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1.
The study of ammonium (NH4
+) transport across plant cell membranes requires accurate measurement of NH4
+ gradients across subcellular gradients. We have developed an ammonium-selective microelectrode based on the ionophore nonactin.
This electrode can detect NH4
+ activities (aNH4) in vivo in the millimolar range in the presence of cytosolic levels of potassium, the main interfering ion. The electrode was used
to measure intracellular aNH4 in internodal cells of the giant alga Chara corallina. Results from cells incubated in media supplemented with 1 mM NH4
+ produced two populations, with means of 7.3 and 30.8 mM, respectively. HPLC analysis of vacuolar sap suggests the higher population represents vacuolar impalements, and the lower
population can thus be assumed to be cytosolic. These results suggest a four-fold accumulation of NH4
+ in the vacuolar compartment of Chara.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
2.
Uptake of inorganic ions and compatible solutes in cultured mangrove cells during salt stress 总被引:6,自引:0,他引:6
Eri Yasumoto Kyoko Adachi Misako Kato Hiroshi Sano Hamako Sasamoto Shigeyuki Baba Hiroshi Ashihara 《In vitro cellular & developmental biology. Plant》1999,35(1):82-85
Summary Callus of the mangrove plant, Sonneratia alba J. Smith, established from pistils of flower buds were cultured on solid Murashige and Skoog medium supplemented with 0 to
500 mM NaCl. Maximum growth was observed with 50 mM NaCl, and net growth of callus occurred for concentrations up to 200 mM NaCl. At 500 mM NaCl, growth of callus was completely inhibited, although a part of the tissue was still alive after 30 d. Cellular levels
of Na+ and Cl− were greatly increased by the treatment with NaCl. Uptake of K+ was also enhanced and was accompanied by increasing levels of Na+ and Cl− so that the Na+/K+ ratio was almost constant (4.1–4.2) in callus grown with 50–200 mM NaCl. Levels of Mg2+ and Ca2+ were not changed significantly with 50–200 mM NaCl, whereas levels of free NH
4
+
, NO
3
−
and SO
4
2−
ions, which are convertible to organic compounds, were lowest in callus grown with 50 mM NaCl. The rate of conversion of 15NH
4
+
into macromolecules during 30 d culture with 0–100 mM NaCl did not vary greatly, but 200 mM NaCl reduced the biosynthesis of macromolecules from this ion. The highest rate of conversion of 15NO
3
−
into macromolecules was observed at 50 mM NaCl. Identification of compatible solutes with NMR-spectroscopy indicated that mannitol is the compatible solute for intact
plants of Sonneratia alba, but no accumulation of mannitol was found in calluses, not even in those grown at high concentrations of NaCl. 相似文献
3.
Summary Free-living or immobilized Chlamydomonas reinhardtii cells photoproduce ammonium from nitrite in a medium containing 1 mM of l-methionine-d,l-sulphoximine (MSX). Ammonium is accumulated in the medium to 8 mM final concentration, which inhibits nitrite uptake by the MSX-treated cells and consequently the excretion of ammonium is blocked. However, if ammonium was removed from the medium and nitrite and MSX periodically restored, the photoproduction process could be maintained over 96 h, with a final ammonium concentration of about 18 mM for free-living cells and 28 mM for immobilized ones. The MSX-treated cells showed a photoproduction productivity of 1300 mol NH
4
+
· mg chlorophyll (Chl)-1, with an average production rate of 14 mol NH
4
+
· mg Chl-1 per hour, for calcium alginate-entrapped cells, while the corresponding data for free-living ones was 650 mol NH
4
+
· mg Chl-1 and 6.7 mol NH
4
+
· mg Chl-1 per hour, respectively. Immobilized cells showed a significant increase in the nitrite uptake rate, probably due to a change in membrane permeability as a consequence of cell-matrix interactions. 相似文献
4.
Summary Net CO2 fixation inLemna gibba L. was inhibited by 0.5 mM L-methionine D,L-sulfoximine (MSX) both under photorespiratory conditions (21% O2) and in 2% O2. The inhibition was noticeably delayed by addition of 5 mM glutamine. Glutamine also delayed MSX-induced inactivation of glutamine synthetase. An increase in intracellular NH
4
+
concentration was noted in the presence of MSX only, and in the presence of 10 mM NH
4
+
only. However, presence of 10 mM NH
4
+
did not cause any inhibition of CO2 fixation. 相似文献
5.
The Potassium Requirement for Growth and Embryogenesis in Wild Carrot Suspension Cultures 总被引:1,自引:0,他引:1
A requirement for potassium for growth and forembryogenesis in suspension cultures of wild carrot (Daucus carota L.) was demonstrated. The concentration of K+ required for maximal growth (1 mM) was less than that required for maximal embryogenesis (20 mM). Neither Na+ nor NH4+ could replace K+. Ammonium ion enhanced embryogenesis when K+ was present at suboptimal levels greater than 1 mM. Nitrogen sources strongly influenced growth and embryogenesis, but the effects of nitrogen were separable from those of K+. Subline differences were noted. Subline CSC-29 produced nearly half the maximum embryo number in 1 mM K+ while CSC-31 produced no embryos at that K+ concentration. Growth of CSC-29 was slightly repressed by Na+, but no more than by similarconcentrations of K+. Growth of CSC-31 in 1 mM K+ was strongly repressed by Na+. Embryogenesis in CSC-29 was unaffected by Na+. In CSC-31, Na+ repressed embryogenesis at lower concentrations of K+. 相似文献
6.
The modelling of ion uptake by plants requires the measurement of kinetic and growth parameters under specific conditions.
The objective of this study was to evaluate the effect of nine NH
inf4
sup+
:NO
inf3
sup−
ratios on onions (Allium cepa L.). Twenty-eight to 84 day-old onion plants were treated with NH
inf4
sup+
:NOf3/sup− ratios ranging from 0 to 100% of each ionic species in one mM solutions in a growth chamber. Maximum N influx (Imax) was assessed using the N depletion method. Except at an early stage, ionic species did not influence significantly Imax, the Michaelis constant (Km) and the minimum concentration for net uptake (Cmin). Imax for ammonium decreased from 101 to 59 pmole cm-2 s-1 while Imax for nitrate increased from 26 to 54 pmole cm-2 s-1 as the plant matured. On average, Km and Cmin values were 14.29 μM, and 5.06 μM for ammonium, and 11.90 μM and 4.54 μM for nitrate, respectively. In general, the effect of NH4
+:NO3
- ratios on root weight, shoot weight and total weight depended on plant age. At an early stage, maximum plant growth and N
uptake were obtained with ammonium as the sole source of N. At later stages, maximum plant growth and N uptake were obtained
as the proportion of nitrate increased in the nutrient solution. The was no apparent nutrient deficiency whatever NH4
+:NO3
- ratio was applied, although ammonium reduced the uptake of cations and increased the uptake of phosphorus.
The research was supported by the Natural Sciences and Engineering Research Council of Canada. 相似文献
7.
G. Vengadesan N. Selvaraj R. Prem Anand V. Gaba A. Ganapathi 《In vitro cellular & developmental biology. Plant》2005,41(6):789-793
Summary Suspension culture of cucumber (Cucumis sativus L.) has been an inefficient method for production of somatic embryos owing to problems with embryo maturation and conversion.
Embryogenic callus of cv. Green Long was induced on semisolid Murashige and Skoog (MS) medium containing 6.8 μM 2,4-dichlorophenoxyacetic acid (2,4-D) and 2.2 μM 6-benzylaminopurine (BA). A large number of globular somatic embryos were obtained on transfer of the callus to MS liquid
medium supplemented with 87.6 mM sucrose, 1.1 μM 2,4-D, and improved by the addition of 342.4 μM
l-glutamine. MS medium supplemented with 87.6 mM sucrose was more effective in somatic embryo production than other sugars. Subsequent development led to the formation of
heart-and torpedo-shaped embryos. Maturation of somatic embryos occurred on plant growth regulator-free MS semi-solid medium
containing 175.2 mM sucrose and 0.5 gl−1 activated charcoal. Conversion of embryos into plants was achieved on half-strength MS semi-solid medium containing 87.6
mM sucrose and 1.4 μM gibberellic acid (GA3) in a 16h photoperiod. Twenty-seven percent of embryos were converted into normal plants. 相似文献
8.
Summary The effects of increasing concentrations of NaCl and CaCl2 on quince (Cydonia oblonga Mill. BA 29 clone) somatic embryogenesis and adventitious root regeneration were investigated. Leaves collected from in vitro-grown shoots were used as explants and induced for 2d in liquid Murashige and Skoog medium containing 11.3 μM 2,4-dichlorophenoxyacetic acid. Explants were then cultured on semisolid Murashige and Skoog medium enriched with 4.7 μM kinetin and 0.5 μM naphthaleneacetic acid under red light for 25 d and under white light for another 25 d. Two experiments were performed: in
the first, NaCl was used at 0,25, 50, 100, and 200 mM in factorial combination with CaCl2 at 3, 9, and 27 mM; in the second, NaCl was applied at 0, 5, 10, 20, 40, and 80 mM in combination with CaCl2 at 0.3, 1.0, and 3.0 mM. Quince leaves revealed the capacity to regenerate somatic embryos and/or adventitious roots. Quantitative and qualitative
regeneration from leaves was affected by NaCl treatments: increasing NaCl concentrations, in combination with CaCl2 at 1 mM, led to an increase in the proportion of leaves producing somatic embryos only, and to a decrease of both leaves regenerating
roots only and leaves simultaneously producing somatic embryos and adventitious roots. This suggests a beneficial effect of
salt stress on the embryogenic process. The regeneration response decreased with increasing salt concentrations and was almost
totally inhibited above 50 mM NaCl and 9 mM CaCl2. The presence of CaCl2 in the culture medium apparently mitigated the effects of salt stress, but only when NaCl was applied at 40 mM. NaCl at 5 mM, in the presence of 0.3 or 1 mM CaCl2, was favorable both to somatic embryo and root production. No value of the ratio Na+/Ca2+ was found to be optimal for the regeneration processes. 相似文献
9.
Differential effect of ammonium on the induction of nitrate and nitrite reductase activities in roots of barley (Hordeum vulgare) seedlings 总被引:2,自引:0,他引:2
Muhammad Aslam Robert L. Travis D. William Rains Ray C. Huffaker 《Physiologia plantarum》1997,101(3):612-619
The effect of exogenous NH4+ on the induction of nitrate reductase activity (NRA; EC 1.6.6.1) and nitrite reductase activity (NiRA; EC 1.7.7.1) in roots of 8-day-old intact barley (Hordeum vulgare L.) seedlings was studied. Enzyme activities were induced with 0.1, 1 or 10 mM NO3+ in the presence of 0, 1 or 10 mM NH4+, Exogenous NH4+ partially inhibited the induction of NRA when roots were exposed to 0.1 mM, but not to 1 or 10 mM NO3+, In contrast, the induction of NiRA was inhibited by NH4+ at all NO3+ levels. Maximum inhibition of the enzyme activities occurred at 1.0 mM NH4+ Pre-treatment with NH4+ had no effect on the subsequent induction of NRA in the absence of additional NH4+ whereas the induction of NiRA in NH4+-pretreated roots was inhibited in the absence of NH4+ At 10 mM NO3+ L-methionine sulfoximine stimulated the induction of NRA whether or not exogenous NH4+ was present. In contrast, the induction of NiRA was inhibited by L-methionine sulfoximine irrespective of NH4+ supply. During the postinduction phase, exogenous NH4+ decreased NRA in roots supplied with 0.1 mM but not with 1mM NH3+ whereas, NiRA was unaffected by NH4+ at either substrate concentration. The results indicate that exogenous NH4+ regulates the induction of NRA in roots by limiting the availability of NO3+. Conversely, it has a direct effect, independent of the availability of NO3+, on the induction of NiRA. The lack of an NH4+ effect on NiRA during the postinduction phase is apparently due to a slower turnover rate of that enzyme. 相似文献
10.
Although mineral N (nitrate and ammonium) is believed to have generally negative effects on nodulation in legume–rhizobia symbioses, previous studies have shown that low, static concentrations of ammonium stimulate nodulation in pea, and that this enhancement may be due to an elevation in cytokinin to auxin levels in roots. Here, the effects of ammonium (0.0, 0.1, 0.5 and 2.5 mM) on nodulation and auxin levels were investigated in wild‐type (WT) white clover (Trifolium repens cv. Haifa) and its transformants (lines 38 and 41) which contain the auxin‐sensitive reporter gene (GH3:gusA). The effects of exogenous application (10?10, 10?9 and 10?8 M) of the cytokinin 6‐benzylaminopurine (BAP) were also assessed. Whole‐plant nodulation (nodules plant?1) and dry weight (DW)‐specific nodulation (nodules g?1 root DW) were stimulated (up to 49%) in all white clover lines by 0.1 mM NH4+. This represents the first confirmation of an NH4+‐induced stimulation of DW‐specific nodulation in a species other than pea. At 2.5 mM NH4+, the effect was lost on whole‐plant nodulation and was inhibitory on DW‐specific nodulation. Rhizobial inoculation resulted in a decline in the expression of GH3:gusA in root tips as expected; however, ammonium treatment did not affect GH3 expression in any root zones. Exogenous application of BAP at 10?9 and 10?8 M stimulated whole‐plant and DW‐specific nodulation in wild‐type white clover to a similar degree as treatment with 0.1 mM NH4+. These results support our previous hypothesis that the stimulation of nodulation by low concentrations of ammonium involves the alteration of the ratio of cytokinin to auxin, specifically by increasing cytokinin. 相似文献
11.
The specific activity of glutamine synthetase (L-glutamate: ammonia ligase, EC 6.3.1.2) in surface grownAspergillus niger was increased 3–5 fold when grown on L-glutamate or potassium nitrate, compared to the activity obtained on ammonium chloride.
The levels of glutamine synthetase was regulated by the availability of nitrogen source like NH
4
+
, and further, the enzyme is repressed by increasing concentrations of NH
4
+
. In contrast to other micro-organisms, theAspergillus niger enzyme was neither specifically inactivated by NH
4
+
or
L-glutamine nor regulated by covalent modification. Glutamine synthetase fromAspergillus niger was purified to homogenity. The native enzyme is octameric with a molecular weight of 385,000±25,000. The enzyme also catalyses
Mn2+ or Mg2+-dependent synthetase and Mn2+-dependent transferase activity.
Aspergillusniger glutamine synthetase was completely inactivated by two mol of phenyl-glyoxal and one mol of N-ethylmaleimide with second
order rate constants of 3.8 M-1 min-1 and 760 M-1 min-1 respectively. Ligands like Mg. ATP, Mg. ADP, Mg. AMP, L-glutamate NH
4
+
, Mn2+ protected the enzyme against inactivation. The pattern of inactivation and protection afforded by different ligands against
N-ethylamaleimide and phenylglyoxal was remarkably similar. These results suggest that metal ATP complex acts as a substrate
and interacts with an arginine ressidue at the active site. Further, the metal ion and the free nucleotide probably interact
at other sites on the enzyme affecting the catalytic activity. 相似文献
12.
Nitrate or ammonium nutrition in french bean 总被引:2,自引:0,他引:2
Summary Bean Plants were grown in a greenhouse in sand irrigated with nutrient solutions containing either 2 mM NO
3
–
or 2 mM NH
4
+
. After 45 days fresh weight of NH
4
+
plants was half that of NO
3
–
plants. Cation concentration in NH
4
+
plants was 30% less than in NO
3
–
plants. Amino acids (SER, ASN, GLN) accummulated 3 to 10 times more in NH
4
+
plants. The concentration of organic acids (malic, malonic, citric) was 10 to 30 times higher in NO
3
–
plants. The ATP-costings for the synthesis of amino acids and organic acids in NH
4
+
plants was half that of NO
3
–
ones: therefore it could not account for the reduction of growth in the ammonium-fed plants. 相似文献
13.
In vivo 15N and 14N nuclear magnetic resonance spectroscopy was used to investigate the assimilation of nitrate and ammonium in seedlings of Norway spruce (Picea abies [L.] Karst.). The main objective was to study accumulation of free NH+4 and examine to what extent the nitrogen source affects the composition of the free amino acid pools in roots, stems and needles. NH+4 concentrations in plants growing in the presence of 0.5–50 mM ammonium were quantified using 14N NMR. The NH+4 values in tissues ranged from 6 to 46 μmol (g fresh weight)?1. with highest concentrations in roots and needles. The tissue NH+4 peaked at 5.0 mM NH+4 in the medium. and failed to increase when NH+4 in the medium was increased to 50 mM, indicating metabolic control of the concentration of this cation in tissues. The 14N NMR spectra were used to estimate pH of the NH+4 storage pools. Based on the pH sensitivity of the quintet of 14NH+4 resonance, we suggest that the pH of the ammonium storage compartments in the roots and stems should be 3.7–3.8, and in needles 3.4–3.5, representing extremely low pH values of the tissue. 15N from nitrate or ammonium was first incorporated into the amide group of glutamine and then into α-amino groups, confirming that the glutamine synthetase/ glutamate synthase cycle is the major route of nitrogen assimilation into amino acids and thus plays a role in lowering the levels of NH+4 in the cytoplasm. NH+4 can also be assimilated in roots in plants growing in darkness. The main 15N-labelled amino acids were glutamine. arginine and alanine. Almost no 15N signals from needles were observed. Double labelling (δN + w, wN) of arginine is consistent with the operation of the ornithine cycle, and enrichment indicates that this cycle is a major sink of newly assimilated nitrogen. Nitrogen assimilation in roots in the presence of added methionine sulphoximine and glutamate indicated the catabolic action of glutamate dehydrogenase. The 15N NMR spectra of plants grown on 15N-urea showed a marked increase in the labelling of ammonium and glutamine. indicating high urease activity. Amino acids were also quantified using high pressure liquid chromatography. Arginine was found to be an important transport form of nitrogen in the stem. 相似文献
14.
Although the inhibitory effects of high concentrations of mineral N (> 1.0 mM) on nodule development and function have often been studied, the effects of low, static concentrations of NH4+ (< 1.0 mM) on nodulation are unknown. In the present experiments we examine the effects of static concentrations of NH4+ at 0, 0.1 and 0.5 mM in flowing, hydroponic culture on nodule establishment and nitrogenase activity in field peas [Pisum sativum L. cv. Express (Svalöf AB)] for the initial 28 days after planting (DAP). Peas grown in the presence of low concentrations of NH4+ had significantly greater nodule numbers (up to 4-fold) than plants grown without NH4+. Nodule dry weight per plant was significantly higher at 14, 21 and 28 DAP in plants grown in the presence of NH4+, but individual nodule mass was lower than in plants grown without NH4+. The nodulation pattern of the plants supplied with NH4+ was similar to that often reported for supernodulating mutants, however the plants did not express other growth habits associated with supernodulation. Estimates of N2 fixation indicate that the plus-NH4+ peas fixed as much or more N2 than the plants supplied with minus-NH4+ nutrient solution. There were no significant differences in nodule numbers, nodule mass or NH4+ uptake between the plants grown at the two concentrations of NH4+. Nodulation appeared to autoregulate by 14 DAP in the minus-NH4+ treatment. Plant growth and N accumulation in the minus-NH4+ plants lagged behind those of the plus-NH4+ treatments prior to N2 fixation becoming well established in the final week of the experiment. The plus-NH4+ treatments appeared not to elicit autoregulation and plants continued to initiate nodules throughout the experiment. 相似文献
15.
Cotyledon expiants ofPanax ginseng were cultured on modified Murashige and Skoog medium with various concentrations of NH4C1 and KNO,. Morphogenesis such as somatic embryo, embryogenic callus, or adventitious root formation from cotyledon expiants differently occurred according to the concentrations of NH/ and NO3. Somatic embryos were actively formed in a moderate concentration of NH4 + (20 mM) in combination of NO3, but in a high concentration of NH4 + (60 mM), only embryogenie calli were formed. In little or no NH4 +, adventitious roots were formed at a high rate. The influence of NO3 on those morphogenesis was slight but combination of NO3 with NH4 + was indispensable since the cotyledon expiants were necrotized on medium containing only NH4 + as a nitrogen source. Histological observation revealed that somatic embryo and embryogénie callus formation occurred from the same origin (cotyledon epidermis), whereas, adventitious roots were originated from the cells near vascular strands. 相似文献
16.
Franklyn G. Healy Charles Latorre Stephan L. Albrecht Pallavolu M. Reddy Keelnatham T. Shanmugam 《Current microbiology》2003,46(6):0423-0431
A L-methionine-D,L-sulfoximine-resistant mutant of the cyanobacterium Anabaena variabilis, strain SA1, excreted the ammonium ion generated from N2 reduction. In order to determine the biochemical basis for the NH4
+-excretion phenotype, glutamine synthetase (GS) was purified from both the parent strain SA0 and from the mutant. GS from
strain SA0 (SA0-GS) had a pH optimum of 7.5, while the pH optimum for GS from strain SA1 (SA1-GS) was 6.8. SA1-GS required
Mn+2 for optimum activity, while SA0-GS was Mg+2 dependent. SA0-GS had the following apparent K
m
values at pH 7.5: glutamate, 1.7 mM; NH4
+, 0.015 mM; ATP, 0.13 mM. The apparent K
m
for substrates was significantly higher for SA1-GS at its optimum pH (glutamate, 9.2 mM; NH4
+, 12.4 mM; ATP, 0.17 mM). The amino acids alanine, aspartate, cystine, glycine, and serine inhibited SA1-GS less severely than the SA0-GS. The nucleotide
sequences of glnA (encoding glutamine synthetase) from strains SA0 and SA1 were identical except for a single nucleotide substitution that
resulted in a Y183C mutation in SA1-GS. The kinetic properties of SA1-GS isolated from E. coli or Klebsiella oxytoca glnA mutants carrying the A. variabilis SA1 glnA gene were also similar to SA1-GS isolated from A. variabilis strain SA1. These results show that the NH4
+-excretion phenotype of A. variabilis strain SA1 is a direct consequence of structural changes in SA1-GS induced by the Y183C mutation, which elevated the K
m
values for NH4
+ and glutamate, and thus limited the assimilation of NH4
+ generated by N2 reduction. These properties and the altered divalent cation-mediated stability of A. variabilis SA1-GS demonstrate the importance of Y183 for NH4
+ binding and metal ion coordination.
Received: 3 July 2002 / Accepted: 29 July 2002 相似文献
17.
Scott E. Hyndman Paul M. Hasegawa Ray A. Bressan 《Plant Cell, Tissue and Organ Culture》1981,1(1):229-238
Cultured shoots ofRosa ‘Improved Blaze’ were used to determine the effects of sucrose and inorganic nitrogen on adventitious root formation. Shoots
grown in media containing high sucrose concentrations (146.07–262.93 mM) produced more and longer roots than those grown in media containing 0–87.64 mM sucrose. This response to sucrose was related to the metabolism of sucrose rather than its osmotic properties since the use
of mannitol and 3-O-methyl-α-D-glucopyranoside as osmotic substitutes did not reproduce the effect on rooting. The number
and length of roots increased when the shoots were grown in media with the nitrogen concentration of the Murashige-Skoog (MS)
salt formulation reduced from 60 to 7.5 mM. Neither nitrate (NO
3
−
) nor ammonium (NH
4
+
) alone at any of the concentrations tested had the effect on rooting that both had together in the ratio of the MS salt formulation.
When the sucrose and nitrogen concentrations were both varied, the greatest rate of root initiation occurred on shoots grown
in media with a high sucrose to nitrogen concentration ratio. 相似文献
18.
Summary Tissue culture isolated from the root ofDaucus carota readily produced embryos when they were grown on a medium (Ms) containing relatively large amounts of NH4
+ and nitrate. Embryos were never formed on a second medium (Mw) which contained only nitrate in a low concentration. The cells of Ms-cultures always had a high level of NH4
+ while it was scarcely detected in Mw-cultures. Addition of either nitrate or glutamate to Mw resulted in the formation of embryos and also in the accumulation of a detectable amount of NH4
+ in the cells of the tissues. Although the occurrence of NH4
+ in the medium is not necessary for embryo formationin vitro it appears that a certain level of intracellular NH4
+ is a prerequisite for this process. Since there is a positive correlation between embryo formation and the contents in the cultures of both soluble and insoluble organic nitrogen, it is probable that NH4
+ is important for embryo formation because it is an essential substrate for the synthesis of organic nitrogen compounds such as amino acids and proteins. The ability to synthesize organic nitrogen from NH4
+ decreased conspicuously in the tissues which were cultured on Ms for long periods of time and had lost the capacity to form embryos. A certain level of K+ in culture media is necessary for optimal production of embryos on media with NH4
+, while Na+ has no effect on embryo formation.The authors are indebted to Mr. E.Mahn, Miss S.Semenoff, and Mrs. E.Bock for their technical assistances. 相似文献
19.
γ‐Aminobutyric acid addition alleviates ammonium toxicity by limiting ammonium accumulation in rice (Oryza sativa) seedlings
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Excessive use of nitrogen (N) fertilizer has increased ammonium (NH4+) accumulation in many paddy soils to levels that reduce rice vegetative biomass and yield. Based on studies of NH4+ toxicity in rice (Oryza sativa, Nanjing 44) seedlings cultured in agar medium, we found that NH4+ concentrations above 0.75 mM inhibited the growth of rice and caused NH4+ accumulation in both shoots and roots. Use of excessive NH4+ also induced rhizosphere acidification and inhibited the absorption of K, Ca, Mg, Fe and Zn in rice seedlings. Under excessive NH4+ conditions, exogenous γ‐aminobutyric acid (GABA) treatment limited NH4+ accumulation in rice seedlings, reduced NH4+ toxicity symptoms and promoted plant growth. GABA addition also reduced rhizosphere acidification and alleviated the inhibition of Ca, Mg, Fe and Zn absorption caused by excessive NH4+. Furthermore, we found that the activity of glutamine synthetase/NADH‐glutamate synthase (GS; EC 6.3.1.2/NADH‐GOGAT; EC1.4.1.14) in root increased gradually as the NH4+ concentration increased. However, when the concentration of NH4+ is more than 3 mM, GABA treatment inhibited NH4+‐induced increases in GS/NADH‐GOGAT activity. The inhibition of ammonium assimilation may restore the elongation of seminal rice roots repressed by high NH4+. These results suggest that mitigation of ammonium accumulation and assimilation is essential for GABA‐dependent alleviation of ammonium toxicity in rice seedlings. 相似文献
20.
Incubation of whole cells of the nitrifying bacterium Nitrosomonas europaea with ethylene led to the formation of ethylene oxide. Ethylene oxide production was prevented by inhibitors of ammonium ion oxidation, and showed properties implying that ethylene is a substrate for the ammonia oxidising enzyme, ammonia monooxygenase. Endogenous substrates, hydroxylamine, hydrazine and ammonium ions were compared as sources of reducing power in terms of rates and stoichiometries of ethylene oxidation. The highest rates of ethylene oxide formation (15 mol h-1 mg protein-1) were obtained with hydrazine as donor. The data suggest that at high concentrations of ethylene the rate of oxidation is limited by the rate at which reducing power can be supplied to the monooxygenase, not by an intrinsic V
max. Ethylene had an inhibitory effect on the rate of ammonium ion utilisation; an approximate K
i of 80 M was derived, but the results deviated from simple competitive behaviour. Measurement of relative rates of ethylene oxide formation and ammonium ion utilization led to a k
cat/K
m value for ethylene of 1.1 relative to NH
4
+
, or 0.04 relative to the true natural substrate, NH3. The effects of higher concentrations of ethylene oxide on oxygen uptake rates were also investigated. The results imply that ethylene oxide is also a substrate for the monooxygenase, but with a much lower affinity than ethylene. 相似文献