The use of milk Fourier-transform infrared spectra for predicting cheesemaking traits in Grana Padano Protected Designation of Origin cheese

The prediction of the cheese yield (%CY) traits for curd, solids, and retained water and the amount of fat, protein, solids, and energy recovered from the milk into the curd (%REC) by Bayesian models, using Fourier-transform infrared spectroscopy (FTIR), can be of significant economic interest to the dairy industry and can contribute to the improvement of the cheese process efficiency. The yields give a quantitative measure of the ratio between weights of the input and output of the process, whereas the nutrient recovery allows to assess the quantitative transfer of a component from milk to cheese (expressed in % of the initial weight). The aims of this study were: (1) to investigate the feasibility of using bulk milk spectra to predict %CY and %REC traits, and (2) to quantify the effect of the dairy industry and the contribution of single-spectrum wavelengths on the prediction accuracy of these traits using vat milk samples destined to the production of Grana Padano Protected Designation of Origin cheese. Information from 72 cheesemaking days (in total, 216 vats) from 3 dairy industries were collected. For each vat, the milk was weighed and analyzed for composition (total solids [TS], lactose, protein, and fat). After 48 h from cheesemaking, each cheese was weighed, and the resulting whey was sampled for composition as well (TS, lactose, protein, and fat). Two spectra from each milk sample were collected in the range between 5,011 and 925 cm-1 and averaged before the data analysis. The calibration models were developed via a Bayesian approach by using the BGLR (Bayesian Generalized Linear Regression) package of R software. The performance of the models was assessed by the coefficient of determination (R2VAL) and the root mean squared error (RMSEVAL) of validation. Random cross-validation (CVL) was applied [80% calibration and 20% validation set] with 10 replicates. Then, a stratified cross-validation (SCV) was performed to assess the effect of the dairy industry on prediction accuracy. The study was repeated using a selection of informative wavelengths to assess the necessity of using whole spectra to optimize prediction accuracy. Results showed the feasibility of using FTIR spectra and Bayesian models to predict cheesemaking traits. The R2VAL values obtained with the CVL procedure were promising in particular for the %CY and %REC for protein, ranging from 0.44 to 0.66 with very low RMSEVAL (from 0.16 to 0.53). Prediction accuracy obtained with the SCV was strongly influenced by the dairy factory industry. The general low values gained with the SCV do not permit a practical application of this approach, but they highlight the importance of building calibration models with a dataset covering the largest possible sample variability. This study also demonstrated that the use of the full FTIR spectra may be redundant for the prediction of the cheesemaking traits and that a specific selection of the most informative wavelengths led to improved prediction accuracy. This could lead to the development of dedicated spectrometers using selected wavelengths with built-in calibrations for the online prediction of these innovative traits.

Thylakoid ultrastructural variations in chlorophyll-deficient wheat: aberrations or structural acclimation?

MAIN CONCLUSION

A structural re-modeling of the thylakoid system, including granum size and regularity, occurs in chlorophyll-deficient wheat mutants affected by photosynthetic membrane over-reduction. In the chloroplast of land plants, the thylakoid system is defined by appressed grana stacks and unstacked stroma lamellae. This study focuses on the variations of the grana organization occurring in outdoor-grown wheat mutants characterized by low chlorophyll content and a tendency for photosynthetic membrane over-reduction. Triticum aestivum ANK-32A and Triticum durum ANDW-7B were compared to their corresponding WT lines, NS67 and LD222, respectively. Electron micrographs of chloroplasts were used to calculate grana ultrastructural parameters. Photosynthetic parameters were obtained by modulated chlorophyll fluorescence and applying Light Curves (LC) and Rapid Light Curves (RLC) protocols. For each photosynthetic parameter, the difference Δ(RLC-LC) was calculated to evaluate the flexible response to light in the examined lines. In the mutants, fewer and smaller disks formed grana stacks characterized by a marked increase in lateral and cross-sectional irregularity, both negatively correlated with the number of layers per granum. A relationship was found between membrane over-reduction and granum structural irregularity. The possible acclimative significance of a greater proportion of stroma-exposed grana domains in relieving the excess electron pressure on PSI is discussed.

Genome-wide identification of GA2ox genes family and analysis of PbrGA2ox1-mediated enhanced chlorophyll accumulation by promoting chloroplast development in pear

BACKGROUND

Chlorophyll (Chl) is an agronomic trait associated with photosynthesis and yield. Gibberellin 2-oxidases (GA2oxs) have previously been shown to be involved in Chl accumulation. However, whether and how the PbrGA2ox proteins (PbrGA2oxs) mediate Chl accumulation in pear (Pyrus spp.) is scarce.

RESULTS

Here, we aimed to elucidate the role of the pear GA2ox gene family in Chl accumulation and the related underlying mechanisms. We isolated 13 PbrGA2ox genes (PbrGA2oxs) from the pear database and identified PbrGA2ox1 as a potential regulator of Chl accumulation. We found that transiently overexpressing PbrGA2ox1 in chlorotic pear leaves led to Chl accumulation, and PbrGA2ox1 silencing in normal pear leaves led to Chl degradation, as evident by the regreening and chlorosis phenomenon, respectively. Meanwhile, PbrGA2ox1-overexpressing (OE) tobacco plants discernably exhibited Chl built-up, as evidenced by significantly higher Pn and Fv/Fm. In addition, RNA sequencing (RNA-seq), physiological and biochemical investigations revealed an increase in abscisic acid (ABA), methyl jasmonate (MeJA), and salicylic acid (SA) concentrations and signaling pathways; a marked elevation in reducing and soluble sugar contents; and a marginal decline in the starch and sucrose levels in OE plants. Interestingly, PbrGA2ox1 overexpression did not prominently affect Chl synthesis. However, it indeed facilitated chloroplast development by increasing chloroplast number per cell and compacting the thylakoid granum stacks. These findings might jointly contribute to Chl accumulation in OE plants.

CONCLUSION

Overall, our results suggested that GA2oxs accelerate Chl accumulation by stimulating chloroplast development and proved the potential of PbrGA2ox1 as a candidate gene for genetically breeding biofortified pear plants with a higher yield.

MFP1 defines the subchloroplast location of starch granule initiation

Starch is one of the major carbohydrate storage compounds in plants. The biogenesis of starch granules starts with the formation of initials, which subsequently expand into granules. Several coiled-coil domain-containing proteins have been previously implicated with the initiation process, but the mechanisms by which they act remain largely elusive. Here, we demonstrate that one of these proteins, the thylakoid-associated MAR-BINDING FILAMENT-LIKE PROTEIN 1 (MFP1), specifically determines the subchloroplast location of initial formation. The expression of MFP1 variants "mis"-targeted to specific locations within chloroplasts in Arabidopsis results in distinctive shifts in not only how many but also where starch granules are formed. Importantly, "re" localizing MFP1 to the stromal face of the chloroplast's inner envelope is sufficient to generate starch granules in this aberrant position. These findings provide compelling evidence that a single protein MFP1 possesses the capacity to direct the initiation and biosynthesis machinery of starch granules.

Singlet Oxygen and Superoxide Anion Radical Detection by EPR Spin Trapping in Thylakoid Preparations

Reactive oxygen species (ROS) are produced by energy transfer and electron transport in plant chloroplast thylakoids at non-toxic levels under normal growth conditions, but at threatening levels under adverse or fluctuating environmental conditions. Among chloroplast ROS, singlet oxygen and superoxide anion radical, respectively, produced by photosystem II (PSII) and PSI, are known to be the major ROS under several stress conditions. Both are very unlikely to diffuse out of chloroplasts, but they are instead capable of triggering ROS-mediated chloroplast operational retrograde signalling to activate defence gene expression in concert with hormones and other molecular compounds. Therefore, their detection, identification and localization in vivo or in biological preparations is a priority for a deeper understanding of their role in (concurrent) regulation of plant growth and defence responses. Here, we present two EPR spin traps, abbreviated as TEMPD-HCl and DEPMPO, to detect and identify ROS in complex systems, such as isolated thylakoids, together with some hints and cautions to perform reliable spin trapping experiments.

The effect of calorie-restriction along with thylakoid membranes of spinach on the gut-brain Axis Pathway and oxidative stress biomarkers in obese women with polycystic ovary syndrome: a Randomized, Double-blind, placebo-controlled clinical trial

BACKGROUND

Women with polycystic ovary syndrome (PCOS) have higher intestinal mucosal permeability, leading to the lipopolysaccharide (LPS) leakage and endotoxemia. This, in turn, leads to oxidative stress (OS) and neuro-inflammation caused by the gut-brain axis, affecting the neurotrophic factors levels such as brain-derived neurotrophic factor (BDNF) and S100 calcium-binding protein B (S100 B) levels. In this study, it was hypothesized that the thylakoid membranes of spinach supplementation along with a hypocaloric diet may have improved the LPS levels, neurotrophic factors, and OS in PCOS patients.

METHODS

In this double-blind, randomized, placebo-controlled, and clinical trial, 48 women with obesity and diagnosed with PCOS based on Rotterdam criteria were randomly assigned to thylakoid (N = 21) and placebo groups (N = 23). A personalized hypocaloric diet with 500 calories less than the total energy expenditure was prescribed to all patients. The participants were daily supplemented with either a 5 g/day thylakoid-rich spinach extract or a placebo (5 g cornstarch) for 12 weeks along with a prescribed low-calorie diet. Anthropometric measurements and biochemical parameters were assessed at baseline and after the 12-week intervention.

RESULTS

A statistically significant decrease in the LPS levels (P < 0.001) and an increase in the BDNF levels (P < 0.001) were recorded for the participants receiving the oral thylakoid supplements and a low-calorie diet. Furthermore, significant decreases were observed in fasting blood glucose, insulin, homeostatic model of assessment for insulin resistance, free testosterone index, and follicle-stimulating hormone / luteinizing hormone ratio in both groups (P < 0.05). No significant differences were detected between the two groups regarding the changes in malondialdehyde, catalase, total antioxidant capacity, and S100B levels (P > 0.05).

CONCLUSIONS

In sum, the thylakoid membranes of spinach supplemented with a hypocaloric diet reduced the LPS levels, increased the BDNF levels, and improved the glycemic profile and sex-hormone levels; however, they had no effects on the OS markers levels after 12 weeks of intervention.

Toxic effects of lanthanum(III) on photosynthetic performance of rice seedlings: Combined chlorophyll fluorescence, chloroplast structure and thylakoid membrane protein assessment

Rare earth elements (REEs) are emerging as an anticipated pollution in the environment due to their active use in many areas. However, the effects of REEs on the photosynthesis of rice have not been thoroughly explored. Therefore, this study emphasizes how high levels of La(III) affect the thylakoid membrane of rice seedlings, thereby inhibiting photosynthesis and growth. Here, we reported that rice plants treated with La(III) exhibited an increase in La accumulation in the leaves, accompanied by a decrease in chlorophyll content and photosynthetic capacity. La(III) exposure decreased Mg content in leaves, but possibly increased other nutrients including Cu, Mn, and Zn through systemic endocytosis. K-band and L-band appeared in the fluorescence OJIP transients, indicating La(III) stress destroyed the donor and receptor sides of photosystem II (PSII). Numerous reaction centers (RC/CSm) were inactivated by La(III) treatment, which resulted in a reduction in electron transport capacity (decreased ETo/RC and ETo/CSm) and an increase in the dissipation of the excess excitation energy by heat (increased DIo/RC and DIo/CSm). The BN-PAGE analysis of thylakoid membrane protein complexes showed that La(III) induced the degradation of supercomplexes, PSII core, LHCII, PSI core, LHCI, and F1-ATPase binding Cyt b6f complex. Collectively, this study revealed that La(III) causes significant degradation of thylakoid membrane proteins, thereby promoting the decomposition of photosynthetic complexes, ultimately destroying the chloroplast structure and reducing the photosynthetic performance of rice seedlings.

Structural and functional properties of different types of siphonous LHCII trimers from an intertidal green alga Bryopsis corticulans

Light-harvesting complexes of photosystem II (LHCIIs) in green algae and plants are vital antenna apparatus for light harvesting, energy transfer, and photoprotection. Here we determined the structure of a siphonous-type LHCII trimer from the intertidal green alga Bryopsis corticulans by X-ray crystallography and cryo-electron microscopy (cryo-EM), and analyzed its functional properties by spectral analysis. The Bryopsis LHCII (Bry-LHCII) structures in both homotrimeric and heterotrimeric form show that green light-absorbing siphonaxanthin and siphonein occupied the sites of lutein and violaxanthin in plant LHCII, and two extra chlorophylls (Chls) b replaced Chls a. Binding of these pigments expands the blue-green light absorption of B. corticulans in the tidal zone. We observed differences between the Bry-LHCII homotrimer crystal and cryo-EM structures, and also between Bry-LHCII homotrimer and heterotrimer cryo-EM structures. These conformational changes may reflect the flexibility of Bry-LHCII, which may be required to adapt to light fluctuations from tidal rhythms.

Altered lipid acyl chain length controls energy dissipation in light-harvesting complex II proteoliposomes by hydrophobic mismatch

In plants, the major light-harvesting antenna complex (LHCII) is vital for both light harvesting and photoprotection in photosystem II. Previously, we proposed that the thylakoid membrane itself could switch LHCII into the photoprotective state, qE, via a process known as hydrophobic mismatch. The decrease in the membrane thickness that followed the formation of ΔpH was a key fact that prompted this idea. To test this, we made proteoliposomes from lipids with altered acyl chain length (ACL). Here, we show that ACL regulates the average chlorophyll fluorescence lifetime of LHCII. For liposomes made of lipids with an ACL of 18 carbons, the lifetime was ∼2 ns, like that for the thylakoid membrane. Furthermore, LHCII appears to be quenched in proteoliposomes with an ACL both shorter and longer than 18 carbons. The proteoliposomes made of short ACL lipids display structural heterogeneity revealing two quenched conformations of LHCII, each having characteristic 77 K fluorescence spectra. One conformation spectrally resembles isolated LHCII aggregates, whilst the other resembles LHCII immobilized in polyacrylamide gels. Overall, the decrease in the ACL appears to produce quenched conformations of LHCII, which renders plausible the idea that the trigger of qE is the hydrophobic mismatch.

Cryo-EM structures of LHCII in photo-active and photo-protecting states reveal allosteric regulation of light harvesting and excess energy dissipation

The major light-harvesting complex of photosystem II (LHCII) has a dual regulatory function in a process called non-photochemical quenching to avoid the formation of reactive oxygen. LHCII undergoes reversible conformation transitions to switch between a light-harvesting state for excited-state energy transfer and an energy-quenching state for dissipating excess energy under full sunshine. Here we report cryo-electron microscopy structures of LHCII in membrane nanodiscs, which mimic in vivo LHCII, and in detergent solution at pH 7.8 and 5.4, respectively. We found that, under low pH conditions, the salt bridges at the lumenal side of LHCII are broken, accompanied by the formation of two local α-helices on the lumen side. The formation of α-helices in turn triggers allosterically global protein conformational change, resulting in a smaller crossing angle between transmembrane helices. The fluorescence decay rates corresponding to different conformational states follow the Dexter energy transfer mechanism with a characteristic transition distance of 5.6 Å between Lut1 and Chl612. The experimental observations are consistent with the computed electronic coupling strengths using multistate density function theory.

Supplementation with spinach-derived thylakoid augments the benefits of high intensity training on adipokines, insulin resistance and lipid profiles in males with obesity

Introduction

This study investigated the effects of 12 weeks of high-intensity functional training (HIFT) combined with spinach-derived thylakoid supplementation on some selected Adipokines and insulin resistance in males with obesity.

Method

Sixty-eight participants (mean age: 27.6 ± 8.4 yrs.; mean height: 168.4 ± 2.6 cm; mean weight: 95.7 ± 3.8 kg, mean BMI: 32.6 ± 2.6 kg/m2) were randomly divided into four groups of 17 per group: Control group (CG), Supplement group (SG), Training group (TG), and Training + supplement group (TSG). Following baseline measurements, the two training groups (TG and TSG) started the 12 weeks of exercise training program (3 sessions per week). A total of 36 sessions lasting up to 60 min were included in the HIFT program using the CrossFit program. The eligible participants received 5 g/day of thylakoid-rich spinach extract or matching placebo as 5 g/day of raw corn starch (one sachet, 30 min before lunch) for 12 weeks. Baseline assessments were obtained 48 hours before the start of the training protocols and 48 hours after the last training session in all groups.

Results

There were significant interactions (p<0.001 for all) between exercise and time for adiponectin (ES:0.48), leptin (ES:0.46), resistin (ES:0.3), omentin (ES:0.65), vaspin (ES:0.46), visfatin (ES:0.62), apelin (ES:0.42), RBP4 (ES:0.63), chemrin (0.36) and semaphorin3c (ES: 0.5). Plasma levels of semaphorin3c were significantly correlated (p<0.05) with body weight (r= 0.57), BMI (r= 0.43), FFM (r= -0.612), FAT (r= 0.768), VO2peak (r=-0.53), insulin (r= 0.756), glucose (r= 0.623), and HOMA-IR (r= 0.727). There were also significant group differences in insulin (ES: 0.77), glucose (ES: 0.21), and HOM-IR (ES: 0.44) (p<0.05).

Discussion

Our findings indicate that 12 weeks of HIFT supplemented with spinach-derived thylakoid reduced levels of leptin, resistin, vaspin, visfatin, apelin, RBP4, chemrin, semaphorin3c and insulin resistance while increasing adiponectin and omentin levels in men with obesity.

Microalgae from Nordic collections demonstrate biostimulant effect by enhancing plant growth and photosynthetic performance

We investigated the biostimulant potential of six microalgal species from Nordic collections extracted with two different procedures: thermal hydrolysis with a weak solution of sulfuric acid accompanied by ultrasonication and bead-milling with aqueous extraction followed by centrifugation. To this aim, we designed a phenotyping pipeline consisting of a root growth assay in the model plant Arabidopsis thaliana, complemented with greenhouse experiments to evaluate lettuce yield (Lactuca sativa L. cv. Finstar) and photosynthetic performance. The best-performing hydrolyzed extracts stimulated Arabidopsis root elongation by 8%-13% and lettuce yield by 12%-15%. The in situ measured photosynthetic performance of lettuce was upregulated in the efficient extracts: PSII quantum yield increased by 26%-34%, and thylakoid proton flux increase was in the range of 34%-60%. In contrast, aqueous extracts acquired by bead-milling showed high dependence on biomass concentration in the extract and an overall plant growth enhancement was not attained in any of the applied dosages. Our results indicate that hydrolysis of the biomass can be a decisive factor for rendering effective plant biostimulants from microalgae.

Structure of Dunaliella photosystem II reveals conformational flexibility of stacked and unstacked supercomplexes

Photosystem II (PSII) generates an oxidant whose redox potential is high enough to enable water oxidation , a substrate so abundant that it assures a practically unlimited electron source for life on earth . Our knowledge on the mechanism of water photooxidation was greatly advanced by high-resolution structures of prokaryotic PSII . Here, we show high-resolution cryogenic electron microscopy (cryo-EM) structures of eukaryotic PSII from the green alga Dunaliella salina at two distinct conformations. The conformers are also present in stacked PSII, exhibiting flexibility that may be relevant to the grana formation in chloroplasts of the green lineage. CP29, one of PSII associated light-harvesting antennae, plays a major role in distinguishing the two conformations of the supercomplex. We also show that the stacked PSII dimer, a form suggested to support the organisation of thylakoid membranes , can appear in many different orientations providing a flexible stacking mechanism for the arrangement of grana stacks in thylakoids. Our findings provide a structural basis for the heterogenous nature of the eukaryotic PSII on multiple levels.

Aggregation of chlorophylls on plant thylakoid membranes using coarse-grained simulations

Chlorophyll a molecules (CLA) in light-harvesting complex are the most essential pigments for photosynthesis. Coarse-grained molecular dynamics simulations of CLA are carried out in the plant thylakoid membranes at $293$...

Mixed population hypothesis of the active and inactive PSII complexes opens a new door for photoinhibition and fluorescence studies: an ecophysiological perspective

The current hypotheses for the mechanisms of photosystem II (PSII) photodamage in vivo remain split on the primary damage site. However, most researchers have considered that PSII is inhibited by a sole mechanism and that the photoinhibited PSII consists of one population. In this perspective, we propose 'the mixed population hypothesis', in which there are four PSII populations: PSII with active/inactive Mn4 CaO5 oxygen-evolving complex respectively with functional/damaged primary quinone (QA ) reduction activity. This hypothesis provides a new insight into not only the PSII photoinhibition/photoprotection studies but also the repair process. We discuss our new data implying that the repair rate differs in the respective PSII populations.

The Structural and Spectral Features of Light-Harvesting Complex II Proteoliposomes Mimic Those of Native Thylakoid Membranes

The major photosystem II light-harvesting antenna (LHCII) is the most abundant membrane protein in nature and plays an indispensable role in light harvesting and photoprotection in the plant thylakoid. Here, we show that "pseudothylakoid characteristics" can be observed in artificial LHCII membranes. In our proteoliposomal system, at high LHCII densities, the liposomes become stacked, mimicking the in vivo thylakoid grana membranes. Furthermore, an unexpected, unstructured emission peak at ∼730 nm appears, similar in appearance to photosystem I emission, but with a clear excimeric character that has never been previously reported. These states correlate with the increasing density of LHCII in the membrane and a decrease in its average fluorescence lifetime. The appearance of these low-energy states can also occur in natural plant membrane structures, which has unique consequences for the interpretation of the spectroscopic and physiological properties of the photosynthetic membrane.

Ultrafast Excitation Energy Transfer Dynamics in the LHCII-CP29-CP24 Subdomain of Plant Photosystem II

We measure the two-dimensional electronic spectra of the LHCII(M)-CP29-CP24 complex in photosystem II (PSII) and provide the first study of the ultrafast excitation energy transfer (EET) processes of an asymmetric and native light-harvesting assembly of the antenna of PSII. With comparisons to LHCII, we observe faster energy equilibrations in the intermediate levels of the LHCII(M)-CP29-CP24 complex at 662 and 670 nm. Notably, the putative "bottleneck" states in LHCII exhibit faster effective dynamics in the LHCII(M)-CP24-CP29 complex, with the average lifetime shortening from 2.5 ps in LHCII to 1.2 ps in the bigger assembly. The observations are supported by high-level structure-based calculations, and the accelerated dynamics can be attributed to the structural change of LHCII(M) in the bigger complex. This study shows that the biological functioning structures of the complexes are important to understand the overall EET dynamics of the PSII supercomplex.

Formation of Supported Thylakoid Membrane Bioanodes for Effective Electron Transfer and Stable Photocurrent

The light-dependent reactions of photosynthesis use light energy to generate photoelectrons traveling through the thylakoid membranes (TMs). Extracting the photoelectrons from the TMs to form bioanodes can have various applications. Most studies focus on modifying the electrode materials to increase the collected photocurrent. Seldom studies have investigated how the orientation of the TMs influences photocurrent collection. In addition, the formation of reactive oxygen species (ROS) during photosynthesis is a challenge for stable photocurrent generation. Here, we enhanced the photoelectron transfer from the TMs to electrodes by depositing expanded thylakoids as planar supported membranes onto an electrode. The high contact area between the external electrodes and TMs per unit mass of thylakoid allows the thylakoid to more effectively transfer electrons to the electrodes, thereby reducing the free electrons available for the ROS generation. We expanded the naturally stacked thylakoids into liposomes through osmotic pressure and dropcasted them onto an Au electrode. The electrochemical impedance measurement showed that the supported membrane bioanode formed by the expanded liposomes had a lower photoelectron transfer resistance. Additionally, we observed that the expanded TM bioanode provided a higher photocurrent and was more durable to air/water interfacial tension. These results suggest that the effective contact between the expanded TM and electrodes can lead to more efficient electron transfer and increase the system robustness. The photo fuel cell (PFC) made by the expanded TM bioanode had a higher open-circuit voltage than the one made by the stacked TM bioanode. Interestingly, we found that PFCs made of high-load TM bioanodes had fast photocurrent decay under continuous operation at high cell voltages. The poor contact of large numbers of TMs with the electrodes at the high-load TM bioanodes could cause more ROS accumulation and therefore decreased the operational stability, supporting the importance of effective contact between TMs and the electrodes.

Photosystem 2 and the oxygen evolving complex: a brief overview

These special issues of photosynthesis research present papers documenting progress in revealing the many aspects of photosystem 2, a unique, one-of-a-kind complex system that can reduce a plastoquinone to a plastoquinol on every second flash of light and oxidize 2 H₂O to an O₂ on every fourth flash. This overview is a brief personal assessment of the progress observed by the author over a four-decade research career, including a discussion of some remaining unsolved issues. It will come as no surprise to readers that there are remaining questions given the complexity of PS2, and the efforts that have been needed so far to uncover its secrets. In fact, most readers will have their own lists of outstanding questions.

The immunophilin CYCLOPHILIN28 affects PSII-LHCII supercomplex assembly and accumulation in Arabidopsis thaliana

In plant chloroplasts, photosystem II (PSII) complexes, together with light-harvesting complex II (LHCII), form various PSII-LHCII supercomplexes (SCs). This process likely involves immunophilins, but the underlying regulatory mechanisms are unclear. Here, by comparing Arabidopsis thaliana mutants lacking the chloroplast lumen-localized immunophilin CYCLOPHILIN28 (CYP28) to wild-type and transgenic complemented lines, we determined that CYP28 regulates the assembly and accumulation of PSII-LHCII SCs. Compared to the wild type, cyp28 plants showed accelerated leaf growth, earlier flowering time, and enhanced accumulation of high molecular weight PSII-LHCII SCs under normal light conditions. The lack of CYP28 also significantly affected the electron transport rate. Blue native-polyacrylamide gel electrophoresis analysis revealed more Lhcb6 and less Lhcb4 in M-LHCII-Lhcb4-Lhcb6 complexes in cyp28 versus wild-type plants. Peptidyl-prolyl cis/trans isomerase (PPIase) activity assays revealed that CYP28 exhibits weak PPIase activity and that its K113 and E187 residues are critical for this activity. Mutant analysis suggested that CYP28 may regulate PSII-LHCII SC accumulation by altering the configuration of Lhcb6 via its PPIase activity. Furthermore, the Lhcb6-P139 residue is critical for PSII-LHCII SC assembly and accumulation. Therefore, our findings suggest that CYP28 likely regulates PSII-LHCII SC assembly and accumulation by altering the configuration of P139 of Lhcb6 via its PPIase activity.

Electron and proton transport in wheat exposed to salt stress: is the increase of the thylakoid membrane proton conductivity responsible for decreasing the photosynthetic activity in sensitive genotypes?

Effects of salinity caused by 150 mM NaCl on primary photochemical reactions and some physiological and biochemical parameters (K⁺/Na⁺ ratio, soluble sugars, proline, MDA) have been studied in five Triticum aestivum L. genotypes with contrasting salt tolerance. It was found that 150 mM NaCl significantly decreased the photosynthetic efficiency of two sensitive genotypes. The K⁺/Na⁺ ratio decreased in all genotypes exposed to salinity stress when compared with the control. Salinity stress also caused lipid peroxidation and accumulation of soluble sugars and proline. The amounts of soluble sugars and proline were higher in tolerant genotypes than sensitive ones, and lipid peroxidation was higher in sensitive genotypes. The noninvasive measurements of photosynthesis-related parameters indicated the genotype-dependent effects of salinity stress on the photosynthetic apparatus. The significant decrease of chlorophyll content (SPAD values) or adverse effects on photosynthetic functions at the PSII level (measured by the chlorophyll fluorescence parameters) were observed in the two sensitive genotypes only. Although the information obtained by different fast noninvasive techniques were consistent, the correlation analyses identified the highest correlation of the noninvasive records with MDA, K⁺/Na⁺ ratio, and free proline content. The lower correlation levels were found for chlorophyll content (SPAD) and F_v/F_m values derived from chlorophyll fluorescence. Performance index (PI_abs) derived from fast fluorescence kinetics, and F₇₃₅/F₆₈₅ ratio correlated well with MDA and Na⁺ content. The most promising were the results of linear electron flow measured by MultispeQ sensor, in which we found a highly significant correlation with all parameters assessed. Moreover, the noninvasive simultaneous measurements of chlorophyll fluorescence and electrochromic band shift using this sensor indicated the apparent proton leakage at the thylakoid membranes resulting in a high proton conductivity (gH⁺), present in sensitive genotypes only. The possible consequences for the photosynthetic functions and the photoprotection are discussed.

Thylakoid proteome variation of Eutrema salsugineum in response to drought and salinity combined stress

It is well known that plant responses to stress involve different events occurring at different places of the cell/leaf and at different time scales in relation with the plant development. In fact, the organelles proteomes include a wide range of proteins that could include a wide range of proteins showing a considerable change in cellular functions and metabolism process. On this basis, a comparative proteomics analysis and fluorescence induction measurements were performed to investigate the photosynthetic performance and the relative thylakoid proteome variation in Eutrema salsugineum cultivated under salt stress (200 mM NaCl), water deficit stress (PEG) and combined treatment (PEG + NaCl) as a hyperosmotic stress. The obtained results showed a significant decrease of plant growth under drought stress conditions, with the appearance of some toxicity symptoms, especially in plants subjected to combined treatment. Application of salt or water stress alone showed no apparent change in the chlorophyll a fluorescence transients, primary photochemistry (fluorescence kinetics of the O-J phase), the PQ pool state (J-I phase changes), (Fv/Fm) and (Fk/Fj) ratios. However, a considerable decrease of all these parameters was observed under severe osmotic stress (PEG + NaCl). The thylakoid proteome analysis revealed 58 proteins showing a significant variation in their abundance between treatments (up or down regulation). The combined treatment (PEG + NaCl) induced a decrease in the expression of the whole PSII core subunit (D1, D2, CP43, CP47, PsbE and PsbH), whereas the OEC subunits proteins remained constant. An increase in the amount of PsaD, PsaE, PsaF, PsaH, PsaK and PsaN was detected under drought stress (PEG5%). No significant change in the accumulation of Cyt b6 and Cyt f was observed. Some regulated proteins involved in cellular redox homeostasis were detected (glutamine synthetase, phosphoglycerate kinase, transketolase), and showed a significant decrease under the combined treatment. Some oxidative stress related proteins were significantly up-regulated under salt or drought stress and could play a crucial role in the PSI photoprotection and the control of ROS production level.

Confocal microscopy reveals alterations of thylakoids in Limnospira fusiformis during prophage induction

The alkaliphilic cyanobacterium Limnospira fusiformis is an integral part in food webs of tropical soda lakes. Recently, sudden breakdowns of Limnospira sp. blooms in their natural environment have been linked to cyanophage infections. We studied ultrastructural details and prophage components in the laboratory by means of confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM). For a comparison at the subcellular level, we included transmission electron microscopy (TEM) material of infected cells collected during a field survey. Compared to TEM, CLSM has the advantage to rapidly providing results for whole, intact cells. Moreover, many cells can be studied at once. We chemically induced lysogenic cyanophages by means of mitomycin C (MMC) treatments and studied the ultrastructural alterations of host cells. In parallel, the number of cyanophages was obtained by flow cytometry. After treatment of the culture with MMC, flow cytometry showed a strong increase in viral counts, i.e., prophage induction. CLSM reflected the re-organization of L. fusiformis with remarkable alterations of thylakoid arrangements after prophage induction. Our study provides a first step towards 3D visualization of ultrastructure of cyanobacteria and showed the high potential of CLSM to investigate viral-mediated modifications in these groups.

Stability of thylakoid protein complexes and preserving photosynthetic efficiency are crucial for the successful recovery of the halophyte Cakile maritima from high salinity

Plants native to extreme habitats often face changes in environmental conditions such as salinity level and water availability. In response, plants have evolved efficient mechanisms allowing them to survive or recover. In the present work, effects of high salinity and salt-stress release were studied on the halophyte Cakile maritima. Four week-old plants were either cultivated at 0 mM NaCl or 200 mM NaCl. After one month of treatment, plants were further irrigated at either 0 mM NaCl, 200 mM NaCl, or rewatered to 0 mM NaCl (stress release). Upon salt stress, C. maritima plants exhibited reduced biomass production and shoot hydration which were associated with a decrease in the amount of chlorophyll a and b. However, under the same stressful conditions a significant increase of anthocyanin and malonyldialdehyde concentrations was noticed. Salt-stressed plants were able to maintain stable protein complexes of thylakoid membranes. Measurement of chlorophyll fluorescence and P700 redox state showed that PSI was more susceptible for damage by salinity than PSII. PSII machinery was significantly enhanced under saline conditions. All measured parameters were partially restored under salt-stress release conditions. Photoinhibition of PSI was also reversible and C. maritima was able to successfully re-establish PSI machinery indicating the high contribution of chloroplasts in salt tolerance mechanisms of C. maritima. Overall, to overcome high salinity stress, C. maritima sets a cascade of physio-biochemical and molecular pathways. Chloroplasts seem to act as metabolic centers as part of this adaptive process enabling growth restoration in this halophyte following salt stress release.

Influence of the compatible solute sucrose on thylakoid membrane organization and violaxanthin de-epoxidation

The compatible solute sucrose reduces the efficiency of the enzymatic de-epoxidation of violaxanthin, probably by a direct effect on the protein parts of violaxanthin de-epoxidase which protrude from the lipid phase of the thylakoid membrane. The present study investigates the influence of the compatible solute sucrose on the violaxanthin cycle of higher plants in intact thylakoids and in in vitro enzyme assays with the isolated enzyme violaxanthin de-epoxidase at temperatures of 30 and 10 °C, respectively. In addition, the influence of sucrose on the lipid organization of thylakoid membranes and the MGDG phase in the in vitro assays is determined. The results show that sucrose leads to a pronounced inhibition of violaxanthin de-epoxidation both in intact thylakoid membranes and the enzyme assays. In general, the inhibition is similar at 30 and 10 °C. With respect to the lipid organization only minor changes can be seen in thylakoid membranes at 30 °C in the presence of sucrose. However, sucrose seems to stabilize the thylakoid membranes at lower temperatures and at 10 °C a comparable membrane organization to that at 30 °C can be observed, whereas control thylakoids show a significantly different membrane organization at the lower temperature. The MGDG phase in the in vitro assays is not substantially affected by the presence of sucrose or by changes of the temperature. We conclude that the presence of sucrose and the increased viscosity of the reaction buffers stabilize the protein part of the enzyme violaxanthin de-epoxidase, thereby decreasing the dynamic interactions between the catalytic site and the substrate violaxanthin. This indicates that sucrose interacts with those parts of the enzyme which are accessible at the membrane surface of the lipid phase of the thylakoid membrane or the MGDG phase of the in vitro enzyme assays.

Tissue-type specific accumulation of the plastoglobular proteome, transcriptional networks, and plastoglobular functions

Plastoglobules are dynamic protein-lipid microcompartments in plastids enriched for isoprenoid-derived metabolites. Chloroplast plastoglobules support formation, remodeling, and controlled dismantling of thylakoids during developmental transitions and environmental responses. However, the specific molecular functions of most plastoglobule proteins are still poorly understood. This review harnesses recent co-mRNA expression data from combined microarray and RNA-seq information in ATTED-II on an updated inventory of 34 PG proteins, as well as proteomics data across 30 Arabidopsis tissue types from ATHENA. Hierarchical clustering based on relative abundance for the plastoglobule proteins across non-photosynthetic and photosynthetic tissue types showed their coordinated protein accumulation across Arabidopsis parts, tissue types, development, and senescence. Evaluation of mRNA-based forced networks at different coefficient thresholds identified a central hub with seven plastoglobule proteins and four peripheral modules. Enrichment of specific nuclear transcription factors (e.g. Golden2-like) and support for crosstalk between plastoglobules and the plastid gene expression was observed, and specific ABC1 kinases appear part of a light signaling network. Examples of other specific findings are that FBN7b is involved with upstream steps of tetrapyrrole biosynthesis and that ABC1K9 is involved in starch metabolism. This review provides new insights into the functions of plastoglobule proteins and an improved framework for experimental studies.

Chloroplast thylakoid ascorbate peroxidase PtotAPX plays a key role in chloroplast development by decreasing hydrogen peroxide in Populus tomentosa

Chloroplast development is a complex process that is critical to the growth and development of plants. However, the detailed mechanism of chloroplast development in woody plants remains unclear. In this study, we showed that chloroplasts with elaborate thylakoids could develop from proplastids in the cells of calli derived from leaf tissues of Populus tomentosa upon exposure to light. Chloroplast development was confirmed at the molecular and cellular levels. Transcriptome analysis revealed that genes related to photoreceptors and photosynthesis were significantly up-regulated during chloroplast development in a time-dependent manner. In light-induced chloroplast development, a key process was the removal of hydrogen peroxide, in which thylakoid-localized PtotAPX played a major role; light-induced chloroplast development was enhanced in PtotAPX-overexpressing transgenic P. tomentosa callus with lower levels of hydrogen peroxide, but was suppressed in PtotAPX antisense transgenic callus with higher levels of hydrogen peroxide. Moreover, the suppression of light-induced chloroplast development in PtotAPX antisense transgenic callus was relieved by the exogenous reactive oxygen species scavenging agent N,N'-dimethylthiourea (DMTU). Based on these results, we propose that PtotAPX-mediated removal of reactive oxygen species plays a key role in chloroplast development from proplastids upon exposure to light in P. tomentosa.

Proline is a quencher of singlet oxygen and superoxide both in in vitro systems and isolated thylakoids

Proline is a versatile plant metabolite, which is produced in large amounts in plants exposed to osmotic and oxidative stress. Proline has been shown to provide protection against various reactive oxygen species (ROS), such as hydrogen peroxide and hydroxyl radicals. On the other hand, its protective effect against singlet oxygen has been debated, and it is considered ineffective against superoxide. Here we used various methods for the detection of singlet oxygen (electron paramagnetic resonance, EPR, spin trapping by 2,2,6,6-tetramethyl-4-piperidone, fluorescence probing by singlet oxygen sensor green, SOSG, and oxygen uptake due to chemical trapping) and superoxide (oxygen uptake due to oxygen reduction) in vitro and in isolated thylakoids. We demonstrated that proline does quench both singlet oxygen and superoxide in vitro. By comparing the effects of chemical scavengers and physical quenchers, we concluded that proline eliminates singlet oxygen via a physical mechanism, with a bimolecular quenching rate of ca. 1.5-4 10⁶  M^-1 s^-1 . Our data also show that proline can eliminate superoxide in vitro in a process that is likely to proceed via an electron transfer reaction. We could also show that proline does quench both singlet oxygen and superoxide produced in isolated thylakoids. The scavenging efficiency of proline is relatively small on a molar basis, but considering its presence in high amounts in plant cells under stress conditions it may provide a physiologically relevant contribution to ROS scavenging, supplementing other nonenzymatic ROS scavengers of plant cells.

Glycinebetaine mitigated the photoinhibition of photosystem II at high temperature in transgenic tomato plants

Photosystem II (PSII), especially the D1 protein, is highly sensitive to the detrimental impact of heat stress. Photoinhibition always occurs when the rate of photodamage exceeds the rate of D1 protein repair. Here, genetically engineered codA-tomato with the capability to accumulate glycinebetaine (GB) was established. After photoinhibition treatment at high temperature, the transgenic lines displayed more thermotolerance to heat-induced photoinhibition than the control line. GB maintained high expression of LeFtsHs and LeDegs and degraded the damaged D1 protein in time. Meanwhile, the increased transcription of synthesis-related genes accelerated the de novo synthesis of D1 protein. Low ROS accumulation reduced the inhibition of D1 protein translation in the transgenic plants, thereby reducing protein damage. The increased D1 protein content and decreased phosphorylated D1 protein (pD1) in the transgenic plants compared with control plants imply that GB may minimize photodamage and maximize D1 protein stability. As D1 protein exhibits a high turnover, PSII maybe repaired rapidly and efficiently in transgenic plants under photoinhibition treatment at high temperature, with the resultant mitigation of photoinhibition of PSII.

Is zeaxanthin needed for desiccation tolerance? Sustained forms of thermal dissipation in tolerant versus sensitive bryophytes

Desiccation tolerant (DT) plants engage and disengage sustained forms of energy dissipation in response to desiccation and rehydration. This project sought to characterize the role of zeaxanthin and thylakoid protein phosphorylation status in sustained energy dissipation during desiccation in bryophytes with varying DT. Tolerant (Polytrichum piliferum, Dicranum species, Calliergon stramineum) and sensitive (Grimmia species, Schistidium rivulare, Sphagnum species) moss were desiccated in darkness or natural light conditions for up to three weeks. Desiccation caused pronounced reductions in F_v /F_m in all cases which was enhanced by light exposure during desiccation. Desiccation in darkness resulted in no accumulation of Z in any species, however, in natural light conditions there was significant accumulation of Z in tolerant but not sensitive species. Desiccation in natural light, relative to darkness, resulted in more pronounced reductions in F_o in tolerant but not sensitive species. Recovery of F_v /F_m upon rehydration occurred in two phases, a rapid phase (minutes) and a slower phase (hours). Increased time of desiccation, and light exposure, resulted in a reduction in the rapid phase. Desiccation in light conditions resulted in some accumulation of the phosphorylated form of the major light harvesting trimer (LHCII). Data are consistent with two mechanisms of sustained quenching, neither of which requires Z. However, when desiccation occurs in natural light conditions, accumulation of Z likely contributes to one or both of the sustained forms of dissipation. Increases in LHCII phosphorylation during desiccation are consistent with increased connectivity between the photosystems. The absence of Z formation in sensitive species may contribute to their lack of desiccation tolerance.

The physiological response of photosynthesis to nitrogen deficiency

Nitrogen (N), as a macro-element, plays a vital role in plant growth and development. N deficiency affects plant productivity by decreasing photosynthesis, leaf area and longevity of green leaf. To date, many studies have reported that the relationship between photosynthesis and N supply. Here, we summarized the physiological response of photosynthesis to N deficiency in leaf structure and N allocation within the leaf. In serious N stress, photosynthetic rate decreases for almost all plants. The reasons as follows:(1) reducing stomatal conductance of mesophyll cell (g_s) and bundle sheath cells (g_bs) which influences intercellular CO₂ concentration; (2) reducing the content of bioenergetics and light-harvesting protein which inhibits electron transport rate and increase the light energy dissipated as heat; (3) reducing the content and/or activity of photosynthetic enzymes which reduces carboxylation rate. During reproductive stage, N stress induces plant senescence and N components degradation, especially photosynthetic enzymes and thylakoid N, and thus reduces photosynthesis. To keep high grain yield in low N deficiency, we should choose the genotype with higher N allocation within bioenergetics and lower degradation of photosynthetic enzymes. This review provides a generalized N allocation in response to N stress and gives a new prospect for breeding N-efficient genotypes.

Functional characterization of a chloroplast-localized Mn2+(Ca2+)/H+ antiporter, ZmmCCHA1 from Zea mays ssp. mexicana L

The annual Zea mays ssp. mexicana L. is a member of the teosinte group and a close wild relative of maize. Thus, Zea mays ssp. mexicana L. can be effectively used in maize breeding. AtCCHA1 is a Mn2+ and/or Ca2+/H+ antiporter localized in chloroplasts in Arabidopsis. In this study, its homolog from Zea mays ssp. mexicana L., ZmmCCHA1, was isolated and characterized. The transcriptional level of ZmmCCHA1 in Zea mays ssp. mexicana L. was repressed in response to excessive Ca2+ or Mn2+. Heterologous functional complementation assays using yeast mutants showed that ZmmCCHA1 mediates Ca2+ and Mn2+ transport. The ZmmCCHA1 protein was localized in the chloroplasts when expressed in tobacco leaves. Furthermore, ectopic overexpression of ZmmCCHA1 in the Arabidopsis ccha1 mutant rescued the mutant defects on growth and the photosynthetic proteins in the thylakoid membranes. Moreover, the photosynthetic phenotypes of Arabidopsis ccha1 mutant at steady-state were greatly but not completely complemented by the overexpression of ZmmCCHA1. In addition, overexpressing the ZmmCCHA1 rescued the sensitives of PSII in the Arabidopsis ccha1 mutant to Mn2+ deficiency or high Ca2+ condition. These results indicate that the isolated ZmmCCHA1 is the homolog of AtCCHA1 and plays a conserved role in maintaining the Mn2+ and/or Ca2+ homeostasis in chloroplasts which is critical for the function of PSII in photosynthesis.

Nanoplastics exposure modulate lipid and pigment compositions in diatoms

The impact of nanoplastics (NP) using model polystyrene nanoparticles amine functionalized (PS-NH₂) has been investigated on pigment and lipid compositions of the marine diatom Chaetoceros neogracile, at two growth phases using a low (0.05 μg mL^-1) and a high (5 μg mL^-1) concentrations for 96 h. Results evidenced an impact on pigment composition associated to the light-harvesting function and photoprotection mainly at exponential phase. NP also impacted lipid composition of diatoms with a re-adjustment of lipid classes and fatty acids noteworthy. Main changes upon NP exposure were observed in galactolipids and triacylglycerol's at both growth phases affecting the thylakoids membrane structure and cellular energy reserve of diatoms. Particularly, exponential cultures exposed to high NP concentration showed an impairment of long chain fatty acids synthesis. Changes in pigment and lipid content of diatom' cells revealed that algae physiology is determinant in the way cells adjust their thylakoid membrane composition to cope with NP contamination stress. Compositions of reserve and membrane lipids are proposed as sensitive markers to assess the impact of NP exposure, including at potential predicted environmental doses, on marine organisms.

Probing functional and optical cross-sections of PSII in leaves during state transitions using fast repetition rate light induced fluorescence transients

Plants adjust the relative sizes of PSII and PSI antennae in response to the spectral composition of weak light favouring either photosystem by processes known as state transitions (ST), attributed to a discrete antenna migration involving phosphorylation of light-harvesting chlorophyll-protein complexes in PSII. Here for the first time we monitored the extent and dynamics of ST in leaves from estimates of optical absorption cross-section (relative PSII antenna size; aPSII). These estimates were obtained from in situ measurements of functional absorption cross-section (σPSII) and maximum photochemical efficiency of PSII (φPSII); i.e. aPSII = σPSII/φPSII (Kolber et al. 1998) and other parameters from a light induced fluorescence transient (LIFT) device (Osmond et al. 2017). The fast repetition rate (FRR) QA flash protocol of this instrument monitors chlorophyll fluorescence yields with reduced QA irrespective of the redox state of plastoquinone (PQ), as well as during strong ~1 s white light pulses that fully reduce the PQ pool. Fitting this transient with the FRR model monitors kinetics of PSII → PQ, PQ → PSI, and the redox state of the PQ pool in the 'PQ pool control loop' that underpins ST, with a time resolution of a few seconds. All LIFT/FRR criteria confirmed the absence of ST in antenna mutant chlorina-f2 of barley and asLhcb2-12 of Arabidopsis, as well as STN7 kinase mutants stn7 and stn7/8. In contrast, wild-type barley and Arabidopsis genotypes Col, npq1, npq4, OEpsbs, pgr5 bkg and pgr5, showed normal ST. However, the extent of ST (and by implication the size of the phosphorylated LHCII pool participating in ST) deduced from changes in a'PSII and other parameters with reduced QA range up to 35%. Estimates from strong WL pulses in the same assay were only ~10%. The larger estimates of ST from the QA flash are discussed in the context of contemporary dynamic structural models of ST involving formation and participation of PSII and PSI megacomplexes in an 'energetically connected lake' of phosphorylated LHCII trimers (Grieco et al. 2015). Despite the absence of ST, asLhcb2-12 displays normal wild-type modulation of electron transport rate (ETR) and the PQ pool during ST assays, reflecting compensatory changes in antenna LHCIIs in this genotype. Impaired LHCII phosphorylation in stn7 and stn7/8 accelerates ETR from PSII →PQ, over-reducing the PQ pool and abolishing the yield difference between the QA flash and WL pulse, with implications for photochemical and thermal phases of the O-J-I-P transient.

Chloroplast Lipids and Their Biosynthesis

Chloroplasts contain high amounts of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) and low levels of the anionic lipids sulfoquinovosyldiacylglycerol (SQDG), phosphatidylglycerol (PG), and glucuronosyldiacylglycerol (GlcADG). The mostly extraplastidial lipid phosphatidylcholine is found only in the outer envelope. Chloroplasts are the major site for fatty acid synthesis. In Arabidopsis, a certain proportion of glycerolipids is entirely synthesized in the chloroplast (prokaryotic lipids). Fatty acids are also exported to the endoplasmic reticulum and incorporated into lipids that are redistributed to the chloroplast (eukaryotic lipids). MGDG, DGDG, SQDG, and PG establish the thylakoid membranes and are integral constituents of the photosynthetic complexes. Phosphate deprivation induces phospholipid degradation accompanied by the increase in DGDG, SQDG, and GlcADG. During freezing and drought stress, envelope membranes are stabilized by the conversion of MGDG into oligogalactolipids. Senescence and chlorotic stress lead to lipid and chlorophyll degradation and the deposition of acyl and phytyl moieties as fatty acid phytyl esters.

Influence of the Carotenoid Composition on the Conformational Dynamics of Photosynthetic Light-Harvesting Complexes

Nonphotochemical quenching (NPQ) is the major self-regulatory mechanism of green plants, performed on a molecular level to protect them from an overexcitation during the direct sunlight. It is believed that NPQ becomes available due to conformational dynamics of the light-harvesting photosynthetic complexes and involves a direct participation of carotenoids. In this work, we perform a single-molecule microscopy on major light-harvesting complexes (LHCII) from different Arabidopsis thaliana mutants exhibiting various carotenoid composition. We show how the distinct carotenoids affect the dynamics of the conformational switching between multiple coexisting light-emitting states of LHCII and demonstrate that properties of the quenched conformation are not influenced by the particular carotenoids available in LHCII. We also discuss the possible origin of different conformational states and relate them to the fluorescence decay kinetics observed during the bulk measurements.

Structure of the plant photosystem I supercomplex at 2.6 Å resolution

Four elaborate membrane complexes carry out the light reaction of oxygenic photosynthesis. Photosystem I (PSI) is one of two large reaction centres responsible for converting light photons into the chemical energy needed to sustain life. In the thylakoid membranes of plants, PSI is found together with its integral light-harvesting antenna, light-harvesting complex I (LHCI), in a membrane supercomplex containing hundreds of light-harvesting pigments. Here, we report the crystal structure of plant PSI-LHCI at 2.6 Å resolution. The structure reveals the configuration of PsaK, a core subunit important for state transitions in plants, a conserved network of water molecules surrounding the electron transfer centres and an elaborate structure of lipids bridging PSI and its LHCI antenna. We discuss the implications of the structure for energy transfer and the evolution of PSI.

The Use of Green Leaf Membranes to Promote Appetite Control, Suppress Hedonic Hunger and Loose Body Weight

On-going research aims at answering the question, which satiety signal is the most potent or which combination of satiety signals is the most potent to stop eating. There is also an aim at finding certain food items or food additives that could be used to specifically reduce food intake therapeutically. Therapeutic attempts to normalize body weight and glycaemia with single agents alone have generally been disappointing. The success of bariatric surgery illustrates the rationale of using several hormones to treat obesity and type-2-diabetes. We have found that certain components from green leaves, the thylakoids, when given orally have a similar rationale in inducing the release of several gut hormones at the same time. In this way satiety is promoted and hunger suppressed, leading to loss of body weight and body fat. The mechanism is a reduced rate of intestinal lipid hydrolysis, allowing the lipolytic products to reach the distal intestine and release satiety hormones. The thylakoids also regulate glucose uptake in the intestine and influences microbiota composition in the intestine in a prebiotic direction. Using thylakoids is a novel strategy for treatment and prevention of obesity.

Photoprotection of photosystems in fluctuating light intensities

Oxygenic photosynthetic organisms experience strong fluctuations in light intensity in their natural terrestrial and aquatic growth environments. Recent studies with both plants and cyanobacteria have revealed that Photosystem (PS) I is the potential target of damage upon abrupt changes in light intensity. Photosynthetic organisms have, however, developed powerful mechanisms in order to protect their photosynthetic apparatus against such potentially hazardous light conditions. Although the electron transfer chain has remained relatively unchanged in both plant chloroplasts and their cyanobacterial ancestors, the photoprotective and regulatory mechanisms of photosynthetic light reactions have experienced conspicuous evolutionary changes. In cyanobacteria, the specific flavodiiron proteins (Flv1 and Flv3) are responsible for safeguarding PSI under rapidly fluctuating light intensities, whilst the thylakoid located terminal oxidases are involved in the protection of PSII during 12h diurnal cycles involving abrupt, square-wave, changes from dark to high light. Higher plants such as Arabidopsis thaliana have evolved different protective mechanisms. In particular, the PGR5 protein controls electron flow during sudden changes in light intensity by allowing the regulation mostly via the Cytochrome b6f complex. Besides the function of PGR5, plants have also acquired other dynamic regulatory mechanisms, among them the STN7-related LHCII protein phosphorylation that is similarly responsible for protection against rapid changes in the light environment. The green alga Chlamydomonas reinhardtii, as an evolutionary intermediate between cyanobacteria and higher plants, probably possesses both protective mechanisms. In this review, evolutionarily different photoprotective mechanisms under fluctuating light conditions are described and their contributions to cyanobacterial and plant photosynthesis are discussed.

Tissue-specific accumulation and regulation of zeaxanthin epoxidase in Arabidopsis reflect the multiple functions of the enzyme in plastids

The enzyme zeaxanthin epoxidase (ZEP) catalyzes the conversion of zeaxanthin to violaxanthin, a key reaction for ABA biosynthesis and the xanthophyll cycle. Both processes are important for acclimation to environmental stress conditions, in particular drought (ABA biosynthesis) and light (xanthophyll cycle) stress. Hence, both ZEP functions may require differential regulation to optimize plant fitness. The key to understanding the function of ZEP in both stress responses might lie in its spatial and temporal distribution in plant tissues. Therefore, we analyzed the distribution of ZEP in plant tissues and plastids under drought and light stress by use of a ZEP-specific antibody. In addition, we determined the pigment composition of the plant tissues and chloroplast membrane subcompartments in response to these stresses. The ZEP protein was detected in all plant tissues (except flowers) concomitant with xanthophylls. The highest levels of ZEP were present in leaf chloroplasts and root plastids. Within chloroplasts, ZEP was localized predominantly in the thylakoid membrane and stroma, while only a small fraction was bound by the envelope membrane. Light stress affected neither the accumulation nor the relative distribution of ZEP in chloroplasts, while drought stress led to an increase of ZEP in roots and to a degradation of ZEP in leaves. However, drought stress-induced increases in ABA were similar in both tissues. These data support a tissue- and stress-specific accumulation of the ZEP protein in accordance with its different functions in ABA biosynthesis and the xanthophyll cycle.