Synthesis and Turnover of Nitrate Reductase in Corn Roots

Self Cite

129

mentioning

confidence: 99%

“…The nitrate-induced NR activity of higher plants also declines when the source of nitrate is removed (5,8,14,26). Alternatively, reversible inactivation of Chlorella NR leads to a decrease in NR activity when induced cells are transferred to media containing ammonia (16).…”

mentioning

confidence: 99%

Synthesis and Degradation of Barley Nitrate Reductase

Somers¹,

Kuo²,

Kleinhofs³

et al. 1983

Self Cite

129

“…Nitrate reductase activity is lost rapidly when NO,-is withdrawn from the induction medium (22). To investigate whether glucose would affect the decay of enzyme activity, the roots were induced for 2.…”

Section: Resultsmentioning

confidence: 99%

“…They were then planted on N03--free 0.9%O agar which contained one-tenth strength Hoagland solution and an additional supplement of 0.08 Mm molybdenum (22,26 were calculated from a standard curve prepared on the day of the assay. The source of the nitrate reductase was a partially purified preparation of the enzyme from 7-day-old corn leaves.…”

mentioning

confidence: 99%

Effect of Glucose on the Induction of Nitrate Reductase in Corn Roots

Aslam

Oaks²

1975

Self Cite

In Zea mays L., addition of glucose to the induction medium has no effect on the induction of nitrate reductase during the initial 3 hours either in root tips (0-10 mm) or mature root sections (25-35 nun). With longer times, higher levels of enzyme activity are recovered from both root segments when glucose is present in the incubation medium. The induction in root tips is saturated by 10 mM NO3-. Higher concentrations of NO3-are required for saturation in mature root sections. The response to glucose is seen over a wide range of external NO3-concentrations.Nitrate reductase activity is lost rapidly when nitrate is withdrawn from the induction medium. Additions of glucose do not prevent this loss. Additions of glucose have no effect on total uptake of NO3-by the root segments but they increase the anaerobic NO2-production in both root tips and mature root segments. This latter measurement is considered to be an estimate of an active NO3-pool in the cytoplasm. Thus the results show that glucose alters the distribution of NO3-within the root sections. This may be an important factor in controlling the in vivo stability of the enzyme or its rate of synthesis.In cereals, carbohydrates represent up to 80%/C of the materials transported to the embryo from the endosperm (5, 17). These sugars are a major source of energy and building blocks for biosynthetic reactions. The endosperm is also a rich source of amino acids which may be incorporated directly into embryo protein (8,17,19). These transported amino acids also inhibit the biosynthesis of certain amino acids in the growing regions of the embryo (15, 16), and thus could limit the contribution of transported sugars to biosynthetic reactions. Little is known about regulation of specific metabolic pathways which might be modulated by soluble carbohydrates. In corn root tips, increased incorporation of acetate-carbon into protein amino acids is seen with additions of glucose or sucrose (20

“…Utilization of nitrate by higher plants involves the basic processes of uptake, reduction and translocation. Nitrate reductase, the enzyme which catalyzes the rate-limiting step in nitrate assimilation, requires continued protein synthesis to maintain its activity (10,11,16,23,24). There is also evidence that uptake and translocation of nitrate are dependent on protein synthesis (9,19,26).…”

Section: Abstracimentioning

confidence: 99%

p-Fluorophenylalanine-Induced Restriction of Ion Uptake and Assimilation by Maize Roots

Morgan¹,

Volk²,

Jackson³

1985

ABSTRACIRoots of decapitated maize seedlings (Zea mays L.) were exposed for 12 hours to 1.0 millimolar KNO3 (98.5 atom per cent 15N) in the presence and absence (control) of 0.1 millimolar p-fluorophenylalanine (FPA), an analog of the amino acid phenylalanine. FPA decreased nitrate uptake but had little effect on potassium uptake. In contrast, accumulation of both ions in the xylem exudate was greatly restricted. Utilization of nitrate by higher plants involves the basic processes of uptake, reduction and translocation. Nitrate reductase, the enzyme which catalyzes the rate-limiting step in nitrate assimilation, requires continued protein synthesis to maintain its activity (10,11,16,23,24). There is also evidence that uptake and translocation of nitrate are dependent on protein synthesis (9,19,26). This latter evidence, however, is based primarily on studies with inhibitors of RNA and protein synthesis, some of which are known to affect cellular processes other than protein synthesis (3, 7, 8). For example, cycloheximide has been reported to stimulate respiration and inhibit chloride uptake by beet root discs within 1 h of its application (8). Because of this lack of specificity, we have reexamined the effect of impaired protein synthesis on nitrate uptake and assimilation, using FPA,3 an analog of phenylalanine, to produce ineffective protein (18,21). Potassium fluxes were also quantified, since this cation is the common counterion associated with nitrate uptake and translocation.'Supported by National Science Foundation Grant PCM 81-18661. MATERIALS AND METHODSGrowth of Maize Seedlings. DeKalb XL-45 maize seeds (Zea mays L.) were germinated on paper towels moistened with 0.1 mM CaSO4 in an incubator maintained in darkness at 30°C and 98% RH. After 3 d, all roots except the primary roots were excised, and the seedlings were assembled into cultures of five seedlings each. The cultures were placed into aerated, nitrogenfree nutrient solution and returned to the incubator. The solution (pH 6.0) contained 0.05 mm K2S04, 0.4 mM KH2PO4, 0.25 mM MgSO4, 1.0 mm CaSO4, trace elements at 0.4 Hoagland concentration (12) and FeEDTA at 1.0 mg Fe 1-'. On the 4th d, the shoots were excised below the first node, and exudate collectors, small plastic pipet tips, were sealed to the mesocotyls with silicone grease. The cultures were placed in fresh, nitrogen-free nutrient solution and returned to the incubator. The experiment was run 20 h later, when the decapitated seedlings were 5-d old.Experimental Procedure. Just prior to the experiment, exudate was removed from the collectors, and the roots were rinsed thoroughly in 1.0 mm CaSO4 (30°C). The roots of each culture were then exposed to 250 ml of uptake solution containing 1.0 mM K'5NO3 (98.5 A % '5N) and 0 (control) or 0.1 mm FPA. Otherwise, the composition of the uptake solution was identical to that of the nitrogen-free, nutrient solution. During the 12-h exposure period, the uptake solutions (pH 6.0) were maintained at 30°C, aerated continuously, and replaced with fresh solu...