PIN5 is involved in regulating NH4+ efflux and primary root growth under high-ammonium stress via mediating intracellular auxin transport

Background and Aims Ammonium (NH4+) is an important nitrogen (N) source in many ecosystems and agricultural systems but excessive NH4+ is toxic to root growth and development, especially when NH4+ 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-NH4+ stress. However, the regulation of subcellular IAA homeostasis under high-NH4+ 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 NH4+ fluxes in the roots were determined.Results We show that high NH4+ disturbs the subcellular IAA homeostasis by upregulating the expression of PIN5. Knockout of PIN5 resulted in elevated cytoplastic IAA accumulation and reduced NH4+ efflux under high-NH4+ stress. Furthermore, we show that NH4+ treatment promotes H+ efflux at the root elongation zone, in turn promoting NH4+ efflux, with the involvement of PIN5-mediated intracellular IAA transport. Moreover, stabilization of rhizosphere pH reduced NH4+ efflux and promoted primary root growth in the pin5 mutant under high NH4+.Conclusion Our findings provide a mechanistic explanation for the role of subcellular IAA homeostasis in response to high-NH4+ stress through the coordinated regulation of NH4+ efflux and H+ efflux.