Résumé
Nitrogen (N) is one of the key mineral nutrients for plants and its availability has a major impact on their growth and development. The main N sources for terrestrial plants in soils is nitrate (NO3-) and most often it is a growth limiting factor. Plants are able to sense NO3- in their environment, allowing them to quickly respond to the dramatic fluctuations of its availability. Significant advances have been made during the recent period concerning the molecular mechanisms of NO3- sensing and signalling in higher plants. The striking action of NO3- as a signal molecule on genome expression has been unravelled. Note worthily, NO3- sensing systems have been identified. These unexpectedly correspond to membrane transporters also ensuring the uptake of NO3- into root cells, thus generalizing the nutrient ‘transceptor’ (transporter/receptor) concept defined in yeast. Furthermore, components of the downstream transduction cascades, such as transcription factors or kinases, have also been isolated. Recently, a major breakthrough arising from this improved knowledge is a better understanding of the integration of NO3- and hormone signalling pathways, that explains the extraordinary developmental plasticity of plants in response to NO3-.