Molecular regulation underlying the mutual interaction between ammonium and nitrate transport in plant roots

为了科普土壤中氮素有效性的波动，植物根系已经发展了不同的硝酸盐和铵离子膜运输系统。为了防止由于这两种带相反电荷的N形式的不平衡吸收而引起的代谢应激，硝酸盐和铵的运输能力需要紧密协调。在低的外部基质浓度下，根系吸收主要由高亲和力AMT 1型转运蛋白介导的铵和高亲和力NRT 2型和低亲和力NRT 1型转运蛋白硝酸盐。在后者中，NRT1.1不仅介导运输，而且还感测硝酸盐，允许调节NRT 2-以及AMT 1-型转运蛋白的表达水平。我们在拟南芥植物中的初步实验显示，在硝酸盐或混合硝酸铵营养下，铵吸收缺陷的四倍amt突变体的生长更好，而流入和基因表达研究提供了NRT 2和AMT 1基因的调控特征的证据，不能单独用NRT1.1的传感器作用来解释。因此，本提案的总体目标是通过以下方式探索铵和硝酸盐之间平衡摄取的潜在调节机制：i）研究AMT1.5在硝酸盐存在下介导高亲和力铵摄取的特定作用; ii）探索NRT 2和AMT 1基因的NRT1.1依赖性和非依赖性调节中N形式触发的pH变化的调节功能;和iii）鉴定和表征在混合N营养下参与调节铵-硝酸盐吸收比的NRT和AMT的转录调节子。预期的结果有望揭示植物如何感知和抵消不平衡的养分吸收，对改善作物氮营养的潜在影响。

To cope with fluctuating nitrogen (N) availabilities in soils, plant roots have developed separate and diverse membrane transport systems for nitrate and ammonium. To prevent metabolic stress due to imbalanced uptake of these two oppositely charged N forms, nitrate and ammonium transport capacities need to be tightly coordinated. At low external substrate concentrations, root uptake is mainly mediated by high-affinity AMT1-type transport proteins for ammonium and by high-affinity NRT2-type and low-affinity NRT1-type transporters for nitrate. Among the latter, the transceptor NRT1.1 mediates not only transport but also sensing of nitrate, allowing to regulate expression levels of NRT2- as well as AMT1-type transporters. Our preliminary experiments in Arabidopsis plants show better growth of an ammonium uptake-defective quadruple amt mutant under nitrate or mixed ammonium nitrate nutrition, while influx and gene expression studies provide evidence for regulatory features of NRT2 and AMT1 genes that cannot be explained by the sensor role of NRT1.1 alone. Thus, the overarching goal of this proposal is to explore the regulatory mechanisms underlying a balanced uptake between ammonium and nitrate by i) investigating a specific role of AMT1.5 in mediating high-affinity ammonium uptake in the presence of nitrate; ii) exploring a regulatory function of N form-triggered pH changes in the NRT1.1-dependent and -independent regulation of NRT2 and AMT1 genes; and iii) identifying and characterizing transcriptional regulators of NRTs and AMTs involved in modulating the ammonium-to-nitrate uptake ratio under mixed N nutrition. The expected results promise to uncover how plants sense and counteract imbalanced nutrient uptake with potential impact on the improvement of N nutrition in crops.