The light reactions of oxygenic photosynthesis are performed by protein complexes embedded in the lipid bilayer of thylakoid membranes (TMs). Bilayers provide optimal conditions for the build-up of the proton motive force (pmf) and ATP synthesis. However, functional plant TMs, besides the bilayer, contain an inverted hexagonal (HII) phase and isotropic phases, a lipid polymorphism due to their major, non-bilayer lipid species, monogalactosyldiacylglycerol (MGDG). The lipid phase behavior of TMs is explained within the framework of the Dynamic Exchange Model (DEM), an extension of the fluid-mosaic model. DEM portrays the bilayer phase as inclusions between photosynthetic supercomplexes - characterized by compromised membrane impermeability and restricted sizes inflicted by the segregation propensity of lipid molecules, safe-guarding the high protein density of TMs. Isotropic phases mediate membrane fusions and are associated with the lumenal lipocalin-like enzyme, violaxanthin de-epoxidase. Stromal-side proteins surrounded by lipids give rise to the HII phase. These features instigate experimentally testable hypotheses: (i) non-bilayer phases mediate functional sub-compartmentalization of plant chloroplasts - a quasi-autonomous energization and ATP synthesis of each granum-stroma TM assembly; and (ii) the generation and utilization of pmf depend on hydrated protein networks and proton-conducting pathways along membrane surfaces - rather than on strict impermeability of the bilayer.

• MeSH
• Models, Biological MeSH
• Photosynthesis MeSH
• Galactolipids metabolism   MeSH
• Lipid Bilayers metabolism   MeSH
• Plants * metabolism   MeSH
• Thylakoids * metabolism   chemistry   MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• Galactolipids MeSH
• Lipid Bilayers MeSH

It has been thoroughly documented, by using 31P-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 (HII) 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. 2022 Progr Lipid Res 101,163). We take advantage of the selectivity of wheat germ lipase (WGL) in eliminating the I phases of TMs (Dlouhý et al. 2022 Cells 11: 2681), and the tendency of the so-called BBY particles, stacked photosystem II (PSII) enriched membrane pairs of 300-500 nm in diameter, to form large laterally fused sheets (Dunahay et al. 1984 BBA 764: 179). Our 31P-NMR spectroscopy data show that BBY membranes contain L and I phases. Similar to TMs, WGL selectively eliminated the I phases, which at the same time exerted no effect on the molecular organization and functional activity of PSII membranes. As revealed by sucrose-density centrifugation, magnetic linear dichroism spectroscopy and scanning electron microscopy, WGL disassembled the large laterally fused sheets. These data provide direct experimental evidence on the involvement of I phase(s) in the fusion of stacked PSII membrane pairs, and strongly suggest the role of non-bilayer lipids in the self-assembly of the TM system.

• Keywords
• 31P-NMR spectroscopy; BBY membrane, Linear dichroism spectroscopy, Membrane fusion; non-bilayer lipids, Wheat germ lipase,
• MeSH
• Photosystem II Protein Complex * metabolism   MeSH
• Membrane Fusion physiology   MeSH
• Lipids chemistry   MeSH
• Magnetic Resonance Spectroscopy MeSH
• Thylakoids * metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Photosystem II Protein Complex * MeSH
• Lipids MeSH

The acclimation of higher plants to different light intensities is associated with a reorganization of the photosynthetic apparatus. These modifications, namely, changes in the amount of peripheral antenna (LHCII) of photosystem (PS) II and changes in PSII/PSI stoichiometry, typically lead to an altered chlorophyll (Chl) a/b ratio. However, our previous studies show that in spruce, this ratio is not affected by changes in growth light intensity. The evolutionary loss of PSII antenna proteins LHCB3 and LHCB6 in the Pinaceae family is another indication that the light acclimation strategy in spruce could be different. Here we show that, unlike Arabidopsis, spruce does not modify its PSII/PSI ratio and PSII antenna size to maximize its photosynthetic performance during light acclimation. Its large PSII antenna consists of many weakly bound LHCIIs, which form effective quenching centers, even at relatively low light. This, together with sensitive photosynthetic control on the level of cytochrome b6f complex (protecting PSI), is the crucial photoprotective mechanism in spruce. High-light acclimation of spruce involves the disruption of PSII macro-organization, reduction of the amount of both PSII and PSI core complexes, synthesis of stress proteins that bind released Chls, and formation of "locked-in" quenching centers from uncoupled LHCIIs. Such response has been previously observed in the evergreen angiosperm Monstera deliciosa exposed to high light. We suggest that, in contrast to annuals, shade-tolerant evergreen land plants have their own strategy to cope with light intensity changes and the hallmark of this strategy is a stable Chl a/b ratio.

• Keywords
• Arabidopsis thaliana, LHCII antenna, Light acclimation, Non-photochemical quenching, Photoprotection, Photosynthetic control, Picea abies, Thylakoid membrane,
• MeSH
• Acclimatization MeSH
• Arabidopsis * metabolism   MeSH
• Chlorophyll A metabolism   MeSH
• Chlorophyll metabolism   MeSH
• Cytochromes b metabolism   MeSH
• Photosystem I Protein Complex metabolism   MeSH
• Photosystem II Protein Complex metabolism   MeSH
• Cytochrome b6f Complex metabolism   MeSH
• Heat-Shock Proteins metabolism   MeSH
• Picea * metabolism   MeSH
• Light MeSH
• Light-Harvesting Protein Complexes metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Chlorophyll A MeSH
• Chlorophyll MeSH
• Cytochromes b MeSH
• Photosystem I Protein Complex MeSH
• Photosystem II Protein Complex MeSH
• Cytochrome b6f Complex MeSH
• Heat-Shock Proteins MeSH
• Light-Harvesting Protein Complexes MeSH

It is well established that plant thylakoid membranes (TMs), in addition to a bilayer, contain two isotropic lipid phases and an inverted hexagonal (HII) phase. To elucidate the origin of non-bilayer lipid phases, we recorded the 31P-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 31P-NMR-detectable lipid phases of isolated TMs, but the weak signal of isolated plastoglobuli was not affected. Parallel with the destabilization of their lamellar phase, TMs lost their impermeability; other effects, mainly on Photosystem-II, lagged behind the destruction of the original phases. Wheat-germ lipase selectively eliminated the isotropic phases but exerted little or no effect on the structural and functional parameters of TMs-indicating that the isotropic phases are located outside the protein-rich regions and might be involved in membrane fusion. Trypsin and Proteinase K selectively suppressed the HII phase-suggesting that a large fraction of TM lipids encapsulate stroma-side proteins or polypeptides. We conclude that-in line with the Dynamic Exchange Model-the non-bilayer lipid phases of TMs are found in subdomains separated from but interconnected with the bilayer accommodating the main components of the photosynthetic machinery.

• Keywords
• 31P-NMR spectroscopy, lipid polymorphism, lipocalins, membrane fusion, membrane models, non-bilayer lipids, plastoglobuli, structural and functional plasticity, thylakoid membrane,
• MeSH
• Lipase metabolism   MeSH
• Lipid Bilayers * metabolism   MeSH
• Magnetic Resonance Spectroscopy MeSH
• Peptide Hydrolases metabolism   MeSH
• Thylakoids * metabolism   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Lipase MeSH
• Lipid Bilayers * MeSH
• Peptide Hydrolases MeSH