Regulation of the root high-affinity iron uptake system by whole-plant signals was investigated at the molecular level in Arabidopsis, through monitoring FRO2 and IRT1 gene expression. These two genes encode the root ferric-chelate reductase and the high-affinity iron transporter, respectively, involved in the iron deficiency-induced uptake system. Recovery from iron-deficient conditions and modulation of apoplastic iron pools indicate that iron itself plays a major role in the regulation of root iron deficiency responses at the mRNA and protein levels. Split-root experiments show that the expression of IRT1 and FRO2 is controlled both by a local induction from the root iron pool and through a systemic pathway involving a shoot-borne signal, both signals being integrated to tightly control production of the root iron uptake proteins. We also show that IRT1 and FRO2 are expressed during the day and down-regulated at night and that this additional control is overruled by iron starvation, indicating that the nutritional status prevails on the diurnal regulation. Our work suggests, for the first time to our knowledge, that like in grasses, the root iron acquisition in strategy I plants may also be under diurnal regulation. On the basis of the new molecular insights provided in this study and given the strict coregulation of IRT1 and FRO2 observed, we present a model of local and long-distance regulation of the root iron uptake system in Arabidopsis.Iron is an essential element for all living organisms, including plants. As a transition metal, its ability to gain and lose an electron confers iron important properties for redox reactions. Although abundant in soils, iron often forms highly insoluble ferrichydroxide precipitates that limit its availability for plants (Guerinot and Yi, 1994). Therefore, mechanisms allowing plants to solubilize and to efficiently take up iron have evolved. However, those mechanisms are carefully regulated to prevent oxidative damages due to iron overload. Angiosperms can be divided into two groups based on their strategies of iron uptake. Non-graminaceous strategy I plants rely on acidification of the rhizosphere to increase solubility of ferric iron complexes through activation of an H ϩ -ATPase activity, transplasma membrane electron transfer to reduce iron into its more soluble ferrous form, and a transport activity to provide iron to root cells (Marschner et al., 1986). Grasses, also called strategy II plants, respond to iron insufficiency by releasing phytosiderophores, which have a strong affinity for ferric iron, and by transporting the Fe(III)-phytosiderophore complex across the plasma membrane of the root epidermal cell (Takagi et al., 1984).A great effort has recently been made to isolate genes involved in this root response to iron deficiency, especially in Arabidopsis. In this species, the acidification function is likely to be performed by a member of the H ϩ -ATPase AHA family (Palmgren, 2001), whereas the root ferric reductase that reduces ferric iron into ferrous iron in ...