TAKAHASHI Michiko The University of Tokyo, Graduate School of Agricultural and Life Sciences, Assistant, 大学院・農学生命科学研究科, 助手 (90345182)
NAKANISHI Hiromi The University of Tokyo, Graduate School of Agricultural and Life Sciences, Assistant, 大学院・農学生命科学研究科, 助手 (80282698)
|Budget Amount *help
¥15,000,000 (Direct Cost: ¥15,000,000)
Fiscal Year 2003: ¥5,300,000 (Direct Cost: ¥5,300,000)
Fiscal Year 2002: ¥9,700,000 (Direct Cost: ¥9,700,000)
Nicotianamine (NA), a chelator of metals, is ubiquitously present in higher plants. In graminaceous plants, NA is a biosynthetic precursor of phytosiderophores and is thus a crucial component for iron (Fe) acquisition. Here we show that three rice NA synthase (NAS) genes, OsNAS1, OsNAS2 and OsNAS3, are expressed in cells involved in long-distance transport of Fe and that the three genes are differentially regulated by Fe. OsNAS1 and OsNAS2 transcripts were detected in Fe-sufficient roots but not in leaves, and levels of both increased markedly in both roots and leaves in response to Fe deficiency. In contrast, the OsNAS3 transcript was present in leaves but was very low in roots of Fe-sufficient plants. Further, OsNAS3 expression was induced in roots but was suppressed in leaves in response to Fe deficiency. Promoter : R-glucuronidase (GUS) analysis revealed that OsNAS 1 and OsNAS2 were expressed in Fe-sufficient roots in companion cells and pericycle cells adjacent to the protoxylem.
With Fe deficiency, OsNAS1 and OsNAS2 expression extended to all root cells along with an increase in phytosiderophore secretion. In Fe-deficient plants, OsNAS1 and OsNAS2 were expressed in the vascular bundles of green leaves and in all cells of leaves showing severe chiorosis. OsNAS3 expression was restricted to the pericycle cells and companion cells of the roots, and in companion cells of leaves irrespective of Fe status. These results strongly suggested that NAS and NA play an important role in long-distance transport of Fe in rice plants in addition to their roles in phytosiderophore secretion from roots. Nicotianamine aminotransferase (NAAT) catalyzes the amino group transfer of NA in the biosynthetic pathway of phytosiderophores, and is essential for iron acquisition in graminaceous plants. The gene encoding NAAT from barley was introduced into the non-graminaceous plant, Nicotiana tabacum, which produces NA but not phytosiderophores. Transgenic tobacco plants (naat-tobacco) that constitutively expressed the NAAT gene had young leaves with interveinal chlorosis and flowers that were abnormally shaped and sterile. Endogenous NA was consumed as a result of NAAT overproduction in naat-tobacco. The resulting NA shortage caused disorders in internal metal transport, leading to these abnormal phenotypes. In addition to the role in long-distance metal transport, NA may be involved in the regulation of metal transfer within the cells. The results of this study suggest that a shortage of NA impaired the functions of metal-requiring proteins, including transcription factors. Less