Reciprocal co-regulation of nitrate and ammonium transporters is modulated by external pH in Arabidopsis

Plants primarily acquire inorganic nitrogen (N) as nitrate (NO3-) and ammonium (NH4+). The uptake of these forms is strongly modulated by external pH, which influences both their availability and the activity of their specific root transporters (NRTs for NO3- and AMTs for NH4+). Moreover, NO3- and NH4+ uptake exert opposite effects on net proton (H+) fluxes, raising the question of how external H+ availability shapes the balance between both N forms and their reciprocal regulation. Using Arabidopsis knock-out mutants deficient in key NO3- and NH4+ transporters, in combination with N and H+ flux assays and gene expression analyses, this study shows that low external pH strongly promotes NO3- uptake but severely constrains plant growth under NH4+ nutrition. The stimulatory effect of external H+ overrides the H+ efflux typically induced by NH4+. Conversely, at higher external pH, an alternative, AMT-independent transport mechanism likely related to K+ transport appears to facilitate NH4+ uptake and mitigate its toxicity. Furthermore, mutants lacking AMTs exhibited enhanced high-affinity NO3- uptake at low pH, while the NRT1.1 mutant (chl1-5) showed increased high-affinity NH4+ acquisition at higher pH. These findings highlight a new and complex interplay between pH and reciprocal N uptake dynamics and point to AMT- and NRT1.1-independent pathways contributing to the acquisition of alternative N forms under contrasting pH conditions.