Oomen Ronald J F J, Wu Jian, Lelièvre Françoise, Blanchet Sandrine, Richaud Pierre, Barbier-Brygoo Hélène, Aarts Mark G M, Thomine Sébastien
Institut des Sciences du Végétal, CNRS, Avenue de la Terrasse, Gif-sur-Yvette, France.
New Phytol. 2009;181(3):637-50. doi: 10.1111/j.1469-8137.2008.02694.x. Epub 2008 Nov 27.
The ability of metal hyperaccumulating plants to tolerate and accumulate heavy metals results from adaptations of metal homeostasis. NRAMP metal transporters were found to be highly expressed in some hyperaccumulating plant species. Here, we identified TcNRAMP3 and TcNRAMP4, the closest homologues to AtNRAMP3 and AtNRAMP4 in Thlaspi caerulescens and characterized them by expression analysis, confocal imaging and heterologous expression in yeast and Arabidopsis thaliana. TcNRAMP3 and TcNRAMP4 are expressed at higher levels than their A. thaliana homologues. When expressed in yeast TcNRAMP3 and TcNRAMP4 transport the same metals as their respective A. thaliana orthologues: iron (Fe), manganese (Mn) and cadmium (Cd) but not zinc (Zn) for NRAMP3; Fe, Mn, Cd and Zn for NRAMP4. They also localize at the vacuolar membrane in A. thaliana protoplasts. Inactivation of AtNRAMP3 and AtNRAMP4 in A. thaliana results in strong Cd and Zn hypersensitivity, which is fully rescued by TcNRAMP3 or TcNRAMP4 expression. However, metal tolerance conferred by TcNRAMP expression in nramp3nramp4 mutant does not exceed that of wild-type A. thaliana. Our data indicate that the difference between TcNRAMP3 and TcNRAMP4 and their A. thaliana orthologues does not lie in a different protein function, but probably resides in a different expression level or expression pattern.
金属超积累植物耐受和积累重金属的能力源于金属稳态的适应性变化。研究发现，天然抗性相关巨噬蛋白（NRAMP）金属转运蛋白在一些超积累植物物种中高度表达。在此，我们鉴定了遏蓝菜中与拟南芥AtNRAMP3和AtNRAMP4最相近的同源基因TcNRAMP3和TcNRAMP4，并通过表达分析、共聚焦成像以及在酵母和拟南芥中的异源表达对它们进行了表征。TcNRAMP3和TcNRAMP4的表达水平高于其拟南芥同源基因。在酵母中表达时，TcNRAMP3和TcNRAMP4转运的金属与其各自拟南芥直系同源基因相同：NRAMP3转运铁（Fe）、锰（Mn）和镉（Cd），但不转运锌（Zn）；NRAMP4转运Fe、Mn、Cd和Zn。它们也定位于拟南芥原生质体的液泡膜上。拟南芥中AtNRAMP3和AtNRAMP4的失活导致对镉和锌的高度敏感，而TcNRAMP3或TcNRAMP4的表达可完全挽救这种敏感性。然而，在nramp3nramp4突变体中，TcNRAMP表达赋予的金属耐受性不超过野生型拟南芥。我们的数据表明，TcNRAMP3和TcNRAMP4与其拟南芥直系同源基因之间的差异不在于蛋白质功能不同，而可能在于表达水平或表达模式不同。
