Coaction of light and the nitrogen substrate in controlling the expression of the tomato genes encoding nitrite reductase and nitrate reductase

The co-action of light and the nitrogen source in the regulation of the expression of the genes encoding nitrite reductase (NIR; EC 1.7.7.1) or nitrate reductase (NR; EC 1.6.6.1) was investigated in the cotyledons of dark-and white light-grown tomato (Lycopersicon esculentum) seedlings. Light acting at red/far-red or at blue regions of the spectrum stimulated both the NiR transcript and the NIR protein level in dark-grown tomato seedling cotyledons, and resulted in an increase in NiR activity. The light-stimulation of tomato NIR gene expression does not depend on re availability of nitrate. Thus, the mode of co-action between light and nitrate in the regulation of NIR gene expression in tomato is different than in all other plant species examined so far. In the cotyledons of tomato seedlings grown during a 16-h photoperiod in white light, nitrate is required for maximum NiR gene expression. Glutamine seems to repress NiR protein expression in green cotyledons. Taken together, our results suggest that the mode of regulation by light and nitrate of the NiR protein level in white light-grown tomato seedling cotyledons is different than during the illumination of etiolated tomato cotyledons. In etiolated and in green tomato seedling cotyledons, both light and nitrate were required for the induction of NR transcripts, NR protein and N R activity. This result confirms a synergistic coaction of both factors in the regulation of NR gene expression.