The PSI-H subunit of photosystem I is essential for state transitions in plant photosynthesis

Immunophilin FKB20-2 participates in oligomerization of Photosystem I in Chlamydomonas

PSI is a sophisticated photosynthesis protein complex that fuels the light reaction of photosynthesis in algae and vascular plants. While the structure and function of PSI have been studied extensively, the dynamic regulation on PSI oligomerization and high light response is less understood. In this work, we characterized a high light-responsive immunophilin gene FKB20-2 (FK506-binding protein 20-2) required for PSI oligomerization and high light tolerance in Chlamydomonas (Chlamydomonas reinhardtii). Biochemical assays and 77-K fluorescence measurement showed that loss of FKB20-2 led to the reduced accumulation of PSI core subunits and abnormal oligomerization of PSI complexes and, particularly, reduced PSI intermediate complexes in fkb20-2. It is noteworthy that the abnormal PSI oligomerization was observed in fkb20-2 even under dark and dim light growth conditions. Coimmunoprecipitation, MS, and yeast 2-hybrid assay revealed that FKB20-2 directly interacted with the low molecular weight PSI subunit PsaG, which might be involved in the dynamic regulation of PSI-light-harvesting complex I supercomplexes. Moreover, abnormal PSI oligomerization caused accelerated photodamage to PSII in fkb20-2 under high light stress. Together, we demonstrated that immunophilin FKB20-2 affects PSI oligomerization probably by interacting with PsaG and plays pivotal roles during Chlamydomonas tolerance to high light.

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.

Structure of Photosystem I Supercomplex Isolated from a Chlamydomonas reinhardtii Cytochrome b6f Temperature-Sensitive Mutant

The unicellular green alga, Chlamydomonas reinhardtii, has been widely used as a model system to study photosynthesis. Its possibility to generate and analyze specific mutants has made it an excellent tool for mechanistic and biogenesis studies. Using negative selection of ultraviolet (UV) irradiation–mutated cells, we isolated a mutant (TSP9) with a single amino acid mutation in the Rieske protein of the cytochrome b6f complex. The W143R mutation in the petC gene resulted in total loss of cytochrome b6f complex function at the non-permissive temperature of 37 °C and recovery at the permissive temperature of 25 °C. We then isolated photosystem I (PSI) and photosystem II (PSII) supercomplexes from cells grown at the non-permissive temperature and determined the PSI structure with high-resolution cryogenic electron microscopy. There were several structural alterations compared with the structures obtained from wild-type cells. Our structural data suggest that the mutant responded by excluding the Lhca2, Lhca9, PsaL, and PsaH subunits. This structural alteration prevents state two transition, where LHCII migrates from PSII to bind to the PSI complex. We propose this as a possible response mechanism triggered by the TSP9 phenotype at the non-permissive temperature.

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.

Temperature mapping of non-photochemical quenching in Chlorella vulgaris

Light intensity and temperature independently impact all parts of the photosynthetic machinery in plants and algae. Yet to date, the vast majority of pulse amplitude modulated (PAM) chlorophyll a fluorescence measurements have been performed at well-defined light intensities, but rarely at well-defined temperatures. In this work, we show that PAM measurements performed at various temperatures produce vastly different results in the chlorophyte Chlorella vulgaris. Using a recently developed Phenoplate technique to map quantum yield of Photosystem II (Y(II)) and non-photochemical quenching (NPQ) as a function of temperature, we show that the fast-relaxing NPQ follows an inverse normal distribution with respect to temperature and appears insensitive to previous temperature acclimation. The slow-relaxing or residual NPQ after 5 minutes of dark recovery follows a normal distribution similar to Y(II) but with a peak in the higher temperature range. Surprisingly, higher slow- and fast-relaxing NPQ values were observed in high-light relative to low-light acclimated cultures. Y(II) values peaked at the adaptation temperature regardless of temperature or light acclimation. Our novel findings show the complete temperature working spectrum of Y(II) and how excess energy quenching is managed across a wide range of temperatures in the model microalgal species C. vulgaris. Finally, we draw attention to the fact that the effect of the temperature component in PAM measurements has been wildly underestimated, and results from experiments at room temperature can be misleading.

Long- and short-term acclimation of the photosynthetic apparatus to salinity in Chlamydomonas reinhardtii. The role of Stt7 protein kinase

Salt stress triggers an Stt7-mediated LHCII-phosphorylation signaling mechanism similar to light-induced state transitions. However, phosphorylated LHCII, after detaching from PSII, does not attach to PSI but self-aggregates instead. Salt is a major stress factor in the growth of algae and plants. Here, our study mainly focuses on the organization of the photosynthetic apparatus to the long-term responses of Chlamydomonas reinhardtii to elevated NaCl concentrations. We analyzed the physiological effects of salt treatment at a cellular, membrane, and protein level by microscopy, protein profile analyses, transcripts, circular dichroism spectroscopy, chlorophyll fluorescence transients, and steady-state and time-resolved fluorescence spectroscopy. We have ascertained that cells that were grown in high-salinity medium form palmelloids sphere-shaped colonies, where daughter cells with curtailed flagella are enclosed within the mother cell walls. Palmelloid formation depends on the presence of a cell wall, as it was not observed in a cell-wall-less mutant CC-503. Using the stt7 mutant cells, we show Stt7 kinase-dependent phosphorylation of light-harvesting complex II (LHCII) in both short- and long-term treatments of various NaCl concentrations-demonstrating NaCl-induced state transitions that are similar to light-induced state transitions. The grana thylakoids were less appressed (with higher repeat distances), and cells grown in 150 mM NaCl showed disordered structures that formed diffuse boundaries with the flanking stroma lamellae. PSII core proteins were more prone to damage than PSI. At high salt concentrations (100-150 mM), LHCII aggregates accumulated in the thylakoid membranes. Low-temperature and time-resolved fluorescence spectroscopy indicated that the stt7 mutant was more sensitive to salt stress, suggesting that LHCII phosphorylation has a role in the acclimation and protection of the photosynthetic apparatus.

State transition is quiet around pyrenoid and LHCII phosphorylation is not essential for thylakoid deformation in Chlamydomonas 137c

Photosynthetic organisms have developed a regulation mechanism called state transition (ST) to rapidly adjust the excitation balance between the two photosystems by light-harvesting complex II (LHCII) movement. Though many researchers have assumed coupling of the dynamic transformations of the thylakoid membrane with ST, evidence of that remains elusive. To clarify the above-mentioned coupling in a model organism Chlamydomonas, here we used two advanced microscope techniques, the excitation-spectral microscope (ESM) developed recently by us and the superresolution imaging based on structured-illumination microscopy (SIM). The ESM observation revealed ST-dependent spectral changes upon repeated ST inductions. Surprisingly, it clarified a less significant ST occurrence in the region surrounding the pyrenoid, which is a subcellular compartment specialized for the carbon-fixation reaction, than that in the other domains. Further, we found a species dependence of this phenomenon: 137c strain showed the significant intracellular inhomogeneity of ST occurrence, whereas 4A+ strain hardly did. On the other hand, the SIM observation resolved partially irreversible fine thylakoid transformations caused by the ST-inducing illumination. This fine, irreversible thylakoid transformation was also observed in the STT7 kinase-lacking mutant. This result revealed that the fine thylakoid transformation is not induced solely by the LHCII phosphorylation, suggesting the highly susceptible nature of the thylakoid ultrastructure to the photosynthetic light reactions.

Role of serine/threonine protein kinase STN7 in the formation of two distinct photosystem I supercomplexes in Physcomitrium patens

Reversible thylakoid protein phosphorylation provides most flowering plants with dynamic acclimation to short-term changes in environmental light conditions. Here, through generating Serine/Threonine protein kinase 7 (STN7)-depleted mutants in the moss Physcomitrella (Physcomitrium patens), we identified phosphorylation targets of STN7 kinase and their roles in short- and long-term acclimation of the moss to changing light conditions. Biochemical and mass spectrometry analyses revealed STN7-dependent phosphorylation of N-terminal Thr in specific Light-Harvesting Complex II (LHCII) trimer subunits (LHCBM2 and LHCBM4/8) and provided evidence that phospho-LHCBM accumulation is responsible for the assembly of two distinct Photosystem I (PSI) supercomplexes (SCs), both of which are largely absent in STN7-depleted mutants. Besides the canonical state transition complex (PSI-LHCI-LHCII), we isolated the larger moss-specific PSI-Large (PSI-LHCI-LHCB9-LHCII) from stroma-exposed thylakoids. Unlike PSI-LHCI-LHCII, PSI-Large did not demonstrate short-term dynamics for balancing the distribution of excitation energy between PSII and PSI. Instead, PSI-Large contributed to a more stable increase in PSI antenna size in Physcomitrella, except under prolonged high irradiance. Additionally, the STN7-depleted mutants revealed altered light-dependent phosphorylation of a monomeric antenna protein, LHCB6, whose phosphorylation displayed a complex regulation by multiple kinases. Collectively, the unique phosphorylation plasticity and dynamics of Physcomitrella monomeric LHCB6 and trimeric LHCBM isoforms, together with the presence of PSI SCs with different antenna sizes and responsiveness to light changes, reflect the evolutionary position of mosses between green algae and vascular plants, yet with clear moss-specific features emphasizing their adaptation to terrestrial low-light environments.

Regulation of photosynthetic light reaction proteins via reversible phosphorylation

The regulation of photosynthesis occurs at different levels including the control of nuclear and plastid genes transcription, RNA processing and translation, protein translocation, assemblies and their post translational modifications. Out of all these, post translational modification enables rapid response of plants towards changing environmental conditions. Among all post-translational modifications, reversible phosphorylation is known to play a crucial role in the regulation of light reaction of photosynthesis. Although, phosphorylation of PS II subunits has been extensively studied but not much attention is given to other photosynthetic complexes such as PS I, Cytochrome b6f complex and ATP synthase. Phosphorylation reaction is known to protect photosynthetic apparatus in challenging environment conditions such as high light, elevated temperature, high salinity and drought. Recent studies have explored the role of photosynthetic protein phosphorylation in conferring plant immunity against the rice blast disease. The evolution of phosphorylation of different subunits of photosynthetic proteins occurred along with the evolution of plant lineage for their better adaptation to the changing environment conditions. In this review, we summarize the progress made in the research field of phosphorylation of photosynthetic proteins and highlights the missing links that need immediate attention.

The role of light-harvesting complex I in excitation energy transfer from LHCII to photosystem I in Arabidopsis

Photosynthesis powers nearly all life on Earth. Light absorbed by photosystems drives the conversion of water and carbon dioxide into sugars. In plants, photosystem I (PSI) and photosystem II (PSII) work in series to drive the electron transport from water to NADP+. As both photosystems largely work in series, a balanced excitation pressure is required for optimal photosynthetic performance. Both photosystems are composed of a core and light-harvesting complexes (LHCI) for PSI and LHCII for PSII. When the light conditions favor the excitation of one photosystem over the other, a mobile pool of trimeric LHCII moves between both photosystems thus tuning their antenna cross-section in a process called state transitions. When PSII is overexcited multiple LHCIIs can associate with PSI. A trimeric LHCII binds to PSI at the PsaH/L/O site to form a well-characterized PSI-LHCI-LHCII supercomplex. The binding site(s) of the "additional" LHCII is still unclear, although a mediating role for LHCI has been proposed. In this work, we measured the PSI antenna size and trapping kinetics of photosynthetic membranes from Arabidopsis (Arabidopsis thaliana) plants. Membranes from wild-type (WT) plants were compared to those of the ΔLhca mutant that completely lacks the LHCI antenna. The results showed that "additional" LHCII complexes can transfer energy directly to the PSI core in the absence of LHCI. However, the transfer is about two times faster and therefore more efficient, when LHCI is present. This suggests LHCI mediates excitation energy transfer from loosely bound LHCII to PSI in WT plants.

Does silicon really matter for the photosynthetic machinery in plants…?

Silicon (Si) is known to alleviate the adverse impact of different abiotic and biotic stresses by different mechanisms including morphological, physiological, and genetic changes. Photosynthesis, one of the most important physiological processes in the plant is sensitive to different stress factors. Several studies have shown that Si ameliorates the stress effects on photosynthesis by protecting photosynthetic machinery and its function. In stressed plants, several photosynthesis-related processes including PSII maximum photochemical quantum yield (Fv/Fm), the yield of photosystem II (φPSII), electron transport rates (ETR), and photochemical quenching (qP) were observed to be regulated when supplemented with Si, which indicates that Si effectively protects the photosynthetic machinery. In addition, studies also suggested that Si is capable enough to maintain the uneven swelling, disintegrated, and missing thylakoid membranes caused during stress. Furthermore, several photosynthesis-related genes were also regulated by Si supplementation. Taking into account the key impact of Si on the evolutionarily conserved process of photosynthesis in plants, this review article is focused on the aspects of silicon and photosynthesis interrelationships during stress and signaling pathways. The assemblages of this discussion shall fulfill the lack of constructive literature related to the influence of Si on one of the most dynamic and important processes of plant life i.e. photosynthesis.

Transcriptional Analysis of Microcystis aeruginosa Co-Cultured with Algicidal Bacteria Brevibacillus laterosporus

Harmful algal blooms caused huge ecological damage and economic losses around the world. Controlling algal blooms by algicidal bacteria is expected to be an effective biological control method. The current study investigated the molecular mechanism of harmful cyanobacteria disrupted by algicidal bacteria. Microcystis aeruginosa was co-cultured with Brevibacillus laterosporus Bl-zj, and RNA-seq based transcriptomic analysis was performed compared to M. aeruginosa, which was cultivated separately. A total of 1706 differentially expressed genes were identified, which were mainly involved in carbohydrate metabolism, energy metabolism and amino acid metabolism. In the co-cultured group, the expression of genes mainly enriched in photosynthesis and oxidative phosphorylation were significantly inhibited. However, the expression of the genes related to fatty acid synthesis increased. In addition, the expression of the antioxidant enzymes, such as 2-Cys peroxiredoxin, was increased. These results suggested that B. laterosporus could block the electron transport by attacking the PSI system and complex I of M. aeruginosa, affecting the energy acquisition and causing oxidative damage. This further led to the lipid peroxidation of the microalgal cell membrane, resulting in algal death. The transcriptional analysis of algicidal bacteria in the interaction process can be combined to explain the algicidal mechanism in the future.

Silicon biology in crops under abiotic stress: A paradigm shift and cross-talk between genomics and proteomics

Silicon is a beneficial element to improve the biological process, growth, development, and crop productivity. The review mainly focuses on the advantage of crops supplemented with silicon, how Si alleviate abiotic stress as well as regulate the genes and proteins involved in metabolic and biological functions in plants. Abiotic stress causes damage to the proteins, nucleic acids, affect transpiration rate, stomatal conductance, alter the nutrient balance, and cell desiccation which could reduce the growth and development of the plants. To overcome from this problem researchers, focus on beneficial element like silicon to protect the plants against various abiotic stresses. The previous review reports are based on the application of silicon on salinity and drought stress, plant defense mechanism, the elevation of plant metabolism, enhancement of the biochemical and physiological properties, regulation of secondary metabolites and plant hormone. Here, we discuss about the silicon uptake and accumulation in plants, and silicon regulates the reactive oxygen species under abiotic stress, further we mainly focus on the genes and proteins which play a vital role in plants with silicon supplementation. The study can help the researchers to focus further on plants to improve the advancement in them under abiotic stress.

Transcriptome analysis revealed the regulation of gibberellin and the establishment of photosynthetic system promote rapid seed germination and early growth of seedling in pearl millet

BACKGROUND

Seed germination is the most important stage for the formation of a new plant. This process starts when the dry seed begins to absorb water and ends when the radicle protrudes. The germination rate of seed from different species varies. The rapid germination of seed from species that grow on marginal land allows seedlings to compete with surrounding species, which is also the guarantee of normal plant development and high yield. Pearl millet is an important cereal crop that is used worldwide, and it can also be used to extract bioethanol. Previous germination experiments have shown that pearl millet has a fast seed germination rate, but the molecular mechanisms behind pearl millet are unclear. Therefore, this study explored the expression patterns of genes involved in pearl millet growth from the germination of dry seed to the early growth stages.

RESULTS

Through the germination test and the measurement of the seedling radicle length, we found that pearl millet seed germinated after 24 h of swelling of the dry seed. Using transcriptome sequencing, we characterized the gene expression patterns of dry seed, water imbibed seed, germ and radicle, and found more differentially expressed genes (DEGs) in radicle than germ. Further analysis showed that different genome clusters function specifically at different tissues and time periods. Weighted gene co-expression network analysis (WGCNA) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that many genes that positively regulate plant growth and development are highly enriched and expressed, especially the gibberellin signaling pathway, which can promote seed germination. We speculated that the activation of these key genes promotes the germination of pearl millet seed and the growth of seedlings. To verify this, we measured the content of gibberellin and found that the gibberellin content after seed imbibition rose sharply and remained at a high level.

CONCLUSIONS

In this study, we identified the key genes that participated in the regulation of seed germination and seedling growth. The activation of key genes in these pathways may contribute to the rapid germination and growth of seed and seedlings in pearl millet. These results provided new insight into accelerating the germination rate and seedling growth of species with slow germination.

The key cyclic electron flow protein PGR5 associates with cytochrome b6f, and its function is partially influenced by the LHCII state transition

In plants and algae, PGR5-dependent cyclic electron flow (CEF) is an important regulator of acclimation to fluctuating environments, but how PGR5 participates in CEF is unclear. In this work, we analyzed two PGR5s in cucumber (Cucumis sativus L.) under different conditions and found that CsPGR5a played the dominant role in PGR5-dependent CEF. The results of yeast two-hybrid, biomolecular fluorescence complementation (BiFC), blue native PAGE, and coimmunoprecipitation (CoIP) assays showed that PGR5a interacted with PetC, Lhcb3, and PsaH. Furthermore, the intensity of the interactions was dynamic during state transitions, and the abundance of PGR5 attached to cyt b₆f decreased during the transition from state 1 to state 2, which revealed that the function of PGR5a is related to the state transition. We proposed that PGR5 is a small mobile protein that functions when attached to protein complexes.

Dynamic thylakoid stacking and state transitions work synergistically to avoid acceptor-side limitation of photosystem I

TAP38/STN7-dependent (de)phosphorylation of light-harvesting complex II (LHCII) regulates the relative excitation rates of photosystems I and II (PSI, PSII) (state transitions) and the size of the thylakoid grana stacks (dynamic thylakoid stacking). Yet, it remains unclear how changing grana size benefits photosynthesis and whether these two regulatory mechanisms function independently. Here, by comparing Arabidopsis wild-type, stn7 and tap38 plants with the psal mutant, which undergoes dynamic thylakoid stacking but lacks state transitions, we explain their distinct roles. Under low light, smaller grana increase the rate of PSI reduction and photosynthesis by reducing the diffusion distance for plastoquinol; however, this beneficial effect is only apparent when PSI/PSII excitation balance is maintained by state transitions or far-red light. Under high light, the larger grana slow plastoquinol diffusion and lower the equilibrium constant between plastocyanin and PSI, maximizing photosynthesis by avoiding PSI photoinhibition. Loss of state transitions in low light or maintenance of smaller grana in high light also both bring about a decrease in cyclic electron transfer and over-reduction of the PSI acceptor side. These results demonstrate that state transitions and dynamic thylakoid stacking work synergistically to regulate photosynthesis in variable light.

PSB33 protein sustains photosystem II in plant chloroplasts under UV-A light

Plants can quickly and dynamically respond to spectral and intensity variations of the incident light. These responses include activation of developmental processes, morphological changes, and photosynthetic acclimation that ensure optimal energy conversion and minimal photoinhibition. Plant adaptation and acclimation to environmental changes have been extensively studied, but many details surrounding these processes remain elusive. The photosystem II (PSII)-associated protein PSB33 plays a fundamental role in sustaining PSII as well as in the regulation of the light antenna in fluctuating light. We investigated how PSB33 knock-out Arabidopsis plants perform under different light qualities. psb33 plants displayed a reduction of 88% of total fresh weight compared to wild type plants when cultivated at the boundary of UV-A and blue light. The sensitivity towards UV-A light was associated with a lower abundance of PSII proteins, which reduces psb33 plants' capacity for photosynthesis. The UV-A phenotype was found to be linked to altered phytohormone status and changed thylakoid ultrastructure. Our results collectively show that PSB33 is involved in a UV-A light-mediated mechanism to maintain a functional PSII pool in the chloroplast.

Demethylation alters transcriptome profiling of buds and leaves in 'Kyoho' grape

BACKGROUND

Grape buds and leaves are directly associated with the physiology and metabolic activities of the plant, which is monitored by epigenetic modifications induced by environment and endogenous factors. Methylation is one of the epigenetic regulators that could be involved in DNA levels and affect gene expression in response to stimuli. Therefore, changes of gene expression profile in leaves and bud through inhibitors of DNA methylation provide a deep understanding of epigenetic effects in regulatory networks.

RESULTS

In this study, we carried out a transcriptome analysis of 'Kyoho' buds and leaves under 5-azacytidine (5-azaC) exposure and screened a large number of differentially expressed genes (DEGs). GO and KEGG annotations showed that they are mainly involved in photosynthesis, flavonoid synthesis, glutathione metabolism, and other metabolic processes. Functional enrichment analysis also provided a holistic perspective on the transcriptome profile when 5-azaC bound to methyltransferase and induced demethylation. Enrichment analysis of transcription factors (TFs) also showed that the MYB, C2H2, and bHLH families are involved in the regulation of responsive genes under epigenetic changes. Furthermore, hormone-related genes have also undergone significant changes, especially gibberellin (GA) and abscisic acid (ABA)-related genes that responded to bud germination. We also used protein-protein interaction network to determine hub proteins in response to demethylation.

CONCLUSIONS

These findings provide new insights into the establishment of molecular regulatory networks according to how methylation as an epigenetic modification alters transcriptome patterns in bud and leaves of grape.

Neoxanthin affects the stability of the C2 S2 M2 -type photosystem II supercomplexes and the kinetics of state transition in Arabidopsis

Neoxanthin (Neo), which is only bound to the peripheral antenna proteins of photosystem (PS) II, is a conserved carotenoid in all green plants. It has been demonstrated that Neo plays an important role in photoprotection and its deficiency fails to impact LHCII stability in vitro and indoor plant growth in vivo. Whether Neo is involved in maintaining the PSII complex structure or adaptive mechanisms for the everchanging environment has not yet been elucidated. In this study, the role of Neo in maintaining the structure and function of the PSII-LHCII supercomplexes was studied using Neo deficient Arabidopsis mutants. Our results show that Neo deficiency had little effect on the electron transport capacity and the plant fitness, but the PSII-LHCII supercomplexes were significantly impacted by the lack of Neo. In the absence of Neo, the M-type LHCII trimer cannot effectively associate with the C₂ S₂ -type PSII-LHCII supercomplexes even in moderate light conditions. Interestingly, Neo deficiency also leads to decreased PSII protein phosphorylation but rapid transition from state 1 to state 2. We suggest that Neo might enforce the interactions between LHCII and the minor antennas and that the absence of Neo makes M-type LHCII disassociate from the PSII complex, leading to the disassembly of the PSII-LHCII C₂ S₂ M₂ supercomplexes, which results in alterations in the phosphorylation patterns of the thylakoid photosynthetic proteins and the kinetics of state transition.

High light intensity aggravates latent manganese deficiency in maize

Manganese (Mn) plays an important role in the oxygen-evolving complex, where energy from light absorption is used for water splitting. Although changes in light intensity and Mn status can interfere with the functionality of the photosynthetic apparatus, the interaction between these two factors and the underlying mechanisms remain largely unknown. Here, maize seedlings were grown hydroponically and exposed to two different light intensities under Mn-sufficient or -deficient conditions. No visual Mn deficiency symptoms appeared even though the foliar Mn concentration in the Mn-deficient treatments was reduced to 2 µg g-1. However, the maximum quantum yield efficiency of PSII and the net photosynthetic rate declined significantly, indicating latent Mn deficiency. The reduction in photosynthetic performance by Mn depletion was further aggravated when plants were exposed to high light intensity. Integrated transcriptomic and proteomic analyses showed that a considerable number of genes encoding proteins in the photosynthetic apparatus were only suppressed by a combination of Mn deficiency and high light, thus indicating interactions between changes in Mn nutritional status and light intensity. We conclude that high light intensity aggravates latent Mn deficiency in maize by interfering with the abundance of PSII proteins.

Working day and night: plastid casein kinase 2 catalyses phosphorylation of proteins with diverse functions in light- and dark-adapted plastids

Casein kinase 2 is a ubiquitous protein kinase that has puzzled researchers for several decades because of its pleiotropic activity. Here, we set out to identify the in vivo targets of plastid casein kinase 2 (pCK2) in Arabidopsis thaliana. Survey phosphoproteome analyses were combined with targeted analyses with wild-type and pck2 knockdown mutants to identify potential pCK2 targets by their decreased phosphorylation state in the mutant. To validate potential substrates, we complemented the pck2 knockdown line with tandem affinity tag (TAP)-tagged pCK2 and found it to restore growth parameters, as well as many, but not all, putative pCK2-dependent phosphorylation events. We further performed a targeted analysis at the end-of-night to increase the specificity of target protein identification. This analysis confirmed light-independent phosphorylation of several pCK2 target proteins. Based on the aforementioned data, we define a set of in vivo pCK2-targets that span different chloroplast functions, such as metabolism, transcription, translation and photosynthesis. The pleiotropy of pCK2 functions is also manifested by altered state transition kinetics during short-term acclimation and significant alterations in the mutant metabolism, supporting its function in photosynthetic regulation. Thus, our data expand our understanding on chloroplast phosphorylation networks and provide insights into kinase networks in the regulation of chloroplast functions.

Antimicrobial solid media for screening non-sterile Arabidopsis thaliana seeds

Stable genetic transformation of plants is a low-efficiency process, and identification of positive transformants usually relies on screening for expression of a co-transformed marker gene. Often this involves germinating seeds on solid media containing a selection reagent. Germination on solid media requires surface sterilization of seeds and careful aseptic technique to prevent microbial contamination, but surface sterilization techniques are time consuming and can cause seed mortality if not performed carefully. We developed an antimicrobial cocktail that can be added to solid media to inhibit bacterial and fungal growth without impairing germination, allowing us to bypass the surface sterilization step. Adding a combination of terbinafine (1 μM) and timentin (200 mg l^-1 ) to Murashige and Skoog agar delayed the onset of observable microbial growth and did not affect germination of non-sterile seeds from 10 different wild-type and mutant Arabidopsis thaliana accessions. We named this antimicrobial solid medium "MSTT agar". Seedlings sown in non-sterile conditions could be maintained on MSTT agar for up to a week without observable contamination. This medium was compatible with rapid screening methods for hygromycin B, phosphinothricin (BASTA) and nourseothricin resistance genes, meaning that positive transformants can be identified from non-sterile seeds in as little as 4 days after stratification, and transferred to soil before the onset of visible microbial contamination. By using MSTT agar we were able to select genetic transformants on solid media without seed surface sterilization, eliminating a tedious and time-consuming step.

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.

Dual lysine and N-terminal acetyltransferases reveal the complexity underpinning protein acetylation

Protein acetylation is a highly frequent protein modification. However, comparatively little is known about its enzymatic machinery. N-α-acetylation (NTA) and ε-lysine acetylation (KA) are known to be catalyzed by distinct families of enzymes (NATs and KATs, respectively), although the possibility that the same GCN5-related N-acetyltransferase (GNAT) can perform both functions has been debated. Here, we discovered a new family of plastid-localized GNATs, which possess a dual specificity. All characterized GNAT family members display a number of unique features. Quantitative mass spectrometry analyses revealed that these enzymes exhibit both distinct KA and relaxed NTA specificities. Furthermore, inactivation of GNAT2 leads to significant NTA or KA decreases of several plastid proteins, while proteins of other compartments were unaffected. The data indicate that these enzymes have specific protein targets and likely display partly redundant selectivity, increasing the robustness of the acetylation process in vivo. In summary, this study revealed a new layer of complexity in the machinery controlling this prevalent modification and suggests that other eukaryotic GNATs may also possess these previously underappreciated broader enzymatic activities.

Structure and energy transfer pathways of the Dunaliella Salina photosystem I supercomplex

Oxygenic photosynthesis evolved more than 3 billion years ago in cyanobacteria. The increased complexity of photosystem I (PSI) became apparent from the high-resolution structures that were obtained for the complexes that were isolated from various organisms, ranging from cyanobacteria to plants. These complexes are all evolutionarily linked. In this paper, the researchers have uncovered the increased complexity of PSI in a single organism demonstrated by the coexistance of two distinct PSI compositions. The Large Dunaliella PSI contains eight additional subunits, six in PSI core and two light harvesting complexes. Two additional chlorophyll a molecules pertinent for efficient excitation energy transfer in state II transition were identified in PsaL and PsaO. Short distances between these newly identified chlorophylls correspond with fast excitation transfer rates previously reported during state II transition. The apparent PSI conformations could be a coping mechanism for the high salinity.

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.

Higher order photoprotection mutants reveal the importance of ΔpH-dependent photosynthesis-control in preventing light induced damage to both photosystem II and photosystem I

Although light is essential for photosynthesis, when in excess, it may damage the photosynthetic apparatus, leading to a phenomenon known as photoinhibition. Photoinhibition was thought as a light-induced damage to photosystem II; however, it is now clear that even photosystem I may become very vulnerable to light. One main characteristic of light induced damage to photosystem II (PSII) is the increased turnover of the reaction center protein, D1: when rate of degradation exceeds the rate of synthesis, loss of PSII activity is observed. With respect to photosystem I (PSI), an excess of electrons, instead of an excess of light, may be very dangerous. Plants possess a number of mechanisms able to prevent, or limit, such damages by safe thermal dissipation of light energy (non-photochemical quenching, NPQ), slowing-down of electron transfer through the intersystem transport chain (photosynthesis-control, PSC) in co-operation with the Proton Gradient Regulation (PGR) proteins, PGR5 and PGRL1, collectively called as short-term photoprotection mechanisms, and the redistribution of light between photosystems, called state transitions (responsible of fluorescence quenching at PSII, qT), is superimposed to these short term photoprotective mechanisms. In this manuscript we have generated a number of higher order mutants by crossing genotypes carrying defects in each of the short-term photoprotection mechanisms, with the final aim to obtain a direct comparison of their role and efficiency in photoprotection. We found that mutants carrying a defect in the ΔpH-dependent photosynthesis-control are characterized by photoinhibition of both photosystems, irrespectively of whether PSBS-dependent NPQ or state transitions defects were present or not in the same individual, demonstrating the primary role of PSC in photoprotection. Moreover, mutants with a limited capability to develop a strong PSBS-dependent NPQ, were characterized by a high turnover of the D1 protein and high values of Y(NO), which might reflect energy quenching processes occurring within the PSII reaction center.

PGR5 and NDH-1 systems do not function as protective electron acceptors but mitigate the consequences of PSI inhibition

Avoidance of photoinhibition at photosystem (PS)I is based on synchronized function of PSII, PSI, Cytochrome b₆f and stromal electron acceptors. Here, we used a special light regime, PSI photoinhibition treatment (PIT), in order to specifically inhibit PSI by accumulating excess electrons at the photosystem (Tikkanen and Grebe, 2018). In the analysis, Arabidopsis thaliana WT was compared to the pgr5 and ndho mutants, deficient in one of the two main cyclic electron transfer pathways described to function as protective alternative electron acceptors of PSI. The aim was to investigate whether the PGR5 (pgr5) and the type I NADH dehydrogenase (NDH-1) (ndho) systems protect PSI from excess electron stress and whether they help plants to cope with the consequences of PSI photoinhibition. First, our data reveals that neither PGR5 nor NDH-1 system protects PSI from a sudden burst of electrons. This strongly suggests that these systems in Arabidopsis thaliana do not function as direct acceptors of electrons delivered from PSII to PSI - contrasting with the flavodiiron proteins that were found to make Physcomitrella patens PSI resistant to the PIT. Second, it is demonstrated that under light-limiting conditions, the electron transfer rate at PSII is linearly dependent on the amount of functional PSI in all genotypes, while under excess light, the PGR5-dependent control of electron flow at the Cytochrome b₆f complex overrides the effect of PSI inhibition. Finally, the PIT is shown to increase the amount of PGR5 and NDH-1 as well as of PTOX, suggesting that they mitigate further damage to PSI after photoinhibition rather than protect against it.

DAY-LENGTH-DEPENDENT DELAYED-GREENING1, the Arabidopsis Homolog of the Cyanobacterial H+-Extrusion Protein, Is Essential for Chloroplast pH Regulation and Optimization of Non-Photochemical Quenching

Plants convert solar energy into chemical energy through photosynthesis, which supports almost all life activities on earth. Because the intensity and quality of sunlight can change dramatically throughout the day, various regulatory mechanisms help plants adjust their photosynthetic output accordingly, including the regulation of light energy accumulation to prevent the generation of damaging reactive oxygen species. Non-photochemical quenching (NPQ) is a regulatory mechanism that dissipates excess light energy, but how it is regulated is not fully elucidated. In this study, we report a new NPQ-regulatory protein named Day-Length-dependent Delayed-Greening1 (DLDG1). The Arabidopsis DLDG1 associates with the chloroplast envelope membrane, and the dldg1 mutant had a large NPQ value compared with wild type. The mutant also had a pale-green phenotype in developing leaves but only under continuous light; this phenotype was not observed when dldg1 was cultured in the dark for ≥8 h/d. DLDG1 is a homolog of the plasma membrane-localizing cyanobacterial proton-extrusion-protein A that is required for light-induced H+ extrusion and also shows similarity in its amino-acid sequence to that of Ycf10 encoded in the plastid genome. Arabidopsis DLDG1 enhances the growth-retardation phenotype of the Escherichia coli K+/H+ antiporter mutant, and the everted membrane vesicles of the E. coli expressing DLDG1 show the K+/H+ antiport activity. Our findings suggest that DLDG1 functionally interacts with Ycf10 to control H+ homeostasis in chloroplasts, which is important for the light-acclimation response, by optimizing the extent of NPQ.

DAY-LENGTH-DEPENDENT DELAYED-GREENING1, the Arabidopsis Homolog of the Cyanobacterial H+-Extrusion Protein, Is Essential for Chloroplast pH Regulation and Optimization of Non-Photochemical Quenching

Plants convert solar energy into chemical energy through photosynthesis, which supports almost all life activities on earth. Because the intensity and quality of sunlight can change dramatically throughout the day, various regulatory mechanisms help plants adjust their photosynthetic output accordingly, including the regulation of light energy accumulation to prevent the generation of damaging reactive oxygen species. Non-photochemical quenching (NPQ) is a regulatory mechanism that dissipates excess light energy, but how it is regulated is not fully elucidated. In this study, we report a new NPQ-regulatory protein named Day-Length-dependent Delayed-Greening1 (DLDG1). The Arabidopsis DLDG1 associates with the chloroplast envelope membrane, and the dldg1 mutant had a large NPQ value compared with wild type. The mutant also had a pale-green phenotype in developing leaves but only under continuous light; this phenotype was not observed when dldg1 was cultured in the dark for ≥8 h/d. DLDG1 is a homolog of the plasma membrane-localizing cyanobacterial proton-extrusion-protein A that is required for light-induced H+ extrusion and also shows similarity in its amino-acid sequence to that of Ycf10 encoded in the plastid genome. Arabidopsis DLDG1 enhances the growth-retardation phenotype of the Escherichia coli K+/H+ antiporter mutant, and the everted membrane vesicles of the E. coli expressing DLDG1 show the K+/H+ antiport activity. Our findings suggest that DLDG1 functionally interacts with Ycf10 to control H+ homeostasis in chloroplasts, which is important for the light-acclimation response, by optimizing the extent of NPQ.

An Organic Molecular Photocatalyst Releasing Oxygen from Water

Artificial photosynthesis employing solar energy is one of the best means of reaching a sustainable energy cycle, in which water is oxidized to oxygen and supplies electrons for fuel generation. The development of suitable oxygen evolution materials driven by sunlight is hence the most challenging research topic in the field of photocatalysis. Herein, photocatalytic O₂ production from water at a rate of 22.6 μmol h^-1 was performed by using a pyrene-based organic molecule constructed through the Ullmann coupling reaction. Its significantly improved light-harvesting ability and notably accelerated separation and transfer of photogenerated charges enhanced the O₂ evolution efficiency. This study highlights the structural design of organic molecules applied to photocatalytic water splitting.

Comparing effects of berberine on the growth and photosynthetic activities of Microcystis aeruginosa and Chlorella pyrenoidosa

Berberine is a potent algicidal allelochemical of Microcystis aeruginosa. To optimize its application in the control of Microcystis blooms, the effects of berberine on the growth and photosynthetic activities of M. aeruginosa and a non-target green alga, Chlorella pyrenoidosa, were compared. The results showed that the algicidal activity of berberine on M. aeruginosa was light dependent. Berberine had no algicidal effects on C. pyrenoidosa with or without light exposure. Under light-dark conditions, berberine significantly decreased the chlorophyll fluorescence parameters in M. aeruginosa while no significant berberine-induced changes were observed under constant darkness. Significant reductions of photosystem II (PSII) and whole chain electron transport activities in M. aeruginosa exposed to berberine suggested that PSII was the important target site attacked by berberine. Contrary to M. aeruginosa, no berberine-induced inhibition in photosynthesis activities were observed in C. pyrenoidosa. The differences in photosynthetic apparatuses of these two algae might be responsible for their different sensitivities to berberine.

Digitonin-sensitive LHCII enlarges the antenna of Photosystem I in stroma lamellae of Arabidopsis thaliana after far-red and blue-light treatment

Light drives photosynthesis. In plants it is absorbed by light-harvesting antenna complexes associated with Photosystem I (PSI) and photosystem II (PSII). As PSI and PSII work in series, it is important that the excitation pressure on the two photosystems is balanced. When plants are exposed to illumination that overexcites PSII, a special pool of the major light-harvesting complex LHCII is phosphorylated and moves from PSII to PSI (state 2). If instead PSI is over-excited the LHCII complex is dephosphorylated and moves back to PSII (state 1). Recent findings have suggested that LHCII might also transfer energy to PSI in state 1. In this work we used a combination of biochemistry and (time-resolved) fluorescence spectroscopy to investigate the PSI antenna size in state 1 and state 2 for Arabidopsis thaliana. Our data shows that 0.7 ± 0.1 unphosphorylated LHCII trimers per PSI are present in the stroma lamellae of state-1 plants. Upon transition to state 2 the antenna size of PSI in the stroma membrane increases with phosphorylated LHCIIs to a total of 1.2 ± 0.1 LHCII trimers per PSI. Both phosphorylated and unphosphorylated LHCII function as highly efficient PSI antenna.

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.

Chlorophyll catabolism precedes changes in chloroplast structure and proteome during leaf senescence

The earliest visual changes of leaf senescence occur in the chloroplast as chlorophyll is degraded and photosynthesis declines. Yet, a comprehensive understanding of the sequence of catabolic events occurring in chloroplasts during natural leaf senescence is still missing. Here, we combined confocal and electron microscopy together with proteomics and biochemistry to follow structural and molecular changes during Arabidopsis leaf senescence. We observed that initiation of chlorophyll catabolism precedes other breakdown processes. Chloroplast size, stacking of thylakoids, and efficiency of PSII remain stable until late stages of senescence, whereas the number and size of plastoglobules increase. Unlike catabolic enzymes, whose level increase, the level of most proteins decreases during senescence, and chloroplast proteins are overrepresented among these. However, the rate of their disappearance is variable, mostly uncoordinated and independent of their inherent stability during earlier developmental stages. Unexpectedly, degradation of chlorophyll-binding proteins lags behind chlorophyll catabolism. Autophagy and vacuole proteins are retained at relatively high levels, highlighting the role of extra-plastidic degradation processes especially in late stages of senescence. The observation that chlorophyll catabolism precedes all other catabolic events may suggest that this process enables or signals further catabolic processes in chloroplasts.

Non-photochemical quenching-dependent acclimation and thylakoid organization of Chlamydomonas reinhardtii to high light stress

Light is essential for all photosynthetic organisms while an excess of it can lead to damage mainly the photosystems of the thylakoid membrane. In this study, we have grown Chlamydomonas reinhardtii cells in different intensities of high light to understand the photosynthetic process with reference to thylakoid membrane organization during its acclimation process. We observed, the cells acclimatized to long-term response to high light intensities of 500 and 1000 µmol m^-2 s^-1 with faster growth and more biomass production when compared to cells at 50 µmol m^-2 s^-1 light intensity. The ratio of Chl a/b was marginally decreased from the mid-log phase of growth at the high light intensity. Increased level of zeaxanthin and LHCSR3 expression was also found which is known to play a key role in non-photochemical quenching (NPQ) mechanism for photoprotection. Changes in photosynthetic parameters were observed such as increased levels of NPQ, marginal change in electron transport rate, and many other changes which demonstrate that cells were acclimatized to high light which is an adaptive mechanism. Surprisingly, PSII core protein contents have marginally reduced when compared to peripherally arranged LHCII in high light-grown cells. Further, we also observed alterations in stromal subunits of PSI and low levels of PsaG, probably due to disruption of PSI assembly and also its association with LHCI. During the process of acclimation, changes in thylakoid organization occurred in high light intensities with reduction of PSII supercomplex formation. This change may be attributed to alteration of protein-pigment complexes which are in agreement with circular dichoism spectra of high light-acclimatized cells, where decrease in the magnitude of psi-type bands indicates changes in ordered arrays of PSII-LHCII supercomplexes. These results specify that acclimation to high light stress through NPQ mechanism by expression of LHCSR3 and also observed changes in thylakoid protein profile/supercomplex formation lead to low photochemical yield and more biomass production in high light condition.

Structure of the maize photosystem I supercomplex with light-harvesting complexes I and II

Plants regulate photosynthetic light harvesting to maintain balanced energy flux into photosystems I and II (PSI and PSII). Under light conditions favoring PSII excitation, the PSII antenna, light-harvesting complex II (LHCII), is phosphorylated and forms a supercomplex with PSI core and the PSI antenna, light-harvesting complex I (LHCI). Both LHCI and LHCII then transfer excitation energy to the PSI core. We report the structure of maize PSI-LHCI-LHCII solved by cryo-electron microscopy, revealing the recognition site between LHCII and PSI. The PSI subunits PsaN and PsaO are observed at the PSI-LHCI interface and the PSI-LHCII interface, respectively. Each subunit relays excitation to PSI core through a pair of chlorophyll molecules, thus revealing previously unseen paths for energy transfer between the antennas and the PSI core.

Organization of Plant Photosystem II and Photosystem I Supercomplexes

In nature, plants are continuously exposed to varying environmental conditions. They have developed a wide range of adaptive mechanisms, which ensure their survival and maintenance of stable photosynthetic performance. Photosynthesis is delicately regulated at the level of the thylakoid membrane of chloroplasts and the regulatory mechanisms include a reversible formation of a large variety of specific protein-protein complexes, supercomplexes or even larger assemblies known as megacomplexes. Revealing their structures is crucial for better understanding of their function and relevance in photosynthesis. Here we focus our attention on the isolation and a structural characterization of various large protein supercomplexes and megacomplexes, which involve Photosystem II and Photosystem I, the key constituents of photosynthetic apparatus. The photosystems are often attached to other protein complexes in thylakoid membranes such as light harvesting complexes, cytochrome b ₆ f complex, and NAD(P)H dehydrogenase. Structural models of individual supercomplexes and megacomplexes provide essential details of their architecture, which allow us to discuss their function as well as physiological significance.

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

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

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.

Structure of the plant photosystem I

Plant photosystem I (PSI) is one of the most intricate membrane complexes in nature. It comprises two complexes, a reaction center and light-harvesting complex (LHC), which together form the PSI-LHC supercomplex. The crystal structure of plant PSI was solved with two distinct crystal forms. The first, crystallized at pH 6.5, exhibited P21 symmetry; the second, crystallized at pH 8.5, exhibited P212121 symmetry. The surfaces involved in binding plastocyanin and ferredoxin are identical in both forms. The crystal structure at 2.6 Å resolution revealed 16 subunits, 45 transmembrane helices, and 232 prosthetic groups, including 143 chlorophyll a, 13 chlorophyll b, 27 β-carotene, 7 lutein, 2 xanthophyll, 1 zeaxanthin, 20 monogalactosyl diglyceride, 7 phosphatidyl diglyceride, 5 digalactosyl diglyceride, 2 calcium ions, 2 phylloquinone, and 3 iron sulfur clusters. The model reveals detailed interactions, providing mechanisms for excitation energy transfer and its modulation in one of nature's most efficient photochemical machine.

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.

Terrestrial Plants Evolve Highly Assembled Photosystem Complexes in Adaptation to Light Shifts

It has been known that PSI and PSII supercomplexes are involved in the linear and cyclic electron transfer, dynamics of light capture, and the repair cycle of PSII under environmental stresses. However, evolutions of photosystem (PS) complexes from evolutionarily divergent species are largely unknown. Here, we improved the blue native polyacrylamide gel electrophoresis (BN-PAGE) separation method and successfully separated PS complexes from all terrestrial plants. It is well known that reversible D1 protein phosphorylation is an important protective mechanism against oxidative damages to chloroplasts through the PSII photoinhibition-repair cycle. The results indicate that antibody-detectable phosphorylation of D1 protein is the latest event in the evolution of PS protein phosphorylation and occurs exclusively in seed plants. Compared to angiosperms, other terrestrial plant species presented much lower contents of PS supercomplexes. The amount of light-harvesting complexes II (LHCII) trimers was higher than that of LHCII monomers in angiosperms, whereas it was opposite in gymnosperms, pteridophytes, and bryophytes. LHCII assembly may be one of the evolutionary characteristics of vascular plants. In vivo chloroplast fluorescence measurements indicated that lower plants (bryophytes especially) showed slower changes in state transition and nonphotochemical quenching (NPQ) in response to light shifts. Therefore, the evolution of PS supercomplexes may be correlated with their acclimations to environments.

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.

Extensive gain and loss of photosystem I subunits in chromerid algae, photosynthetic relatives of apicomplexans

In oxygenic photosynthesis the initial photochemical processes are carried out by photosystem I (PSI) and II (PSII). Although subunit composition varies between cyanobacterial and plastid photosystems, the core structures of PSI and PSII are conserved throughout photosynthetic eukaryotes. So far, the photosynthetic complexes have been characterised in only a small number of organisms. We performed in silico and biochemical studies to explore the organization and evolution of the photosynthetic apparatus in the chromerids Chromera velia and Vitrella brassicaformis, autotrophic relatives of apicomplexans. We catalogued the presence and location of genes coding for conserved subunits of the photosystems as well as cytochrome b₆f and ATP synthase in chromerids and other phototrophs and performed a phylogenetic analysis. We then characterised the photosynthetic complexes of Chromera and Vitrella using 2D gels combined with mass-spectrometry and further analysed the purified Chromera PSI. Our data suggest that the photosynthetic apparatus of chromerids underwent unique structural changes. Both photosystems (as well as cytochrome b₆f and ATP synthase) lost several canonical subunits, while PSI gained one superoxide dismutase (Vitrella) or two superoxide dismutases and several unknown proteins (Chromera) as new regular subunits. We discuss these results in light of the extraordinarily efficient photosynthetic processes described in Chromera.

A Spatial Interactome Reveals the Protein Organization of the Algal CO2-Concentrating Mechanism

Approximately one-third of global CO2 fixation is performed by eukaryotic algae. Nearly all algae enhance their carbon assimilation by operating a CO2-concentrating mechanism (CCM) built around an organelle called the pyrenoid, whose protein composition is largely unknown. Here, we developed tools in the model alga Chlamydomonas reinhardtii to determine the localizations of 135 candidate CCM proteins and physical interactors of 38 of these proteins. Our data reveal the identity of 89 pyrenoid proteins, including Rubisco-interacting proteins, photosystem I assembly factor candidates, and inorganic carbon flux components. We identify three previously undescribed protein layers of the pyrenoid: a plate-like layer, a mesh layer, and a punctate layer. We find that the carbonic anhydrase CAH6 is in the flagella, not in the stroma that surrounds the pyrenoid as in current models. These results provide an overview of proteins operating in the eukaryotic algal CCM, a key process that drives global carbon fixation.

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.

Different adaptation strategies of two citrus scion/rootstock combinations in response to drought stress

Scion/rootstock interaction is important for plant development and for breeding programs. In this context, polyploid rootstocks presented several advantages, mainly in relation to biotic and abiotic stresses. Here we analyzed the response to drought of two different scion/rootstock combinations presenting different polyploidy: the diploid (2x) and autotetraploid (4x) Rangpur lime (Citrus limonia, Osbeck) rootstocks grafted with 2x Valencia Delta sweet orange (Citrus sinensis) scions, named V/2xRL and V/4xRL, respectively. Based on previous gene expression data, we developed an interactomic approach to identify proteins involved in V/2xRL and V/4xRL response to drought. A main interactomic network containing 3,830 nodes and 97,652 edges was built from V/2xRL and V/4xRL data. Exclusive proteins of the V/2xRL and V/4xRL networks (2,056 and 1,001, respectively), as well as common to both networks (773) were identified. Functional clusters were obtained and two models of drought stress response for the V/2xRL and V/4xRL genotypes were designed. Even if the V/2xRL plant implement some tolerance mechanisms, the global plant response to drought was rapid and quickly exhaustive resulting in a general tendency to dehydration avoidance, which presented some advantage in short and strong drought stress conditions, but which, in long terms, does not allow the plant survival. At the contrary, the V/4xRL plants presented a response which strong impacts on development but that present some advantages in case of prolonged drought. Finally, some specific proteins, which presented high centrality on interactomic analysis were identified as good candidates for subsequent functional analysis of citrus genes related to drought response, as well as be good markers of one or another physiological mechanism implemented by the plants.