Identification and localisation of the rice nicotianamine aminotransferase gene OsNAAT1 expression suggests the site of phytosiderophore synthesis in rice
Identification and localisation of the rice nicotianamine aminotransferase gene OsNAAT1 expression suggests the site of phytosiderophore synthesis in rice
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DOI：
10.1007/s11103-007-9262-8
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发表时间：
2008-01-01
影响因子：
5.1
通讯作者：
Nishizawa, Naoko K.
中科院分区：
文献类型：
--
作者：
Inoue, Haruhiko;Takahashi, Michiko;Nishizawa, Naoko K.
水稻植物（Oryza sativa L.）利用铁螯合化合物mugineic acid family phytosiderophores （MAs）吸收铁。在MAs的生物合成途径中，烟胺转氨酶（nicotianamine amintransferase， NAAT）催化了从烟胺到3 ''-酮的关键步骤。本研究通过筛选缺铁水稻根系cDNA文库和检索数据库，鉴定出6个水稻NAAT基因（OsNAAT1-6）。在NAAT同源物中，OsNAAT1属于大麦功能NAAT亚群（HvNAAT-A和HvNAAT-B），以及cDNA文库筛选获得的玉米同源物（ZmNAAT1）。Northern blot和RT-PCR分析显示，缺铁对OsNAAT1、OsNAAT2-6、osnaat2和OsNAAT2-6均有显著上调。OsNAAT1蛋白在体外具有NAAT酶活性，证实OsNAAT1基因编码功能性NAAT。启动子- gus分析显示，OsNAAT1在富铁根原生木质部附近的伴根细胞和中柱鞘细胞中表达。此外，在缺铁根的所有细胞中都能诱导表达，其中伴生细胞和中柱鞘细胞的GUS活性特别强。OsNAAT1在富铁芽的伴生细胞中也有表达，在缺铁叶的所有细胞中都有明显的诱导表达。这些表达模式与OsNAS1、OsNAS2和OsDMAS1基因高度相似，这些基因负责MAs生物合成以获取铁。这些发现有力地表明，水稻在整个缺铁根系中合成MAs以从根际获取铁，并在韧皮部细胞中合成MAs以维持MAs介导的长距离运输所促进的金属稳态。
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 synthesises 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.