Abstract
This study of the Arabidopsis thaliana nitrate transporter NRT1.9 reveals an important function for a NRT1 family member in phloem nitrate transport. Functional analysis in Xenopus laevis oocytes showed that NRT1.9 is a low-affinity nitrate transporter. Green fluorescent protein and β-glucuronidase reporter analyses indicated that NRT1.9 is a plasma membrane transporter expressed in the companion cells of root phloem. In nrt1.9 mutants, nitrate content in root phloem exudates was decreased, and downward nitrate transport was reduced, suggesting that NRT1.9 may facilitate loading of nitrate into the root phloem and enhance downward nitrate transport in roots. Under high nitrate conditions, the nrt1.9 mutant showed enhanced root-to-shoot nitrate transport and plant growth. We conclude that phloem nitrate transport is facilitated by expression of NRT1.9 in root companion cells. In addition, enhanced root-to-shoot xylem transport of nitrate in nrt1.9 mutants points to a negative correlation between xylem and phloem nitrate transport.
Highlights
Nitrate is one of the major nitrogen sources for the synthesis of amino acids and nucleic acids in higher plants
We report that mutation of the Arabidopsis nitrate transporter NRT1.9 reduced nitrate content in root phloem and increased shoot nitrate content
According to phylogenetic analysis of the Arabidopsis NRT1 family, NRT1.9 is classified, together with NRT1.6 and NRT1.7, in subgroup IV, while CHL1, NRT1.2, NRT1.3, and NRT1.4 are in subgroup I, and NRT1.5 is in subgroup II (Tsay et al, 2007)
Summary
Nitrate is one of the major nitrogen sources for the synthesis of amino acids and nucleic acids in higher plants. Nitrate assimilation starts with the reduction of nitrate to nitrite by nitrate reductase (NR) in the cytoplasm and the subsequent reduction of nitrite to ammonium by nitrite reductase in chloroplasts (or plastids in the roots). Besides serving as a nutrient source and osmoticum, nitrate functions as a signal molecule regulating nitrogen and carbon metabolism and coordinating whole-plant development (Forde, 2002; Sakakibara et al, 2006; Vidal and Gutierrez, 2008). Since nitrate is a key component in the regulation of plant development and growth, the regulation of nitrate distribution, for example, root-to-shoot nitrate transport, is important for plants to modulate their growth in response to various environmental conditions
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