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

Abstract

Several quantitative trait locus analyses have suggested that grain yield and nitrogen use efficiency are well correlated with nitrate storage capacity and efficient remobilization. This study of the Arabidopsis thaliana nitrate transporter NRT1.7 provides new insights into nitrate remobilization. Immunoblots, quantitative RT-PCR, b-glucuronidase reporter analysis, and immunolocalization indicated that NRT1.7 is expressed in the phloem of the leaf minor vein and that its expression levels increase coincidentally with the source strength of the leaf. In nrt1.7 mutants, more nitrate was present in the older leaves, less 15 NO 3 2 spotted on old leaves was remobilized into N-demanding tissues, and less nitrate was detected in the phloem exudates of old leaves. These data indicate that NRT1.7 is responsible for phloem loading of nitrate in the source leaf to allow nitrate transport out of older leaves and into younger leaves. Interestingly, nrt1.7 mutants showed growth retardation when external nitrogen was depleted. We conclude that (1) nitrate itself, in addition to organic forms of nitrogen, is remobilized, (2) nitrate remobilization is important to sustain vigorous growth during nitrogen deficiency, and (3) sourceto-sink remobilization of nitrate is mediated by phloem.

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.…”

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

“…So far, both the low-affinity nitrate transporter NRT2.7 and the chloride channel, CLCa, were identified as responsible for shortdistance nitrate translocation into vacuoles (De Angeli et al, 2006;Chopin et al, 2007). For long-distance transport, recent studies found that NRT1.5 represents a major mechanism for loading nitrate into xylem vessels (Lin et al, 2008) and NRT1.7 for loading nitrate into the phloem (Fan et al, 2009).…”

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

“…Root-to-shoot nitrate transport is probably governed by three members of AtNRT1 family: two closely related transporters NRT1.5 and NRT1.8, are responsible for loading or retrieving nitrate from xylem sap, respectively, whereas root stele-expressed AtNRT1.9 is involved in the loading of nitrate into the root phloem to enhance downward nitrate transport in roots (Wang and Tsay 2011). The remaining AtNRTs operate in shoots, where they are involved in nitrate storage in leaf petioles (NRT1.4), NO 3 − translocation from maternal tissue to developing embryos (NRT1.6) or the remobilization of NO 3 − from older to younger leaves (NRT1.7) (Chiu et al 2004;Almagro et al 2008;Fan et al 2009). The molecular features of nitrate transport within plants and the regulation of this process have been thoroughly studied mostly in the model plant Arabidopsis thaliana.…”