Nitrate Efflux at the Root Plasma Membrane: Identification of anArabidopsisExcretion Transporter

Abstract

Root NO 3 À efflux to the outer medium is a component of NO 3 À 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 NO 3 À transport activity (K m ¼ 5 mM) in isolated root PM, electrically coupled to the ATPdependant H þ -pumping activity, is inhibited by anti-NAXT antibodies. In standard culture conditions, NO 3 À contents were altered in plants expressing NAXT-interfering RNAs but not in naxt1 mutant plants. Upon acid load, unidirectional root NO 3 À efflux markedly increased in wild-type plants, leading to a prolonged NO 3 À excretion regime concomitant with a decrease in root NO 3 À 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 NO 3 À (but not of Cl À ) was strongly impaired in naxt1 mutant PM. This identification of NO 3 À efflux transporters at the PM of plant cells opens the way to molecular studies of the physiological role of NO 3 À efflux in stressed or unstressed plants.

Identification of arbuscular mycorrhiza-inducible Nitrate Transporter 1/Peptide Transporter Family (NPF) genes in rice

“…Recently, the Arabidopsis NPF7.3/NRT1.5 was shown to be a K + transporter and to NO 3 − -dependently control K + homeostasis in shoots (Drechsler et al 2015;Li et al 2017). Other family members, for example, function as a NO 3 − excretion transporter or in actinorhizal and rhizobial N-fixing symbioses (Jeong et al 2004;Segonzac et al 2007;Yendrek et al 2010), whereas a role of NPF proteins in plant-AMF interactions has not been observed yet.…”

Control of root architecture and nodulation by theLATD/NIPtransporter

“…16 Thus, the physiological role of LATD/NIP, which is required for meristem function and rhizobium infection, is likely to be quite different from that of the characterized members of clade IV.…”

“…Of the 19 Arabidopsis NRT1(PTR) members that fall into this clade, only 3, NRT1.6, NRT1.7 and NAXT1, have a biochemically demonstrated function: all are low-affinity nitrate transporters. [14][15][16] Clade IV includes one of the most closely related Arabidopsis NRT1(PTR) members to LATD/NIP, encoded by At1g52190, which has been reported in a review to be a low-affinity nitrate transporter, 6 but the data are unpublished. The physiological roles of these proteins are likely to be quite diverse as they each have a distinct tissue-specific expression pattern.…”

The genomic organization and transcriptional pattern of genes encoding nitrate transporters 1 (NRT1) in cucumber

“…It has been established that nitrate uptake by plants is an active, H + -coupled process facilitated through the highand low-affinity transport systems (HATS and LATS) which could be constitutive or inducible (Glass and Siddiqi 1995;Glass et al 2001Glass et al , 2002Zhao et al 1999;Tischner 2000;Touraine and Gojon 2001;Forde 2002). Also the nitrate influx into vacuole (Blumwald and Poole 1985;Schumaker and Sze 1987;Kabała et al 2003;de Angeli et al 2006;von der Fecht-Bartenbach et al 2010) as well as its efflux out of the plant cell (Segonzac et al 2007;Lin et al 2008) have been shown to be driven by a proton motive force. Active nitrate transport across plant cell membranes has been attributed to three families including the proton-coupled symporters and antiporters belonging to NRT1, NRT2 (Tsay et al 2007) and CLC multigenic families (de Angeli et al 2006Angeli et al , 2009von der Fecht-Bartenbach et al 2010).…”