EFFECTS OF CONSTITUTIVE EXPRESSION OF NITRATE REDUCTASE IN TRANSGENICNICOTIANA-PLUMBAGINIFOLIA L IN RESPONSE TO VARYING NITROGEN SUPPLY

Transformed Nicotiana plumbaginifolia plants with constitutive express ion of nitrate reductase (NR) activity were grown at different levels of nitrogen nutrition. The gradients in foliar NO3- content and maximu m 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 co nditions. Constitutive expression of NR did not influence the foliar p rotein and chlorophyll contents under any circumstances. A reciprocal relationship between the observed maximal extractable NR activity of t he leaves and their NO3- content was observed in plants grown in nitro gen-replete conditions at low irradiance (170 mu mol photons . m(-2). s(-1)). This relationship disappeared at higher irradiance (450 mu mol photons . m(-2). s(-1)) because the maximal extractable NR activity i n the leaves of the wild-type plants in these conditions increased to a level that was similar to, or greater than that found in constitutiv e NR-expressors. Much more NO3- accumulated in the leaves of plants gr own at 450 mu mol photons . m(-2). s(-1) than in those grown at 170 mu mol photons . m(-2). s(-1) in N-replete conditions. The foliar NO3- l evel and maximal NR activity decreased with the imposition of N-defici ency in all plant types such that after prolonged exposure to nitrogen depletion very little NO3- was found in the leaves and NR activity ha d decreased to almost zero. The activity of NR decreased under conditi ons of nitrogen deficiency. This regulation is multifactoral since the re is no regulation of NR gene expression by NO3- in the constitutive NR-expressors. We conclude that the NR protein is specifically targett ed for destruction under nitrogen deficiency. Consequently, constituti ve expression of NR activity does not benefit the plant in terms of in creased biomass production in conditions of limiting nitrogen.