Добірка наукової літератури з теми "Photosystem antenna size"

Abstract 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.

Abstract The main chlorophyll a/b protein complex of the chloroplast thylakoid membrane is organized into two subpopulations; one inner which is tightly bound to the photosystem II core and one outer which is bound more loosely or peripherally. In this study, changes in the LHC II com position due to long-term light acclimation were analyzed and quantified in spinach thylakoids and isolated stroma lamellae vesicles. The results show that; photosystem II located in the appressed thylakoid regions (α-centres) which have a relatively large antenna size, contains both the inner and outer LHC II with a predominance of the latter (58-70%). At low light the antenna size o f the α-center becomes larger due to a specific increase of the outer pool o f LHC II. The antenna size of photosystem II in the stroma thylakoids (β-centres) is smaller and contains mainly or only the inner LHC II pool. In contrast to the α-centres the β-centres centres do not undergo adaptive changes in their size in response to long-term changes in the light intensities.

The heterogeneity of photosystem II with respect to α and β centers was investigated in triazine-resistant and susceptible biotypes of Chenopodium album . In both biotypes the light harvesting antenna sizes of photosystem II α centers was larger than those of β centers. In the resistant biotype the antenna size of the α centers was smaller than those in the susceptible one. There was not much difference in the antenna sizes of the β centers. The proportion of β centers was larger in the resistant biotype compared with the sensitive one.

The heterogeneity of photosystem II with respect to a and β centers was investigated in triazine-resistant and susceptible biotypes of Chenopodium album. In both biotypes the light harvesting antenna sizes of photosystem II a centers was larger than those of β centers. In the resistant biotype the antenna size of the a centers was smaller than those in the susceptible one. There was not much difference in the antenna sizes of the β centers. The proportion of β centers was larger in the resistant biotype compared with the sensitive one.

Abstract Thylakoids isolated from SAN 9785 (4-chloro-5-dimethylamino-2-phenyl-3(2H)-pyridazi-none)-treated pea plants showed an inhibition of “state transition” and the light-harvesting complex II (LHC II) phosphorylation-mediated changes in the energy distribution between photosystem II (PS II) and photosystem I (PS I) as measured by a decrease in PS II and an increase in PS I fluorescence yield. Interestingly, in these thylakoids the extent of phosphorylation-induced migration of light-harvesting complex (LHC II-P) to non-appressed mem­brane regions was only marginally inhibited. We propose that the suppression in the ability for “state transition” by SANDOZ 9785 (SAN 9785) treatment occurs due to a lack of effec­tive coupling of the migrated LHC II-P and PS I. Since we observed a decrease in the antenna size of PS I of the treated plants, the lack of effective coupling is attributed to this decrease in the antenna size of PS I.

Le alghe sono definite organismi fotosintetici ossigenici, procarioti o eucarioti, con un’organizzazione da unicellulare a pluricellulare, che non sviluppano foglie o radici giustificando la classificazione di ‘piante inferiori’. Le alghe presentano diverse potenziali applicazioni commerciali, come la produzione di biomassa per l’alimentazione umana/animale o per essere usata come fertilizzante, l’estrazione di molecole ad elevato valore aggiunto con un mercato nell’industria chimica o farmaceutica, infine, anche se ancora lontano dalla commercializzazione, la produzione di bio-combustibili. Fornire un substrato per la crescita eterotrofa potrebbe essere una possibile strategia per la crescita delle alghe, tuttavia uno dei maggiori vantaggi delle alghe rispetto ad organismi non fotosintetici è la possibilità di convertire l’energia solare, l’acqua e l’anidride carbonica in biomassa, attraverso il processo fotosintetico. D’altra parte, la coltivazione attuale di ceppi selvatici prospetta rese in biomassa nettamente inferiori rispetto a stime teoriche basate su una fotosintesi ottimale. Tale risultato riflette un problema produttivo reale che dipende sostanzialmente da un utilizzo inefficiente della luce. In particolare, i ceppi selvatici sono in genere dotati di ampi complessi antenna dei fotosistemi per la raccolta della luce, un vantaggio nell’ambiente naturale dove la luce può essere limitante e le cellule crescono a bassa densità. Tale caratteristica è stata proposta diventare invece controproducente durante la coltivazione di massa. Questo perché la fotosintesi è saturata ad intensità di luce relativamente basse, con conseguente dissipazione dell’energia assorbita in eccesso, e la luce è rapidamente attenuata all’interno della coltura. Diversamente, fenotipi a ridotta capacità d’assorbimento della luce potrebbero incrementare l’efficienza di conversione della luce in biomassa. Il principale vantaggio sarebbe di saturare la fotosintesi a più alte intensità luminose, minimizzando lo smorzamento non fotochimico dell’eccitazione. D’altra parte, tali fenotipi non sopravvivrebbero nell’ambiente naturale e non potrebbero essere isolati in natura, ma devono essere generati tramite ingegneria genetica. Chlamydomonas reinhardtii è un’alga verde unicellulare che si presta alla trasformazione e di cui sono disponibili informazioni di sequenza per tutti e tre i genomi (nucleare, mitocondriale e cloroplastico). Tecniche d’ingegneria genetica possono quindi essere applicate a questo organismo modello per generare mutanti con differenti livelli di riduzione della capacità di assorbire la luce e per verificare le promesse di questi ultimi nell’ottimizzare l’efficienza di utilizzo della luce. In seguito, conoscenze acquisite dallo studio di organismi modello possono essere d’aiuto per avanzare il miglioramento genetico di altre specie algali produttive e attraenti per applicazioni commerciali. Tramite mutagenesi inserzionale casuale del genoma nucleare di C. reinhardtii, tre mutanti ‘verde pallido’, con un ridotto contenuto in clorofilla per cellula rispetto al wild type, sono stati isolati: as1, as2 e gun4. Un ceppo a ridotta antenna deve soddisfare specifici criteri di saturazione della fotosintesi ad alte intensità luminose ed elevata efficienza quantica e un fenotipo ‘verde pallido’ non corrisponde necessariamente ad una maggiore produttività fotosintetica. Ad esempio, il mutante gun4 è compromesso nella biosintesi della clorofilla, accumula una porfirina precursore ed è fotosensibile. E’ evidente che tale ceppo non può essere coltivato come produttore di biomassa. Alternativamente, modificare il targeting delle proteine e la biogenesi dei complessi coordinanti la clorofilla potrebbe essere una strategia per regolare la capacità di assorbire la luce senza compromettere la fotoprotezione, come suggerito dal mutante as1. Quest’ultimo ha una mutazione inserzionale in un gene omologo ad arsA, possibilmente coinvolto nell’importazione delle proteine nel cloroplasto mediante regolazione della biogenesi del traslocone della membrana esterna del cloroplasto. Il fenotipo ‘verde pallido’ in as1 e as2 deriva da una riduzione sia nella taglia d’antenna dei singoli fotosistemi sia nella quantità di fotosistemi. Agire solo sulla dimensione dell’antenna per fotosistema non è fattibile, considerando la devozione dei complessi antenna alla fotoprotezione oltre che alla raccolta della luce e i limiti strutturali di una dimensione minima dell’antenna che permetta l’assemblaggio e la funzionalità del fotosistema stesso. Pertanto, diminuire la densità di fotosistemi nei tilacoidi potrebbe essere una valida strategia complementare alla sola riduzione della dimensione dell’antenna per fotosistema per ottenere fenotipi a ridotto contenuto in pigmenti. D’altra parte, i complessi fotosintetici stessi costituiscono un apparato tutt’altro che rigido e l’acclimatazione a lungo termine in grado di modulare la capacità di assorbire la luce come risposta a diverse intensità luminose in C. reinhardtii include la regolazione del contenuto in clorofilla per cellula. Fattori putativamente coinvolti nella foto-acclimatazione, come LHL4, un membro della famiglia LHC, potrebbero essere ingegnerizzati, di fatto una riduzione del contenuto in clorofilla è stata osservata nelle linee sovra-esprimenti in modo costitutivo LHL4. Sebbene il gene responsabile del fenotipo osservato in as2 non sia stato tuttora identificato, la modificazione della curva di risposta alla luce della fotosintesi sembra la più promettente per incrementare la produttività ad alte intensità luminose rispetto al wild type. as2 ha infatti dimostrato rese maggiori del wild type in termini di densità cellulare sia in piccola scala sia in un fotobioreattore di 65 litri. Tuttavia, per osservare i benefici attesi di produttività fotosintetica soprattutto su larga scala, bisogna prestare attenzione alla geometria del fotobioreattore e alle condizioni di coltura. In particolare, esiste una concentrazione ottimale di clorofilla, e quindi di cellule, per avere la massima produttività fotosintetica integrata per l’intera coltura ad una determinata radiazione luminosa. Tale concentrazione ottimale permette di assorbire il più possibile dell’energia disponibile, limitando nel contempo l’attenuazione della luce all’interno della coltura la quale risulterebbe in una perdita di biomassa dovuta alla respirazione nelle zone non sufficientemente illuminate. Al di sotto della concentrazione ottimale, la produttività fotosintetica risulta ridotta da una limitazione in clorofilla nell’assorbire la luce.
Algae are defined as oxygenic photosynthetic organisms, prokaryotic or eukaryotic, with organization ranging from unicellular to multicellular, that don’t have true stems, roots and leaves thus leading to their classification as ‘lower’ plants. Algae have several potential commercial applications, such as production of biomass for human/animal feeding or to be used as fertilizer, extraction of high-value chemicals and pharmaceuticals and, although still far from being on the market, as a biofuels feedstock. Supplying a substrate for heterotrophic growth could be a possible strategy for algae-based biorefineries, however the major advantage of using algae over non photosynthetic organisms is the possibility to convert solar energy, water and carbon dioxide into biomass, through photosynthesis. Conversely, present cultivation of wild type strains yields biomass productivities that are far below theoretical estimations based on optimal photosynthesis, enlightening an existing problem that mainly relies on light utilization inefficiency. In particular, large light-harvesting antenna systems, an advantage in the wild where light could be limiting and cells grow at low density, have been proposed to be instead detrimental during mass cultivation because of photosynthesis saturation occurring at relatively low light intensities, with dissipation of excess absorbed energy, and rapid light extinction within the culture. In contrast, phenotypes of reduced absorption cross section could improve solar-to-biomass conversion efficiency. The main advantage would be that photosynthesis saturation occurs at higher light intensities, minimizing non photochemical quenching. On the other hand, such phenotypes would not survive in the wild and could not be encountered in nature but have to be generated by genetic engineering. Chlamydomonas reinhardtii is a unicellular green alga that is suitable to transformation and whose genomes are sequenced. Techniques of genetic engineering could thus be applied in this model organism to generate mutants with different extents in absorption cross section reduction and to verify their promises of improved light use efficiency. Then, knowledge from intensively studied organisms could help advancing in genetic improvement of other productive algal species that are attractive for commercial applications. From random insertion mutagenesis of the nuclear genome of C. reinhardtii, three ‘pale green’ strains have been isolated, namely antenna size mutant 1 (as1), antenna size mutant 2 (as2) and gun4. A truncated antenna strain must meet specific criteria of high saturation light and quantum yield of photosynthesis and not all ‘pale green’ strains are truly useful mutants for improved productivity. For instance, the gun4 mutant is compromised in chlorophyll biosynthesis, accumulating a chlorophyll precursor porphyrin and displaying photosensitivity. It’s understandable that it could not be grown as a biomass producer. Alternatively, to act on protein targeting and biogenesis of chlorophyll-binding complexes could be a mean to regulate the absorption cross section of the cell without leading to photosensitivity, as suggested by mutant as1. The latter has an insertion mutation in an arsA-homolog gene possibly involved in chloroplast protein import by mediating biogenesis of the translocon of chloroplast outer membrane. Remarkably, the ‘pale green’ phenotype of as1 and as2 derives from reduction in both photosystems antenna size and amount of photosystem core complexes. Acting only on the chlorophyll antenna size per photosystem is not feasible, considering devotion of antenna systems to both light harvesting and photoprotection and structural constrains of a minimal antenna size to allow for folding and function of photosystem core complex. Reducing the density of photosystems in thylakoids could be a valuable complementary strategy as compared to the sole reduction in photosystem antenna size to obtain phenotypes of lower absorption cross section. At the other hand, photosynthetic complexes constitute themselves an apparatus that is far from rigid and long term acclimation to adjust the light harvesting capacity to changing light conditions in C. reinhardtii relies on regulating the chlorophyll content per cell. Factors possibly involved in photo-acclimation, as LHL4, a LHC-like protein, could be target for genetic engineering and constitutive up-regulation of LHL4 has led to reduction in the chlorophyll content. Although the gene responsible for the observed phenotype is still unknown in as2, modification of the light response curve of photosynthesis seems to be the most promising to improve productivity during cultivation in high light. as2 has indeed yielded higher cell densities than wild type both in a small-scale apparatus and in a 65L-photobioreactor. However, in order to observe the expected benefits on photosynthetic productivity during scale-up, attention must be paid to photobioreactor design and growth conditions. In particular, optimum chlorophyll (cell) concentration for maximal integrated net photosynthesis exists at a given irradiance value, which would be such that most of the incident light will be absorbed while avoiding too strong light attenuation that would result in biomass loss through respiration in sub-illuminated zones. Below optimum chlorophyll concentration, limitation in chlorophyll in absorbing light could restrict overall photosynthetic productivity.

Light capturing and energy conversion by PSI is one of the most fundamental processes in nature. In the heart of these adaptations stand PSI, PSII and their light harvesting antenna complexes. The main goal of this grant proposal was to obtain by X-ray crystallography information on the structure of plant photosystem I (PSI) and photosystem II (PSII) supercomplexes. We achieved several milestones along this line but as yet, like several strong laboratories around the world, we have no crystal structure of plant PSII. We have redesigned the purification and crystallization procedures and recently solved the crystal structure of the PSI supercomplex at 3.3 Å resolution. Even though this advance in resolution appears to be relatively small, we obtained a significantly improved model of the supercomplex. The work was published in J. Biol. Chem. (Amunts et al., 2010). The improved electron density map yielded identification and tracing of the PsaK subunit. The location of an additional 10 ß-carotenes, as well as 5 chlorophylls and several loop regions that were previously uninterruptable have been modeled. This represents the most complete plant PSI structure obtained thus far, revealing the locations of and interactions among 17 protein subunits and 193 non-covalently bound photochemical cofactors. We have continued extensive experimental efforts to improve the structure of plant PSI and to obtain PSII preparation amenable to crystallization. Most of our efforts were devoted to obtain well-defined subcomplexes of plant PSII preparations that are amenable to crystallization. We studied the apparent paradox of the high sensitivity of oxygen evolution of isolated thylakoids while BBY particles exhibit remarkable resilience to the same treatment. The integrity of the photosystem II (PSII) extrinsic protein complement as well as calcium effects arise from the Ca2+ atom associated with the site of photosynthetic water oxidation were investigated. This work provides deeper insights into the interaction of PsbO with PSII. Sight-directed mutagenesis indicated the location of critical sites involved in the stability of the water oxidation reaction. When combined with previous results, the data lead to a more detailed model for PsbO binding in eukaryotic PSII.

In this project, we studied the photosynthetic apparatus during dehydration and rehydration of the homoiochlorophyllous resurrection plant Craterostigmapumilum (retains most of the photosynthetic components during desiccation). Resurrection plants have the remarkable capability to withstand desiccation, being able to revive after prolonged severe water deficit in a few days upon rehydration. Homoiochlorophyllous resurrection plants are very efficient in protecting the photosynthetic machinery against damage by reactive oxygen production under drought. The main purpose of this BARD project was to unravel these largely unknown protection strategies for C. pumilum. In detail, the specific objectives were: (1) To determine the distribution and local organization of photosynthetic protein complexes and formation of inverted hexagonal phases within the thylakoid membranes at different dehydration/rehydration states. (2) To determine the 3D structure and characterize the geometry, topology, and mechanics of the thylakoid network at the different states. (3) Generation of molecular models for thylakoids at the different states and study the implications for diffusion within the thylakoid lumen. (4) Characterization of inter-system electron transport, quantum efficiencies, photosystem antenna sizes and distribution, NPQ, and photoinhibition at different hydration states. (5) Measuring the partition of photosynthetic reducing equivalents between the Calvin cycle, photorespiration, and the water-water cycle. At the beginning of the project, we decided to use C. pumilum instead of C. wilmsii because the former species was available from our collaborator Dr. Farrant. In addition to the original two dehydration states (40 relative water content=RWC and 5% RWC), we characterized a third state (15-20%) because some interesting changes occurs at this RWC. Furthermore, it was not possible to detect D1 protein levels by Western blot analysis because antibodies against other higher plants failed to detect D1 in C. pumilum. We developed growth conditions that allow reproducible generation of different dehydration and rehydration states for C. pumilum. Furthermore, advanced spectroscopy and microscopy for C. pumilum were established to obtain a detailed picture of structural and functional changes of the photosynthetic apparatus in different hydrated states. Main findings of our study are: 1. Anthocyan accumulation during desiccation alleviates the light pressure within the leaves (Fig. 1). 2. During desiccation, stomatal closure leads to drastic reductions in CO2 fixation and photorespiration. We could not identify alternative electron sinks as a solution to reduce ROS production. 3. On the supramolecular level, semicrystalline protein arrays were identified in thylakoid membranes in the desiccated state (see Fig. 3). On the electron transport level, a specific series of shut downs occur (summarized in Fig. 2). The main events include: Early shutdown of the ATPase activity, cessation of electron transport between cyt. bf complex and PSI (can reduce ROS formation at PSI); at higher dehydration levels uncoupling of LHCII from PSII and cessation of electron flow from PSII accompanied by crystal formation. The later could severe as a swift PSII reservoir during rehydration. The specific order of events in the course of dehydration and rehydration discovered in this project is indicative for regulated structural transitions specifically realized in resurrection plants. This detailed knowledge can serve as an interesting starting point for rationale genetic engineering of drought-tolerant crops.