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

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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.