Is boron involved solely in structural roles in vascular plants?

It is very well proved that boron (B) plays a primary structural role in the plant cell wall. In addition, this micronutrient has been involved in a great variety of physiological processes in vascular plants. It has been reported that B deficiency induces stress-responsive genes and, in tobacco plants, it seems to decrease net nitrate uptake by repressing expression of root plasmalemma H(+)-ATPase gene. Moreover, root asparagine concentration is clearly increased under B deficiency, as also observed for other abiotic stresses. Accumulation of asparagine in response to abiotic stresses could be an ammonium detoxification mechanism when high amounts of ammonium are internally generated by deamination of soluble amino acids released from enhanced proteolysis under stress conditions. Nevertheless, the mechanisms underlying the several effects caused by B deficiency are unknown. Although a mechanism has been reported to explain B effects based on signals via the cell wall-plasma membrane-cytoskeleton continuum, we propose and discuss the possible role of B as a cellular signal through transcription factors. This hypothetical mechanism could explain not only its diverse effects on so many physiological processes, but also that a negligible amount of boron into the protoplast can be decisive for the normal development of such events.

Role of asparagine and asparagine synthetase genes in sunflower (Helianthus annuus) germination and natural senescence

Sunflower (Helianthus annuus) contains three active asparagine synthetase (EC 6.3.5.4, AS) genes: HAS1, HAS1.1 and HAS2. Asparagine content and AS gene expression were determined during germination and leaf and cotyledon natural senescence to assess the role of asparagine as well as the extent of participation of each AS gene in different nitrogen mobilizing processes. Asparagine accumulated in the dry seed and was the predominant amide throughout germination. During cotyledon senescence, the asparagine level was slightly higher than that of glutamine. The opposite was true for leaf senescence. According to transcript accumulation data, most of the asparagine newly synthesized for germination and cotyledon expansion was due to HAS2 activity, with little contribution of the other AS genes. However, all three genes work together to synthesize asparagine for leaf senescence. The absence of significant AS gene expression in cotyledon senescence differentiates leaf and cotyledon senescence, and suggests a cotyledon-specific regulation.

Light and metabolic regulation of HAS1, HAS1.1 and HAS2, three asparagine synthetase genes in Helianthus annuus

The role of light, carbon and nitrogen availability on the regulation of three asparagine synthetase (AS, EC 6.3.5.4)-coding genes, HAS1, HAS1.1 and HAS2, has been investigated in sunflower (Helianthus annuus). The response of each gene to different illumination conditions and to treatments that modify the carbon and nitrogen status of the plant was evaluated by Northern analysis with gene-specific probes. Light represses the expression of HAS1 and HAS1.1. Phytochrome and photosynthesis-derived carbohydrates mediate this repression. On the contrary, maintained HAS2 expression requires light and is positively affected by sucrose. HAS1 and HAS1.1 expression is dependent on nitrogen availability, while HAS2 transcripts are still found in N-starved plants. High ammonium level induces all three AS genes and partially reverts sucrose repression of HAS1 and HAS1.1. In summary, light, carbon and nitrogen availability control asparagine synthesis in sunflower by regulating three AS-coding genes. Illumination and carbon sufficiency maintain HAS2 active to supply asparagine that can be used for growth. Darkness and low C/N ratio conditions trigger the response of the specialized HAS1 and HAS1.1 genes which contribute to store the excess nitrogen as asparagine. Ammonium induces all three AS-genes which may favor its detoxification.

The rate of CO(2) assimilation controls the expression and activity of glutamine synthetase through sugar formation in sunflower (Helianthus annuus L) leaves

The expression and activity of glutamine synthetase (GS, EC 6.3.1.2) were examined in relation to the rate of CO2 assimilation in sunflower (Helianthus annuus L.) leaves. Intact plants were kept in the dark for 72 h and subsequently exposed to light under different atmospheric CO2 concentrations (100, 400 and 1200 microl l-1) for 6 h. The in vivo rates of net CO2 assimilation correlated with atmospheric CO2 concentrations. Stomatal conductances and transpiration rates remained largely unaffected by CO2 levels. Exposure of the plants to increasing CO2 concentrations in the light caused concomitant increases in the contents of starch and soluble sugars and a decrease in the nitrate content in leaves. Both cytosolic and chloroplastic (GS2) GS activities were higher at elevated CO2. A greater accumulation of GS2 mRNA was also observed under high CO2. Exogenous supply of sucrose to detached leaves greatly increased the levels of GS enzyme activity and of mRNA for chloroplastic GS in the dark. These results indicate that GS expression and activity in sunflower leaves are modulated by the rate of CO2 assimilation, and that photosynthesized sugars are presumably involved as regulatory metabolites.

Three genes showing distinct regulatory patterns encode the asparagine synthetase of sunflower (Helianthus annuus)

•  Asparagine metabolism in sunflower (Helianthus annuus) was investigated by cDNA cloning, sequence characterization and expression analysis of three genes encoding different isoforms of asparagine synthetase (AS, EC 6.3.5.4). •  The AS-coding sequences were searched for in leaves, roots and cotyledons by using a methodology based on the simultaneous amplification of different cDNAs. Three distinct AS-coding genes, HAS1, HAS1.1 and HAS2, were identified. •  HAS1 and HAS1.1 are twin genes with closely related sequences that share some regulatory features. By contrast, HAS2 is a singular sequence that encodes an incomplete AS polypeptide and shows an unusual regulation. The functionality of both the complete HAS1 and the truncated HAS2 proteins was demonstrated by complementation assays. Northern analysis revealed that HAS1, HAS1.1 and HAS2 were differentially regulated dependent on the organ, the physiological status, the developmental stage and the light conditions. •  Asparagine synthetase from sunflower is encoded by a small gene family whose members have achieved a significant degree of specialization to cope with the major situations requiring asparagine synthesis.

A short-term exposure of cucumber plants to rising atmospheric CO2 increases leaf carbohydrate content and enhances nitrate reductase expression and activity

Nitrate reductase (NR; EC 1.6.6.1) is the first enzyme of the nitrate-assimilatory pathway and is regulated transcriptionally and post-translationally by several metabolic and environmental signals. To investigate whether NR is controlled by the rate of photosynthetic CO2 assimilation in cucumber (Cucumis sastivus L.), intact plants were exposed, after the dark period, to light under different atmospheric CO2 concentrations (100, 400 and 2,000 microL x L(-1)) for 2 h. The in-vivo rates of net CO2 assimilation correlated with atmospheric CO2 concentrations. The CO2-fixation rate under 2,000 microL x L(-1) CO2 was 2.4- and 5.4-fold higher than under 400 and 100 microL x L(-1), respectively. Stomatal conductances and transpiration rates were almost identical after the 2-h light period under the various CO2 concentrations tested. Increasing atmospheric CO2 concentrations caused concomitant increases in the contents of starch and soluble sugars in the leaves and a decrease in the nitrate content. The activity and activation state of NR were both higher under elevated CO2 than under low CO2. High CO2 also enhanced NR-gene expression in the leaves. Sugars were supplied via roots to intact carbohydrate-starved plants and NR mRNA levels were analysed after 7 h. Fructose markedly stimulated NR-gene transcription in both leaves and roots. It is concluded that, in cucumber plants, the rate of CO2 assimilation controls the rate of nitrate assimilation by modulation of NR expression and activity, and that sugars are presumably involved as regulatory metabolites.

Echocardiographic observations during in hospital cardiopulmonary resuscitation

OBJECTIVE

To assess echocardiographic observations during in hospital cardiopulmonary resuscitation (CPR) and the utility of this information in the management of cardiac arrest.

DESIGN

Echocardiographic system brought to the hospital site of cardiac arrest and applied to patients in conjunction with conventional CPR.

SETTING

Large community-hospital with portable echocardiographic system and an alerted cardiology team skilled in this technique.

MEASUREMENTS AND MAIN RESULTS

Mechanical asystole was initially observed in 18 (90%) of 20 cardiac arrest patients during CPR, including four patients with severe bradyarrhythmia as the arrest rhythm. The return of ventricular contractions in four of these 18 patients a short time after starting CPR prompted positive inotropic therapy. Ventricular wall motion was noted in two patients with severe bradyarrhythmia (pseudo-electromechanical dissociation) and the causes of cardiac arrest identified as massive pulmonary embolism and hypovolemia, respectively. A gel-like, coalescent echo contrast within the cardiac chambers was observed 20 to 30 mins after CPR in ten patients with unrelenting cardiac arrest and uniformly associated with an adverse outcome. Six patients survived resuscitation but only two patients survived to hospital discharge.

CONCLUSIONS

An echocardiographic examination is feasible during CPR and may offer useful information in the management of the individual patients with cardiac arrest. It may depict the proximate cause of cardiac arrest, e.g., pulmonary embolism, cardiac tamponade, or hypovolemia, and signal the return of ventricular contractions in patients with initially absent mechanical activity. The appearance of intracardiac coalescent echo contrast in our patients with unrelenting cardiac arrest was associated with a failed outcome. The role of echocardiographic imaging in the setting of advanced cardiac life support requires further study.

Glutamine-synthetase isoforms appearing in sunflower cotyledons during germination : Effects of light and nitrate

Ion-exchange chromatography has been used to separate the isoforms of glutamine synthetase (GS; EC 6.3.1.2) appearing in sunflower (Helianthus annuus L. cv. Peredovic) cotyledons during seedling growth under different light and nitrogen conditions. Both in dry and imbibed seeds, only a single form of GS (GSs) was detected. Upon seed germination, the GSs isoform was gradually replaced by cytosolic (GS1) and plastidic (GS2) isoforms. Light and nitrate decreased the levels of GS1. In contrast, the appearance of GS2 was greatly stimulated by light. Nitrate also had a positive effect, particularly in the light. Light and nitrate acted synergistically on the appearance of GS2. The GS2:GS1 ratio in cotyledons of 9-d-old seedlings ranged from about 2, in darkness and nitrate-deprivation conditions, to 16 under light and nitrate application. The possible physiological roles of the distinct GS isoforms appearing in the epigeal cotyledons of sunflower during germination, and their differential regulation by light and nitrate, are discussed.

Nitrate and nitrite uptake and reduction by intact sunflower plants

Nitrogen-starved sunflower plants (Helianthus annuus L. cv. Peredovic) cannot absorb NO 3 (-) or NO 2 (-) upon initial exposure to these anions. Ability of the plants to take up NO 3 (-) and NO 2 (-) at high rates from the beginning was induced by a pretreatment with NO 3 (-) . Nitrite also acted as inducer of the NO 2 (-) -uptake system. The presence of cycloheximide during NO 3 (-) -pretreatment prevented the subsequent uptake of NO 3 (-) and NO 2 (-) , indicating that both uptake systems are synthesized de novo when plants are exposed to NO 3 (-) . Cycloheximide also suppressed nitrate-reductase (EC 1.6.6.1) and nitrite-reductase (EC 1.7.7.1) activities in the roots. The sulfhydryl-group reagent N-ethylmaleimide greatly inhibited the uptake of NO 3 (-) and NO 2 (-) . Likewise, N-ethylmaleimide promoted in vivo the inactivation of nitrate reductase without affecting nitrite-reductase activity. Rates of NO 3 (-) and NO 2 (-) uptake as a function of external anion concentration exhibited saturation kinetics. The calculated Km values for NO 3 (-) and NO 2 (-) uptake were 45 and 23 μM, respectively. Rates of NO 3 (-) uptake were four to six times higher than NO 3 (-) -reduction rates in roots. In contrast, NO 2 (-) -uptake rates, found to be very similar to NO 3 (-) -uptake rates, were much lower (about 30 times) than NO 2 (-) -reduction rates. Removal of oxygen from the external solution drastically suppressed NO 3 (-) and NO 2 (-) uptake without affecting their reduction. Uptake and reduction were also differentially affected by pH. The results demonstrate that uptake of NO 3 (-) and NO 2 (-) into sunflower plants is mediated by energy-dependent inducible-transport systems distinguishable from the respective enzymatic reducing systems.

Development of nitrogen-assimilating enzymes in sunflower cotyledons during germination as affected by the exogenous nitrogen source

Activities of nitrate reductase (NR; EC 1.6.6.1), nitrite reductase (NiR; EC 1.7.7.1), glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GDH; EC 1.4.1.3) were measured in cotyledons of sunflower (Helianthus annuus L. cv Peredovic) seedlings during germination and early growth under various external nitrogen sources. The presence of NO 3 (-) in the medium promoted a gradual increase in the levels of NR and NiR activities during the first 7 d of germination. Neither NR nor NiR activities were increased in a nitrogen-free medium or in media with either NH 4 (+) or urea as nitrogen sources. Moreover, the presence of NH 4 (+) did not abolish the NO 3 (-) -dependent appearance of NR and NiR activities. The increase of NR activity was impaired both by cycloheximide and chloramphenicol, which indicates that both cytoplasmic 80S and plastidic 70S ribosomes are involved in the synthesis of the NR molecule. By contrast, the appearance of NiR activity was only inhibited by cycloheximide, indicating that NiR seems to be exclusively synthesized on the cytoplasmic 80S ribosomes. Glutamine-synthetase activity was also strongly increased by external NO 3 (-) but not by NH 4 (+) or urea. The appearance of GS activity was more efficiently suppressed by cycloheximide than chloramphenicol. This indicates that GS is mostly synthesized in the cytoplasm. The cotyledons of the dry seed contain high levels of GDH activity which decline during germination independently of the presence or absence of a nitrogen source. Cycloheximide, but not chloramphenicol, greatly prevented the decrease of GDH activity.

The reactivation of nitrate reductase from spinach (Spinacea oleracea L.) inactivated by NADH and cyanide: effects of peroxidase and associated systems

Nitrate reductase of spinach (Spinacea oleracea L.) leaves which had been inactivated in vitro by treatment with NADH and cyanide, was reactivated by incubation with oxidant systems and measured as FMNH2-dependent activity. Ferricyanide, a purely chemical oxidant, produced rapid maximal reactivation (100%) which was 90% complete in less than 3 min. Reactivation occurred slowly and less completely (30-75% in 30 or 60 min) when the enzyme was incubated with pure horseradish peroxidase alone, depending on using one or 20 units and time. Addition of glucose and glucose oxidase to generate hydrogen peroxide increased reactivation slightly (10-15%) with 20 units of peroxidase but more (30-50%) with one unit and to 75-90% of ferricyanide values. Adding catalase decreased reactivation by more than half either with or without glucose oxidase. Glucose and glucose oxidase alone did not cause reactivation. Addition of superoxide dismutase increased reactivation from 50-75% of ferricyanide values with one unit of peroxidase alone but had no effect on greater reactivation obtained in the presence of glucose oxidase. The addition of p-cresol and manganese together increased reactivation with one unit of peroxidase and in the presence of glucose oxidase by about double, but omission of manganese had no effect. However, as shown previously, although trivalent manganese was formed, the residual presence of manganous ions inhibited reactivation. Nevertheless, peroxidase systems either alone or with additionally generated hydrogen peroxide can induce substantial reactivation of nitrate reductase in physiologically relevant conditions.

The reactivation of nitrate reductase from spinach (Spinacia oleracea L.) inactivated by NADH and cyanide, using trivalent manganese either generated by illuminated chloroplasts or as manganipyrophosphate

Nitrate reductase of spinach (Spinacia oleracea L.) leaves which had been inactivated in vitro by treatment with NADH and cyanide, was reactivated by incubation with oxidant systems and measured as FMNH2-dependent activity. Reactivation was produced with trivalent manganese compounds represented either by manganipyrophosphate or produced by oxidation of Mn(2+) ions in the presence of illuminated chloroplasts and compared with reactivation obtained with ferricyanide. Reactivation in the chloroplast system was equivalent to that with ferricyanide when orthophosphate was used but was variable and weak in the presence of pyrophosphate, although manganipyrophosphate was formed, freely. Reactivation by manganipyrophosphate in dark reaction conditions was less effective than with ferricyanide but was not inhibited by the addition of pyrophosphate. Reactivation with illuminated unheated chloroplasts was dependent on added manganese and oxidation of manganese in the presence of pyrophosphate was abolished by boiling the chloroplasts. In the presence of orthophosphate however, boiled, illuminated chloroplasts reactivated the enzyme in the absence of added manganese. Reactivation occurred spontaneously in air, more slowly than with the other oxidants, but to a similar extent to that produced by manganipyrophosphate. The results provide a possible model for physiological reactivation mechanisms.