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 AtNrt2.2 genes, encoding putative NO 3 Ϫ transporters in Arabidopsis, in the regulation of high-affinity NO 3 Ϫ uptake has been investigated in the atnrt2 mutant, where these two genes are deleted. Our initial analysis of the atnrt2 mutant (S. Filleur, M. Ϫ 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 NO 3 Ϫ , indicating that the mutant is more specifically impaired in the inducible component of the HATS. Thus, high-affinity NO 3 Ϫ uptake in this genotype is likely to be due to the constitutive HATS. Root 15 NO 3 Ϫ influx in the atnrt2 mutant is no more derepressed by nitrogen starvation or decrease in the external NO 3 Ϫ availability. Moreover, the mutant also lacks the usual compensatory up-regulation of NO 3 Ϫ uptake in NO 3 Ϫ -fed roots, in response to nitrogen deprivation of another portion of the root system. Finally, exogenous supply of NH 4 ϩ in the nutrient solution fails to inhibit 15 NO 3 Ϫ influx in the mutant, whereas it strongly decreases that in the wild type. This is not explained by a reduced activity of NH 4 ϩ 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 NO 3 Ϫ uptake, and in the adaptative responses of the plant to both spatial and temporal changes in nitrogen availability in the environment. The uptake of NO 3Ϫ by roots cells is a key process for higher plants because it is the first step of the assimilatory pathway providing most of organic nitrogen required for synthesis of biomolecules, including proteins and nucleic acids (Beevers and Hageman, 1980). More than 30 years of physiological investigations have led to the conclusion that at least three uptake systems are responsible for the influx of NO 3 Ϫ into the roots (for review, see Clarkson, 1986;Glass and Siddiqi, 1995;Crawford and Glass, 1998;Daniel-Vedele et al., 1998;Forde, 2000). Two highaffinity transport systems (HATS) are able to take up NO 3 Ϫ at low concentrations in the external medium, and display saturable kinetics as a function of the external NO 3 Ϫ concentration ([NO 3 Ϫ ] o ), with saturation in the range of 0.2 to 0.5 mm [NO 3 Ϫ ] o . One of these systems appears to be present even in plants never supplied with NO 3 Ϫ , and thus is considered as constitutive (cHATS). The other HATS is specifically stimulated by NO 3 Ϫ , and is consequently assumed to be inducible (iHATS). The maximum activity (V max ) recorded for the iHATS is generally much larger than that of the cHATS, suggesting that the former system plays a key role in the root uptake of NO 3 Ϫ from external media where [NO 3 Ϫ ] o does not exceed 1 mm. The iHATS and cHATS appear to be genetically distinct because a mutant defective in the cHATS, but not in the iHATS, has been isolated in Arabidopsis (Wang and Crawford, 1996)....

Identification of genes associated with perianth senescence in Daffodil (Narcissus pseudonarcissus L. ‘Dutch Master’)

“…Some of these, such as the nitrate transporters, have been isolated from the roots of A. thaliana where they were shown to mediate nitrate uptake from the soil [43]. No nitrate transporters have been characterized in senescing flowers.…”

Nitrate transporters and peptide transporters

“…AtNRT2.1 and AtNRT2.2 are next to each other on the chromosome, and both are involved in high affinity nitrate uptake [26][27][28]. In the nrt2.1 and nrt2.2 mutants, iHATS is reduced by 50-72% and 19%, respectively, indicating that AtNRT2.1 plays a more dominant role and AtNRT2.2 a minor role in iHATS [28].…”

“…This could be due to there being multiple genes involved in nitrate uptake. For example, in Arabidopsis, AtNRT1.1 (CHL1), AtNRT2.1, and AtNRT2.2 are known to be involved in high-affinity nitrate uptake [1,24,[26][27][28], and AtNRT1.1 (CHL1) and AtNRT1.2 are known to be involved in low-affinity nitrate , and HvPTR1 were performed using the BLOSUM protein weight matrix and the phylogenetic tree was constructed using the neighbor-joining method of the ClustalX program [87]. The tree was displayed and manipulated using the MEGA3 program [88].…”

TheArabidopsisNitrate Transporter NRT1.8 Functions in Nitrate Removal from the Xylem Sap and Mediates Cadmium Tolerance

“…The NRT2 family consists of seven members in the Arabidopsis genome. NRT2.1 and NRT2.2 are involved in inducible high-affinity nitrate uptake (Cerezo et al, 2001;Li et al, 2007). Though functionally and phylogenetically distinct, the nitrate transport functions of both NRT1 and NRT2 are believed to be proton dependent (Paulsen and Skurray, 1994;Orsel et al, 2002a).…”