Quality control of Photosystem II: Where and how does the degradation of the D1 protein by FtsH proteases start under light stress? – Facts and hypotheses

Genome-wide identification of the ATP-dependent zinc metalloprotease (FtsH) in Triticeae species reveals that TaFtsH-1 regulates cadmium tolerance in Triticum aestivum

The ATP-dependent zinc metalloprotease (FtsH) protein gene family is essential for plant growth, development, and stress responses. Although FtsH genes have been identified in various plant species, the FtsH gene family in wheat (Triticum aestivum) remains unstudied. In this study, we identified 11 TaFtsH genes with uneven chromosomal distribution, significant variations in gene sequence length, and differing intron numbers among individual members. Additionally, these proteins exhibit similar physicochemical characteristics as well as secondary and tertiary structures. The FtsH genes can be classified into eight groups, each characterized by similar structures and conserved motifs. Intraspecific and interspecific comparisons further revealed extensive gene duplications within the TaFtsH gene family, indicating a closer relationship to maize. Analysis of cis-acting elements in the promoter regions of TaFtsH genes revealed developmental and stress-responsive elements in most of the genes. Expression pattern analysis showed that TaFtsH genes are expressed in all wheat tissues, though with varying patterns. TaFtsH genes displayed differential responses to CdCl2, ZnSO4, and MnSO4 stress treatments. Gene Ontology (GO) enrichment analysis indicated that TaFtsH genes are involved in protein hydrolysis. Barley stripe mosaic virus-induced gene silencing (BSMV-VIGS) technology confirmed the function of TaFtsH-1, indicating that silencing TaFtsH-1 enhances common wheat's resistance to cadmium (Cd) toxicity. In summary, this study offers an in-depth understanding of the FtsH gene family in wheat, establishing a solid basis for comprehending its functions, genetic mechanisms, and improving wheat's tolerance to heavy metal contamination.

Are tomato plants co-exposed to heat and salinity able to ensure a proper carbon metabolism? - An insight into the photosynthetic hub

Abiotic stress combinations, such as high temperatures and soil/water salinization, severely threaten crop productivity worldwide. In this work, an integrative insight into the photosynthetic metabolism of tomato plants subjected to salt (100 mM NaCl) and/or heat (42 °C; 4 h/day) was performed. After three weeks, the stress combination led to more severe consequences on growth and photosynthetic pigments than the individual stresses. Regarding the photochemical efficiency, transcript accumulation and protein content of major actors (CP47 and D1) were depleted in all stressed plants, although the overall photochemical yield was not negatively affected under the co-exposure. Gas-exchange studies revealed to be mostly affected by salt (single or combined), which harshly compromised carbon assimilation. Additionally, transcript levels of stress-responsive genes (e.g., HsfA1 and NHX2) were differentially modulated by the single and combined treatments, suggesting the activation of stress-signature responses. Overall, by gathering an insightful overview of the main regulatory hub of photosynthesis, we show that the impacts on the carbon metabolism coming from the combination of heat and salinity, two major conditioners of crop yields, were not harsher than those of single stresses, indicating that the growth impairment might be attributed to a proficient distribution of resources towards defense mechanisms.

Chlorophyll fluorescence-based high-throughput phenotyping facilitates the genetic dissection of photosynthetic heat tolerance in African (Oryza glaberrima) and Asian (Oryza sativa) rice

Rising temperatures and extreme heat events threaten rice production. Half of the global population relies on rice for basic nutrition, and therefore developing heat-tolerant rice is essential. During vegetative development, reduced photosynthetic rates can limit growth and the capacity to store soluble carbohydrates. The photosystem II (PSII) complex is a particularly heat-labile component of photosynthesis. We have developed a high-throughput chlorophyll fluorescence-based screen for photosynthetic heat tolerance capable of screening hundreds of plants daily. Through measuring the response of maximum PSII efficiency to increasing temperature, this platform generates data for modelling the PSII-temperature relationship in large populations in a small amount of time. Coefficients from these models (photosynthetic heat tolerance traits) demonstrated high heritabilities across African (Oryza glaberrima) and Asian (Oryza sativa, Bengal Assam Aus Panel) rice diversity sets, highlighting valuable genetic variation accessible for breeding. Genome-wide association studies were performed across both species for these traits, representing the first documented attempt to characterize the genetic basis of photosynthetic heat tolerance in any species to date. A total of 133 candidate genes were highlighted. These were significantly enriched with genes whose predicted roles suggested influence on PSII activity and the response to stress. We discuss the most promising candidates for improving photosynthetic heat tolerance in rice.

Photosynthetic acclimation to changing environments

Plants are exposed to environments that fluctuate of timescales varying from seconds to months. Leaves that develop in one set of conditions optimise their metabolism to the conditions experienced, in a process called developmental acclimation. However, when plants experience a sustained change in conditions, existing leaves will also acclimate dynamically to the new conditions. Typically this process takes several days. In this review, we discuss this dynamic acclimation process, focussing on the responses of the photosynthetic apparatus to light and temperature. We briefly discuss the principal changes occurring in the chloroplast, before examining what is known, and not known, about the sensing and signalling processes that underlie acclimation, identifying likely regulators of acclimation.

Genome-wide identification and expression analysis of the ftsH protein family and its response to abiotic stress in Nicotiana tabacum L

BACKGROUND

The filamentous temperature-sensitive H protease (ftsH) gene family plays an important role in plant growth and development. FtsH proteins belong to the AAA protease family. Studies have shown that it is a key gene for plant chloroplast development and photosynthesis regulation. In addition, the ftsH gene is also involved in plant response to stress. At present, the research and analysis of the ftsH gene family are conducted in microorganisms such as Escherichia coli and Oenococcus and various plants such as Arabidopsis, pear, rice, and corn. However, analysis reports on ftsH genes from tobacco (Nicotiana tabacum L.), an important model plant, are still lacking. Since ftsH genes regulate plant growth and development, it has become necessary to systematically study this gene in an economically important plant like tobacco.

RESULTS

This is the first study to analyze the ftsH gene from Nicotiana tabacum L. K326 (NtftsH). We identified 20 ftsH genes from the whole genome sequence, renamed them according to their chromosomal locations, and divided them into eight subfamilies. These 20 NtftsH genes were unevenly distributed across the 24 chromosomes. We found four pairs of fragment duplications. We further investigated the collinearity between these genes and related genes in five other species. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis identified differential expression patterns of NtftsH in different tissues and under various abiotic stress conditions.

CONCLUSIONS

This study provides a comprehensive analysis of the NtftsH gene family. The exon-intron structure and motif composition are highly similar in NtftsH genes that belong to the same evolutionary tree branch. Homology analysis and phylogenetic comparison of ftsH genes from several different plants provide valuable clues for studying the evolutionary characteristics of NtftsH genes. The NtftsH genes play important roles in plant growth and development, revealed by their expression levels in different tissues as well as under different stress conditions. Gene expression and phylogenetic analyses will provide the basis for the functional analysis of NtftsH genes. These results provide a valuable resource for a better understanding of the biological role of the ftsH genes in the tobacco plant.

Reactive oxygen species in photosystem II: relevance for oxidative signaling

Reactive oxygen species (ROS) are formed in photosystem II (PSII) under various types of abiotic and biotic stresses. It is considered that ROS play a role in chloroplast-to-nucleus retrograde signaling, which changes the nuclear gene expression. However, as ROS lifetime and diffusion are restricted due to the high reactivity towards biomolecules (lipids, pigments, and proteins) and the spatial specificity of signal transduction is low, it is not entirely clear how ROS might transduce signal from the chloroplasts to the nucleus. Biomolecule oxidation was formerly connected solely with damage; nevertheless, the evidence appears that oxidatively modified lipids and pigments are be involved in chloroplast-to-nucleus retrograde signaling due to their long diffusion distance. Moreover, oxidatively modified proteins show high spatial specificity; however, their role in signal transduction from chloroplasts to the nucleus has not been proven yet. The review attempts to summarize and evaluate the evidence for the involvement of ROS in oxidative signaling in PSII.

Responses of Phragmites australis to copper stress: A combined analysis of plant morphology, physiology and proteomics

Few relevant research attempts have been made to determine heavy metal resistance mechanisms of rhizomatous perennial plants. Thus, it is pertinent to investigate the physiological and biochemical changes in Phragmites australis under metal-stressed conditions to facilitate the development of strategies to enhance copper (Cu) tolerance. We measured parameters related to plant growth and development, metal translocation and physiological responses of P. australis subjected to Cu stress. In addition, the differentially expressed proteins (DEP) were evaluated using the isobaric tag for relative and absolute quantification (iTRAQ) system. A large amount of copper accumulates in the roots of P.australis, but the growth parameters were not sensitive to Cu. However, the high concentration of Cu reduced the content of chlorophyll a and chlorophyll b, and the expression of important photosynthesis proteins PsbD, PsbO and PsaA were all down-regulated, so photosynthesis was inhibited. In contrast, the content of ascorbic acid and proline both increased with the increase of copper stress. P.australis fixed a large amount of Cu in its roots, limiting the migration of Cu to other parts of the plant. Moreover, Cu stress can affect photosynthesis by inhibiting the activity of PSI, PSII and LHCII. In addition, P.australis synthesizes ascorbic acid through the D-mannose/L-galactose pathway, and synthesizes proline through the ornithine pathway. Ascorbic acid and proline can increase Cu tolerance and protect photosynthesis. These results provide a theoretical basis for understanding the tolerance and repair mechanisms of plants in response to heavy metal pollution.

Chlorophyll a Fluorescence Transient and 2-Dimensional Electrophoresis Analyses Reveal Response Characteristics of Photosynthesis to Heat Stress in Malus. 'Prairifire'

Flowering crabapples are a series of precious ornamental woody plants. However, their growth and development are inhibited in the subtropical regions due to the weak photosynthesis under high-temperature environment in the summer. Chlorophyll a fluorescence transient and 2-dimensional electrophoresis (2-DE) analyses were conducted to investigate the response characteristics of photosynthesis under simulated 38 °C heat stress in leaves of Malus. ‘Prairifire’, a spring-red leaf cultivar of flowering crabapple with strong thermal adaptability. In the present study, the net photosynthetic rate (Pn) was significantly decreased during the heat shock process, which showed a similar trend to the stomatal conductance (Gs), indicating a sensitive stomatal behavior to heat stress. Moreover, an efficient reaction center in photosystem II (PSII), and a functionally intact oxygen-evolving complex (OEC) conferred strong photosynthetic adaptability under heat stress. The higher level of transketolase (TK) under 48-h heat shock treatment was considered a protective mechanism of photosynthetic apparatus. However, heat stress inhibited the functions of light harvesting complex II (LHCII), electron transport in PSII, and the levels of key enzymes in the Calvin cycle, which were considered as the reasons causing an increase in the proportion of non-stomatal restrictions.

The structural and functional domains of plant thylakoid membranes

In plants, the stacking of part of the photosynthetic thylakoid membrane generates two main subcompartments: the stacked grana core and unstacked stroma lamellae. However, a third distinct domain, the grana margin, has been postulated but its structural and functional identity remains elusive. Here, an optimized thylakoid fragmentation procedure combined with detailed ultrastructural, biochemical, and functional analyses reveals the distinct composition of grana margins. It is enriched with lipids, cytochrome b₆ f complex, and ATPase while depleted in photosystems and light-harvesting complexes. A quantitative method is introduced that is based on Blue Native Polyacrylamide Gel Electrophoresis (BN-PAGE) and dot immunoblotting for quantifying various photosystem II (PSII) assembly forms in different thylakoid subcompartments. The results indicate that the grana margin functions as a degradation and disassembly zone for photodamaged PSII. In contrast, the stacked grana core region contains fully assembled and functional PSII holocomplexes. The stroma lamellae, finally, contain monomeric PSII as well as a significant fraction of dimeric holocomplexes that identify this membrane area as the PSII repair zone. This structural organization and the heterogeneous PSII distribution support the idea that the stacking of thylakoid membranes leads to a division of labor that establishes distinct membrane areas with specific functions.

Metabolic Reprogramming in Chloroplasts under Heat Stress in Plants

Increases in ambient temperatures have been a severe threat to crop production in many countries around the world under climate change. Chloroplasts serve as metabolic centers and play a key role in physiological adaptive processes to heat stress. In addition to expressing heat shock proteins that protect proteins from heat-induced damage, metabolic reprogramming occurs during adaptive physiological processes in chloroplasts. Heat stress leads to inhibition of plant photosynthetic activity by damaging key components functioning in a variety of metabolic processes, with concomitant reductions in biomass production and crop yield. In this review article, we will focus on events through extensive and transient metabolic reprogramming in response to heat stress, which included chlorophyll breakdown, generation of reactive oxygen species (ROS), antioxidant defense, protein turnover, and metabolic alterations with carbon assimilation. Such diverse metabolic reprogramming in chloroplasts is required for systemic acquired acclimation to heat stress in plants.

The Zinc-Finger Thylakoid-Membrane Protein FIP Is Involved With Abiotic Stress Response in Arabidopsis thaliana

Many plant genes have their expression modulated by stress conditions. Here, we used Arabidopsis FtsH5 protease, which expression is regulated by light stress, as bait in a yeast two-hybrid screen to search for new proteins involved in the stress response. As a result, we found FIP (FtsH5 Interacting Protein), which possesses an amino proximal cleavable transit peptide, a hydrophobic membrane-anchoring region, and a carboxyl proximal C₄-type zinc-finger domain. In vivo experiments using FIP fused to green fluorescent protein (GFP) showed a plastid localization. This finding was corroborated by chloroplast import assays that showed FIP inserted in the thylakoid membrane. FIP expression was down-regulated in plants exposed to high light intensity, oxidative, salt, and osmotic stresses, whereas mutant plants expressing low levels of FIP were more tolerant to these abiotic stresses. Our data shows a new thylakoid-membrane protein involved with abiotic stress response in Arabidopsis thaliana.

The High Light Response and Redox Control of Thylakoid FtsH Protease in Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii, the major protease involved in the maintenance of photosynthetic machinery in thylakoid membranes, the FtsH protease, mostly forms large hetero-oligomers (∼1 MDa) comprising FtsH1 and FtsH2 subunits, whatever the light intensity for growth. Upon high light exposure, the FtsH subunits display a shorter half-life, which is counterbalanced by an increase in FTSH1/2 mRNA levels, resulting in the modest upregulation of FtsH1/2 proteins. Furthermore, we found that high light increases the protease activity through a hitherto unnoticed redox-controlled reduction of intermolecular disulfide bridges. We isolated a Chlamydomonas FTSH1 promoter-deficient mutant, ftsh1-3, resulting from the insertion of a TOC1 transposon, in which the high light-induced upregulation of FTSH1 gene expression is largely lost. In ftsh1-3, the abundance of FtsH1 and FtsH2 proteins are loosely coupled (decreased by 70% and 30%, respectively) with no formation of large and stable homo-oligomers. Using strains exhibiting different accumulation levels of the FtsH1 subunit after complementation of ftsh1-3, we demonstrate that high light tolerance is tightly correlated with the abundance of the FtsH protease. Thus, the response of Chlamydomonas to light stress involves higher levels of FtsH1/2 subunits associated into large complexes with increased proteolytic activity.

Proteome Analysis Reveals Extensive Light Stress-Response Reprogramming in the Seagrass Zostera muelleri (Alismatales, Zosteraceae) Metabolism

Seagrasses are marine ecosystem engineers that are currently declining in abundance at an alarming rate due to both natural and anthropogenic disturbances in ecological niches. Despite reports on the morphological and physiological adaptations of seagrasses to extreme environments, little is known of the molecular mechanisms underlying photo-acclimation, and/or tolerance in these marine plants. This study applies the two-dimensional isoelectric focusing (2D-IEF) proteomics approach to identify photo-acclimation/tolerance proteins in the marine seagrass Zostera muelleri. For this, Z. muelleri was exposed for 10 days in laboratory mesocosms to saturating (control, 200 μmol photons m^-2 s^-1), super-saturating (SSL, 600 μmol photons m^-2 s^-1), and limited light (LL, 20 μmol photons m^-2 s^-1) irradiance conditions. Using LC-MS/MS analysis, 93 and 40 protein spots were differentially regulated under SSL and LL conditions, respectively, when compared to the control. In contrast to the LL condition, Z. muelleri robustly tolerated super-saturation light than control conditions, evidenced by their higher relative maximum electron transport rate and minimum saturating irradiance values. Proteomic analyses revealed up-regulation and/or appearances of proteins belonging to the Calvin-Benson and Krebs cycle, glycolysis, the glycine cleavage system of photorespiration, and the antioxidant system. These proteins, together with those from the inter-connected glutamate-proline-GABA pathway, shaped Z. muelleri photosynthesis and growth under SSL conditions. In contrast, the LL condition negatively impacted the metabolic activities of Z. muelleri by down-regulating key metabolic enzymes for photosynthesis and the metabolism of carbohydrates and amino acids, which is consistent with the observation with lower photosynthetic performance under LL condition. This study provides novel insights into the underlying molecular photo-acclimation mechanisms in Z. muelleri, in addition to identifying protein-based biomarkers that could be used as early indicators to detect acute/chronic light stress in seagrasses to monitor seagrass health.

Born in 1949 in postwar Japan

In this article, I would like to look back at my life as a researcher of photosynthesis. I was born in 1949, and grew up and was educated in postwar Japan in the 1950s and 1960s. I have studied photosynthesis, in particular Photosystem II, after research experiences in the USA and UK. My study of Photosystem II has continued over 43 years until now. Through the present retrospection, I would like to suggest that all photosynthesis researchers, including the members of the "49ers", many other established scientists, and young students as well, should not simply stay in the lab working hard on their studies and writing papers; but should also do something for the public. People want to learn from us about many critical social issues such as the environment, food, energy and, most importantly, peace. I believe that our knowledge must form an important basis for people to take action to create a peaceful and harmonious human society.

Photosystem II repair in plant chloroplasts--Regulation, assisting proteins and shared components with photosystem II biogenesis

Photosystem (PS) II is a multisubunit thylakoid membrane pigment-protein complex responsible for light-driven oxidation of water and reduction of plastoquinone. Currently more than 40 proteins are known to associate with PSII, either stably or transiently. The inherent feature of the PSII complex is its vulnerability in light, with the damage mainly targeted to one of its core proteins, the D1 protein. The repair of the damaged D1 protein, i.e. the repair cycle of PSII, initiates in the grana stacks where the damage generally takes place, but subsequently continues in non-appressed thylakoid domains, where many steps are common for both the repair and de novo assembly of PSII. The sequence of the (re)assembly steps of genuine PSII subunits is relatively well-characterized in higher plants. A number of novel findings have shed light into the regulation mechanisms of lateral migration of PSII subcomplexes and the repair as well as the (re)assembly of the complex. Besides the utmost importance of the PSII repair cycle for the maintenance of PSII functionality, recent research has pointed out that the maintenance of PSI is closely dependent on regulation of the PSII repair cycle. This review focuses on the current knowledge of regulation of the repair cycle of PSII in higher plant chloroplasts. Particular emphasis is paid on sequential assembly steps of PSII and the function of the number of PSII auxiliary proteins involved both in the biogenesis and repair of PSII. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Differential protein expression in Phalaenopsis under low temperature

A comparative proteomic analysis was carried out to explore the molecular mechanisms of responses to cold stress in Phalaenopsis after treated by low temperature (13/8 °C day/night) for 15 days. Differentially expressed proteins were examined using two-dimensional electrophoresis (2-DE) and matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-TOF/MS). Among 85 differentially expressed proteins, 73 distinct proteins were identified. Comparative analysis revealed that the identified proteins mainly participate in photosynthesis, protein synthesis, folding and degradation, respiration, defense response, amino acid metabolism, energy pathway, cytoskeleton, transcription regulation, signal transduction, and seed storage protein, while the functional classification of the remaining four proteins was not determined. These data suggested that the proteins might work cooperatively to establish a new homeostasis under cold stress; 37 % of the identified cold-responsive proteins were associated with various aspects of chloroplast physiology, and 56 % of them were predicted to be located in the chloroplasts, implying that the cold stress tolerance of Phalaenopsis was achieved, at least partly, by regulation of chloroplast function. Moreover, the protein destination control, which was mediated by chaperones and proteases, plays an important role in tolerance to cold stress.

Influence of thylakoid membrane lipids on the structure and function of the plant photosystem II core complex

MGDG leads to a dimerization of isolated, monomeric PSII core complexes. SQDG and PG induce a detachment of CP43 from the PSII core, thereby disturbing the intrinsic PSII electron transport. The influence of the four thylakoid membrane lipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) on the structure and function of isolated monomeric photosystem (PS) II core complexes was investigated. Incubation with the negatively charged lipids SQDG and PG led to a loss of the long-wavelength 77 K fluorescence emission at 693 nm that is associated with the inner antenna proteins. The neutral galactolipids DGDG and MGDG had no or only minor effects on the fluorescence emission spectra of the PSII core complexes, respectively. Pigment analysis, absorption and 77 K fluorescence excitation spectroscopy showed that incubation with SQDG and PG led to an exposure of chlorophyll molecules to the surrounding medium followed by conversion to pheophytin under acidic conditions. Size-exclusion chromatography and polypeptide analysis corroborated the findings of the spectroscopic measurements and pigment analysis. They showed that the negatively charged lipid SQDG led to a dissociation of the inner antenna protein CP43 and the 27- and 25-kDa apoproteins of the light-harvesting complex II, that were also associated with a part of the PSII core complexes used in the present study. Incubation of PSII core complexes with MGDG, on the other hand, induced an almost complete dimerization of the monomeric PSII. Measurements of the fast PSII fluorescence induction demonstrated that MGDG and DGDG only had a minor influence on the reduction kinetics of plastoquinone QA and the artificial PSII electron acceptor 2,5-dimethyl-p-benzoquinone (DMBQ). SQDG and, to a lesser extent, PG perturbed the intrinsic PSII electron transport significantly.

Quality control of photosystem II: direct imaging of the changes in the thylakoid structure and distribution of FtsH proteases in spinach chloroplasts under light stress

Under light stress, the reaction center-binding protein D1 of PSII is photo-oxidatively damaged and removed from PSII complexes by proteases located in the chloroplast. A protease considered to be responsible for degradation of the damaged D1 protein is the metalloprotease FtsH. We showed previously that the active hexameric FtsH protease is abundant at the grana margin and the grana end membranes, and this homo-complex removes the photodamaged D1 protein in the grana. Here, we showed a change in the distribution of FtsH in spinach thylakoids during excessive illumination by transmission electron microscopy (TEM) and immunogold labeling of FtsH. The change in distribution of the protease was accompanied by structural changes to the thylakoids, which we detected using spinach leaves by TEM after chemical fixation of the samples. Quantitative analyses showed several characteristic changes in the structure of the thylakoids, including shrinkage of the grana, outward bending of the marginal portions of the thylakoids and an increase in the height of the grana stacks under excessive illumination. The increase in the height of the grana stacks may include swelling of the thylakoids and an increase in the partition gaps between the thylakoids. These data strongly suggest that excessive illumination induces partial unstacking of the thylakoids, which enables FtsH to access easily the photodamaged D1 protein. Finally three-dimensional tomography of the grana was recorded to observe the effect of light stress on the overall structure of the thylakoids.

Deciphering thylakoid sub-compartments using a mass spectrometry-based approach

Photosynthesis has shaped atmospheric and ocean chemistries and probably changed the climate as well, as oxygen is released from water as part of the photosynthetic process. In photosynthetic eukaryotes, this process occurs in the chloroplast, an organelle containing the most abundant biological membrane, the thylakoids. The thylakoids of plants and some green algae are structurally inhomogeneous, consisting of two main domains: the grana, which are piles of membranes gathered by stacking forces, and the stroma-lamellae, which are unstacked thylakoids connecting the grana. The major photosynthetic complexes are unevenly distributed within these compartments because of steric and electrostatic constraints. Although proteomic analysis of thylakoids has been instrumental to define its protein components, no extensive proteomic study of subthylakoid localization of proteins in the BBY (grana) and the stroma-lamellae fractions has been achieved so far. To fill this gap, we performed a complete survey of the protein composition of these thylakoid subcompartments using thylakoid membrane fractionations. We employed semiquantitative proteomics coupled with a data analysis pipeline and manual annotation to differentiate genuine BBY and stroma-lamellae proteins from possible contaminants. About 300 thylakoid (or potentially thylakoid) proteins were shown to be enriched in either the BBY or the stroma-lamellae fractions. Overall, present findings corroborate previous observations obtained for photosynthetic proteins that used nonproteomic approaches. The originality of the present proteomic relies in the identification of photosynthetic proteins whose differential distribution in the thylakoid subcompartments might explain already observed phenomenon such as LHCII docking. Besides, from the present localization results we can suggest new molecular actors for photosynthesis-linked activities. For instance, most PsbP-like subunits being differently localized in stroma-lamellae, these proteins could be linked to the PSI-NDH complex in the context of cyclic electron flow around PSI. In addition, we could identify about a hundred new likely minor thylakoid (or chloroplast) proteins, some of them being potential regulators of the chloroplast physiology.

Thylakoid FtsH protease contributes to photosystem II and cytochrome b6f remodeling in Chlamydomonas reinhardtii under stress conditions

FtsH is the major thylakoid membrane protease found in organisms performing oxygenic photosynthesis. Here, we show that FtsH from Chlamydomonas reinhardtii forms heterooligomers comprising two subunits, FtsH1 and FtsH2. We characterized this protease using FtsH mutants that we identified through a genetic suppressor approach that restored phototrophic growth of mutants originally defective for cytochrome b6f accumulation. We thus extended the spectrum of FtsH substrates in the thylakoid membranes beyond photosystem II, showing the susceptibility of cytochrome b6f complexes (and proteins involved in the ci heme binding pathway to cytochrome b6) to FtsH. We then show how FtsH is involved in the response of C. reinhardtii to macronutrient stress. Upon phosphorus starvation, photosynthesis inactivation results from an FtsH-sensitive photoinhibition process. In contrast, we identified an FtsH-dependent loss of photosystem II and cytochrome b6f complexes in darkness upon sulfur deprivation. The D1 fragmentation pattern observed in the latter condition was similar to that observed in photoinhibitory conditions, which points to a similar degradation pathway in these two widely different environmental conditions. Our experiments thus provide extensive evidence that FtsH plays a major role in the quality control of thylakoid membrane proteins and in the response of C. reinhardtii to light and macronutrient stress.

Architectural switch in plant photosynthetic membranes induced by light stress

Unavoidable side reactions of photosynthetic energy conversion can damage the water-splitting photosystem II (PSII) holocomplex embedded in the thylakoid membrane system inside chloroplasts. Plant survival is crucially dependent on an efficient molecular repair of damaged PSII realized by a multistep repair cycle. The PSII repair cycle requires a brisk lateral protein traffic between stacked grana thylakoids and unstacked stroma lamellae that is challenged by the tight stacking and low protein mobility in grana. We demonstrated that high light stress induced two main structural changes that work synergistically to improve the accessibility between damaged PSII in grana and its repair machinery in stroma lamellae: lateral shrinkage of grana diameter and increased protein mobility in grana thylakoids. It follows that high light stress triggers an architectural switch of the thylakoid network that is advantageous for swift protein repair. Studies of the thylakoid kinase mutant stn8 and the double mutant stn7/8 demonstrate the central role of protein phosphorylation for the structural alterations. These findings are based on the elaboration of mathematical tools for analyzing confocal laser-scanning microscopic images to study changes in the sophisticated thylakoid architecture in intact protoplasts.

Identifying core features of adaptive metabolic mechanisms for chronic heat stress attenuation contributing to systems robustness

The contribution of metabolism to heat stress may play a significant role in defining robustness and recovery of systems; either by providing the energy and metabolites required for cellular homeostasis, or through the generation of protective osmolytes. However, the mechanisms by which heat stress attenuation could be adapted through metabolic processes as a stabilizing strategy against thermal stress are still largely unclear. We address this issue through metabolomic and transcriptomic profiles for populations along a thermal cline where two seagrass species, Zostera marina and Zostera noltii, were found in close proximity. Significant changes captured by these profile comparisons could be detected, with a larger response magnitude observed in northern populations to heat stress. Sucrose, fructose, and myo-inositol were identified to be the most responsive of the 29 analyzed organic metabolites. Many key enzymes in the Calvin cycle, glycolysis and pentose phosphate pathways also showed significant differential expression. The reported comparison suggests that adaptive mechanisms are involved through metabolic pathways to dampen the impacts of heat stress, and interactions between the metabolome and proteome should be further investigated in systems biology to understand robust design features against abiotic stress.

HCF243 encodes a chloroplast-localized protein involved in the D1 protein stability of the arabidopsis photosystem II complex

Numerous auxiliary nuclear factors have been identified to be involved in the dynamics of the photosystem II (PSII) complex. In this study, we characterized the high chlorophyll fluorescence243 (hcf243) mutant of Arabidopsis (Arabidopsis thaliana), which shows higher chlorophyll fluorescence and is severely deficient in the accumulation of PSII supercomplexes compared with the wild type. The amount of core subunits was greatly decreased, while the outer antenna subunits and other subunits were hardly affected in hcf243. In vivo protein-labeling experiments indicated that the synthesis rate of both D1 and D2 proteins decreased severely in hcf243, whereas no change was found in the rate of other plastid-encoded proteins. Furthermore, the degradation rate of the PSII core subunit D1 protein is higher in hcf243 than in the wild type, and the assembly of PSII is retarded significantly in the hcf243 mutant. HCF243, a nuclear gene, encodes a chloroplast protein that interacts with the D1 protein. HCF243 homologs were identified in angiosperms with one or two copies but were not found in lower plants and prokaryotes. These results suggest that HCF243, which arose after the origin of the higher plants, may act as a cofactor to maintain the stability of D1 protein and to promote the subsequent assembly of the PSII complex.

Thylakoid protein phosphorylation in dynamic regulation of photosystem II in higher plants

In higher plants, the photosystem (PS) II core and its several light harvesting antenna (LHCII) proteins undergo reversible phosphorylation cycles according to the light intensity. High light intensity induces strong phosphorylation of the PSII core proteins and suppresses the phosphorylation level of the LHCII proteins. Decrease in light intensity, in turn, suppresses the phosphorylation of PSII core, but strongly induces the phosphorylation of LHCII. Reversible and differential phosphorylation of the PSII-LHCII proteins is dependent on the interplay between the STN7 and STN8 kinases, and the respective phosphatases. The STN7 kinase phosphorylates the LHCII proteins and to a lesser extent also the PSII core proteins D1, D2 and CP43. The STN8 kinase, on the contrary, is rather specific for the PSII core proteins. Mechanistically, the PSII-LHCII protein phosphorylation is required for optimal mobility of the PSII-LHCII protein complexes along the thylakoid membrane. Physiologically, the phosphorylation of LHCII is a prerequisite for sufficient excitation of PSI, enabling the excitation and redox balance between PSII and PSI under low irradiance, when excitation energy transfer from the LHCII antenna to the two photosystems is efficient and thermal dissipation of excitation energy (NPQ) is minimised. The importance of PSII core protein phosphorylation is manifested under highlight when the photodamage of PSII is rapid and phosphorylation is required to facilitate the migration of damaged PSII from grana stacks to stroma lamellae for repair. The importance of thylakoid protein phosphorylation is highlighted under fluctuating intensity of light where the STN7 kinase dependent balancing of electron transfer is a prerequisite for optimal growth and development of the plant. This article is part of a Special Issue entitled: Photosystem II.