Composition of the photosystems and chloroplast structure in extreme shade plants

Changes in Metabolism and Content of Chlorophyll in Common Duckweed (Lemna minor L.) Caused by Environmental Contamination with Fluorides

The impact of fluorine on plants remains poorly understood. We examined duckweed growth in extracts of soil contaminated with fluorine leached from chicken manure. Additionally, fluorine levels were analyzed in fresh manure, outdoor-stored manure, and soil samples at varying distances from the manure pile. Fresh manure contained 37-48 mg F- × kg-1, while soil extracts contained 2.1 to 4.9 mg F- × kg-1. We evaluated the physiological effects of fluorine on duckweed cultured on soil extracts or in 50% Murashige-Skoog (MS) medium supplemented with fluorine concentrations matching those in soil samples (2.1 to 4.9 mg F- × L-1), as well as at 0, 4, and 210 mg × L-1. Duckweed exposed to fluorine displayed similar toxicity symptoms whether in soil extracts or supplemented medium. Fluoride at concentrations of 2.1 to 4.9 mg F- × L-1 reduced the intact chlorophyll content, binding the porphyrin ring at position 32 without affecting Mg2+. This reaction resulted in chlorophyll a absorption peak shifted towards shorter wavelengths and formation of a new band of the F--chlorophyll a complex at λ = 421 nm. Moreover, plants exposed to low concentrations of fluorine exhibited increased activities of aminolevulinic acid dehydratase and chlorophyllase, whereas the activities of both enzymes sharply declined when the fluoride concentration exceeded 4.9 mg × L-1. Consequently, fluorine damages chlorophyll a, disrupts the activity of chlorophyll-metabolizing enzymes, and diminishes the plant growth rate, even when the effects of these disruptions are too subtle to be discerned by the naked human eye.

Expansion microscopy resolves the thylakoid structure of spinach

The light-harvesting reactions of photosynthesis take place on the thylakoid membrane inside chloroplasts. The thylakoid membrane is folded into appressed membranes, the grana, and nonappressed membranes that interconnect the grana, the stroma lamellae. This folding is essential for the correct functioning of photosynthesis. Electron microscopy and atomic force microscopy are commonly used to study the thylakoid membrane, but these techniques have limitations in visualizing a complete chloroplast and its organization. To overcome this limitation, we applied expansion microscopy (ExM) on isolated chloroplasts. ExM is a technique that involves physically expanding a sample in a swellable hydrogel to enhance the spatial resolution of fluorescence microscopy. Using all-protein staining, we visualized the 3D structure of spinach (Spinacia oleracea) thylakoids in detail. We were able to resolve stroma lamellae that were 60 nm apart and observe their helical wrapping around the grana. Furthermore, we accurately measured the dimensions of grana from top views of chloroplasts, which allow for precise determination of the granum diameter. Our results demonstrate that ExM is a fast and reliable technique for studying thylakoid organization in great detail.

Thylakoid membrane appression in the giant chloroplast of Selaginella martensii Spring: A lycophyte challenges grana paradigms in shade-adapted species

In vascular plants, the thylakoid architecture is dominated by the highly structured multiple membrane layers known as grana. The structural diversity of the thylakoid system among plant species is mainly determined by the adaptation to the growth light regime, according to a paradigm stating that shade-tolerant species are featured by a high membrane extension with an enhanced number of thylakoid layers per granum. In this study, the thylakoid system was analysed in Selaginella martensii Spring, a shade-adapted rainforest species belonging to lycophytes, a diminutive plant lineage, sister clade of all other vascular plants (euphyllophytes, including ferns and seed plants). The species is characterized by giant cup-shaped chloroplasts in the upper epidermis and, quantitatively less important, disk-shaped chloroplasts in the mesophyll and lower epidermis. The study aimed at the quantitative assessment of the thylakoid appression exploiting a combination of complementary methods, including electron microscopy, selective thylakoid solubilisation, electron paramagnetic resonance, and simultaneous analysis of fast chlorophyll a fluorescence and P700 redox state. With a chlorophyll a/b ratio of 2.6 and PSI/PSII ratio of 0.31, the plant confirmed two typical hallmarks of shade-adaptation. The morphometric analysis of electron micrographs revealed a 33% fraction of non-appressed thylakoid domains. However, contrasting with the structural paradigm of thylakoid shade-adaptation in angiosperms, S. martensii privileges the increase in the granum diameter in place of the increase in the number of layers building the granum. The very wide grana diameter, 727 nm on average, largely overcame the threshold of 500 nm currently hypothesized to allow an effective diffusion of long-range electron carriers. The fraction of non-appressed membranes based on the selective solubilisation of thylakoids with digitonin was 26%, lower than the morphometric determination, indicating the presence of non-appressed domains inaccessible to the detergent, most probably because of the high three-dimensional complexity of the thylakoid system in S. martensii. Particularly, strong irregularity of grana stacks is determined by assembling thylakoid layers of variable width that tend to slide apart from each other as the number of stacked layers increases.

Structure and function of bark and wood chloroplasts in a drought-tolerant tree (Fraxinus ornus L.)

Leaves are the most important photosynthetic organs in most woody plants, but chloroplasts are also found in organs optimized for other functions. However, the actual photosynthetic efficiency of these chloroplasts is still unclear. We analyzed bark and wood chloroplasts of Fraxinus ornus L. saplings. Optical and spectroscopic methods were applied to stem samples and compared with leaves. A sharp light gradient was detected along the stem radial direction, with blue light mainly absorbed by the outer bark, and far-red-enriched light reaching the underlying xylem and pith. Chlorophylls were evident in the xylem rays and the pith and showed an increasing concentration gradient toward the bark. The stem photosynthetic apparatus showed features typical of acclimation to a low-light environment, such as larger grana stacks, lower chlorophyll a/b and photosystem I/II ratios compared with leaves. Despite likely receiving very few photons, wood chloroplasts were photosynthetically active and fully capable of generating a light-dependent electron transport. Our data provide a comprehensive scenario of the functional features of bark and wood chloroplasts in a woody species and suggest that stem photosynthesis is coherently optimized to the prevailing micro-environmental conditions at the bark and wood level.

GmFtsH25 overexpression increases soybean seed yield by enhancing photosynthesis and photosynthates

Increasing plant photosynthetic capacity is a promising approach to boost yields, but it is particularly challenging in C₃ crops, such as soybean (Glycine max (L.) Merr.). Here, we identified GmFtsH25, encoding a member of the filamentation temperature-sensitive protein H protease family, as a major gene involved in soybean photosynthesis, using linkage mapping and a genome-wide association study. Overexpressing GmFtsH25 resulted in more grana thylakoid stacks in chloroplasts and increased photosynthetic efficiency and starch content, while knocking out GmFtsH25 produced the opposite phenotypes. GmFtsH25 interacted with photosystem I light harvesting complex 2 (GmLHCa2), and this interaction may contribute to the observed enhanced photosynthesis. GmFtsH25 overexpression lines had superior yield traits, such as yield per plant, compared to the wild type and knockout lines. Additionally, we identified an elite haplotype of GmFtsH25, generated by natural mutations, which appears to have been selected during soybean domestication. Our study sheds light on the molecular mechanism by which GmFtsH25 modulates photosynthesis and provides a promising strategy for improving the yields of soybean and other crops.

How to Measure Grana - Ultrastructural Features of Thylakoid Membranes of Plant Chloroplasts

Granum is a basic structural unit of the thylakoid membrane network of plant chloroplasts. It is composed of multiple flattened membranes forming a stacked arrangement of a cylindrical shape. Grana membranes are composed of lipids and tightly packed pigment-protein complexes whose primary role is the catalysis of photosynthetic light reactions. These membranes are highly dynamic structures capable of adapting to changing environmental conditions by fine-tuning photochemical efficiency, manifested by the structural reorganization of grana stacks. Due to a nanometer length scale of the structural granum features, the application of high-resolution electron microscopic techniques is essential for a detailed analysis of the granum architecture. This mini-review overviews recent approaches to quantitative grana structure analyses from electron microscopy data, highlighting the basic manual measurements and semi-automated workflows. We outline and define structural parameters used by different authors, for instance, granum height and diameter, thylakoid thickness, end-membrane length, Stacking Repeat Distance, and Granum Lateral Irregularity. This article also presents insights into efficient and effective measurements of grana stacks visualized on 2D micrographs. The information on how to correctly interpret obtained data, taking into account the 3D nature of grana stacks projected onto 2D space of electron micrograph, is also given. Grana ultrastructural observations reveal key features of this intriguing membrane arrangement, broadening our knowledge of the thylakoid network's remarkable plasticity.

Photosynthetic Efficiency and Anatomical Structure of Pepper Leaf (Capsicum annuum L.) Transplants Grown under High-Pressure Sodium (HPS) and Light-Emitting Diode (LED) Supplementary Lighting Systems

The aim of this study was to evaluate the effects of various supplemental greenhouse lighting systems, i.e., high-pressure sodium lamps and mixtures of red and blue light-emitting diodes, on the photochemical efficiency, anatomical leaf structure, and growth of the two pepper cultivars. The intensity levels of the photosynthetically active radiation were the same for both light treatments. In this study, the relative chlorophyll content was measured. Additionally, certain parameters of chlorophyll a fluorescence were measured under ambient light or after dark adaptation. The obtained results showed that the application of light-emitting diodes (LEDs) as supplemental lighting positively affected the anatomical leaf characteristics and plant growth. The leaves of both pepper cultivars were thicker and had larger palisade parenchyma cells under LED supplemental lighting compared to leaves grown under high-pressure sodium (HPS) lamps. Moreover, the mesophyll cells of seedlings grown under LEDs contained more chloroplasts than those growing under HPS lighting. The chlorophyll a fluorescence measurements of pepper seedlings grown under LEDs showed significant increases in photosynthetic apparatus performance index (PI) values compared to plants grown under HPS lamps; however, the values for this index were higher in cv. ‘Aifos’ as compared to cv. ‘Palermo’. We recommend that supplemental lighting systems are applied with caution, as their performance appears to depend not only on the light spectrum but also on the cultivar.

Competitive advantages of Ulva prolifera from Pyropia aquaculture rafts in Subei Shoal and its implication for the green tide in the Yellow Sea

The physiological characteristics of Ulva prolifera and Blidingia sp. during two pre-bloom stages (March & May) were compared to evaluate the competitive advantage of U. prolifera on Pyropia aquaculture rafts in Subei Shoal. (1) Compared to Blidingia sp., U. prolifera had a lower growth rate, chlorophyll content, photosynthetic efficiency, and antioxidant capacity in March. (2) In May, various indicators of U. prolifera's physiological function improved significantly, while the antioxidant capacity of Blidingia sp. decreased significantly. Large lipidic globules in U. prolifera cells became scattered small lipidic globules in May, which indicated a decrease in lipid membrane peroxidation. (3) In U. prolifera, the ratio of buoyancy to gravity of per unit volume was 1.73, and the bubbles inside the thalli provided 60% of the total buoyancy. Buoyancy generated by the inflatable structure of U. prolifera allowed this species to float after being separated from the rafts.

Circular spectropolarimetric sensing of higher plant and algal chloroplast structural variations

Photosynthetic eukaryotes show a remarkable variability in photosynthesis, including large differences in light-harvesting proteins and pigment composition. In vivo circular spectropolarimetry enables us to probe the molecular architecture of photosynthesis in a non-invasive and non-destructive way and, as such, can offer a wealth of physiological and structural information. In the present study, we have measured the circular polarizance of several multicellular green, red, and brown algae and higher plants, which show large variations in circular spectropolarimetric signals with differences in both spectral shape and magnitude. Many of the algae display spectral characteristics not previously reported, indicating a larger variation in molecular organization than previously assumed. As the strengths of these signals vary by three orders of magnitude, these results also have important implications in terms of detectability for the use of circular polarization as a signature of life.

Effect of Paclobutrazol Application on Plant Photosynthetic Performance and Leaf Greenness of Herbaceous Peony

Paclobutrazol (PBZ) has been associated with effects on the photosynthetic capacity of plants. PBZ affects the growth and development of plants in general. However, little is known about the effects of PBZ on photosynthetic performance and related anatomical features of herbaceous peony (Paeonia lactiflora Pall.) leaves. In the present study, PBZ application resulted in a significant reduction in peony plant height. Furthermore, PBZ application significantly increased photosynthetic rate (Pn), transpiration rate (Tr) and water use efficiency (WUE), but significantly decreased intercellular CO2 concentration (Ci) at some stages from the bolting stage to the bud stage of the plants, compared to controls. Moreover, PBZ application increased the maximum quantum yield of PSII photochemistry (Fv/Fm), coefficient of photochemical quenching (qP) and intrinsic PSII efficiency (ΦPSII), but decreased the coefficient of non-photochemical quenching (qN) and non-photochemical quenching (NPQ). Leaves treated with PBZ had a heavy aggregation of chloroplasts close to the cell wall, with distinct grana lamellae, more and bigger starch grains (on average for a chloroplast), and fewer plastoglobuli, as compared to the control. PBZ increased chlorophyll content (SPAD) and the number of chloroplasts in individual cells in the foliar ultrastructure. PBZ-treated leaves had a darker green color with decreased luminosity (L*) and increased hue angle (h°). The results indicated that plants treated with PBZ were superior in terms of increased photosynthetic characteristics when compared with untreated controls. The direct cause of the increase in Pn and leaf greenness of PBZ-treated P. lactiflora may be the increase in chlorophyll content.

Plant Growth under Natural Light Conditions Provides Highly Flexible Short-Term Acclimation Properties toward High Light Stress

Efficient acclimation to different growth light intensities is essential for plant fitness. So far, most studies on light acclimation have been conducted with plants grown under different constant light regimes, but more recent work indicated that acclimation to fluctuating light or field conditions may result in different physiological properties of plants. Thale cress (Arabidopsis thaliana) was grown under three different constant light intensities (LL: 25 μmol photons m^-2 s^-1; NL: 100 μmol photons m^-2 s^-1; HL: 500 μmol photons m^-2 s^-1) and under natural fluctuating light (NatL) conditions. We performed a thorough characterization of the morphological, physiological, and biochemical properties focusing on photo-protective mechanisms. Our analyses corroborated the known properties of LL, NL, and HL plants. NatL plants, however, were found to combine characteristics of both LL and HL grown plants, leading to efficient and unique light utilization capacities. Strikingly, the high energy dissipation capacity of NatL plants correlated with increased dynamics of thylakoid membrane reorganization upon short-term acclimation to excess light. We conclude that the thylakoid membrane organization and particularly the light-dependent and reversible unstacking of grana membranes likely represent key factors that provide the basis for the high acclimation capacity of NatL grown plants to rapidly changing light intensities.

Effects of changes in micro-weather conditions on structural features, total protein and carbohydrate content in leaves of the Atlantic rain forest tree golden trumpet (Tabebuia chrysotricha)

Golden trumpet, Tabebuia chrysotricha, is a native tree from the Brazilian Atlantic rain forest, with a broad latitudinal distribution. In this study, we investigated the potential effects of short-term changes in micro-weather conditions on structural features, and total protein and carbohydrate content of golden trumpet leaves, using structural and histochemical approaches. Leaves were harvested in four different micro-weather conditions: 1. Afternoon, after a hot, sunny day; 2. at dawn, after a previously hot, sunny day; 3. at noon, of a hot, sunny day; and 4. at noon, of a cold, cloudy day. Leaflets exposed to low light irradiance showed flattened chloroplasts, uniformly distributed within the cells, throughout the palisade parenchyma. Conversely, leaflets exposed to high light irradiance presented flattened and rounded chloroplasts, in the upper and lower palisade parenchyma cells, respectively. The strongest protein staining was found for leaves harvested at the coldest period, whereas the weakest protein staining was found for leaves harvested after a hot, sunny day. The largest and most numerous starch grains were found for leaves harvested in the afternoon, after a hot, sunny day. Conversely, the smallest and less numerous starch grains were found for leaves harvested at dawn. Analysis of the data reported herein suggests that the leaflet responses to transient changes in micro-weather conditions are likely to contribute to the golden trumpet successful establishment in the broad latitudinal distribution in which the species is found.

Photosynthetic adaptation strategy of Ulva prolifera floating on the sea surface to environmental changes

For 8 consecutive years, a green tide has originated in the southern Yellow Sea and spread to the Qingdao offshore area. The causative species, Ulva prolifera, always forms a very thick thallus mat that is capable of drifting long distances over long periods. During this process, although the thalli face disturbance by complex environmental factors, they maintain high biomass and proliferation. We hypothesized that some form of photosynthetic adaptation strategy must exist to protect the thalli. Therefore, we studied the different photosynthetic response characteristics of the surface and lower layers of the floating thallus mats, and investigated the physiological and molecular-level adaptation mechanisms. The results showed that: (1) U. prolifera has strong photosynthetic capability that ensures it can gain sufficient energy to increase its biomass and adapt to long-distance migration. (2) Surface layer thalli adapt to the complex environment by dissipating excess energy via photosynthetic quantum control (energy quenching and energy redistribution between PSII/PSI) to avoid irreversible damage to the photosynthetic system. (3) Lower layer thalli increase their contents of Chlorophyll a (Chl a) and Chlorophyll b (Chl b) and decrease their Chl a/Chl b ratio to improve their ability to use light energy. (4) U. prolifera has strong photosynthetic plasticity and can adapt to frequent exchange between the surface and lower layer environments because of wave disturbance. Pigment component changes, energy quenching, and energy redistribution between PSII/PSI contribute to this photosynthetic plasticity.

Specific interaction of IM30/Vipp1 with cyanobacterial and chloroplast membranes results in membrane remodeling and eventually in membrane fusion

The photosynthetic light reaction takes place within the thylakoid membrane system in cyanobacteria and chloroplasts. Besides its global importance, the biogenesis, maintenance and dynamics of this membrane system are still a mystery. In the last two decades, strong evidence supported the idea that these processes involve IM30, the inner membrane-associated protein of 30kDa, a protein also known as the vesicle-inducing protein in plastids 1 (Vipp1). Even though we just only begin to understand the precise physiological function of this protein, it is clear that interaction of IM30 with membranes is crucial for biogenesis of thylakoid membranes. Here we summarize and discuss forces guiding IM30-membrane interactions, as the membrane properties as well as the oligomeric state of IM30 appear to affect proper interaction of IM30 with membrane surfaces. Interaction of IM30 with membranes results in an altered membrane structure and can finally trigger fusion of adjacent membranes, when Mg²⁺ is present. Based on recent results, we finally present a model summarizing individual steps involved in IM30-mediated membrane fusion. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Overexpression of SlRBZ Results in Chlorosis and Dwarfism through Impairing Chlorophyll, Carotenoid, and Gibberellin Biosynthesis in Tomato

ZFPs play important roles in many biological processes, including plant development, stress response, and phytohormone response. RanBP2-type zinc finger transcription factors have been characterized in animals and humans. However, their functions remain largely unknown in plants. In this study, we identified a RanBP2-type zinc finger protein gene (SlRBZ) in tomato. SlRBZ was constitutively expressed in roots, stems, leaves, flowers, and fruits. The SlRBZ-GFP fused protein was localized in the nucleus. Overexpression of SlRBZ resulted in chlorosis and dwarf phenotypes in tomato. Determination of physiological index showed that chlorophyll, carotenoid, and GAs contents were evidently decreased in transgenic plants. Furthermore, the qRT-PCR and RNA-Seq analyses demonstrated that the transcription of the genes involved in these biosynthesis pathways obviously decreased in SlRBZ-OE plants. In addition, ultrastructural observation by transmission electron microscopy indicated that plastids could not develop into mature chloroplasts with normal chloroplast membrane and thylakoid membrane system in SlRBZ-OE plants. The results suggest that overexpression of SlRBZ may impair the biosynthesis of chlorophyll, carotenoid, and gibberellin through blocking chloroplast development, resulting in chlorosis and dwarfism in tomato.

A variation on chloroplast development: the bizonoplast and photosynthetic efficiency in the deep-shade plant Selaginella erythropus

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PREMISE OF THE STUDY

Chloroplast development and structure are highly conserved in vascular plants, but the bizonoplast of Selaginella is a notable exception. In the shade plant S. erythropus, each dorsal epidermal cell contains one bizonoplast, while other cells have normal chloroplasts. Our quest was to (1) determine the origin of bizonoplasts, (2) explore developmental plasticity, and (3) correlate developmental changes with photosynthetic activity to provide insights unavailable in other green plants with more constrained development.•

METHODS

Bizonoplast development was studied in juvenile prostrate and older erect shoots of S. erythropus. Plastid plasticity was studied in plants cultivated under different light conditions. Chlorophyll fluorescence was measured and correlated with photosynthetic activity.•

KEY RESULTS

The bizonoplast originates from a proplastid, forming a distinctive upper zone rapidly after exposure to low light. In the prostrate shoots, the proplastid develops through early stages only. When the shoot becomes erect, the proplastid soon develops into a mature bizonoplast. Erect shoots have significantly higher photosynthetic efficiency than prostrate shoots. No bizonoplasts were found in the plants growing in high light, where 2-4 spheroidal chloroplasts formed, or with light from below.•

CONCLUSIONS

The upper zone develops above a normal-looking chloroplast structure to produce a bizonoplast. Bizonoplast developmental plasticity suggests that regular lamellar structure and monoplastidy are adaptations to deep shade environments. Such novel variation in S. erythropus is in stark contrast to known plastid development in other vascular plants, possibly reflecting retention of developmental flexibility in the basal clade, Lycophyta, to which it belongs.

Towards elucidation of dynamic structural changes of plant thylakoid architecture

Long-term acclimation of shade versus sun plants modulates the composition, function and structural organization of the architecture of the thylakoid membrane network. Significantly, these changes in the macroscopic structural organization of shade and sun plant chloroplasts during long-term acclimation are also mimicked following rapid transitions in irradiance: reversible ultrastructural changes in the entire thylakoid membrane network increase the number of grana per chloroplast, but decrease the number of stacked thylakoids per granum in seconds to minutes in leaves. It is proposed that these dynamic changes depend on reversible macro-reorganization of some light-harvesting complex IIb and photosystem II supracomplexes within the plant thylakoid network owing to differential phosphorylation cycles and other biochemical changes known to ensure flexibility in photosynthetic function in vivo. Some lingering grana enigmas remain: elucidation of the mechanisms involved in the dynamic architecture of the thylakoid membrane network under fluctuating irradiance and its implications for function merit extensive further studies.

Post-genomic insight into thylakoid membrane lateral heterogeneity and redox balance

Photosynthetic machinery requires balanced distribution of excitation energy from the light-harvesting complexes to photosystems. The efficiency of light-harvesting is regulated by thermal dissipation of excess energy, while the distribution of energy between photosystems is dependent on STN7 kinase and phosphorylation of thylakoid proteins. The regulation of excitation energy transfer has been linked to the lateral segregation of photosynthetic complexes along the thylakoid membrane. The study of photosynthetic regulation mechanisms using Arabidopsis mutants, which have been available for the last ten years, has challenged traditional views on regulation of excitation energy distribution. Here, we discuss an urgent need to create a holistic view of the dynamics of the thylakoid membrane using systematic research of the mutants available today.

Molecular dynamics of the diatom thylakoid membrane under different light conditions

During the last years significant progress was achieved in unraveling molecular characteristics of the thylakoid membrane of different diatoms. With the present review it is intended to summarize the current knowledge about the structural and functional changes within the thylakoid membrane of diatoms acclimated to different light conditions. This aspect is addressed on the level of the organization and regulation of light-harvesting proteins, the dissipation of excessively absorbed light energy by the process of non-photochemical quenching, and the lipid composition of diatom thylakoid membranes. Finally, a working hypothesis of the domain formation of the diatom thylakoid membrane is presented to highlight the most prominent differences of heterokontic thylakoids in comparison to vascular plants and green algae during the acclimation to low and high light conditions.

Vipp1: a very important protein in plastids?!

As a key feature in oxygenic photosynthesis, thylakoid membranes play an essential role in the physiology of plants, algae, and cyanobacteria. Despite their importance in the process of oxygenic photosynthesis, their biogenesis has remained a mystery to the present day. A decade ago, vesicle-inducing protein in plastids 1 (Vipp1) was described to be involved in thylakoid membrane formation in chloroplasts and cyanobacteria. Most follow-up studies clearly linked Vipp1 to membranes and Vipp1 interactions as well as the defects observed after Vipp1 depletion in chloroplasts and cyanobacteria indicate that Vipp1 directly binds to membranes, locally stabilizes bilayer structures, and thereby retains membrane integrity. Here current knowledge about the structure and function of Vipp1 is summarized with a special focus on its relationship to the bacterial phage shock protein A (PspA), as both proteins share a common origin and appear to have retained many similarities in structure and function.

Three-dimensional architecture of grana and stroma thylakoids of higher plants as determined by electron tomography

We have investigated the three-dimensional (3D) architecture of the thylakoid membranes of Arabidopsis (Arabidopsis thaliana), tobacco (Nicotiana tabacum), and spinach (Spinacia oleracea) with a resolution of approximately 7 nm by electron tomography of high-pressure-frozen/freeze-substituted intact chloroplasts. Higher-plant thylakoids are differentiated into two interconnected and functionally distinct domains, the photosystem II/light-harvesting complex II-enriched stacked grana thylakoids and the photosystem I/ATP synthase-enriched, nonstacked stroma thylakoids. The grana thylakoids are organized in the form of cylindrical stacks and are connected to the stroma thylakoids via tubular junctions. Our data confirm that the stroma thylakoids are wound around the grana stacks in the form of multiple, right-handed helices at an angle of 20° to 25° as postulated by a helical thylakoid model. The junctional connections between the grana and stroma thylakoids all have a slit-like architecture, but their size varies tremendously from approximately 15 × 30 nm to approximately 15 × 435 nm, which is approximately 5 times larger than seen in chemically fixed thylakoids. The variable slit length results in less periodicity in grana/stroma thylakoid organization than proposed in the original helical model. The stroma thylakoids also exhibit considerable architectural variability, which is dependent, in part, on the number and the orientation of adjacent grana stacks to which they are connected. Whereas some stroma thylakoids form solid, sheet-like bridges between adjacent grana, others exhibit a branching geometry with small, more tubular sheet domains also connecting adjacent, parallel stroma thylakoids. We postulate that the tremendous variability in size of the junctional slits may reflect a novel, active role of junctional slits in the regulation of photosynthetic function. In particular, by controlling the size of junctional slits, plants could regulate the flow of ions and membrane molecules between grana and stroma thylakoid membrane domains.

The GDC1 gene encodes a novel ankyrin domain-containing protein that is essential for grana formation in Arabidopsis

In land-plant chloroplasts, the grana play multiple roles in photosynthesis, including the potential increase of photosynthetic capacity in light and enhancement of photochemical efficiency in shade. However, the molecular mechanisms of grana formation remain elusive. Here, we report a novel gene, Grana-Deficient Chloroplast1 (GDC1), required for chloroplast grana formation in Arabidopsis (Arabidopsis thaliana). In the chloroplast of knockout mutant gdc1-3, only stromal thylakoids were observed, and they could not stack together to form appressed grana. The mutant exhibited seedling lethality with pale green cotyledons and true leaves. Further blue native-polyacrylamide gel electrophoresis analysis indicated that the trimeric forms of Light-Harvesting Complex II (LHCII) were scarcely detected in gdc1-3, confirming previous reports that the LHCII trimer is essential for grana formation. The Lhcb1 protein, the major component of the LHCIIb trimer, was substantially reduced, and another LHCIIb trimer component, Lhcb2, was slightly reduced in the gdc1-3 mutant, although their transcription levels were not altered in the mutant. This suggests that defective LHCII trimer formation in gdc1-3 is due to low amounts of Lhcb1 and Lhcb2. GDC1 encodes a chloroplast protein with an ankyrin domain within the carboxyl terminus. It was highly expressed in Arabidopsis green tissues, and its expression was induced by photosignaling pathways. Immunoblot analysis of the GDC1-green fluorescent protein (GFP) fusion protein in 35S::GDC1-GFP transgenic plants with GFP antibody indicates that GDC1 is associated with an approximately 440-kD thylakoid protein complex instead of the LHCII trimer. This shows that GDC1 may play an indirect role in LHCII trimerization during grana formation.

Effects of exogenous epibrassinolide on photosynthetic characteristics in tomato (Lycopersicon esculentum Mill) seedlings under weak light stress

The effects of three concentrations (0.1, 0.01, 0.001 mg/kg) of exogenous 24-epibrassinolide on leaf photosynthesis, chlorophyll content, chlorophyll fluorescence, and parameters of light response curve in tomato seedlings under 150 micromol x m(-2) x s(-1) weak light stress were studied, with two tomato cultivars, 'Zhongza9', tolerant, and 'Zhongshu6', sensitive to weak light stress. The results showed that the net photosynthetic rate (Pn), maximal photochemical quantum efficiency of PSII (Fv/Fm), light saturation point (LSP), and dark respiration rate (Rd) decreased remarkably under weak light, but the chlorophyll content, especially chlorophyll b (chlb) content, increased obviously compared with normal light intensity control. However, exogenous 24-epibrassinolide alleviated the decrease of leaf Pn and Fv/Fm and induced the further increase of chlb content as well as the further decrease of Rd and chla/chlb under weak light stress, which indicated that exogenous 24-epibrassinolide could enhance plant tolerance to weak light and diminish damage from weak light. However, the optimum concentrations were different between the two cultivars; 0.1 mg/kg 24-epibrassinolide showed the best induction effects in 'Zhongshu6', and the best level for 'Zhongza9' was 0.01 mg/kg 24-epibrassinolide.

Effects of glycerol on the fluorescence spectra and chloroplast ultrastructure of Phaeodactylum tricornutum (Bacillariophyta)

Responses of the photosynthetic activity of Phaeodactylum tricornutum (Bacillariophyta) to organic carbon glycerol were investigated. The growth rate, photosynthetic pigments, 77 K fluorescence spectra, and chloroplast ultrastructure of P. tricornutum were examined under photoautotrophic, mixotrophic, and photoheterotrophic conditions. The results showed that the specific growth rate was the fastest under mixotrophic conditions. The cell photosynthetic pigment content and values of Chl a/Chl c were reduced under mixotrophic and photoheterotrophic conditions. The value of carotenoid/Chl a was enhanced under mixotrophic conditions, but was decreased under photoheterotrophic conditions. In comparison with photoautotrophic conditions, the fluorescence emission peaks and fluorescence excitation peaks were not shifted. The relative fluorescence of photosystem (PS) I and PS II and the values of F685/F710 and F685/F738 were decreased. Chloroplast thylakoid pairs were less packed under mixotrophic and photoheterotrophic conditions. There was a strong correlation between degree of chloroplast thylakoid packing and the excitation energy kept in PS II. These results suggested that the PS II activity was reduced by glycerol under mixotrophic conditions, thereby leading to repression of the photosynthetic activity.

Molecular recognition in thylakoid structure and function

In photosynthesis, light-harvesting chlorophyll molecules are shunted between photosystems by phosphorylation of the protein to which they are bound. An anchor for the phosphorylated chlorophyll-protein complex has now been identified in the reaction centre of chloroplast photosystem I. This finding supports the idea that molecular recognition, not membrane surface charge, governs the architecture of the chloroplast thylakoid membrane. We describe a model for the chloroplast thylakoid membrane that is consistent with recent structural data that specify the relative dimensions of intrinsic protein complexes and their dispositions within the membrane. Control of molecular recognition accommodates membrane stacking, lateral heterogeneity and regulation of light-harvesting function by means of protein phosphorylation during state transitions--adaptations that compensate for selective excitation of photosystem I or photosystem II. High-resolution structural description of membrane protein-protein interactions is now required to understand thylakoid structure and regulation of photosynthesis.

Gametophyte morphology and ultrastructure of the extremely deep shade fern, Trichomanes speciosum

•  The extent to which macro- and micromorphological features might contribute to tolerance of extremely deep shade by Trichomanes speciosum, a member of the filmy ferns (Hymenophyllaceae), is reported here. •  Confocal laser scanning, transmission and scanning electron microscopy were used to study the ultrastructure of gametophytes and sporophyte leaves. •  Gametophyte filament cells contain numerous small, spherical or ovoid chloroplasts, whereas sporophyte leaf cells have fewer, slightly larger, disc-shaped chloroplasts. The chloroplast grana of gametophytic cells have fewer thylakoids than sporophyte cells, although grana are not numerous in either. Gametophyte filament cell walls resemble those of sporophyte leaf cells, with two or more layers of electron-opaque material and covered in a thin cuticle. Gemma cell wall ultrastructure does not differ from that of gametophyte filament cells; rhizoid cell walls are thick and several-layered. •  Neither gametophyte filaments nor sporophyte leaves have chloroplasts of the extreme forms reported for deep shade fern or angiosperm leaves. The success of the fern is attributed to a low metabolic rate and inability of other species to cope with extreme low light.

Balancing the two photosystems: photosynthetic electron transfer governs transcription of reaction centre genes in chloroplasts

Chloroplasts are cytoplasmic organelles whose primary function is photosynthesis, but which also contain small, specialized and quasi-autonomous genetic systems. In photosynthesis, two energy converting photosystems are connected, electrochemically, in series. The connecting electron carriers are oxidized by photosystem I (PS I) and reduced by photosystem II (PS II). It has recently been shown that the oxidation reduction state of one connecting electron carrier, plastoquinone, controls transcription of chloroplast genes for reaction centre proteins of the two photosystems. The control counteracts the imbalance in electron transport that causes it: oxidized plastoquinone induces PS II and represses PS I; reduced plastoquinone induces PS I and represses PS II. This complementarity is observed both in vivo, using light favouring one or other photosystem, and in vitro, when site-specific electron transport inhibitors are added to transcriptionally and photosynthetically active chloroplasts. There is thus a transcriptional level of control that has a regulatory function similar to that of purely post-translational 'state transitions' in which the redistribution of absorbed excitation energy between photosystems is mediated by thylakoid membrane protein phosphorylation. The changes in rates of transcription that are induced by spectral changes in vivo can be detected even before the corresponding state transitions are complete, suggesting the operation of a branched pathway of redox signal transduction. These findings suggest a mechanism for adjustment of photosystem stoichiometry in which initial events involve a sensor of the redox state of plastoquinone, and may thus be the same as the initial events of state transitions. Redox control of chloroplast transcription is also consistent with the proposal that a direct regulatory coupling between electron transport and gene expression determines the function and composition of the chloroplast's extra-nuclear genetic system.

STRUCTURE AND MEMBRANE ORGANIZATION OF PHOTOSYSTEM II IN GREEN PLANTS

Photosystem II (PSII) is the pigment protein complex embedded in the thylakoid membrane of higher plants, algae, and cyanobacteria that uses solar energy to drive the photosynthetic water-splitting reaction. This chapter reviews the primary, secondary, tertiary, and quaternary structures of PSII as well as the function of its constituent subunits. The understanding of in vivo organization of PSII is based in part on freeze-etched and freeze-fracture images of thylakoid membranes. These images show a resolution of about 40-50 A and so provide information mainly on the localization, heterogeneity, dimensions, and shapes of membrane-embedded PSII complexes. Higher resolution of about 15-40 A has been obtained from single particle images of isolated PSII complexes of defined and differing subunit composition and from electron crystallography of 2-D crystals. Observations are discussed in terms of the oligomeric state and subunit organization of PSII and its antenna components.

Why do thylakoid membranes from higher plants form grana stacks?

Chloroplasts contain a system of membrane sacs, the thylakoids, some of which are stacked to form grana (singular, granum), whereas others float freely in the stroma. It is on the thylakoid membranes that the electron carriers necessary for photosynthesis reside. There has been continuous speculation and discussion about the function of the grana ever since Menke postulated their lamellar nature in 1939. On the basis of new insights into the biophysics of the two photosystems and the molecular organization of thylakoid membranes of algae that exhibit a different lateral heterogeneity from that of higher plants, we propose that the membrane stacking found in the chloroplasts of higher plants and green algae is just one way in which Nature implements a general principle, namely that of physically separating a slow (PS II) and a fast (PS I) photosystem.

The chloroplast thylakoid membrane system is a molecular conveyor belt

Light drives photosynthesis, but paradoxically light is also the most variable environmental factor influencing photosynthesis both qualitatively and quantitatively. The photosynthetic apparatus of higher plants is adaptable in the extreme, as exemplified by its capacity for acclimation to very bright sunny or deeply shaded conditions. It can also respond to rapid changes in light such as sunflecks. In this paper I offer a model that i) explains the thylakoid membrane organisation into grana stacks and stroma lamellae, ii) proposes a role for rapid D1 protein turnover and LHCII phosphorylation, and iii) suggests a mechanism for photoinhibition. I argue that the photosynthetic membrane system is dynamic in three dimensions, so much so that, in the light, it is in constant motion and operates in a manner somewhat analogous to a conveyor belt. D1 protein degradation is proposed to be the motor that drives this system. Photoinhibition is suggested to be due to the arrest of D1 protein turnover.

PHOTOSYNTHETIC AND RESPIRATORY CHARACTERISTICS OF MALAYAN SUN AND SHADE FERNS

A comparative study of four Malayan ferns, Christensenia aesculifolia (Bl.) Maxon, Tectaria singaporeana (Wall.) Ching, Abacopteris multilineata (Wall.) Ching and Hymenophyllum polyanthos Sw. from shady habitats and another four, Dicranopteris linearis (Burm.) Und., Lygodium scandens (L.) Sw., Blechnum orientate Linn, and Stenochlaena palustris (Burm.) Bedd. from sunlit habitats showed that the total chlorophyll content expressed on a gram fresh weight basis was greater in the shade ferns. There was little difference in the chlorophyll content between the sun and shade ferns when it was expressed on a per unit leaf area basis. The protein and protohaem content was greater in the sun ferns. Measurements of the in vitro photochemical activities of the photosystems I and II in isolated chloroplasts by means of an oxygen electrode showed higher rates in the sun ferns. As determined by spectrophotometric analysis, the photosynthetic cytochrome content from isolated chloroplasts was greater in the sun ferns. The results indicate that the sun ferns have physiological characteristics favouring greater capacity for photosynthesis. Mitochondria isolated from the sun ferns showed faster rates of electron transport using exogenous NADH as substrate.

PHYSIOLOGICAL AND ANATOMICAL ASPECTS OF PHOTOSYNTHESIS OF TWO AGROSTIS SPECIES AT A SUB-ANTARCTIC ISLAND

Leaf anatomy, rates of photosynthesis, leaf N, chlorophyll, RuBP carboxylase and nitrate reductase were studied on the indigenous Agrostis magellanica Lam. and the invasive alien Agrostis stolonifera L. on Marion Island (46° 54' S, 37° 45' E). Leaves of A. magellanica were more deeply ridged, thicker and more sclerophyllus than those of A. stolonifera. Mesophyll cells of A. magellanica were larger but the number of cells per unit leaf area and the total area of chloroplast per unit leaf area were the same for the two species. Maximum CO₂ assimilation rates for the two species did not differ (mean maxima of 9.5 and 9 μmol CO₂ m^-2 s^-1 for A. magellanica and A. stolonifera respectively). At low photon flux densities, A. stolonifera showed a greater response of CO₂ assimilation to photon flux density. A. magellanica exhibited temperature-dependent photoinhibition. Leaf N, chlorophyll and RuBP carboxylase on a fresh or dry weight basis were higher in A. stolonifera but on a leaf area basis there was little difference between the species. The competitive ability of A. stolonifera on Marion Island may be related to its response to low photon flux densities or to its carbon allocation patterns (less sclerophyllous tissue means that a greater leaf area may be produced per unit carbon fixed). The lack of support tissue may limit it to sites partly sheltered from frequent gale force winds.

Photosynthetic acclimation and water-use efficiency of three species of understory herbaceous bamboo (Gramineae) in Panama

To assess the role of photosynthetic acclimation in the response of tropical understory herbs to treefall light gaps, photosynthetic response curves were determined for three species of herbaceous bamboo growing in treatments of sun and shade at Barro Calorado Island, Panama. Increased maximum photosynthetic capacity did not always accompany higher ramet production in the sun treatment. Pharus latifolius reproduced abundantly in both treatments, and produced more ramets and developed higher maximum photosynthetic capacity under higher irradiance. Streptochaeta spicata also produced a high percentage of reproductive ramets in both treatments and produced more ramets in the sun, did not show any significant differences in photosynthetic parameters between treatments. Streptochaeta sodiroana did not change maximum photosynthetic capacity in the sun, and had higher photosynthetic efficiency and lower mortality in the shade. Stable carbon isotope composition of leaves indicated that all three species developed higher water-use efficiency under higher irradiance. Photosynthetic flexibility may contribute to the ability of P. latifolius to reproduce in treefall gaps, whereas S. spicata and S. sodiroana may maintain the ability to fix carbon efficiently in low irradiance even when growing or persisting in gaps.

EFFECT OF IRRADIANCE AND LIGHT QUALITY ON STARCH SYNTHESIS BY ISOLATED CHLOROPLASTS OF CODIUM FRAGILE

Net starch synthesis by isolated chloroplasts of the intertidal alga Codium fragile (Suringar) Hariot was determined by enzymic hydrolysis (amyloglucosidase/α-amylase) of solubilized chloroplast samples followed by the measurement of the resultant glucose yields by hexokinase/glucose-6-phosphate dehydrogenase-coupled reactions. Net starch synthesis was only observed between 100 and 375 μmol m^-2 s^-1 (PAR) corresponding to irradiance values which supported saturating or super-saturating rates of photosynthesis. Light filter inactivation of the light-harvesting complex carotenoids siphonoxanthin and siphonein reduced net starch synthesis by 68 %. These findings are discussed in relation to photosynthesis in an intertidal zone environment.

Stoichiometries of electron transport complexes in spinach chloroplasts

The stoichiometric relationship among photosystem II complexes, photosystem I complexes, cytochrome b/f complexes, high-potential cytochrome b-559, and chlorophyll in spinach chloroplasts has been determined. Two features of this data stand out, in contrast to currently proposed stoichiometries in which the ratio of photosystem II to photosystem I is reported to be 2:1 and the chlorophyll to reaction center ratio to be as low as 260:1. Using a variety of techniques it was found that the stoichiometry of photosystem II:photosystem I:cytochrome b/f complex was 1:1:1, within 10%, and that the ratio of total chlorophyll to these components was 600:1, also within 10%. A ratio of two high-potential cytochrome b-559 molecules per 640 chlorophyll, or two molecules per photosystem II reaction center, was found. These ratios were remarkably constant regardless of the time of year or the source of the spinach. The concentration of photosystem II complexes was determined using a pH electrode to measure the flash-induced proton release resulting from water oxidation. The photosystem I reaction center concentration was measured by two different techniques that compared favorably. In the first method a pH electrode was used to measure the amount of flash-induced proton consumption associated with the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive oxidation of N,N,N',N'- tetramethylphenylenediamine , resulting in the production of hydrogen peroxide. In the second method the amount of P700 oxidized by far-red light was determined using dual-wavelength spectroscopy. The concentration of the cytochrome b/f complex was determined assuming 1 mol of cytochrome f per complex. The concentration of cytochrome f was measured spectroscopically by its light-induced turnover and by chemical difference spectra. The concentration of high-potential cytochrome b-559 was determined by chemical difference spectra. In addition to these studies, the light-induced absorbance change exhibiting a peak at 323 nm that has been attributed to the reduction of the primary quinone acceptor of photosystem II has been investigated. This measurement frequently has been used to quantitate the photosystem II to chlorophyll ratio. However, in view of these results it is argued that this technique significantly overestimates the photosystem II concentration.