PIN5 is involved in regulating NH 4 + efflux and primary root growth under high-ammonium stress via mediating intracellular auxin transport

Background and Aims Ammonium (NH 4 + ) is an important nitrogen (N) source in many ecosystems and agricultural systems but excessive NH 4 + is toxic to root growth and development, especially when NH 4 + is the sole N source. Previous studies have shown that polar auxin (indole-3-acetic acid, IAA) transport mediated by PIN2 and AUX1 is critical for maintaining lateral root development under high-NH 4 + stress. However, the regulation of subcellular IAA homeostasis under high-NH 4 + stress has remained unclear. Methods Knockout mutants for the intracellular IAA transporter PIN5 and the plasma-membrane H + -ATPases AHA1 and AHA2 were used, and primary root length, transmembrane H + fluxes, and NH 4 + fluxes in the roots were determined. Results We show that high NH 4 + disturbs the subcellular IAA homeostasis by upregulating the expression of PIN5. Knockout of PIN5 resulted in elevated cytoplastic IAA accumulation and reduced NH 4 + efflux under high-NH 4 + stress. Furthermore, we show that NH 4 + treatment promotes H + efflux at the root elongation zone, in turn promoting NH 4 + efflux, with the involvement of PIN5-mediated intracellular IAA transport. Moreover, stabilization of rhizosphere pH reduced NH 4 + efflux and promoted primary root growth in the pin5 mutant under high NH 4 + . Conclusion Our findings provide a mechanistic explanation for the role of subcellular IAA homeostasis in response to high-NH 4 + stress through the coordinated regulation of NH 4 + efflux and H + efflux.