The Arabidopsis root stele transporter NPF2.3 contributes to nitrate translocation to shoots under salt stress

Abstract

In most plants, NO(3)(-) constitutes the major source of nitrogen, and its assimilation into amino acids is mainly achieved in shoots. Furthermore, recent reports have revealed that reduction of NO(3)(-) translocation from roots to shoots is involved in plant acclimation to abiotic stress. NPF2.3, a member of the NAXT (nitrate excretion transporter) sub-group of the NRT1/PTR family (NPF) from Arabidopsis, is expressed in root pericycle cells, where it is targeted to the plasma membrane. Transport assays using NPF2.3-enriched Lactococcus lactis membranes showed that this protein is endowed with NO(3)(-) transport activity, displaying a strong selectivity for NO(3)(-) against Cl(-). In response to salt stress, NO(3)(-) translocation to shoots is reduced, at least partly because expression of the root stele NO(3)(-) transporter gene NPF7.3 is decreased. In contrast, NPF2.3 expression was maintained under these conditions. A loss-of-function mutation in NPF2.3 resulted in decreased root-to-shoot NO(3)(-) translocation and reduced shoot NO(3)(-) content in plants grown under salt stress. Also, the mutant displayed impaired shoot biomass production when plants were grown under mild salt stress. These mutant phenotypes were dependent on the presence of Na(+) in the external medium. Our data indicate that NPF2.3 is a constitutively expressed transporter whose contribution to NO(3)(-) translocation to the shoots is quantitatively and physiologically significant under salinity.

AtNPF2.5 Modulates Chloride (Cl−) Efflux from Roots of Arabidopsis thaliana

“…It is reasonable to hypothesize that the function-uncharacterised NAXT members may also encode anion transporters that are permeable to Cl − or/and ${\text{NO}}_3^{-}$. However, the selectivity of these proteins for Cl − and ${\text{NO}}_3^{-}$ needs to be determined with care, given that anion selectivity can be changed by a single residue mutation (Wege et al, 2010; Maierhofer et al, 2014), and that NPF2.3 is selective to ${\text{NO}}_3^{-}$ (Taochy et al, 2015). A comprehensive elucidation of the physiological roles of all NAXT members would gain interesting insights into functions of the NRT1/PTR family, a family with diverse transport specificities and physiological roles [e.g., transport of Cl − , ${\text{NO}}_3^{-}$, ABA, and glucosinolates (Tsay et al, 2007; Kanno et al, 2012; Nour-Eldin et al, 2012; Chiba et al, 2015; Li et al, 2016)].…”

“…As one member of the NRT1/PTRs in Arabidopsis, NPF2.4 belongs to the NAXT (for Nitrate Excretion Transporter) subfamily (a seven-gene clade) that was named after NAXT1 ( NPF2.7 ), a plasma membrane transporter involved in the efflux of ${\text{NO}}_3^{-}$ from the root surface (Segonzac et al, 2007; Tsay et al, 2007; Léran et al, 2014). Another NAXT family member, NPF2.3, was identified following functional characterization as a transporter that was more selective for ${\text{NO}}_3^{-}$ than for Cl − and was important for root-to-shoot transfer of ${\text{NO}}_3^{-}$ (Taochy et al, 2015). NPF2.3 and NPF2.4 were both shown to be regulated by NaCl, indicating the possible involvement of the NAXT subfamily in plant salinity responses (Taochy et al, 2015; Li et al, 2016).…”

“…Another NAXT family member, NPF2.3, was identified following functional characterization as a transporter that was more selective for ${\text{NO}}_3^{-}$ than for Cl − and was important for root-to-shoot transfer of ${\text{NO}}_3^{-}$ (Taochy et al, 2015). NPF2.3 and NPF2.4 were both shown to be regulated by NaCl, indicating the possible involvement of the NAXT subfamily in plant salinity responses (Taochy et al, 2015; Li et al, 2016). The other four NAXT genes, NPF2.1, NPF2.2, NPF2.5 , and NPF2.6 , are yet to be functionally characterized.…”

Source and sink mechanisms of nitrogen transport and use

“…However, release of nitrate from the Arabidopsis root pericycle is mediated by a proton-coupled transport mechanism through NPF7.3/NRT1.5 (Lin et al, 2008), and in rice via OsNPF2.4 , potentailly pointing to a link between transporter activity, xylem pH, and root-to-shoot nitrate transport. Arabidopsis NPF2.3 is expressed in the pericycle and exports nitrate under salt stress (Taochy et al, 2015). NPF7.2/NRT1.8 and NPF2.9/NRT1.9 regulate root-to-shoot nitrate translocation by importing xylem nitrate back into root cells.…”

Identification of a Stelar-Localized Transport Protein That Facilitates Root-to-Shoot Transfer of Chloride in Arabidopsis

“…2). The protein sequence of NPF2.4 was found to have a 64% identity and a 78% similarity to NAXT1/NPF2.7 and a 76% identity and a 85% similarity to NPF2.3, a NO 3 -selective xylem loader in Arabidopsis root (Taochy et al, 2015;Supplemental Fig. S1).…”