Regulation of nitrate reductase activity in higher plants

The roles that leaf nitrate content and nitrate flux play in regulating the levels of nitrate reductase activity (NRA) were investigated in 8- to 14-day old maize (Zea mays L.) plants containing high nitrate levels while other environmental and endogenous factors were constant. The nitrate flux of intact plants was measured from the product of the transpiration rate and the concentration of nitrate in the xylem. NRA decreased when the seedlings were deprived of nitrate. The nitrate flux and the leaf nitrate content also decreased. When nitrate was resupplied to the roots, all three parameters increased.Attempts to alter the nitrate flux by varying transpiration rates were unsuccessful due to a relatively constant rate of delivery of nitrate to the xylem as transpiration rates fell. However, cooling the roots resulted in a decrease in the nitrate flux. Plants with a lower nitrate flux rapidly lost NRA, although the leaf nitrate content was initially unaffected. If the roots remained cool for a long enough time, the leaf nitrate content eventually decreased. Rewarming the roots increased the nitrate flux, leaf nitrate content, and NRA to control levels. When the nitrate flux in excised shoots was varied in three separate ways, decreasing the nitrate flux to the leaves resulted in a rapid decrease in NRA, although leaf nitrate contents were unchanged.These experiments show that the nitrate flux to the leaves from the roots plays a much larger regulatory role than the leaf nitrate content in controlling the level of NRA in intact plants.

Nitrate Reductase Activity in Maize (Zea mays L.) Leaves: I. Regulation by Nitrate Flux.

Publisher Summary This chapter describes nitrate and nitrite reduction. Nitrate is the predominant form of nitrogen available to most cultivated plants grown under normal field conditions. Although ammoniacal fertilizers are used almost exclusively, the ammonia derived from these fertilizers is oxidized to nitrate by soil organisms. Nitrification is extremely rapid when the soil is well aerated, moist, and above 7°–10°C. Certain bacteria can utilize nitrate nitrogen as the sole nitrogen source for the synthesis of all nitrogen containing compounds of the cell. This nitrate assimilation can occur under both aerobic and anaerobic conditions. Nitrate reduction by partially purified enzyme preparations is frequently stimulated by phosphate. Nitrate reduction, mediated by either reduced pyridine nucleotides or reduced viologen dyes, is inhibited by cyanide and azide; however, pyridine-nucleotide-mediated reduction of cytochrome c is insensitive to these inhibitors. A reactivation of an inactive nitrate reductase apoenzyme extracted from molybdenum-deficient plants can be achieved by the addition of acid-treated nitrate reductase or by addition of phosphate buffer washes of nitrate reductase absorbed on adenosine monophosphate -Sepharose.

Nitrate and Nitrite Reduction

Ammonium and nitrate nutrition of horticultural crops.

In higher plants, light is crucial for regulation of nitrate uptake, translocation and assimilation into organic compounds. Part of this metabolism is tightly coupled to photosynthesis because the enzymes involved, nitrite reductase and glutamate synthase, are localized to the chloroplasts and receive reducing power from photosynthetic electron transport. However, important enzymes in nitrate acquisition and reduction are localized to cellular compartments other than chloroplasts and are also up-regulated by light, i.e. transporters in cell and organellar membranes and nitrate reductase in the cytosol. This review describes the different light-dependent signalling cascades regulating nitrate metabolism at the transcriptional as well as post-transcriptional level, and how reactions in different compartments of the cell are co-ordinated. Essential players in this network are phytochrome and HY5 (long hypocotyls 5)/HYH (HY5 homologue)-dependent signalling pathways, the energy-related AMPK (AMP-activated protein kinase) protein kinase homologue SNRK1 (sucrose non-fermenting kinase 1-related kinase), chloroplastic thioredoxins and the prokaryotically originated PII protein. A complex light-dependent network of regulation emerges, which appears to be necessary for optimal nitrogen assimilation and for avoiding the accumulation of toxic intermediates and side products, such as nitrite and reactive oxygen compounds.

Signalling cascades integrating light-enhanced nitrate metabolism

The role of phytochrome in the induction of nitrate reductase of etiolated field peas (Pisum arvense L.) was examined. Terminal bud nitrate concentration increased in darkness, and the increase correlated with induction of nitrate reductase following brief exposure of intact plants to red, blue, far red, and white lights. Brief light exposure of intact plants stimulated nitrate uptake and induction of nitrate reductase by terminal buds subsequently excised and incubated on nitrate solution in darkness; exposure of excised buds in contact with nitrate led to less uptake but more induction. Nitrate and nitrate reductase activity both declined during incubation with water, irrespective of light treatment. Nitrate enrichment of intact terminal buds and uptake into excised buds and increases in nitrate reductase activity were all red/far red reversible. Dimethyl sulfoxide (1%, v/v) and sugars (sucrose 0.5%, glucose 1, w/v), although stimulating nitrate uptake into excised tissue in darkness, failed to enhance nitrate reductase activity over dark controls. Phytochrome may regulate nitrate reductase via both nitrate movement and a general mechanism such as enhancement of protein synthesis.

Phytochrome, Nitrate Movement, and Induction of Nitrate Reductase in Etiolated Pea Terminal Buds

THE extractable activity of nitrate reductase from higher plant leaves is inducible by light and shows, under natural growth conditions, a pattern of diurnal variation1. Studies on the nature of light involvement have generally used the green leaf as experimental material, implying that photosynthesis supports the induction process1,2. We have examined the role of light for the induction of nitrate reductase activity in the etiolated terminal buds of field peas (Pisum arvense cv. Century). Treatments consisted of brief exposure of intact plants to broad bands of light, followed by a period in darkness before extraction for enzyme assay. These light treatments exclude the possibility of photosynthesis as a process contributing to induction. Under these conditions, induction is shown to be reversibly controlled by red and far red light, an effect ascribable to the pigment phytochrome.

Nitrate reductase activity: phytochrome mediation of induction in etiolated peas.

Accumulation and stability of nitrite in intact aerial leaves

Nitrite production during darkness by vacuum-infiltrated strips of foliar material was investigated in 6-day-old corn, field peas, wheat, barley, and marrow and in 10-week-old Gomphrena globosa In

Robert W. Sheard

Papers

Phytochrome, Nitrate Movement, and Induction of Nitrate Reductase in Etiolated Pea Terminal Buds

Nitrate reductase activity: phytochrome mediation of induction in etiolated peas.

Accumulation and stability of nitrite in intact aerial leaves

Conditions affecting in vivo nitrate reductase activity in chlorophyllous tissues

Effects of blue and red light on nitrate reductase level in leaves of maize and pea seedlings