Nitrate reductase higher plants

Fig. 1. The nitrate assimilation pathway in higher plants. The pathway of nitrate assimilation in the tobacco leaf is illustrated. In some other species an additional cytosolic GS is found in the leaf. The pathway in plant roots is more poorly documented and more variable GS in roots is mostly cytosolic, and some enzymes such as GOGAT are found as isoforms utilising alternate reducing substrates. T, expected nitrate carrier NR, nitrate reductase NiR, nitrite reductase GS, glutamine synthetase GOGAT, glutamate synthase Fd, ferredoxin Gin, glutamine Glu, glutamate.

Caboche, M. Rouze, P. (1990). Nitrate reductase a target for molecular and cellular studies in higher plants. Trends in Genetics 6,187-92. 

Duke, S.H. Duke, S.O. (1984). Light control of extractable nitrate reductase activity in higher plants. Physiologia Plantarum 62, 485-93. 

Evans, H.J. Nason, A. (1953). Pyridine nucleotide nitrate reductase from extracts of higher plants. Plant Physiology 28, 233-54. 

Rajasekhar, V.K. Oelmiiller, R. (1987). Regulation of nitrate reductase and nitrite reductase in higher plants. Physiologia Plantarum 71, 517-21. 

Nitrate Reductases. These enzymes have two important functions (38). They serve as a terminal oxidation system in some microorganisms, and in many microorganisms, molds, fungi, and higher plants they are utilized in the assimilation of nitrogen into the cell via production of ammonia, which is subsequently metabolized into cell components. 

Kaiser, W. M., and Huber, S. C., 1994, Posttranslational regulation of nitrate reductase in higher plants. Plant Physiol. 106 8179821. 

This family includes the sulfite oxidases and dehydrogenases of prokaryotes Thiobacilli sp.), plants, birds, and animals, and the assimilatory nitrate reductases from bacteria, algae, fungi, and plants. The sulfite oxidases of higher eukaryotes are 100-110kDa homodimers (Table 1) they are located in the mitochrondrial intermembrane space and catalyze the oxidation of toxic sulfite to innocuous sulfate (equation 7). Human sulfite oxidase deficiency leads to major neurological abnormalities, mental retardation, dislocation of the ocular lenses, and early death. ... 

Molecular mechanisms of nitrate accumulation depend not only on the nitrate reductase system, but also on the ability of roots to take from the soil, nitrate or ammonium ions, and on the plant s capacity for their conversion by assimilation processes to higher products. Besides this, the assimilation depends on the ability of a given genotype to transport substances necessary for the synthesis. It was shown that genotype differences of the nitrate reductase level do not depend on the nitrate content in tissues [25]. Nitrates are accumulated in plant organisms at high concentrations when aU the nitrogen accepted cannot be utilized for the production of amino acids and for subsequent protein synthesis [26]. This occurs when the plant, in the course of its metabolism, is unable to reduce the accepted nitrates into the assimilable ammonia form. 

Nitrate reductase NO3 + NADH + H > NOj + NAD + HjO Bacteria, fungi, higher plants ... 

Nitrate reductase from higher plants is a tetramer with molecular weight of 160,000-500,000, with 0.04-0.06% of the Mo present as a prosthetic group (Notion and Hewitt, 1979 Hewitt and Notion, 1980). Each enzyme molecule has two flavine-adenine dinucleotides (FADs), two hemes, and one Mo atom. The nitrate reductases present in lower plants differ slightly in molecular weight, protein subunits, and number of flavine or heme units (Hewitt and Notion, 1980). 

Notton B. A., and Hewitt, E. J. (1979). Structure and properties of higher plant nitrate reductase especially Spinacea oleracia. In Nitrogen Assimilation of Plants, ed. E. J. Hewitt and C. V. Cutting, pp. 227-44. London Academic Press. 

The primary role of Mo in plant nutrition is associated with N metabolism, where it acts as an enzyme activator. Under most soil conditions, the source of N is NO3", and Mo is required by all crop species when N is provided as NOj-. Molybdenum is a critical constituent of the enzyme nitrate reductase. Different crop species have different requirements for Mo. In general, legumes have high requirements, but Mo accumulations in some of the Brassica crop species can be several times higher than those in legumes (Nayyar et al., 1977b). 

Although most studies of nitrate metabolism in higher plants have been done with whole leaves because leaves of most species have high levels of extractable activity and are readily available, nitrate and nitrite reductases can be extracted from roots, especially when appropriate protectants are... 

In cyanobacteria, fungi and higher plants with assimilatory nitrate reductase, nitrate enhancement of nitrate reductase levels has been observed. The nitrate-dependent appearance of nitrate reductase is prevented by inhibitors of protein and nucleic acid synthesis (Beevers and Hageman, I%9 Hewitt, 1975). Inhibitor studies indicate that the influence of nitrate is one of induction of protein synthesis rather than an activation of some inert precursor and Zielke and Filner (1971) have demonstrated de novo synthesis of nitrate reductase in response to inducer nitrate. Concurrent degradation of the enzyme was also noted. 

A similar situation may occur in higher plants as Notton et al. (1974) and Rucklidge et al. (1976) have demonstrated that inactive nitrate reductase is produced in molybdenum-deficient plants. 

The production of an active nitrate reductase in response to nitrate is dependent upon a supply of molybdate. Molybdate-deficient fungi (Dantzig etal., 1978), higher plants (Notton et o/., 1974), algae (Vega et a/., 1971), and bacteria (Lester and DeMoss, 1971 Enoch and Lester, 1972) produce aber-... 

In other higher plants it has been reported that amino acids may prevent the nitrate stimulated appearance of nitrate reductase (Filner, 1%6 Heimer and Filner, 1971 Radin, 1975 Oaks era/., 1977). However, interpretation of these results is often difficult because the applied amino acids may interfere with growth and metabolism of the tissue and may also prevent nitrate uptake or distribution of nitrate within the cell and hence interfere with the inductimi process. 

Further understanding of the genetic regulation of nitrate reductase in higher plants would be advanced by the availability of mutants. Such material has been developed in Arabidopsis by the use of N-... 

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