The use of mutants and transgenic plants to study nitrate assimilation

Abstract

The nitrate assimilatory pathway has been the matter of intensive genetic and molecular analysis over the past decade. Mutants impaired in the expression of nitrate reductase have been characterized in a number of plant species. Molecular analysis of the Nia gene coding for nitrate reductase has been the basis for a three‐domain model of the structure of the enzyme, in agreement with biochemical and genetic data. Mutagenesis and antisense strategies have led to the description of nitrite reductase deficiencies. The molecular analysis of the corresponding Nii genes has provided invaluable information on the structure of nitrite reductase. Recently, a gene involved in nitrate uptake has also been identified. The regulation of the nitrate assimilatory pathway has been investigated. Analysis of the regulation of the pathway at the molecular level has shown evidence for the involvement of nitrate, light and/or sucrose, and reduced nitrogen in the regulation. Surprisingly, no bona fide regulatory mutant specific to this pathway has been identified so far in higher plants. This may reflect the redundancy of regulatory genes. The deregulated expression of one or the other step of the pathway obtained by ectopic expression of the corresponding genes is a new approach to study the physiological role of these regulations. Elements of the pathway have also been successfully used as transposon traps, or negatively selectable markers for other purposes. Finally, the identification at the molecular level of regulatory genes and structural elements involved in transport and storage of nitrate, or in the biosynthesis of cofactors of nitrate and nitrite reductases, will be the goal of the next decade.

Influence of malate and 2-oxoglutarate on the NIA transcript level and nitrate reductase activity in tobacco leaves

“…Glutamine increased slightly during the first part of the light period and more markedly from the middle of the light period onwards (Fig. 1d) (see also Hoff et al 1994;Scheible et al 1997b;Geiger et al 1998). The glutamine : 2-oxoglutarate ratio remained fairly constant or even declined slightly during the first 4 h of the light period, rose progressively during the remainder of the light period, and decreased slightly during the first part and markedly towards the end of the night (not shown).…”

“…Nitrate reductase (NIA) is induced by nitrate (Pouteau et al 1989;Vaucheret et al 1990;Cheng et al 1992;Scheible et al 1997a), repressed by glutamine (Vincentz et al 1993;Hoff, Truong & Caboche 1994) and stimulated by sugars (Cheng et al 1992;Vincentz et al 1993). Expression is totally inhibited when sugars fall below a critical threshold (Matt et al 1998).…”

“…Although plants contain a P II homolog (Hsieh et al 1998) it lacks the uridylylation site that facilitates posttranslational regulation by 2-oxoglutarate and glutamine in fungi. 2-Oxoglutarate levels were not monitored in the studies that established a correlation between glutamine levels and NIA expression (summarized in Hoff et al 1994), or in more recent studies where this correlation broke down (Dzuibany et al 1998).…”

The nitrate and ammonium nitrate supply have a major influence on the response of photosynthesis, carbon metabolism, nitrogen metabolism and growth to elevated carbon dioxide in tobacco

“…As a first example, nitrate itself induces nitrate transporters and NR, leading to higher rates of nitrate uptake and assimilation (Hoff, Truong & Caboche 1994;Crawford 1995;Scheible et al 1997a;Krapp et al 1997). The decreased concentration of total and organic nitrogen and decreased activity of NR in elevated [CO 2 ] (see above) might therefore be due to a depletion of nitrate in the soil and plant, rather than a fundamental inability to increase nitrate uptake and assimilation to match the increased rates of photosynthesis in elevated [CO 2 ].…”

Nitrogen assimilation in Citrus based on CitEST data mining

“…This metabolic system is regulated at different levels by many factors, including nitrogen source and light, in order to adjust nitrate assimilation to the plant's needs. Nitrate acts as an elicitor inducing the transcription of nitrate transporters and NR/NiR genes (Hoff et al, 1994;Forde, 2000) and regulating their expression together with light (Bouton et al, 2002).…”