Revisiting the nonregulatory, constitutive nonphotochemical quenching of the absorbed light energy in oxygenic photosynthetic organisms.

The present paper aims to open discussion on the information content, physical mechanism(s), and measuring protocols to determine the partitioning of the absorbed light energy in oxygenic photosynthetic organisms. Revisiting these questions is incited by recent findings discovering that PSII, in addition to its open and closed state, assumes a light-adapted charge-separated state and that chlorophyll fluorescence induction (ChlF), besides the photochemical activity of PSII, reflects the structural dynamics of its reaction center complex. Thus, the photochemical quantum yield of PSII cannot be determined from the conventional ChlF-based protocol.

Remembering Jan Amesz (1934-2001): a great gentleman, a major discoverer, and an internationally renowned biophysicist of both oxygenic and anoxygenic photosynthesis.

We present here the research contributions of Jan Amesz (1934-2001) on deciphering the details of the early physico-chemical steps in oxygenic photosynthesis in plants, algae and cyanobacteria, as well as in anoxygenic photosynthesis in purple, green, and heliobacteria. His research included light absorption and the mechanism of excitation energy transfer, primary photochemistry, and electron transfer steps until the reduction of pyridine nucleotides. Among his many discoveries, we emphasize his 1961 proof, with L.

Role of isotropic lipid phase in the fusion of photosystem II membranes.

It has been thoroughly documented, by using P-NMR spectroscopy, that plant thylakoid membranes (TMs), in addition to the bilayer (or lamellar, L) phase, contain at least two isotropic (I) lipid phases and an inverted hexagonal (H) phase. However, our knowledge concerning the structural and functional roles of the non-bilayer phases is still rudimentary. The objective of the present study is to elucidate the origin of I phases which have been hypothesized to arise, in part, from the fusion of TMs (Garab et al.

Effects of lipids on the rate-limiting steps in the dark-to-light transition of Photosystem II core complex of .

In our earlier works, we have shown that the rate-limiting steps, associated with the dark-to-light transition of Photosystem II (PSII), reflecting the photochemical activity and structural dynamics of the reaction center complex, depend largely on the lipidic environment of the protein matrix. Using chlorophyll- fluorescence transients (ChlF) elicited by single-turnover saturating flashes, it was shown that the half-waiting time (Δ ) between consecutive excitations, at which 50% of the fluorescence increment was reached, was considerably larger in isolated PSII complexes of () than in the native thylakoid membrane (TM). Further, it was shown that the addition of a TM lipid extract shortened Δ of isolated PSII, indicating that at least a fraction of the 'missing' lipid molecules, replaced by detergent molecules, caused the elongation of Δ .

Etioplasts are more susceptible to salinity stress than chloroplasts and photosynthetically active etio-chloroplasts of wheat (Triticum aestivum L.).

High soil salinity is a global problem in agriculture that directly affects seed germination and the development of the seedlings sown deep in the soil. To study how salinity affected plastid ultrastructure, leaf segments of 11-day-old light- and dark-grown (etiolated) wheat (Triticum aestivum L. cv.

New foundations for the physical mechanism of variable chlorophyll a fluorescence. Quantum efficiency versus the light-adapted state of photosystem II.

Photosystem II (PSII) uses solar energy to oxidize water and delivers electrons to fix CO2. Although the structure at atomic resolution and the basic photophysical and photochemical functions of PSII are well understood, many important questions remain. The activity of PSII in vitro and in vivo is routinely monitored by recording the induction kinetics of chlorophyll a fluorescence (ChlF).

Long- and short-term acclimation of the photosynthetic apparatus to salinity in . 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.

Honoring two stalwarts of photosynthesis research: Eva-Mari Aro and Govindjee.

On behalf of the entire photosynthesis community, it is an honor, for us, to write about two very eminent scientists who were recently recognised with a Lifetime Achievement Award from the International Society of Photosynthesis Research (ISPR) on August 5, 2022; this prestigious Award was given during the closing ceremony of the 18th International Congress on Photosynthesis Research in Dunedin, New Zealand. The awardees were: Professor Eva-Mari Aro (Finland) and Professor Emeritus Govindjee Govindjee (USA). One of the authors, Anjana Jajoo, is especially delighted to be a part of this tribute to professors Aro and Govindjee as she was lucky enough to have worked with both of them.

Light-induced reversible reorganizations in closed Type II reaction centre complexes: physiological roles and physical mechanisms.

The purpose of this review is to outline our understanding of the nature, mechanism and physiological significance of light-induced reversible reorganizations in closed Type II reaction centre (RC) complexes. In the so-called 'closed' state, purple bacterial RC (bRC) and photosystem II (PSII) RC complexes are incapable of generating additional stable charge separation. Yet, upon continued excitation they display well-discernible changes in their photophysical and photochemical parameters.

Structural Entities Associated with Different Lipid Phases of Plant Thylakoid Membranes-Selective Susceptibilities to Different Lipases and Proteases.

It is well established that plant thylakoid membranes (TMs), in addition to a bilayer, contain two isotropic lipid phases and an inverted hexagonal (H) phase. To elucidate the origin of non-bilayer lipid phases, we recorded the P-NMR spectra of isolated spinach plastoglobuli and TMs and tested their susceptibilities to lipases and proteases; the structural and functional characteristics of TMs were monitored using biophysical techniques and CN-PAGE. Phospholipase-A1 gradually destroyed all P-NMR-detectable lipid phases of isolated TMs, but the weak signal of isolated plastoglobuli was not affected.

Short-Chained Alcohols Make Membrane Surfaces Conducive for Melittin Action: Implication for the Physiological Role of Alcohols in Cells.

Alcohols are a part of cellular metabolism, but their physiological roles are not well understood. We investigated the effects of short-chain alcohols on and model membranes mimicking the lipid composition of eukaryotic inner mitochondrial membranes. We also studied the synergistic effects of alcohols with the bee venom membrane-active peptide, melittin, which is structurally similar to endogenous membrane-active peptides.

Ultrafast excitation quenching by the oxidized photosystem II reaction center.

Photosystem II (PSII) is the pigment-protein complex driving the photoinduced oxidation of water and reduction of plastoquinone in all oxygenic photosynthetic organisms. Excitations in the antenna chlorophylls are photochemically trapped in the reaction center (RC) producing the chlorophyll-pheophytin radical ion pair P Pheo. When electron donation from water is inhibited, the oxidized RC chlorophyll P acts as an excitation quencher, but knowledge on the kinetics of quenching is limited.

Structural and functional roles of non-bilayer lipid phases of chloroplast thylakoid membranes and mitochondrial inner membranes.

The 'standard' fluid-mosaic membrane model can provide a framework for the operation of the photosynthetic and respiratory electron transport systems, the generation of the proton motive force (pmf) and its utilization for ATP synthesis according to the chemiosmotic theory. However, this model, with the bilayer organization of all lipid molecules, assigns no function to non-bilayer lipids - while in recent years it became clear that the two fundamental energy transducing membranes of the biosphere, chloroplast thylakoid membranes (TMs) and inner mitochondrial membranes (IMMs), contain large amounts of non-bilayer (non-lamellar) lipid phases. In this review, we summarize our understanding on the role of non-lamellar phases in TMs and IMMs: (i) We propose that for these membrane vesicles the dynamic exchange model (DEM) provides a more suitable framework than the 'standard' model; DEM complements the 'standard' model by assuming the co-existence of bilayer and non-bilayer phases and their interactions, which contribute to the structural dynamics of the membrane systems and safe-guard the membranes' high protein:lipid ratios.

Dependence of the rate-limiting steps in the dark-to-light transition of photosystem II on the lipidic environment of the reaction center.

In our earlier works, we have identified rate-limiting steps in the dark-to-light transition of PSII. By measuring chlorophyll fluorescence transients elicited by single-turnover saturating flashes (STSFs) we have shown that in diuron-treated samples an STSF generates only F (< F) fluorescence level, and to produce the maximum (F) level, additional excitations are required, which, however, can only be effective if sufficiently long Δ waiting times are allowed between the excitations. Biological variations in the half-rise time (Δ ) of the fluorescence increment suggest that it may be sensitive to the physicochemical environment of PSII.

Salt Stress Induces Paramylon Accumulation and Fine-Tuning of the Macro-Organization of Thylakoid Membranes in Cells.

The effects of salt stress condition on the growth, morphology, photosynthetic performance, and paramylon content were examined in the mixotrophic, unicellular, flagellate . We found that salt stress negatively influenced cell growth, accompanied by a decrease in chlorophyll (Chl) content. Circular dichroism (CD) spectroscopy revealed the changes in the macro-organization of pigment-protein complexes due to salt treatment, while the small-angle neutron scattering (SANS) investigations suggested a reduction in the thylakoid stacking, an effect confirmed by the transmission electron microscopy (TEM).

Bee Venom Melittin Disintegrates the Respiration of Mitochondria in Healthy Cells and Lymphoblasts, and Induces the Formation of Non-Bilayer Structures in Model Inner Mitochondrial Membranes.

In this paper, we examined the effects of melittin, a bee venom membrane-active peptide, on mitochondrial respiration and cell viability of healthy human lymphocytes (HHL) and Jurkat cells, as well as on lymphoblasts from acute human T cell leukemia. The viability of melittin-treated cells was related to changes in O consumption and in the respiratory control index (RCI) of mitochondria isolated from melittin-pretreated cells as well as of mitochondria first isolated from cells and then directly treated with melittin. It was shown that melittin is three times more cytotoxic to Jurkat cells than to HHL, but O consumption and RCI values of mitochondria from both cell types were equally affected by melittin when melittin was directly added to mitochondria.

Lipid Polymorphism of the Subchloroplast-Granum and Stroma Thylakoid Membrane-Particles. II. Structure and Functions.

In Part I, by using P-NMR spectroscopy, we have shown that isolated granum and stroma thylakoid membranes (TMs), in addition to the bilayer, display two isotropic phases and an inverted hexagonal (H) phase; saturation transfer experiments and selective effects of lipase and thermal treatments have shown that these phases arise from distinct, yet interconnectable structural entities. To obtain information on the functional roles and origin of the different lipid phases, here we performed spectroscopic measurements and inspected the ultrastructure of these TM fragments. Circular dichroism, 77 K fluorescence emission spectroscopy, and variable chlorophyll-a fluorescence measurements revealed only minor lipase- or thermally induced changes in the photosynthetic machinery.

Lipid Polymorphism of the Subchloroplast-Granum and Stroma Thylakoid Membrane-Particles. I. P-NMR Spectroscopy.

Build-up of the energized state of thylakoid membranes and the synthesis of ATP are warranted by organizing their bulk lipids into a bilayer. However, the major lipid species of these membranes, monogalactosyldiacylglycerol, is a non-bilayer lipid. It has also been documented that fully functional thylakoid membranes, in addition to the bilayer, contain an inverted hexagonal (H) phase and two isotropic phases.

Cardiolipin, Non-Bilayer Structures and Mitochondrial Bioenergetics: Relevance to Cardiovascular Disease.

The present review is an attempt to conceptualize a contemporary understanding about the roles that cardiolipin, a mitochondrial specific conical phospholipid, and non-bilayer structures, predominantly found in the inner mitochondrial membrane (IMM), play in mitochondrial bioenergetics. This review outlines the link between changes in mitochondrial cardiolipin concentration and changes in mitochondrial bioenergetics, including changes in the IMM curvature and surface area, cristae density and architecture, efficiency of electron transport chain (ETC), interaction of ETC proteins, oligomerization of respiratory complexes, and mitochondrial ATP production. A relationship between cardiolipin decline in IMM and mitochondrial dysfunction leading to various diseases, including cardiovascular diseases, is thoroughly presented.

Differential Polarization Imaging of Plant Cells. Mapping the Anisotropy of Cell Walls and Chloroplasts.

Modern light microscopy imaging techniques have substantially advanced our knowledge about the ultrastructure of plant cells and their organelles. Laser-scanning microscopy and digital light microscopy imaging techniques, in general-in addition to their high sensitivity, fast data acquisition, and great versatility of 2D-4D image analyses-also opened the technical possibilities to combine microscopy imaging with spectroscopic measurements. In this review, we focus our attention on differential polarization (DP) imaging techniques and on their applications on plant cell walls and chloroplasts, and show how these techniques provided unique and quantitative information on the anisotropic molecular organization of plant cell constituents: (i) We briefly describe how laser-scanning microscopes (LSMs) and the enhanced-resolution Re-scan Confocal Microscope (RCM of Confocal.

Accumulation of geranylgeranylated chlorophylls in the pigment-protein complexes of Arabidopsis thaliana acclimated to green light: effects on the organization of light-harvesting complex II and photosystem II functions.

Light quality significantly influences plant metabolism, growth and development. Recently, we have demonstrated that leaves of barley and other plant species grown under monochromatic green light (500-590 nm) accumulated a large pool of chlorophyll a (Chl a) intermediates with incomplete hydrogenation of their phytyl chains. In this work, we studied accumulation of these geranylgeranylated Chls a and b in pigment-protein complexes (PPCs) of Arabidopsis plants acclimated to green light and their structural-functional consequences on the photosynthetic apparatus.

Light-adapted charge-separated state of photosystem II: structural and functional dynamics of the closed reaction center.

Photosystem II (PSII) uses solar energy to oxidize water and delivers electrons for life on Earth. The photochemical reaction center of PSII is known to possess two stationary states. In the open state (PSIIO), the absorption of a single photon triggers electron-transfer steps, which convert PSII into the charge-separated closed state (PSIIC).

Thylakoid membrane reorganizations revealed by small-angle neutron scattering of leaves associated with non-photochemical quenching.

Non-photochemical quenching (NPQ) is an important photoprotective mechanism in plants and algae. Although the process is extensively studied, little is known about its relationship with ultrastructural changes of the thylakoid membranes. In order to better understand this relationship, we studied the effects of illumination on the organization of thylakoid membranes in leaves.