Expression of rice (Oryza sativa L.) genes involved in high-affinity nitrate transport during the period of nitrate induction

When breeding crops that utilize nitrogen efficiently, it is important to reveal the regulation of nitrate uptake at the molecular level. We focused on the expression of two nitrate uptake-related genes, NRT2 and NAR2, and nitrate uptake during the nitrate induction. Four rice NRT2s (OsNRT2.1 similar to 2.4) and two NAR2s (OsNAR2.1 similar to 2.2) were identified in the rice genome database. We analyzed the expression of the genes in the roots and shoots after supply of nitrate with or without ammonium pretreatment by the RT-PCR method. The apparent nitrate uptake was also measured. Differential expression of OsNRT2.1 and OsNRT2.2, which have the same ORF with different 5'- and 3'-UTR, was observed in the seedlings pretreated with ammonium. The apparent nitrate uptake synchronously increased after supply of nitrate with the expression pattern of OsNRT2.1 in roots either with or without ammonium pretreatment. The expression of OsNAR2.1, which is thought to be an activator gene for the function of NRT2, was also coordinated with that of OsNRT2.1. On the other hand, OsNRT2.2 was already expressed after ammonium pretreatment, suggesting that OsNRT2.2 would be transcribed to take up nitrate in flooded soils. The present results indicate that OsNRT2.1, OsNRT2.2 and OsNAR2.1 are candidate genes when breeding for efficient use of nitrate in rice.