Regulation of sulfate assimilation by nitrogen and sulfur nutrition in poplar trees

Plants cover their need for sulfur by taking up inorganic sulfate, reducing it to sulfide, and incorporating it into the amino acid cysteine. In herbaceous plants the pathway of assimilatory sulfate reduction is highly regulated by the availability of the nutrients sulfate and nitrate. To investigate the regulation of sulfate assimilation in deciduous trees we used the poplar hybrid Populus tremula × P. alba as a model. The enzymes of the pathway are present in several isoforms, except for sulfite reductase and -glutamylcysteine synthetase; the genomic organization of the pathway is thus similar to herbaceous plants. The mRNA level of APS reductase, the key enzyme of the pathway, was induced by 3 days of sulfur deficiency and reduced by nitrogen deficiency in the roots, whereas in the leaves it was affected only by the withdrawal of nitrogen. When both nutrients were absent, the mRNA levels did not differ from those in control plants. Four weeks of sulfur deficiency did not affect growth of the poplar plants, but the content of glutathione, the most abundant low molecular thiol, was reduced compared to control plants. Sulfur limitation resulted in an increase in mRNA levels of ATP sulfurylase, APS reductase, and sulfite reductase, probably as an adaptation mechanism to increase the efficiency of the sulfate assimilation pathway. Altogether, although distinct differences were found, e.g. no effect of sulfate deficiency on APR in poplar leaves, the regulation of sulfate assimilation by nutrient availability observed in poplar was similar to the regulation described for herbaceous plants.     

The transformation of inorganic sulfur compounds and the assimilation of organic and inorganic carbon by the sulfur disproportionating bacterium Desulfocapsa sulfoexigens

The physiology of the sulfur disproportionator Desulfocapsa sulfoexigens was investigated in batch cultures and in a pH-regulated continuously flushed fermentor system. It was shown that a sulphide scavanger in the form of ferric iron was not obligatory and that the control of pH allowed production of more biomass than was possible in carbonate buffered but unregulated batch cultures. Small amounts of sulphite were produced during disproportionation of elemental sulfur and thiosulphate. In addition, it was shown that in the presence of hydrogen, a respiratory type of process is favored before the disproportionation of sulphite, thiosulphate and elemental sulfur. Sulphate reduction was not observed. D. sulfoexigens assimilated inorganic carbon even in the presence of organic carbon sources. Inorganic carbon assimilation was probably catalyzed by the reverse CO-dehydrogenase pathway, which was supported by the constitutive expression of the gene encoding CO-dehydrogenase in cultures grown in the presence of acetate and by the high carbon fractionation values that are indicative of this pathway.     

Growth regulation by GTP. Regulation of nucleotide pools in Neurospora by nitrogen and sulfur control systems

Purine nucleotide pools in the fungus Neurospora crassa decline in response to carbon, nitrogen, or sulfur deprivation. There is, in addition, a decline in GTP/ATP ratios on nitrogen or sulfur deprivation in wild type. The GTP/ATP decline is missing on nitrogen deprivation of the nitrogen control mutant, nit-2, and on sulfur deprivation of the sulfur control mutant, cys-3. The nit-2 mutant also shows elevated UTP pools on nitrogen deprivation when compared with similarly treated wild type. Six-hour sulfur-deprived cys-3 shows multiple aberrations in nucleotide pools when compared with similarly treated wild type. These include very low energy charge and depletion of pools of most nucleotides. ATP in sulfur-deprived cys-3 drops by about 88%. Sulfur-deprived cys-3 is also greatly impaired in comparison with wild type in its ability to resume growth when restored to nutritional sufficiency after a period of sulfur deprivation. These results clearly demonstrate that the nitrogen (nit-2) and sulfur (cys-3 regulatory systems are not limited to control of catabolism of exogenous nitrogen and sulfur sources, respectively, but rather influence, a broader range of cellular properties than has been previously thought. The pattern of GTP pool control is consistent with a positive role for GTP in growth control. Evidence in other systems supporting such a growth-regulating role for GTP is discussed.     

氮、锰、硫缺乏对蛋白核小球藻Chlorella pyrenoidosa光合产氢及其生长的影响

本实验通过研究缺氮、缺锰和缺硫对蛋白核小球藻Chlorella pyrenoidosa产氢的影响,发现缺氮、缺锰及缺硫条件下该藻均能产氢,但在缺氮条件下产氢量最高,约为88.613μL H2/mgChla,分别是对照组、缺锰和缺硫实验组产氢量的4.61倍、1.92倍和3.63倍。通过对光合、呼吸及生长的比较研究,发现缺锰对该藻光合、呼吸及生长的影响要小于缺氮和缺硫;与正常培养条件相比,缺锰、缺硫抑制藻细胞的光合放氧和生长,对呼吸影响小,而缺氮不仅最大程度抑制光合放氧和生长,同时使呼吸作用增强,这为进一步优化该藻产氢条件及研究其产氢机制提供了线索。     

Role of O-acetyl-l-serine in the coordinated regulation of the expression of a soybean seed storage-protein gene by sulfur and nitrogen nutrition

The composition of seed storage proteins is regulated by sulfur and nitrogen supplies. Under conditions of a low sulfur-to-nitrogen ratio, accumulation of the β-subunit of β-conglycinin, a sulfur-poor seed storage protein of soybean (Glycine max [L.] Merr.), is elevated, whereas that of glycinin, a sulfur-rich storage protein, is reduced. Using transgenic Arabidopsis thaliana [L.] Heynh., it was found that the promoter from the gene encoding the β-subunit of β-conglycinin up-regulates gene expression under sulfur deficiency and down-regulates gene expression under nitrogen deficiency. To obtain an insight into the metabolic control of this regulation, the concentrations of metabolites related to the sulfur assimilation pathway were determined. Among the metabolites, O-acetyl-l-serine (OAS), one of the precursors of cysteine biosynthesis, accumulated to higher levels under low-sulfur and high-nitrogen conditions in siliques of transgenic A. thaliana. The pattern of OAS accumulation in response to various levels of sulfur and nitrogen was similar to that of gene expression driven by the β-subunit promoter. Elevated levels of OAS accumulation were also observed in soybean cotyledons cultured under sulfur deficiency. Moreover, OAS applied to in-vitro cultures of immature soybean cotyledons under normal sulfate conditions resulted in a high accumulation of the β-subunit mRNA and protein, whereas the accumulation of glycinin was reduced. These changes were very similar to the responses observed under conditions of sulfur deficiency. Our results suggest that the level of free OAS mediates sulfur- and nitrogen-regulation of soybean seed storage-protein composition. Received: 6 February 1999 / Accepted: 16 March 1999     

Prokaryotic sulfur oxidation

Recent biochemical and genomic data differentiate the sulfur oxidation pathway of Archaea from those of Bacteria. From these data it is evident that members of the Alphaproteobacteria harbor the complete sulfur-oxidizing Sox enzyme system, whereas members of the beta and gamma subclass and the Chlorobiaceae contain sox gene clusters that lack the genes encoding sulfur dehydrogenase. This indicates a different pathway for oxidation of sulfur to sulfate. Acidophilic bacteria oxidize sulfur by a system different from the Sox enzyme system, as do chemotrophic endosymbiotic bacteria.     

Regulation of alternative oxidase 1 in Chlamydomonas reinhardtii during sulfur starvation

The mitochondrial respiratory chain in plants, some protists and many fungi consists of the ATP-coupling cyanide-sensitive cytochrome pathway and the cyanide-resistant alternative respiratory pathway. The alternative pathway is mediated by alternative oxidase (AOX). Although AOX has been proposed to play essential roles in nutrient stress tolerance of plants and protists, the effects of sulfur (S) deprivation, on AOX are largely unknown. The unicellular green alga Chlamydomonas reinhardtii reacts to S limitation conditions with the induced expression of many genes. In this work, we demonstrated that exposure of C. reinhardtii to S deprivation results in the up-regulation of AOX1 expression and an increased AOX1 protein. Furthermore, S-deprived C. reinhardtii cells display the enhanced AOX1 capacity. Moreover, nitrate assimilation regulatory protein (NIT2) is involved in the control of the AOX1 gene expression in the absence of S. Together, the results clearly indicate that AOX1 relates to S limitation stress responses and is regulated in a NIT2-dependent manner, probably together with yet-unknown regulatory factor(s).     

Utilization of glutathione as an exogenous sulfur source is independent of gamma-glutamyl transpeptidase in the yeast Saccharomyces cerevisiae: evidence for an alternative gluathione degradation pathway

gamma-Glutamyl transpeptidase (gamma-GT) is the only enzyme known to be responsible for glutathione degradation in living cells. In the present study we provide evidence that the utilization of glutathione can occur in the absence of gamma-GT. When disruptions in the CIS2 gene encoding gamma-GT were created in met15Delta strains, which require organic sulfur sources for growth, the cells were able to grow well with glutathione as the sole sulfur source suggesting that a gamma-GT-independent pathway for glutathione degradation exists in yeast cells. The CIS2 gene was strongly repressed by ammonium and derepressed in glutamate medium, and was found to be regulated by the nitrogen regulatory circuit. The utilization of glutathione as a sulfur source was, however, independent of the nitrogen source in the medium, further underlining that the two degradatory pathways were distinct.     

Changes in sulfate transport characteristics and protein composition of Anacystis nidulans R2 during sulfur deprivation.

Sulfur-starved cells of Anacystis nidulans have an increased capacity to take up sulfate. The apparent Vmax for sulfate uptake increased at least 10-fold after 24 h of sulfur deprivation, whereas the K1/2 remained unchanged at approximately 1.35 microM. The initial rate of sulfate uptake increased between 2 and 6 h after transfer of the cells to sulfur-free medium, in concert with elevated levels of three cytoplasmic membrane polypeptides with molecular masses of 43, 42, and 36 kilodaltons (kDa). The amounts of these polypeptides did not increase in response to nitrogen or phosphorus deprivation. A fourth cytoplasmic membrane polypeptide of 17 kDa did not appear until 24 h after transfer to sulfur-deficient medium. In the total soluble fraction, three polypeptides with masses of 36.5, 33.5, and 28.5 kDa increased dramatically in response to sulfur deprivation, but not in response to nitrogen or phosphorus deprivation. The specificity and abundance of these polypeptides indicate that they could play an important role in the response of A. nidulans to sulfur deprivation.     

Interaction of sulfur and nitrogen nutrition in tobacco (Nicotiana tabacum) plants: significance of nitrogen source and root nitrate reductase

Abstract: The significance of root nitrate reductase for sulfur assimilation was studied in tobacco (Nicotiana tabacum) plants. For this purpose, uptake, assimilation, and long-distance transport of sulfur were compared between wild-type tobacco and transformants lacking root nitrate reductase, cultivated either with nitrate or with ammonium nitrate. A recently developed empirical model of plant internal nitrogen cycling was adapted to sulfur and applied to characterise whole plant sulfur relations in wild-type tobacco and the transformant. Both transformation and nitrogen nutrition strongly affected sulfur pools and sulfur fluxes. Transformation decreased the rate of sulfate uptake in nitrate-grown plants and root sulfate and total sulfur contents in root biomass, irrespective of N nutrition. Nevertheless, glutathione levels were enhanced in the roots of transformed plants. This may be a consequence of enhanced APR activity in the leaves that also resulted in enhanced organic sulfur content in the leaves of the tranformants. The lack of nitrate reductase in the roots in the transformants caused regulatory changes in sulfur metabolism that resembled those observed under nitrogen deficiency. Nitrate nutrition reduced total sulfur content and all the major fractions analysed in the leaves, but not in the roots, compared to ammonium nitrate supply. The enhanced organic sulfur and glutathione levels in ammonium nitrate-fed plants corresponded well to elevated APR activity. But foliar sulfate contents also increased due to decreased re-allocation of sulfate into the phloem of ammonium nitrate-fed plants. Further studies will elucidate whether this decrease is achieved by downregulation of a specific sulfate transporter in vascular tissues.     

Sulfite oxidase controls sulfur metabolism under SO2 exposure in Arabidopsis thaliana

In the present study, the significance of sulfite oxidase (SO) for sulfite detoxification and sulfur assimilation was investigated. In response to sulfur dioxide (SO(2)) exposure, a remarkable expansion of sulfate and a significant increase of GSH pool were observed in wild-type and SO-overexpressing Arabidopsis. These metabolic changes were connected with a negative feedback inhibition of adenosine 5'-phosphosulfate reductase (APR), but no alterations in gas exchange parameters or visible symptoms of injury. However, Arabidopsis SO-KO mutants were consistently negatively affected upon 600 nL L(-1) SO(2) exposure for 60 h and showed phenotypical symptoms of injury with small necrotic spots on the leaves. The mean g(H2O) was reduced by about 60% over the fumigation period, accompanied by a reduction of net CO(2) assimilation and SO(2) uptake of about 50 and 35%. Moreover, sulfur metabolism was completely distorted. Whereas sulfate pool was kept constant, thiol-levels strongly increased. This demonstrates that SO should be the only protagonist for back-oxidizing and detoxification of sulfite. Based on these results, it is suggested that co-regulation of SO and APR controls sulfate assimilation pathway and stabilizes sulfite distribution into organic sulfur compounds. In conclusion, a sulfate-sulfite cycle driven by APR and SO can be postulated for fine-tuning of sulfur distribution that is additionally used for sulfite detoxification, when plants are exposed to atmospheric SO(2).