Nitrate Reductase Activity in ipt-Expressing Tobacco.

Using a novel system for expressing ipt gene in tobacco, we were able to increase cytokinins in plant tissue. This enabled us to study new regulatory aspects of nitrate (NO3-) reduction. We have grown plants with elevated cytokinins on MS with variable NO3- and benzyladenine (BA). We determined nitrate reductase activity (NRA) in this plant material. Initially, we found that ipt-expressing plants always display lower NRA than wild-type SR1 plants. We also found that long-term exposure of tobacco to cytokinins caused up to 60% decrease in NRA. However, addition of BA to N-starved plants caused induction of NRA to levels up to 100% higher than those before induction. In ipt-expressing plants this effect was only present if BA was administered together with NO3-. Exposure to 40mM NO3- caused up to 550% induction of NRA. This effect was always more pronounced in wild-type plants. We propose that regulation of NR by NO3- and cytokinins is a result of overlaying regulatory processes, with short-term ones having positive effect on NRA and those acting over extended periods having inhibitory effects on NRA.