of EST-SSR073 as a diagnostic marker amplifying three alleles of the sizes, 168, 170 and 190bp in F1. Of them, alleles 190 and 168bp were confirmed to concur BB resistance and susceptibility, respectively. The alleles were validated in a panel of 64 tea cultivars, resulting in the amplification of 12 alleles at EST-SSR073. The EST-SSR073 allele sequences matched with Camellia sinensis photosystem-I reaction center subunit-II. The marker EST-SSR073 can be effectively used in breeding tea against BB, recording a milestone in MAS in tea.

Diclofenac and atrazine restrict the growth of a synchronous Chlamydomonas reinhardtii population via various mechanisms

Non-steroidal anti-inflammatory drug diclofenac (DCF) is commonly found in freshwater bodies and can have adverse effects on non-target organisms. Among the studies on DCF toxicity, several ones have reported its harmful effects on plants and algae. To gain a better understanding of the mechanisms of DCF toxicity towards green algae, we used a synchronous Chlamydomonas reinhardtii cc-1690 culture and compared DCF (135 mg/L) effects with effects caused by atrazine (ATR; 77.6 μg/L), an herbicide with a well-known mechanism of toxic action. To achieve our goal, cell number and size, photosynthetic oxygen consumption/evolution, chlorophyll a fluorescence in vivo, H2O2 production by the cells, antioxidative enzymes encoding genes expression were analyzed during light phase of the cell cycle.We have found, that DCF and ATR affect C. reinhardtii through different mechanisms. ATR inhibited the photosynthetic electron transport chain and induced oxidative stress in chloroplast. Such chloroplastic energetics disruption indirectly influenced respiration, the intensification of which could partially mitigate low efficiency of photosynthetic energy production. As a result, ATR inhibited the growth of single cell leading to limitation in C. reinhardtii population development. In contrast to ATR-treated algae, in DCF-treated cells the fraction of active PSII reaction centers was diminished without drastic changes in electron transport or oxidative stress symptoms in chloroplast. However, significant increase in transcript level of gene encoding for mitochondria-located catalase indicates respiratory processes as a source of H2O2 overproduced in the DCF-treated cells. Because the single cell growth was not strongly affected by DCF, its adverse effect on progeny cell number seemed to be related rather to arresting of cell divisions. Concluding, although the DCF phytotoxic action appeared to be different from the action of the typical herbicide ATR, it can act as algal growth-inhibiting factor in the environment.

Phenolic acids inhibit the photosynthetic productivity of poplar

Barriers to continuous cropping of poplar plantations are closely related to the accumulation of phenolic acids in the soil. The purpose of this study was to explore the mechanism through which phenolic acid stress affects poplar productivity. The results showed that phenolic acids had a significant inhibitory effect on the photosynthesis of poplar. The inhibition of photosynthesis due to phenolic acids occurred mainly because, during electron transfer at the PSII reaction center, the primary quinone acceptor of PSII (QA) in the oxidized state was reduced, and excess light energy was lost in the form of heat dissipation; thus, poplar productivity decreased. At low phenolic acid concentrations (0X-1.5X), the activity of the PSII reaction center was temporarily inactivated, mainly because of stomatal limiting factors. At high phenolic acid concentrations (1.5X-3.0X), the PSII reaction centers were damaged, and photoinhibition occurred, mainly because of nonstomatal limiting factors.

Disentangling the photosynthesis performance in japonica rice during natural leaf senescence using OJIP fluorescence transient analysis

Rice undergoes leaf senescence accompanied with grain filling when the plants reach the end of their temporal niche, and a delay in leaf senescence ultimately improves the yield and quality of grain. To estimate the decline in photosynthesis during leaf senescence and to find an efficient and useful tool to identify rice genotypes with a longer duration of active photosynthesis, we examined PSII photosynthetic activity in the flag leaves of japonica rice Shennong265 (SN265) and Beigeng3 (BG3) during leaf senescence using chlorophyll a fluorescence kinetics. The results show that inhibition occurred in the electron transport chains, but the energetic connectivity of PSII units was not affected as dramatically during leaf senescence. PSII reaction centres (RCs) were transformed into 'silent RCs,' and the chlorophyll content decreased during leaf senescence. However the size of the 'economic' antennae increased. Further, the percentage of variation of the specific energy flux parameters can rationally be used to indicate leaf senescence from the perspective of energy balance. Although the performance indices were more sensitive than other functional and structural JIP-test parameters, they still did not serve as an indicator of crop yield.