背景+问题：Compared to root development regulated by external nutrients, less is known about how internal nutrients are monitored to control plasticity of shoot development.

主要研究：In this study, we characterize an Arabidopsis thaliana transceptor, NRT1.13 (NPF4.4), of the NRT1/PTR/NPF family. 

结果1-结构特征：Different from most NRT1 transporters, NRT1.13 does not have the conserved proline residue between transmembrane domains 10 and 11; an essential residue for nitrate transportactivity in CHL1/NRT1.1/NPF6.3. 

结果2-转运活性：As expected, when expressed in oocytes, NRT1.13 showed no nitrate transport activity. However, when Ser 487 at the corresponding position was converted back to proline, NRT1.13 S487P regained nitrate uptake activity, suggesting that wild-type NRT1.13 cannot transport nitrate but can bind it. 

结果3-表达部位：Subcellular localization and β-glucuronidase reporter analyses indicated that NRT1.13 is a plasma membrane protein expressed at the parenchyma cells next to xylem in the petioles and the stem nodes. 

结果4-突变体功能试验：When plants were grown with a normal concentration of nitrate, nrt1.13 showed no severe growth phenotype. However, when grown under low-nitrate conditions, nrt1.13 showed delayed flowering, increased node number, retarded branch outgrowth, and reduced lateral nitrate allocation to nodes. 

结论：Our results suggest that NRT1.13 is required for low-nitrate acclimation and that internal nitrate is monitored near the xylem by NRT1.13 to regulate shoot architecture and flowering time.

与外部营养物质调控根发育相比，植物如何监测内部营养物质以控制地上组织发育可塑性的还不清楚。本文中，作者鉴定了拟南芥中NRT1/PTR/NPF家族的一个硝酸盐感受器NRT1.13。与大多数的NRT1转运蛋白不一样，NRT1.13蛋白在跨膜结构域10和11之间并没有保守的脯氨酸残基；该残基对于CHL1/NRT1.1/NPF6.3的硝酸盐转运活性是必需的。与预期一致，NRT1.13在卵母细胞中没有表现出硝酸盐转运活性。但是，NRT1.13在487号位上对应的丝氨酸被修改为脯氨酸后，NRT1.13 S487P重新获得了硝酸盐转运活性，说明野生型的NRT1.13虽然能够结合硝酸盐，但是不能转运硝酸盐。亚细胞定位和β-葡萄糖醛酸酶报告分析显示，NRT1.13是一个质膜蛋白，在叶柄和茎节中靠近木质部的薄壁细胞中表达。当nrt1.13突变体生长在正常的硝酸盐浓度下时，其没有严重的生长缺陷。但是，当nrt1.13突变体生长在低硝酸盐浓度下时，其表现出延迟开花、茎节增加、分枝向外生长迟缓、以及向茎节的硝酸盐侧向分配减少等表型。本文的结果表明，NRT1.13作用于植物的低硝酸盐适应，以及NRT1.13负责监控木质部附近的内部硝酸盐浓度，从而调控株型结构和开花时间。