Protein turnover in plant biology

Accounting for photosystem I photoinhibition sheds new light on seasonal acclimation strategies of boreal conifers

The photosynthetic acclimation of boreal evergreen conifers is controlled by regulatory and photoprotective mechanisms that allow conifers to cope with extreme environmental changes. However, the underlying dynamics of photosystem II (PSII) and photosystem I (PSI) remain unresolved. Here, we investigated the dynamics of PSII and PSI during the spring recovery of photosynthesis in Pinus sylvestris and Picea abies using a combination of chlorophyll a fluorescence, P700 difference absorbance measurements, and quantification of key thylakoid protein abundances. In particular, we derived a new set of PSI quantum yield equations, correcting for the effects of PSI photoinhibition. Using the corrected equations, we found that the seasonal dynamics of PSII and PSI photochemical yields remained largely in balance, despite substantial seasonal changes in the stoichiometry of PSII and PSI core complexes driven by PSI photoinhibition. Similarly, the previously reported seasonal up-regulation of cyclic electron flow was no longer evident, after accounting for PSI photoinhibition. Overall, our results emphasize the importance of considering the dynamics of PSII and PSI to elucidate the seasonal acclimation of photosynthesis in overwintering evergreens. Beyond the scope of conifers, our corrected PSI quantum yields expand the toolkit for future studies aimed at elucidating the dynamic regulation of PSI.

Chemical Protein Crosslinking-Coupled Mass Spectrometry Reveals Interaction of LHCI with LHCII and LHCSR3 in Chlamydomonas reinhardtii

The photosystem I (PSI) of the green alga Chlamydomonas reinhardtii associates with 10 light-harvesting proteins (LHCIs) to form the PSI-LHCI complex. In the context of state transitions, two LHCII trimers bind to the PSAL, PSAH and PSAO side of PSI to produce the PSI-LHCI-LHCII complex. In this work, we took advantage of chemical crosslinking of proteins in conjunction with mass spectrometry to identify protein-protein interactions between the light-harvesting proteins of PSI and PSII. We detected crosslinks suggesting the binding of LHCBM proteins to the LHCA1-PSAG side of PSI as well as protein-protein interactions of LHCSR3 with LHCA5 and LHCA3. Our data indicate that the binding of LHCII to PSI is more versatile than anticipated and imply that LHCSR3 might be involved in the regulation of excitation energy transfer to the PSI core via LHCA5/LHCA3.

Toxicity of tigecycline on the freshwater microalga Scenedesmus obliquus: Photosynthetic and transcriptional responses

Tigecycline (TGC) is a new tetracycline antibiotic medication against multidrug-resistant bacteria. However, the toxicity of TGC to microalgae remains largely unknown. In this study, the toxicity of TGC on Scenedesmus obliquus was examined, focusing on changes in algal growth, photosynthetic activity, and transcriptome. According to an acute toxicity test, the IC10 and IC50 values were 0.72 mg/L and 4.15 mg/L, respectively. Analyses of photosynthetic efficiency and related parameters, such as light absorption, energy capture, and electron transport, identified a 35% perturbation in the IC50 group, while the IC10 group remained largely unaffected. Transcriptomic analysis showed that in the IC10 and IC50 treatment groups, there were 874 differentially expressed genes (DEGs) (220 upregulated and 654 downregulated) and 4289 DEGs (2660 upregulated and 1629 downregulated), respectively. Gene Ontology enrichment analysis showed that TGC treatment markedly affected photosynthesis, electron transport, and chloroplast functions. In the IC50 group, a clear upregulation of genes related to photosynthesis and chloroplast functions was observed, which could be an adaptive stress response. In the IC10 group, significant downregulation of DEGs involved in ribosomal pathways and peptide biosynthesis processes was observed. Kyoto Encyclopedia of Gene and Genomes enrichment analysis showed that treatment with TGC also disrupted energy production, protein synthesis, and metabolic processes in S. obliquus. Significant downregulation of key proteins related to Photosystem II was observed under the IC10 TGC treatment. Conversely, IC50 TGC treatment resulted in substantial upregulation across a broad array of photosystem-related proteins from both Photosystems II and I. IC10 and IC50 TGC treatments differentially influenced proteins involved in the photosynthetic electron transport process. This study emphasizes the potential risks of TGC pollution to microalgae, which contributes to a better understanding of the effects of antibiotic contamination in aquatic ecosystems.

Generation and physiological characterization of genome-edited Nicotiana benthamiana plants containing zeaxanthin as the only leaf xanthophyll

MAIN CONCLUSION

Simultaneous genome editing of the two homeologous LCYe and ZEP genes of Nicotiana benthamiana results in plants in which all xanthophylls are replaced by zeaxanthin. Plant carotenoids act both as photoreceptors and photoprotectants in photosynthesis and as precursors of apocarotenoids, which include signaling molecules such as abscisic acid (ABA). As dietary components, the xanthophylls lutein and zeaxanthin have photoprotective functions in the human macula. We developed transient and stable combinatorial genome editing methods, followed by direct LC-MS screening for zeaxanthin accumulation, for the simultaneous genome editing of the two homeologous Lycopene Epsilon Cyclase (LCYe) and the two Zeaxanthin Epoxidase (ZEP) genes present in the allopolyploid Nicotiana benthamiana genome. Editing of the four genes resulted in plants in which all leaf xanthophylls were substituted by zeaxanthin, but with different ABA levels and growth habits, depending on the severity of the ZEP1 mutation. In high-zeaxanthin lines, the abundance of the major photosystem II antenna LHCII was reduced with respect to wild-type plants and the LHCII trimeric state became unstable upon thylakoid solubilization. Consistent with the depletion in LHCII, edited plants underwent a compensatory increase in PSII/PSI ratios and a loss of the large-size PSII supercomplexes, while the level of PSI-LHCI supercomplex was unaffected. Reduced activity of the photoprotective mechanism NPQ was shown in high-zeaxanthin plants, while PSII photoinhibition was similar for all genotypes upon exposure to excess light, consistent with the antioxidant and photoprotective role of zeaxanthin in vivo.

Characterizing compensatory mechanisms in the absence of photoprotective qE in Chlamydomonas reinhardtii

Rapid fluctuations in the quantity and quality of natural light expose photosynthetic organisms to conditions when the capacity to utilize absorbed quanta is insufficient. These conditions can result in the production of reactive oxygen species and photooxidative damage. Non-photochemical quenching (NPQ) and alternative electron transport are the two most prominent mechanisms which synergistically function to minimize the overreduction of photosystems. In the green alga Chlamydomonas reinhardtii, the stress-related light-harvesting complex (LHCSR) is a required component for the rapid induction and relaxation of NPQ in the light-harvesting antenna. Here, we use simultaneous chlorophyll fluorescence and oxygen exchange measurements to characterize the acclimation of the Chlamydomonas LHCSR-less mutant (npq4lhcsr1) to saturating light conditions. We demonstrate that, in the absence of NPQ, Chlamydomonas does not acclimate to sinusoidal light through increased light-dependent oxygen consumption. We also show that the npq4lhcsr1 mutant has an increased sink capacity downstream of PSI and this energy flow is likely facilitated by cyclic electron transport. Furthermore, we show that the timing of additions of mitochondrial inhibitors has a major influence on plastid/mitochondrial coupling experiments.

Analysis of state 1-state 2 transitions by genome editing and complementation reveals a quenching component independent from the formation of PSI-LHCI-LHCII supercomplex in Arabidopsis thaliana

BACKGROUND

The light-harvesting antennae of photosystem (PS) I and PSII are pigment-protein complexes responsible of the initial steps of sunlight conversion into chemical energy. In natural environments plants are constantly confronted with the variability of the photosynthetically active light spectrum. PSII and PSI operate in series but have different optimal excitation wavelengths. The prompt adjustment of light absorption by photosystems is thus crucial to ensure efficient electron flow needed to sustain downstream carbon fixing reactions. Fast structural rearrangements equilibrate the partition of excitation pressure between PSII and PSI following the enrichment in the red (PSII-favoring) or far-red (PSI-favoring) spectra. Redox imbalances trigger state transitions (ST), a photoacclimation mechanism which involves the reversible phosphorylation/dephosphorylation of light harvesting complex II (LHCII) proteins by the antagonistic activities of the State Transition 7 (STN7) kinase/TAP38 phosphatase enzyme pair. During ST, a mobile PSII antenna pool associates with PSI increasing its absorption cross section. LHCII consists of assorted trimeric assemblies of Lhcb1, Lhcb2 and Lhcb3 protein isoforms (LHCII), several being substrates of STN7. However, the precise roles of Lhcb phosphorylation during ST remain largely elusive.

RESULTS

We inactivated the complete Lhcb1 and Lhcb2 gene clades in Arabidopsis thaliana and reintroduced either wild type Lhcb1.3 and Lhcb2.1 isoforms, respectively, or versions lacking N-terminal phosphorylatable residues proposed to mediate state transitions. While the substitution of Lhcb2.1 Thr-40 prevented the formation of the PSI-LHCI-LHCII complex, replacement of Lhcb1.3 Thr-38 did not affect the formation of this supercomplex, nor did influence the amplitude or kinetics of PSII fluorescence quenching upon state 1-state 2 transition.

CONCLUSIONS

Phosphorylation of Lhcb2 Thr-40 by STN7 alone accounts for ≈ 60% of PSII fluorescence quenching during state transitions. Instead, the presence of Thr-38 phosphosite in Lhcb1.3 was not required for the formation of the PSI-LHCI-LHCII supercomplex nor for re-equilibration of the plastoquinone redox state. The Lhcb2 phosphomutant was still capable of ≈ 40% residual fluorescence quenching, implying that a yet uncharacterized, STN7-dependent, component of state transitions, which is unrelated to Lhcb2 Thr-40 phosphorylation and to the formation of the PSI-LHCI-LHCII supercomplex, contributes to the equilibration of the PSI/PSII excitation pressure upon plastoquinone over-reduction.

STN7 is not essential for developmental acclimation of Arabidopsis to light intensity

Plants respond to changing light intensity in the short term through regulation of light harvesting, electron transfer, and metabolism to mitigate redox stress. A sustained shift in light intensity leads to a long-term acclimation response (LTR). This involves adjustment in the stoichiometry of photosynthetic complexes through de novo synthesis and degradation of specific proteins associated with the thylakoid membrane. The light-harvesting complex II (LHCII) serine/threonine kinase STN7 plays a key role in short-term light harvesting regulation and was also suggested to be crucial to the LTR. Arabidopsis plants lacking STN7 (stn7) shifted to low light experience higher photosystem II (PSII) redox pressure than the wild type or those lacking the cognate phosphatase TAP38 (tap38), while the reverse is true at high light, where tap38 suffers more. In principle, the LTR should allow optimisation of the stoichiometry of photosynthetic complexes to mitigate these effects. We used quantitative label-free proteomics to assess how the relative abundance of photosynthetic proteins varied with growth light intensity in wild-type, stn7, and tap38 plants. All plants were able to adjust photosystem I, LHCII, cytochrome b6 f, and ATP synthase abundance with changing white light intensity, demonstrating neither STN7 nor TAP38 is crucial to the LTR per se. However, stn7 plants grown for several weeks at low light (LL) or moderate light (ML) still showed high PSII redox pressure and correspondingly lower PSII efficiency, CO2 assimilation, and leaf area compared to wild-type and tap38 plants, hence the LTR is unable to fully ameliorate these symptoms. In contrast, under high light growth conditions the mutants and wild type behaved similarly. These data are consistent with the paramount role of STN7-dependent LHCII phosphorylation in tuning PSII redox state for optimal growth in LL and ML conditions.

Dynamic Regulation of the Light-Harvesting System through State Transitions in Land Plants and Green Algae

Photosynthesis constitutes the only known natural process that captures the solar energy to convert carbon dioxide and water into biomass. The primary reactions of photosynthesis are catalyzed by the photosystem II (PSII) and photosystem I (PSI) complexes. Both photosystems associate with antennae complexes whose main function is to increase the light-harvesting capability of the core. In order to maintain optimal photosynthetic activity under a constantly changing natural light environment, plants and green algae regulate the absorbed photo-excitation energy between PSI and PSII through processes known as state transitions. State transitions represent a short-term light adaptation mechanism for balancing the energy distribution between the two photosystems by relocating light-harvesting complex II (LHCII) proteins. The preferential excitation of PSII (state 2) results in the activation of a chloroplast kinase which in turn phosphorylates LHCII, a process followed by the release of phosphorylated LHCII from PSII and its migration to PSI, thus forming the PSI-LHCI-LHCII supercomplex. The process is reversible, as LHCII is dephosphorylated and returns to PSII under the preferential excitation of PSI. In recent years, high-resolution structures of the PSI-LHCI-LHCII supercomplex from plants and green algae were reported. These structural data provide detailed information on the interacting patterns of phosphorylated LHCII with PSI and on the pigment arrangement in the supercomplex, which is critical for constructing the excitation energy transfer pathways and for a deeper understanding of the molecular mechanism of state transitions progress. In this review, we focus on the structural data of the state 2 supercomplex from plants and green algae and discuss the current state of knowledge concerning the interactions between antenna and the PSI core and the potential energy transfer pathways in these supercomplexes.

Single chloroplast in folio imaging sheds light on photosystem energy redistribution during state transitions

Oxygenic photosynthesis is driven by light absorption in photosystem I (PSI) and photosystem II (PSII). A balanced excitation pressure between PSI and PSII is required for optimal photosynthetic efficiency. State transitions serve to keep this balance. If PSII is overexcited in plants and green algae, a mobile pool of light-harvesting complex II (LHCII) associates with PSI, increasing its absorption cross-section and restoring the excitation balance. This is called state 2. Upon PSI overexcitation, this LHCII pool moves to PSII, leading to state 1. Whether the association/dissociation of LHCII with the photosystems occurs between thylakoid grana and thylakoid stroma lamellae during state transitions or within the same thylakoid region remains unclear. Furthermore, although state transitions are thought to be accompanied by changes in thylakoid macro-organization, this has never been observed directly in functional leaves. In this work, we used confocal fluorescence lifetime imaging to quantify state transitions in single Arabidopsis (Arabidopsis thaliana) chloroplasts in folio with sub-micrometer spatial resolution. The change in excitation-energy distribution between PSI and PSII was investigated at a range of excitation wavelengths between 475 and 665 nm. For all excitation wavelengths, the PSI/(PSI + PSII) excitation ratio was higher in state 2 than in state 1. We next imaged the local PSI/(PSI + PSII) excitation ratio for single chloroplasts in both states. The data indicated that LHCII indeed migrates between the grana and stroma lamellae during state transitions. Finally, fluorescence intensity images revealed that thylakoid macro-organization is largely unaffected by state transitions. This single chloroplast in folio imaging method will help in understanding how plants adjust their photosynthetic machinery to ever-changing light conditions.

Spectral diversity of photosystem I from flowering plants

Photosystem I and II (PSI and PSII) work together to convert solar energy into chemical energy. Whilst a lot of research has been done to unravel variability of PSII fluorescence in response to biotic and abiotic factors, the contribution of PSI to in vivo fluorescence measurements has often been neglected or considered to be constant. Furthermore, little is known about how the absorption and emission properties of PSI from different plant species differ. In this study, we have isolated PSI from five plant species and compared their characteristics using a combination of optical and biochemical techniques. Differences have been identified in the fluorescence emission spectra and at the protein level, whereas the absorption spectra were virtually the same in all cases. In addition, the emission spectrum of PSI depends on temperature over a physiologically relevant range from 280 to 298 K. Combined, our data show a critical comparison of the absorption and emission properties of PSI from various plant species.

Structure-function correlations and system dynamics in oxygenic photosynthesis: classical perspectives and murburn precepts

HIGHLIGHTS

Contemporary beliefs on oxygenic photosynthesis are critiqued.Murburn model is suggested as an alternative explanation.In the new model, diffusible reactive species are the main protagonists.All pigments are deemed photo-redox active in the new stochastic mechanism.NADPH synthesis occurs via simple electron transfers, not via elaborate ETC.Oxygenesis is delocalized and not just centered at Mn-Complex.Energetics of murburn proposal for photophosphorylation is provided.The proposal ushers in a paradigm shift in photosynthesis research.

Plants acclimate to Photosystem I photoinhibition by readjusting the photosynthetic machinery

Photosynthetic light reactions require strict regulation under dynamic environmental conditions. Still, depending on environmental constraints, photoinhibition of Photosystem (PSII) or PSI occurs frequently. Repair of photodamaged PSI, in sharp contrast to that of PSII, is extremely slow and leads to a functional imbalance between the photosystems. Slow PSI recovery prompted us to take advantage of the PSI-specific photoinhibition treatment and investigate whether the imbalance between functional PSII and PSI leads to acclimation of photosynthesis to PSI-limited conditions, either by short-term or long-term acclimation mechanisms as tested immediately after the photoinhibition treatment or after 24 h recovery in growth conditions, respectively. Short-term acclimation mechanisms were induced directly upon inhibition, including thylakoid protein phosphorylation that redirects excitation energy to PSI as well as changes in the feedback regulation of photosynthesis, which relaxed photosynthetic control and excitation energy quenching. Longer-term acclimation comprised reprogramming of the stromal redox system and an increase in ATP synthase and Cytochrome b₆ f abundance. Acclimation to PSI-limited conditions restored the CO₂ assimilation capacity of plants without major PSI repair. Response to PSI inhibition demonstrates that plants efficiently acclimate to changes occurring in the photosynthetic apparatus, which is likely a crucial component in plant acclimation to adverse environmental conditions.

A kaleidoscope of photosynthetic antenna proteins and their emerging roles

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts to the light available. High-resolution microscopy and spectroscopic studies on membrane dynamics demonstrate the crosstalk between antennae and other thylakoid membrane components. With the increased understanding of light-harvesting mechanisms and their regulation, efforts are focusing on the development of sustainable processes for effective conversion of sunlight into functional bio-products. The major challenge in this approach lies in the application of fundamental discoveries in light-harvesting systems for the improvement of plant or algal photosynthesis. Here, we underline some of the latest fundamental discoveries on the molecular mechanisms and regulation of light harvesting that can potentially be exploited for the optimization of photosynthesis.

Oxidative modification of LHC II associated with photosystem II and PS I-LHC I-LHC II membranes

Under aerobic conditions the production of Reactive Oxygen Species (ROS) by electron transport chains is unavoidable, and occurs in both autotrophic and heterotrophic organisms. In photosynthetic organisms both Photosystem II (PS II) and Photosystem I (PS I), in addition to the cytochrome b₆/f complex, are demonstrated sources of ROS. All of these membrane protein complexes exhibit oxidative damage when isolated from field-grown plant material. An additional possible source of ROS in PS I and PS II is the distal, chlorophyll-containing light-harvesting array LHC II, which is present in both photosystems. These serve as possible sources of ¹O₂ produced by the interaction of ³O₂ with ³chl* produced by intersystem crossing. We have hypothesized that amino acid residues close to the sites of ROS generation will be more susceptible to oxidative modification than distant residues. In this study, we have identified oxidized amino acid residues in a subset of the spinach LHC II proteins (Lhcb1 and Lhcb2) that were associated with either PS II membranes (i.e. BBYs) or PS I-LHC I-LHC II membranes, both of which were isolated from field-grown spinach. We identified oxidatively modified residues by high-resolution tandem mass spectrometry. Interestingly, two different patterns of oxidative modification were evident for the Lhcb1 and Lhcb2 proteins from these different sources. In the LHC II associated with PS II membranes, oxidized residues were identified to be located on the stromal surface of Lhcb1 and, to a much lesser extent, Lhcb2. Relatively few oxidized residues were identified as buried in the hydrophobic core of these proteins. The LHC II associated with PS I-LHC I-LHC II membranes, however, exhibited fewer surface-oxidized residues but, rather a large number of oxidative modifications buried in the hydrophobic core regions of both Lhcb1 and Lhcb2, adjacent to the chlorophyll prosthetic groups. These results appear to indicate that ROS, specifically ¹O₂, can modify the Lhcb proteins associated with both photosystems and that the LHC II associated with PS II membranes represent a different population from the LHC II associated with PS I-LHC I-LHC II membranes.

Qualitative and quantitative evaluation of thylakoid complexes separated by Blue Native PAGE

BACKGROUND

Blue Native polyacrylamide gel electrophoresis (BN PAGE) followed by denaturing PAGE is a widely used, convenient and time efficient method to separate thylakoid complexes and study their composition, abundance, and interactions. Previous analyses unravelled multiple monomeric and dimeric/oligomeric thylakoid complexes but, in certain cases, the separation of complexes was not proper. Particularly, the resolution of super- and megacomplexes, which provides important information on functional interactions, still remained challenging.

RESULTS

Using a detergent mixture of 1% (w/V) n-dodecyl-β-D-maltoside plus 1% (w/V) digitonin for solubilisation and 4.3-8% gel gradients for separation as methodological improvements in BN PAGE, several large photosystem (PS) I containing bands were detected. According to BN(/BN)/SDS PAGE and mass spectrometry analyses, these PSI bands proved to be PSI-NADH dehydrogenase-like megacomplexes more discernible in maize bundle sheath thylakoids, and PSI complexes with different light-harvesting complex (LHC) complements (PSI-LHCII, PSI-LHCII*) more abundant in mesophyll thylakoids of lincomycin treated maize. For quantitative determination of the complexes and their comparison across taxa and physiological conditions, sample volumes applicable to the gel, correct baseline determination of the densitograms, evaluation methods to resolve complexes running together, calculation of their absolute/relative amounts and distribution among their different forms are proposed.

CONCLUSIONS

Here we report our experience in Blue/Clear-Native polyacrylamide gel electrophoretic separation of thylakoid complexes, their identification, quantitative determination and comparison in different samples. The applied conditions represent a powerful methodology for the analysis of thylakoid mega- and supercomplexes.

Thylakoid Protein Phosphorylation in Chloroplasts

Because of their abundance and extensive phosphorylation, numerous thylakoid proteins stand out amongst the phosphoproteins of plants and algae. In particular, subunits of light-harvesting complex II (LHCII) and of photosystem II (PSII) are dynamically phosphorylated and dephosphorylated in response to light conditions and metabolic demands. These phosphorylations are controlled by evolutionarily conserved thylakoid protein kinases and counteracting protein phosphatases, which have distinct but partially overlapping substrate specificities. The best characterized are the kinases STATE TRANSITION 7 (STN7/STT7) and STATE TRANSITION 8 (STN8), and the antagonistic phosphatases PROTEIN PHOSPHATASE 1/THYLAKOID-ASSOCIATED PHOSPHATASE 38 (PPH1/TAP38) and PHOTOSYSTEM II CORE PHOSPHATASE (PBCP). The phosphorylation of LHCII is mainly governed by STN7 and PPH1/TAP38 in plants. LHCII phosphorylation is essential for state transitions, a regulatory feedback mechanism that controls the allocation of this antenna to either PSII or PSI, and thus maintains the redox balance of the electron transfer chain. Phosphorylation of several core subunits of PSII, regulated mainly by STN8 and PBCP, correlates with changes in thylakoid architecture, the repair cycle of PSII after photodamage as well as regulation of light harvesting and of alternative routes of photosynthetic electron transfer. Other kinases, such as the PLASTID CASEIN KINASE II (pCKII), also intervene in thylakoid protein phosphorylation and take part in the chloroplast kinase network. While some features of thylakoid phosphorylation were conserved through the evolution of photosynthetic eukaryotes, others have diverged in different lineages possibly as a result of their adaptation to varied environments.

Dynamic Changes in Protein-Membrane Association for Regulating Photosynthetic Electron Transport

Photosynthesis has to work efficiently in contrasting environments such as in shade and full sun. Rapid changes in light intensity and over-reduction of the photosynthetic electron transport chain cause production of reactive oxygen species, which can potentially damage the photosynthetic apparatus. Thus, to avoid such damage, photosynthetic electron transport is regulated on many levels, including light absorption in antenna, electron transfer reactions in the reaction centers, and consumption of ATP and NADPH in different metabolic pathways. Many regulatory mechanisms involve the movement of protein-pigment complexes within the thylakoid membrane. Furthermore, a certain number of chloroplast proteins exist in different oligomerization states, which temporally associate to the thylakoid membrane and modulate their activity. This review starts by giving a short overview of the lipid composition of the chloroplast membranes, followed by describing supercomplex formation in cyclic electron flow. Protein movements involved in the various mechanisms of non-photochemical quenching, including thermal dissipation, state transitions and the photosystem II damage–repair cycle are detailed. We highlight the importance of changes in the oligomerization state of VIPP and of the plastid terminal oxidase PTOX and discuss the factors that may be responsible for these changes. Photosynthesis-related protein movements and organization states of certain proteins all play a role in acclimation of the photosynthetic organism to the environment.

Supramolecular associations between atypical oxidative phosphorylation complexes of Euglena gracilis

In vivo associations of respiratory complexes forming higher supramolecular structures are generally accepted nowadays. Supercomplexes (SC) built by complexes I, III and IV and the so-called respirasome (I/III₂/IV) have been described in mitochondria from several model organisms (yeasts, mammals and green plants), but information is scarce in other lineages. Here we studied the supramolecular associations between the complexes I, III, IV and V from the secondary photosynthetic flagellate Euglena gracilis with an approach that involves the extraction with several mild detergents followed by native electrophoresis. Despite the presence of atypical subunit composition and additional structural domains described in Euglena complexes I, IV and V, canonical associations into III₂/IV, III₂/IV₂ SCs and I/III₂/IV respirasome were observed together with two oligomeric forms of the ATP synthase (V₂ and V₄). Among them, III₂/IV SC could be observed by electron microscopy. The respirasome was further purified by two-step liquid chromatography and showed in-vitro oxygen consumption independent of the addition of external cytochrome c.

Composition, phosphorylation and dynamic organization of photosynthetic protein complexes in plant thylakoid membrane

The photosystems (PS), catalyzing the photosynthetic reactions of higher plants, are unevenly distributed in the thylakoid membrane: PSII, together with its light harvesting complex (LHC)II, is enriched in the appressed grana stacks, while PSI-LHCI resides in the non-appressed stroma thylakoids, which wind around the grana stacks. The two photosystems interact in a third membrane domain, the grana margins, which connect the grana and stroma thylakoids and allow the loosely bound LHCII to serve as an additional antenna for PSI. The light harvesting is balanced by reversible phosphorylation of LHCII proteins. Nevertheless, light energy also damages PSII and the repair process is regulated by reversible phosphorylation of PSII core proteins. Here, we discuss the detailed composition and organization of PSII-LHCII and PSI-LHCI (super)complexes in the thylakoid membrane of angiosperm chloroplasts and address the role of thylakoid protein phosphorylation in dynamics of the entire protein complex network of the photosynthetic membrane. Finally, we scrutinize the phosphorylation-dependent dynamics of the protein complexes in context of thylakoid ultrastructure and present a model on the reorganization of the entire thylakoid network in response to changes in thylakoid protein phosphorylation.

Beyond 'seeing is believing': the antenna size of the photosystems in vivo

Photosystems I and II are the central components of the solar energy conversion machinery in oxygenic photosynthesis. They are large functional units embedded in the photosynthetic membranes, where they harvest light and use its energy to drive electrons from water to NADPH. Their composition and organization change in response to different environmental conditions, making these complexes dynamic units. Some of the interactions between subunits survive purification, resulting in the well-defined structures that were recently resolved by cryo-electron microscopy. Other interactions instead are weak, preventing the possibility of isolating and thus studying these complexes in vitro. This review focuses on these supercomplexes of vascular plants, which at the moment cannot be 'seen' but that represent functional units in vivo.

Light harvesting in oxygenic photosynthesis: Structural biology meets spectroscopy

Oxygenic photosynthesis is the main process that drives life on earth. It starts with the harvesting of solar photons that, after transformation into electronic excitations, lead to charge separation in the reaction centers of photosystems I and II (PSI and PSII). These photosystems are large, modular pigment-protein complexes that work in series to fuel the formation of carbohydrates, concomitantly producing molecular oxygen. Recent advances in cryo-electron microscopy have enabled the determination of PSI and PSII structures in complex with light-harvesting components called "supercomplexes" from different organisms at near-atomic resolution. Here, we review the structural and spectroscopic aspects of PSI and PSII from plants and algae that directly relate to their light-harvesting properties, with special attention paid to the pathways and efficiency of excitation energy transfer and the regulatory aspects.

Comparative analysis of thylakoid protein complexes in state transition mutants nsi and stn7: focus on PSI and LHCII

The photosynthetic machinery of plants can acclimate to changes in light conditions by balancing light-harvesting between the two photosystems (PS). This acclimation response is induced by the change in the redox state of the plastoquinone pool, which triggers state transitions through activation of the STN7 kinase and subsequent phosphorylation of light-harvesting complex II (LHCII) proteins. Phosphorylation of LHCII results in its association with PSI (state 2), whereas dephosphorylation restores energy allocation to PSII (state 1). In addition to state transition regulation by phosphorylation, we have recently discovered that plants lacking the chloroplast acetyltransferase NSI are also locked in state 1, even though they possess normal LHCII phosphorylation. This defect may result from decreased lysine acetylation of several chloroplast proteins. Here, we compared the composition of wild type (wt), stn7 and nsi thylakoid protein complexes involved in state transitions separated by Blue Native gel electrophoresis. Protein complex composition and relative protein abundances were determined by LC-MS/MS analyses using iBAQ quantification. We show that despite obvious mechanistic differences leading to defects in state transitions, no major differences were detected in the composition of PSI and LHCII between the mutants. Moreover, both stn7 and nsi plants show retarded growth and decreased PSII capacity under fluctuating light as compared to wt, while the induction of non-photochemical quenching under fluctuating light was much lower in both nsi mutants than in stn7.

Light-harvesting complex II is an antenna of photosystem I in dark-adapted plants

Photosystem I (PSI) is a major player in the light reactions of photosynthesis. In higher plants, it consists of a core complex and four external antennae, Lhca1-4 forming the PSI-light-harvesting complex I (LHCI) supercomplex. The protein and pigment composition as well as the spectroscopic properties of this complex are considered to be identical in different higher plant species. In addition to the four Lhca, a pool of mobile LHCII increases the antenna size of PSI under most light conditions. In this work, we have first investigated purified PSI complexes and then PSI in vivo upon long-term dark-adaptation of four well-studied plant species: Arabidopsis thaliana, Zea mays, Nicotiana tabacum and Hordeum vulgare. By performing time-resolved fluorescence measurements, we show that LHCII is associated with PSI also in a dark-adapted state in all the plant species investigated. The number of LHCII subunits per PSI is plant-dependent, varying between one and three. Furthermore, we show that the spectroscopic properties of PSI-LHCI supercomplexes differ in different plants.

Modeling the Role of LHCII-LHCII, PSII-LHCII, and PSI-LHCII Interactions in State Transitions

The light-dependent reactions of photosynthesis take place in the plant chloroplast thylakoid membrane, a complex three-dimensional structure divided into the stacked grana and unstacked stromal lamellae domains. Plants regulate the macro-organization of photosynthetic complexes within the thylakoid membrane to adapt to changing environmental conditions and avoid oxidative stress. One such mechanism is the state transition that regulates photosynthetic light harvesting and electron transfer. State transitions are driven by changes in the phosphorylation of light harvesting complex II (LHCII), which cause a decrease in grana diameter and stacking, a decrease in energetic connectivity between photosystem II (PSII) reaction centers, and an increase in the relative LHCII antenna size of photosystem I (PSI) compared to PSII. Phosphorylation is believed to drive these changes by weakening the intramembrane lateral PSII-LHCII and LHCII-LHCII interactions and the intermembrane stacking interactions between these complexes, while simultaneously increasing the affinity of LHCII for PSI. We investigated the relative roles and contributions of these three types of interaction to state transitions using a lattice-based model of the thylakoid membrane based on existing structural data, developing a novel algorithm to simulate protein complex dynamics. Monte Carlo simulations revealed that state transitions are unlikely to lead to a large-scale migration of LHCII from the grana to the stromal lamellae. Instead, the increased light harvesting capacity of PSI is largely due to the more efficient recruitment of LHCII already residing in the stromal lamellae into PSI-LHCII supercomplexes upon its phosphorylation. Likewise, the increased light harvesting capacity of PSII upon dephosphorylation was found to be driven by a more efficient recruitment of LHCII already residing in the grana into functional PSII-LHCII clusters, primarily driven by lateral interactions.

Isolation and characterization of a large photosystem I-light-harvesting complex II supercomplex with an additional Lhca1-a4 dimer in Arabidopsis

The biological conversion of light energy into chemical energy is performed by a flexible photosynthetic machinery located in the thylakoid membranes. Photosystems I and II (PSI and PSII) are the two complexes able to harvest light. PSI is the last complex of the electron transport chain and is composed of multiple subunits: the proteins building the catalytic core complex that are well conserved between oxygenic photosynthetic organisms, and, in green organisms, the membrane light-harvesting complexes (Lhc) necessary to increase light absorption. In plants, four Lhca proteins (Lhca1-4) make up the antenna system of PSI, which can be further extended to optimize photosynthesis by reversible binding of LHCII, the main antenna complex of photosystem II. Here, we used biochemistry and electron microscopy in Arabidopsis to reveal a previously unknown supercomplex of PSI with LHCII that contains an additional Lhca1-a4 dimer bound on the PsaB-PsaI-PsaH side of the complex. This finding contradicts recent structural studies suggesting that the presence of an Lhca dimer at this position is an exclusive feature of algal PSI. We discuss the features of the additional Lhca dimer in the large plant PSI-LHCII supercomplex and the differences with the algal PSI. Our work provides further insights into the intricate structural plasticity of photosystems.

Formation of a PSI-PSII megacomplex containing LHCSR and PsbS in the moss Physcomitrella patens

Mosses are one of the earliest land plants that diverged from fresh-water green algae. They are considered to have acquired a higher capacity for thermal energy dissipation to cope with dynamically changing solar irradiance by utilizing both the "algal-type" light-harvesting complex stress-related (LHCSR)-dependent and the "plant-type" PsbS-dependent mechanisms. It is hypothesized that the formation of photosystem (PS) I and II megacomplex is another mechanism to protect photosynthetic machinery from strong irradiance. Herein, we describe the analysis of the PSI-PSII megacomplex from the model moss, Physcomitrella patens, which was resolved using large-pore clear-native polyacrylamide gel electrophoresis (lpCN-PAGE). The similarity in the migration distance of the Physcomitrella PSI-PSII megacomplex to the Arabidopsis megacomplex shown during lpCN-PAGE suggested that the Physcomitrella PSI-PSII and Arabidopsis megacomplexes have similar structures. Time-resolved chlorophyll fluorescence measurements show that excitation energy was rapidly and efficiently transferred from PSII to PSI, providing evidence of an ordered association of the two photosystems. We also found that LHCSR and PsbS co-migrated with the Physcomitrella PSI-PSII megacomplex. The megacomplex showed pH-dependent chlorophyll fluorescence quenching, which may have been induced by LHCSR and/or PsbS proteins with the collaboration of zeaxanthin. We discuss the mechanism that regulates the energy distribution balance between two photosystems in Physcomitrella.

The relevance of dynamic thylakoid organisation to photosynthetic regulation

The higher plant chloroplast thylakoid membrane system performs the light-dependent reactions of photosynthesis. These provide the ATP and NADPH required for the fixation of CO₂ into biomass by the Calvin-Benson cycle and a range of other metabolic reactions in the stroma. Land plants are frequently challenged by fluctuations in their environment, such as light, nutrient and water availability, which can create a mismatch between the amounts of ATP and NADPH produced and the amounts required by the downstream metabolism. Left unchecked, such imbalances can lead to the production of reactive oxygen species that damage the plant and harm productivity. Fortunately, plants have evolved a complex range of regulatory processes to avoid or minimize such deleterious effects by controlling the efficiency of light harvesting and electron transfer in the thylakoid membrane. Generally the regulation of the light reactions has been studied and conceptualised at the microscopic level of protein-protein and protein-ligand interactions, however in recent years dynamic changes in the thylakoid macrostructure itself have been recognised to play a significant role in regulating light harvesting and electron transfer. Here we review the evidence for the involvement of macrostructural changes in photosynthetic regulation and review the techniques that brought this evidence to light.

The PSI-PSII Megacomplex in Green Plants

Energy dissipation is crucial for land and shallow-water plants exposed to direct sunlight. Almost all green plants dissipate excess excitation energy to protect the photosystem reaction centers, photosystem II (PSII) and photosystem I (PSI), and continue to grow under strong light. In our previous work, we reported that about half of the photosystem reaction centers form a PSI-PSII megacomplex in Arabidopsis thaliana, and that the excess energy was transferred from PSII to PSI fast. However, the physiological function and structure of the megacomplex remained unclear. Here, we suggest that high-light adaptable sun-plants accumulate the PSI-PSII megacomplex more than shade-plants. In addition, PSI of sun-plants has a deep trap to receive excitation energy, which is low-energy chlorophylls showing fluorescence maxima longer than 730 nm. This deep trap may increase the high-light tolerance of PSI by improving excitation energy dissipation. Electron micrographs suggest that one PSII dimer is directly sandwiched between two PSIs with 2-fold rotational symmetry in the basic form of the PSI-PSII megacomplex in green plants. This structure should enable fast energy transfer from PSII to PSI and allow energy in PSII to be dissipated via the deep trap in PSI.

The Kinase STATE TRANSITION 8 Phosphorylates Light Harvesting Complex II and Contributes to Light Acclimation in Arabidopsis thaliana

Phosphorylation of the light-harvesting complex II (LHCII) is a central trigger for the reorganization of the photosynthetic complexes in the thylakoid membrane during short-term light acclimation. The major kinase involved in LHCII phosphorylation is STATE TRANSITION 7 (STN7), and its activity is mostly counteracted by a thylakoid-associated phosphatase, PROTEIN PHOSPHATASE 1/THYLAKOID ASSOCIATED PHOSPHATASE 38 (PPH1/TAP38). This kinase/phosphatase pair responds to the redox status of the photosynthetic electron transport chain. In Arabidopsis thaliana, Lhcb1 and Lhcb2 subunits of the LHCII trimers are the major targets of phosphorylation and have different roles in the acclimation of the photosynthetic machinery. Another antagonistic kinase and phosphatase pair, STATE TRANSITION 8 (STN8) and PHOTOSYSTEM II PHOSPHATASE (PBCP) target a different set of thylakoid proteins. Here, we analyzed double, triple, and quadruple knockout mutants of these kinases and phosphatases. In multiple mutants, lacking STN7, in combination with one or both phosphatases, but not STN8, the phosphorylation of LHCII was partially restored. The recovered phosphorylation favors Lhcb1 over Lhcb2 and results in a better adaptation of the photosynthetic apparatus and increased plant growth under fluctuating light. This set of mutants allowed to unveil a contribution of STN8-dependent phosphorylation in the acclimation to rapid light variations.

Growth Light Environment Changes the Sensitivity of Photosystem I Photoinhibition Depending on Common Wheat Cultivars

Light is an important factor for determining photosynthetic performance in land plants. At high light intensity, land plants develop photosynthetic activity by increasing electron sinks, such as the Calvin cycle and photorespiration and photoprotective mechanisms in photosystem II (PSII), to effectively utilize light and protect them from photoinhibition. In addition to PSII, photosystem I (PSI) has a risk of undergoing photoinhibition under high light intensity because of the reactive oxygen species (ROS) produced within PSI. However, the acclimation response has hardly been evaluated in the relationship of PSI photoprotection to growth light. In this study, we studied the effect of growth light intensity on the photoprotective mechanisms in PSI using six wheat cultivars. To evaluate the susceptibility of PSI to its photoinhibition, we used the repetitive short-pulse (rSP) illumination method to cause O₂-dependent PSI photoinhibition. We found that PSI photoinhibition induced by rSP illumination was much more alleviated in wheat cultivars grown under high-light conditions compared to those grown under low-light conditions. Here, we observed that wheat plant grown under high-light conditions lowered the susceptibility of PSI to its photoinhibition compared to those grown under low-light conditions. Furthermore, the acclimation response toward PSI photoinhibition was significantly different among the studied wheat cultivars, although the quantum yields both of PSII and PSI were increased by high-light acclimation in all wheat cultivars as reported previously. Interestingly, we observed that total chlorophyll content in leaves correlated with the susceptibility of PSI to its photoinhibition. On the basis of these results, we suggest that high-light acclimation induces protection mechanisms against PSI photoinhibition in land plants, and the increase in the leaf chlorophyll content relates to the susceptibility of PSI photoinhibition in wheat plants.

Why is chlorophyll b only used in light-harvesting systems?

Chlorophylls (Chl) are important pigments in plants that are used to absorb photons and release electrons. There are several types of Chls but terrestrial plants only possess two of these: Chls a and b. The two pigments form light-harvesting Chl a/b-binding protein complexes (LHC), which absorb most of the light. The peak wavelengths of the absorption spectra of Chls a and b differ by c. 20 nm, and the ratio between them (the a/b ratio) is an important determinant of the light absorption efficiency of photosynthesis (i.e., the antenna size). Here, we investigated why Chl b is used in LHCs rather than other light-absorbing pigments that can be used for photosynthesis by considering the solar radiation spectrum under field conditions. We found that direct and diffuse solar radiation (PAR_dir and PAR_diff, respectively) have different spectral distributions, showing maximum spectral photon flux densities (SPFD) at c. 680 and 460 nm, respectively, during the daytime. The spectral absorbance spectra of Chls a and b functioned complementary to each other, and the absorbance peaks of Chl b were nested within those of Chl a. The absorption peak in the short wavelength region of Chl b in the proteinaceous environment occurred at c. 460 nm, making it suitable for absorbing the PAR_diff, but not suitable for avoiding the high spectral irradiance (SIR) waveband of PAR_dir. In contrast, Chl a effectively avoided the high SPFD and/or high SIR waveband. The absorption spectra of photosynthetic complexes were negatively correlated with SPFD spectra, but LHCs with low a/b ratios were more positively correlated with SIR spectra. These findings indicate that the spectra of the photosynthetic pigments and constructed photosystems and antenna proteins significantly align with the terrestrial solar spectra to allow the safe and efficient use of solar radiation.

A LHCB9-dependent photosystem I megacomplex induced under low light in Physcomitrella patens

Photosystem I of the moss Physcomitrella patens has special properties, including the capacity to undergo non-photochemical fluorescence quenching. We studied the organization of photosystem I under different light and carbon supply conditions in wild-type moss and in moss with the lhcb9 (light-harvesting complex) knockout genotype, which lacks an antenna protein endowed with red-shifted absorption forms. Wild-type moss, when grown on sugars and in low light, accumulated LHCB9 proteins and a large form of the photosystem I supercomplex, which, besides the canonical four LHCI subunits, included a LHCII trimer and four additional LHC monomers. The lhcb9 knockout produced an angiosperm-like photosystem I supercomplex with four LHCI subunits irrespective of the growth conditions. Growth in the presence of sublethal concentrations of electron transport inhibitors that caused oxidation or reduction of the plastoquinone pool prevented or promoted, respectively, the accumulation of LHCB9 and the formation of the photosystem I megacomplex. We suggest that LHCB9 is a key subunit regulating the antenna size of photosystem I and the ability to avoid the over-reduction of plastoquinone: this condition is potentially dangerous in the shaded and sunfleck-rich environment typical of mosses, whose plastoquinone pool is reduced by both photosystem II and the oxidation of sugar substrates.

AtrbohD functions downstream of ROP2 and positively regulates waterlogging response in Arabidopsis

NADPH oxidase AtrbohD plays very important roles in modulating many cellular processes through production of signal molecules reactive oxygen species in Arabidopsis. However, whether it regulates the response to waterlogging stress is unclear. In this report, we showed that expression of AtrbohD was markedly induced by waterlogging stress, and mutation in AtrbohD led to clear sensitivity of Arabidopsis plants to waterlogging stress. Moreover, waterlogging-promoted increases in alcohol dehydrogenase (ADH) activity, ADH1 expression and H2O2 accumulation were significantly attenuated in two mutant lines of AtrbohD. These results indicate that AtrbohD is required for Arabidopsis tolerance to waterlogging stress. Besides, GUS staining experiments revealed that disruption of small G protein ROP2 encoding gene evidently suppressed the increase of AtrbohD expression while defect of AtrbohD did not prominently affect the abundance enhancements of ROP2 transcripts under waterlogged conditions. Together, these data suggest that AtrbohD functions downstream of ROP2 to positively regulate the response to waterlogging stress in Arabidopsis.

Julius von Sachs' forgotten 1897-article: sexuality and gender in plants vs. humans

One hundred and fifty years ago, Julius von Sachs' (1832-1897) monumental Lehrbuch der Botanik (Textbook of Botany) was published, which signified the origin of physiological botany and its integration with evolutionary biology. Sachs regarded the physiology of photoautotrophic organisms as a sub-discipline of botany, and introduced a Darwinian perspective into the emerging plant sciences. Here, we summarize Sachs' achievements and his description of sexuality with respect to the cellular basis of plant and animal biparental reproduction. We reproduce and analyze a forgotten paper (Gutachten) of Sachs dealing with Die Akademische Frau (The Academic Woman), published during the year of his death on the question concerning gender equality in humans. Finally, we summarize his endorsement of woman's rights to pursue academic studies in the natural sciences at the University level, and conclude that Sachs was a humanist as well as a great scientist.

Loss of LHCI system affects LHCII re-distribution between thylakoid domains upon state transitions

LHCI, the peripheral antenna system of Photosystem I, includes four light-harvesting proteins (Lhca1-Lhca4) in higher plants, all of which are devoid in the Arabidopsis thaliana knock-out mutant ΔLhca. PSI absorption cross-section was reduced in the mutant, thus affecting the redox balance of the photosynthetic electron chain and resulting in a more reduced PQ with respect to the wild type. ΔLhca plants developed compensatory response by enhancing LHCII binding to PSI. However, the amplitude of state transitions, as measured from changes of chlorophyll fluorescence in vivo, was unexpectedly low than the high level of PSI-LHCII supercomplex established. In order to elucidate the reasons for discrepancy, we further analyzed state transition in ΔLhca plants. The STN7 kinase was fully active in the mutant as judged from up-regulation of LHCII phosphorylation in state II. Instead, the lateral heterogeneity of thylakoids was affected by lack of LHCI, with LHCII being enriched in stroma membranes with respect to the wild type. Re-distribution of this complex affected the overall fluorescence yield of thylakoids already in state I and minimized changes in RT fluorescence yield when LHCII did connect to PSI reaction center. We conclude that interpretation of chlorophyll fluorescence analysis of state transitions becomes problematic when applied to mutants whose thylakoid architecture is significantly modified with respect to the wild type.

Impairment of Lhca4, a subunit of LHCI, causes high accumulation of chlorophyll and the stay-green phenotype in rice

Chlorophyll is an essential molecule for acquiring light energy during photosynthesis. Mutations that result in chlorophyll retention during leaf senescence are called 'stay-green' mutants. One of the several types of stay-green mutants, Type E, accumulates high levels of chlorophyll in the pre-senescent leaves, resulting in delayed yellowing. We isolated delayed yellowing1-1 (dye1-1), a rice mutant whose yellowing is delayed in the field. dye1-1 accumulated more chlorophyll than the wild-type in the pre-senescent and senescent leaves, but did not retain leaf functionality in the 'senescent green leaves', suggesting that dye1-1 is a Type E stay-green mutant. Positional cloning revealed that DYE1 encodes Lhca4, a subunit of the light-harvesting complex I (LHCI). In dye1-1, amino acid substitution occurs at the location of a highly conserved amino acid residue involved in pigment binding; indeed, a severely impaired structure of the PSI-LHCI super-complex in dye1-1 was observed in a blue native PAGE analysis. Nevertheless, the biomass and carbon assimilation rate of dye1-1 were comparable to those in the wild-type. Interestingly, Lhcb1, a trimeric LHCII protein, was highly accumulated in dye1-1, in the chlorophyll-protein complexes. The high accumulation of LHCII in the LHCI mutant dye1 suggests a novel functional interaction between LHCI and LHCII.

Dynamic thylakoid stacking regulates the balance between linear and cyclic photosynthetic electron transfer

Upon transition of plants from darkness to light the initiation of photosynthetic linear electron transfer (LET) from H₂O to NADP⁺ precedes the activation of CO₂ fixation, creating a lag period where cyclic electron transfer (CET) around photosystem I (PSI) has an important protective role. CET generates ΔpH without net reduced NADPH formation, preventing overreduction of PSI via regulation of the cytochrome b ₆ f (cytb ₆ f) complex and protecting PSII from overexcitation by inducing non-photochemical quenching. The dark-to-light transition also provokes increased phosphorylation of light-harvesting complex II (LHCII). However, the relationship between LHCII phosphorylation and regulation of the LET/CET balance is not understood. Here, we show that the dark-to-light changes in LHCII phosphorylation profoundly alter thylakoid membrane architecture and the macromolecular organization of the photosynthetic complexes, without significantly affecting the antenna size of either photosystem. The grana diameter and number of membrane layers per grana are decreased in the light while the number of grana per chloroplast is increased, creating a larger contact area between grana and stromal lamellae. We show that these changes in thylakoid stacking regulate the balance between LET and CET pathways. Smaller grana promote more efficient LET by reducing the diffusion distance for the mobile electron carriers plastoquinone and plastocyanin, whereas larger grana enhance the partition of the granal and stromal lamellae plastoquinone pools, enhancing the efficiency of CET and thus photoprotection by non-photochemical quenching.

Improved recombinant protein production in Arabidopsis thaliana

Production and isolation of recombinant proteins are key steps in modern Molecular Biology. Expression vectors and platforms for various hosts, including both prokaryotic and eukaryotic systems, have been used. In basic plant research, Arabidopsis thaliana is the central model for which a wealth of genetic and genomic resources is available, and enormous knowledge has been accumulated over the past years - especially since elucidation of its genome in 2000. However, until recently an Arabidopsis platform had been lacking for preparative-scale production of homologous recombinant proteins. We recently established an Arabidopsis-based super-expression system, and used it for a structural pilot study of a multi-subunit integral membrane protein complex. This review summarizes the benefits and further potential of the model plant system for protein productions.

ABBREVIATIONS

Nb, Nicotiana benthamiana; OT, oligosaccharyltransferase.

Proteomic characterization of hierarchical megacomplex formation in Arabidopsis thylakoid membrane

Conversion of solar energy into chemical energy in plant chloroplasts concomitantly modifies the thylakoid architecture and hierarchical interactions between pigment-protein complexes. Here, the thylakoids were isolated from light-acclimated Arabidopsis leaves and investigated with respect to the composition of the thylakoid protein complexes and their association into higher molecular mass complexes, the largest one comprising both photosystems (PSII and PSI) and light-harvesting chlorophyll a/b-binding complexes (LHCII). Because the majority of plant light-harvesting capacity is accommodated in LHCII complexes, their structural interaction with photosystem core complexes is extremely important for efficient light harvesting. Specific differences in the strength of LHCII binding to PSII core complexes and the formation of PSII supercomplexes are well characterized. Yet, the role of loosely bound L-LHCII that disconnects to a large extent during the isolation of thylakoid protein complexes remains elusive. Because L-LHCII apparently has a flexible role in light harvesting and energy dissipation, depending on environmental conditions, its close interaction with photosystems is a prerequisite for successful light harvesting in vivo. Here, to reveal the labile and fragile light-dependent protein interactions in the thylakoid network, isolated membranes were subjected to sequential solubilization using detergents with differential solubilization capacity and applying strict quality control. Optimized 3D-lpBN-lpBN-sodium dodecyl sulfate-polyacrylamide gel electrophoresis system demonstrated that PSII-LHCII supercomplexes, together with PSI complexes, hierarchically form larger megacomplexes via interactions with L-LHCII trimers. The polypeptide composition of LHCII trimers and the phosphorylation of Lhcb1 and Lhcb2 were examined to determine the light-dependent supramolecular organization of the photosystems into megacomplexes.

Dual-Located WHIRLY1 Interacting with LHCA1 Alters Photochemical Activities of Photosystem I and Is Involved in Light Adaptation in Arabidopsis

Plastid-nucleus-located WHIRLY1 protein plays a role in regulating leaf senescence and is believed to associate with the increase of reactive oxygen species delivered from redox state of the photosynthetic electron transport chain. In order to make sure whether WHIRLY1 plays a role in photosynthesis, in this study, the performances of photosynthesis were detected in Arabidopsis whirly1 knockout (kowhy1) and plastid localized WHIRLY1 overexpression (oepWHY1) plants. Loss of WHIRLY1 leads to a higher photochemical quantum yield of photosystem I Y(I) and electron transport rate (ETR) and a lower non-photochemical quenching (NPQ) involved in the thermal dissipation of excitation energy of chlorophyll fluorescence than the wild type. Further analyses showed that WHIRLY1 interacts with Light-harvesting protein complex I (LHCA1) and affects the expression of genes encoding photosystem I (PSI) and light harvest complexes (LHCI). Moreover, loss of WHIRLY1 decreases chloroplast NAD(P)H dehydrogenase-like complex (NDH) activity and the accumulation of NDH supercomplex. Several genes encoding the PSI-NDH complexes are also up-regulated in kowhy1 and the whirly1whirly3 double mutant (ko1/3) but steady in oepWHY1 plants. However, under high light conditions (800 μmol m-2 s-1), both kowhy1 and ko1/3 plants show lower ETR than wild-type which are contrary to that under normal light condition. Moreover, the expression of several PSI-NDH encoding genes and ERF109 which is related to jasmonate (JA) response varied in kowhy1 under different light conditions. These results indicate that WHIRLY1 is involved in the alteration of ETR by affecting the activities of PSI and supercomplex formation of PSI with LHCI or NDH and may acting as a communicator between the plastids and the nucleus.

Deficiency of the Stroma-Lamellar Protein LIL8/PSB33 Affects Energy Transfer Around PSI in Arabidopsis

Light-harvesting-like (LIL) proteins are a group of proteins that share a consensus amino acid sequence with light-harvesting Chl-binding (LHC) proteins. We hypothesized that they might be involved in photosynthesis-related processes. In order to gain a better understanding of a potential role in photosynthesis-related processes, we examined the most recently identified LIL protein, LIL8/PSB33. Recently, it was suggested that this protein is an auxiliary PSII core protein which is involved in organization of the PSII supercomplex. However, we found that the majority of LIL8/PSB33 was localized in stroma lamellae, where PSI is predominant. Moreover, the PSI antenna sizes measured under visible light were slightly increased in the lil8 mutants which lack LIL8/PSB33 protein. Analysis of fluorescence decay kinetics and fluorescence decay-associated spectra indicated that energy transfer to quenching sites within PSI was partially hampered in these mutants. On the other hand, analysis of the steady-state fluorescence spectra in these mutants indicates that a population of LHCII is energetically disconnected from PSII. Taken together, we suggest that LIL8/PSB33 is involved in the fine-tuning of light harvesting and/or energy transfer around both photosystems.

Why we need to know the structure of phosphorylated chloroplast light-harvesting complex II

In oxygenic photosynthesis there are two 'light states' - adaptations of the photosynthetic apparatus to spectral composition that otherwise favours either photosystem I or photosystem II. In chloroplasts of green plants the transition to light state 2 depends on phosphorylation of apoproteins of a membrane-intrinsic antenna, the chlorophyll-a/b-binding, light-harvesting complex II (LHC II), and on the resulting redistribution of absorbed excitation energy from photosystem II to photosystem I. The transition to light state 1 reverses these events and requires a phospho-LHC II phosphatase. Current structures of LHC II reveal little about possible steric effects of phosphorylation. The surface-exposed N-terminal domain of an LHC II polypeptide contains its phosphorylation site and is disordered in its unphosphorylated form. A molecular recognition hypothesis proposes that state transitions are a consequence of movement of LHC II between binding sites on photosystems I and II. In state 1, LHC II forms part of the antenna of photosystem II. In state 2, a unique but as yet unidentified 3-D structure of phospho-LHC II may attach it instead to photosystem I. One possibility is that the LHC II N-terminus becomes ordered upon phosphorylation, adopting a local alpha-helical secondary structure that initiates changes in LHC II tertiary and quaternary structure that sever contact with photosystem II while securing contact with photosystem I. In order to understand redistribution of absorbed excitation energy in photosynthesis we need to know the structure of LHC II in its phosphorylated form, and in its complex with photosystem I.

Multiple LHCII antennae can transfer energy efficiently to a single Photosystem I

Photosystems I and II (PSI and PSII) work in series to drive oxygenic photosynthesis. The two photosystems have different absorption spectra, therefore changes in light quality can lead to imbalanced excitation of the photosystems and a loss in photosynthetic efficiency. In a short-term adaptation response termed state transitions, excitation energy is directed to the light-limited photosystem. In higher plants a special pool of LHCII antennae, which can be associated with either PSI or PSII, participates in these state transitions. It is known that one LHCII antenna can associate with the PsaH site of PSI. However, membrane fractions were recently isolated in which multiple LHCII antennae appear to transfer energy to PSI. We have used time-resolved fluorescence-streak camera measurements to investigate the energy transfer rates and efficiency in these membrane fractions. Our data show that energy transfer from LHCII to PSI is relatively slow. Nevertheless, the trapping efficiency in supercomplexes of PSI with ~2.4 LHCIIs attached is 94%. The absorption cross section of PSI can thus be increased with ~65% without having significant loss in quantum efficiency. Comparison of the fluorescence dynamics of PSI-LHCII complexes, isolated in a detergent or located in their native membrane environment, indicates that the environment influences the excitation energy transfer rates in these complexes. This demonstrates the importance of studying membrane protein complexes in their natural environment.

The function of LHCBM4/6/8 antenna proteins in Chlamydomonas reinhardtii

In eukaryotic autotrophs, photosystems are composed of a core moiety, hosting charge separation and electron transport reactions, and an antenna system, enhancing light harvesting and photoprotection. In Chlamydomonas reinhardtii, the major antenna of PSII is a heterogeneous trimeric complex made up of LHCBM1-LHCBM9 subunits. Despite high similarity, specific functions have been reported for several members including LHCBM1, 2, 7, and 9. In this work, we analyzed the function of LHCBM4 and LHCBM6 gene products in vitro by synthesizing recombinant apoproteins from individual sequences and refolding them with pigments. Additionally, we characterized knock-down strains in vivo for LHCBM4/6/8 genes. We show that LHCBM4/6/8 subunits could be found as a component of PSII supercomplexes with different sizes, although the largest pool was free in the membranes and poorly connected to PSII. Impaired accumulation of LHCBM4/6/8 caused a decreased LHCII content per PSII and a reduction in the amplitude of state 1-state 2 transitions. In addition, the reduction of LHCBM4/6/8 subunits caused a significant reduction of the Non-photochemical quenching activity and in the level of photoprotection.

Recent advances in understanding photosynthesis

Photosynthesis is central to all life on earth, providing not only oxygen but also organic compounds that are synthesized from atmospheric CO 2 and water using light energy as the driving force. The still-increasing world population poses a serious challenge to further enhance biomass production of crop plants. Crop yield is determined by various parameters, inter alia by the light energy conversion efficiency of the photosynthetic machinery. Photosynthesis can be looked at from different perspectives: (i) light reactions and carbon assimilation, (ii) leaves and canopy structure, and (ii) source-sink relationships. In this review, we discuss opportunities and prospects to increase photosynthetic performance at the different layers, taking into account the recent progress made in the respective fields.