Effects of salt stress on expression of nitrate transporter and assimilation-related genes in tomato roots

Nitrate is an essential nitrogen source for plant growth and development. The experiments on nitrate uptake by tomato (Solanum esculentum L., cv. L402) seedlings pretreated with 48-h nitrogen starvation and 24-h 75 mM NaCl stress were performed at four different NO3- concentrations. The results showed that salt stress decreased NO3- uptake regardless of the nitrate concentration. In order to study the effect of salt stress on nitrate transporters (NRT), nitrate reductase (NR), and glutamine synthetase (GS) gene expression patterns in young tomato roots, we analyzed the transcript levels of LeNRT, LeNR, and LeGS1 under 75 mM NaCl stress by the technique of real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR). The results indicated that transcripts of LeNRT1.1 and LeNRT1.2 were significantly decreased, suggesting that LeNRT1 may be at least partly responsible for the reduction in nitrate uptake. The transcription of LeNRT2.1 was also decreased; but the mRNA levels of LeNRT2.2 and LeGS1.1 were not influenced dramatically; the transcription of LeNRT2.3 was slightly increased after 12-h salt stress. LeNR transcription in tomato roots exhibited transient up-regulation at 4-h salt treatment. However, the transcription of LeGS1.2 was significantly decreased under the salt treatment. Our results suggest that the down-regulation of LeNRTJ gene expression may be mainly involved in the reduction of nitrate uptake under severe salt stress.