Regulation of nitrate and nitrite reductase expression in Nicotiana plumbaginifolia leaves by nitrogen and carbon metabolites

Abstract

SummaryNitrate (NR) and nitrite reductase (NiR) catalyse the reduction of nitrate to ammonium. The regulation of NR and NiR gene expression by carbohydrates (C) and nitrogen (N) metabolites was studied using detached leaves. In the dark, glucose, fructose and sucrose supplied to detached green leaves of dark-adapted Nicotiana plurnbaginifolia plants resulted in NR mRNA and protein accumulation and the loss of circadian rhythmicity in the size of the transcript pool. The characterization of transgenic plants expressing either a NR cDNA controlled by the 35s CaMV promoter or a transcriptional fusion between the tobacco n i a l (NR structural gene) promoter and the pglucuronidase reporter gene, led us to conclude that C metabolite control is taking place at the transcriptional level. Under low light conditions (limiting photosynthetic conditions), the supply of glutamine or glutamate resulted in a drop in the level of NR mRNA. Exogenously supplied carbohydrates partially antagonized this inhibitory effect suggesting that the availability of N and C metabolites affects the expression of the NR gene. The effects of carbohydrates and glutamine on NiR expression were also studied. NiR mRNA levels in the dark were relatively insensitive to feeding with glucose. Glutamate and glutamine were less efficient at decreasing NiR mRNA than NR mRNA levels. In contrast to NR, NiR mRNA levels were significantly increased by light treatments, indicating that NiR display regulatory characteristics reminiscent of photosynthetic genes such as the small subunit of ribulose bisphosphate carbowlase than to NR.

Light Regulation of Nitrate uptake, Assimilation and Metabolism

“…Light acts on yet another level, which became clear when examining expression of the NR gene linked to the 35S CaMV promoter (Vincentz et al 1993). As expected, NR was constitutively expressed and NR mRNA levels were high in darkness (after 56 h) as well as in the light.…”

“…NIA and NII genes were similarly regulated in Nicotiana with respect to inhibition by glutamine and glutamate. An interesting difference was that in green leaves glucose apparently could replace light for the enhancement of NIA expression, while a positive effect on NII expression was much less pronounced (Vincentz et al 1993).…”

Inter-subunit interactions between Glutamate-Like Receptors inArabidopsis

“…2,3 In fact, amino acids have long been suggested as signals that regulate genes involved in N transport and metabolism in plants, but the exact mechanism of amino acid sensing in plants is not well understood. 4,5 Some mechanisms for amino acid sensing have been documented in bacteria, yeast, and mammals, and potential homologs of such amino acid sensors from other organisms do exist in plants. One example is Glb1, an Arabidopsis homolog of bacterial PII proteins that are considered N "master regulators," [6][7][8][9] although the role of Glb1 seems to be constrained compared with the bacterial counterparts.…”

Atividade das enzimas redutase do nitrato e glutamina sintetase em cafeeiro arábica.

“…Segundo Galangau et al (1988) e Vincentz et al (1993) a atividade da enzima RN nas folhas e raízes pode ser induzida pela presença do substrato…”

“…A luz e o teor de nitrato são fatores importantes na indução e manutenção da atividade da enzima RN nos tecidos vegetais (Hageman & Flesher, 1960;Beevers & Hageman, 1969;Galangau et al, 1988;Vincentz et al, 1993). De acordo com diversos autores a atividade da RN nas folhas de várias espécies, aumenta durante as horas de luz e decresce no escuro (Harper & Hageman, 1972;Lewis et al, 1982;Hipkin et al, 1984).…”