AtAMT1 gene expression and NH4+ uptake in roots of Arabidopsis thaliana:: evidence for regulation by root glutamine levels

The mechanisms involved in regulating high-affinity ammonium (NH4+) uptake and the expression of the AtAMT1 gene encoding a putative high-affinity NH4+ transporter were investigated in the roots of Arabidopsis thaliana. Under conditions of steady-state nitrogen (N) supply, transcript levels of the AtAMT1 gene and V-max values for high-affinity (NH4+)-N-13 influx were inversely correlated with levels of hi provision. Following resupply of NH,NO, to N-starved plants, AtAMT1 mRNA levels and 13NH4+ influx declined rapidly but remained high when the conversion of NH4+ to glutamine (Gln) was blocked with methionine sulfoximine (MSX). This result demonstrates that end products of NH4+ assimilation, rather than NH4+ itself, are responsible for regulating AtAMT1 gene expression. Consistent with this hypothesis, AtAMT1 gene expression and NH4+ influx were suppressed by provision of Gin alone, or together with NH4NO3 plus MSX. Furthermore, AtAMT1 transcript levels and (NH4+)-N-13 influx were negatively correlated with root Gin concentrations, following resupply of N to N-starved plants. In addition to this level of control, the data suggest that high cytoplasmic [NH4+] may inhibit NH4+ influx.