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Nitrate Efflux at the Root Plasma Membrane: Identification of an Arabidopsis Excretion Transporter^[W]

^a Biochimie et Physiologie Moléculaire des Plantes, Agro-M/Centre National de la Recherche Scientifique/Institut National de la Recherche Agronomique/Université Montpellier 2, Unité Mixte de Recherche 5004, F-34060 Montpellier Cedex 1, France
^b Proteomique, Institut National de la Recherche Agronomique, Unité de Recherche 1199, F-34060 Montpellier Cedex 1, France

² Address correspondence to gibrat{at}supagro.inra.fr.

Root NO₃^– efflux to the outer medium is a component of NO₃^– net uptake and can even overcome influx upon various stresses. Its role and molecular basis are unknown. Following a functional biochemical approach, NAXT1 (for NITRATE EXCRETION TRANSPORTER1) was identified by mass spectrometry in the plasma membrane (PM) of Arabidopsis thaliana suspension cells, a localization confirmed using a NAXT1–Green Fluorescent Protein fusion protein. NAXT1 belongs to a subclass of seven NAXT members from the large NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER family and is mainly expressed in the cortex of mature roots. The passive NO₃^– transport activity (K_m = 5 mM) in isolated root PM, electrically coupled to the ATP-dependant H⁺-pumping activity, is inhibited by anti-NAXT antibodies. In standard culture conditions, NO₃^– contents were altered in plants expressing NAXT-interfering RNAs but not in naxt1 mutant plants. Upon acid load, unidirectional root NO₃^– efflux markedly increased in wild-type plants, leading to a prolonged NO₃^– excretion regime concomitant with a decrease in root NO₃^– content. In vivo and in vitro mutant phenotypes revealed that this response is mediated by NAXT1, whose expression is upregulated at the posttranscriptional level. Strong medium acidification generated a similar response. In vitro, the passive efflux of NO₃^– (but not of Cl^–) was strongly impaired in naxt1 mutant PM. This identification of NO₃^– efflux transporters at the PM of plant cells opens the way to molecular studies of the physiological role of NO₃^– efflux in stressed or unstressed plants.

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