Constitutive expression of a putative high-affinity nitrate transporter inNicotiana plumbaginifolia: evidence for post-transcriptional regulation bya reduced nitrogen source

The NpNRT2.1 gene encodes a putative inducible component of the high-affini ty nitrate (NO3-) uptake system in Nicotiana plumbaginifolia. Here we repor t functional and physiological analyses of transgenic plants expressing the NpNRT2.1 coding sequence fused to the CaMV 35S or rolD promoters. Irrespec tive of the level of NO3- supplied, NO3- contents were found to be remarkab ly similar in wild-type and transgenic plants. Under specific conditions (g rowth on 10 mM NO3-), the steady-state NpNRT2.1 mRNA level resulting from t he deregulated transgene expression was accompanied by an increase in (NO3- )-N-15 influx measured in the low concentration range. This demonstrates fo r the first time that the NRT2.1 sequence codes a limiting element of the i nducible high-affinity transport system. Both (NO3-)-N-15 influx and mRNA l evels decreased in the wild type after exposure to ammonium, in agreement w ith previous results from many species. Surprisingly, however, influx was a lso markedly decreased in transgenic plants, despite stable levels of trans gene expression in independent transformants after ammonium addition. We co nclude that the conditions associated with the supply of a reduced nitrogen source such as ammonium, or with the generation of a further downstream me tabolite, probably exert a repressive effect on NO3- influx at both transcr iptional and post-transcriptional levels.