The role of the nitrate transporter NRT1.1 in plant iron homeostasis and toxicity on ammonium

Ammonium (NH4+) is toxic to root growth in most plants, and NH4+ toxicity has been linked to disruptions in plant Fe homeostasis. However, only a few genes have been linked to the disruption of Fe homeostasis under NH4+ nutrition, and pathway details have as yet to be resolved. Here, using RNA-seq analysis and RT-qPCR, we explore the response of different genes expressed in the roots of Fe-replete and Fe-starved Arabidopsis plants under NH4+ conditions. The Nitrate Transporter 1.1 (NRT1.1) gene, known to code for a dual-affinity nitrate transporter, but not other NRTs genes, was specifically induced in Fe-replete plants in response to NH4+ provision. NRT1.1 antagonizes NH4+-dependent Fe accumulation, and this antagonism requires NO3- supply. Constitutively expressing NRT1.1 confers higher NO3- uptake and reduces NH4+-dependent Fe accumulation by increasing pH in the rhizosphere. Building on previous evidence establishing the involvement of root Fe accumulation in the root growth response to elevated NH4+, our study shows that NRT1.1-mediated nitrate uptake curtails symptoms of NH4+ toxicity under elevated NH4+ and in the presence of NO3-, by increasing rhizospheric pH, offering new insights into possible pathways for improving the tolerance to NH4+ toxicity in plants.