Showing papers on "Photosynthesis published in 2001"
TL;DR: Plants and algae have a love/hate relationship with light; however, too much light can lead to increased production of damaging reactive oxygen species as byproducts of photosynthesis.
Abstract: Plants and algae have a love/hate relationship with light. As oxygenic photoautotrophic organisms, they require light for life; however, too much light can lead to increased production of damaging reactive oxygen species as byproducts of photosynthesis. In extreme cases, photooxidative damage can
2,492 citations
TL;DR: The crystal structure of photosystem I from the thermophilic cyanobacterium Synechococcus elongatus described in this paper provides a picture at atomic detail of 12 protein subunits and 127 cofactors comprising 96 chlorophylls, 2 phylloquinones, 3 Fe4S4 clusters, 22 carotenoids, 4 lipids, a putative Ca2+ ion and 201 water molecules.
Abstract: Life on Earth depends on photosynthesis, the conversion of light energy from the Sun to chemical energy. In plants, green algae and cyanobacteria, this process is driven by the cooperation of two large protein-cofactor complexes, photosystems I and II, which are located in the thylakoid photosynthetic membranes. The crystal structure of photosystem I from the thermophilic cyanobacterium Synechococcus elongatus described here provides a picture at atomic detail of 12 protein subunits and 127 cofactors comprising 96 chlorophylls, 2 phylloquinones, 3 Fe4S4 clusters, 22 carotenoids, 4 lipids, a putative Ca2+ ion and 201 water molecules. The structural information on the proteins and cofactors and their interactions provides a basis for understanding how the high efficiency of photosystem I in light capturing and electron transfer is achieved.
2,237 citations
TL;DR: Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance, and the relative importance of both of these changes in maximizing carbon gain is quantified.
Abstract: Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C 3 species grown in photon irradiances of 200 and 1000 μ mol m - 2 s - 1 . Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high-light-grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light-saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low-light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low-light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.
1,055 citations
TL;DR: The results represent an improved ability to model leaf photosynthesis over a wide range of temperatures necessary for predicting carbon uptake by terrestrial C3 systems.
Abstract: Predicting the environmental responses of leaf photosynthesis is central to many models of changes in the future global carbon cycle and terrestrial biosphere. The steadystate biochemical model of C3 photosynthesis of Farquhar et al .( Planta 149, 78‐90, 1980) provides a basis for these larger scale predictions; but a weakness in the application of the model as currently parameterized is the inability to accurately predict carbon assimilation at the range of temperatures over which significant photosynthesis occurs in the natural environment. The temperature functions used in this model have been based on in vitro measurements made over a limited temperature range and require several assumptions of in vivo conditions. Since photosynthetic rates are often Rubisco-limited (ribulose, 1-5 bisphosphate carboxylase/oxygenase) under natural steady-state conditions, inaccuracies in the functions predicting Rubisco kinetic properties at different temperatures may cause significant error. In this study, transgenic tobacco containing only 10% normal levels of Rubisco were used to measure Rubisco-limited photosynthesis over a large range of CO2 concentrations. From the responses of the rate of CO2 assimilation at a wide range of temperatures, and CO2 and O2 concentrations, the temperature functions of Rubisco kinetic properties were estimated in vivo. These differed substantially from previously published functions. These new functions were then used to predict photosynthesis in lemon and found to faithfully mimic the observed pattern of temperature response. There was also a close correspondence with published C3 photosynthesis temperature responses. The results represent an improved ability to model leaf photosynthesis over a wide range of temperatures (10‐40 °C) necessary for predicting carbon uptake by terrestrial C3 systems.
1,036 citations
TL;DR: This work has suggested that carbohydrate accumulation in leaves when there is an imbalance between source and sink at the whole plant level can lead to decreased expression of photosynthetic genes and accelerated leaf senescence, and in a high CO2 world this may become a more prevalent feature of photosynthesis regulation.
Abstract: The concept that photosynthetic flux is influenced by the accumulation of photo-assimilate persisted for 100 years before receiving any strong experimental support. Precise analysis of the mechanisms of photosynthetic responses to sink activity required the development of a battery of appropriate molecular techniques and has benefited from contemporary interest in the effects of elevated CO2 on photosynthesis. Photosynthesis is one of the most highly integrated and regulated metabolic processes to maximize the use of available light, to minimize the damaging effects of excess light and to optimize the use of limiting carbon and nitrogen resources. Hypotheses of feedback regulation must take account of this integration. In the short term, departure from homeostasis can lead to redox signals, which cause rapid changes in the transcription of genes encoding photosystems I and II. End-product synthesis can exert short-term metabolic feedback control through Pi recycling. Beyond this, carbohydrate accumulation in leaves when there is an imbalance between source and sink at the whole plant level can lead to decreased expression of photosynthetic genes and accelerated leaf senescence. In a high CO2 world this may become a more prevalent feature of photosynthetic regulation. However, sink regulation of photosynthesis is highly dependent on the physiology of the rest of the plant. This physiological state regulates photosynthesis through signal transduction pathways that co-ordinate the plant carbon : nitrogen balance, which match photosynthetic capacity to growth and storage capacity and underpin and can override the direct short-term controls of photosynthesis by light and CO2. Photosynthate supply and phytohormones, particularly cytokinins, interact with nitrogen supply to control the expression of photosynthesis genes, the development of leaves and the whole plant nitrogen distribution, which provides the dominant basis for sink regulation of photosynthesis.
1,016 citations
TL;DR: The primary components of photosynthesis that are affected by a short chill are highlighted, in both the dark and the light, and what is known of the mechanisms involved is discussed.
874 citations
869 citations
TL;DR: The molecular understanding of the guard cell signal transduction network opens possibilities for engineering stomatal responses to control CO2 intake and plant water loss.
Abstract: Guard cells are located in the epidermis of plant leaves, and in pairs surround stomatal pores These control both the influx of CO2 as a raw material for photosynthesis and water loss from plants through transpiration to the atmosphere Guard cells have become a highly developed system for dissecting early signal transduction mechanisms in plants In response to drought, plants synthesize the hormone abscisic acid, which triggers closing of stomata, thus reducing water loss Recently, central regulators of guard cell abscisic acid signalling have been discovered The molecular understanding of the guard cell signal transduction network opens possibilities for engineering stomatal responses to control CO2 intake and plant water loss
743 citations
TL;DR: Nitrogen control in cyanobacteria is mediated by NtcA, a transcriptional regulator which belongs to the CAP (the catabolite gene activator or cyclic AMP [cAMP] receptor protein) family and is therefore different from the well-characterized Ntr system.
Abstract: Nitrogen is a quantitatively important bioelement which is incorporated into the biosphere through assimilatory processes carried out by microorganisms and plants. Numerous nitrogencontaining compounds can be used by different organisms as sources of nitrogen. These include, for instance, inorganic ions like nitrate or ammonium and simple organic compounds like urea, amino acids, and some nitrogen-containing bases. Additionally, many bacteria are capable of fixing N 2. Nitrogen control is a phenomenon that occurs widely among microorganisms and consists of repression of the pathways of assimilation of some nitrogen sources when some other, more easily assimilated source of nitrogen is available to the cells. Ammonium is the preferred nitrogen source for most bacteria, but glutamine is also a very good source of nitrogen for many microorganisms. Two thoroughly investigated nitrogen control systems are the NtrB-NtrC two-component regulatory system found in enterics and some other proteobacteria (80) and the GATA family global nitrogen control transcription factors of yeast and some fungi (75). Novel nitrogen control systems have, however, been identified in bacteria other than the proteobacteria, like Bacillus subtilis (26), Corynebacterium glutamicum (52), and the cyanobacteria. The cyanobacterial system is the subject of this review. The cyanobacteria are prokaryotes that belong to the Bacteria domain and are characterized by the ability to perform oxygenic photosynthesis. Cyanobacteria have a wide ecological distribution, and they occupy a range of habitats, which includes vast oceanic areas, temperate soils, and freshwater lakes, and even extreme habitats like arid deserts, frigid lakes, or hot springs. Photoautotrophy, fixing CO 2 through the Calvin cycle, is the dominant mode of growth of these organisms (109). A salient feature of the intermediary metabolism of cyanobacteria is their lack of 2-oxoglutarate dehydrogenase (109). As a consequence, they use 2-oxoglutarate mainly as a substrate for the incorporation of nitrogen, a metabolic arrangement that may have regulatory consequences. Notwithstanding their rather homogeneous metabolism, cyanobacteria exhibit remarkable morphological diversity, being found as either unicellular or filamentous forms and exhibiting a number of cell differentiation processes, some of which take place in response to defined environmental cues, as is the case for the differentiation of N 2-fixing heterocysts (109). Nitrogen control in cyanobacteria is mediated by NtcA, a transcriptional regulator which belongs to the CAP (the catabolite gene activator or cyclic AMP [cAMP] receptor protein) family and is therefore different from the well-characterized Ntr system. Interestingly, however, the signal transduction P II protein, which plays a key role in Ntr regulation, is found in cyanobacteria but with characteristics which differentiate it from proteobacterial P II. In the following paragraphs, we shall first briefly summarize our current knowledge of the cyanobacterial nitrogen assimilation pathways and of what is known about their regulation at the protein level. This description will introduce most of the known cyanobacterial nitrogen assimilation genes. We shall then describe the ntcA gene and the NtcA protein themselves to finally discuss NtcA function through a survey of the NtcA-regulated genes which participate in simple nitrogen assimilation pathways or in heterocyst differentiation and function.
648 citations
TL;DR: The relationship between soil reduction and plant photosynthesis is largely unknown, but the literature reveals a range of photosynthetic sensitivities to the intensity of soil reduction among wetland species as discussed by the authors.
564 citations
09 Nov 2001
TL;DR: The photosynthetic acclimation responses of overwintering evergreens represent specific evolutionary adaptations for plant species that invest in the long-term maintenance of leaf structure in cold climatic zones as exemplified by the boreal forests of the Northern Hemisphere.
Abstract: In this review we focus on photosynthetic behavior of overwintering evergreens with an emphasis on both the acclimative responses of photosynthesis to cold and the winter behavior of photosynthesis in conifers. Photosynthetic acclimation is discussed in terms of the requirement for a balance between the energy absorbed through largely temperature-insensitive photochemical processes and the energy used for temperature-sensitive biochemical processes and growth. Cold acclimation transforms the xanthophyll-mediated nonphotochemical antenna quenching of absorbed light from a short-term dynamic response to a long-term sustained quenching for the whole winter period. This acclimative response helps protect the evergreen foliage from photooxidative damage during the winter when photosynthesis is restricted or prevented by low temperatures. Although the molecular mechanisms behind the sustained winter excitation quenching are largely unknown, it does involve major alterations in the organization and composition of the photosystem II antenna. In addition, photosystem I may play an important role in overwintering evergreens not only by quenching absorbed light photochemically via its support of cyclic electron transport at low temperatures, but also by nonphotochemical quenching of absorbed light irrespective of temperature. The possible role of photosystem II reaction centers in nonphotochemical quenching of absorbed energy in overwintering evergreens is also discussed. Processes like chlororespiration and cyclic electron transport may also be important for maintaining the functional integrity of the photosynthetic apparatus of overwintering evergreens both during periods of thawing in winter and during recovery from winter stress in spring. We suggest that the photosynthetic acclimation responses of overwintering evergreens represent specific evolutionary adaptations for plant species that invest in the long-term maintenance of leaf structure in cold climatic zones as exemplified by the boreal forests of the Northern Hemisphere.
TL;DR: Evidence is provided for red-osier dogwood that anthocyanins form a pigment layer in the palisade mesophyll layer that decreases light capture by chloroplasts that reduces risk of photo-oxidative damage to leaf cells as they senesce, which otherwise may lower the efficiency of nutrient retrieval from senescing autumn leaves.
Abstract: Why the leaves of many woody species accumulate anthocyanins prior to being shed has long puzzled biologists because it is unclear what effects anthocyanins may have on leaf function. Here, we provide evidence for red-osier dogwood (Cornus stolonifera) that anthocyanins form a pigment layer in the palisade mesophyll layer that decreases light capture by chloroplasts. Measurements of leaf absorbance demonstrated that red-senescing leaves absorbed more light of blue-green to orange wavelengths (495-644 nm) compared with yellow-senescing leaves. Using chlorophyll a fluorescence measurements, we observed that maximum photosystem II (PSII) photon yield of red-senescing leaves recovered from a high-light stress treatment, whereas yellow-senescing leaves failed to recover after 6 h of dark adaptation, which suggests photo-oxidative damage. Because no differences were observed in light response curves of effective PSII photon yield for red- and yellow-senescing leaves, differences between red- and yellow-senescing cannot be explained by differences in the capacities for photochemical and non-photochemical light energy dissipation. A role of anthocyanins as screening pigments was explored further by measuring the responses PSII photon yield to blue light, which is preferentially absorbed by anthocyanins, versus red light, which is poorly absorbed. We found that dark-adapted PSII photon yield of red-senescing leaves recovered rapidly following illumination with blue light. However, red light induced a similar, prolonged decrease in PSII photon yield in both red- and yellow-senescing leaves. We suggest that optical masking of chlorophyll by anthocyanins reduces risk of photo-oxidative damage to leaf cells as they senesce, which otherwise may lower the efficiency of nutrient retrieval from senescing autumn leaves.
TL;DR: This review describes how a—still elusive—regulatory kinase senses the physiological state of the photosynthetic cell and triggers an extensive supramolecular reorganization of the Photosynthetic membranes, resulting in a highly flexible energy convertor that adapts to the ever‐changing intracellular demand for ATP and/or reducing power.
Abstract: The chloroplast-based photosynthetic apparatus of plants and algae associates various redox cofactors and pigments with ∼70 polypeptides to form five major transmembrane protein complexes. Among these are two photosystems that have distinct light absorption properties but work in series to produce reducing equivalents aimed at the fixation of atmospheric carbon. A short term chromatic adaptation known as ‘State transitions’ was discovered thirty years ago that allows photosynthetic organisms to adapt to changes in light quality and intensity which would otherwise compromise the efficiency of photosynthetic energy conversion. A two-decade research effort has finally unraveled the major aspects of the molecular mechanism responsible for State transitions, and their physiological significance has been revisited. This review describes how a—still elusive—regulatory kinase senses the physiological state of the photosynthetic cell and triggers an extensive supramolecular reorganization of the photosynthetic membranes. The resulting picture of the photosynthetic apparatus is that of a highly flexible energy convertor that adapts to the ever-changing intracellular demand for ATP and/or reducing power.
TL;DR: In this article, the effects of shading wheat plants on rhizosphere respiration and root priming of soil organic matter decomposition were investigated by using a natural abundance 13C tracer method and 14C pulse labeling simultaneously.
Abstract: The effects of shading wheat plants on rhizosphere respiration and rhizosphere priming of soil organic matter decomposition were investigated by using a natural abundance 13C tracer method and 14C pulse labeling simultaneously. Seven days with strongly reduced photosynthesis (12/60 h day/night period) resulted in only half of the total CO2 efflux from soil compared to the treatment with a 12/12 h day/night period. The CO2 efflux from unplanted soil amounted to only 12 and 20% of the total CO2 efflux from the soil with non-shaded and shaded plants, respectively. On average 75% of total CO2 efflux from the planted soil with prolonged night periods was root-derived. Rhizosphere respiration was tightly coupled with plant photosynthetic activity. Any factor affecting photosynthesis, or substrate supply to roots and rhizosphere microorganisms, is an important determinant of root-derived CO2 efflux, and thereby, total CO2 efflux from soils. Clear diurnal dynamics of the total CO2 efflux intensity indicate the existence of an endogenous control mechanism of rhizosphere respiration. The light-on events after prolonged dark periods lead to strong increases of root-derived and therefore of total CO2 efflux from soil. After 14C pulse labeling, two maxima of the root-derived 14CO2 efflux were measured (6 and 24 h). This result demonstrated the diurnal dynamics of the rhizosphere respiration of recently-assimilated C in both the normal light conditions and shaded plants as well. The total amount of root-derived C respired in the rhizosphere was 17.3 and 20.6% of the total assimilated C for non-shaded and shaded plants, respectively. Both methods used, 13C natural abundance and 14C pulse labeling, gave similar estimates of root-derived CO2 during the whole observation period: 1.80±0.27 and 1.67±0.37 mg C kg−1 h−1 (±SD), respectively. Both tracer methods show that the cultivation of wheat led to the increasing decomposition intensity of soil organic matter (priming effect). Additionally, 13C natural abundance allows tracing of the dynamics of the priming effect depending on the light-on events.
TL;DR: It is revealed that distinct strategies for growth at low and high irradiance underlie the discontinuous response of Arabidopsis thaliana during growth over a broad range of irradiance.
Abstract: The capacity for photosynthetic acclimation in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta was assessed during growth over a broad range of irradiance. Discontinuities in the response to growth irradiance were revealed for the light- and CO2-saturated rate of photosynthesis (Pmax) and the ratio of chlorophyll a to chlorophyll b (Chl a/b). Three separate phases in the response of Pmax and Chl a/b to growth light were evident, with increases at low and high irradiance ranges and a plateau at intermediate irradiance. By measuring all chlorophyll-containing components of the thylakoid membrane that contribute to Chl a/b we reveal that distinct strategies for growth at low and high irradiance underlie the discontinuous response. These strategies include, in addition to changes in the major light-harvesting complexes of photosystem II (LHCII), large shifts in the amounts of both reaction centres as well as significant changes in the levels of minor LHCII and LHCI components.
TL;DR: Spectroscopic and chromatographic analyses of the photosynthetic pigments indicated that chlorosis was not due to a direct interaction of Cd with the chlorophyll biosynthesis pathway, and mineral deficiency and oxidative stress were apparently not involved in the pigment loss.
Abstract: Brassica napus L. (oilseed rape) was grown from seeds on a reconstituted soil contaminated with cadmium (100 mg Cd kg-1 dry soil), resulting in a marked chlorosis of the leaves which was investigated using a combination of biochemical, biophysical and physiological methods. Spectroscopic and chromatographic analyses of the photosynthetic pigments indicated that chlorosis was not due to a direct interaction of Cd with the chlorophyll biosynthesis pathway. In addition, mineral deficiency and oxidative stress were apparently not involved in the pigment loss. Leaf chlorosis was attributable to a marked decrease in the chloroplast density caused by a reduction in the number of chloroplasts per cell and a change in cell size, suggesting that Cd interfered with chloroplast replication and cell division. Relatively little Cd was found in the chloroplasts and the properties of the photosynthetic apparatus (electron transport, protein composition, chlorophyll antenna size, chloroplast ultrastructure) were not affected appreciably in plants grown on Cd-polluted soil. Depth profiling of photosynthetic pigments by phase-resolved photoacoustic spectroscopy revealed that the Cd-induced decrease in pigment content was very pronounced at the leaf surface (stomatal guard cells) compared to the leaf interior (mesophyll). This observation was consistent with light transmission and fluorescence microscopy analyses, which revealed that stomata density in the epidermis was noticeably reduced in Cd-exposed leaves. Concomitantly, the stomatal conductance estimated from gas-exchange measurements was strongly reduced with Cd. When plants were grown in a high-CO2 atmosphere (4,000 microliters CO2 l-1), the inhibitory effect of Cd on growth was not cancelled, suggesting that the reduced availability of CO2 at the chloroplast level associated with the low stomatal conductance was not the main component of Cd toxicity in oilseed rape.
TL;DR: K deficiency during squaring also dramatically reduced leaf area and dry matter accumulation, and affected assimilate partitioning among plant tissues.
Abstract: In cotton (Gossypium hirsutum L) grown in controlled-environment growth chamber the effects of K deficiency during floral bud development on leaf photosynthesis, contents of chlorophyll (Chl) and nonstructural saccharides, leaf anatomy, chloroplast ultrastructure, and plant dry matter accumulation were studied After cotton plants received 35-d K-free nutrient solution at the early square stage, net photosynthetic rate (PN) of the uppermost fully expanded main-stem leaves was only 23 % of the control plants receiving a full K supply Decreased leaf PN of K-deficient cotton was mainly associated with dramatically low Chl content, poor chloroplast ultrastructure, and restricted saccharide translocation, rather than limited stomata conductance in K-deficient leaves Accumulation of sucrose in leaves of K-deficient plants might be associated with reduced entry of sucrose into the transport pool or decreased phloem loading K deficiency during squaring also dramatically reduced leaf area and dry matter accumulation, and affected assimilate partitioning among plant tissues
TL;DR: It is postulate that in the early evolutionary phase of oxygenic photosynthesis, nitrogenase served as an electron acceptor for anaerobic heterotrophic metabolism and that PSI was favored by selection because it provided a micro-anaerobic environment for N2 fixation in cyanobacteria.
Abstract: In the modern ocean, a significant amount of nitrogen fixation is attributed to filamentous, nonheterocystous cyanobacteria of the genus Trichodesmium. In these organisms, nitrogen fixation is confined to the photoperiod and occurs simultaneously with oxygenic photosynthesis. Nitrogenase, the enzyme responsible for biological N2 fixation, is irreversibly inhibited by oxygen in vitro. How nitrogenase is protected from damage by photosynthetically produced O2 was once an enigma. Using fast repetition rate fluorometry and fluorescence kinetic microscopy, we show that there is both temporal and spatial segregation of N2 fixation and photosynthesis within the photoperiod. Linear photosynthetic electron transport protects nitrogenase by reducing photosynthetically evolved O2 in photosystem I (PSI). We postulate that in the early evolutionary phase of oxygenic photosynthesis, nitrogenase served as an electron acceptor for anaerobic heterotrophic metabolism and that PSI was favored by selection because it provided a micro-anaerobic environment for N2 fixation in cyanobacteria.
TL;DR: The utilization of a PSII-like protein as an extra antenna for PSI emphasises the flexibility of cyanobacterial light-harvesting systems, and seems to be a strategy which compensates for the lowering of phycobilisome and PSI levels in response to iron deficiency.
Abstract: Although iron is the fourth most abundant element in the Earth's crust, its concentration in the aquatic ecosystems-particularly the open oceans-is sufficiently low to limit photosynthetic activity and phytoplankton growth. Cyanobacteria, a major class of phytoplankton, respond to iron deficiency by expressing the 'iron-stress-induced' gene, isiA(ref. 3). The protein encoded by this gene has an amino-acid sequence that shows significant homology with one of the chlorophyll a-binding proteins (CP43) of photosystem II (PSII). The precise function of the CP43-like protein, here called CP43', has not been elucidated, although there have been many suggestions. Here we show that CP43' associates with photosystem I (PSI) to form a complex that consists of a ring of 18 CP43' molecules around a PSI trimer. This significantly increases the size of the light-harvesting system of PSI. The utilization of a PSII-like protein as an extra antenna for PSI emphasises the flexibility of cyanobacterial light-harvesting systems, and seems to be a strategy which compensates for the lowering of phycobilisome and PSI levels in response to iron deficiency
TL;DR: Hydrogen production by three photosynthetic bacterial strains from four different short-chain organic acids (lactate, malate, acetate and butyrate) was investigated and the effect of light intensity on hydrogen production was studied.
TL;DR: The purification of a specific PSI–IsiA supercomplex is reported, which is abundant under conditions of iron limitation and provides a structural characterization of an additional chlorophyll-containing, membrane-integral antenna in a cyanobacterial photosystem.
Abstract: Cyanobacteria are abundant throughout most of the world's water bodies and contribute significantly to global primary productivity through oxygenic photosynthesis. This reaction is catalysed by two membrane-bound protein complexes, photosystem I (PSI) and photosystem II (PSII), which both contain chlorophyll-binding subunits functioning as an internal antenna1. In addition, phycobilisomes act as peripheral antenna systems, but no additional light-harvesting systems have been found under normal growth conditions. Iron deficiency, which is often the limiting factor for cyanobacterial growth in aquatic ecosystems2, leads to the induction of additional proteins such as IsiA (ref. 3). Although IsiA has been implicated in chlorophyll storage, energy absorption and protection against excessive light, its precise molecular function and association to other proteins is unknown. Here we report the purification of a specific PSI–IsiA supercomplex, which is abundant under conditions of iron limitation. Electron microscopy shows that this supercomplex consists of trimeric PSI surrounded by a closed ring of 18 IsiA proteins binding around 180 chlorophyll molecules. We provide a structural characterization of an additional chlorophyll-containing, membrane-integral antenna in a cyanobacterial photosystem.
TL;DR: This is the first report in which expression of a single plastid-targeted enzyme has been shown to improve carbon fixation and growth in transgenic plants.
Abstract: Transgenic tobacco plants expressing a cyanobacterial fructose-1,6/sedoheptulose-1,7-bisphosphatase targeted to chloroplasts show enhanced photosynthetic efficiency and growth characteristics under atmospheric conditions (360 p.p.m. CO2). Compared with wild-type tobacco, final dry matter and photosynthetic CO2 fixation of the transgenic plants were 1.5-fold and 1.24-fold higher, respectively. Transgenic tobacco also showed a 1.2-fold increase in initial activity of ribulose 1,5 bisphosphate carboxylase/oxygenase (Rubisco) compared with wild-type plants. Levels of intermediates in the Calvin cycle and the accumulation of carbohydrates were also higher than those in wild-type plants. This is the first report in which expression of a single plastid-targeted enzyme has been shown to improve carbon fixation and growth in transgenic plants.
TL;DR: A thermodynamic analysis is presented showing that bicarbonate (formed by dissolution of CO2) is a more efficient alternative substrate than water for O2 production by oxygenic phototrophs, and it is proposed that bricarbonate was the thermodynamically preferred reductant before water in the evolution of oxygenic photosynthesis.
Abstract: The evolution of O2-producing cyanobacteria that use water as terminal reductant transformed Earth's atmosphere to one suitable for the evolution of aerobic metabolism and complex life. The innovation of water oxidation freed photosynthesis to invade new environments and visibly changed the face of the Earth. We offer a new hypothesis for how this process evolved, which identifies two critical roles for carbon dioxide in the Archean period. First, we present a thermodynamic analysis showing that bicarbonate (formed by dissolution of CO2) is a more efficient alternative substrate than water for O2 production by oxygenic phototrophs. This analysis clarifies the origin of the long debated “bicarbonate effect” on photosynthetic O2 production. We propose that bicarbonate was the thermodynamically preferred reductant before water in the evolution of oxygenic photosynthesis. Second, we have examined the speciation of manganese(II) and bicarbonate in water, and find that they form Mn-bicarbonate clusters as the major species under conditions that model the chemistry of the Archean sea. These clusters have been found to be highly efficient precursors for the assembly of the tetramanganese-oxide core of the water-oxidizing enzyme during biogenesis. We show that these clusters can be oxidized at electrochemical potentials that are accessible to anoxygenic phototrophs and thus the most likely building blocks for assembly of the first O2 evolving photoreaction center, most likely originating from green nonsulfur bacteria before the evolution of cyanobacteria.
TL;DR: The main targets of Cr can be listed as a decrease in the number of active reaction centers and damage to the oxygen-evolving complex and the performance index of PSII, PIABS, decreased due to Cr treatment.
TL;DR: Photosynthesis in plants involves photosystem I and photosystem II, both of which use light energy to drive redox processes, and the distribution of absorbed light energy between the two photosystems depends on an altered binding equilibrium between the light harvesting complex II-photosystem II and light harvestingcomplex II-Photosystem I complexes.
TL;DR: The results establish, for the first time, the suite of biophysical mechanisms that optimize photosynthesis while simultaneously providing photoprotection in symbiotic corals in situ.
Abstract: In zooxanthellate corals, excess excitation energy can be dissipated as heat (nonphotochemical quenching), thereby providing protection against oxidative damage by supraoptimal light in shallow reefs. To identify and quantify the photoprotective mechanisms, we studied the diel variability of chlorophyll fluorescence yields and photosynthetic parameters in situ in corals, using moored and SCUBA-based fast-repetition-rate fluorometers. The results reveal that nonphotochemical quenching is triggered prior to saturation of photosynthetic electron transport by downregulation of the reaction centers of Photosystem II (PSII). This process dissipates up to 80% of the excitation energy. On a sunny day in shallow waters, the daily integrated flux of photons absorbed, and subsequently dissipated as heat, is ;4 times that used for photosynthesis. Fluorescence quenching is further accompanied by a slight reduction in the functional absorption cross section for PSII that results from thermal dissipation of excitation energy in the light-harvesting antennae. These two processes are highly dynamic and adjust to irradiance changes on timescales consistent with the passage of clouds across the sky. Under supraoptimal irradiance, however, up to 30% of PSII reaction centers become photoinhibited, and these are repaired only after several hours of low irradiance. In shallow corals, between 10% and 20% of the reactions centers are chronically photoinhibited and appear to remain permanently nonfunctional throughout the year. Our results establish, for the first time, the suite of biophysical mechanisms that optimize photosynthesis while simultaneously providing photoprotection in symbiotic corals in situ.
TL;DR: Evidence that C4 photosynthesis can function within a single photosynthetic cell in terrestrial plants is provided and it is shown that Borszczowia aralocaspica (Chenopodiaceae) has the photosynthesis features of C4 plants, yet lacks Kranz anatomy.
Abstract: An important adaptation to CO2-limited photosynthesis in cyanobacteria, algae and some plants was development of CO2-concentrating mechanisms (CCM). Evolution of a CCM occurred many times in flowering plants, beginning at least 15-20 million years ago, in response to atmospheric CO2 reduction, climate change, geological trends, and evolutionary diversification of species. In plants, this is achieved through a biochemical inorganic carbon pump called C4 photosynthesis, discovered 35 years ago. C4 photosynthesis is advantageous when limitations on carbon acquisition are imposed by high temperature, drought and saline conditions. It has been thought that a specialized leaf anatomy, composed of two, distinctive photosynthetic cell types (Kranz anatomy), is required for C4 photosynthesis. We provide evidence that C4 photosynthesis can function within a single photosynthetic cell in terrestrial plants. Borszczowia aralocaspica (Chenopodiaceae) has the photosynthetic features of C4 plants, yet lacks Kranz anatomy. This species accomplishes C4 photosynthesis through spatial compartmentation of photosynthetic enzymes, and by separation of two types of chloroplasts and other organelles in distinct positions within the chlorenchyma cell cytoplasm.
TL;DR: The finding that several defence genes have antioxidant responsive elements or GSSG binding sites in their regulatory regions supports the idea that redox signalling is involved in regulating gene expression in response to low temperature.
Abstract: Glutathione is an important component of the ascorbate-glutathione cycle, which is involved in the regulation of hydrogen peroxide (H2O2) concentrations in plants. During chilling and cold acclimation, i.e. exposure to temperatures between 0 and 15 degrees C, H2O2 may accumulate. Excess electrons from the photosynthetic and respiratory electron transport chains can be used for the reduction of oxygen, thus producing superoxide radicals (O2.-); these are subsequently transformed to H2O2 via superoxide dismutase (SOD; EC 1.15.1.1). During the removal of excess H2O2, reduced glutathione (GSH) is converted to its oxidised form (GSSG), and GSH is regenerated by the activity of NADPH-dependent glutathione reductase (GR; EC 1.6.4.2). At low non-freezing temperatures, high GSH content and GR activity were detected in several plant species, indicating a possible contribution to chilling tolerance and cold acclimation. Changes in H2O2 concentration and GSH/GSSG ratio alter the redox state of the cells and may activate special defence mechanisms through a redox signalling chain. The finding that several defence genes have antioxidant responsive elements or GSSG binding sites in their regulatory regions supports the idea that redox signalling is involved in regulating gene expression in response to low temperature.
TL;DR: Crassulacean acid metabolism is an important elaboration of photosynthetic carbon fixation that allows chloroplast-containing cells to fix CO2initially at night using phospho enol pyruvate carboxylase in the cytosol.
Abstract: Crassulacean acid metabolism (CAM) is an important elaboration of photosynthetic carbon fixation that allows chloroplast-containing cells to fix CO2initially at night using phospho enol pyruvate carboxylase (PEPC) in the cytosol. This leads to the formation of C4 organic acids (usually malate),