Thomine Sébastien, Lelièvre Françoise, Debarbieux Elise, Schroeder Julian I, Barbier-Brygoo Hélène
Institut des Sciences du Végétal, CNRS UPR2355, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France.
Plant J. 2003 Jun;34(5):685-95. doi: 10.1046/j.1365-313x.2003.01760.x.
Metal homeostasis is critical for the survival of living organisms, and metal transporters play central roles in maintaining metal homeostasis in the living cells. We have investigated the function of a metal transporter of the NRAMP family, AtNRAMP3, in Arabidopsis thaliana. A previous study showed that AtNRAMP3 expression is upregulated by iron (Fe) starvation and that AtNRAMP3 protein can transport Fe. In the present study, we used AtNRAMP3 promoter beta-glucoronidase (GUS) fusions to show that AtNRAMP3 is expressed in the vascular bundles of roots, stems, and leaves under Fe-sufficient conditions. This suggests a function in long-distance metal transport within the plant. Under Fe-starvation conditions, the GUS activity driven by the AtNRAMP3 promoter is upregulated without any change in the expression pattern. We analyze the impact of AtNRAMP3 disruption and overexpression on metal accumulation in plants. Under Fe-sufficient conditions, AtNRAMP3 overexpression or disruption does not lead to any change in the plant metal content. Upon Fe starvation, AtNRAMP3 disruption leads to increased accumulation of manganese (Mn) and zinc (Zn) in the roots, whereas AtNRAMP3 overexpression downregulates Mn accumulation. In addition, overexpression of AtNRAMP3 downregulates the expression of the primary Fe uptake transporter IRT1 and of the root ferric chelate reductase FRO2. Expression of AtNRAMP3::GFP fusion protein in onion cells or Arabidopsis protoplasts shows that AtNRAMP3 protein localizes to the vacuolar membrane. To account for the results presented, we propose that AtNRAMP3 influences metal accumulation and IRT1 and FRO2 gene expression by mobilizing vacuolar metal pools to the cytosol.
金属稳态对于生物体的生存至关重要，金属转运蛋白在维持活细胞中的金属稳态方面发挥着核心作用。我们研究了拟南芥中天然抗性相关巨噬细胞蛋白（NRAMP）家族的一种金属转运蛋白AtNRAMP3的功能。先前的一项研究表明，AtNRAMP3的表达受铁（Fe）饥饿诱导上调，且AtNRAMP3蛋白能够转运铁。在本研究中，我们使用AtNRAMP3启动子与β-葡萄糖醛酸酶（GUS）的融合体，以证明在铁充足的条件下，AtNRAMP3在根、茎和叶的维管束中表达。这表明其在植物体内长距离金属运输中发挥作用。在铁饥饿条件下，由AtNRAMP3启动子驱动的GUS活性上调，而表达模式没有任何变化。我们分析了AtNRAMP3基因敲除和过表达对植物中金属积累的影响。在铁充足的条件下，AtNRAMP3过表达或基因敲除不会导致植物金属含量发生任何变化。在铁饥饿时，AtNRAMP3基因敲除导致根中锰（Mn）和锌（Zn）的积累增加，而AtNRAMP3过表达则下调锰的积累。此外，AtNRAMP3过表达下调了主要铁吸收转运蛋白IRT1和根铁螯合物还原酶FRO2的表达。AtNRAMP3::GFP融合蛋白在洋葱细胞或拟南芥原生质体中的表达表明，AtNRAMP3蛋白定位于液泡膜。为了解释所呈现的结果，我们提出AtNRAMP3通过将液泡中的金属池转运到细胞质中，影响金属积累以及IRT1和FRO2基因的表达。
