Plant Cell Advance Online Publication
Published on August 19, 2004; 10.1105/tpc.104.024380
Received May 17, 2004
Accepted June 21, 2004
Transcript Profiling in the chl1-5 Mutant of Arabidopsis Reveals a Role of the Nitrate Transporter NRT1.1 in the Regulation of Another Nitrate Transporter, NRT2.1
Stéphane Muños 1, Céline Cazettes 1, Cécile Fizames 1, Frédéric Gaymard 1, Pascal Tillard 1, Marc Lepetit 1, Laurence Lejay 1, and Alain Gojon 1*
1 Biochimie et Physiologie Moléculaire des Plantes, Unité Mixte de Recherche 5004, Agro-M/Centre National de la Recherche Scientifique/Institut National de la Recherche Agronomique/Université Montpellier 2, 34060 Montpellier, Cedex 1, France
* To whom correspondence should be addressed. E-mail: gojon{at}ensam.inra.fr.
Arabidopsis thaliana mutants deficient for the NRT1.1 NO3- transporter display complex phenotypes, including lowered NO3- uptake, altered development of nascent organs, and reduced stomatal opening. To obtain further insight at the molecular level on the multiple physiological functions of NRT1.1, we performed large-scale transcript profiling by serial analysis of gene expression in the roots of the chl1-5 deletion mutant of NRT1.1 and of the Columbia wild type. Several hundred genes were differentially expressed between the two genotypes, when plants were grown on NH4NO3 as N source. Among these genes, the N satiety-repressed NRT2.1 gene, encoding a major component of the root high-affinity NO3- transport system (HATS), was found to be strongly derepressed in the chl1-5 mutant (as well as in other NRT1.1 mutants). This was associated with a marked stimulation of the NO3- HATS activity in the mutant, suggesting adaptive response to a possible N limitation resulting from NRT1.1 mutation. However, derepression of NRT2.1 in NH4NO3-fed chl1-5 plants could not be attributed to lowered production of N metabolites. Rather, the results show that normal regulation of NRT2.1 expression is strongly altered in the chl1-5 mutant, where this gene is no more repressible by high N provision to the plant. This indicates that NRT1.1 plays an unexpected but important role in the regulation of both NRT2.1 expression and NO3- HATS activity. Overexpression of NRT2.1 was also found in wild-type plants supplied with 1 mM NH4+ plus 0.1 mM NO3-, a situation where NRT1.1 is likely to mediate very low NO3- transport. Thus, we suggest that it is the lack of NRT1.1 activity, rather than the absence of this transporter, that derepresses NRT2.1 expression in the presence of NH4+. Two hypotheses are discussed to explain these results: (1) NRT2.1 is upregulated by a NO3- demand signaling, indirectly triggered by lack of NRT1.1-mediated uptake, which overrides feedback repression by N metabolites, and (2) NRT1.1 plays a more direct signaling role, and its transport activity generates an unknown signal required for NRT2.1 repression by N metabolites. Both mechanisms would warrant that either NRT1.1 or NRT2.1 ensure significant NO3- uptake in the presence of NH4+ in the external medium, which is crucial to prevent the detrimental effects of pure NH4+ nutrition.
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