The Arabidopsis Nitrate Transporter NRT1.7, Expressed in Phloem, Is Responsible for Source-to-Sink Remobilization of Nitrate

Shu-Chun Fan ¹, Choun-Sea Lin ², Po-Kai Hsu ¹, Shan-Hua Lin ², and Yi-Fang Tsay ²^*

¹ Graduate Institute of Life Sciences, National Defense Medical Center, Taipei 112, Taiwan; Institute of Molecular Biology, Academia Sinica, Taipei 115, Taiwan
² Institute of Molecular Biology, Academia Sinica, Taipei 115, Taiwan

^* To whom correspondence should be addressed. E-mail: yftsay{at}gate.sinica.edu.tw.

Several quantitative trait locus analyses have suggested thatgrain yield and nitrogen use efficiency are well correlatedwith nitrate storage capacity and efficient remobilization.This study of the Arabidopsis thaliana nitrate transporter NRT1.7provides new insights into nitrate remobilization. Immunoblots,quantitative RT-PCR, ${beta}$ -glucuronidase reporter analysis, and immunolocalizationindicated that NRT1.7 is expressed in the phloem of the leafminor vein and that its expression levels increase coincidentallywith the source strength of the leaf. In nrt1.7 mutants, morenitrate was present in the older leaves, less ¹⁵NO₃^- spottedon old leaves was remobilized into N-demanding tissues, andless nitrate was detected in the phloem exudates of old leaves.These data indicate that NRT1.7 is responsible for phloem loadingof nitrate in the source leaf to allow nitrate transport outof older leaves and into younger leaves. Interestingly, nrt1.7mutants showed growth retardation when external nitrogen wasdepleted. We conclude that (1) nitrate itself, in addition toorganic forms of nitrogen, is remobilized, (2) nitrate remobilizationis important to sustain vigorous growth during nitrogen deficiency,and (3) source-to-sink remobilization of nitrate is mediatedby phloem.