Structural Investigation of PsbO from Plant and Cyanobacterial Photosystem II

Higher Intensity of Salt Stress Accompanied by Heat Inhibits Stomatal Conductance and Induces ROS Accumulation in Tomato Plants

Under natural conditions, abiotic stresses that limit plant growth and development tend to occur simultaneously, rather than individually. Due to global warming and climate change, the frequency and intensity of heat and salt stresses are becoming more frequent. Our aim is to determine the response mechanisms of tomato to different intensities of combined heat and salt stresses. The physiological and morphological responses and photosynthesis/reactive oxygen species (ROS)-related genes of tomato plants were compared under a control, heat stress, salt stress (50/100/200/400 mM NaCl), and a combination of salt and heat stresses. The stomatal conductance (gs) of tomato leaves significantly increased at a heat + 50 mM NaCl treatment on day 4, but significantly decreased at heat + 100/200/400 mM NaCl treatments, compared with the control on days 4 and 8. The O2·- production rate of tomato plants was significantly higher at heat + 100/200/400 mM NaCl than the control, which showed no significant difference between heat + 50 mM NaCl treatment and the control on days 4 and 8. Ascorbate peroxidase 2 was significantly upregulated by heat + 100/200/400 mM NaCl treatment as compared with heat + 50 mM NaCl treatment on days 4 and 8. This study demonstrated that the dominant effect ratio of combined heat and salt stress on tomato plants can shift from heat to salt, when the intensity of salt stress increased from 50 mM to 100 mM or above. This study provides important information for tomato tolerance improvement at combined heat and salt stresses.

Transcriptome analysis of the bloom-forming dinoflagellate Prorocentrum donghaiense exposed to Ginkgo biloba leaf extract, with an emphasis on photosynthesis

Ginkgo biloba leaf extract (GBE) can effectively treat bloom-forming freshwater algae. However, there is limited information about the underlying suppression mechanism of the marine bloom-forming Prorocentrum donghaiense-the most dominant algal bloom species in the East China Sea. We investigated the effect of GBE on P. donghaiense in terms of its response to photosynthesis at the molecular/omic level. In total, 93,743 unigenes were annotated using six functional databases. Furthermore, 67,203 differentially expressed genes (DEGs) were identified in algae treated with 1.8 g∙L-1 GBE. Among these DEGs, we identified the genes involved in photosynthesis. PsbA, PsbB and PsbD in photosystem II, PsaA in photosystem I, and PetB and PetD in the cytochrome b6/f complex were downregulated. Other related genes, such as PsaC, PsaE, and PsaF in photosystem I; PetA in the cytochrome b6/f complex; and atpA, atpD, atpH, atpG, and atpE in the F-type H+-ATPase were upregulated. These results suggest that the structure and activity of the complexes were destroyed by GBE, thereby inhibiting the electron flow between the primary and secondary quinone electron acceptors, primary quinone electron acceptor, and oxygen-evolving complex in the PSII complex, and interrupting the electron flow between PSII and PSI, ultimately leading to a decline in algal cell photosynthesis. These findings provide a basis for understanding the molecular mechanisms underlying P. donghaiense exposure to GBE and a theoretical basis for the prevention and control of harmful algal blooms.

Exogenous melatonin enhances tomato heat resistance by regulating photosynthetic electron flux and maintaining ROS homeostasis

Heat stress reduces plant growth and reproduction and increases agricultural risks. As a natural compound, melatonin modulates broad aspects of the responses of plants to various biotic and abiotic stresses. However, regulation of the photosynthetic electron transfer, reactive oxygen species (ROS) homeostasis and the redox state of redox-sensitive proteins in the tolerance to heat stress induced by melatonin remain largely unknown. The oxygen evolution complex activity on the electron-donating side of photosystem II (PSII) is inhibited, and the electron transfer process from QA to QB on the electron-accepting side of PSII is inhibited. In this case, heat stress decreased the chlorophyll content, carbon assimilation rate, PSII activity, and the proportion of light absorbed by tomato seedlings during electron transfer. The ROS burst led to the breakdown of the PSII core protein. However, exogenous melatonin increased the net photosynthetic rate by 11.3% compared with heat stress, substantially reducing the restriction of photosynthetic systems induced by heat stress. Additionally, melatonin reduces the oxidative damage to PSII by balancing electron transfer on the donor, reactive center, and acceptor sides. Melatonin was used under heat stress to increase the activity of the antioxidant enzyme and preserve ROS equilibrium. In addition, redox proteomics also showed that melatonin controls the redox levels of proteins involved in photosynthesis, and stress and defense processes, which enhances the expression of oxidative genes. In conclusion, melatonin via controlling the photosynthetic electron transport and antioxidant, melatonin increased tomato heat stress tolerance and aided plant growth.

Light-Harvesting Complex II Adopts Different Quaternary Structures in Solution as Observed Using Small-Angle Scattering

The high-resolution crystal structure of the trimeric major light-harvesting complex of photosystem II (LHCII) is often perceived as the basis for understanding its light-harvesting and photoprotective functions. However, the LHCII solution structure and its oligomerization or aggregation state may generally differ from the crystal structure and, moreover, also depend on its functional state. In this regard, small-angle scattering experiments provide the missing link by offering structural information in aqueous solution at physiological temperatures. Herein, we use small-angle scattering to investigate the solution structures of two different preparations of solubilized LHCII employing the nonionic detergents n-octyl-β-d-glucoside (OG) and n-dodecyl-β-D-maltoside (β-DM). The data reveal that the LHCII-OG complex is equivalent to the trimeric crystal structure. Remarkably, however, we observe─for the first time─a stable oligomer composed of three LHCII trimers in the case of the LHCII-β-DM preparation, implying additional pigment-pigment interactions. The latter complex is assumed to mimic trimer-trimer interactions which play an important role in the context of photoprotective nonphotochemical quenching.

Insights into Solution Structures of Photosynthetic Protein Complexes from Small-Angle Scattering Methods

High-resolution structures of photosynthetic pigment–protein complexes are often determined using crystallography or cryo-electron microscopy (cryo-EM), which are restricted to the use of protein crystals or to low temperatures, respectively. However, functional studies and biotechnological applications of photosystems necessitate the use of proteins isolated in aqueous solution, so that the relevance of high-resolution structures has to be independently verified. In this regard, small-angle neutron and X-ray scattering (SANS and SAXS, respectively) can serve as the missing link because of their capability to provide structural information for proteins in aqueous solution at physiological temperatures. In the present review, we discuss the principles and prototypical applications of SANS and SAXS using the photosynthetic pigment–protein complexes phycocyanin (PC) and Photosystem I (PSI) as model systems for a water-soluble and for a membrane protein, respectively. For example, the solution structure of PSI was studied using SAXS and SANS with contrast matching. A Guinier analysis reveals that PSI in solution is virtually free of aggregation and characterized by a radius of gyration of about 75 Å. The latter value is about 10% larger than expected from the crystal structure. This is corroborated by an ab initio structure reconstitution, which also shows a slight expansion of Photosystem I in buffer solution at room temperature. In part, this may be due to conformational states accessible by thermally activated protein dynamics in solution at physiological temperatures. The size of the detergent belt is derived by comparison with SANS measurements without detergent match, revealing a monolayer of detergent molecules under proper solubilization conditions.

Solution Structure of the Detergent-Photosystem II Core Complex Investigated by Small-Angle Scattering Techniques

Albeit achieving the X-ray diffraction structure of dimeric photosystem II core complexes (dPSIIcc) at the atomic resolution, the nature of the detergent belt surrounding dPSIIcc remains ambiguous. Therefore, the solution structure of the whole detergent-protein complex of dPSIIcc of Thermosynechococcus elongatus (T. elongatus) solubilized in n-dodecyl-ß-d-maltoside (ßDM) was investigated by a combination of small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) with contrast variation. First, the structure of dPSIIcc was studied separately in SANS experiments using a contrast of 5% D₂O. Guinier analysis reveals that the dPSIIcc solution is virtually free of aggregation in the studied concentration range of 2-10 mg/mL dPSIIcc, and characterized by a radius of gyration of 62 Å. A structure reconstitution shows that dPSIIcc in buffer solution widely retains the crystal structure reported by X-ray free electron laser studies at room temperature with a slight expansion of the entire protein. Additional SANS experiments on dPSIIcc samples in a buffer solution containing 75% D₂O provide information about the size and shape of the whole detergent-dPSIIcc. The maximum position of P(r) function increases to 68 Å, i.e., it is about 6 Å larger than that of dPSIIcc only, thus indicating the presence of an additional structure. Thus, it can be concluded that dPSIIcc is surrounded by a monomolecular belt of detergent molecules under appropriate solubilization conditions. The homogeneity of the ßDM-dPSIIcc solutions was also verified using dynamic light scattering. Complementary SAXS experiments indicate the presence of unbound detergent micelles by a separate peak consistent with a spherical shape possessing a radius of about 40 Å. The latter structure also contributes to the SANS data but rather broadens the SANS curve artificially. Without the simultaneous inspection of SANS and SAXS data, this effect may lead to an apparent underestimation of the size of the PS II-detergent complex. The formation of larger unbound detergent aggregates in solution prior to crystallization may have a significant effect on the crystal formation or quality of the ßDM-dPSIIcc.

Systematic identification and comparative analysis of lysine succinylation between the green and white parts of chimeric leaves of Ananas comosus var. bracteatus

Background

Lysine succinylation, an important protein posttranslational modification (PTM), is widespread and conservative. The regulatory functions of succinylation in leaf color has been reported. The chimeric leaves of Ananas comosus var. bracteatus are composed of normal green parts and albino white parts. However, the extent and function of lysine succinylation in chimeric leaves of Ananas comosus var. bracteatus has yet to be investigated.

Results

Compared to the green (Gr) parts, the global succinylation level was increased in the white (Wh) parts of chimeric leaves according to the Western blot and immunohistochemistry analysis. Furthermore, we quantitated the change in the succinylation profiles between the Wh and Gr parts of chimeric leaves using label-free LFQ intensity. In total, 855 succinylated sites in 335 proteins were identified, and 593 succinylated sites in 237 proteins were quantified. Compared to the Gr parts, 232 (61.1%) sites in 128 proteins were quantified as upregulated targets, and 148 (38.9%) sites in 70 proteins were quantified as downregulated targets in the Wh parts of chimeric leaves using a 1.5-fold threshold (P < 0.05). These proteins with altered succinylation level were mainly involved in crassulacean acid metabolism (CAM) photosynthesis, photorespiration, glycolysis, the citric acid cycle (CAC) and pyruvate metabolism.

Conclusions

Our results suggested that the changed succinylation level in proteins might function in the main energy metabolism pathways—photosynthesis and respiration. Succinylation might provide a significant effect in the growth of chimeric leaves and the relationship between the Wh and Gr parts of chimeric leaves. This study not only provided a basis for further characterization on the function of succinylated proteins in chimeric leaves of Ananas comosus var. bracteatus but also provided a new insight into molecular breeding for leaf color chimera.

Photosystem II Extrinsic Proteins and Their Putative Role in Abiotic Stress Tolerance in Higher Plants

Abiotic stress remains one of the major challenges in managing and preventing crop loss. Photosystem II (PSII), being the most susceptible component of the photosynthetic machinery, has been studied in great detail over many years. However, much of the emphasis has been placed on intrinsic proteins, particularly with respect to their involvement in the repair of PSII-associated damage. PSII extrinsic proteins include PsbO, PsbP, PsbQ, and PsbR in higher plants, and these are required for oxygen evolution under physiological conditions. Changes in extrinsic protein expression have been reported to either drastically change PSII efficiency or change the PSII repair system. This review discusses the functional role of these proteins in plants and indicates potential areas of further study concerning these proteins.

Effects of PSII Manganese-Stabilizing Protein Succinylation on Photosynthesis in the Model Cyanobacterium Synechococcus sp. PCC 7002

Lysine succinylation is a newly identified protein post-translational modification and plays important roles in various biological pathways in both prokaryotes and eukaryotes, but its extent and function in photosynthetic organisms remain largely unknown. Here, we performed the first systematic studies of lysine succinylation in cyanobacteria, which are the only prokaryotes capable of oxygenic photosynthesis and the established model organisms for studying photosynthetic mechanisms. By using mass spectrometry analysis in combination with the enrichment of succinylated peptides from digested cell lysates, we identified 1,704 lysine succinylation sites on 691 proteins in a model cyanobacterium Synechococcus sp. PCC 7002. Bioinformatic analysis revealed that a large proportion of the succinylation sites were present on proteins in photosynthesis and metabolism. Among all identified succinylated proteins involved in photosynthesis, the PSII manganese-stabilizing protein (PsbO) was found to be succinylated on Lys99 and Lys234. Functional studies of PsbO were performed by site-directed mutagenesis, and mutants mimicking either constitutively succinylated (K99E and K234E) or non-succinylated states (K99R and K234R) were constructed. The succinylation-mimicking K234E mutant exhibited a decreased oxygen evolution rate of the PSII center and the efficiency of energy transfer during the photosynthetic reaction. Molecular dynamics simulations suggested a mechanism that may allow succinylation to influence the efficiency of photosynthesis by altering the conformation of PsbO, thereby hindering the interaction between PsbO and the PSII core. Our findings suggest that reversible succinylation may be an important regulatory mechanism during photosynthesis in Synechococcus, as well as in other photosynthetic organisms.

Solution structure of monomeric and trimeric photosystem I of Thermosynechococcus elongatus investigated by small-angle X-ray scattering

The structure of monomeric and trimeric photosystem I (PS I) of Thermosynechococcus elongatus BP1 (T. elongatus) was investigated by small-angle X-ray scattering (SAXS). The scattering data reveal that the protein-detergent complexes possess radii of gyration of 58 and 78 Å in the cases of monomeric and trimeric PS I, respectively. The results also show that the samples are monodisperse, virtually free of aggregation, and contain empty detergent micelles. The shape of the protein-detergent complexes can be well approximated by elliptical cylinders with a height of 78 Å. Monomeric PS I in buffer solution exhibits minor and major radii of the elliptical cylinder of about 50 and 85 Å, respectively. In the case of trimeric PS I, both radii are equal to about 110 Å. The latter model can be shown to accommodate three elliptical cylinders equal to those describing monomeric PS I. A structure reconstitution also reveals that the protein-detergent complexes are larger than their respective crystal structures. The reconstituted structures are larger by about 20 Å mainly in the region of the hydrophobic surfaces of the monomeric and trimeric PS I complexes. This seeming contradiction can be resolved by the addition of a detergent belt constituted by a monolayer of dodecyl-β-D-maltoside molecules. Assuming a closest possible packing, a number of roughly 1024 and 1472 detergent molecules can be determined for monomeric and trimeric PS I, respectively. Taking the monolayer of detergent molecules into account, the solution structure can be almost perfectly modeled by the crystal structures of monomeric and trimeric PS I.

Solution structure and excitation energy transfer in phycobiliproteins of Acaryochloris marina investigated by small angle scattering

The structure of phycobiliproteins of the cyanobacterium Acaryochloris marina was investigated in buffer solution at physiological temperatures, i.e. under the same conditions applied in spectroscopic experiments, using small angle neutron scattering. The scattering data of intact phycobiliproteins in buffer solution containing phosphate can be well described using a cylindrical shape with a length of about 225Å and a diameter of approximately 100Å. This finding is qualitatively consistent with earlier electron microscopy studies reporting a rod-like shape of the phycobiliproteins with a length of about 250 (M. Chen et al., FEBS Letters 583, 2009, 2535) or 300Å (J. Marquart et al., FEBS Letters 410, 1997, 428). In contrast, phycobiliproteins dissolved in buffer lacking phosphate revealed a splitting of the rods into cylindrical subunits with a height of 28Å only, but also a pronounced sample aggregation. Complementary small angle neutron and X-ray scattering experiments on phycocyanin suggest that the cylindrical subunits may represent either trimeric phycocyanin or trimeric allophycocyanin. Our findings are in agreement with the assumption that a phycobiliprotein rod with a total height of about 225Å can accommodate seven trimeric phycocyanin subunits and one trimeric allophycocyanin subunit, each of which having a height of about 28Å. The structural information obtained by small angle neutron and X-ray scattering can be used to interpret variations in the low-energy region of the 4.5K absorption spectra of phycobiliproteins dissolved in buffer solutions containing and lacking phosphate, respectively.

The extrinsic proteins of Photosystem II

In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn(4)CaO(5) cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.

The involvement of ferredoxin-NADP+ reductase in cyclic electron transport in chloroplasts

The sites of action, in spinach thylakoid, of known inhibitors of electron transport at the reducing end of photosystem I have been more accurately located by parallel investigation of effects on three partial reactions: photo-reduction (from water) of added NADP+, photoreduction of added cytochrome c, and dark reduction of cyto-chrome c by added NADPH. Comparison with inhibitory effects on cyclic electron flow (registered by the slow phase of the electrochromic response during repetitive flash excitation) permitted assessment of the role of ferredoxin and ferredoxin-NADP+ reductase (ferredoxin: NADP+ oxidoreductase, EC 1.18.1.3) in the cyclic electron transport pathway around photosystem I. Disulfodisalicylidenepropane-1,1-diamine inhibited all the above partial reactions except the ferredoxin-dependent photoreduction of cytochrome C. thereby indicating its interference with the reductase or the complexation between reductase and ferredoxin. Studies with purified ferredoxin-NADP+ reductase established it as the sensitive component. Cyclic flow is also sensitive to the above inhibitor and thus presumably involves the reductase. Supporting evidence for this came from studies of inhibition by substituted maleimides, which are inhibitors of electron transfer through the isolated reductase; these also inhibited the slow phase of the electrochromic response and all partial reactions except the photoreduction of cytochrome c. In contrast, an antiserum against the reductase affected only reactions involving NADP. The conclusion is drawn that the pathway of cyclic electron transport includes both ferredoxin and ferredoxin-NADP+ reductase, but not the NADP-binding site on the reductase.