Department of Life Sciences and Institute of Genome Sciences, National Yang-Ming University, Taipei, Taiwan.
Plant Cell. 2011 May;23(5):1945-57. doi: 10.1105/tpc.111.083618. Epub 2011 May 13.
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.
这项关于拟南芥硝酸盐转运蛋白 NRT1.9 的研究揭示了 NRT1 家族成员在韧皮部硝酸盐运输中的重要功能。在非洲爪蟾卵母细胞中的功能分析表明，NRT1.9 是一种低亲和力硝酸盐转运蛋白。绿色荧光蛋白和β-葡萄糖醛酸酶报告基因分析表明，NRT1.9 是一种在根韧皮部伴胞中表达的质膜转运蛋白。在 nrt1.9 突变体中，根韧皮部渗出物中的硝酸盐含量减少，向下的硝酸盐转运减少，表明 NRT1.9 可能有助于将硝酸盐装载到根韧皮部，并增强根中的向下硝酸盐转运。在高硝酸盐条件下，nrt1.9 突变体表现出增强的根到地上部硝酸盐转运和植物生长。我们的结论是，NRT1.9 在根伴胞中的表达促进了韧皮部硝酸盐的转运。此外，nrt1.9 突变体中硝酸盐向木质部的根到地上部的转运增强表明木质部和韧皮部硝酸盐转运之间存在负相关。
