Nitrate transporter NRT1.1 and anion channel SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity

Ammonium (NH4+) and nitrate (NO3-) are major inorganic nitrogen (N) sources for plants. When serving as the sole or dominant N supply, NH4+ often causes root inhibition and shoot chlorosis in plants, known as ammonium toxicity. NO3- usually causes no toxicity and can mitigate ammonium toxicity even at low concentrations, referred to as nitrate-dependent alleviation of ammonium toxicity. Our previous studies indicated a NO3- efflux channel SLAH3 is involved in this process. However, whether additional components contribute to NO3--mediated NH4+ detoxification is unknown. Previously, mutations in NO3- transporter NRT1.1 were shown to cause enhanced resistance to high concentrations of NH4+. Whereas, in this study, we found when the high-NH4+ medium was supplemented with low concentrations of NO3-, nrt1.1 mutant plants showed hyper-sensitive phenotype instead. Furthermore, mutation in NRT1.1 caused enhanced medium acidification under high-NH4+/low-NO3- condition, suggesting NRT1.1 regulates ammonium toxicity by facilitating H+ uptake. Moreover, NRT1.1 was shown to interact with SLAH3 to form a transporter-channel complex. Interestingly, SLAH3 appeared to affect NO3- influx while NRT1.1 influenced NO3- efflux, suggesting NRT1.1 and SLAH3 regulate each other at protein and/or gene expression levels. Our study thus revealed NRT1.1 and SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity through regulating NO3- transport and balancing rhizosphere acidification.