Effects of constitutive expression of nitrate reductase in transgenic Nicotiana plumbaginifolia L. in response to varying nitrogen supply

Transformed Nicotiana plumbaginifolia plants with constitutive expression of nitrate reductase (NR) activity were grown at different levels of nitrogen nutrition. The gradients in foliar NO3(-) content and maximum extractable NR activity observed with leaf order on the shoot, from base to apex, were much decreased as a result of N-deficiency in both the transformed plants and wild-type controls grown under identical conditions. Constitutive expression of NR did not influence the foliar protein and chlorophyll contents under any circumstances. A reciprocal relationship between the observed maximal extractable NR activity of the leaves and their NO3(-) content was observed in plants grown in nitrogen-replete conditions at low irradiance (170 micromoles photons.m-2.s-1). This relationship disappeared at higher irradiance (450 micromoles photons.m-2.s-1) because the maximal extractable NR activity in the leaves of the wild-type plants in these conditions increased to a level that was similar to, or greater than that found in constitutive NR-expressors. Much more NO3(-) accumulated in the leaves of plants grown at 450 micromoles photons.m-2.s-1 than in those grown at 170 micromoles photons.m-2.s-1 in N-replete conditions. The foliar NO3(-) level and maximal NR activity decreased with the imposition of N-deficiency in all plant types such that after prolonged exposure to nitrogen depletion very little NO3(-) was found in the leaves and NR activity had decreased to almost zero. The activity of NR decreased under conditions of nitrogen deficiency. This regulation is multifactoral since there is no regulation of NR gene expression by NO3(-) in the constitutive NR-expressors. We conclude that the NR protein is specifically targeted for destruction under nitrogen deficiency. Consequently, constitutive expression of NR activity does not benefit the plant in terms of increased biomass production in conditions of limiting nitrogen.