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The Arabidopsis Nitrate Transporter NRT1.8 Functions in Nitrate Removal from the Xylem Sap and Mediates Cadmium Tolerance
Jian-Yong Lia,1, Yan-Lei Fua,1, Sharon M. Pikeb, Juan Baoa, Wang Tianc, Yu Zhanga, Chun-Zhu Chena, Yi Zhanga, Hong-Mei Lia, Jing Huanga, Le-Gong Lic, Julian I. Schroederd, Walter Gassmannb and Ji-Ming Gonga,2
a National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, People's Republic of China
b Division of Plant Sciences, C.S. Bond Life Sciences Center and Interdisciplinary Plant Group, University of Missouri, Columbia, Missouri 65211-7310
c College of Life Sciences, Capital Normal University, Beijing 100037, People's Republic of China
d Division of Biological Sciences and Center for Molecular Genetics, Cell and Developmental Biology Section, University of California, San Diego, California 92093-0116
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.
