Functional crosstalk between nitrate and ammonium transporters in N acquisition and pH homeostasis

In quantitative terms, nitrogen (N) is the most important essential mineral element for plants, acquired mainly in the form of ammonium (NH₄ ⁺) and nitrate (NO₃ ^-). Despite fluctuations in soil NH₄ ⁺and NO₃ ^-availability, plants seek to balance their NH₄ ⁺-to- NO₃ ^-uptake ratio to avoid the metabolic burden associated with the compensation of an intracellular proton excess or deficit. However, while the molecular mechanisms by which plants mediate and modulate the activity of their uptake systems for NO₃ ^-and NH₄ ⁺have been well characterized, it has remained unclear to what extent these transport systems could interact. In this review, the potential contributions of AMTs and NRT1.1 to the overall acquisition of N are highlighted. Both NO₃ ^-independent and -dependent signaling of NRT1.1 in modulating NH₄ ⁺tolerance, as well as the underestimated role of AMTs in nutrient and cellular pH homeostasis, are discussed. The interdependency between AMTs and NRT1.1 is considered highly relevant for optimized N uptake in field conditions, where both N forms typically coexist and act complementarily to maintain balanced pH and nutrient homeostasis for optimal plant growth.