Photosynthetic electron transport and anaerobic metabolism in purple non-sulfur phototrophic bacteria

Purple non-sulfur phototrophic bacteria, exemplifed byRhodobacter capsulatus andRhodobacter sphaeroides, exhibit a remarkable versatility in their anaerobic metabolism. In these bacteria the photosynthetic apparatus, enzymes involved in CO₂ fixation and pathways of anaerobic respiration are all induced upon a reduction in oxygen tension. Recently, there have been significant advances in the understanding of molecular properties of the photosynthetic apparatus and the control of the expression of genes involved in photosynthesis and CO₂ fixation. In addition, anaerobic respiratory pathways have been characterised and their interaction with photosynthetic electron transport has been described. This review will survey these advances and will discuss the ways in which photosynthetic electron transport and oxidation-reduction processes are integrated during photoautotrophic and photoheterotrophic growth.     

Research advances on aquaporins and vascular function regulation and related diseases

Aquaporins (AQPs), mainly distributed in epithelial and endothelial cells, are a family of channel-forming membrane proteins, originally confirmed to mediate the cellular water-transportation. AQPs are essential for maintaining homeostasis of the body water. Recently, studies have shown that AQPs may be involved in vascular function regulation and the development of related diseases, especially in cerebral ischemia, congestion heart failure, hypertension and tumor angiogenesis. Therefore, further studies are needed to elucidate mechanism accounting for the association between AQPs and vascular function related diseases, which may lead to novel approaches to the prevention and treatment of these diseases. In this review, we will discuss the expression and physiological roles of AQPs in vascular tissues and summarize recent progress in the relationship between AQPs and vascular function related diseases.     

Prediction of Aquaporin Function by Integrating Evolutionary and Functional Analyses

Aquaporins (AQPs) are a family of channel proteins, which transport water and/or small solutes across cell membranes. AQPs are present in Bacteria, Eukarya, and Archaea. The classical AQP evolution paradigm explains the inconsistent phylogenetic trees by multiple transfer events and emphasizes that the assignment of orthologous AQPs is not possible, making it difficult to integrate functional information. Recently, a novel phylogenetic framework of eukaryotic AQP evolution showed congruence between eukaryotic AQPs and organismal trees identifying 32 orthologous clusters in plants and animals (Soto et al. Gene 503:165–176, 2012). In this article, we discuss in depth the methodological strength, the ability to predict functionality and the AQP community perception about the different paradigms of AQP evolution. Moreover, we show an updated review of AQPs transport functions in association with phylogenetic analyses. Finally, we discuss the possible effect of AQP data integration in the understanding of water and solute transport in eukaryotic cells.     

Aquaporins as targets of pharmacological plant-derived compounds

Aquaporins (AQPs) are integral membrane proteins that serve as selective pores through which water and small solutes cross the plasma membranes of many human tissue and cell types. They have been identified in epithelia and endothelia involved in fluid transport, such as kidney tubules and glandular epithelia, glial cells, epidermis, and adipocytes. The pathophysiological roles of these proteins and the primary and secondary involvement of AQPs are becoming apparent in diverse clinical disorders, from diabetes insipidus to various forms of edema. The advanced understanding of aquaporin biology, from the structural determinants of channel permeability to the assignment of their physiological function in different organs, will allow the use of AQPs as targets for the therapy of a wide array of diseases. In this review, the mode of action of clinically-effective plant formulae on human AQPs-related diseases at the molecular, cellular, and organism levels is explored. The use of pharmacological plant-derived compounds as a possible strategy in the therapy of diseases related to altered water homeostasis should stimulate debate and further research objectives.     

Uptake and utilization of inorganic carbon by cyanobacteria

In the cyanobacteria, mechanisms exist that allow photosynthetic CO₂ reduction to proceed efficiently even at very low levels of inorganic carbon. These inducible, active transport mechanisms enable the cyanobacteria to accumulate large internal concentrations of inorganic carbon that may be up to 1000-fold higher than the external concentration. As a result, the external concentration of inorganic carbon required to saturate cyanobacterial photosynthesis in vivo is orders of magnitude lower than that required to saturate the principal enzyme (ribulose bisphosphate carboxylase) involved in the fixation reactions. Since CO₂ is the substrate for carbon fixation, the cyanobacteria somehow perform the neat trick of concentrating this small, membrane permeable molecule at the site of CO₂ fixation. In this review, we will describe the biochemical and physiological experiments that have outlined the phenomenon of inorganic carbon accumulation, relate more recent genetic and molecular biological observations that attempt to define the constituents involved in this process, and discuss a speculative theory that suggests a unified view of inorganic carbon utilization by the cyanobacteria.Abbreviations C_i Inorganic carbon - H-cells Cells grown under high CO₂ - L-cells Cells grown under low CO₂ - RuBP Ribulose-1,5-bisphosphate - WT Wild type     

Genetic and physiological analysis of the CO2-concentrating system of Chlamydomonas reinhardii

When grown photoautotrophically at air levels of CO₂, Chlamydomonas reinhardii possesses a system involving active transport of inorganic carbon which increases the intracellular CO₂ concentration considerably above ambient, thereby stimulating photosynthetic CO₂ assimilation. In previous investigations, two mutant strains of this unicellular green alga deficient in some component of this CO₂-concentrating system were recovered as strains requiring high levels of CO₂ to support photoautotrophic growth. One of the mutants, ca-1-12-1C, is a leaky (nonstringent) CO₂-requiring strain deficient in carbonic anhydrase (EC 4.2.1.1) activity, while the other, pmp-1-16-5K, is a stringent CO₂-requiring strain deficient in inorganic carbon transport. In the present study a double mutant (ca pmp) was constructed to investigate the physiological and biochemical interaction of the two mutations. The two mutations are unlinked and inherited in a Mendelian fashion. The double mutant was found to have a leaky CO₂-requiring phenotype, indicating that the mutation ca-1 overcomes the stringent CO₂-requirement conferred by the mutation pmp-1. Several physiological characteristics of the double mutant were very similar to the carbonic-anhydrase-deficient mutant, including high CO₂ compensation concentration, photosynthetic CO₂ response curve, and deficiency of carbonic-anhydrase activity. However, the labeling pattern of metabolites during photosynthesis in ¹⁴CO₂ was more like that of the bicarbonatetransport-deficient mutant, and accumulation of internal inorganic carbon was intermediate between that of the two original mutants. These data indicate a previously unsuspected complexity in the Chlamydomonas CO₂-concentrating system.     

Hidden impacts of ocean acidification to live and dead coral framework

Cold-water corals, such as Lophelia pertusa, are key habitat-forming organisms found throughout the world''s oceans to 3000 m deep. The complex three-dimensional framework made by these vulnerable marine ecosystems support high biodiversity and commercially important species. Given their importance, a key question is how both the living and the dead framework will fare under projected climate change. Here, we demonstrate that over 12 months L. pertusa can physiologically acclimate to increased CO₂, showing sustained net calcification. However, their new skeletal structure changes and exhibits decreased crystallographic and molecular-scale bonding organization. Although physiological acclimatization was evident, we also demonstrate that there is a negative correlation between increasing CO₂ levels and breaking strength of exposed framework (approx. 20–30% weaker after 12 months), meaning the exposed bases of reefs will be less effective ‘load-bearers’, and will become more susceptible to bioerosion and mechanical damage by 2100.     

Rising CO2 levels and their potential significance for carbon flow in photosynthetic cells

Abstract. In the first part of this review, I discuss how we can predict the direct short-term effect of enhanced CO₂ on photosynthetic rate in C₃ terrestrial plants. To do this, I consider: (1) to what extent enhanced CO₂ will stimulate or relieve demand on partial processes like carboxylation, light harvesting and electron transport, the Calvin cycle, and end-product synthesis; and (2) the extent to which these various processes actually control the rate of photosynthesis. I conclude that control is usually shared between Rubisco (which responds sensitively to CO₂) and other components (which respond less sensitively), and that photosynthesis will be stimulated by 25–75% when the CO₂ concentration is doubled from 35 to 70 Pa. This is in good agreement with the published responses. In the next part of the review, I discuss the evidence that most plants undergo a gradual inhibition of photosynthesis during acclimation to enhanced CO₂. I argue that this is related to an inadequate demand for carbohydrate in the remainder of the plant. Differences in the long-term response to CO₂ may be explained by differences in the sink-source status of plants, depending upon the species, the developmental stage, and the developmental conditions. In the third part of the review, I consider the biochemical mechanisms which are involved in ‘sink’ regulation of photosynthesis. Accumulating carbohydrate could lead to a direct inhibition of photosynthesis, involving mechanical damage by large starch grains or Pi-limitation due to inhibition of sucrose synthesis. I argue that Pi is important in the short-term regulation of partitioning to sucrose and starch, but that its contribution to ‘sink’ regulation has not yet been conclusively demonstrated. Indirect or ‘adaptive’ regulation of photosynthesis is probably more important, involving decreases in amounts of key photosynthetic enzymes, including Rubisco. This decreases the rate of photosynthesis, and potentially would allow resources (e.g. amino acids) to be remobilized from the leaves and reinvested in sink growth to readjust the sink-source balance. In the final part of the review, I argue that similar changes of Rubisco and, possibly, other proteins are probably also involved during acclimation to high CO₂.     

Effects of UV-B radiation on terrestrial plants and ecosystems: interaction with CO2 enrichment

UV-B radiation is just one of the environmental factors, that affect plant growth. It is now widely accepted that realistic assessment of plant responses to enhanced UV-B should be performed at sufficiently high Photosynthetically Active Radiation (PAR), preferably under field conditions. This will often imply, that responses of plants to enhanced UV-B in the field will be assessed under simultaneous water shortage, nutrient deficiency and variation of temperature. Since atmospheric CO₂ enrichment, global warming and increasing UV-B radiation represent components of global climatic change, interactions of UV-B with CO₂ enrichment and temperature are particularly relevant. Only few relevant UV-B× CO₂ interaction studies have been published. Most of these studies refer to greenhouse experiments. We report a significant CO₂ × UV-B interaction for the total plant dry weight and root dry weight of the C₃-grass Elymus athericus. At elevated CO₂ (720 mol mol^-1, plant growth was much less reduced by enhanced UV-B than at ambient atmospheric CO₂ although there were significant (positive) CO₂ effects and (negative) UV-B effects on plant growth. Most other CO₂ × UV-B studies do not report significant interactions on total plant biomass. This lack of CO₂ × UV-B interactions may result from the fact that primary metabolic targets for CO₂ and UVB are different. UV-B and CO₂ may differentially affect plant morphogenetic parameters: biomass allocation, branching, flowering, leaf thickness, emergence and senescence. Such more subtle interactions between CO₂ and UV-B need careful and long term experimentation to be detected. In the case of no significant CO₂× UV-B interactions, combined CO₂ and UV-B effects will be additive. Plants differ in their response to CO₂ and UV-B, they respond in general positively to elevated CO₂ and negatively to enhanced UV-B. Moreover, plant species differ in their responsiveness to CO₂ and UV-B. Therefore, even in case of additive CO₂ and UV-B effects, plant competitive relationships may change markedly under current climatic change with simultaneous enhanced atmospheric CO₂ and solar UV-B radiation.     

Emerging role of DYRK family protein kinases as regulators of protein stability in cell cycle control

Dual-specificity tyrosine phosphorylation-regulated kinases (DYRKs) constitute an evolutionarily conserved family of protein kinases with key roles in the control of cell proliferation and differentiation. Members of the DYRK family phosphorylate many substrates, including critical regulators of the cell cycle. A recent report revealed that human DYRK2 acts as a negative regulator of G₁/S transition by phosphorylating c-Jun and c-Myc, thereby inducing ubiquitination-mediated degradation. Other DYRKs also function as cell cycle regulators by modulating the turnover of their target proteins. DYRK1B can induce reversible cell arrest in a quiescent G₀ state by targeting cyclin D1 for proteasomal degradation and stabilizing p27^Kip1. The DYRK2 ortholog of C. elegans, MBK-2, triggers the proteasomal destruction of oocyte proteins after meiosis to allow the mitotic divisions in embryo development. This review summarizes the accumulating results that provide evidence for a general role of DYRKs in the regulation of protein stability.     

MAPK的细胞内定位与激活后移位机制

信号蛋白的亚细胞定位和激活后移位已成为细胞信号转导研究中的重要内容.MAPK信号通路是真核细胞中的重要信号转导系统.MAPK在细胞中有着相对固定的定位,在适宜的刺激作用下会移位入核并产生相应的生理效应.目前认为,MAPK的磷酸化状态及与其他蛋白质,如上游激酶、磷酸酶和下游底物之间的相互作用,可能在其特异性定位与激活后移位中起作用.MAPK的定位与移位机制的阐明,有助于进一步揭示MAPK的生理功能.     

The impact of elevated carbon dioxide on the phosphorus nutrition of plants: a review

Background Increasing attention is being focused on the influence of rapid increases in atmospheric CO₂ concentration on nutrient cycling in ecosystems. An understanding of how elevated CO₂ affects plant utilization and acquisition of phosphorus (P) will be critical for P management to maintain ecosystem sustainability in P-deficient regions.Scope This review focuses on the impact of elevated CO₂ on plant P demand, utilization in plants and P acquisition from soil. Several knowledge gaps on elevated CO₂-P associations are highlighted.Conclusions Significant increases in P demand by plants are likely to happen under elevated CO₂ due to the stimulation of photosynthesis, and subsequent growth responses. Elevated CO₂ alters P acquisition through changes in root morphology and increases in rooting depth. Moreover, the quantity and composition of root exudates are likely to change under elevated CO₂, due to the changes in carbon fluxes along the glycolytic pathway and the tricarboxylic acid cycle. As a consequence, these root exudates may lead to P mobilization by the chelation of P from sparingly soluble P complexes, by the alteration of the biochemical environment and by changes to microbial activity in the rhizosphere. Future research on chemical, molecular, microbiological and physiological aspects is needed to improve understanding of how elevated CO₂ might affect the use and acquisition of P by plants.     

Fast Induction of High-Affinity HCO3− Transport in Cyanobacteria

The induction of a high-affinity state of the CO₂-concentration mechanism was investigated in two cyanobacterial species, Synechococcus sp. strain PCC7002 and Synechococcus sp. strain PCC7942. Cells grown at high CO₂ concentrations were resuspended in low-CO₂ buffer and illuminated in the presence of carbonic anhydrase for 4 to 10 min until the inorganic C compensation point was reached. Thereafter, more than 95% of a high-affinity CO₂-concentration mechanism was induced in both species. Mass-spectrometric analysis of CO₂ and HCO₃⁻ fluxes indicated that only the affinity of HCO₃⁻ transport increased during the fast-induction period, whereas maximum transport activities were not affected. The kinetic characteristics of CO₂ uptake remained unchanged. Fast induction of high-affinity HCO₃⁻ transport was not inhibited by chloramphenicol, cantharidin, or okadaic acid. In contrast, fast induction of high-affinity HCO₃⁻ transport did not occur in the presence of K252a, staurosporine, or genistein, which are known inhibitors of protein kinases. These results show that induction of high-affinity HCO₃⁻ transport can occur within minutes of exposure to low-inorganic-C conditions and that fast induction may involve posttranslational phosphorylation of existing proteins rather than de novo synthesis of new protein components.     

Aquaporins in cancer

Background

The aquaporins (AQPs) are a family of 13 small hydrophobic integral transmembrane water channel proteins involved in transcellular and transepithelial water movement, transport of fluid and cell migration.

Scope of the review

This review article summarizes our knowledge concerning the involvement of AQPs in tumor growth, angiogenesis and metastatic process.

Major conclusions

Tumor cells types express AQPs and a positive correlation exists between histological tumor grade and the AQP expression. Moreover, AQPs are involved also in tumor edema formation and angiogenesis in several solid and hematological tumors.

General significance

AQPs inhibition in endothelial and tumor cells might limit tumor growth and spread, suggesting a potential therapeutic use in the treatment of tumors. This article is part of a Special Issue entitled Aquaporins.     

Rates and properties of endogenous cyclic photophosphorylation of isolated intact chloroplasts measured by CO2 fixation in the presence of dihydroxyacetone phosphate

1. Dihydroxyacetone phosphate in concentrations ? 2.5 mM completely inhibits CO₂-dependent O₂ evolution in isolated intact spinach chloroplasts. This inhibition is reversed by the addition of equimolar concentrations of P_i, but not by addition of 3-phosphoglycerate. In the absence of P_i, 3-phosphoglycerate and dihydroxyacetone phosphate, only about 20% of the ¹⁴C-labelled intermediates are found in the supernatant, whereas in the presence of each of these substances the percentage of labelled intermediates in the supernatant is increased up to 70–95%. Based on these results the mechanism of the inhibition of O₂ evolution by dihydroxyacetone phosphate is discussed with respect to the function of the known phosphate translocator in the envelope of intact chloroplasts.2. Although O₂ evolution is completely suppressed by dihydroxyacetone phosphate, CO₂ fixation takes place in air with rates of up to 65μ mol · mg^?1 chlorophyll · h^?1. As non-cyclic electron transport apparently does not occur under these conditions, these rates must be due to endogenous pseudocyclic and/or cyclic photophosphorylation.3. Under anaerobic conditions, the rates of CO₂ fixation in presence of dihydroxyacetone phosphate are low (2.5–7 μmol · mg^?1 chlorophyll · h^?1), but they are strongly stimulated by addition of dichlorophenyl-dimethylurea (e.g. 2 · 10^?7 M) reaching values of up to 60 μmol · mg^?1 chlorophyll · h^?1. As under these conditions the ATP necessary for CO₂ fixation can be formed by an endogenous cyclic photophosphorylation, the capacity of this process seems to be relatively high, so it might contribute significantly to the energy supply of the chloroplast. As dichlorophenyl-dimethylurea stimulates CO₂ fixation in presence of dihydroxyacetone phosphate under anaerobic but not under aerobic conditions, it is concluded that only under anaerobic conditions an “overreduction” of the cyclic electron transport system takes place, which is removed by dichlorophenyl-dimethylurea in suitable concentrations. At concentrations above 5 · 10^?7 M dichlorophenyl-dimethylurea inhibits dihydroxyacetone phosphate-dependent CO₂ fixation under anaerobic as well as under aerobic conditions in a similar way as normal CO₂ fixation. Therefore, we assume that a properly poised redox state of the electron transport chain is necessary for an optimal occurrence of endogenous cyclic photophosphorylation.4. The inhibition of dichlorophenyl-dimethylurea-stimulated CO₂ fixation in presence of dihydroxyacetone phosphate by dibromothymoquinone under anaerobic conditions indicates that plastoquinone is an indispensible component of the endogenous cyclic electron pathway.     

An Excel tool for deriving key photosynthetic parameters from combined gas exchange and chlorophyll fluorescence: theory and practice

Combined photosynthetic gas exchange and modulated fluorometres are widely used to evaluate physiological characteristics associated with phenotypic and genotypic variation, whether in response to genetic manipulation or resource limitation in natural vegetation or crops. After describing relatively simple experimental procedures, we present the theoretical background to the derivation of photosynthetic parameters, and provide a freely available Excel‐based fitting tool (EFT) that will be of use to specialists and non‐specialists alike. We use data acquired in concurrent variable fluorescence–gas exchange experiments, where A/C_i and light–response curves have been measured under ambient and low oxygen. From these data, the EFT derives light respiration, initial PSII (photosystem II) photochemical yield, initial quantum yield for CO₂ fixation, fraction of incident light harvested by PSII, initial quantum yield for electron transport, electron transport rate, rate of photorespiration, stomatal limitation, Rubisco (ribulose 1·5‐bisphosphate carboxylase/oxygenase) rate of carboxylation and oxygenation, Rubisco specificity factor, mesophyll conductance to CO₂ diffusion, light and CO₂ compensation point, Rubisco apparent Michaelis–Menten constant, and Rubisco CO₂‐saturated carboxylation rate. As an example, a complete analysis of gas exchange data on tobacco plants is provided. We also discuss potential measurement problems and pitfalls, and suggest how such empirical data could subsequently be used to parameterize predictive photosynthetic models.     

Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions

C4 plants contribute ≈ 20% of global gross primary productivity, and uncertainties regarding their responses to rising atmospheric CO₂ concentrations may limit predictions of future global change impacts on C4-dominated ecosystems. These uncertainties have not yet been considered rigorously due to expectations of C4 low responsiveness based on photosynthetic theory and early experiments. We carried out a literature review (1980–97) and meta-analysis in order to identify emerging patterns of C4 grass responses to elevated CO₂, as compared with those of C3 grasses. The focus was on nondomesticated Poaceae alone, to the exclusion of C4 dicotyledonous and C4 crop species. This provides a clear test, controlled for genotypic variability at family level, of differences between the CO₂-responsiveness of these functional types. Eleven responses were considered, ranging from physiological behaviour at the leaf level to carbon allocation patterns at the whole plant level. Results were also assessed in the context of environmental stress conditions (light, temperature, water and nutrient stress), and experimental growing conditions (pot size, experimental duration and fumigation method). Both C4 and C3 species increased total biomass significantly in elevated CO₂, by 33% and 44%, respectively. Differing tendencies between types in shoot structural response were revealed: C3 species showed a greater increase in tillering, whereas C4 species showed a greater increase in leaf area in elevated CO₂. At the leaf level, significant stomatal closure and increased leaf water use efficiency were confirmed in both types, and higher carbon assimilation rates were found in both C3 and C4 species (33% and 25%, respectively). Environmental stress did not alter the C4 CO₂-response, except for the loss of a significant positive CO₂-response for above-ground biomass and leaf area under water stress. In C3 species, stimulation of carbon assimilation rate was reduced by stress (overall), and nutrient stress tended to reduce the mean biomass response to elevated CO₂. Leaf carbohydrate status increased and leaf nitrogen concentration decreased significantly in elevated CO₂ only in C3 species. We conclude that the relative responses of the C4 and C3 photosynthetic types to elevated CO₂ concur only to some extent with expectations based on photosynthetic theory. The significant positive responses of C4 grass species at both the leaf and the whole plant level demand a re-evaluation of the assumption of low responsiveness in C4 plants at both levels, and not only with regard to water relations. The combined shoot structural and water use efficiency responses of these functional types will have consequential implications for the water balance of important catchments and range-lands throughout the world, especially in semiarid subtropical and temperate regions. It may be premature to predict that C4 grass species will lose their competitive advantage over C3 grass species in elevated CO₂.     

New insight into the evolution of aquaporins from flowering plants and vertebrates: orthologous identification and functional transfer is possible

Aquaporins (AQPs) represent a family of channel proteins that transport water and/or small solutes across cell membranes in the three domains of life. In all previous phylogenetic analysis of aquaporin, trees constructed using proteins with very low amino acid identity (<15%) were incongruent with rRNA data. In this work, restricting the evolutionary study of aquaporins to proteins with high amino acid identity (>25%), we showed congruence between AQPs and organismal trees. On the basis of this analysis, we defined 19 orthologous gene clusters in flowering plant species (3 PIP-like, 7 TIP-like, 6 NIP-like and 3 SIP-like). We described specific conserved motifs for each subfamily and each cluster, which were used to develop a method for automatic classification. Analysis of amino acid identity between orthologous monocotyledon and dicotyledon AQPs from each cluster, suggested that PIPs are under high evolutionary constraint. The phylogenetic analysis allowed us the assignment of orthologous aquaporins for very distant animal lineages (tetrapods-fishes). We also demonstrated that the location of all vertebrate AQPs in the ortholog clusters could be predicted by comparing their amino acid identity with human AQPs. We defined four AQP subfamilies in animals: AQP1-like, AQP8-like, AQP3-like and AQP11-like. Phylogenetic analysis showed that the four animal AQPs subfamilies are related with PIP-like, TIP-like, NIP-like and SIP-like subfamilies, respectively. Thus, this analysis would allow the prediction of individual AQPs function on the basis of orthologous genes from Arabidopsis thaliana and Homo sapiens.