外源γ-氨基丁酸（GABA）对低氧胁迫下甜瓜幼苗根系GABA代谢及氨基酸含量的影响

采用水培法,通过准确控制营养液溶氧浓度,研究了外源γ-氨基丁酸（GABA）对低氧胁迫0~8 d ‘西域一号’甜瓜幼苗根系GABA代谢及氨基酸含量的影响.结果表明：与通气对照相比,低氧处理的甜瓜幼苗正常生长受到严重抑制,其根系谷氨酸脱羧酶（GAD）、谷氨酸脱氢酶（GDH）、谷氨酸合成酶（GOGAT）、谷氨酰胺合成酶（GS）、丙氨酸氨基转移酶（ALT）、天门冬氨酸氨基转移酶（AST）活性以及GABA、丙酮酸、丙氨酸、天冬氨酸含量均显著提高,而谷氨酸和α 酮戊二酸含量在处理4~8 d均显著降低.与低氧处理相比,外源GABA处理有效缓解了低氧胁迫对幼苗根系生长的抑制作用,同时甜瓜根系内源GABA、谷氨酸、α-酮戊二酸、天冬氨酸含量显著提高,但GAD、GDH、GOGAT、GS、ALT、AST活性在整个处理过程中均显著降低,丙酮酸和丙氨酸含量也显著降低.低氧同时添加GABA和γ-乙烯基 γ-氨基丁酸（VGB）处理显著降低了低氧胁迫下GABA的缓解效应.低氧胁迫下外源GABA被植物根系吸收后,通过反馈抑制GAD活性维持较高的Glu含量,保持植物体内碳、氮代谢平衡,维持正常生理代谢,从而缓解低氧胁迫对甜瓜幼苗的伤害.     

NaCl对水稻谷氨酸合酶和谷氨酸脱氢酶的胁迫作用

在ＮａＣｌ的胁迫下,水稻幼苗根和叶的谷氨酸合酶和谷氨酸脱氢酶的活性随着营养液中的ＮａＣｌ浓度的升高而降低;游离ＮＨ４＾＋在叶中积累,在根中未见明显变化。与根相比,叶对ＮａＣｌ的胁迫作用更为敏感。叶的ＮＡＤＨ－ＧＯＧＡＴ和ＮＡＤＨ－ＧＤＨ活性在ＮａＣｌ胁迫降低的程度明显大于根。无论是否有ＮａＣｌ存在,根的ＮＡＤＨ－ＧＤＨ活性明显高于叶。ＧＳ／ＧＤＨ比值分析提示,对对照下,根中的ＮＨ４＾存在,根的ＮＡ     

Nickel-induced depression of nitrogen assimilation in wheat roots

The influence of 50 and 100 μM Ni on the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), alanine aminotransferase (AlaAT) and aspartate aminotransferase (AspAT) was studied in the wheat roots. Root fresh weight, tissue Ni, nitrate, ammonium, glutamate and protein concentrations were also determined. Exposure to Ni resulted in a marked reduction in fresh weight of the roots accompanied by a rapid accumulation of Ni in these organs. Both nitrate and ammonium contents in the root tissue were considerably enhanced by Ni stress. While protein content was not significantly influenced by Ni application, glutamate concentration was slightly reduced on the first day after treatment with the higher Ni dose. Treatment of the wheat seedlings with 100 μM Ni led to a decrease in NR activity; however, it did not alter the activation state of this enzyme. Decline in NiR activity observed after application of 100 μM Ni was more pronounced than that in NR. The activities of GS and NADH-GOGAT also showed substantial decreases in response to Ni stress with the latter being more susceptible to this metal. Starting from the fourth day, both aminating and deaminating GDH activities in the roots of the seedlings supplemented with Ni were lower in comparison to the control. While the activity of AspAT remained unaltered after Ni application that of AlaAT showed a considerable enhancement. The results indicate that exposure of the wheat seedlings to Ni resulted in a general depression of nitrogen assimilation in the roots. Increase in the glutamate-producing activity of AlaAT may suggest its involvement in supplying the wheat roots with this amino acid under Ni stress.     

Impact of nitrogen assimilation on regulation of antibiotic production in Streptomyces hygroscopicus 155

The enzymes of the assimilation pathways in cultures of S. hygroscopicus grown in the presence of various nitrogen sources were investigated. No assimilation activity of glutamate dehydrogenase (GDH) was observed. Activities of alanine dehydrogenase (ADH), GDH, glutamine: 2-oxoglutarate aminotransferase (GOGAT) and glutamate synthetase (GS) were studied. High concentrations of ammonium and alanine induced ADH formation. The levels of GS remained low in media with NH4Cl. Various nitrogen sources had no impact on the activity of GOGAT which suggested the involvement of constitutive synthesis. ADH was likely to play an alternative role. Determination of the quantitative and qualitative composition of the free amino acids confirmed the involvement of the GS-GOGAT pathway in nitrogen assimilation. The concentration of ammonium ions in the media with one amino acid or in the presence of several amino acids lowered the antibiotic activity while in the media with alanine and the other nitrogen compounds it increased the antibiotic activity.     

浑球红假单胞菌氨同化途径的研究

本文测定了浑球红假单胞菌(Rhodobacter sphaeroides)菌株601谷氨酰胺合成酶(GS)、谷氨酸合酶(GOGAT)、谷氨酸脱氢酶(GDH)和丙氨酸脱氢酶(ADH)的活性。低氨时,GS/GOGAT活力高,GDH活力低,高氨时,GS/GOGAT活力低,GDH活力高。在以分子氮或低浓度氨为氮源的培养条件下,加入GS抑制刑MSX(L—methionine—DL—sulphoximine),细菌生长受到抑制。但是,生长在以谷氨酸为氮源的细菌则不受影响。上述结果表明,浑球红假单胞菌菌株601氨同化是通过GS/GOGAT途径和GDH途径。     

Nitrogen metabolism in the facultative methylotroph Arthrobacter P1 grown with various amines or ammonia as nitrogen sources

The metabolism of trimethylamine (TMA) and dimethylamine (DMA) in Arthrobacter P1 involved the enzymes TMA monooxygenase and trimethylamine-N-oxide (TMA-NO) demethylase, and DMA monooxygenase, respectively. The methylamine and formaldehyde produced were further metabolized via a primary amine oxidase and the ribulose monophosphate (RuMP) cycle. The amine oxidase showed activity with various aliphatic primary amines and benzylamine. The organism was able to use methylamine, ethylamine and propylamine as carbon-and nitrogen sources for growth. Butylamine and benzylamine only functioned as nitrogen sources. Growth on glucose with ethylamine, propylamine, butylamine and benzylamine resulted in accumulation of the respective aldehydes. In case of ethylamine and propylamine this was due to repression by glucose of the synthesis of the aldehyde dehydrogenase(s) required for their further metabolism. Growth on glucose/methylamine did not result in repression of the RuMP cycle enzyme hexulose-6-phosphate synthase (HPS). High levels of this enzyme were present in the cells and as a result formaldehyde did not accumulate. Ammonia assimilation in Arthrobacter P1 involved NADP-dependent glutamate dehydrogenase (GDH), NAD-dependent alanine dehydrogenase (ADH) and glutamine synthetase (GS) as key enzymes. In batch cultures both GDH and GS displayed highest levels during growth on acetate with methylamine as the nitrogen source. A further increase in the levels of GS, but not GDH, was observed under ammonia-limited growth conditions in continuous cultures with acetate or glucose as carbon sources.Abbreviations HPS hexulose-6-phosphate synthase - RuMP ribulose monophosphate - DMA dimethylamine - TMA trimethylamine - TMA-NO trimethylamine-N-oxide - ICL isocitrate lyase - GS glutamine synthetase - GDH glutamate dehydrogenase - ADH alanine dehydrogenase - GOGAT glutamate synthase     

Enzymes of ammonium assimilation inStreptomyces avermitilis

Glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), alanine dehydrogenase (ADH) and alanine aminotransferase (GPT) were detected in the cell-free homogenate ofStreptomyces avermitilis grown in a defined medium containing ammonium sulfate as the only nitrogen source. At an initial NH₄ ⁺ concentration of 7.5 mmol/L, high activities of GS, GOGAT and GDH were found while that of ADH was low. The ADH activity was markedly increased at initially millimolar NH₄ ⁺ concentrations. In some characteristics of its NH₄ ⁺-assimilating system (e.g. control of some enzyme activities, the NADPH specificity of GOGAT, the presence of alanine aminotransferase),S. avermitilis differs from other known streptomycetes.     

The two senescence-related markers, GS1 (cytosolic glutamine synthetase) and GDH (glutamate dehydrogenase), involved in nitrogen mobilization, are differentially regulated during pathogen attack and by stress hormones and reactive oxygen species in Nicotiana tabacum L. leaves

To investigate the role of stress in nitrogen management in plants, the effect of pathogen attack, elicitors, and phytohormone application on the expression of the two senescence-related markers GS1 (cytosolic glutamine synthetase EC 6.3.1.2) and GDH (glutamate dehydrogenase, EC 1.4.1.2) involved in nitrogen mobilization in senescing leaves of tobacco (Nicotiana tabacum L.) plants, was studied. The expression of genes involved in primary nitrogen assimilation such as GS2 (chloroplastic glutamine synthetase) and Nia (nitrate reductase, EC 1.6.1.1) was also analysed. The Glubas gene, coding a beta-1,3-glucanase, was used as a plant-defence gene control. As during natural senescence, the expression of GS2 and Nia was repressed under almost all stress conditions. By contrast, GS1 and GDH mRNA accumulation was increased. However, GS1 and GDH showed differential patterns of expression depending on the stress applied. The expression of GS1 appeared more selective than GDH. Results indicate that the GDH and GS1 genes involved in leaf senescence are also a component of the plant defence response during plant-pathogen interaction. The links between natural plant senescence and stress-induced senescence are discussed, as well as the potential role of GS1 and GDH in a metabolic safeguard process.     

渗透胁迫下不同抗旱性小麦幼苗氨同化差异

在渗透胁迫下,测定了不同抗旱性小麦（抗旱性强的品种洛旱6号和抗旱性弱的品种周麦18）幼苗氨同化酶及相关参数的变化.结果表明:小麦生物量在渗透胁迫下明显降低,且抗旱性弱的周麦18降幅较大.铵态氮含量随胁迫程度的增加而增加,且周麦18增加较明显;谷氨酰胺合成酶(GS)活性在不同抗旱性品种间表现不同,抗旱性强的洛旱6号在低渗透胁迫下显著增加,在高渗透胁迫下明显降低,而周麦18随胁迫程度的增加逐渐降低;依赖还原型辅酶Ⅰ的谷氨酸脱氢酶(NADH-GDH)活性随胁迫程度的增加逐渐加大,低渗透胁迫下周麦18增加较明显,高渗透胁迫下洛旱6号增幅较大;依赖氧化型辅酶Ⅰ的谷氨酸脱氢酶(NAD^+-GDH)和依赖氧化型辅酶Ⅱ的异柠檬酸脱氢酶(NADP-ICDH)活性均随胁迫程度的增加而增加,周麦18的NAD^+-GDH活性、洛旱6号的NADP-ICDH活性增幅较大.表明小麦抗旱性的提高与铵态氮同化的增强有关,低渗透和高渗透胁迫下分别依赖GS和NADH-GDH活性的增加.     

Isolation of auxotrophic mutants in support of ammonia assimilation via glutamine synthetase in Methanobacterium ivanovii

Various enzymes involved in the initial metabolic pathway for ammonia assimilation by Methanobacterium ivanovii were examined. M. ivanovii showed significant activity of glutamine synthetase (GS). Glutamate synthase (GOGAT) and alanine dehydrogenase (ADH) were present, wheras, glutamate dehydrogenase (GDH) was not detected. When M. ivanovii was grown with different levels of NH ₊ ⁴ (i.e. 2, 20 or 200 mM), GS, GOGAT and ADH activities varied in response to NH ₊ ⁴ concentration. ADH was not detected at 2 mM level, but its activity increased with increased levels of NH ₊ ⁴ in the medium. Both GS and GOGAT activities increased with decreasing concentrations of NH ₊ ⁴ and were maximum when ammonia was limiting, suggesting that at low NH ₊ ⁴ levels, GS and GOGAT are responsible for ammonia assimilation and at higher NH ₊ ⁴ levels, ADH might play a role. Metabolic mutants of M. ivanovii that were auxotrophic for glutamine were obtained and analyzed for GS activity. Results indicate two categories of mutants: i) GS-deficient auxotrophic mutants and ii) GS-impaired auxotrophic mutants.Abbreviations GS Glutamine synthetase - GOGAT glutamate synthase - GDH glutamate dehydrogenase - ADH alanine dehydrogenase     

Growth of Bacillus fastidiosus on glycerol and the enzymes of ammonia assimilation

Bacillus fastidiosus was able to grow on glycerol as a carbon source when allantoin or urate was used as nitrogen source. The primary assimilatory enzyme for glycerol was glycerol kinase; glycerol dehydrogenase could not be detected. The glycerol kinase activity was increased 30-fold in allantoin/glycerol-grown cells as compared to alantoin-grown cells. Under both growth conditions high levels of glutamate dehydrogenase were found. Glutamine synthetase and glutamate synthase activities could not be demonstrated, while low levels of alanine dehydrogenase were present. It is concluded that B. fastidiosus assimilates ammonia by the NADP-dependent glutamate dehydrogenase.Abbreviations GS glutamine synthetase - GOGAT glutamate synthase - GDH glutamate dehydrogenase - ADH alanine dehydrogenase     

南瓜种子萌发及子叶发育时谷氨酰胺合成酶和其它氨同化酶的变化

在发育的新生组织中 ,来自种子胚乳储存蛋白的降解和氨基酸分解代谢产生的氨由谷氨酰胺合成酶 ( Glutamine synthetase,GS)重新同化 ,生成的谷氨酰胺 ( Gln)被转运到正在生长着的部分。GS是高等植物氮素代谢的关键酶 [1] ,这个酶能同化不同来源的氨。 GS有多种同工酶 ,存在于植物的各种组织和器官中。它们是由一小的同源但分离的核基因家族编码的 [2 3 ] ,这些不同的 GS在植物氮素同化中起着非重叠的作用 [4] ,它们的表达受到环境、发育进程以及组织或细胞类型等许多因素的影响。在大多数已研究过的植物叶片中存在两种 GS,即胞液型GS(…     

Ammonium assimilation in Pinus radiata seedlings: effects of storage treatments,transplanting stress and water regimes after planting under simulated field conditions

This study is concerned with the effects of storage conditions, planting shock and water availability at planting sites on nitrogen assimilation in Pinus radiata. Seedlings were stored for 1, 8 or 15 days at 4 or 10 °C with or without soil around the roots and afterwards planted in well-irrigated or dry soils. Our analysis of protein content and the activities of glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GDH; EC 1.4.1.2) showed that, as storage duration increased, there was a gradual reduction in soluble protein content and an inhibition of GS, whereas GDH activity increased. Thus there appears to be a close relationship between desiccation and changes in nitrogen assimilation. These effects were more pronounced in bare-root seedlings than in seedlings with soil around the roots, indicating that the rooting-plug medium provides protection against desiccation. Seedlings were susceptible to planting shock even under well-watered conditions. Drought enhanced the severity of planting shock and both protein content (80%) and GS activity (75%) were drastically reduced when water was withheld, whereas GDH activity increased by more than 170%. Thus, the decline in needle relative water content (RWC) appeared closely related to a decrease in protein content and GS activity, and on the other hand, to an increase in GDH. There was a remarkable recovery of these parameters after rewatering, depending however, on stock quality at the moment of planting.     

Bottle gourd rootstock-grafting affects nitrogen metabolism in NaCl-stressed watermelon leaves and enhances short-term salt tolerance

The plant growth, nitrogen absorption, and assimilation in watermelon (Citrullus lanatus [Thunb.] Mansf.) were investigated in self-grafted and grafted seedlings using the salt-tolerant bottle gourd rootstock Chaofeng Kangshengwang (Lagenaria siceraria Standl.) exposed to 100 mM NaCl for 3 d. The biomass and NO₃⁻ uptake rate were significantly increased by rootstock while these values were remarkably decreased by salt stress. However, compared with self-grafted plants, rootstock-grafted plants showed higher salt tolerance with higher biomass and NO₃⁻ uptake rate under salt stress. Salinity induced strong accumulation of nitrate, ammonium and protein contents and a significant decrease of nitrogen content and the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate synthase (GOGAT) in leaves of self-grafted seedlings. In contrast, salt stress caused a remarkable decrease in nitrate content and the activities of GS and GOGAT, and a significant increase of ammonium, protein, and nitrogen contents and NR activity, in leaves of rootstock-grafted seedlings. Compared with that of self-grafted seedlings, the ammonium content in leaves of rootstock-grafted seedlings was much lower under salt stress. Glutamate dehydrogenase (GDH) activity was notably enhanced in leaves of rootstock-grafted seedlings, whereas it was significantly inhibited in leaves of self-grafted seedlings, under salinity stress. Three GDH isozymes were isolated by native gel electrophoresis and their expressions were greatly enhanced in leaves of rootstock-grafted seedlings than those of self-grafted seedlings under both normal and salt-stress conditions. These results indicated that the salt tolerance of rootstock-grafted seedlings might (be enhanced) owing to the higher nitrogen absorption and the higher activities of enzymes for nitrogen assimilation induced by the rootstock. Furthermore, the detoxification of ammonium by GDH when the GS/GOGAT pathway was inhibited under salt stress might play an important role in the release of salt stress in rootstock-grafted seedlings.     

Ammonium assimilation inNocardia asteroides

Specific enzymes of ammonium assimilation were measured in cell-free extracts ofNocardia asteroides grown in a synthetic medium with glutamate as the nitrogen source. Cell-free extracts had active glutamine synthetase (GS) and glutamate synthase (GOGAT) and alanine dehydrogenase (ADH) but glutamate dehydrogenase (GDH) could not be detected in the enzyme preparation. This shows that GS/GOGAT is the major pathway of ammonium assimilation inN. asteroides.     

Effect of root-applied spermidine on growth and respiratory metabolism in roots of cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis>) seedlings under hypoxia

The effects of exogenous spermidine (Spd) application to hypoxic nutrient solution on the contents of endogenous polyamines (PAs) and respiratory metabolism in the roots of cucumber (Cucumis sativus L.) seedlings were investigated. Cucumber seedlings were grown hydroponically in control and hypoxic nutrient solutions with and without addition of Spd at a concentration of 0.05 mM. The activities of key enzymes involved in the tricarboxylic acid cycle (TCAC), such as succinate dehydrogenase (SDH) and isocitrate dehydrogenase (IDH), were significantly inhibited under root-zone hypoxia with dissolved oxygen (DO) at 1 mg/l. In contrast, the activities of enzymes involved in the process of fermentation, such as pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH), lactate dehydrogenase (LDH), and alanine aminotransferase (AlaAT), were significantly increased. Thus, aerobic respiration was inhibited and fermentation was enhanced in the roots of cucumber seedlings as a result of decreasing ATP content to inhibit the dry weight of seedlings under hypoxic stress. Moreover, the contents of free, soluble conjugated, and insoluble bound putrescine (Put), Spd, and spermine (Spm) in the roots of cucumber seedlings were significantly increased under hypoxia stress. Interestingly, application of Spd to hypoxic roots markedly suppressed the accumulation of free Put and, in contrast, promoted an increase in free Spd and Spm, as well as soluble conjugated and insoluble bound Put, Spd, and Spm contents. From these data, we deduced that exogenous Spd promotes the conversion of free Put into free Spd and Spm, and soluble conjugated and insoluble bound PAs under hypoxia stress. Furthermore, the activities of LDH, PDC, and ADH were suppressed and, in contrast, the activities of SDH and IDH were enhanced by application of exogenous Spd to hypoxic roots. As a result, aerobic respiration was enhanced but fermentation metabolism was inhibited in the roots of cucumber seedlings, leading to an increase in ATP content to alleviate the inhibited dry weight of seedlings due to hypoxia stress. These results suggest that application of Spd to hypoxic nutrient solution promoted conversion of free Put into free Spd and Spm as well as soluble conjugated and insoluble bound PAs, further enhanced IDH and SDH activities, and inhibited ethanol fermentation and lactate fermentation, resulting in increased ATP content and eventually enhanced tolerance of cucumber plants to root-zone hypoxia.     

Glutamine synthetase and glutamate dehydrogenase in cadmium-stressed triticale seedlings

The studies were performed on young triticale seedlings grown on a mineral medium containing 5 mM NO ₃ ⁻ as the nitrogen source, with the addition of 0.5 mM CdCl₂. It was determined that cadmium ions accumulated mainly in the plant roots. Decreases in nitrate concentrations both in the roots and shoots of seedlings, as well as decreases in soluble protein contents with simultaneous increases in endopeptidase activity were also observed. Both in roots and shoots significant decreases in glutamic acid were noted. Toxic cadmium ion accumulation in seedlings significantly modified activity of primary nitrogen assimilating enzymes, i.e. glutamine synthetase (GS, EC 6.3.1.2) and glutamate dehydrogenase (GDH, EC 1.4.1.2). There was a significant decrease in GS activity both in roots and in shoots of the stressed plants, in comparison to plants grown without cadmium. In shoots of the control plants and plants subjected to stress two GS isoforms were discovered: cytoplasmatic (GS₁) and chloroplastic (GS₂). Substantial decreases in total glutamine synthetase activity in green parts of seedlings, occurring under stress conditions, result from dramatic decrease in GS₂ activity (by 60 % in relation to the control plants); despite simultaneous increases in the cytoplasmatic isoform (GS₁) activity by approx. 96 %. Cadmium ions accumulating in roots and shoots of seedlings not only increased GDH activity, but also modified its coenzymatic specificity.