Dual Regulation of the Arabidopsis High-Affinity Root Iron Uptake System by Local and Long-Distance Signals

Abstract

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 ...

Iron deficiency-mediated stress regulation of four subgroup Ib BHLH genes in Arabidopsis thaliana

“…Hence, we conclude that FIT was controlled by a diVerent systemic signal than BHLH100. FIT might have been induced on the iron supply side in response to a systemic signal produced after sensing local iron deWciency on the iron deWciency side, as was previously described for IRT1 (Vert et al 2003). Hence, FIT and IRT1 were expressed to achieve iron mobilization in the root parts where iron was most available, whereas up-regulation of BHLH100 presumably only occurred in response to stress.…”

Modulation of iron responsive gene expression and enzymatic activities in response to changes of the iron nutritional status in Cucumis sativus L.

“…We can assume that iron itself, through its movements in the xylematic and phloematic saps, may signal the iron status of the plant. It can act, according to the dual regulation model proposed by Vert et al (2003), either as a local inducer signal and as a repressive systemic signal once its concentration inside the plant reaches a satisfactory level. Whether iron acts directly or in association with other molecules is not yet 15 known.…”

“…3A and Table 1). For AtFRO2 and AtIRT1 a coordinate control of the expression was formulated (Connolly et al, 2003;Vert et al, 2003). We may extend this hypothesis also to the CsFRO1, CsIRT1, CsHA1 and the Cspepc1 (Fig.…”

“…3B). This is particularly true for what concern the CsFRO1 and CsIRT1, that are directly involved in iron acquisition, and IRT1 (Connolly et al, 2002;Vert et al, 2003).…”

“…The increase in the reduction activity after Fe supply to Fe-starved roots, is a well known response and is referred to as a substrate induction effect, i.e. iron acts as a local inducer (Vert et al, 2003). This difference in the activity of Fe(III) reduction is well visible in Figure 2B where In previous papers we have demonstrated the implication of the PEPC in the response to Fe deficiency and several hypothesis were proposed to explain the strict correlation existing between the induction of iron deficiency responses and the PEPC activation (Rabotti et al, 1995;.…”