Adaptation of photosynthesis under iron deficiency in maize

This paper explores the effects of high light stress on Fe-deficient plants. Maize (Zea mays) plants were grown under conditions of Fe deficiency and complete nutrition. Attached, intact leaves of Fe-deficient and control plants were used for gas exchange experiments under suboptimal, optimal and photoinhibitory illumination. Isolated chloroplasts were used to study photosynthetic electron transport system, compromised by the induction of Fe deficiency. The reaction centers of PS II (measured as reduction of Q, the primary electron acceptor of P 680) and PS I (measured as oxidation of P 700) were estimated from the amplitude of light induced absorbance change at 320 and 700 nm, respectively. Plants were subjected to photoinhibitory treatment for different time periods and isolated chloroplasts from these plants were used for electron transport studies. Carbon dioxide fixation in control as well as in Fe-deficient plants decreased in response to high light intensities. Total chlorophyll, P 700 and Q content in Fe-deficient chloroplasts decreased, while Chl a/b ratio and Q/P 700 ratio increased. However, electron transport through PS II suffered more after photoinhibitory treatment as compared to electron transport through PS I or whole chain. Electron transfer through PS I+PS II, excluding the water oxidation complex showed a decrease in Fe-deficient plants. However, electron transport through this part of the chain did not suffer much as a result of photoinhibition, suggesting a defect in the oxidising side of PS II.     

Long- and short-term effects of decreased sink demand on carbohydrate levels and photosynthesis in wheat leaves

Wheat (Triticum aestivum L.) ears were removed to investigate long-term regulation of photosynthesis by sink demand at ambient CO₂ and 22 °C. The CO₂ level was also increased to 660 μmol mol^?1 and temperature was lowered to 5 °C to examine short-term responses of photosynthesis to low sink demand. Sink removal inhibited photosynthesis and increased leaf levels of glucose, fructose and ribulose-1, 5-bisphosphate (RuBP), and the glucose-6-phosphate (G6P)/fructose-6-phosphate (F6P) and RuBP/3-phosphoglycerate (PGA) ratios under growth conditions, but had no effect on the activity and activation state of ribulose-1, 5-bisphosphate carboxylase oxygenase (Rubisco) either under growth or short-term conditions, suggesting an inhibition of photosynthesis by decreased in vivo catalysis of Rubisco. Photosynthesis increased similarly in eared and earless shoots after a rise in CO₂ concentration, and the ratio of triose-phosphates (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP) to PGA was similar or higher for removed than intact ears, suggesting that feedback inhibition of photosynthesis was not caused by a limitation of ATP synthesis in chloroplasts. Under short-term conditions (660 μmol mol^?1 CO₂, 5 °C), TP and RuBP levels and the TP/PGA and TP/RuBP ratios were increased by sink removal, indicating an additional limitation of photosynthesis by the rate of RuBP regeneration.     

Long- and short-term responses of leaf carbohydrate levels and photosynthesis to decreased sink demand in soybean

Axillary buds and the apical portion of shoots of soybean [Glycine max (L.) Merr. cultivar Turchina] plants were trimmed to investigate long-term regulation of photosynthesis by sink demand at ambient CO₂ and 22 °C. Also, in intact and trimmed shoots, the CO₂ level was increased to 660 μmol mol^?1 and temperature was lowered to 5°C to examine the superimposed short-term responses of photosynthesis to low sink demand. Under growth conditions, trimming the shoots increased leaf photosynthesis and the levels of sucrose, glucose-6-phosphate (G6P) and 3-phosphoglycerate (PGA), as well as the G6P/fructose-6-phosphate (F6P) and sucrose/starch ratios, while it decreased the level of starch and the triose-phosphate (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP)/PGA ratio. Photosynthesis enhancement was accompanied by increased chlorophyll contents and ribulose-l,5-bisphosphate carboxylase oxygenase (Rubisco) activity. Sink removal consistently increased photosynthesis measured under a variety of conditions (growth CO₂ or a short-term change to 660 μmol mol_-1 CO₂; growth temperature or a short-term change to 5 °C), except when low temperature was combined with ambient CO₂; the increase in photosynthesis was higher under short-term elevated CO₂ than at ambient CO₂. In contrast with its effect at ambient CO₂, shoot trimming increased the levels of TP and ribulose-1,5-bisphosphate (RuBP) and the TP/PGA ratio under high-CO₂ conditions.     

Infection with virulent and avirulent P. syringae strains differentially affects photosynthesis and sink metabolism in Arabidopsis leaves

Infection of plants with pathogens leads not only to the induction of defence reactions but also to changes in carbohydrate metabolism. In this study, the effects of infection by a virulent and an avirulent strain of P. syringae on spatio-temporal changes in photosynthesis were compared using chlorophyll fluorescence imaging. The maximum PSII quantum yield, effective PSII quantum yield and nonphotochemical quenching were decreased in Arabidopsis leaves infected with either strain. At the same time, the quantum yield of nonregulated energy dissipation was increased. These changes could be detected by chlorophyll fluorescence imaging before symptoms were visible by eye. The effects were restricted to the vicinity of the infection site and did not spread to uninfected areas of the leaf. Qualitatively similar changes in photosynthetic parameters were observed in both interactions. Major differences between the responses to both strains were evident in the onset and time course of changes. A decrease in photosynthesis was detectable already at 3 h only after challenge with the avirulent strain while after 48 h the rate of photosynthesis was lower with the virulent strain. In contrast to photosynthesis, the regulation of marker genes for source/sink relations and the activities of invertase isoenzymes showed qualitative differences between both interactions. Inoculation of the virulent but not the avirulent strain resulted in downregulation of photosynthetic genes and upregulation of vacuolar invertases. The activity of vacuolar invertases transiently increased upon infection with the virulent strain but decreased with the avirulent strain while extracellular invertase activity was downregulated in both interactions.     

Getting a GR(i)P on oligodendrocyte development

One of the most extensively studied of mammalian cells is the oligodendrocyte, the myelin-forming cell of the central nervous system. The ancestry and development of this cell have been studied with every approach utilized by developmental biologists. Such detailed efforts have the potential of providing paradigms of relevance to those interested in analyzing the ancestry and development of any cell type.One of the striking features of studies on the development of oligodendrocytes is that different analytical approaches have led to strikingly different theoretical views regarding the ancestry of these cells. On one extreme is the hypothesis that the steps leading to the generation of oligodendrocytes begin with the generation of a glial-restricted precursor (GRP) cell from neuroepithelial stem cells. GRP cells are thought to be capable of giving rise to all glial cells (including oligodendrocytes and multiple astrocyte populations), but not to neurons, a process that appears to require progression through further stages of greater lineage restriction. On the other extreme is the hypothesis that oligodendrocytes are derived from a precursor cell that generates only motor neurons and oligodendrocytes, with astrocytes being generated through a separate lineage. In this review, we critically consider the various contributions to understanding the ancestry of oligodendrocytes, with particular attention to the respective merits of the GRP cell vs. the motor neuron-oligodendrocyte precursor (MNOP) cell hypothesis. We draw the conclusion that, at present, the strengths of the GRP cell hypothesis outweigh those of the MNOP hypothesis and other hypotheses suggesting oligodendrocytes are developmentally more related to motor neurons than to astrocytes. Moreover, it is clear from existing data that, following the period of motor neuron generation, the major glial precursor cell in the embryonic spinal cord is the GRP cell, and that multiple previous studies on the earliest stages of oligodendrocyte generation in the developing spinal cord have been focused on a differentiation stage of GRP cells.     

Sugar repression of photosynthesis: the role of carbohydrates in signalling nitrogen deficiency through source:sink imbalance

The aim of this work was to examine whether carbohydrates are involved in signalling N deficiency through source:sink imbalance. Photosynthetic metabolism in tobacco was studied over 8 d during the withdrawal of N from previously N-sufficient plants in which the source:sink ratio was manipulated by shading leaves on some of the plants. In N-sufficient plants over this time-scale, there was a small decline in photosynthetic rate, Rubisco protein and amino acid content, with a larger decrease in carbohydrate content. Withdrawal of N from the growing medium induced a large decrease in the rate of photosynthesis (35% reduction after 8 d under the growing conditions, with a reduction also apparent at high and low measuring CO₂), which was caused by a large decrease in the amount of Rubisco protein (62% after 8 d) and Rubisco activity. Higher amounts; of hexoses preceded the loss of photosynthetic activity and sucrose and starch accumulation. Reduction of the sourcersink ratio by shading prevented the loss of photosynthetic activity and the increase in hexoses and other carbohydrates. These data indicate that the reduction of photosynthesis that accompanies N deficiency in intact plants has the characteristics of sugar repression of photosynthesis observed in model systems, but that the accumulation of hexose prior to the decline in photosynthesis is small. The possibility that sugar repression of photosynthesis under physiological conditions depends more crucially on the C:N status of leaves than the carbohydrate status alone is discussed.     

Low energy photosynthesis of gold-titania catalysts.

By identifying the Electron Partitioning Effect (EPE) as responsible for the large gold deposits usually formed in the conventional photodeposition method, a low energy UV light-based method for the preparation of comparatively high-activity gold-titania catalysts was developed. These materials were tested in the carbon monoxide (CO) oxidation reaction and returned markedly higher levels of activity at room temperature, when compared to catalysts prepared by the traditional photodeposition method. This is the first instance of using a light-mediated process for preparing catalysts active for the CO oxidation reaction.     

铁胁迫下大豆光合和磷/铁性状对磷素的响应

为了明确低铁胁迫下磷素用量对大豆光合和磷/铁性状的影响及基因型差异,为磷、铁肥的合理施用提供理论依据,以前期筛选的6个磷高效基因型和6个磷低效基因型大豆为供试材料,设4个P∶Fe配比处理,分别为0∶30、30∶30、150 ∶30和300∶30(μmol·L-1),对大豆叶绿素荧光特性和磷、铁利用率进行了测定,利用单株...     

ADP+orthophosphate (P(i)) stimulates an Na/K pump-mediated coefflux of P(i) and Na in human red blood cell ghosts

The Na/K pump in human red blood cells that normally exchanges 3 Nai for 2 Ko is known to continue to transport Na in a ouabain-sensitive and ATP-dependent manner when the medium is made free of both Nao and Ko. Although this Na efflux is called "uncoupled" because of removal of ions to exchange with, the efflux has been shown to be comprised of a coefflux with cellular anions. The work described in this paper presents a new mode of operation of uncoupled Na efflux. This new mode not only depends upon the combined presence of ADP and intracellular orthophosphate (P(i))i but the Na efflux that is stimulated to occur is coeffluxed with (P(i))i. These studies were carried out with DIDS- treated resealed red cell ghosts, suspended in buffered (NMG)2SO4, that were made to contain, in addition to other constituents, varying concentrations of ADP and P(i) together with Na2 SO4, MgSO4 and hexokinase. While neither ADP nor P(i) was effective alone, ouabain- sensitive uncoupled Na efflux, (measured with 22Na) could be activated by [ADP+P(i)] where the K0.5 for ADP in the presence of 10 mmol (P(i))i/liter ghosts was 100-200 mumol/liter ghosts and the K0.5 for (P(i))i, in the presence of 500 mumol ADP/liter ghosts was 3-4 mmol/liter ghosts. [ADP+P(i)] activation of this Na efflux could be inhibited by as little as 2 mumol ATP/liter ghosts but the inhibition could be relieved by the addition of 50 mM glucose, given entrapped hexokinase. While ouabain-sensitive Na efflux was found to be coeffluxed with P(i) (measured with entrapped [32P]H3PO4), this was not so for SO4 (measured with 35SO4). The stoichiometry of Na to P(i) efflux was found to be approximately 2 to 1. Na efflux as well as (P(i))i efflux were both inhibited by 10 mM Nao (K0.5 approximately equal to 4 mM). But, whereas 20 mM Ko (K0.5 approximately equal to 6 mM) inhibited the efflux of (P(i))i, as would be expected from previous work, Na efflux was actually increased. When Ko influx was measured in this situation there was a 1 for 1 exchange of Nai for Ko, that is, of course, downhill with respect to the gradient of each ion. Surprisingly AsO4 was unable to replace P(i) for activation of Na efflux but Na efflux could be inhibited by vanadate and oligomycin. In terms of mechanism, it is likely that ADP acts to promote the formation of the phosphoenzyme (EP) by (P(i))i that would otherwise be inhibited by Nai.(ABSTRACT TRUNCATED AT 400 WORDS)     

Increased concentrations of P(i) and lactic acid reduce creatine-stimulated respiration in muscle fibers.

We tested the hypothesis that the respiratory function of skeletal muscle mitochondria is impaired by lactic acidosis and elevated concentrations of P(i). The rate of respiration of chemically skinned fiber bundles from rat soleus muscle was measured at [P(i)] (brackets denote concentration) and pH values similar to those at rest (3 mM P(i), pH 7.0) and high-intensity exercise (20 mM P(i), pH 6.6). Respiration was measured in the absence of ADP and after sequential additions of 0.1 mM ADP, 20 mM creatine (Cr; V(Cr)), and 4 mM ADP. Respiration at 0.1 mM ADP increased after addition of Cr. However, V(Cr) was 23% lower (P < 0.05) during high-intensity conditions than during resting conditions. V(Cr) was also reduced when P(i) or H(+) was increased separately (P < 0.05). Respiration in the absence of ADP and after additions of 0.1 mM ADP and 4 mM ADP was not affected by changes in [P(i)] or [H(+)]. The response was similar, irrespective of when acidosis was induced (i.e., quiescent or actively respiring mitochondria). In conclusion, Cr-stimulated respiration is impaired by increases in [H(+)] and [P(i)] corresponding to those in exercising muscle. Although the reduced Cr-stimulated respiration could be compensated for by increased [ADP], this might have implications for intracellular homeostasis.     

Phosphate starvation signaling: a threesome controls systemic P(i) homeostasis

Systemic signaling between roots and shoots is required to maintain mineral nutrient homeostasis for optimal metabolism under varying environmental conditions. Recent work has revealed molecular components of a signaling module that controls systemic phosphate homeostasis, modulates uptake and transport in Arabidopsis. This module comprises PHO2, a protein that controls protein stability, the phloem-mobile microRNA-399 and a ribo-regulator that squelches the activity of miR399 towards PHO2 by a novel mechanism. This advance is a significant step for the design of future rational approaches to improve crop phosphate use efficiency.     

Creatine reduces human muscle PCr and pH decrements and P(i) accumulation during low-intensity exercise.

The purpose of this study was to examine with (31)P-magnetic resonance spectroscopy energy metabolism during repeated plantar flexion isometric exercise (Ex-1-Ex-4) at 32 +/- 1 and 79 +/- 4% of maximal voluntary contraction (MVC) before and during a creatine (Cr) feeding period of 5 g/day for 11 days. Eight trained male subjects participated in the study. ATP was unchanged with Cr supplementation at rest and during exercise at both intensities. Resting muscle phosphocreatine (PCr) increased (P < 0.05) from 18.3 +/- 0.9 (before) to 19.6 +/- 1.0 mmol/kg wet wt after 9 days. At 79% MVC, PCr used, P(i) accumulated, and pH at the end of Ex-1-Ex-4 were similar after 4 and 11 days of Cr supplementation. In contrast, PCr utilization and P(i) accumulation were lower and pH was higher for exercise at 32% MVC with Cr supplementation, suggesting aerobic resynthesis of PCr was more rapid during exercise. These results suggest that elevating muscle Cr enhances oxidative phosphorylation during mild isometric exercise, where it is expected that oxygen delivery matches demands and predominantly slow-twitch motor units are recruited.     

Functional studies on a split type II Na/P(i)-cotransporter

Analysis of rat and mouse proximal tubular brush-border membrane expression of the type IIa Na/P(i)-cotransporter provides evidence for its cleavage in the large extracellular loop (ECL-2). To study functional properties and membrane distribution of this split NaP(i)-IIa transporter we followed two strategies. In one strategy we expressed the transporter as two complementary parts (p40 and p45) in Xenopus laevis oocytes and as another strategy we cleaved the WT protein with trypsin. Both strategies resulted in a split NaP(i)-IIa protein located in the plasma membrane. The two domains were tied together by a disulfide bridge, most likely involving the cysteines 306 and 334. Surface expression of the NaP(i)-IIa fragments was dependent on the presence of both domains. If both domains were coexpressed, the transporter was functional and transport characteristics were identical to those of the WT-NaP(i)-IIa protein. Corresponding to this, the transporter cleaved by trypsin also retains its transport capacity. These data indicate that cleavage of the type IIa Na/P(i)-cotransporter at ECL-2 is compatible with its cotransport function.     

Identification of an F-box protein that negatively regulates P(i) starvation responses

The Arabidopsis gene At5g21040 encodes a protein containing both WD40 and F-box motifs, termed FBX2. A T-DNA insertional mutant in this gene was obtained. Analysis of this mutant line showed that FBX2 is a negative regulator of several P(i) starvation responses. FBX2 interacts with BHLH32, another negative regulator of P(i) starvation responses. We suggest that FBX2 may be part of an SCF-like complex that recruits BHLH32 and its partners, potentially targeting the latter for proteolysis.     

Cell-matrix interactions modulate transepithelial phosphate transport in P(i)-deprived OK cells

In opossum kidney (OK) cells as well as in kidney proximal tubules, P(i) depletion increases apical (A) and basolateral (B) Na(+)-dependent P(i) cell influxes. In OK cells' monolayers in contrast to proximal tubules, there is no increase in transepithelial P(i) transport. This limitation may be due to altered cell-matrix interactions. A and B cell (32)P(i) uptakes and transepithelial (32)P(i) and [(14)C]mannitol fluxes were measured in OK cells grown on uncoated or on Matrigel-coated filter inserts. Cells were exposed overnight to solution of either low (0.25 mM) or high (2.5 mM) P(i). When grown on Matrigel, immunofluorescence of apical NaPi4 (an isoform of the sodium-phosphate cotransporter) transporters increased and A and B (32)P(i) uptakes into P(i) depleted cells were five and threefold higher than in P(i) replete cells (P < 0.001). P(i) deprivation resulted in larger increase in A to B (4.6x, P < 0.001) than in B to A (3.5x, P < 0.001) P(i) flux and net P(i) transport from A to B increased 10-fold (P < 0.001). With P(i) depletion increases in B to A (3.4x) and A to B (3.3x) paracellular [(14)C]mannitol fluxes were similar, and its net flux was opposite to that of P(i). In cells grown on uncoated filters, transepithelial and paracellular unidirectional and net P(i) fluxes decreased or did not change with P(i) depletion, despite twofold increases in apical and basolateral P(i) cell influxes. In summary, Matrigel-OK cell interactions, particularly in P(i)-depleted cells, led to enhanced expression of apical NaPi4 transporters resulting in higher P(i) transport rates across cell boundaries; apical P(i) readily entered the transcellular transport pool and paracellular fluxes were smaller fractions of transepithelial P(i) fluxes. These Matrigel-induced changes led to an increase in net transepithelial apical to basolateral P(i) transport.     

66. Jahresversammlung (1952) zu Freiburg i. Breisgau

Ohne Zusammenfassung     

ms2i6A deficiency enhances proofreading in translation.

The hypermodified base 2-methylthio-N6-isopentenyladenosine (ms2i6A) at position 37 occurs frequently in tRNAs that read codons starting with uridine. Here we have studied how ms2i6A affects the accuracy of poly(U) translation in vitro. Deficiency leads to a higher rejection rate of tRNA4(Leu) by more aggressive proofreading on the wild-type ribosome, but with the initial selection step unchanged. Our data indicate that ms2i6A has no effect on codon-anticodon interactions on wild-type ribosomes as long as aminoacyl-tRNA is in ternary complex with EF-Tu and GTP. ms2i6A deficiency in the cognate poly(U) reader tRNA(Phe) leads to increased misreading when the near-cognate competitor tRNA4(Leu) is wild-type. ms2i6A deficiency in tRNA4(Leu) gives a decreased error level in competition with wild-type tRNA(Phe).