TheArabidopsisNitrate Transporter NRT1.8 Functions in Nitrate Removal from the Xylem Sap and Mediates Cadmium Tolerance  

Abstract

Long-distance transport of nitrate requires xylem loading and unloading, a successive process that determines nitrate distribution and subsequent assimilation efficiency. Here, we report the functional characterization of NRT1.8, a member of the nitrate transporter (NRT1) family in Arabidopsis thaliana. NRT1.8 is upregulated by nitrate. Histochemical analysis using promoter-β-glucuronidase fusions, as well as in situ hybridization, showed that NRT1.8 is expressed predominantly in xylem parenchyma cells within the vasculature. Transient expression of the NRT1.8:enhanced green fluorescent protein fusion in onion epidermal cells and Arabidopsis protoplasts indicated that NRT1.8 is plasma membrane localized. Electrophysiological and nitrate uptake analyses using Xenopus laevis oocytes showed that NRT1.8 mediates low-affinity nitrate uptake. Functional disruption of NRT1.8 significantly increased the nitrate concentration in xylem sap. These data together suggest that NRT1.8 functions to remove nitrate from xylem vessels. Interestingly, NRT1.8 was the only nitrate assimilatory pathway gene that was strongly upregulated by cadmium (Cd2+) stress in roots, and the nrt1.8-1 mutant showed a nitrate-dependent Cd2+-sensitive phenotype. Further analyses showed that Cd2+ stress increases the proportion of nitrate allocated to wild-type roots compared with the nrt1.8-1 mutant. These data suggest that NRT1.8-regulated nitrate distribution plays an important role in Cd2+ tolerance.

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

“…Both AtNRT1.5 and AtNRT1.8 were shown to be involved in long-distance transport of NO 3 -. AtNRT1.5 appears to mediate nitrate efflux out of root cells and loading into the xylem for transport to the shoot , whereas AtNRT1.8 is involved in retrieving NO 3 -from the xylem parenchyma in the roots and shoots (Li et al 2010). Hence, both proteins cooperate to regulate long-distance nitrate transport.…”

“…CsNRT1.5A function seems to be more complex and related to some adaptive response of plants to a variable external nitrate supply. In Arabidopsis NRT1.8 was shown to play a significant role in cadmium tolerance leading to nitrate retention in roots to enhance root nitrate assimilation under heavy metal stress (Li et al 2010). Hence, it was suggested that the coordinated regulation of NRT1.5 and NRT1.8 expression resulting in a reduction of nitrate translocation to shoots could be a common adaptive response of plant to a wide range of stresses (Li et al 2010;Gojon and Gaymard 2010).…”

Nitrate transporters: an overview in legumes

“…Indeed, nitrate transporters are expressed in all plant organs, allowing both long-distance transport of nitrate, and tailored delivery to cell types. Provision of nitrate to aerial organs first requires xylem loading by nitrate transporters (Lin et al 2008;Li et al 2010;Wang and Tsay 2011) and, therefore, identification of the transporters responsible may be assisted by knowledge of the tissue patterning of gene expression. MtNPF6.8 is expressed in shoots as well as in the pericycle region in roots (Pellizzaro et al 2014).…”

NRT1.5/NPF7.3 Functions as a Proton-Coupled H+/K+ Antiporter for K+ Loading into the Xylem in Arabidopsis

“…Previous reports have shown that NRT1.8/NPF7.2 and NRT1.7/ NPF2.13 from the NRT1/PTR family play roles in NO 3 2 removal from the xylem and NO 3 2 loading into the phloem, respectively (Fan et al, 2009;Li et al, 2010). Thus, we examined the phenotypes of the nrt1.7 and nrt1.8 mutants under the LK condition.…”