Expression analysis of a high-affinity nitrate transporter isolated from Arabidopsis thaliana by differential display

We used the differential display technique on total RNAs from roots of Arab idopsis thaliana (L.) Heynh. plants which had or had not been induced for 2 h by nitrate. One isolated cDNA clone, designated Nrt2:1At, was found to c ode for a putative high-affinity nitrate transporter. Two genomic sequences homologous to Nrt2:1At were found to be localized on the same fragment of chromosome 1 in the Arabidopsis genome. Expression analyses of both low- an d high-affinity nitrate transporter genes, respectively Nrt1:1At (previousl y named Chl1) and Nrt2:1At, were carried out on plants grown under differen t nitrogen regimes. In this paper, we show that both genes are induced by v ery low levels of nitrate (50 mu M KNO3). However, stronger induction was o bserved with Nrt2:1At than with Nrt1:1At. Moreover, these two genes, althou gh both over-expressed in a nitrate-reductase-deficient mutant, were differ ently regulated when N-sufficient wild-type or mutant plants were transferr ed to an N-free medium. Indeed, the steady-state amounts of Nrt1:1At mRNA d eclined whereas the amount of Nrt2:1At mRNA increased, probably reflecting the de-repression of the high-affinity transport system during N-starvation .