studying-supramolecular-organization-photosynthetic-membranes-within

Studying the Supramolecular Organization of Photosynthetic Membranes within Freeze-fractured Leaf Tissues by Cryo-scanning Electron Microscopy

Cryo-scanning electron microscopy (SEM) of freeze-fractured samples allows investigation of biological structures at near native conditions. Here, we ...

Weizmann Institute of Science

Agricultural Research Organization, Volcani Center

Cyanobacteria, the progenitors of plant and algal chloroplasts, enabled aerobic life on earth by introducing oxygenic photosynthesis. In most cyanobacteria, the photosynthetic membranes are arranged in multiple, seemingly disconnected, concentric shells. In such an arrangement, it is unclear how intracellular trafficking proceeds and how different layers of the photosynthetic membranes communicate with each other to maintain photosynthetic homeostasis. Using electron microscope tomography, we show that the photosynthetic membranes of two distantly related cyanobacterial species contain multiple perforations. These perforations, which are filled with particles of different sizes including ribosomes, glycogen granules and lipid bodies, allow for traffic throughout the cell. In addition, different layers of the photosynthetic membranes are joined together by internal bridges formed by branching and fusion of the membranes. The result is a highly connected network, similar to that of higher-plant chloroplasts, allowing water-soluble and lipid-soluble molecules to diffuse through the entire membrane network. Notably, we observed intracellular membrane-bounded vesicles, which were frequently fused to the photosynthetic membranes and may play a role in transport to these membranes.

Thylakoid membrane perforations and connectivity enable intracellular traffic in cyanobacteria.

Endoplasmic reticulum-associated degradation (ERAD) eliminates aberrant proteins from the secretory pathway. Such proteins are retained in the endoplasmic reticulum and targeted for degradation by the ubiquitin-proteasome system. Cis-acting motifs can function in ERAD as retention signals, preventing vesicular export from the endoplasmic reticulum, or as degrons, targeting proteins for degradation. Here, we show that microstp, the C-terminal 20-residue tailpiece of the secretory IgM mus heavy chain, functions both as a portable retention signal and as an ERAD degron. Retention of microstp fusions of secreted versions of thyroid peroxidase and yellow fluorescent protein in the endoplasmic reticulum requires the presence of the penultimate cysteine of microstp. In its role as a portable degron, the microstp targets the retained proteins for ERAD but does not serve as an obligatory ubiquitin-conjugation site. Abolishing microstp glycosylation accelerates the degradation of both microstpCys-fused substrates, yet absence of the N-glycan eliminates the requirement for the penultimate cysteine in the retention and degradation of the unglycosylated yellow fluorescent protein. Hence, the dual role played by the microstpCys motif as a retention signal and as a degron can be attributed to distinct elements within this sequence.

Distinguishing between retention signals and degrons acting in ERAD.

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&#x7C7B;&#x56CA;&#x4F53;&#x819C;&#x819C;&#x91CD;&#x5851;&#x5728;&#x62DF;&#x5357;&#x82A5;&#x4E2D;&#x7684;&#x72B6;&#x6001;&#x8F6C;&#x6362;&#x3002;

Adaptability of oxygenic photosynthetic organisms to fluctuations in light spectral composition and intensity is conferred by state transitions, short-term regulatory processes that enable the photosynthetic apparatus to rapidly adjust to variations in light quality. In green algae and higher plants, these processes are accompanied by reversible structural rearrangements in the thylakoid membranes. We studied these structural changes in the thylakoid membranes of Arabidopsis thaliana chloroplasts using atomic force microscopy, scanning and transmission electron microscopy, and confocal imaging. Based on our results and on the recently determined three-dimensional structure of higher-plant thylakoids trapped in one of the two major light-adapted states, we propose a model for the transitions in membrane architecture. The model suggests that reorganization of the membranes involves fission and fusion events that occur at the interface between the appressed (granal) and nonappressed (stroma lamellar) domains of the thylakoid membranes. Vertical and lateral displacements of the grana layers presumably follow these localized events, eventually leading to macroscopic rearrangements of the entire membrane network.

Thylakoid membrane remodeling during state transitions in Arabidopsis.

Adaptabilit&#xE9; des oxyg&#xE9;nique organismes photosynth&#xE9;tiques aux fluctuations de la composition spectrale de lumi&#xE8;re et de l&#x26;#39;intensit&#xE9; est conf&#xE9;r&#xE9;e par les transitions d&#x26;#39;&#xE9;tat &#xE0; court terme des processus de r&#xE9;glementation qui permettent &#xE0; l&#x26;#39;appareil photosynth&#xE9;tique de s&#x26;#39;adapter rapidement aux variations de qualit&#xE9; de la lumi&#xE8;re. Chez les algues vertes et les plantes sup&#xE9;rieures, ces processus sont accompagn&#xE9;s par r&#xE9;versibles r&#xE9;arrangements structuraux dans les membranes des thylako&#xEF;des. Nous avons &#xE9;tudi&#xE9; ces changements structurels dans les membranes des thylako&#xEF;des des chloroplastes d&#x26;#39;Arabidopsis thaliana en utilisant la microscopie &#xE0; force atomique, la num&#xE9;risation et la microscopie &#xE9;lectronique &#xE0; transmission, et de l&#x26;#39;imagerie confocale. Sur la base de nos r&#xE9;sultats et sur la r&#xE9;cemment d&#xE9;termin&#xE9; la structure tridimensionnelle de l&#x26;#39;enseignement sup&#xE9;rieur-plantes thylako&#xEF;des pris au pi&#xE8;ge dans l&#x26;#39;un des deux principaux &#xE9;tats de lumi&#xE8;re adapt&#xE9;s, nous proposons un mod&#xE8;le pour les transitions de l&#x26;#39;architecture membrane. Le mod&#xE8;le sugg&#xE8;re que la r&#xE9;organisation des membranes implique des &#xE9;v&#xE9;nements de fission et de fusion qui se produisent &#xE0; l&#x26;#39;interface entre le apprim&#xE9;es (Granal) et nonappressed (stroma lamellaire) domaines de la membrane des thylaco&#xEF;des. Les d&#xE9;placements verticaux et lat&#xE9;raux des couches grana vraisemblablement suivre ces &#xE9;v&#xE9;nements localis&#xE9;s, pour aboutir finalement &#xE0; des r&#xE9;arrangements macroscopiques du r&#xE9;seau membrane enti&#xE8;re.

Remodelage des membranes thylako&#xEF;des Pendant transitions d&#x26;#39;&#xE9;tat dans Arabidopsis

Anpassungsf&auml;higkeit der Kalkstabilisierung photosynthetischen Organismen zu Schwankungen bei leichten spektralen Zusammensetzung und Intensit&auml;t ist von Zustands&uuml;berg&auml;ngen, kurzfristige regulatorischen Prozesse vergeben, mit denen der photosynthetischen Apparat Variationen in leichter Qualit&auml;t schnell anpassen k&ouml;nnen. Gr&uuml;nalgen und h&ouml;heren Pflanzen sind diese Prozesse von reversible strukturellen Umlagerungen in der Thylakoid-Membranen begleitet. Wir studierten diese strukturellen Ver&auml;nderungen in den Thylakoid Membranen Arabidopsis Thaliana Chloroplasten mittels Rasterkraftmikroskopie, Scannen und &Uuml;bertragung Elektronenmikroskopie und konfokale Bildgebung. Basierend auf unsere Ergebnisse und &uuml;ber die vor kurzem ermittelte dreidimensionale Struktur der h&ouml;heren Pflanze Granalamelle gefangen in einem der zwei gro&szlig;en Licht-angepasst, schlagen wir ein Modell f&uuml;r die &Uuml;berg&auml;nge in der Membran-Architektur. Das Modell legt nahe, dass Reorganisation der Membranen Spalt- und Ereignisse umfasst, die an der Schnittstelle zwischen der linealisch (granal) auftreten und nonappressed (Stroma lamellar) Dom&auml;nen der Thylakoid Membranen. Vertikale und laterale Verschiebungen der Grana-Schichten folgen vermutlich diese lokalisierte Ereignisse, makroskopische Umlagerungen der gesamten Membran-Netzwerks und letztlich.

Thylakoid Membrane Umbau w&auml;hrend der Zustands&uuml;berg&auml;nge in Arabidopsis.

Adattabilit&agrave; degli organismi fotosintetici ossigenati a fluttuazioni nella composizione spettrale luce e intensit&agrave; &egrave; conferito da transizioni di stato, a breve termine dei processi normativi che consentono l&#39;apparato fotosintetico regolare rapidamente alle variazioni della qualit&agrave; della luce. In alghe verdi e piante superiori, questi processi sono accompagnati da riarrangiamenti strutturali reversibili nelle membrane del thylakoid. Abbiamo studiato questi cambiamenti strutturali nelle membrane dei cloroplasti thaliana di Arabidopsis mediante microscopia a forza atomica, microscopia elettronica a scansione e trasmissione e imaging confocale tilacoide. Basato sui nostri risultati e sulla struttura tridimensionale determinata di recente della pianta superiore granario intrappolato in uno dei due principali stati adattati di luce, vi proponiamo un modello per le transizioni in architettura di membrana. Il modello suggerisce che la riorganizzazione delle membrane implica fissione e fusione di eventi che si verificano a livello di interfaccia tra il appressed (granal) e nonappressed domini (stroma lamellare) delle membrane del thylakoid. Spostamenti verticali e laterali degli strati grana presumibilmente seguono questi eventi localizzati, finalmente conducendo al macroscopiche riorganizzazioni della rete intera membrana.

Tilacoide membrana rimodellamento durante le transizioni di stato in Arabidopsis.

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&#x30B7;&#x30ED;&#x30A4;&#x30CC;&#x30CA;&#x30BA;&#x30CA;&#x306B;&#x304A;&#x3051;&#x308B;&#x72B6;&#x614B;&#x9077;&#x79FB;&#x6642;&#x306B;&#x30C1;&#x30E9;&#x30B3;&#x30A4;&#x30C9;&#x819C;&#x30EA;&#x30D5;&#x30A9;&#x30FC;&#x30E0;

&#xBCC0;&#xB3D9; &#xBE5B; &#xC2A4;&#xD399;&#xD2B8;&#xB7FC; &#xAD6C;&#xC131;&#xACFC; &#xAC15;&#xB3C4;&#xC5D0; oxygenic &#xAD11;&#xD569;&#xC131; &#xC0DD;&#xBB3C;&#xC758; &#xC801;&#xC751;&#xC131;&#xC740; &#xBE5B; &#xD488;&#xC9C8;&#xC758; &#xBCC0;&#xD654;&#xB97C; &#xBE60;&#xB974;&#xAC8C; &#xC870;&#xC815; &#xD558; &#xAD11;&#xD569;&#xC131; &#xAE30;&#xAD6C;&#xB97C; &#xC0AC;&#xC6A9;&#xD560; &#xC218; &#xC788;&#xB294; &#xB2E8;&#xAE30; &#xADDC;&#xC81C; &#xD504;&#xB85C;&#xC138;&#xC2A4;&#xC758; &#xC0C1;&#xD0DC; &#xBCC0;&#xD654;&#xC5D0; &#xC758;&#xD574; &#xC218; &#xC5EC;. &#xC870;&#xB958;&#xC640; &#xB192;&#xC740; &#xC2DD;&#xBB3C;, &#xC774;&#xB7EC;&#xD55C; &#xD504;&#xB85C;&#xC138;&#xC2A4; &#xAC00;&#xC5ED; &#xAD6C;&#xC870;&#xC801; &#xC7AC;&#xBC30;&#xC5F4; thylakoid &#xB9C9;&#xC740; &#xD568;&#xAED8; &#xC81C;&#xACF5; &#xB429;&#xB2C8;&#xB2E4;. &#xC6B0;&#xB9AC;&#xB294; &#xC774;&#xB7EC;&#xD55C; &#xAD6C;&#xC870;&#xC801; &#xBCC0;&#xD654;&#xB294; Arabidopsis thaliana chloroplasts &#xC6D0;&#xC790; &#xD798; &#xD604;&#xBBF8;&#xACBD;, &#xC2A4;&#xCE94; &#xBC0F; &#xC804;&#xC1A1; &#xC804;&#xC790; &#xD604;&#xBBF8;&#xACBD; &#xAC80;&#xC0AC; &#xBC95;, confocal &#xC601;&#xC0C1; &#xC0AC;&#xC6A9;&#xC758; thylakoid &#xB9C9;&#xC740; &#xACF5;&#xBD80;. &#xC6B0;&#xB9AC;&#xC758; &#xACB0;&#xACFC;&#xC5D0; &#xB530;&#xB77C; &#xB450; &#xAC00;&#xC9C0; &#xC8FC;&#xC694; &#xBE5B; &#xC801;&#xC751; &#xC0C1;&#xD0DC; &#xC911; &#xD558;&#xB098;&#xC5D0; &#xAC07;&#xD600; &#xB354; &#xB192;&#xC740; &#xC2DD;&#xBB3C; thylakoids&#xC758; &#xCD5C;&#xADFC;&#xC5D0; &#xD655;&#xC778;&#xB41C; &#xB41C; 3 &#xCC28;&#xC6D0; &#xAD6C;&#xC870;, &#xC6B0;&#xB9AC; &#xB9C9; &#xC544;&#xD0A4;&#xD14D;&#xCC98;&#xC5D0;&#xC11C; &#xC804;&#xD658;&#xC5D0; &#xB300; &#xD55C; &#xBAA8;&#xB378;&#xC744; &#xC81C;&#xC548; &#xD569;&#xB2C8;&#xB2E4;. &#xBAA8;&#xB378; &#xC138;&#xD3EC; &#xB9C9;&#xC758; &#xC870;&#xC9C1; &#xAC1C;&#xD3B8; (granal) &#xC0AC;&#xC774;&#xB294; appressed &#xC778;&#xD130;&#xD398;&#xC774;&#xC2A4;&#xC5D0;&#xC11C; &#xBC1C;&#xC0DD; &#xD558;&#xB294; &#xBD84;&#xC5F4; &#xBC0F; &#xC735;&#xD569; &#xC774;&#xBCA4;&#xD2B8; &#xAD00;&#xB828; &#xC81C;&#xC548; &#xBC0F; nonappressed thylakoid &#xC138;&#xD3EC; &#xB9C9;&#xC758; (&#xAE30;&#xC9C8; &#xD50C;&#xB808;&#xC774;&#xD2B8;) &#xB3C4;&#xBA54;&#xC778;. Grana &#xB808;&#xC774;&#xC5B4;&#xC758; &#xC218;&#xC9C1; &#xBC0F; &#xC218;&#xD3C9; &#xBCC0;&#xC704;&#xB294; &#xC544;&#xB9C8;&#xB3C4; &#xACB0;&#xAD6D; &#xC804;&#xCCB4; &#xB9C9; &#xB124;&#xD2B8;&#xC6CC;&#xD06C;&#xC758; &#xAC70;&#xC2DC;&#xC801;&#xC778; &#xC7AC;&#xBC30;&#xC5F4;&#xC73C;&#xB85C; &#xC774;&#xC5B4;&#xC9C0;&#xB294; &#xC774;&#xB7EC;&#xD55C; &#xC9C0;&#xC5ED;&#xD654; &#xB41C; &#xC774;&#xBCA4;&#xD2B8;&#xB97C; &#xB530;&#xB985;&#xB2C8;&#xB2E4;.

Thylakoid Arabidopsis&#xC758; &#xC0C1;&#xD0DC;&#xB97C; &#xC804;&#xD658; &#xD558;&#xB294; &#xB3D9;&#xC548; &#xB9C9; &#xAC1C;&#xC7A5; &#xB418;&#xC5C8;&#xC2B5;&#xB2C8;&#xB2E4;.

Capacidad de adaptaci&#xF3;n de los organismos fotosint&#xE9;ticos oxig&#xE9;nica a las fluctuaciones en la composici&#xF3;n espectral de la luz y la intensidad es conferida por las transiciones de estado a corto plazo, los procesos de regulaci&#xF3;n que permiten que el aparato fotosint&#xE9;tico de adaptarse con rapidez a las variaciones en la calidad de la luz. En las algas verdes y plantas superiores, estos procesos se acompa&#xF1;an de alteraciones estructurales reversibles en las membranas de los tilacoides. Se estudiaron los cambios estructurales en las membranas de los tilacoides de los cloroplastos de Arabidopsis thaliana mediante microscop&#xED;a de fuerza at&#xF3;mica, la exploraci&#xF3;n y la microscop&#xED;a electr&#xF3;nica de transmisi&#xF3;n y visualizaci&#xF3;n de imagen confocal. En base a los resultados y en la recientemente determin&#xF3; la estructura tridimensional de los tilacoides de plantas de mayor atrapados en uno de los dos estados principales de luz adecuadas, se propone un modelo para las transiciones de la arquitectura de la membrana. El modelo sugiere que la reorganizaci&#xF3;n de las membranas que implica eventos de fisi&#xF3;n y de fusi&#xF3;n que ocurren en la interfase entre el adpreso (grana) y nonappressed (estroma lamelar) dominios de las membranas de los tilacoides. Desplazamientos verticales y laterales de las capas de grana, presumiblemente sigue estos eventos localizados, llevando eventualmente a reordenamientos macrosc&#xF3;picas de la red completa de la membrana.

Remodelaci&#xF3;n de membrana tilacoide Durante transiciones de estado en Arabidopsis

&#x5173;&#x4E8E;&#x9AD8;&#x7B49;&#x690D;&#x7269;&#x7C7B;&#x56CA;&#x4F53;&#x819C;&#x7F51;&#x7EDC;&#x7684;&#x4E09;&#x7EF4;&#x6A21;&#x578B;&#x8BF4;&#x660E;&#x3002;

A note on three-dimensional models of higher-plant thylakoid networks.

Une note sur les mod&#xE8;les en trois dimensions des r&#xE9;seaux thylako&#xEF;de sup&#xE9;rieur de plantes

Anmerkung zu dreidimensionalen Modellen der h&ouml;heren Pflanze Thylakoid-Netzwerke.

Una nota su modelli tridimensionali di reti pi&ugrave; alta pianta tilacoide.

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Una nota sobre modelos tridimensionales de las redes de la planta superior-tilacoides

Aerobic life on Earth depends on oxygenic photosynthesis. This fundamentally important process is carried out within an elaborate membranous system, called the thylakoid network. In angiosperms, thylakoid networks are constructed almost from scratch by an intricate, light-dependent process in which lipids, proteins, and small organic molecules are assembled into morphologically and functionally differentiated, three-dimensional lamellar structures. In this review, we summarize the major events that occur during this complex, largely elusive process, concentrating on those that are directly involved in network formation and potentiation and highlighting gaps in our knowledge, which, as hinted by the title, are substantial.

Biogenesis of thylakoid networks in angiosperms: knowns and unknowns.

The machinery that conducts the light-driven reactions of oxygenic photosynthesis is hosted within specialized paired membranes called thylakoids. In higher plants, the thylakoids are segregated into two morphological and functional domains called grana and stroma lamellae. A large fraction of the luminal volume of the granal thylakoids is occupied by the oxygen-evolving complex of photosystem II. Electron microscopy data we obtained on dark- and light-adapted Arabidopsis thylakoids indicate that the granal thylakoid lumen significantly expands in the light. Models generated for the organization of the oxygen-evolving complex within the granal lumen predict that the light-induced expansion greatly alleviates restrictions imposed on protein diffusion in this compartment in the dark. Experiments monitoring the redox kinetics of the luminal electron carrier plastocyanin support this prediction. The impact of the increase in protein mobility within the granal luminal compartment in the light on photosynthetic electron transport rates and processes associated with the repair of photodamaged photosystem II complexes is discussed.

Dynamic control of protein diffusion within the granal thylakoid lumen.

Chloroplasts of higher plants develop from proplastids, which are undifferentiated plastids that lack photosynthetic (thylakoid) membranes. In flowering plants, the proplastid-chloroplast transition takes place at the shoot apex, which consists of the shoot apical meristem (SAM) and the flanking leaf primordia. It has been believed that the SAM contains only proplastids and that these become chloroplasts only in the primordial leaves. Here, we show that plastids of the SAM are neither homogeneous nor necessarily null. Rather, their developmental state varies with the specific region and/or layer of the SAM in which they are found. Plastids throughout the L1 and L3 layers of the SAM possess fairly developed thylakoid networks. However, many of these plastids eventually lose their thylakoids during leaf maturation. By contrast, plastids at the central, stem cell-harboring region of the L2 layer of the SAM lack thylakoid membranes; these appear only at the periphery, near the leaf primordia. Thus, plastids in the SAM undergo distinct differentiation processes that, depending on their lineage and position, lead to either development or loss of thylakoid membranes. These processes continue along the course of leaf maturation.

Gain and loss of photosynthetic membranes during plastid differentiation in the shoot apex of Arabidopsis.

The process of oxygenic photosynthesis enabled and still sustains aerobic life on Earth. The most elaborate form of the apparatus that carries out the primary steps of this vital process is the one present in higher plants. Here, we review the overall composition and supramolecular organization of this apparatus, as well as the complex architecture of the lamellar system within which it is harbored. Along the way, we refer to the genetic, biochemical, spectroscopic and, in particular, microscopic studies that have been employed to elucidate the structure and working of this remarkable molecular energy conversion device. As an example of the highly dynamic nature of the apparatus, we discuss the molecular and structural events that enable it to maintain high photosynthetic yields under fluctuating light conditions. We conclude the review with a summary of the hypotheses made over the years about the driving forces that underlie the partition of the lamellar system of higher plants and certain green algae into appressed and non-appressed membrane domains and the segregation of the photosynthetic protein complexes within these domains.

Composition, architecture and dynamics of the photosynthetic apparatus in higher plants.

The chromatophores of Rhodobacter (Rb.) sphaeroides represent a minimal bio-energetic system, which efficiently converts light energy into usable chemical energy. Despite extensive studies, several issues pertaining to the morphology and molecular architecture of this elemental energy conversion system remain controversial or unknown. To tackle these issues, we combined electron microscope tomography, immuno-electron microscopy and atomic force microscopy. We found that the intracellular Rb. sphaeroides chromatophores form a continuous reticulum rather than existing as discrete vesicles. We also found that the cytochrome bc1 complex localizes to fragile chromatophore regions, which most likely constitute the tubular structures that interconnect the vesicles in the reticulum. In contrast, the peripheral light-harvesting complex 2 (LH2) is preferentially hexagonally packed within the convex vesicular regions of the membrane network. Based on these observations, we propose that the bc1 complexes are in the inter-vesicular regions and surrounded by reaction center (RC) core complexes, which in turn are bounded by arrays of peripheral antenna complexes. This arrangement affords rapid cycling of electrons between the core and bc1 complexes while maintaining efficient excitation energy transfer from LH2 domains to the RCs.

The architecture of Rhodobacter sphaeroides chromatophores.

During desiccation, homoiochlorophyllous resurrection plants retain most of their photosynthetic apparatus, allowing them to resume photosynthetic activity quickly upon water availability. These plants rely on various mechanisms to prevent the formation of reactive oxygen species and/or protect their tissues from the damage they inflict. In this work, we addressed the issue of how homoiochlorophyllous resurrection plants deal with the problem of excessive excitation/electron pressures during dehydration using Craterostigma pumilum as a model plant. To investigate the alterations in the supramolecular organization of photosynthetic protein complexes, we examined cryoimmobilized, freeze-fractured leaf tissues using (cryo)scanning electron microscopy. These examinations revealed rearrangements of photosystem II (PSII) complexes, including a lowered density during moderate dehydration, consistent with a lower level of PSII proteins, as shown by biochemical analyses. The latter also showed a considerable decrease in the level of cytochrome f early during dehydration, suggesting that initial regulation of the inhibition of electron transport is achieved via the cytochrome b6f complex. Upon further dehydration, PSII complexes are observed to arrange into rows and semicrystalline arrays, which correlates with the significant accumulation of sucrose and the appearance of inverted hexagonal lipid phases within the membranes. As opposed to PSII and cytochrome f, the light-harvesting antenna complexes of PSII remain stable throughout the course of dehydration. Altogether, these results, along with photosynthetic activity measurements, suggest that the protection of retained photosynthetic components is achieved, at least in part, via the structural rearrangements of PSII and (likely) light-harvesting antenna complexes into a photochemically quenched state.

Photoprotection conferred by changes in photosynthetic protein levels and organization during dehydration of a homoiochlorophyllous resurrection plant.

The group of homoiochlorophyllous resurrection plants evolved the unique capability to survive severe drought stress without dismantling the photosynthetic machinery. This implies that they developed efficient strategies to protect the leaves from reactive oxygen species (ROS) generated by photosynthetic side reactions. These strategies, however, are poorly understood. Here, we performed a detailed study of the photosynthetic machinery in the homoiochlorophyllous resurrection plant Craterostigma pumilum during dehydration and upon recovery from desiccation. During dehydration and rehydration, C. pumilum deactivates and activates partial components of the photosynthetic machinery in a specific order, allowing for coordinated shut down and subsequent reinstatement of photosynthesis. Early responses to dehydration are the closure of stomata and activation of electron transfer to oxygen accompanied by inactivation of the cytochrome b6 f complex leading to attenuation of the photosynthetic linear electron flux (LEF). The decline in LEF is paralleled by a gradual increase in cyclic electron transport to maintain ATP production. At low water contents, inactivation and supramolecular reorganization of photosystem II becomes apparent, accompanied by functional detachment of light harvesting complexes and interrupted access to plastoquinone. This well-ordered sequence of alterations in the photosynthetic thylakoid membranes helps prepare the plant for the desiccated state and minimize ROS production. This article is protected by copyright. All rights reserved.

Dana Charuvi

Department of Biological Chemistry

Weizmann Institute of Science

Studying the Supramolecular Organization of Photosynthetic Membranes within Freeze-fractured Leaf Tissues by Cryo-scanning Electron Microscopy