Iron-Nicotianamine Transporters Are Required for Proper Long Distance Iron Signaling.

The mechanisms of root iron uptake and the transcriptional networks that control root-level regulation of iron uptake have been well studied, but the mechanisms by which shoots signal iron status to the roots remain opaque. Here, we characterize an Arabidopsis (Arabidopsis thaliana) double mutant, yellow stripe1-like yellow stripe3-like (ysl1ysl3), which has lost the ability to properly regulate iron deficiency-influenced gene expression in both roots and shoots. In spite of markedly low tissue levels of iron, the double mutant does not up- and down-regulate iron deficiency-induced and -repressed genes. We have used grafting experiments to show that wild-type roots grafted to ysl1ysl3 shoots do not initiate iron deficiency-induced gene expression, indicating that the ysl1ysl3 shoots fail to send an appropriate long-distance signal of shoot iron status to the roots. We present a model to explain how impaired iron localization in leaf veins results in incorrect signals of iron sufficiency being sent to roots and affecting gene expression there. Improved understanding of the mechanism of long-distance iron signaling will allow improved strategies for the engineering of staple crops to accumulate additional bioavailable iron in edible parts, thus improving the iron nutrition of the billions of people worldwide whose inadequate diet causes iron deficiency anemia.

Brachypodium distachyon as a new model system for understanding iron homeostasis in grasses: phylogenetic and expression analysis of Yellow Stripe-Like (YSL) transporters.

BACKGROUND AND AIMS: Brachypodium distachyon is a temperate grass with a small stature, rapid life cycle and completely sequenced genome that has great promise as a model system to study grass-specific traits for crop improvement. Under iron (Fe)-deficient conditions, grasses synthesize and secrete Fe(III)-chelating agents called phytosiderophores (PS). In Zea mays, Yellow Stripe1 (ZmYS1) is the transporter responsible for the uptake of Fe(III)-PS complexes from the soil. Some members of the family of related proteins called Yellow Stripe-Like (YSL) have roles in internal Fe translocation of plants, while the function of other members remains uninvestigated. The aim of this study is to establish brachypodium as a model system to study Fe homeostasis in grasses, identify YSL proteins in brachypodium and maize, and analyse their expression profiles in brachypodium in response to Fe deficiency. METHODS: The YSL family of proteins in brachypodium and maize were identified based on sequence similarity to ZmYS1. Expression patterns of the brachypodium YSL genes (BdYSL genes) were determined by quantitative RT-PCR under Fe-deficient and Fe-sufficient conditions. The types of PS secreted, and secretion pattern of PS in brachypodium were analysed by high-performance liquid chromatography. KEY RESULTS: Eighteen YSL family members in maize and 19 members in brachypodium were identified. Phylogenetic analysis revealed that some YSLs group into a grass-specific clade. The Fe status of the plant can regulate expression of brachypodium YSL genes in both shoots and roots. 3-Hydroxy-2'-deoxymugineic acid (HDMA) is the dominant type of PS secreted by brachypodium, and its secretion is diurnally regulated. CONCLUSIONS: PS secretion by brachypodium parallels that of related crop species such as barley and wheat. A single grass species-specific YSL clade is present, and expression of the BdYSL members of this clade could not be detected in shoots or roots, suggesting grass-specific functions in reproductive tissues. Finally, the Fe-responsive expression profiles of several YSLs suggest roles in Fe homeostasis.