title: Identification and localisation of the rice nicotianamine aminotransferase gene OsNAAT1 expression suggests the site of phytosiderophore synthesis in rice.
abstract: Rice plants (Oryza sativa L.) take up iron using iron-chelating compounds known as mugineic acid family phytosiderophores (MAs). In the biosynthetic pathway of MAs, nicotianamine aminotransferase (NAAT) catalyses the key step from nicotianamine to the 3''-keto form. In the present study, we identified six rice NAAT genes (OsNAAT1-6) by screening a cDNA library made from Fe-deficient rice roots and by searching databases. Among the NAAT homologues, OsNAAT1 belongs to a subgroup containing barley functional NAAT (HvNAAT-A and HvNAAT-B) as well as a maize homologue cloned by cDNA library screening (ZmNAAT1). Northern blot and RT-PCR analysis showed that OsNAAT1, but not OsNAAT2-6, was strongly up-regulated by Fe deficiency, both in roots and shoots. The OsNAAT1 protein had NAAT enzyme activity in vitro, confirming that the OsNAAT1 gene encodes functional NAAT. Promoter-GUS analysis revealed that OsNAAT1 was expressed in companion and pericycle cells adjacent to the protoxylem of Fe-sufficient roots. In addition, expression was induced in all cells of Fe-deficient roots, with particularly strong GUS activity evident in the companion and pericycle cells. OsNAAT1 expression was also observed in the companion cells of Fe-sufficient shoots, and was clearly induced in all the cells of Fe-deficient leaves. These expression patterns highly resemble those of OsNAS1, OsNAS2 and OsDMAS1, the genes responsible for MAs biosynthesis for Fe acquisition. These findings strongly suggest that rice synthesizes MAs in whole Fe-deficient roots to acquire Fe from the rhizosphere, and also in phloem cells to maintain metal homeostasis facilitated by MAs-mediated long-distance transport.
authors.affiliation: Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan.
authors.forename: Haruhiko
authors.initials: H
authors.lastname: Inoue
authors.forename: Michiko
authors.initials: M
authors.lastname: Takahashi
authors.forename: Takanori
authors.initials: T
authors.lastname: Kobayashi
authors.forename: Motofumi
authors.initials: M
authors.lastname: Suzuki
authors.forename: Hiromi
authors.initials: H
authors.lastname: Nakanishi
authors.forename: Satoshi
authors.initials: S
authors.lastname: Mori
authors.forename: Naoko K
authors.initials: NK
authors.lastname: Nishizawa
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D017931:DNA Primers; D008969:Molecular Sequence Data; D012275:Oryza / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017386:Sequence Homology, Amino Acid; D017262:Siderophores / Q000096:biosynthesis*; D000637:Transaminases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D017262:Siderophores; D000637:Transaminases