Ammonium-dependent regulation of ammonium transporter ZmAMT1s expression conferred by glutamine levels in roots of maize

Abstract

In maize, two root epidermis-expressed ammonium transporters ZmAMT1;1a and ZmAMT1;3 play major roles in high-affinity ammonium uptake. However, the transcriptional regulation of ZmAMT1s in roots for ensuring optimal ammonium acquisition remains largely unknown. Here, using a split root system we showed that ZmAMT1;1a and ZmAMT1;3 transcript levels were induced by localized ammonium supply to nitrogen-deficient roots. This enhanced expression of ZmAMT1s correlated with increases in 15NH4+ influx rates and tissue glutamine concentrations in roots. When ammonium was supplied together with methionine sulfoximine, an inhibitor of glutamine synthase, ammonium-induced expression of ZmAMT1s disappeared, suggesting that glutamine rather than ammonium itself regulated ZmAMT1s expression. When glutamine was supplied to nitrogen-deficient roots, expression levels of ZmAMT1s were enhanced, and negative feedback regulation could subsequently occur by supply of glutamine at a high level. Thus, our results indicated an ammonium-dependent regulation of ZmAMT1s at transcript levels, and a dual role of glutamine was suggested in the regulation of ammonium uptake in maize roots.