Major Alterations of the Regulation of Root NO3 − Uptake Are Associated with the Mutation of Nrt2.1 and Nrt2.2 Genes in Arabidopsis

Abstract

The role of AtNrt2.1 and AtNrt2.2genes, encoding putative NO₃ ⁻ transporters in Arabidopsis, in the regulation of high-affinity NO₃ ⁻ uptake has been investigated in theatnrt2 mutant, where these two genes are deleted. Our initial analysis of the atnrt2 mutant (S. Filleur, M.F. 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. Root¹⁵NO₃ ⁻ influx in theatnrt2 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 ¹⁵NO₃ ⁻ 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 thatAtNrt2.1 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.