Reference "Identification and localisation of the rice nicotianamine aminotransferase gene OsNAAT1 expression suggests the site of phytosiderophore synthesis in rice"

Reference ID

25479

Title

Identification and localisation of the rice nicotianamine aminotransferase gene OsNAAT1 expression suggests the site of phytosiderophore synthesis in rice

Source

Plant molecular biology, 2008, vol. 66, pp. 193-203

Authors (7)

Inoue-H	Takahashi-M
Kobayashi-T	Suzuki-M
Nakanishi-H	Mori-S
Nishizawa-N-K

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.