Nitrate efflux at the root plasma membrane:: Identification of an Arabidopsis excretion transporter

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.