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1.
The effect of exogenous NH 4+ on NO 3− uptake and in vivo NO 3− reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO 3− uptake. Pretreatment with NH 4Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO 3− uptake. When NH4+ was given at the start of NO3− nutrition, the time pattern of NO3− uptake was the same as in plants receiving no NH4+. After 6 hours, however, the NO3− uptake rate (NUR) and root NRA were inhibited by NH4+ to a maximum of 45% and 60%, respectively. The response of the NUR of NO3−-induced plants depended on the NH4Cl concentration. Below 1 millimolar NH4+, the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH4+ at 2 millimolar and above caused a rapid and transient stimulation of NO3− uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO3−-induced plants, root NRA responded less and more slowly to NH4+ than did NUR. Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH4+ inhibition of the NUR of NO3−-induced plants. We conclude that repression of the NUR by NH4+ depends on NH4+ assimilation. The repression by NH4+ was least at the lowest and highest NH4+ levels tested (0.04 and 25 millimolar). 相似文献
2.
A protein agglutinin, trifoliin, was purified from white clover seeds and seedling roots. Trifoliin specifically agglutinates the symbiont of clover, Rhizobium trifolii, at concentrations as low as 0.2 μg protein/ml, and binds to the surface of encapsulated R. trifolii 0403. This clover protein has a subunit with Mr ≈ 50 000, an isoelectric point of 7.3, and contains carbohydrate. Antibody to purified trifoliin binds to the root hair region of 24-h-old clover seedlings, but does not bind to alfalfa, birdsfoot trefoil or joint vetch. The highest concentration of trifoliin on a clover root is present at sites where material in the capsule of R. trifolii binds. 2-Deoxy-d-glucose elutes trifoliin from intact clover-seedling roots, suggesting that this protein is anchored to root cell walls through its carbohydrate binding sites. We propose that trifoliin on the root hair surface plays an important role in the recognition of R. trifolii by clover. 相似文献
3.
Summary Nitrate added at critical concentrations to plant growth medium inhibits the infection of legume roots by Rhizobium. The direct
interaction, of nitrate and trifoliin A, a Rhizobium-recognition lection from white clover ( Trifolium repens L.), was examined as a possible basis for this regulation. Selective molecular ultrafiltration studies to detect ligand-protein
interactions showed that radioactive 13NO 3
− did not bind directly to trifoliin A when incubated at two molar ratios. Immunoprecipitation of trifoliin A by its homologous
antibody was unaffected by 15 m M NO 3
−. In addition, there was no apparent reduction in attachment of R. trifolii 0403 to root hairs of clover seedings during 1 h of incubation in the presence of 15 m M NO 3
−. These results show that nitrate inhibition of these early steps of the infection process is not due to a direct interaction
of nitrate with trifoliin A or its glycosylated receptors. 相似文献
4.
The inducibility and kinetics of the NO 3−, NO 2−, and NH 4+ transporters in roots of wheat seedlings ( Triticum aestivum cv Yercora Rojo) were characterized using precise methods approaching constant analysis of the substrate solutions. A microcomputer-controlled automated high performance liquid chromatography system was used to determine the depletion of each N species (initially at 1 millimolar) from complete nutrient solutions. Uptake rate analyses were performed using computerized curve-fitting techniques. More precise estimates were obtained for the time required for and the extent of the induction of each transporter. Up to 10 and 6 hours, respectively, were required to achieve apparent full induction of the NO 3− and NO 2− transporters. Evidence for substrate inducibility of the NH 4+ transporters requiring 5 hours is presented. The transport of NO 3− was mediated by a dual system (or dual phasic), whereas only single systems were found for transport of NO 2− and NH 4+. The Km values for NO 3−, NO 2−, and NH 4+ were, respectively, 0.027, 0.054, and 0.05 millimolar. The Km for mechanism II of NO 3− transport could not be defined in this study as it exhibited only apparent first order kinetics up to 1 millimolar. 相似文献
5.
Sites on white clover and alfalfa roots that bind Rhizobium trifolii and R. meliloti capsular polysaccharides, respectively, were examined by fluorescence microscopy. Fluorescein isothiocyanate-labeled capsular material from R. trifolii bound specifically to root hairs of clover but not alfalfa. Binding was most intense at the root hair tips. Treatment of clover roots with 2-deoxyglucose (2-dG) prevented binding of R. trifolii capsular material to the roots. The sugar 2-dG enhanced the elution of clover root protein, which could bind to and specifically agglutinate R. trifolii but not R. meliloti or R. japonicum. The mild elution procedure left the roots intact. Agglutination of R. trifolii and passive hemagglutination of rabbit erythrocytes coated with the capsular material of R. trifolii were specifically inhibited by 2-dG. These results suggest that clover roots contain proteins that cross-link complementary polysaccharides on the surface of clover root hairs and infective R. trifolii through 2-dG-sensitive binding sites. Alfalfa root hairs were shown to specifically bind to a surface polysaccharide from R. meliloti. 相似文献
6.
The influence of NH 4+, in the external medium, on fluxes of NO 3− and K + were investigated using barley ( Hordeum vulgare cv Betzes) plants. NH 4+ was without effect on NO 3− ( 36ClO 3−) influx whereas inhibition of net uptake appeared to be a function of previous NO 3− provision. Plants grown at 10 micromolar NO 3− were sensitive to external NH 4+ when uptake was measured in 100 micromolar NO 3−. By contrast, NO 3− uptake (from 100 micromolar NO 3−) by plants previously grown at this concentration was not reduced by NH 4+ treatment. Plants pretreated for 2 days with 5 millimolar NO 3− showed net efflux of NO 3− when roots were transferred to 100 micromolar NO 3−. This efflux was stimulated in the presence of NH 4+. NH 4+ also stimulated NO 3− efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO 3− occur via effects upon efflux. By contrast to the situation for NO 3−, net K + uptake and influx of 36Rb +-labeled K + was inhibited by NH 4+ regardless of the nutrient history of the plants. Inhibition of net K + uptake reached its maximum value within 2 minutes of NH 4+ addition. It is concluded that the latter ion exerts a direct effect upon K + influx. 相似文献
7.
Rhizobium-Azospirillum interactions during establishment of Rhizobium-clover symbiosis were studied. When mixed cultures of Azospirillum and Rhizobium trifolii strains were simultaneously inoculated onto clover plants, no nodulation by R. trifolii was observed. R. trifolii ANU1030, which nodulated clover plants without attacking root hairs, i.e., does not cause root hair curling (Hac −), did not show inhibition of nodulation when inoculated together with Azospirillum strains. Isolation of bacteria from surface-sterilized roots showed that azospirilla could be isolated both from within root segments and from nodules. Inhibition of nodulation could be mimicked by the addition of auxins to the plant growth medium. 相似文献
8.
We examined nitrate assimilation and root gas fluxes in a wild-type barley ( Hordeum vulgare L. cv Steptoe), a mutant ( nar1a) deficient in NADH nitrate reductase, and a mutant ( nar1a; nar7w) deficient in both NADH and NAD(P)H nitrate reductases. Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the nar1a mutation influences assimilation only in the shoot and that exposure to NO 3− induced shoot nitrate reduction more slowly than root nitrate reduction in all three genotypes. When plants that had been deprived of nitrogen for several days were exposed to ammonium, root carbon dioxide evolution and oxygen consumption increased markedly, but respiratory quotient—the ratio of carbon dioxide evolved to oxygen consumed—did not change. A shift from ammonium to nitrate nutrition stimulated root carbon dioxide evolution slightly and inhibited oxygen consumption in the wild type and nar1a mutant, but had negligible effects on root gas fluxes in the nar1a; nar7w mutant. These results indicate that, under NH 4+ nutrition, 14% of root carbon catabolism is coupled to NH 4+ absorption and assimilation and that, under NO 3− nutrition, 5% of root carbon catabolism is coupled to NO 3− absorption, 15% to NO 3− assimilation, and 3% to NH 4+ assimilation. The additional energy requirements of NO 3− assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH 4+ and NO 3− absorption and assimilation constitute a significant portion of root respiration. 相似文献
9.
Net uptakes of K + and NO 3− were monitored simultaneously and continuously for two barley ( Hordeum vulgare) cultivars, Prato and Olli. The cultivars had similar rates of net K + and NO 3− uptake in the absence of NH 4+ or Cl −. Long-term exposure (over 6 hours) to media which contained equimolar mixtures of NH 4+, K +, Cl −, or NO 3− affected the cultivars very differently: (a) the presence of NH 4+ as NH 4Cl stimulated net NO 3− uptake in Prato barley but inhibited net NO 3− uptake in Olli barley; (b) Cl − inhibited net NO 3− uptake in Prato but had little effect in Olli; and (c) NH 4+ as (NH 4) 2SO 4 inhibited net K + uptake in Prato but had little effect in Olli. Moreover, the immediate response to the addition of an ion often varied significantly from the long-term response; for example, the addition of Cl − initially inhibited net K + uptake in Olli barley but, after a 4 hour exposure, it was stimulatory. For both cultivars, net NH 4+ and Cl − uptake did not change significantly with time after these ions were added to the nutrient medium. These data indicate that, even within one species, there is a high degree of genotypic variation in the control of nutrient absorption. 相似文献
10.
Dissimilatory reduction of NO 2− to N 2O and NH 4+ by a soil Citrobacter sp. was studied in an attempt to elucidate the physiological and ecological significance of N 2O production by this mechanism. In batch cultures with defined media, NO 2− reduction to NH 4+ was favored by high glucose and low NO 3− concentrations. Nitrous oxide production was greatest at high glucose and intermediate NO 3− concentrations. With succinate as the energy source, little or no NO 2− was reduced to NH 4+ but N 2O was produced. Resting cell suspensions reduced NO 2− simultaneously to N 2O and free extracellular NH 4+. Chloramphenicol prevented the induction of N 2O-producing activity. The Km for NO 2− reduction to N 2O was estimated to be 0.9 mM NO 2−, yet the apparent Km for overall NO 2− reduction was considerably lower, no greater than 0.04 mM NO 2−. Activities for N 2O and NH 4+ production increased markedly after depletion of NO 3− from the media. Amendment with NO 3− inhibited N 2O and NH 4+ production by molybdate-grown cells but not by tungstate-grown cells. Sulfite inhibited production of NH 4+ but not of N 2O. In a related experiment, three Escherichia coli mutants lacking NADH-dependent nitrite reductase produced N 2O at rates equal to the wild type. These observations suggest that N 2O is produced enzymatically but not by the same enzyme system responsible for dissimilatory reduction of NO 2− to NH 4+. 相似文献
11.
Assimilation of NO 3− and NH 4+ by perennial ryegrass ( Lolium perenne L.) turf, previously deprived of N for 7 days, was examined. Nitrogen uptake rate was increased up to four- to five-fold for both forms of N by N-deprivation as compared to N-sufficient controls, with the deficiency-enhanced N absorption persisting through a 48 hour uptake period. Nitrate, but not NH 4+, accumulated in the roots and to a lesser degree in shoots. By 48 hours, 53% of the absorbed NO 3− had been reduced, whereas 97% of the NH 4+ had been assimilated. During the early stages (0 to 8 hours) of NO 3− uptake by N-deficient turf, reduction occurred primarily in the roots. Between 8 and 16 hours, however, the site of reduction shifted to the shoots. Nitrogen form did not affect partitioning of the absorbed N between roots (40%) and shoots (60%) but did affect growth. Compared to NO 3−, NH 4+ uptake inhibited root, but not shoot, growth. Total soluble carbohydrates decreased in both roots and shoots during the uptake period, principally the result of fructan metabolism. Ammonium uptake resulted in greater total depletion of soluble carbohydrates in the root compared to NO 3− uptake. The data indicate that N assimilation by ryegrass turf utilizes stored sugars but is also dependent on current photosynthate. 相似文献
12.
The oxidation of NH 4+ by Nitrosomonas europaea was insensitive to 10 mM NaClO 3 (sodium chlorate) but was strongly inhibited by NaClO 2 (sodium chlorite; Ki, 2 μM). The oxidation of NO 2− by Nitrobacter winogradskyi was inhibited by both ClO 3− and ClO 2− ( Ki for ClO 2−, 100 μM). N. winogradskyi reduced ClO 3− to ClO 2− under both aerobic and anaerobic conditions, and as much as 0.25 mM ClO 2− was detected in the culture filtrate. In mixed N. europaea-N. winogradskyi cell suspensions, the oxidation of both NH 4+ and NO 2− was inhibited in the presence of 10 mM ClO 3− after a 2-h lag period, despite the fact that, under these conditions, ClO 2− was not detected in the filtrate. The data are consistent with the hypothesis that, in mixed culture, NH 4+ oxidation is inhibited by ClO 2− produced by reduction of ClO 3− by the NO 2− oxidizer. The use of ClO 3− inhibition of NO 2− oxidation in assays of nitrification by mixed populations necessitates cautious interpretation unless it can be shown that the oxidation of NH 4+ is not affected. 相似文献
13.
At root temperature below 14 C the absorption of 15N from NH 4+ greatly exceeded that from NO 2− by tillers of Lolium multiflorum and Lolium perenne under conditions where pH, external concentration, plant N status, and pretreatment temperature were varied. There was a marked increase in the temperature sensitivity of NO 3− transport below 14 C, irrespective of the temperature at which plants were grown previously. A marked increase in the temperature sensitivity was also seen for NH 4+ transport, but this occurred at the lower temperature of 10 C. Pretreatment of roots at 8 C lowered this still further to 5 C. Above and below these transition temperatures the Q 10 values for NO 3− and NH 4+ transport were similar. Thus, the increased absorption of NH 4+ relative to NO 3− at low temperatures seems to be related primarily to the difference in transition temperatures. 相似文献
14.
Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO 3− or NH 4+, the solutions being maintained at pH 5.5. In NO 3−-fed plants excess nutrient anion over cation uptake was equivalent to net OH − efflux, and the total charge from NO 3− and SO 42− reduction equated to the sum of organic anion accumulation plus net OH − efflux. In NH 4+-fed plants a large H + efflux was recorded in close agreement with excess cation over anion uptake. This H + efflux equated to the sum of net cation (NH 4+ minus SO 42−) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO 3− that is taken up and reduced in NO 3−-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH 4+-fed plants absorbed NH 4+ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO 3−-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients. 相似文献
15.
Aquaspirillum magnetotacticum MS-1 grew microaerobically but not anaerobically with NO 3− or NH 4+ as the sole nitrogen source. Nevertheless, cell yields varied directly with NO 3− concentration under microaerobic conditions. Products of NO 3− reduction included NH 4+, N 2O, NO, and N 2. NO 2− and NH 2OH, each toxic to cells at 0.2 mM, were not detected as products of cells growing on NO 3−. NO 3− reduction to NH 4+ was completely repressed by the addition of 2 mM NH 4+ to the growth medium, whereas NO 3− reduction to N 2O or to N 2 was not. C 2H 2 completely inhibited N 2O reduction to N 2 by growing cells. These results indicate that A. magnetotacticum is a microaerophilic denitrifier that is versatile in its nitrogen metabolism, concomitantly reducing NO 3− by assimilatory and dissimilatory means. This bacterium appears to be the first described denitrifier with an absolute requirement for O 2. The process of NO 3− reduction appears well adapted for avoiding accumulation of several nitrogenous intermediates that are toxic to cells. 相似文献
16.
Young bean plants ( Phaseolus vulgaris L. var Saxa) were fed with 3.5 or 10 millimolar N in either the form of NO 3− or NH 4+, after being grown on N-free nutrient solution for 8 days. The pH of the nutrient solutions was either 6 or 4. The cell sap pH and the extractable activities of phosphoenolpyruvate carboxylase and of pyruvate kinase from roots and primary leaves were measured over several days. The extractable activity of phosphoenolpyruvate carboxylase (based on soluble protein) from primary leaves increased with NO3− nutrition, whereas with NH4+ nutrition and on N-free nutrient solution the activity remained at a low level. Phosphoenopyruvate carboxylase activity from the roots of NH4+-fed plants at pH 4 was finally somewhat higher than from the roots of plants grown on NO3− at the same pH. There was no difference in activity from the root between the N treatments when pH in the nutrient solutions was 6. The extractable activity of pyruvate kinase from roots and primary leaves seemed not to be influenced by the N nutrition of the plants. The results are discussed in relation to the physiological function of both enzymes with special regard to the postulated functions of phosphoenolpyruvate carboxylase in C3 plants as an anaplerotic enzyme and as part of a cellular pH stat. 相似文献
17.
The relative effects of water stress on growth parameters of subterranean clover ( Trifolium subterraneum L. cv. Woogenellup) dependent on either N 2 or 8 millimolar NH 4NO 3 for N were examined. Whole-plant carbon exchange rate (CER), acetylene reduction (AR), dry matter production, and Kjeldahl N accumulation were measured on uniform, intact swards of clover that were maintained under adequately watered conditions or were subjected to three cycles of water stress (leaf water potential ≤−30 bar) over an 18-day period. In the absence or presence of water stress, growth rate, net N accumulation rate, and total N concentration of plants dependent on N 2 were 25 to 26, 45 to 50, and 20 to 21% less, respectively, than plants supplied with 8 millimolar NH 4NO 3. The water stress treatment produced less than a 50% decrease in CER regardless of plant N source, a 90% inhibition of AR in plants dependent on N 2, and a 41% decline in dry matter production on both N sources. Water stress decreased reduced N accumulation 55% in N 2-dependent plants and 50% in NH 4NO 3-dependent plants. Changes in growth and N accumulation caused a 10 to 11% decrease in total plant N concentration of water-stressed plants compared to adequately irrigated controls, but water stress decreased the N concentration of tissue synthesized during the 18-day treatment period in N 2-grown plants more than in plants supplied 8 millimolar NH 4NO 3. Thus, the relative effect of water stress on growth under the two N regimes was similar, but N accumulation by N 2-dependent clover was inhibited to a slightly greater extent ( P ≤ 0.001) than in NH 4NO 3-dependent plants. 相似文献
18.
Neutral carrier-based liquid membrane ion-selective microelectrodes for NH 4+ and NO 3− were developed and used to investigate inorganic nitrogen acquisition in two varieties of barley, Hordeum vulgare L. cv Olli and H. vulgare L. cv Prato, originating in cold and warm climates, respectively. In the present paper, the methods used in the fabrication of ammonium- and nitrate-selective microelectrodes are described, and their application in the study of inorganic nitrogen uptake is demonstrated. Net ionic fluxes of NH 4+ and NO 3− were measured in the unstirred layer of solution immediately external to the root surface. The preference for the uptake of a particular ionic form was examined by measuring the net flux of the predominant form of inorganic nitrogen, with and without the alternative ion in solution. Net flux of NH 4+ into the cold-adapted variety remained unchanged when equimolar concentrations (200 micromolar) of NH 4+ and NO 3− were present. Similarly, net flux of NO 3− into the warm-adapted variety was not affected when NH 4+ was also present in solution. The high temporal and spatial resolution afforded by ammonium- and nitrate-selective microelectrodes permits a detailed examination of inorganic nitrogen acquisition and its component ionic interactions. 相似文献
19.
Chlamydomonas reinhardii cells, growing photoautotrophically under air, excreted to the culture medium much higher amounts of NO 2− and NH 4+ under blue than under red light. Under similar conditions, but with NO 2− as the only nitrogen source, the cells consumed NO 2− and excreted NH 4+ at similar rates under blue and red light. In the presence of NO 3− and air with 2% CO 2 (v/v), no excretion of NO 2− and NH 4+ occurred and, moreover, if the bubbling air of the cells that were currently excreting NO 2− and NH 4+ was enriched with 2% CO 2 (v/v), the previously excreted reduced nitrogen ions were rapidly reassimilated. The levels of total nitrate reductase and active nitrate reductase increased several times in the blue-light-irradiated cells growing on NO 3− under air. When tungstate replaced molybdate in the medium (conditions that do not allow the formation of functional nitrate reductase), blue light activated most of the preformed inactive enzyme of the cells. Furthermore, nitrate reductase extracted from the cells in its inactive form was readily activated in vitro by blue light. It appears that under high irradiance (90 w m −2) and low CO 2 tensions, cells growing on NO 3− or NO 2− may not have sufficient carbon skeletons to incorporate all the photogenerated NH 4+. Because these cells should have high levels of reducing power, they might use NO 3− or, in its absence, NO 2− as terminal electron acceptors. The excretion of the products of NO 2− and NH 4+ to the medium may provide a mechanism to control reductant level in the cells. Blue light is suggested as an important regulatory factor of this photorespiratory consumption of NO 3− and possibly of the whole nitrogen metabolism in green algae. 相似文献
20.
A potential for heterotrophic nitrification was identified in soil from a mature conifer forest and from a clear-cut site. Potential rates of NO 2− production were determined separately from those of NO 3− by using acetylene to block autotrophic NH 4+ oxidation and chlorate to block NO 2− oxidation to NO 3− in soil slurries. Rates of NO 2− production were similar in soil from the forest and the clear-cut site and were strongly inhibited by acetylene. The rate of NO 3− production was much greater than that of NO 2− production, and NO 3− production was not significantly affected by acetylene or chlorate. Nitrate production was partially inhibited by cycloheximide, but was not significantly reduced by streptomycin. Neither the addition of ammonium nor the addition of peptone stimulated NO 3− production. 15N labeling of the NH 4+ pool demonstrated that NO 3− was not coming from NH 4+. The potential for heterotrophic nitrification in these forest soils was greater than that for autotrophic nitrification. 相似文献
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