Volume 58 (2008) Issue 3 Pages 201-207
We previously reported two rice ammonium transporter (OsAMT) genes, OsAMT1;2 and OsAMT1;3, which show root-specific expression at the vegetative stage. To further understand the functional differences between the two genes, we used the promoter::gusA reporter gene to investigate their detailed expression patterns. We show that in transgenic rice plants carrying individual 4-kb promoter::gusA fusion genes, the expression of the two genes was confined to roots during the entire plant life cycle, suggesting the housekeeping roles of the two transporters in rice. Detailed observation of the above transgenic lines further shows that OsAMT1;3 was preferentially expressed in the apices of seminal and lateral roots under nitrogen-deficient conditions, and was repressed when supplied with ammonium. By contrast, OsAMT1;2 expression was induced most intensely in the root elongation zone under nitrogen-sufficient conditions. These observations suggest that the two genes have distinct roles in nitrogen utilization: OsAMT1;3 appears to function as a nitrogen sensor and OsAMT1;2 as an assimilator. In contrast to 4-kb promoters, gusA-reporter expression was not detected in the roots under both nitrogen-deficient and -sufficient conditions when using individual 2-kb promoter sequences proximal to transcription initiation sites for the two genes, indicating that distal 2-kb promoter sequences are essential for the root-specific expression and conditional responsiveness to nitrogen of OsAMT1;2 and OsAMT1;3 genes.