Vacuolar-Iron-Transporter1-Like Proteins Mediate Iron Homeostasis in Arabidopsis

Iron deficiency is a nutritional problem in plants and reduces crop productivity, quality and yield. With the goal of improving the iron (Fe) storage properties of plants, we have investigated the function of three Arabidopsis proteins with homology to Vacuolar Iron Transporter1 (AtVIT1). Heterologous expression of Vacuolar Iron Transporter-Like1 (AtVTL1; At1g21140), AtVTL2 (At1g76800) or AtVTL5 (At3g25190) in the yeast vacuolar Fe transport mutant, Δccc1, restored growth in the presence of 4 mM Fe. Isolated vacuoles from yeast expressing either of the VTL genes in the Δccc1 background had a three- to four-fold increase in Fe concentration compared to vacuoles isolated from the untransformed mutant. Transiently expressed GFP-tagged AtVTL1 was localized exclusively and AtVTL2 was localized primarily to the vacuolar membrane of onion epidermis cells. Seedling root growth of the Arabidopsis nramp3/nramp4 and vit1-1 mutants was decreased compared to the wild type when seedlings were grown under Fe deficiency. When expressed under the 35S promoter in the nramp3/nramp4 or vit1-1 backgrounds, AtVTL1, AtVTL2 or AtVTL5 restored root growth in both mutants. The seed Fe concentration in the nramp3/nramp4 mutant overexpressing AtVTL1, AtVTL2 or AtVTL5 was between 50 and 60% higher than in non-transformed double mutants or wild-type plants. We conclude that the VTL proteins catalyze Fe transport into vacuoles and thus contribute to the regulation of Fe homeostasis in planta.

缺铁是植物面临的一类营养障碍问题，会降低作物的生产性能、品质与籽粒产量。为改良植物的铁（Fe）储存特性，我们对3种与液泡铁转运蛋白1（Vacuolar Iron Transporter1, AtVIT1）同源的拟南芥蛋白功能开展了研究。我们将液泡铁转运蛋白样1（Vacuolar Iron Transporter-Like1, AtVTL1; At1g21140）、AtVTL2（At1g76800）或AtVTL5（At3g25190）在酵母液泡铁转运缺陷突变体Δccc1中进行异源表达，结果显示其在4 mM铁浓度条件下可恢复酵母生长。相较于未转化的突变体，在Δccc1遗传背景中表达任一VTL基因的酵母所分离得到的液泡，其铁浓度提升了3至4倍。瞬时表达绿色荧光蛋白（Green Fluorescent Protein, GFP）标签融合的AtVTL1时，其仅定位于洋葱表皮细胞的液泡膜；而AtVTL2则主要定位于该膜结构。在缺铁培养条件下，拟南芥nramp3/nramp4与vit1-1突变体的幼苗根系生长相较于野生型受到抑制。当在nramp3/nramp4或vit1-1遗传背景中通过35S启动子驱动表达AtVTL1、AtVTL2或AtVTL5时，两种突变体的根系生长均得到恢复。过表达上述任一VTL基因的nramp3/nramp4突变体，其种子铁浓度较未转化的双突变体或野生型植株高出50%至60%。综上，我们认为VTL蛋白可催化铁向液泡内的转运，从而参与调控植物体内的铁稳态。