Ammonium homeostasis and signaling in plant cells

Nitrogen is an essential macronutrient in plant growth and development. Ammonium is one of the major inorganic nitrogen forms for root uptake. The homeostasis of ammonium in the plant cell is under tight control to prevent ammonium toxicity when in excess. In the ammonium signaling pathway, internal and external ammonium can be detected by specific sensors, which in turn triggers a series of proper plant ammonium responses including transcription regulation and phosphorylation. Ammonium absorption is mainly mediated by the root-located ammonium transporters, which are key regulators in the nitrogen signaling pathway. Many researchers have attempted to unravel the mechanisms of ammonium uptake by the transporters. Fine-tuned modulation of ammonium homeostasis is necessary to maintain an appropriate level of ammonium in the cytoplasm, which is a balance of ammonium efflux, assimilation and compartmentation. Recently, there has been important progress in revealing the ammonium sensing and signaling mechanisms. In this review, we focus on the homeostatic regulation and signaling of cytosolic ammonium.