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Improving photosynthesis, the fundamental process by which plants convert light energy into chemical energy, is a key area of research with great potential for enhancing sustainable agricultural productivity and addressing global food security challenges. This perspective delves into the latest advancements and approaches aimed at optimizing photosynthetic efficiency. Our discussion encompasses the entire process, beginning with light harvesting and its regulation and progressing through the bottleneck of electron transfer. We then delve into the carbon reactions of photosynthesis, focusing on strategies targeting the enzymes of the Calvin–Benson–Bassham (CBB) cycle. Additionally, we explore methods to increase carbon dioxide (CO2) concentration near the Rubisco, the enzyme responsible for the first step of CBB cycle, drawing inspiration from various photosynthetic organisms, and conclude this section by examining ways to enhance CO2 delivery into leaves. Moving beyond individual processes, we discuss two approaches to identifying key targets for photosynthesis improvement: systems modeling and the study of natural variation. Finally, we revisit some of the strategies mentioned above to provide a holistic view of the improvements, analyzing their impact on nitrogen use efficiency and on canopy photosynthesis. [ABSTRACT FROM AUTHOR]

Photosynthesis is tightly regulated in order to withstand dynamic light environments. Under high light intensities, a mechanism known as non-photochemical quenching (NPQ) dissipates excess excitation energy, protecting the photosynthetic machinery from damage. An obstacle that lies in the way of understanding the molecular mechanism of NPQ is the large gap between in vitro and in vivo studies. On the one hand, the complexity of the photosynthetic membrane makes it challenging to obtain molecular information from in vivo experiments. On the other hand, a suitable in vitro system for the study of quenching is not available. Here we have developed a minimal NPQ system using proteoliposomes. With this, we demonstrate that the combination of low pH and PsbS is both necessary and sufficient to induce quenching in LHCII, the main antenna complex of plants. This proteoliposome system can be further exploited to gain more insight into how PsbS and other factors (e.g. zeaxanthin) influence the quenching mechanism observed in LHCII. [ABSTRACT FROM AUTHOR]

The article informs about the need for news stories to account for gender bias in reporting dismissals, demotions, and conflicts involving prominent women in academic research. Topic include It emphasizes the importance of pursuing the question of possible gender bias in the treatment of women in academic leadership positions and in the reporting of cases of leadership conflicts, considering the biases that delay women's advancement and compromise their effectiveness in such roles.

Summary: 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. [ABSTRACT FROM AUTHOR]

We model the energy transfer dynamics in the Lhca4 peripheral antenna of photosystem I from higher plants. Equilibration between the bulk exciton levels of the antenna and the red-shifted charge-transfer (CT) states is described using the numerically inexpensive Redfield-Förster approach and exact hierarchical equation (HEOM) method. We propose a compartmentalization scheme allowing a quantitatively correct description of the dynamics with the Redfield-Förster theory, including the exciton-type relaxation within strongly coupled compartments and hopping-type migration between them. The Redfield-Förster method gives the kinetics close to the HEOM solution when treating the CT state as dynamically localized. We also demonstrate that the excited states strongly coupled with the CT should be considered as localized as well. [ABSTRACT FROM AUTHOR]

Nonphotochemical quenching (NPQ) is the process that protects photosynthetic organisms from photodamage by dissipating the energy absorbed in excess as heat. In the model green alga Chlamydomonas reinhardtii, NPQ is abolished in the knock-out mutants of the pigment–protein complexes LHCSR3 and LHCBM1. However, while LHCSR3 is a pH sensor and switches to a quenched conformation at low pH, the role of LHCBM1 in NPQ has not been elucidated yet. In this work, we combined biochemical and physiological measurements to study short-term high-light acclimation of npq5, the mutant lacking LHCBM1. In low light in the absence of this complex, the antenna size of PSII was smaller than in its presence; this effect was marginal in high light (HL), implying that a reduction of the antenna was not responsible for the low NPQ. The mutant expressed LHCSR3 at the wild-type level in HL, indicating that the absence of this complex is also not the reason. Finally, NPQ remained low in the mutant even when the pH was artificially lowered to values that can switch LHCSR3 to the quenched conformation. We concluded that both LHCSR3 and LHCBM1 are required for the induction of NPQ and that LHCBM1 is the interacting partner of LHCSR3. This interaction can either enhance the quenching capacity of LHCSR3 or connect this complex with the PSII supercomplex. In the alga C. reinhardtii, the presence of the pigment–protein complexes LHCSR3 and LHCBM1 is crucial for protecting against excess light, with LHCBM1 potentially enhancing the function of LHCSR3. [ABSTRACT FROM AUTHOR]

Oxygenic photosynthesis, the process that converts light energy into chemical energy, is traditionally associated with the absorption of visible light by chlorophyll molecules. However, recent studies have revealed a growing number of organisms capable of using far-red light (700–800 nm) to drive oxygenic photosynthesis. This phenomenon challenges the conventional understanding of the limits of this process. In this review, we briefly introduce the organisms that exhibit far-red photosynthesis and explore the different strategies they employ to harvest far-red light. We discuss the modifications of photosynthetic complexes and their impact on the delivery of excitation energy to photochemical centers and on overall photochemical efficiency. Finally, we examine the solutions employed to drive electron transport and water oxidation using relatively low-energy photons. The findings discussed here not only expand our knowledge of the remarkable adaptation capacities of photosynthetic organisms but also offer insights into the potential for enhancing light capture in crops. [ABSTRACT FROM AUTHOR]

The article discusses report of dismissals, demotions, and conflicts involving prominent women in academic research.

Nine genes (LHCBM1-9) encode the major light-harvesting system of Chlamydomonas reinhardtii. Transcriptomic and proteomic analyses have shown that those genes are all expressed albeit in different amounts and some of them only in certain conditions. However, little is known about the properties and specific functions of the individual gene products because they have never been isolated. Here we have purified several complexes from native membranes and/or we have reconstituted them in vitro with pigments extracted from C. reinhardtii. It is shown that LHCBM1 and -M2/7 represent more than half of the LHCBM population in the membrane. LHCBM2/7 forms homotrimers while LHCBM1 seems to be present in heterotrimers. Trimers containing only type I LHCBM (M3/4/6/8/9) were also observed. Despite their different roles, all complexes have very similar properties in terms of pigment content, organization, stability, absorption, fluorescence and excited-state lifetimes. Thus the involvement of LHCBM1 in non-photochemical quenching is suggested to be due to specific interactions with other components of the membrane and not to the inherent quenching properties of the complex. Similarly, the overexpression of LHCBM9 during sulfur deprivation can be explained by its low sulfur content as compared with the other LHCBMs. Considering the highly conserved biochemical and spectroscopic properties, the major difference between the complexes may be in their capacity to interact with other components of the thylakoid membrane. [ABSTRACT FROM AUTHOR]

The first step of photosynthesis in plants is performed by the light-harvesting complexes (LHC), a large family of pigment-binding proteins embedded in the photosynthetic membranes. These complexes are conserved across species, suggesting that each has a distinct role. However, they display a high degree of sequence homology and their static structures are almost identical. What are then the structural features that determine their different properties? In this work, we compared the two best-characterized LHCs of plants: LHCII and CP29. Using molecular dynamics simulations, we could rationalize the difference between them in terms of pigment-binding properties. The data also show that while the loops between the helices are very flexible, the structure of the transmembrane regions remains very similar in the crystal and the membranes. However, the small structural differences significantly affect the excitonic coupling between some pigment pairs. Finally, we analyzed in detail the structure of the long N-terminus of CP29, showing that it is structurally stable and it remains on top of the membrane even in the absence of other proteins. Although the structural changes upon phosphorylation are minor, they can explain the differences in the absorption properties of the pigments observed experimentally. [ABSTRACT FROM AUTHOR]

Photosynthetic organisms are crucial for life on Earth as they provide food and oxygen and are at the basis of most energy resources. They have a large variety of light-harvesting strategies that allow them to live nearly everywhere where sunlight can penetrate. They have adapted their pigmentation to the spectral composition of light in their habitat, they acclimate to slowly varying light intensities and they rapidly respond to fast changes in light quality and quantity. This is particularly important for oxygen-producing organisms because an overdose of light in combination with oxygen can be lethal. Rapid progress is being made in understanding how different organisms maximize light harvesting and minimize deleterious effects. Here we summarize the latest findings and explain the main design principles used in nature. The available knowledge can be used for optimizing light harvesting in both natural and artificial photosynthesis to improve light-driven production processes. [ABSTRACT FROM AUTHOR]

Abstract: Photosystem II (PSII) is responsible for the water oxidation in photosynthesis and it consists of many proteins and pigment-protein complexes in a variable composition, depending on environmental conditions. Sunlight-induced charge separation lies at the basis of the photochemical reactions and it occurs in the reaction center (RC). The RC is located in the PSII core which also contains light-harvesting complexes CP43 and CP47. The PSII core of plants is surrounded by external light-harvesting complexes (lhcs) forming supercomplexes, which together with additional external lhcs, are located in the thylakoid membrane where they perform their functions. In this paper we provide an overview of the available information on the structure and organization of pigment-protein complexes in PSII and relate this to experimental and theoretical results on excitation energy transfer (EET) and charge separation (CS). This is done for different subcomplexes, supercomplexes, PSII membranes and thylakoid membranes. Differences in experimental and theoretical results are discussed and the question is addressed how results and models for individual complexes relate to the results on larger systems. It is shown that it is still very difficult to combine all available results into one comprehensive picture. [Copyright &y& Elsevier]

In this work, the spectroscopic characteristics of carotenoids associated with the antenna complexes of Photosystem I have been studied. Pigment composition, absorption spectra, and laser-induced triplet-minus-singlet (T-S) spectra were determined for native LHCI from the wild type (WT) and lut2 mutant from Arabidopsis thaliana as well as for reconstituted individual Lhca WT and mutated complexes. All WT complexes bind lutein and violaxanthin, while β-carotene was found to be associated only with the native LHCI preparation and recombinant Lhca3. In the native complexes, the main lutein absorption bands are located at 492 and 510 nm. It is shown that violaxanthin is able to occupy all lutein binding sites, but its absorption is blue-shifted to 487 and 501 nm. The ‘red’ lutein absorbing at 510 nm was found to be associated with Lhca3 and Lhca4 which also show a second carotenoid, peaking around 490 nm. Both these xanthophylls are involved in triplet quenching and show two T-S maxima: one at 507 nm (corresponding to the 490 nm singlet absorption) and the second at 525 nm (with absorption at 510 nm). The ‘blue’-absorbing xanthophyll is located in site L1 and can receive triplets from chlorophylls (Chl) 1012, 1011, and possibly 1013. The red-shifted spectral component is assigned to a lutein molecule located in the L2 site. A 510 nm lutein was also observed in the trimers of LHCII but was absent in the monomers. In the case of Lhca, the 510 nm band is present in both the monomeric and dimeric complexes. We suggest that the large red shift observed for this xanthophyll is due to interaction with the neighbor Chl 1015. In the native T-S spectrum, the contribution of carotenoids associated with Lhca2 is visible while the one of Lhcal is not. This suggests that in the Lhca2-Lhca3 heterodimeric complex energy equilibration is not complete at least on a fast time scale. [ABSTRACT FROM AUTHOR]

Photosystem I of higher plants is characterized by red-shifted spectral forms deriving from chlorophyll chromophores. Each of the four Lhca1 to -4 subunits exhibits a specific fluorescence emission spectrum, peaking at 688, 701, 725, and 733 nm, respectively. Recent analysis revealed the role of chlorophyll-chlorophyll interactions of the red forms in Lhca3 and Lhca4, whereas the basis for the fluorescence emission at 701 nm in Lhca2 is not yet clear. We report a detailed characterization of the Lhca2 subunit using molecular biology, biochemistry, and spectroscopy and show that the 701-nm emission form originates from a broad absorption band at 690 nm. Spectroscopy on recombinant mutant proteins assesses that this band represents the low energy form of an excitonic interaction involving two chlorophyll a molecules bound to sites A5 and B5, the same protein domains previously identified for Lhca3 and Lhca4. The resulting emission is, however, substantially shifted to higher energies. These results are discussed on the basis of the structural information that recently became available from x-ray crystallography (Ben Shem, A., Frolow, F., and Nelson, N. (2003) Nature 426, 630–635). We suggest that, within the Lhca subfamily, spectroscopic properties of chromophores are modulated by the strength of the excitonic coupling between the chromophores A5 and B5, thus yielding fluorescence emission spanning a large wavelength interval. It is concluded that the interchromophore distance rather than the transition energy of the individual chromophores or the orientation of transition vectors represents the critical factor in determining the excitonic coupling in Lhca pigment-protein complexes. [ABSTRACT FROM AUTHOR]

The Lhca antenna complexes of photosystem I (PSI) have been characterized by comparison of native and recombinant preparations. Eight Lhca polypeptides have been found to be all organized as dimers in the PSI–LHCI complex. The red emission fluorescence is associated not only with Lhca1–4 heterodimer, but also with dimers containing Lhca2 and/or Lhca3 complexes. Reconstitution of Lhca1 and Lhca4 monomers as well as of the Lhca1–4 dimer in vitro was obtained. The biochemical and spectroscopic features of these three complexes are reported. The monomers Lhca1 and Lhca4 bind 10 Chls each, while the Chl a/b ratio is lower in Lhca4 as compared to Lhca1. Three carotenoid binding sites have been found in Lhca1, while only two are present in Lhca4. Both complexes contain lutein and violaxanthin while β-carotene is selectively bound to the Lhca1–4 dimer in substoichiometric amounts upon dimerization. Spectral analysis revealed the presence of low energy absorption forms in Lhca1 previously thought to be exclusively associated with Lhca4. It is shown that the process of dimerization changes the spectroscopic properties of some chromophores and increases the amplitude of the red absorption tail of the complexes. The origin of these spectroscopic features is discussed. [Copyright &y& Elsevier]

Characterizes the chromophore organization in protein CP26. Role of CP26 in chlorophylls-xantophylls binding; Importance of Chl alb-xantophyll-protein complex in photosynthesis; Implication of CP26 for pigment-protein complex folding.

Studies a photosystem I preparation from maize, containing its full antenna complement and in which detergent effects on chlorophyll coupling are almost completely absent. Use of time-resolved fluorescence techniques; Analysis of data in terms of both the decay-associated spectra and the time-resolved emission spectra.

The article reports that plants protect themselves from high-light conditions by dissipating a large part of their absorbed energy as heat in a process, which need the protein PsbS. PsbS's structure opens new possibilities for understanding the mechanism of photoprotection in plants protection. PsbS in its inactive form is a dimer at neutral pH.

Focuses on a study conducted to determine the relationship of the length of antennae with its absorption capability for the long rays of the sun. Factors which influences the capacity of the antennae to absorb rays; Materials and methods; Results of the study; Discussion.

Focuses on a study which explored the chromophore-binding properties of the higher plant light-harvesting protein CP29 by using site-directed mutagenesis of pigment-binding residues. Materials and methods of the study; Stability of mutant proteins; Biochemical properties of recombinant wild-type and mutant photosystem II LHC and CP29; Description of the xanthophyll-binding sites; Spectral characteristics of CP29 protein.

Investigates conformational changes induced by phosphorylation in the Cp24 pigment binding subunit of photosystem II, a photosynthetic component most sensitive to photoinhibition. Role of pigment binding subunits in the regulation of the chlorophyll excited states concentration; Modulation of the spectral properties of photosystem II.

Reports on a preliminary polypeptide analysis of the photosystem I light-harvesting complex of maize thylakoids. Purification of the photosystem I-light-harvesting complex; Electron transport; Kinetic models of photosystem I; Indications that the photosystem I-light-harvesting complex is isoenergetic with P700 among red chlorophylls.

The article talks about several aspects of light harvesting in oxygenic photosynthesis. It mentions that harvesting of photons is the first step in photosynthesis, the biological process that transforms solar energy into chemical energy; and photosynthetic membranes of algae and plants are packed with protein complexes binding many chlorophyll and carotenoid pigments, that are combined to form functional units.

Vascular plants use carotenoids and chlorophylls a and b to harvest solar energy in the visible region (400–700 nm), but they make little use of the far‐red (FR) light. Instead, some cyanobacteria have developed the ability to use FR light by redesigning their photosynthetic apparatus and synthesizing red‐shifted chlorophylls. Implementing this strategy in plants is considered promising to increase crop yield. To prepare for this, a characterization of the FR light‐induced changes in plants is necessary. Here, we explore the behaviour of Arabidopsis thaliana upon exposure to FR light by following the changes in morphology, physiology and composition of the photosynthetic complexes. We found that after FR‐light treatment, the ratio between the photosystems and their antenna size drastically readjust in an attempt to rebalance the energy input to support electron transfer. Despite a large increase in PSBS accumulation, these adjustments result in strong photoinhibition when FR‐adapted plants are exposed to light again. Crucially, FR light‐induced changes in the photosynthetic membrane are not the result of senescence, but are a response to the excitation imbalance between the photosystems. This indicates that an increase in the FR absorption by the photosystems should be sufficient for boosting photosynthetic activity in FR light. The photosynthetic apparatus of plants can undergo massive changes. Arabidopsis plants acclimate to far‐red light by changing the photosystems ratio and their antenna sizes. These changes are not due to dark‐induced senescence but are triggered by the redox state of the electron carriers. [ABSTRACT FROM AUTHOR]

The article discusses research by Xiaochun Qin and colleagues, reported in the current issue, on the molecular biology of photosynthesis in the pea plant, species Pisum sativum, focusing on the photosystem I (PSI) pigment-protein complex, which it notes is highly efficient in converting solar energy. It notes the Qin study provides high-resolution data on the PSI. Topics include red forms, or chlorophylls with red-shifted spectra.

Here, we described the first amyotrophic lateral sclerosis patient presenting the c.881 G > T p.G294V TARDBP mutation in homozygous status. The patient belongs to a large pedigree from Morocco. Except for one older affected brother his parents and remaining 8 sibs are referred to be healthy and do not show any neurological sign or symptom. The lack of evidence of TARDBP deletions of any sizes, together with the presence of several AOH segments, strongly suggests that the homozygosity status of p.G294V in the proband derived from parental consanguinity. A revision of the literature and our cohorts indicates that the p.G294V mutation has been detected in only 15 additional ALS patients in heterozygosity and, except for one additional Moroccan patient, all were of Italian origin. The analysis of microsatellite markers surrounding the TARDBP gene in 8 individuals carrying the p.G294V mutation showed that the haplotypic context of the Moroccan proband is shared with most patients of European origin indicating that they carry the p.G294V mutation inherited from a common ancestor. The analysis of the 15 ALS pedigrees (from literature data and present study), strongly suggests a reduced penetrance of the p.G294V mutation since for 13 of the 15 described p.G294V ALS cases the parents did not show any neurological symptoms. This result has potentially important implications in genetic counseling, since genetic testing of a reduced penetrance mutation on pre-symptomatic individuals proves very difficult to predict the outcome based on the genotype. [ABSTRACT FROM AUTHOR]

In several systems, from plant's canopy to algal bioreactors, the decrease of the antenna size has been proposed as a strategy to increase the photosynthetic efficiency. However, still little is known about possible secondary effects of such modifications. This is particularly relevant because the modulation of the antenna size is one of the most important light acclimation responses in photosynthetic organisms. In our study, we used an Arabidopsis thaliana mutant (dLhcb2), which has a 60% decrease of Lhcb1 and Lhcb2, the two main components of the major Photosystem II antenna complex. We show that the mutant maintains the photosynthetic and photoprotective capacity of the Wild Type (WT) and adapts to different light conditions by remodelling its photosynthetic apparatus, but the regulatory mechanism differs from that of the WT. Surprisingly, it does not compensate for the decreased light‐harvesting capacity by increasing other pigment‐protein complexes. Instead, it lowers the ratio of the cytochrome b6f and ATP synthase to the photosystems, regulating linear electron flow and maintaining the photosynthetic control at the level of these complexes as in the WT. We show that targeting the reduction of two specific antenna proteins, Lhcb1 and Lhcb2, represents a viable solution to obtain plants with a truncated antenna size, which still maintain the capacity to acclimate to different light conditions. Plants with reduced light‐harvesting complexes II (LHCII) content adapt to different light conditions using strategies different from WT plants, without modulating the pigment content. The reduction of LHCII can thus be a viable strategy to increase light‐use efficiency in the canopy. [ABSTRACT FROM AUTHOR]

The photosynthetic apparatus must be able to withstand light conditions that exceed its capacity for carbon fixation. Photosynthetic organisms developed nonphotochemical quenching (NPQ), a process that dissipates excess absorbed light energy as heat and limits the production of reactive oxygen species and cellular damage. In the green alga Chlamydomonas reinhardtii, the LHCSR pigment-binding proteins are essential for NPQ. These complexes are not constitutively present in the thylakoid membranes; however, in laboratory conditions their expression depends on prior high light exposure of cells. To investigate the role of NPQ, we measured cells grown under a day-night cycle with a high light peak at mid-day. LHCSRs are present and NPQ is active consistently throughout the day, likely due to their slow degradation in vivo. This suggests that in physiologically relevant conditions, Chlamydomonas cells are prepared to immediately activate photoprotection, as is the case in vascular plants. We further reveal that state transitions are fully functional under these conditions and that PsbS is highly expressed throughout the day, suggesting it might have a more impactful role than previously thought. [ABSTRACT FROM AUTHOR]

For many applications that involve measuring ultrafast optical phenomena, the streak camera is the device of choice because of its excellent time resolution, its high sensitivity, the possibility to simultaneously measure lifetimes and spectra, and because it can capture the temporal dynamics in a single shot. Nevertheless, to obtain a good time resolution, often narrow slits have to be employed that restrict the image source area and, therefore, limit the light collection efficiency in the experiment. For some applications, it is therefore challenging to find an acceptable balance between the time resolution and signal-to-noise ratio. To overcome this limitation, we have devised the propagation synchronous integration principle for the streak camera, in which an effective spatio-dependent time-shift in the excitation of a sample is introduced and counteracted by the streak sweep, thereby effectively allowing for an increased image source area while maintaining the optimal time resolution. Using the Optronis streak camera with tunable streak sweep and large (1 mm) photocathode width, we could achieve a sevenfold increase in light collection efficiency without affecting the time resolution. Furthermore, we were also able to achieve an 11-fold increase in light collection at the cost of a 26% decrease in the time resolution. [ABSTRACT FROM AUTHOR]

The article discusses study by X. Su and colleagues published in the periodical about cryo–electron microscopy structure of the photosystem II (PSII) complex of vascular plants. Topics discussed include structure of the oxygen-evolving complex (OEC) of plants reported by Wei and colleagues; small membrane proteins called light-harvesting complexes (LHCs) in eukaryotes and arrangement of pigments in the structure.

Summary: Drought is a major abiotic stress that impairs plant growth and development. Despite this, a comprehensive understanding of drought effects on the photosynthetic apparatus is lacking. In this work, we studied the consequences of 14‐d drought treatment on Arabidopsis thaliana.We used biochemical and spectroscopic methods to examine photosynthetic membrane composition and functionality.Drought led to the disassembly of PSII supercomplexes and the degradation of PSII core. The light‐harvesting complexes (LHCII) instead remain in the membrane but cannot act as an antenna for active PSII, thus representing a potential source of photodamage. This effect can also be observed during nonphotochemical quenching (NPQ) induction when even short pulses of saturating light can lead to photoinhibition. At a later stage, under severe drought stress, the PSI antenna size is also reduced and the PSI‐LHCI supercomplexes disassemble. Surprisingly, although we did not observe changes in the PSI core protein content, the functionality of PSI is severely affected, suggesting the accumulation of nonfunctional PSI complexes.We conclude that drought affects both photosystems, although at a different stage, and that the operative quantum efficiency of PSII (ΦPSII) is very sensitive to drought and can thus be used as a parameter for early detection of drought stress. [ABSTRACT FROM AUTHOR]

The photosynthetic light-harvesting complexes (LHCs) are responsible for light absorption due to their pigment-binding properties. These pigments are primarily Chlorophyll (Chl) molecules of type a and b , which ensure an excellent coverage of the visible light spectrum. To date, it is unclear which factors drive the selective binding of different Chl types in the LHC binding pockets. To gain insights into this, we employed molecular dynamics simulations on LHCII binding different Chl types. From the resulting trajectories, we have calculated the binding affinities per each Chl-binding pocket using the Molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) model. To further examine the importance of the nature of the axial ligand in tuning the Chl selectivity of the binding sites, we used Density Functional Theory (DFT) calculations. The results indicate that some binding pockets have a clear Chl selectivity, and the factors governing these selectivities are identified. Other binding pockets are promiscuous, which is consistent with previous in vitro reconstitution studies. DFT calculations show that the nature of the axial ligand is not a major factor in determining the Chl binding pocket selectivity, which is instead probably controlled by the folding process. [ABSTRACT FROM AUTHOR]

In the first step of the photosynthetic process, light is absorbed by the pigments associated with the antenna proteins, known as light-harvesting complexes (Lhcs), which in vivo are functionally organized as hetero-oligomers. The architecture of the pigments, chlorophylls, and carotenoids bound to each LHC is responsible for the efficient excitation energy transfer resulting in photochemistry. So far, the only LHC studied in depth was LHCII, the most abundant membrane protein of plants, while less information was available for the other antennae. In particular, despite the availability of the structure of CP29 obtained at near atomic resolution in 2011 (Pan et al., 2011), the mismatch in pigment content and spectroscopic properties between CP29 in solution and in the crystal has hampered the possibility to use the structure to interpret the experimental data. In this work, we purified CP29 and its larger assembly (CP29-LHCII-CP24) from the membrane in very mild conditions using a His-tag, and we have studied their pigment binding and spectroscopic properties. In addition, we have performed mutation analysis in vivo to obtain mutants of CP29 lacking individual chlorophylls. The peculiar properties of this antenna support its role in directing the energy flow from the external antennae to the reaction center. [ABSTRACT FROM AUTHOR]

Under excess light, photosynthetic organisms employ feedback mechanisms to avoid photodamage. Photoprotection is triggered by acidification of the lumen of the photosynthetic membrane following saturation of the metabolic activity. A low pH triggers thermal dissipation of excess absorbed energy by the light-harvesting complexes (LHCs). LHCs are not able to sense pH variations, and their switch to a dissipative mode depends on stress-related proteins and allosteric cofactors. In green algae the trigger is the pigment-protein complex LHCSR3. Its C-terminus is responsible for a pH-driven conformational change from a light-harvesting to a quenched state. Here, we show that by replacing the C-terminus of the main LHC of plants with that of LHCSR3, it is possible to regulate its excited-state lifetime solely via protonation, demonstrating that the protein template of LHCs can be modified to activate reversible quenching mechanisms independent of external cofactors and triggers. [ABSTRACT FROM AUTHOR]

The efficient use of excitation energy in photosynthetic membranes is achieved by a dense network of pigment–protein complexes. These complexes fulfill specific functions and interact dynamically with each other in response to rapidly changing environmental conditions. Here, we studied how in the intact cells of Chlamydomonas reinhardtii ( C.r. ) the lack of the photosystem I (PSI) core or the photosystem II (PSII) core affects these interactions. To that end the mutants F15 and M18 (both PSI-deficient) and FUD7 (PSII-deficient) were incubated under conditions known to promote state transitions in wild-type. The intact cells were then instantly frozen to 77 K and the full-spectrum time-resolved fluorescence emission of the cells was measured by means of streak camera. In the PSI-deficient mutants excitation energy transfer (EET) towards light-harvesting complexes of PSI (Lhca) occurs in less than 0.5 ns, and fluorescence from Lhca decays in 3.1 ns. Decreased trapping by PSII and increased fluorescence of Lhca upon state 1 (S1) → state 2 (S2) transition appears in the F15 and less in the M18 mutant. In the PSII-deficient mutant FUD7, quenched (0.5 ns) and unquenched (2 ns) light-harvesting complexes of PSII (LHCII) are present in both states, with the quenched form more abundant in S2 than in S1. Moreover, EET of 0.4 ns from the remaining LHCII to PSI increases upon S1 → S2 transition. We relate the excitation energy kinetics observed in F15, M18 and FUD7 to the remodeling of the photosynthetic apparatus in these mutants under S1 and S2 conditions. [ABSTRACT FROM AUTHOR]

A repressible/inducible chloroplast gene expression system has been used to conditionally inhibit chloroplast protein synthesis in the unicellular alga Chlamydomonas reinhardtii. This system allows one to follow the fate of photosystem II and photosystem I and their antennae upon cessation of chloroplast translation. The main results are that the levels of the PSI core proteins decrease at a slower rate than those of PSII. Amongst the light-harvesting complexes, the decrease of CP26 proceeds at the same rate as for the PSII core proteins whereas it is significantly slower for CP29, and for the antenna complexes of PSI this rate is comprised between that of CP26 and CP29. In marked contrast, the components of trimeric LHCII, the major PSII antenna, persist for several days upon inhibition of chloroplast translation. This system offers new possibilities for investigating the biosynthesis and turnover of individual photosynthetic complexes in the thylakoid membranes. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy. [ABSTRACT FROM AUTHOR]

Plants and green algae optimize photosynthesis in changing light conditions by balancing the amount of light absorbed by photosystems I and II. These photosystems work in series to extract electrons from water and reduce NADP+ to NADPH. Light-harvesting complexes (LHCs) are held responsible for maintaining the balance by moving from one photosystem to the other in a process called state transitions. In the green alga Chlamydomonas reinhardtii, a photosynthetic model organism, state transitions are thought to involve 80% of the LHCs. Here, we demonstrate with picosecond-fluorescence spectroscopy on C. reinhardtii cells that, although LHCs indeed detach from photosystem II in state 2 conditions, only a fraction attaches to photosystem I. The detached antenna complexes become protected against photodamage via shortening of the excited-state lifetime. It is discussed how the transition from state 1 to state 2 can protect C. reinhardtii in high-light conditions and how this differs from the situation in plants. [ABSTRACT FROM AUTHOR]

We have studied thylakoid membranesof Arabidopsis thalianaacclimated to different lightconditions and have related proteincomposition to excitation energy transfer and trapping kinetics inPhotosystem II (PSII). In high light: the plants have reduced amountsof the antenna complexes LHCII and CP24, the overall trapping timeof PSII is only ∼180 ps, and the quantum efficiency reachesa value of 91%. In low light: LHCII is upregulated, the PSII lifetimebecomes ∼310 ps, and the efficiency decreases to 84%. Thisdifference is largely caused by slower excitation energy migrationto the reaction centers in low-light plants due to the LHCII trimersthat are not part of the C2S2M2supercomplex.This pool of “extra” LHCII normally transfers energyto both photosystems, whereas it transfers only to PSII upon far-redlight treatment (state 1). It is shown that in high light the reductionof LHCII mainly concerns the LHCII-M trimers, while the pool of “extra”LHCII remains intact and state transitions continue to occur. Theobtained values for the efficiency of PSII are compared with the valuesof Fv/Fm,a parameter that is widely used to indicate the PSII quantum efficiency,and the observed differences are discussed. [ABSTRACT FROM AUTHOR]

Abstract: LHCII, the most abundant membrane protein on earth, is the major light-harvesting complex of plants. It is generally accepted that LHCII is associated with Photosystem II and only as a short-term response to overexcitation of PSII a subset moves to Photosystem I, triggered by its phosphorylation (state1 to state2 transition). However, here we show that in most natural light conditions LHCII serves as an antenna of both Photosystem I and Photosystem II and it is quantitatively demonstrated that this is required to achieve excitation balance between the two photosystems. This allows for acclimation to different light intensities simply by regulating the expression of LHCII genes only. It is demonstrated that indeed the amount of LHCII that is bound to both photosystems decreases when growth light intensity increases and vice versa. Finally, time-resolved fluorescence measurements on the photosynthetic thylakoid membranes show that LHCII is even a more efficient light harvester when associated with Photosystem I than with Photosystem II. [Copyright &y& Elsevier]

Abstract: The first event of photosynthesis is the harvesting of solar energy by a large array of pigments. These pigments are coordinated to proteins that organize them to assure efficient excitation energy transfer. The protein plays an essential role in tuning the spectroscopic properties of the pigments, by determining their site energy and/or by favoring pigment–pigments interactions. Here we investigate how the protein modulates the pigment properties by using a single-point-mutation approach. We monitor changes in the low-energy absorption/emission band of Lhca4, which is well separated from the bulk absorption and thus represents an attractive model system. Moreover, it was recently shown that Lhca4 exists in at least two conformations, a dominating one emitting at 720nm and a second one emitting at 685nm (Kruger et al. PNAS 2011). Here we show that a single amino-acid substitution (from Asn to Gln, which are both chlorophyll-binding residues and only differ for one C―C bond), moves the equilibrium almost completely towards the 685-nm conformation. This indicates that small changes in the protein can have a large effect on the properties of the pigments. We show that His99, which was suggested to coordinate a red-absorbing chlorophyll (Melkozernov and Blankenship, JBC 2003), is not a chlorophyll ligand. We also show that single amino-acid substitutions nearby the chlorophylls allow to tune the emission spectrum of the pigments over a wide range of wavelengths and to modulate the excited-state lifetimes of the complex. These findings are discussed in the light of previously proposed non-photochemical quenching models. [Copyright &y& Elsevier]

The light-harvesting complexes of photosystem I and II (Lhcas and Lhcbs) of plants display a high structural homology and similar pigment content and organization. Yet, the spectroscopic properties of these complexes, and accordingly their functionality, differ substantially. This difference is primarily due to the charge-transfer (CT) character of a chlorophyll dimer in all Lhcas, which mixes with the excitonic states of these complexes, whereas this CT character is generally absent in Lhcbs. By means of single-molecule spectroscopy near room temperature, we demonstrate that the presence or absence of such a CT state in Lhcas and Lhcbs can occasionally be reversed; i.e., these complexes are able to interconvert conformationally to quasi-stable spectral states that resemble the Lhcs of the other photosystem. The high structural similarity of all the Lhca and Lhcb proteins suggests that the stable conformational states that give rise to the mixed CT-excitonic state are similar for all these proteins. andsimilarly for the conformations that involve no CT state. This indicates that the specific functions related to Lhca and Lhcb complexes are realized by different stable conformations of a single generic protein structure. We propose that this functionality is modulated and controlled by the protein environment. [ABSTRACT FROM AUTHOR]

Lhcb5 is an antenna protein that is highly conserved in plants and green algae. It is part of the inner layer of photosystem II antenna system retained in high light acclimated plants. To study the structure-function relation and the role of individual pigments in this complex, we (i) "knocked out" each of the chromophores bound to multiple (nine total) chlorophyll sites and (ii) exchanged the xanthophylls bound to the three xanthophyll sites. The occupancy and associated energy of the pigment binding sites were determined. The role of the individual pigments in protein folding, stability, energy transfer, and dissipation was studied in vitro. The results indicate that lutein has a primary role in the folding and stability of the complex, whereas violaxanthin and zeaxanthin have a negative effect on folding yield and stability, respectively. The data showed a distinct function for the Li and L2 carotenoid binding sites, the former preferentially involved in gathering the excitation energy to chlorophyll a (Chi a), whereas the latter modulates the concentration of chlorophyll singlet excited states dependent on the xanthophylls bound to it, likely via an interaction with Chl-603. Our results also underscored the role of zeaxanthin and lutein in quenching the excitation energy, whereas violaxanthin was shown to be very effective in energy transfer. The characteristics of the isolated proteins were consistent with the observed role of Lhcb5 in vivo in catalyzing fluorescence quenching upon zeaxanthin binding. [ABSTRACT FROM AUTHOR]

Studies the xanthophyll binding sites of the CP29 antenna protein of higher plant photosystem II using recombitant proteins folded in vitro. Reconstitution of the recombitant CP29 with different xanthophyll species; Light harvesting and photoprotection; Binding sequences.

The average fluorescence decay lifetimes, due to reaction centre photochemical trapping, were calculated for wavelengths in the 690- to 770-nm interval from the published fluorescence decay-associated emission spectra for Photosystem I (PSI)–light-harvesting complex of Photosystem I (LHCI) [Biochemistry 39 (2000) 6341] at 280 and 170 K. For 280 K, the overall trapping time at 690 nm is 81 ps and increases with wavelength to reach 103 ps at 770 nm. For 170 K, the 690-nm value is 115 ps, increasing to 458 ps at 770 nm. This underlines the presence of kinetically limiting processes in the PSI antenna (diffusion limited). The explanation of these nonconstant values for the overall trapping time band is sought in terms of thermally activated transfer from the red absorbing states to the “bulk” acceptor chlorophyll (chl) states in the framework of the Arrhenius–Eyring theory. It is shown that the wavelength-dependent “activation energies” come out in the range between 1.35 and 2.7 kcal mol−1, increasing with the emission wavelength within the interval 710–770 nm. These values are in good agreement with the Arrhenius activation energy determined for the steady-state fluorescence yield over the range 130–280 K for PSI–LHCI. We conclude that the variable trapping time in PSI–LHCI can be accounted for entirely by thermally activated transfer from the low-energy chl states to the bulk acceptor states and therefore that the position of the various red states in the PSI antenna seems not to be of significant importance. The analysis shows that the bulk antenna acceptor states are on the low-energy side of the bulk antenna absorption band. [Copyright &y& Elsevier]

Investigates the central chlorophyll a binding sites in minor photosystem antenna protein. Examination of mutant proteins from light-harvesting complexes of higher plants; Importance of determining the absorption position for chlorophyll molecules; Evaluation on the quality of protein folding in mutated polypeptides.

The minor light-harvesting chlorophyll-a/b-binding protein CP29 (Lhcb4), overexpressed in Escherichia coli, has been reconstituted in vitro with pigments. The recombinant pigment-protein complexes show biochemical and spectral properties identical to the native CP29 purified from maize thylakoids. The xanthophyll lutein is the only carotenoid necessary for reconstitution, a finding consistent with the structural role of two lutein molecules/polypeptide suggested by the crystallographic data for the homologous protein light-harvesting chlorophyll-a/b-binding protein of photosystem II (LHCII). The CP29 protein scaffold can accommodate different chromophores. This conclusion was deduced by the observation that the pigment composition of the reconstituted protein depends on the pigments present in the reconstitution mixture. Thus, in addition to a recombinant CP29 identical to the native one, two additional forms of the complex could be obtained by increasing chlorophyll b content. This finding is typical of CP29 because the major LHCII complex shows an absolute selectivity for chromophore binding [Plumley, F. G. & Schmidt, G. W. (1987) Proc. Natl Acad. Sci. USA 84, 146-150; Paulsen, H., Rumler, U. & Rudiger, W. (1990) Planta (Heidelb.) 181, 204-211], and it is consistent with the higher stability of CP29 during greening and in chlorophyll b mutants compared with LHCII. [ABSTRACT FROM AUTHOR]