Journal Article

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

Li, Hong-Mei, Zhang, Yu, Li, Jian-Yong, Pike, Sharon M, Chen, Chun-Zhu, Zhang, Yi, Schroeder, Julian I, Fu, Yan-Lei, Gassmann, Walter, Tian, Wang, Huang, Jing, Li, Le-Gong, Bao, Juan, Gong, Ji-Ming

2010

Overview

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.

• Publisher: American Society of Plant Biologists
• Subject: Adaptation, Physiological / Adaptation, Physiological - drug effects / Adaptation, Physiological - genetics / Anion Transport Proteins / Anion Transport Proteins - genetics / Anion Transport Proteins - metabolism / Arabidopsis / Arabidopsis - cytology / Arabidopsis - drug effects / Arabidopsis - enzymology / Arabidopsis - genetics / Arabidopsis Proteins / Arabidopsis Proteins - genetics / Arabidopsis Proteins - metabolism / Arabidopsis thaliana / Cadmium / Cadmium - toxicity / Cell Membrane / Cell Membrane - drug effects / Cell Membrane - enzymology / cytology / drug effects / enzymology / Epidermal cells / Gene Expression Profiling / Gene Expression Regulation, Plant / Gene Expression Regulation, Plant - drug effects / genes / genetics / green fluorescent protein / Hydrogen-Ion Concentration / Hydrogen-Ion Concentration - drug effects / in situ hybridization / metabolism / Mutation / Mutation - genetics / Nitrates / Nitrates - metabolism / Nutrient removal / onions / Oocytes / Parenchyma / Plant cells / Plant Exudates / Plant Exudates - genetics / Plant Exudates - metabolism / Plant roots / Plants / plasma membrane / Pollution tolerance / Protein Transport / Protein Transport - drug effects / Reverse transcriptase polymerase chain reaction / Roots / sap / Stress, Physiological / Stress, Physiological - drug effects / Stress, Physiological - genetics / Subcellular Fractions / Subcellular Fractions - drug effects / Subcellular Fractions - enzymology / toxicity / transporters / Up-Regulation / Up-Regulation - drug effects / Xenopus laevis / Xylem / Xylem - cytology / Xylem - drug effects / Xylem - enzymology / Xylem - genetics / xylem vessels