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

Abstract

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.

Bypassing Iron Storage in Endodermal Vacuoles Rescues the Iron Mobilization Defect in the natural resistance associated-macrophage protein3natural resistance associated-macrophage protein4 Double Mutant

“…S4). Previous studies have shown that the Dccc1 growth defect may be rescued by transformation with AtVIT1 and its homologs AtVTL1 (Gollhofer et al, 2014), LeVIT1 (Kim et al, 2006), TgVIT1 (Momonoi et al, 2009), and OsVIT1 or OsVIT2 (Zhang et al, 2012). Expression in yeast indicates that the G77D mutation prevents Dccc1 rescue by AtVIT1 but does not alter the AtVIT1 protein level or subcellular localization (Fig.…”

“…VTL gene expression is down-regulated under Fe deficiency (Gollhofer et al, 2011). At least one of the VTLs also functions in Fe sequestration in the vacuole (Gollhofer et al, 2014). VIT1 homologs have been identified in most plant species for which genome sequencing data are available.…”

A WRKY Transcription Factor Regulates Fe Translocation under Fe Deficiency

“…Nodulin-like3/21) increased the ratios of shoot-to-root Fe content, suggesting that these nodulin-like genes are probably involved in Fe distribution between shoots and roots (Gollhofer et al, 2011). VITL1 (also named Nodulin-like1) localized to the vacuolar membrane and is responsible for Fe uptake into vacuoles (Gollhofer et al, 2011(Gollhofer et al, , 2014. We speculate that VITL1 is the major contribution to the root-toshoot Fe translocation regulated by WRKY46.…”

“…NAS2 encodes an enzyme that synthesizes nicotianamine and is responsible for Fe transport inside the body of plants (Schuler et al, 2012). VITL1 encodes a potential Fe transporter involved in Fe uptake into vacuole (Gollhofer et al, 2014) and is proposed to function in Fe homeostasis in vascular tissues (Gollhofer et al, 2011). To confirm whether these two genes (NAS2 and VITL1) are indeed regulated by WRKY46, we detected their expression in wild-type and wrky46-1 mutant plants under Fe-sufficient or -deficient conditions.…”

“…Arabidopsis ferroportin1/iron regulated 1 (AtFPN1/ AtIREG1), a potential novel effluxer, is expected to be responsible for free Fe transport into xylem, although the transport activity for IREG1 has not been reported (Morrissey et al, 2009). Besides, a group of nodulin-like genes, whose expression is dramatically down-regulated by Fe deficiency, show high similarity of protein sequence to AtVIT1 (vacuolar Fe uptake transporter 1; Kim et al, 2006) and are thought to function in Fe homeostasis in root vascular tissue, thus controlling the distribution of Fe between roots and shoots (Gollhofer et al, 2011(Gollhofer et al, , 2014. Nevertheless, little is known about the molecular regulatory mechanism of Fe translocation in plants.…”