Structural constraints for protein repair in plant photosynthetic membranes

The photosystem-II repair cycle: updates and open questions

MAIN CONCLUSION

The photosystem-II (PSII) repair cycle is essential for the maintenance of photosynthesis in plants. A number of novel findings have illuminated the regulatory mechanisms of the PSII repair cycle. Photosystem II (PSII) is a large pigment-protein complex embedded in the thylakoid membrane. It plays a vital role in photosynthesis by absorbing light energy, splitting water, releasing molecular oxygen, and transferring electrons for plastoquinone reduction. However, PSII, especially the PsbA (D1) core subunit, is highly susceptible to oxidative damage. To prevent irreversible damage, plants have developed a repair cycle. The main objective of the PSII repair cycle is the degradation of photodamaged D1 and insertion of newly synthesized D1 into the PSII complex. While many factors are known to be involved in PSII repair, the exact mechanism is still under investigation. In this review, we discuss the primary steps of PSII repair, focusing on the proteolytic degradation of photodamaged D1 and the factors involved.

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.

Hacking the thylakoid proton motive force for improved photosynthesis: modulating ion flux rates that control proton motive force partitioning into Δψ and ΔpH

There is considerable interest in improving plant productivity by altering the dynamic responses of photosynthesis in tune with natural conditions. This is exemplified by the 'energy-dependent' form of non-photochemical quenching (q_E), the formation and decay of which can be considerably slower than natural light fluctuations, limiting photochemical yield. In addition, we recently reported that rapidly fluctuating light can produce field recombination-induced photodamage (FRIP), where large spikes in electric field across the thylakoid membrane (Δψ) induce photosystem II recombination reactions that produce damaging singlet oxygen (¹O₂). Both q_E and FRIP are directly linked to the thylakoid proton motive force (pmf), and in particular, the slow kinetics of partitioning pmf into its ΔpH and Δψ components. Using a series of computational simulations, we explored the possibility of 'hacking' pmf partitioning as a target for improving photosynthesis. Under a range of illumination conditions, increasing the rate of counter-ion fluxes across the thylakoid membrane should lead to more rapid dissipation of Δψ and formation of ΔpH. This would result in increased rates for the formation and decay of q_E while resulting in a more rapid decline in the amplitudes of Δψ-spikes and decreasing ¹O₂ production. These results suggest that ion fluxes may be a viable target for plant breeding or engineering. However, these changes also induce transient, but substantial mismatches in the ATP : NADPH output ratio as well as in the osmotic balance between the lumen and stroma, either of which may explain why evolution has not already accelerated thylakoid ion fluxes. Overall, though the model is simplified, it recapitulates many of the responses seen in vivo, while spotlighting critical aspects of the complex interactions between pmf components and photosynthetic processes. By making the programme available, we hope to enable the community of photosynthesis researchers to further explore and test specific hypotheses.This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.

Photosystem I-LHCII megacomplexes respond to high light and aging in plants

Photosystem II is known to be a highly dynamic multi-protein complex that participates in a variety of regulatory and repair processes. In contrast, photosystem I (PSI) has, until quite recently, been thought of as relatively static. We report the discovery of plant PSI-LHCII megacomplexes containing multiple LHCII trimers per PSI reaction center. These PSI-LHCII megacomplexes respond rapidly to changes in light intensity, as visualized by native gel electrophoresis. PSI-LHCII megacomplex formation was found to require thylakoid stacking, and to depend upon growth light intensity and leaf age. These factors were, in turn, correlated with changes in PSI/PSII ratios and, intriguingly, PSI-LHCII megacomplex dynamics appeared to depend upon PSII core phosphorylation. These findings suggest new functions for PSI and a new level of regulation involving specialized subpopulations of photosystem I which have profound implications for current models of thylakoid dynamics.

Fine-Tuning of Photosynthesis Requires CURVATURE THYLAKOID1-Mediated Thylakoid Plasticity

The thylakoid membrane system of higher plant chloroplasts consists of interconnected subdomains of appressed and nonappressed membrane bilayers, known as grana and stroma lamellae, respectively. CURVATURE THYLAKOID1 (CURT1) protein complexes mediate the shape of grana stacks in a dosage-dependent manner and facilitate membrane curvature at the grana margins, the interface between grana and stroma lamellae. Although grana stacks are highly conserved among land plants, the functional relevance of grana stacking remains unclear. Here, we show that inhibiting CURT1-mediated alteration of thylakoid ultrastructure in Arabidopsis (Arabidopsis thaliana) reduces photosynthetic efficiency and plant fitness under adverse, controlled, and natural light conditions. Plants that lack CURT1 show less adjustment of grana diameter, which compromises regulatory mechanisms like the photosystem II repair cycle and state transitions. Interestingly, CURT1A suffices to induce thylakoid membrane curvature in planta and thylakoid hyperbending in plants overexpressing CURT1A. We suggest that CURT1 oligomerization is regulated at the posttranslational level in a light-dependent fashion and that CURT1-mediated thylakoid plasticity plays an important role in fine-tuning photosynthesis and plant fitness during challenging growth conditions.

Chloroplast Proteases: Updates on Proteolysis within and across Suborganellar Compartments

Chloroplasts originated from the endosymbiosis of ancestral cyanobacteria and maintain transcription and translation machineries for around 100 proteins. Most endosymbiont genes, however, have been transferred to the host nucleus, and the majority of the chloroplast proteome is composed of nucleus-encoded proteins that are biosynthesized in the cytosol and then imported into chloroplasts. How chloroplasts and the nucleus communicate to control the plastid proteome remains an important question. Protein-degrading machineries play key roles in chloroplast proteome biogenesis, remodeling, and maintenance. Research in the past few decades has revealed more than 20 chloroplast proteases, which are localized to specific suborganellar locations. In particular, two energy-dependent processive proteases of bacterial origin, Clp and FtsH, are central to protein homeostasis. Processing endopeptidases such as stromal processing peptidase and thylakoidal processing peptidase are involved in the maturation of precursor proteins imported into chloroplasts by cleaving off the amino-terminal transit peptides. Presequence peptidases and organellar oligopeptidase subsequently degrade the cleaved targeting peptides. Recent findings have indicated that not only intraplastidic but also extraplastidic processive protein-degrading systems participate in the regulation and quality control of protein translocation across the envelopes. In this review, we summarize current knowledge of the major chloroplast proteases in terms of type, suborganellar localization, and diversification. We present details of these degradation processes as case studies according to suborganellar compartment (envelope, stroma, and thylakoids). Key questions and future directions in this field are discussed.

Close Relationships Between the PSII Repair Cycle and Thylakoid Membrane Dynamics

In chloroplasts, a three-dimensional network of thylakoid membranes is formed by stacked grana and interconnecting stroma thylakoids. The grana are crowded with photosynthetic proteins, where PSII-light harvesting complex II (LHCII) supercomplexes often show semi-crystalline arrays for efficient energy trapping, transfer and use. Although light is essential for photosynthesis, PSII is damaged by reactive oxygen species that are generated from primary photochemical reactions when plants are exposed to excess light. Because PSII complexes are embedded in the lipid bilayers of thylakoid membranes, their functions are affected by the conditions of the lipids. Electron paramagnetic resonance (EPR) spin trapping measurements showed that singlet oxygen was formed through peroxidation of thylakoid lipids, suggesting that lipid peroxidation can damage proteins, including the D1 protein. After photodamage, PSII is restored by a specific repair system in thylakoid membranes. In the PSII repair cycle, phosphorylation and dephosphorylation of the PSII proteins control the timing of PSII disassembly and subsequent degradation of the D1 protein. Under light stress, stacked grana turn into unstacked thylakoids with bent grana margins. These structural changes may be closely linked to the mechanisms of the PSII repair cycle because PSII can move more easily from the grana core to the stroma thylakoids through an expanded stromal gap between each thylakoid. Thus, plants modulate the structure of thylakoid membranes under high light to carry out efficient PSII repair. This review focuses on the behavior of the PSII complex and the active role of structural changes to thylakoid membranes under light stress.

Functional Implications of Photosystem II Crystal Formation in Photosynthetic Membranes

The structural organization of proteins in biological membranes can affect their function. Photosynthetic thylakoid membranes in chloroplasts have the remarkable ability to change their supramolecular organization between disordered and semicrystalline states. Although the change to the semicrystalline state is known to be triggered by abiotic factors, the functional significance of this protein organization has not yet been understood. Taking advantage of an Arabidopsis thaliana fatty acid desaturase mutant (fad5) that constitutively forms semicrystalline arrays, we systematically test the functional implications of protein crystals in photosynthetic membranes. Here, we show that the change into an ordered state facilitates molecular diffusion of photosynthetic components in crowded thylakoid membranes. The increased mobility of small lipophilic molecules like plastoquinone and xanthophylls has implications for diffusion-dependent electron transport and photoprotective energy-dependent quenching. The mobility of the large photosystem II supercomplexes, however, is impaired, leading to retarded repair of damaged proteins. Our results demonstrate that supramolecular changes into more ordered states have differing impacts on photosynthesis that favor either diffusion-dependent electron transport and photoprotection or protein repair processes, thus fine-tuning the photosynthetic energy conversion.

Differential effects of ambient or diminished CO2 and O2 levels on thylakoid membrane structure in light-stressed plants

Over-reduction of the photosynthetic electron transport chain may severely damage the photosynthetic apparatus as well as other constituents of the chloroplast and the cell. Here, we exposed Arabidopsis leaves to saturating light either under normal atmospheric conditions or under CO2--and O2 -limiting conditions, which greatly increase excitation and electron pressures by draining terminal electron acceptors. The two treatments were found to have very different, often opposing, effects on the structure of the thylakoid membranes, including the width of the granal lumenal compartment. Modulation of the latter is proposed to be related to movements of ions across the thylakoid membrane, which alter the relative osmolarity of the lumen and stroma and affect the partitioning of the proton motive force into its electrical and osmotic components. The resulting changes in thylakoid organization and lumenal width should facilitate the repair of photodamaged photosystem II complexes in response to light stress under ambient conditions, but are expected to inhibit the repair cycle when the light stress occurs concurrently with CO2 and O2 depletion. Under the latter conditions, the changes in thylakoid structure are predicted to complement other processes that restrict the flow of electrons into the high-potential chain, thus moderating the production of deleterious reactive oxygen species at photosystem I.

Significance of the photosystem II core phosphatase PBCP for plant viability and protein repair in thylakoid membranes

PSII undergoes photodamage, which results in photoinhibition-the light-induced loss of photosynthetic activity. The main target of damage in PSII is the reaction center protein D1, which is buried in the massive 1.4 MDa PSII holocomplex. Plants have evolved a PSII repair cycle that degrades the damaged D1 subunit and replaces it with a newly synthesized copy. PSII core proteins, including D1, are phosphorylated in high light. This phosphorylation is important for the mobilization of photoinhibited PSII from stacked grana thylakoids to the repair machinery in distant unstacked stroma lamellae. It has been recognized that the degradation of the damaged D1 is more efficient after its dephosphorylation by a protein phosphatase. Recently a protein phosphatase 2C (PP2C)-type PSII core phosphatase (PBCP) has been discovered, which is involved in the dephosphorylation of PSII core proteins. Its role in PSII repair, however, is unknown. Using a range of spectroscopic and biochemical techniques, we report that the inactivation of the PBCP gene affects the growth characteristic of plants, with a decreased biomass and altered PSII functionality. PBCP mutants show increased phosphorylation of core subunits in dark and photoinhibitory conditions and a diminished degradation of the D1 subunit. Our results on D1 turnover in PBCP mutants suggest that dephosphorylation of PSII subunits is required for efficient D1 degradation.

Structure and dynamics of thylakoids in land plants

Thylakoids of land plants have a bipartite structure, consisting of cylindrical grana stacks, made of membranous discs piled one on top of the other, and stroma lamellae which are helically wound around the cylinders. Protein complexes predominantly located in the stroma lamellae and grana end membranes are either bulky [photosystem I (PSI) and the chloroplast ATP synthase (cpATPase)] or are involved in cyclic electron flow [the NAD(P)H dehydrogenase (NDH) and PGRL1-PGR5 heterodimers], whereas photosystem II (PSII) and its light-harvesting complex (LHCII) are found in the appressed membranes of the granum. Stacking of grana is thought to be due to adhesion between Lhcb proteins (LHCII or CP26) located in opposed thylakoid membranes. The grana margins contain oligomers of CURT1 proteins, which appear to control the size and number of grana discs in a dosage- and phosphorylation-dependent manner. Depending on light conditions, thylakoid membranes undergo dynamic structural changes that involve alterations in granum diameter and height, vertical unstacking of grana, and swelling of the thylakoid lumen. This plasticity is realized predominantly by reorganization of the supramolecular structure of protein complexes within grana stacks and by changes in multiprotein complex composition between appressed and non-appressed membrane domains. Reversible phosphorylation of LHC proteins (LHCPs) and PSII components appears to initiate most of the underlying regulatory mechanisms. An update on the roles of lipids, proteins, and protein complexes, as well as possible trafficking mechanisms, during thylakoid biogenesis and the de-etiolation process complements this review.

Quality control of Photosystem II: the molecular basis for the action of FtsH protease and the dynamics of the thylakoid membranes

The reaction center-binding D1 protein of Photosystem II is damaged by excessive light, which leads to photoinhibition of Photosystem II. The damaged D1 protein is removed immediately by specific proteases, and a metalloprotease FtsH located in the thylakoid membranes is involved in the proteolytic process. According to recent studies on the distribution and organization of the protein complexes/supercomplexes in the thylakoid membranes, the grana of higher plant chloroplasts are crowded with Photosystem II complexes and light-harvesting complexes. For the repair of the photodamaged D1 protein, the majority of the active hexameric FtsH proteases should be localized in close proximity to the Photosystem II complexes. The unstacking of the grana may increase the area of the grana margin and facilitate easier access of the FtsH proteases to the damaged D1 protein. These results suggest that the structural changes of the thylakoid membranes by light stress increase the mobility of the membrane proteins and support the quality control of Photosystem II.

Integrative regulatory network of plant thylakoid energy transduction

Highly flexible regulation of photosynthetic light reactions in plant chloroplasts is a prerequisite to provide sufficient energy flow to downstream metabolism and plant growth, to protect light reactions against photodamage, and to ensure controlled cellular signaling from the chloroplast to the nucleus. Such comprehensive regulation occurs via the control of excitation energy transfer to and between the two photosystems (PSII and PSI), of the electrochemical gradient across the thylakoid membrane (ΔpH), and of electron transfer from PSII to PSI electron acceptors. In this opinion article, we propose that these regulatory mechanisms, functioning at different levels of photosynthetic energy conversion, might be interconnected and describe how the concomitant and integrated function of these mechanisms might enable plants to acclimate to a full array of environmental changes.

Diffusion of molecules and macromolecules in thylakoid membranes

The survival and fitness of photosynthetic organisms is critically dependent on the flexible response of the photosynthetic machinery, harbored in thylakoid membranes, to environmental changes. A central element of this flexibility is the lateral diffusion of membrane components along the membrane plane. As demonstrated, almost all functions of photosynthetic energy conversion are dependent on lateral diffusion. The mobility of both small molecules (plastoquinone, xanthophylls) as well as large protein supercomplexes is very sensitive to changes in structural boundary conditions. Knowledge about the design principles that govern the mobility of photosynthetic membrane components is essential to understand the dynamic response of the photosynthetic machinery. This review summarizes our knowledge about the factors that control diffusion in thylakoid membranes and bridges structural membrane alterations to changes in mobility and function. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.

Loss-of-function of OsSTN8 suppresses the photosystem II core protein phosphorylation and interferes with the photosystem II repair mechanism in rice (Oryza sativa)

STN8 kinase is involved in photosystem II (PSII) core protein phosphorylation (PCPP). To examine the role of PCPP in PSII repair during high light (HL) illumination, we characterized a T-DNA insertional knockout mutant of the rice (Oryza sativa) STN8 gene. In this osstn8 mutant, PCPP was significantly suppressed, and the grana were thin and elongated. Upon HL illumination, PSII was strongly inactivated in the mutants, but the D1 protein was degraded more slowly than in wild-type, and mobilization of the PSII supercomplexes from the grana to the stromal lamellae for repair was also suppressed. In addition, higher accumulation of reactive oxygen species and preferential oxidation of PSII reaction center core proteins in thylakoid membranes were observed in the mutants during HL illumination. Taken together, our current data show that the absence of STN8 is sufficient to abolish PCPP in osstn8 mutants and to produce all of the phenotypes observed in the double mutant of Arabidopsis, indicating the essential role of STN8-mediated PCPP in PSII repair.

Architectural switches in plant thylakoid membranes

Recent progress in elucidating the structure of higher plants photosynthetic membranes provides a wealth of information. It allows generation of architectural models that reveal well-organized and complex arrangements not only on whole membrane level, but also on the supramolecular level. These arrangements are not static but highly responsive to the environment. Knowledge about the interdependency between dynamic structural features of the photosynthetic machinery and the functionality of energy conversion is central to understanding the plasticity of photosynthesis in an ever-changing environment. This review summarizes the architectural switches that are realized in thylakoid membranes of green plants.

Glycerolipids in photosynthesis: composition, synthesis and trafficking

Glycerolipids constituting the matrix of photosynthetic membranes, from cyanobacteria to chloroplasts of eukaryotic cells, comprise monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol and phosphatidylglycerol. This review covers our current knowledge on the structural and functional features of these lipids in various cellular models, from prokaryotes to eukaryotes. Their relative proportions in thylakoid membranes result from highly regulated and compartmentalized metabolic pathways, with a cooperation, in the case of eukaryotes, of non-plastidic compartments. This review also focuses on the role of each of these thylakoid glycerolipids in stabilizing protein complexes of the photosynthetic machinery, which might be one of the reasons for their fascinating conservation in the course of evolution. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.