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

为了应对土壤中氮素有效性的波动，植物根系为硝酸盐和氨氮发展了不同的膜运输系统。为了防止由于对这两种相反电荷的氮素的不平衡吸收而造成的代谢胁迫，硝酸盐和铵的运输能力需要密切协调。在低底物浓度下，根系对氨氮的吸收主要由高亲和力AMT1型转运蛋白和硝酸盐的高亲和力NRT2型和低亲和力NRT1型转运蛋白介导。在后者中，NRT1.1不仅参与硝酸盐的转运，而且还参与硝酸盐的感知，从而调节NRT2和AMT1类转运蛋白的表达水平。我们在拟南芥植物上的初步实验表明，一个铵吸收缺陷的四重AMT突变体在硝酸盐或混合硝酸铵营养下生长更好，而内流和基因表达研究为NRT2和AMT1基因的调控特征提供了证据，这些特征不能仅用NRT1.1的传感器作用来解释。因此，这项建议的总体目标是通过以下方式来探索铵和硝酸盐之间平衡吸收的调控机制：i)研究AMT1.5在硝酸盐存在下调节高亲和力铵吸收中的特定作用；ii)探索N型触发的pH变化在NRT1.1依赖和独立调控NRT2和AMT1基因中的调节功能；以及iii)识别和表征参与调节混合N营养下铵/硝酸盐吸收比率的NRT和AMT的转录调控因子。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，并对作物氮素营养的改善产生潜在影响。

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.