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 th e 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 cotyled ons of dark-and white light-grown tomato (Lycopersicon esculentum) see dlings. 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 a ctivity. The light-stimulation of tomato NIR gene expression does not depend on re availability of nitrate. Thus, the mode of co-action betw een light and nitrate in the regulation of NIR gene expression in toma to is different than in all other plant species examined so far. In th e cotyledons of tomato seedlings grown during a 16-h photoperiod in wh ite light, nitrate is required for maximum NiR gene expression. Glutam ine seems to repress NiR protein expression in green cotyledons. Taken together, our results suggest that the mode of regulation by light an d nitrate of the NiR protein level in white light-grown tomato seedlin g cotyledons is different than during the illumination of etiolated to mato cotyledons. In etiolated and in green tomato seedling cotyledons, both light and nitrate were required for the induction of NR transcri pts, NR protein and N R activity. This result confirms a synergistic c oaction of both factors in the regulation of NR gene expression.