Differential Expression of the Two Arabidopsis Nitrate Reductase Genes

Abstract

The differential regulation of the two nitrate reductase (NR, EC 1.6.6.1) genes of Arabidopsis thaliana L. Heynh was examined. cDNAs corresponding to each of the NR genes (NR1 and NR2) were used to measure changes in the steady-state levels of NR mRNA in response to nitrate, light, circadian rhythm, and tissue specificity. Although nitrate-induction kinetics of the two genes are very similar, NRI is expressed in the absence of nitrate at a higher basal level than NR2. Nitrate induction is transient both in the roots and leaves, however the kinetics are different: the induction and decline in the roots precede that in the leaves. Light induces the expression of each of the genes with significantly different kinetics: NR2 reached saturation more rapidly than did NR1. Both genes showed similar diurnal patterns of circadian rhythm, with NR2 mRNA accumulating earlier in the morning.

5[prime] Proximal Regions of Arabidopsis Nitrate Reductase Genes Direct Nitrate-Induced Transcription in Transgenic Tobacco

“…We have shown previously that the expression of two Arabidopsis NR genes is regulated in a coordinated fashion (Cheng et al, 1991). We demonstrated further that SUC can replace photosynthesis to increase NR gene transcription in Arabidopsis, thus separating the direct effect of light from that of the photosynthesis ).…”

“…To obtain an adequate amount of nitrogen when available, it may be advantageous for the plants to elevate the othenvise low capacity for nitrate reduction to a much higher level. In fact, the response of higher plant NR genes to environmental nitrate is rapid; steady-state mRNA levels increase within 15 to 30 min of nitrate induction (Gowri and Campbell, 1989;Melzer et al, 1989;Cheng et al, 1991). In addition to the two determining factors nitrate and carbohydrates, other factors also regulate NR gene expression.…”

“…It is probable that these factors play important roles both during development of a plant and when it encounters environmental stresses. Finally, the expression of NR mRNA shows a diumal pattem and is under the control of a circadian rhythm (Deng et al, 1990;Cheng et al, 1991). Although the nature of this oscillation is far from clear (Vincentz and Caboche, 1991;P i l e et al, 1993), it may fine tune the daily energy balance of the plant.…”

“…Explarits from transformants chosen for nitrak induction experiments were transferred to ammonium medium (containing no nitrate) as previously described (Cheng et al, 1991). Each explant consisted of one or two shooi: nodes.…”

Light Regulation of Nitrate uptake, Assimilation and Metabolism

“…Interestingly, different NIA genes within a species may show different responses to irradiation as demonstrated for Arabidospis NIA1 and NIA2 in the wild type (Cheng et al 1991), and the cr88 mutant . Cr88 is a chlorate resistant mutant of Arabidopsis that exhibits the etiolated phenotype not only in darkness but also in continuous red light; with long hypocotyls and closed, or partially expanded cotyledons, and delayed greening.…”