Major alterations of the regulation of root NO3- uptake are associated with the mutation of Nrt2.1 and Nrt2.2 genes in arabidopsis

The role of AtNrt2.1 and AtNrt2.2 genes, encoding putative NO3- transporters in Arabidopsis, in the regulation of high-affinity NO3- uptake has been investigated in the atnrt2 mutant, where these two genes are deleted. Our initial analysis of the atnrt2 mutant (S. Filleur, M.P. Dorbe, M. Cerezo, M. Orsel, F. Granier, A. Gojon, F. Daniel-Vedele [2001] FEBS Lett 489: 220-224) demonstrated that root NO3- uptake is affected in this mutant due to the alteration of the high-affinity transport system (HATS), but not of the low-affinity transport system. In the present work, we show that the residual HATS activity in atnrt2 plants is not inducible by NO3-, indicating that the mutant is more specifically impaired in the inducible component of the HATS. Thus, high-affinity NO3- uptake in this genotype is likely to be due to the constitutive HATS. Root (NO3-)-N-15 influx in the atnrt2 mutant is no more derepressed by nitrogen starvation or decrease in the external NO3- availability. Moreover, the mutant also lacks the usual compensatory up-regulation of NO3- uptake in NO3--fed roots, in response to nitrogen deprivation of another portion of the root system. Finally, exogenous supply of NH4+ in the nutrient solution fails to inhibit (NO3-)-N-15 influx in the mutant, whereas it strongly decreases that in the wild type. This is not explained by a reduced activity of NH4+ uptake systems in the mutant. These results collectively indicate that AtNrt2.1 and/or AtNrt2.2 genes play a key role in the regulation of the high-affinity NO3- uptake, and in the adaptative responses of the plant to both spatial and temporal changes in nitrogen availability in the environment.