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1.
Nitrate and NO 2− transport by roots of 8-day-old uninduced and induced intact barley ( Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of NO 3− and NO 2− was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low Km, possibly a constitutive uptake system, and a linear system for both NO 3− and NO 2−. The low Km system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both NO 3− and NO 2− uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low Kms of NO 3− and NO 2− uptake were 7 and 9 micromolar, respectively, and were similar to those of the low Km system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high Km values of NO 3− uptake ranged from 34 to 36 micromolar and of NO 2− uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that NO 3− and NO 2− inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, Ki values showed that NO 3− was a more effective inhibitor than NO 2−. Nitrate and NO 2− transport by both the low and high Km systems were greatly inhibited by p-hydroxymercuribenzoate, whereas the linear system was only slightly inhibited. 相似文献
2.
Corn shoot mitochondria possess an energy-linked transport system for sulfate uptake as demonstrated by osmotic swelling and [ 35S]SO 42− accumulation. Maximum uptake is secured in the presence of Mg 2+ and oligomycin with sucrose for osmotic support. Neither phosphate nor dicarboxylate anions are required. When added simultaneously, millimolar concentrations of phosphate block [ 35S]SO 42− uptake after the initial minute. Mersalyl, N-ethylmaleimide, and 2,4-dinitrophenol are strong inhibitors of sulfate uptake; n-butylmalonate is a weak inhibitor. These inhibitors act in the same fashion on phosphate uptake. It is concluded that sulfate uptake in the absence of phosphate is by the phosphate transporter. 相似文献
3.
Cultures of a purple nonsulfur bacterium, Rhodobacter sphaeroides, amended with ~1 or ~100 ppm selenate or selenite, were grown phototrophically to stationary phase. Analyses of culture headspace, separated cells, and filtered culture supernatant were carried out using gas chromatography, X-ray absorption spectroscopy, and inductively coupled plasma spectroscopy-mass spectrometry, respectively. While selenium-amended cultures showed much higher amounts of SeO 32− bioconversion than did analogous selenate experiments (94% uptake for SeO 32− as compared to 9.6% for SeO 42−-amended cultures from 100-ppm solutions), the chemical forms of selenium in the microbial cells were not very different except at exposure to high concentrations of selenite. Volatilization accounted for only a very small portion of the accumulated selenium; most was present in organic forms and the red elemental form. 相似文献
4.
The influence of NO 3− uptake and reduction on ionic balance in barley seedlings ( Hordeum vulgare, cv. Compana) was studied. KNO 3 and KCl treatment solutions were used for comparison of cation and anion uptake. The rate of Cl − uptake was more rapid than the rate of NO 3− uptake during the first 2 to 4 hours of treatment. There was an acceleration in rate of NO 3− uptake after 4 hours resulting in a sustained rate of NO 3− uptake which exceeded the rate of Cl − uptake. The initial (2 to 4 hours) rate of K + uptake appeared to be independent of the rate of anion uptake. After 4 hours the rate of K + uptake was greater with the KNO 3 treatment than with the KCl treatment, and the solution pH, cell sap pH, and organic acid levels with KNO 3 increased, relative to those with the KCl treatment. When absorption experiments were conducted in darkness, K + uptake from KNO 3 did not exceed K + uptake from KCl. We suggest that the greater uptake and accumulation of K + in NO 3−-treated plants resulted from ( a) a more rapid, sustained uptake and transport of NO 3− providing a mobile counteranion for K + transport, and ( b) the synthesis of organic acids in response to NO 3− reduction increasing the capacity for K + accumulation by providing a source of nondiffusible organic anions. 相似文献
5.
Interstitial water profiles of SeO 42−, SeO 32−, SO 42−, and Cl − in anoxic sediments indicated removal of the seleno-oxyanions by a near-surface process unrelated to sulfate reduction. In sediment slurry experiments, a complete reductive removal of SeO 42− occurred under anaerobic conditions, was more rapid with H 2 or acetate, and was inhibited by O 2, NO 3−, MnO 2, or autoclaving but not by SO 42− or FeOOH. Oxidation of acetate in sediments could be coupled to selenate but not to molybdate. Reduction of selenate to elemental selenium was determined to be the mechanism for loss from solution. Selenate reduction was inhibited by tungstate and chromate but not by molybdate. A small quantity of the elemental selenium precipitated into sediments from solution could be resolublized by oxidation with either nitrate or FeOOH, but not with MnO 2. A bacterium isolated from estuarine sediments demonstrated selenate-dependent growth on acetate, forming elemental selenium and carbon dioxide as respiratory end products. These results indicate that dissimilatory selenate reduction to elemental selenium is the major sink for selenium oxyanions in anoxic sediments. In addition, they suggest application as a treatment process for removing selenium oxyanions from wastewaters and also offer an explanation for the presence of selenite in oxic waters. 相似文献
6.
The patterns of molybdenum (MoO 42−) absorption and transport were investigated in intact bean ( Phaseolus vulgaris L.) and rice ( Oryza sativa L. cv. I.R.8) plants. The mobility of MoO 42− absorbed by roots and by leaves was compared with that of a freely mobile element, Rb +. Although MoO 42− absorption by bean roots was nearly as high as that of Rb +, its transport to the shoot was considerably less. When MoO 42− was fed to one of the primary leaves, most of it was transported to the stem and root. Evidence obtained here showed that MoO 42− was mobile. Experiments with intact rice seedlings revealed large differences in the absorption and transport of MoO 42− between the plants grown in CaSO 4 and those in Hoagland solution. Molybdate uptake by excised rice roots was suggested to be an active process since it was greatly inhibited by a metabolic inhibitor. The presence of Mn 2+, Zn 2+, Cu 2+, CI −, or SO 42− in the absorption medium reduced MoO 42− uptake which was markedly enhanced by the presence of Fe 2+. 相似文献
7.
The influence of the allelopathic compound ferulic acid (FA) on nitrogen uptake from solutions containing both NO 3− and NH 4+ was examined in 8-day-old nitrogen-depleted corn ( Zea mays L.) seedlings. Concurrent effects on uptake of Cl − and K + also were assessed. The presence of 250 micromolar FA inhibited the initial (0-1 hours) rate of NO 3− uptake and also prevented development of the NO 3−-inducible accelerated rate. The pattern of recovery when FA was removed was interpreted as indicating a rapid relief of FA-restricted NO 3− uptake activity, followed by a reinitiation of the induction of that activity. No inhibition of NO 3− reduction was detected. Ammonium uptake was less sensitive than NO 3− uptake to inhibition by FA. An inhibition of Cl − uptake occurred as induction of the NO 3− transport system developed in the absence of FA. Alterations of Cl − uptake in the presence of FA were, therefore, a result of a beneficial effect, because NO 3− uptake was restricted, and a direct inhibitory effect. The presence of FA increased the initial net K + loss from the roots during exposure to the low K, ammonium nitrate uptake solution and delayed the recovery to positive net uptake, but it did not alter the general pattern of the response. The implications of the observations are discussed for growth of plants under natural conditions and cultural practices that foster periodic accumulation of allelopathic substances. 相似文献
8.
The effects of several amino-reactive disulfonic stilbene derivatives and N-(4-azido-2-nitrophenyl)-2-aminoethylsulfonate on Cl −, SO 42−, and inorganic phosphate (Pi) uptake in protoplasts isolated from corn root tissue were studied. 4-Acetamido-4′-isothiocyano-2,2′-stilbenedisulfonic acid, 4,4′-diisothiocyano-2,2′-stilbenedisulfonic acid, 4,4′-diamino-2,2′-stilbenedisulfonic acid, and NAP-taurine inhibited Cl − and SO 42− but not Pi and K + uptake in corn root protoplasts; whereas mersalyl inhibited Pi but not Cl − or SO 42− uptake. The rate of uptake of all anions decreased with increasing external pH. In addition, these reagents markedly inhibited plasmalemma ATPase activity isolated from corn root tissue. Excised root segments were less sensitive to Cl − and SO 42− transport inhibitors. 相似文献
9.
We measured potential rates of bacterial dissimilatory reduction of 75SeO 42− to 75Se 0 in a diversity of sediment types, with salinities ranging from freshwater (salinity = 1 g/liter) to hypersaline (salinity = 320 g/liter and with pH values ranging from 7.1 to 9.8. Significant biological selenate reduction occurred in all samples with salinities from 1 to 250 g/liter but not in samples with a salinity of 320 g/liter. Potential selenate reduction rates (25 nmol of SeO 42− per ml of sediment added with isotope) ranged from 0.07 to 22 μmol of SeO 42− reduced liter −1 h −1. Activity followed Michaelis-Menten kinetics in relation to SeO 42− concentration ( Km of selenate = 7.9 to 720 μM). There was no linear correlation between potential rates of SeO 42− reduction and salinity, pH, concentrations of total Se, porosity, or organic carbon in the sediments. However, potential selenate reduction was correlated with apparent Km for selenate and with potential rates of denitrification ( r = 0.92 and 0.81, respectively). NO 3−, NO 2−, MoO 42−, and WO 42− inhibited selenate reduction activity to different extents in sediments from both Hunter Drain and Massie Slough, Nev. Sulfate partially inhibited activity in sediment from freshwater (salinity = 1 g/liter) Massie Slough samples but not from the saline (salinity = 60 g/liter) Hunter Drain samples. We conclude that dissimilatory selenate reduction in sediments is widespread in nature. In addition, in situ selenate reduction is a first-order reaction, because the ambient concentrations of selenium oxyanions in the sediments were orders of magnitude less than their Kms. 相似文献
10.
The nature of the injury and recovery of nitrate uptake (net uptake) from NaCl stress in young barley ( Hordeum vulgare L, var CM 72) seedlings was investigated. Nitrate uptake was inhibited rapidly by NaCl, within 1 minute after exposure to 200 millimolar NaCl. The duration of exposure to saline conditions determined the time of recovery of NO 3− uptake from NaCl stress. Recovery was dependent on the presence of NO 3− and was inhibited by cycloheximide, 6-methylpurine, and cerulenin, respective inhibitors of protein, RNA, and sterol/fatty acid synthesis. These inhibitors also prevented the induction of the NO 3− uptake system in uninduced seedlings. Uninduced seedlings exhibited endogenous NO 3− transport activity that appeared to be constitutive. This constitutive activity was also inhibited by NaCl. Recovery of constitutive NO 3− uptake did not require the presence of NO 3−. 相似文献
11.
Uptake of label from solutions containing 35SO 2, H 35SO 3− and 35SO 32− into mesophyll protoplasts, vacuoles, and chloroplasts isolated from young barley leaves was measured at different pH values. Uptake was fast at low pH, when the concentration of SO 2 was high, and low at high pH, when the concentration of SO 2 was low. When the resistance ( R) of plasmalemma, tonoplast, and chloroplast envelope to the penetration of SO 2 was calculated from rates of uptake of label, comparable values were obtained for the different biomembranes at low pH values. R was close to 8000 seconds per meter and permeability coefficients were close to 1.25 × 10 −4 meters per second. Under these conditions R may describe resistance to SO 2 diffusion across a lipid bilayer. At higher pH values, R decreased. As R was calculated on the assumption that SO 2 is the only penetrating molecular species, the data suggest that carrier-mediated anion transport contributes to the uptake of sulfur at physiological pH values thereby decreasing apparent RSO2. The contribution of anion transport appeared to be smaller for transfer across the plasmalemma than for transfer across the tonoplast. It was large for transfer across the chloroplast envelope. The phosphate translocator of the chloroplast envelope catalyzed uptake of SO 32− into chloroplasts at neutral pH. Uptake was decreased in the presence of high levels of phosphate or sulfate and by pyridoxal phosphate. SO 2 transfer into cells leads to the intracellular liberation of one or two protons, depending on pH and oxidizing conditions. When the divalent sulfite anion is exchanged across the chloroplast envelope, bisulfite formation results in proton uptake in the chloroplast stroma, whereas SO 2 uptake into chloroplasts lowers the stroma pH. 相似文献
12.
The effect of NaCl and Na 2SO 4 salinity on NO 3− assimilation in young barley ( Hordeum vulgare L. var Numar) seedlings was studied. The induction of the NO 3− transporter was affected very little; the major effect of the salts was on its activity. Both Cl − and SO 42− salts severely inhibited uptake of NO 3−. When compared on the basis of osmolality of the uptake solutions, Cl − salts were more inhibitory (15-30%) than SO 42− salts. At equal concentrations, SO 42− salts inhibited NO 3− uptake 30 to 40% more than did Cl − salts. The absolute concentrations of each ion seemed more important as inhibitors of NO 3− uptake than did the osmolality of the uptake solutions. Both K + and Na + salts inhibited NO 3− uptake similarly; hence, the process seemed more sensitive to anionic salinity than to cationic salinity. Unlike NO3− uptake, NO3− reduction was not affected by salinity in short-term studies (12 hours). The rate of reduction of endogenous NO3− in leaves of seedlings grown on NaCl for 8 days decreased only 25%. Nitrate reductase activity in the salt-treated leaves also decreased 20% but its activity, determined either in vitro or by the `anaerobic' in vivo assay, was always greater than the actual in situ rate of NO3− reduction. When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in situ nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO3− transporter, rather than that of the NO3− reduction system, may be a critical factor to plant survival during salt stress. 相似文献
13.
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
14.
A computer-controlled multichannel data acquisition system was employed to obtain continuous measurements of net nitrate or chlorate uptake by roots of intact barley plants ( Hordeum vulgare cv Betzes) using nitrate-specific electrodes. Plants, previously grown in solutions maintained at 10 or 200 micromolar NO 3− (low N or high N conditions, respectively), were provided with 200 micromolar NO 3− or ClO 3− during the uptake period. Initial rates of NO 3− uptake were several times higher in low N plants than in high N plants. Within 10 min, uptake in the former plants declined to a new steady rate which was sustained for the remainder of the experiment. No such time-dependent changes were evident in the high N plants. Rates and patterns of net chlorate uptake exhibited almost identical dependence upon previous nitrate provision. NO 3− ( 36ClO 3−) influx, by contrast, appeared to be independent of NO 3− pretreatment prior to influx determination. Nitrate efflux, estimated by several different methods, was strongly correlated with internal nitrate concentration of the roots. 相似文献
15.
Potassium ions at low concentrations stimulate cytokinin-dependent betacyanin synthesis in Amaranthus tricolor seedlings more than other alkali metal ions when tested as the chloride salts. The sequence of relative stimulation is K + > Rb + > (Na + = Li +). Calcium and Mg 2+ ions are inhibitory at concentrations > 1 millimolar when tested as chlorides. Anions also have an effect on the degree of alkali metal stimulation in the order PO 43− > NO 3− > Cl −. The high activity of phosphate may be partly due to its chelating effect on inhibitory Ca 2+ ions, or to effects on K + uptake. A mixture of Na + and K + in the presence of phosphate is more effective than either cation alone. This result may be due either to effects on tyrosine transport or on the potassium uptake system. Phytochrome-dependent betacyanin synthesis shows the same stimulation by Na + plus K +. The effect of a number of inhibitors of transport systems on betacyanin accumulation is reported. The possible role of the ionic environment of cells in their metabolic regulation is discussed, particularly in relation to cytokinin action. 相似文献
16.
The net influx (uptake) rates of NO 3−, NH 4+, NO 2−, and urea into roots of wheat ( Triticum aestivum cv Yecora Rojo) seedlings from complete nutrient solutions containing all four compounds were monitored simultaneously. Although urea uptake was too slow to monitor, its presence had major inhibitory effects on the uptake of each of the other compounds. Rates of NO 3−, NH 4+, and NO 2− uptake depended in a complex fashion on the concentration of all four N compounds. Equations were developed which describe the uptake rates of each of the compounds, and of total N, as functions of concentrations of all N sources. Contour plots of the results show the interactions over the range of concentrations employed. The coefficients of these equations provide quantitative values for evaluating primary and interactive effects of each compound on N uptake. 相似文献
17.
Membranes from homogenates of growing and of dormant storage roots of red beet ( Beta vulgaris L.) were centrifuged on linear sucrose gradients. Vanadate-sensitive ATPase activity, a marker for plasma membrane, peaked at 38% to 40% sucrose (1.165-1.175 grams per cubic centimeter) in the case of growing material but moved to as low as 30% sucrose (1.127 grams per cubic centimeter) during dormancy. A band of nitrate-sensitive ATPase was found at sucrose concentrations of 25% to 28% or less (around 1.10 grams per cubic centimeter) for both growing and dormant material. This band showed proton transport into membrane vesicles, as measured by the quenching of fluorescence of acridine orange in the presence of ATP and Mg2+. The vesicles were collected on a 10/23% sucrose step gradient. The phosphate hydrolyzing activity was Mg dependent, relatively substrate specific for ATP (ATP > GTP > UTP > CTP = 0) and increased up to 4-fold by ionophores. The ATPase activity showed a high but variable pH optimum, was stimulated by Cl−, but was unaffected by monovalent cations. It was inhibited about 50% by 10 nanomolar mersalyl, 20 micromolar N,N′-dicyclohexylcarbodiimide, 80 micromolar diethylstilbestrol, or 20 millimolar NO3−; but was insensitive to molybdate, vanadate, oligomycin, and azide. Proton transport into vesicles from the 10/23% sucrose interface was stimulated by Cl−, inhibited by NO3−, and showed a high pH optimum and a substrate specificity similar to the ATPase, including some proton transport driven by GTP and UTP. The low density of the vesicles (1.10 grams per cubic centimeter) plus the properties of H+ transport and ATPase activity are similar to the reported properties of intact vacuoles of red beet and other materials. We conclude that the low density, H+-pumping ATPase of red beets originated from the tonoplast. Tonoplast H+-ATPases with similar properties appear to be widely distributed in higher plants and fungi. 相似文献
18.
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
19.
Enterobacter cloacae SLD1a-1 is capable of reductive detoxification of selenate to elemental selenium under aerobic growth conditions. The initial reductive step is the two-electron reduction of selenate to selenite and is catalyzed by a molybdenum-dependent enzyme demonstrated previously to be located in the cytoplasmic membrane, with its active site facing the periplasmic compartment (C. A. Watts, H. Ridley, K. L. Condie, J. T. Leaver, D. J. Richardson, and C. S. Butler, FEMS Microbiol. Lett. 228:273-279, 2003). This study describes the purification of two distinct membrane-bound enzymes that reduce either nitrate or selenate oxyanions. The nitrate reductase is typical of the NAR-type family, with α and β subunits of 140 kDa and 58 kDa, respectively. It is expressed predominantly under anaerobic conditions in the presence of nitrate, and while it readily reduces chlorate, it displays no selenate reductase activity in vitro. The selenate reductase is expressed under aerobic conditions and expressed poorly during anaerobic growth on nitrate. The enzyme is a heterotrimeric (αβγ) complex with an apparent molecular mass of ~600 kDa. The individual subunit sizes are ~100 kDa (α), ~55 kDa (β), and ~36 kDa (γ), with a predicted overall subunit composition of α 3β 3γ 3. The selenate reductase contains molybdenum, heme, and nonheme iron as prosthetic constituents. Electronic absorption spectroscopy reveals the presence of a b-type cytochrome in the active complex. The apparent Km for selenate was determined to be ~2 mM, with an observed Vmax of 500 nmol SeO 42− min −1 mg −1 ( kcat, ~5.0 s −1). The enzyme also displays activity towards chlorate and bromate but has no nitrate reductase activity. These studies report the first purification and characterization of a membrane-bound selenate reductase. 相似文献
20.
The kinetics of vanadium absorption by excised barley ( Hordeum vulgare L., cv. Eire) roots were investigated with respect to ionic species of V in solution, time and concentration dependence, Ca sensitivity, and interaction with various anions, cations, and pH levels. The role of metabolism in V absorption was also studied using anaerobic treatment (N 2 gas) and chemical inhibitors (NaN 3, KCN, or 2,4-dinitrophenol). Approximately one-third of the labeled V initially taken up by excised roots was desorbed to a constant level after 45 min in unlabeled V solutions. The rate of absorption of labeled V from 5 μ m NH 4VO 3 solutions containing 0.5 m m CaSO 4 was constant for at least 3 hours. Omission of Ca resulted in a 72% reduction in V uptake when compared to controls with 0.5 m m CaSO 4. The rate of uptake of V was highest at pH 4 but dropped to a very low level at pH 10. It was relatively constant between the pH levels of 5 and 8 at which the VO 3− ion is the predominant ionic species in solution. The rate of absorption of V was followed as a function of concentrations from 0.5 to 100 μ m NH 4VO 3. It was found to be a linear function of concentration and did not follow saturation kinetics. Absorption experiments carried out with labeled V from either N aVO 3 or NH 4VO 3 sources gave similar results. No anion studied ( i.e. HPO 42−, HAsO 42−, MoO 42−, SeO 42−, SeO 32−, CrO 42−, BO 33−, No 3−, and Cl −) interfered appreciably ( i.e. less than 30% inhibition) with the absorption of labeled V. Anaerobic treatment of absorption solution with N 2 gas did not inhibit V absorption by excised roots. The results obtained using chemical inhibitors were not consistent. It was concluded that V is not actively absorbed by excised barley roots. 相似文献
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