Overexpression of the FRO2 Ferric Chelate Reductase Confers Tolerance to Growth on Low Iron and Uncovers Posttranscriptional Control

Abstract

The Arabidopsis FRO2 gene encodes the low-iron-inducible ferric chelate reductase responsible for reduction of iron at the root surface. Here, we report that FRO2 and IRT1, the major transporter responsible for high-affinity iron uptake from the soil, are coordinately regulated at both the transcriptional and posttranscriptional levels. FRO2 and IRT1 are induced together following the imposition of iron starvation and are coordinately repressed following iron resupply. Steady-state mRNA levels of FRO2 and IRT1 are also coordinately regulated by zinc and cadmium. Like IRT1, FRO2 mRNA is detected in the epidermal cells of roots, consistent with its proposed role in iron uptake from the soil. FRO2 mRNA is detected at high levels in the roots and shoots of 35S-FRO2 transgenic plants. However, ferric chelate reductase activity is only elevated in the 35S-FRO2 plants under conditions of iron deficiency, indicating that FRO2 is subject to posttranscriptional regulation, as shown previously for IRT1. Finally, the 35S-FRO2 plants grow better on low iron as compared with wild-type plants, supporting the idea that reduction of ferric iron to ferrous iron is the rate-limiting step in iron uptake.Iron is an essential element for plants. It functions to accept and donate electrons and thus serves as an important cofactor for a number of metalloenzymes involved in respiration and photosynthesis. Although iron is abundant in the earth's crust, it is often unavailable to plants because it tends to form insoluble ferric hydroxide complexes in aerobic environments at neutral or basic pH (Guerinot and Yi, 1994). In addition to the solubility problem, the chemical properties of iron require cells to place limitations on its accumulation. Fe(II) and Fe(III) act catalytically to generate hydroxyl radicals that can damage cellular constituents such as DNA and lipids (Halliwell and Gutteridge, 1992). Thus, iron uptake (Eide et al., 1996;Robinson et al., 1999;Connolly et al., 2002) and storage (Lescure et al., 1991;Briat and Lobréaux, 1997;Wei and Theil, 2000) are carefully regulated processes.Dicots and non-grass monocots employ the strategy I response to mobilize iron from the soil (Marschner and Rö mheld, 1994). This response includes release of protons and subsequent acidification of the rhizosphere, which serves to drive more Fe(III) into solution, reduction of Fe(III) to Fe(II) at the root surface, and transport of Fe(II) across the root epidermal cell membrane. We have previously identified the genes in Arabidopsis that encode the ironregulated ferric chelate reductase (FRO2) and the ferrous iron transporter (IRT1) that function to take up iron from the soil as part of the strategy I response (Eide et al., 1996;Yi and Guerinot, 1996;Robinson et al., 1999;Vert et al., 2002). FRO2 is predicted to encode a polypeptide of 725 amino acids, with conserved FAD-and NADPH-binding sites. FRO2 has six hydrophobic domains within its amino-terminal region along with two additional carboxyl-terminal hydrophobic domains, all of which are predict...

Increased de novo riboflavin synthesis and hydrolysis of FMN are involved in riboflavin secretion from Hyoscyamus albus hairy roots under iron deficiency

“…In fact, up-regulation of H + -ATPase I transcript accumulation in M. truncatula roots [20] and of ferric chelate reductase (FRO2) and ferrous iron transporter (IRT) transcript accumulations in A. thaliana roots have been reported to occur within 1 day under Fe deficiency [21]. On the other hand, H. albus roots did not exhibit detectable riboflavin secretion until 3 days from transfer to Fe-deficient conditions (Fig.…”

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

“…0 Fe means 0 M Fe plus 50 M ferrozine. Mean values § SD were determined for three to eight measurements tip and in the upper root zones, induced by low iron and down-regulated in Wt loss of function mutants (Eide et al 1996;Robinson et al 1999;Vert et al 2002;Connolly et al 2003;Colangelo and Guerinot 2004;Jakoby et al 2004). FIT was also mainly expressed in roots near the root tip and in the upper root zones, induced by low iron and its promoter was down regulated in Wt loss of function mutants (Colangelo and Guerinot 2004;Jakoby et al 2004;this work).…”

Iron deficiency induces changes in riboflavin secretion and the mitochondrial electron transport chain in hairy roots of Hyoscyamus albus

“…In parallel with this change, biochemical changes have been observed, including proton extrusion by the activation of a proton-pump H + -ATPase, thereby reducing soil pH and increasing the solubility of Fe 3+ Waters et al, 2002;Hell and Stephan, 2003;Schmidt, 2003;Santi et al, 2005). Concomitantly, organic acids such as citric and malic acids that act as Fe 3+ -chelators (Ryan et al, 2001;Dakora and Phillips, 2002) are released, chelated Fe 3+ is reduced to Fe 2+ by the action of a membrane-bound Fe 3+ chelate reductase, and then Fe 2+ is transported into cells by a Fe-regulated transporter (Waters et al, 2002;Connolly et al, 2003). To support all these processes directly connected with the enhancement of Fe uptake, a supply of ATP as well as of 5 reducing agents such as NAD(P)H is essential (Zocchi, 2007).…”