Abstract
Plant evolved a complex profile of responses to cope with changes of nutrient availability in the soil. These are based on a stringent control of expression and/or activity of proteins involved in nutrients transport and assimilation. Furthermore, a sensing and signaling system for scanning the concentration of substrates in the rooted area and for transmitting this information to the plant machinery controlling root development can be extremely useful for an efficient plant response. Ammonium represents for plants either a preferential nitrogen source or the trigger for toxicity symptoms depending by its concentration. We propose a role for the high affinity Lotus japonicus ammonium transporter LjAMT1;3 as an intracellular ammonium sensor to achieve a convenient modulation of the root development in conditions of potentially toxic external ammonium concentration.