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.
Arabidopsis thaliana mutants deficient for the NRT1.1 NO(3)(-) transporter display complex phenotypes, including lowered NO(3)(-) 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 NH(4)NO(3) as N source. Among these genes, the N satiety-repressed NRT2.1 gene, encoding a major component of the root high-affinity NO(3)(-) 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 NO(3)(-) HATS activity in the mutant, suggesting adaptive response to a possible N limitation resulting from NRT1.1 mutation. However, derepression of NRT2.1 in NH(4)NO(3)-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 NO(3)(-) HATS activity. Overexpression of NRT2.1 was also found in wild-type plants supplied with 1 mM NH(4)(+) plus 0.1 mM NO(3)(-), a situation where NRT1.1 is likely to mediate very low NO(3)(-) 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 NH(4)(+). Two hypotheses are discussed to explain these results: (1) NRT2.1 is upregulated by a NO(3)(-) 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 NO(3)(-) uptake in the presence of NH(4)(+) in the external medium, which is crucial to prevent the detrimental effects of pure NH(4)(+) nutrition.
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.
Stéphane
S
Muños
Céline
C
Cazettes
Cécile
C
Fizames
Frédéric
F
Gaymard
Pascal
P
Tillard
Marc
M
Lepetit
Laurence
L
Lejay
Alain
A
Gojon
D027321:Anion Transport Proteins / Q000235:genetics / Q000378:metabolism*; D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism*; D004269:DNA, Bacterial; D015536:Down-Regulation / Q000235:genetics; D025461:Feedback, Physiological / Q000235:genetics; D020869:Gene Expression Profiling; D018506:Gene Expression Regulation, Plant / Q000235:genetics*; D018745:Genome, Plant; D015698:Genomic Library; D016254:Mutagenesis, Insertional; D000091285:Nitrate Transporters; D009566:Nitrates / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation / Q000235:genetics*; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion; D014158:Transcription, Genetic / Q000235:genetics; D015854:Up-Regulation / Q000235:genetics
D027321:Anion Transport Proteins; D004269:DNA, Bacterial; C101492:NRT1.1 protein, Arabidopsis; D000091285:Nitrate Transporters; D009566:Nitrates; D010940:Plant Proteins; D000644:Quaternary Ammonium Compounds; D012097:Repressor Proteins; C036483:T-DNA; D009584:Nitrogen