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1.
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
2.
The effect of amino acids on nitrate transport was studied in Zea mays cell suspension cultures and in Zea mays excised roots. The inclusion of aspartic acid, arginine, glutamine and glycine (15m M total amino acids) in a complete cell-culture media containing 1.0 m M NO 3
- strongly inhibited nitrate uptake and the induction of accelerated uptake rates. The nitrate uptake rate increased sharply once solution amino acid levels fell below detection limits. Glutamine alone inhibited induction in the cell suspension culture. Maize seedlings germinated and grown for 7 days in a 15 m M mixture of amino acids also had lower nitrate uptake rates than seedlings grown in 0.5 m M Ca(NO 3) 2 or 1 m M CaCl 2. As amino acids are the end product of nitrate assimilation, the results suggest an end-product feed-back mechanism for the regulation of nitrate uptake. 相似文献
3.
Summary Short-term absorption experiments were conducted with intact barley ( Hordeum vulgare L.) seedlings to observe the effects of the osmotic potential (Ψ π) and salt species on nitrate uptake and in vivo nitrate reduction. The experiments consisted of growing barley seedlings for 5 days in complete nutrient solutions salinized
to (Ψ π) levels of −0.6, −1.8, −3.0, −4.2, and −5.4 bars with NaCl, CaCl 2 or Na 2SO 4. After the absorption period, the seedlings were separated into shoots and roots, weighed, then analyzed for NO 3. The nutrient solutions were sampled for NO 3 analysis each day immediately before renewing the solutions. The accumulative loss of NO 3 from the solutions was considered to be uptake whereas NO 3 reduction was the difference between uptake and seedling content. Lowering the (Ψ π) of the nutrient solutions resulted in decreased concentrations of NO 3 in the plant, little or no effect (except at the lowest (Ψ π) level) on uptake, and increased nitrate reductase activity. Increased rates of NO 3 reduction were in particular associated with the Cl concentration of the nutrient solution. 相似文献
4.
Radish ( Raphanus sativus L.) seedlings pretreated with different hormones viz. kinetin, gibberellic acid and abscisic acid were subjected to different
N-forms. The seedlings were treated with different concentrations of KNO 3, NH 4Cl and NH 4NO 3 and the changes in nitrate reductase activity were seen in light and dark conditions in the cotyledons. Nitrate reductase
activity was affected differently by hormone application. Nitrate increased and ammonia decreased nitrate reductase activity;
in both light and dark-grown seedlings KNO 3 induced more in vitro nitrate reductase activity. NH
4
+
when combined with NO
3
−
, however, could level up to some extent, with KNO 3 in light, except in kinetin. A transient response of induction of NR activity was evident with decreased levels after a certain
specific ambient N-concentration, despite the presence of high N in the medium. However, the pattern of transition varied
with the hormones applied. Further, hormones are found to affect induction of different isoforms of nitrate reductase by NH
4
+
and NO
3
−
. NH
4
+
induced isoform was prominently promoted by kinetin treatment in dark. The data documents a particular kind of interaction
between controlling factors (light, N-source and phytohormones) which affect nitrate reductase levels. 相似文献
5.
Soybean ( Glycine max [L.] Merr.) seeds were imbibed and germinated with or without NO 3−, tungstate, and norflurazon (San 9789). Norflurazon is a herbicide which causes photobleaching of chlorophyll by inhibiting carotenoid synthesis and which impairs normal chloroplast development. After 3 days in the dark, seedlings were placed in white light to induce extractable nitrate reductase activity. The induction of maximal nitrate reductase activity in greening cotyledons did not require NO 3− and was not inhibited by tungstate. Induction of nitrate reductase activity in norflurazon-treated cotyledons had an absolute requirement for NO 3− and was completely inhibited by tungstate. Nitrate was not detected in seeds or seedlings which had not been treated with NO 3−. The optimum pH for cotyledon nitrate reductase activity from norflurazon-treated seedlings was at pH 7.5, and near that for root nitrate reductase activity, whereas the optimum pH for nitrate reductase activity from greening cotyledons was pH 6.5. Induction of root nitrate reductase activity was also inhibited by tungstate and was dependent on the presence of NO 3−, further indicating that the isoform of nitrate reductase induced in norflurazon-treated cotyledons is the same or similar to that found in roots. Nitrate reductases with and without a NO 3− requirement for light induction appear to be present in developing leaves. In vivo kinetics (light induction and dark decay rates) and in vitro kinetics (Arrhenius energies of activation and NADH:NADPH specificities) of nitrate reductases with and without a NO 3− requirement for induction were quite different. Km values for NO 3− were identical for both nitrate reductases. 相似文献
6.
The effect of light and exogenously supplied sucrose on NO 3
− uptake was studied in 9-day-old intact C 3 (barley) and C 4 (corn) seedlings. The seedlings used were uninduced for nitrate uptake system (i.e. had never seen nitrogen during germination
and growth) and were exposed to continuous light for 3 days to avoid any diurnal variation and to load the seedlings fully
with photosynthates. The uptake assay was conducted either in light or in darkness. Prior to assay, seedlings were treated
with darkness or light for 24 h. Accordingly, four sets of seedlings, i.e. pretreated with light and assayed in light (LL);
pretreated and assayed in darkness (DD); pretreated with light and assayed in darkness (LD); and pretreated with darkness
and assayed in light (DL) were formed. Barley exhibited 55% higher NO 3
− uptake than corn during light (LL) and 91% higher during darkness (DD). Shifting barley seedlings from light to dark (LD)
or dark to light (DL) for uptake assay, did not affect NO 3
− uptake, i.e. in LD the uptake was similar to LL and in DL it was similar to DD. However, in corn, the light conditions during
the assay determined the uptake regardless of the conditions during the period preceding the assay. One percent sucrose in
the medium increased NO 3
− uptake by 31% in barley and 70% in corn during light (LL). The corresponding increase during darkness (DD) was 38% in both
barley and corn. Removal of the corn residual endosperm decreased NO 3
− uptake by 40% during darkness. Etiolated seedlings (those having never seen light) of both barley and corn were able to take
up significant amount of NO 3
− during darkness. Externally supplied sucrose in the assay medium of etiolated seedlings increased the NO 3
− uptake to about 4 and 2 fold in barley and corn, respectively. The data presented here provide evidence that: 1. In intact
seedlings, light per se is not obligatory for NO 3
− uptake and that the carbohydrate supply may mimic light. 2. Light affected the NO 3
− uptake differently in barley and corn.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
7.
The mechanism of nitrate uptake for assimilation in procaryotes is not known. We used the radioactive isotope, 13N as NO 3
-, to study this process in a prevalent soil bacterium, Pseudomonas fluorescens. Cultures grown on ammonium sulfate or ammonium nitrate failed to take up labeled nitrate, indicating ammonium repressed synthesis of the assimilatory enzymes. Cultures grown on nitrite or under ammonium limitation had measurable nitrate reductase activity, indicating that the assimilatory enzymes need not be induced by nitrate. In cultures with an active nitrate reductase, the form of 13N internally was ammonium and amino acids; the amino acid labeling pattern indicated that 13NO 3
- was assimilated via glutamine synthetase and glutamate synthase. Cultures grown on tungstate to inactivate the reductase concentrated NO 3
- at least sixfold. Chlorate had no effect on nitrate transport or assimilation, nor on reduction in cell-free extracts. Ammonium inhibited nitrate uptake in cells with and without active nitrate reductases, but had no effect on cell-free nitrate reduction, indicating the site of inhibition was nitrate transport into the cytoplasm. Nitrate assimilation in cells grown on nitrate and nitrate uptake into cells grown with tungstate on nitrite both followed Michaelis-Menten kinetics with similar K
mvalues, 7 M. Both azide and cyanide inhibited nitrate assimilation. Our findings suggest that Pseudomonas fluorescens can take up nitrate via active transport and that nitrate assimilation is both inhibited and repressed by ammonium. 相似文献
8.
The effect of exogenous NH 4+ 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 m M NO 3+ in the presence of 0, 1 or 10 m M NH 4+, Exogenous NH 4+ partially inhibited the induction of NRA when roots were exposed to 0.1 m M, but not to 1 or 10 m M NO 3+, In contrast, the induction of NiRA was inhibited by NH 4+ at all NO 3+ levels. Maximum inhibition of the enzyme activities occurred at 1.0 m M NH 4+ Pre-treatment with NH 4+ had no effect on the subsequent induction of NRA in the absence of additional NH 4+ whereas the induction of NiRA in NH 4+-pretreated roots was inhibited in the absence of NH 4+ At 10 m M NO 3+ L-methionine sulfoximine stimulated the induction of NRA whether or not exogenous NH 4+ was present. In contrast, the induction of NiRA was inhibited by L-methionine sulfoximine irrespective of NH 4+ supply. During the postinduction phase, exogenous NH 4+ decreased NRA in roots supplied with 0.1 m M but not with 1m M NH 3+ whereas, NiRA was unaffected by NH 4+ at either substrate concentration. The results indicate that exogenous NH 4+ regulates the induction of NRA in roots by limiting the availability of NO 3+. Conversely, it has a direct effect, independent of the availability of NO 3+, on the induction of NiRA. The lack of an NH 4+ effect on NiRA during the postinduction phase is apparently due to a slower turnover rate of that enzyme. 相似文献
9.
Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen-depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium. The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m ?3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m ?3). When plants were decapitated after an 18 h NO 3? pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG ?3 uptake. Presumably due to a difference in NO 3? due to a difference in NO 3? uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar-stimulation of, oxygen consumption as well as the release of 14CO 2 from freshly absorbed (U- 14C) sugar was the same for glucose and fructose. Therefore, we propose a glucose-specific effect on NO 3? uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose-fructose effect on nitrate reductase activity independent of the effect on NO 3? uptake was not indicated. A constant level of NRA occurred in roots of NO 3? induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half-life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots. 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.
Mustard ( Brassica juncea Coss cv. T-59 ‘Varuna’) seedlings pretreated with gibberellic acid (GA) and kinetin (KiN) were grown in light. In vivo nitrate reductase (NR) activity was estimated and effect of tungsten on light-induced and NO
3
su−
-induced NR activity was investigated. Different concentrations of GA did not show any effect on induction of light-induced
NR; addition of nitrate promoted in vivo NR activity but no concentration effect of GA was evident. Light-induced NR was promoted by KiN and like in GA treatment,
addition of nitrate increased NR activity. Addition of Na-tungstate inhibited NO
3
−
induced NR while light-induced NR was not much affected in both GA and KiN treated seedlings. The two forms of NRs were further
characterized by studying the decay kinetics using Na-tungstate. In light-induced NR, tungstate did not affect NR activity
up to 11 h, while at later periods, a slight decay was observed. On the other hand, NO
3
−
-induced NR activity increased up to 4 h and subsequently a rapid fall was observed. It was therefore apparent that light-induced
NR had a very low turnover rate as compared to NO
3
−
-induced NR. These results further support the earlier conclusion that in mustard seedlings two distinct types of NR enzyme
exist and that nitrate requirement for NR induction is not absolute. 相似文献
12.
Exogenously applied GABA modulates root growth by inhibition of root elongation when seedlings were grown in vitro on full-strength Murashige and Skoog (MS) salts, but root elongation was stimulated when seedlings were grown on 1/8 strength MS salts. When the concentration of single ions in MS salts was individually varied, the control of growth between inhibition and stimulation was found to be related to the level of nitrate (NO 3?) in the growth medium. At NO 3? concentrations below 40 mM (full-strength MS salts level), root growth was stimulated by the addition of GABA to the growth medium; whereas at concentrations above 40 mM NO 3?, the addition of GABA to the growth medium inhibited root elongation. GABA promoted NO 3? uptake at low NO 3?, while GABA inhibited NO 3? uptake at high NO 3?. Activities of several enzymes involved in nitrogen and carbon metabolism including nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (NADH-GOGAT), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and phosphoenol pyruvate carboxylase (PEPCase) were regulated by GABA in the growth medium. Supplementing 1/8 strength MS medium with 50 mM GABA enhanced the activities of all of the above enzymes except ICDH activities in root tissues. However, at full-strength MS, GABA showed no inhibitory effect on the activities of these enzymes, except on GS in both root and shoot tissues, and PEPCase activity in shoot tissues. Exogenous GABA increased the amount of NR protein rather than its activation status in the tissues. This study shows that GABA affects the growth of Arabidopsis, possibly by acting as a signaling molecule, modulating the activity of enzymes involved in primary nitrogen metabolism and nitrate uptake. 相似文献
13.
The rate of nitrate uptake by N-depleted French dwarf bean ( Phaseolus vulgaris L. cv. Witte Krombek) increased steadily during the first 6 h after addition of NO 3 -After this initial phase the rale remained constant for many hours. Detached root systems showed the same time-course of uptake as roots of intact plants. In vivo nitrate reductase activity (NRA) was assayed with or without exogenous NO 3- in the incubation medium and the result ing activities were denoted potential and actual level, respectively. In roots the difference between actual and potential NRA disappeared within 15 min after addition of nitrate, and NRA increased for about 15 h. Both potential and actual NRA were initially very low. In leaves, however, potential NRA was initially very high and was not affected by ambient nitrate (0.1–5 mol m -3) for about 10 h. Actual and potential leaf NRA became equal after the same period of time. In the course of nitrate nutrition, the two nitrate reductase activities in leaves were differentially inhibited by cycloheximide (3.6 mmol m -3) and tungstate (1 mol m -3). We suggest that initial potential NRA reflects the activity of pre-existing enzyme, whereas actual NRA depends on enzyme assembly during NO 3- supply. Apparent induction of nitrate uptake and most (85%) of the actual in vivo NRA occurred in the root system during the first 6 h of nitrate utilization by dwarf bean. 相似文献
14.
Net nitrate uptake, 36ClO ?3/NO ?3 influx and 36Cl ? influx into Pisum sativum L. cv. Feltham First seedlings have been examined following growth in culture medium containing different combinations of chloride and nitrate. When young (6 days old) seedlings, that had been grown in the absence of N were used, nitrate accumulation stimulated net nitrate uptake and 36ClO ?3/NO ?3 influx (r 2= 0.99) while chloride accumulation inhibited nitrate uptake and 36ClO ?3/NO ?3 influx (r 2= 0.65). When nitrate was provided during growth there was no effect of chloride pretreatment on net nitrate uptake and there was little effect of total [NO ?3+ Cl ?] i on 36ClO ?3/NO ?3 influx (r 2= 0.26). A direct effect of Cl ? on 36ClO ?3/NO ?3 influx was only found when seedlings had been starved of N for more prolonged periods (14 days). When moderate chloride was supplied during growth, 36Cl ? influx was insensitive to nitrate or chloride accumulated, but significantly correlated with log e [NO ?3+ Cl ?] i (r 2= 0.75). When trace amounts of Cl ? were supplied during growth 36Cl ? influx was inhibited by (a) NO ?3 in the external medium and (b) Cl ? pretreatment, but was insensitive to NO ?3 pretreatment. The sensitivity of 36Cl ? influx to external nitrate was not found following Cl ? pretreatment in the absence of nitrate. The possibility that there are two populations of chloride carriers which differ in their sensitivity to external nitrate is discussed. Tentative schematic models to account for the regulation of nitrate and chloride uptake are proposed in the context of current hypotheses for regulation of ion transport and control systems theory. 相似文献
15.
Abstract Effect of ammonium on in vivo activity of nitrate reductase in roots, shoots and leaves of maize ( Zea mays L.) seedlings was studied in relation to light/dark conditions and EDTA supply. Supply of 5 m M (NH 4) 2SO 4 increased the steady state level of enzyme only in leaves and in light, while it had no effect in roots and shoots and in the dark. The substrate induction of enzyme was also little affected by 1 to 10 m M (NH 4) 2SO 4 in roots and shoots. In the leaves the activity in the dark was either inhibited (minus EDTA) or stimulated (plus EDTA) by 5 to 10 m M (NH 4) 2SO 4. The activity was stimulated in the light also in the presence of EDTA at higher concentrations of ammonium. When different concentrations of ammonium were supplied without any exogenous nitrate in the light, the enzyme activity increased at low concentration and was either inhibited or unaffected at higher concentrations depending upon the tissue used. Supply of EDTA with ammonium modified its effect to some extent. It is suggested that the effect of ammonium on nitrate reductase activity depends upon the tissue used and the effective concentration of the ammonium. 相似文献
17.
Summary Nitrate reductase activity has been assayed in corn seedlings grown in a nutrient solution containing different concentrations of nitrate and inoculated with Arthrobacter sp. The enzymatic activity was greatly enhanced especially at the levels of NO 3-ions suboptimal for the maximum induction. The ability of synthetic phytohormones (IAA, IPA and GA 3) in inducting nitrate reductase activity has been also texted. 相似文献
18.
Nitrate reductase activity in excised embryos of Agrostemma githago increases in response to both NO 3− and cytokinins. We asked the question whether cytokinins affected nitrate reductase activity directly or through NO 3−, either by amplifying the effect of low endogenous NO 3− levels, or by making NO 3− available for induction from a metabolically inactive compartment. Nitrate reductase activity was enhanced on the average by 50% after 1 hour of benzyladenine treatment. In some experiments, the cytokinin response was detectable as early as 30 minutes after addition of benzyladenine. Nitrate reductase activity increased linearly for 4 hours and began to decay 13 hours after start of the hormone treatment. When embryos were incubated in solutions containing mixtures of NO 3− and benzyladenine, additive responses were obtained. The effects of NO 3− and benzyladenine were counteracted by abscisic acid. The increase in nitrate reductase activity was inhibited at lower abscisic acid concentrations in embryos which were induced with NO 3−, as compared to embryos treated with benzyladenine. Casein hydrolysate inhibited the development of nitrate reductase activity. The response to NO 3− was more susceptible to inhibition by casein hydrolysate than the response to the hormone. When NO 3− and benzyladenine were withdrawn from the medium after maximal enhancement of nitrate reductase activity, the level of the enzyme decreased rapidly. Nitrate reductase activity increasd again as a result of a second treatment with benzyladenine but not with NO 3−. At the time of the second exposure to benzyladenine, no NO 3− was detectable in extracts of Agrostemma embryos. This is taken as evidence that cytokinins enhance nitrate reductase activity directly and not through induction by NO 3−. 相似文献
19.
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
20.
Membrane associated nitrate reductase (NR) was detected in plasma membrane (PM) fractions isolated by aqueous two-phase partitioning from barley ( Hordeum vulgare L. var CM 72) roots. The PM associated NR was not removed by washing vesicles with 500 millimolar NaCl and 1 millimolar EDTA and represented up to 4% of the total root NR activity. PM associated NR was stimulated up to 20-fold by Triton X-100 whereas soluble NR was only increased 1.7-fold. The latency was a function of the solubilization of NR from the membrane. NR, solubilized from the PM fraction by Triton X-100 was inactivated by antiserum to Chlorella sorokiniana NR. Anti-NR immunoglobulin G fragments purified from the anti-NR serum inhibited NO 3− uptake by more than 90% but had no effect on NO 2− uptake. The inhibitory effect was only partially reversible; uptake recovered to 50% of the control after thorough rinsing of roots. Preimmune serum immunoglobulin G fragments inhibited NO 3− uptake 36% but the effect was completely reversible by rinsing. Intact NR antiserum had no effect on NO 3− uptake. The results present the possibility that NO 3− uptake and NO 3− reduction in the PM of barley roots may be related. 相似文献
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