Résumé
Nitrogen (N) availability in soils is highly variable in both time and space. To cope with this constraint, plants develop a wide range of adaptive responses, which modulate their N use efficiency according to external N availability and internal N status. Many of these responses are triggered by the action of nitrate as a signal molecule. Plants are able to sense nitrate in the environment, and increasing evidence indicate that this is at least in part due to plasma membrane nitrate transceptors which fulfil a dual transport/sensing function. The best known example is the nitrate transporter NRT1.1(CHL1) of Arabidopsis thaliana. NRT1.1 has been shown to govern several responses of the plant to nitrate, including regulation of other nitrate transporters and of enzymes of nitrate metabolism, relief of seed dormancy, and changes in root system architecture. Nitrate signalling mediated by NRT1.1 is independent of its nitrate transport activity, and the two functions can be uncoupled genetically. The mechanisms of nitrate sensing by NRT1.1 are still not clearly understood. However, it has been proposed that NRT1.1 is responsible for the nitrate-induced changes in lateral root growth because it is able to facilitate transport of auxin in addition to nitrate. As a consequence, NRT1.1 modulates auxin accumulation in lateral roots as a function of nitrate availability, thereby coupling nitrate sensing with the hormonal control of organ development. Besides NRT1.1, other nitrate/nitrogen transporters with a signalling role probably exist. Thus, transceptor-mediated nutrient signalling may be of more general occurrence in plants.