Root growth inhibition by NH4+ in Arabidopsis is mediated by the root tip and is linked to NH4+ efflux and GMPase activity

Root growth in higher plants is sensitive to excess ammonium (NH4+). Our study shows that contact of NH4+ with the primary root tip is both necessary and sufficient to the development of arrested root growth under NH4+ nutrition in Arabidopsis. We show that cell elongation and not cell division is the principal target in the NH4+ inhibition of primary root growth. Mutant and expression analyses using DR5:GUS revealed that the growth inhibition is furthermore independent of auxin and ethylene signalling. NH4+ fluxes along the primary root, measured using the Scanning Ion-selective Electrode Technique, revealed a significant stimulation of NH4+ efflux at the elongation zone following treatment with elevated NH4+, coincident with the inhibition of root elongation. Stimulation of NH4+ efflux and inhibition of cell expansion were significantly more pronounced in the NH4+-hypersensitive mutant vtc1-1, deficient in the enzyme GDP-mannose pyrophosphorylase (GMPase). We conclude that both restricted transmembrane NH4+ fluxes and proper functioning of GMPase in roots are critical to minimizing the severity of the NH4+ toxicity response in Arabidopsis.