Nitrogen metabolism of Spirodela oligorrhiza : I. Utilization of ammonium, nitrate and nitrite

Nitrate transport system in Neurospora crassa

Nitrate uptake in Neurospora crassa has been investigated under various conditions of nitrogen nutrition by measuring the rate of disappearance of nitrate from the medium and by determining mycelial nitrate accumulation. The nitrate transport system is induced by either nitrate or nitrite, but is not present in mycelia grown on ammonia or Casamino Acids. The appearance of nitrate uptake activity is prevented by cycloheximide, puromycin, or 6-methyl purine. The induced nitrate transport system displays a K(m) for nitrate of 0.25 mM. Nitrate uptake is inhibited by metabolic poisons such as 2,4-dinitrophenol, cyanide, and antimycin A. Furthermore, mycelia can concentrate nitrate 50-fold. Ammonia and nitrite are non-competitive inhibitors with respect to nitrate, with K(i) values of 0.13 and 0.17 mM, respectively. Ammonia does not repress the formation of the nitrate transport system. In contrast, the nitrate uptake system is repressed by Casamino Acids. All amino acids individually prevent nitrate accumulation, with the exception of methionine, glutamine, and alanine. The influence of nitrate reduction and the nitrate reductase protein on nitrate transport was investigated in wild-type Neurospora lacking a functional nitrate reductase and in nitrate non-utilizing mutants, nit-1, nit-2, and nit-3. These mycelia contain an inducible nitrate transport system which displays the same characteristics as those found in the wild-type mycelia having the functional nitrate reductase. These findings suggest that nitrate transport is not dependent upon nitrate reduction and that these two processes are separate events in the assimilation of nitrate.

The effect of the source of inorganic nitrogen on growth and enzymes of nitrogen assimilation in soybean and wheat cells in suspension cultures

Soybean (Glycine max L. cv. Mandarin) and wheat (Triticum monococcum L.) cells were grown in media with NO3 (-) plus NH4 (+) (B5) and NO3 (-) without NH4 (+) (B5-NH4) as nitrogen sources. Changes in pH, [NO3 (-)] and [NH4 (+)] in media, and dry weight, protein content, nitrate reductase (NR) and glutamate dehydrogenase (GDH) in the cells were followed for about 170 h. With both NH4 (+) and NO3 (-) in the medium, NH4 (+) was utilized very quickly. Soybean cells grew poorly in the absence of NH4 (+) while wheat cells grew equally well on media with or without NH4 (+). When soybean cells were grown in medium with NO3 (-) plus NH4 (+), dry weight and NR activity remained relatively low for several hours after which both increased rapidly. This coincided with the time NH4 (+) was depleted from the medium. In the absence of NH4 (+), soybean cell growth and NR activity remained low. NR activity in wheat cells, and GDH activity in soybean and wheat cells, did not vary significantly in the presence or absence of NH4 (+).

Nitrogen metabolism of Spirodela oligorrhiza : III. Amino acids and the utilization of nitrate

Wheras ammonium inhibited almost completely utilization of nitrate (NO3) by Spirodela oligorrhiza, asparagine, aspartate, glutamine, and glutamate had only a sparing effect. Asparagine and glutamine were used more rapidly than NO3; aspartate and glutamate at about the same rate. These differences in utilization of NO3 were not determined by differences in the levels of nitrate reductase.

The nitrogen metabolism of Spirodela oligorrhiza : II. Control of the enzymes of nitrate assimilation

Nitrate reductase (NO3R) in Spirodela oligorrhiza has a specific requirement for NAD, and no activity could be detected when reduced NADP was supplied as a cofactor. NO3R and nitrite reductase (NO2R) are adaptive enzymes, being present only when the plant is supplied with NO3 or NO2. There is significant correlation between the concentration of NO3 in plants, and the levels of NO3R that they contain. When plants not containing NO3R or NO2R are supplied with NO3, there was a rapid increase in the levels of the two enzymes and in the concentration of NO3 in the plants. These increases were retarded but not prevented by NH4 in the medium. When NO3-grown plants were depleted of N, there was a rapid decrease in the levels of NO3R and NO2R.The utilization of NH4 prevented the assimilation of NO3 even by plants that contained high levels of NO3R and NO2R. It was therefore concluded that NH4, or the processes or products of its assimilation must inhibit the activity of NO3R. NH4, arginine, asparagine or glutamine, tested separately or together, had little effect on the in vitro activity of NO3R.