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

Ammonium (NH 4 + ) and nitrate (NO 3 - ) are major inorganic nitrogen (N) sources for plants. When serving as the sole or dominant N supply, NH 4 + often causes root inhibition and shoot chlorosis in plants, known as ammonium toxicity. NO 3 - 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 NO 3 - efflux channel SLAH3 is involved in this process. However, whether additional components contribute to NO 3 - -mediated NH 4 + detoxification is unknown. Previously, mutations in NO 3 - transporter NRT1.1 were shown to cause enhanced resistance to high concentrations of NH 4 + . Whereas, in this study, we found when the high-NH 4 + medium was supplemented with low concentrations of NO 3 - , nrt1.1 mutant plants showed hyper-sensitive phenotype instead. Furthermore, mutation in NRT1.1 caused enhanced medium acidification under high-NH 4 + /low-NO 3 - 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 NO 3 - influx while NRT1.1 influenced NO 3 - 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 NO 3 - transport and balancing rhizosphere acidification.