The Essential Basic Helix-Loop-Helix Protein FIT1 Is Required for the Iron Deficiency Response

Abstract

Regulation of iron uptake is critical for plant survival. Although the activities responsible for reduction and transport of iron at the plant root surface have been described, the genes controlling these activities are largely unknown. We report the identification of the essential gene Fe-deficiency Induced Transcription Factor 1 (FIT1), which encodes a putative transcription factor that regulates iron uptake responses in Arabidopsis thaliana. Like the Fe(III) chelate reductase FRO2 and high affinity Fe(II) transporter IRT1, FIT1 mRNA is detected in the outer cell layers of the root and accumulates in response to iron deficiency. fit1 mutant plants are chlorotic and die as seedlings but can be rescued by the addition of supplemental iron, pointing to a defect in iron uptake. fit1 mutant plants accumulate less iron than wild-type plants in root and shoot tissues. Microarray analysis shows that expression of many (72 of 179) iron-regulated genes is dependent on FIT1. We demonstrate that FIT1 regulates FRO2 at the level of mRNA accumulation and IRT1 at the level of protein accumulation. We propose a new model for iron uptake in Arabidopsis where FRO2 and IRT1 are differentially regulated by FIT1.

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

“…Finally, single exon insertion mutants had no detectable phenotype. One of our main aims was to analyze whether these four subgroup Ib BHLH genes might be involved in a regulatory network controlling the essential iron acquisition regulator gene FIT that also belongs to the BHLH gene family (Colangelo and Guerinot 2004;Jakoby et al 2004). Our second question regarded the potential function of these four genes.…”

“…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). In contrast to that, BHLH038, BHLH039 and BHLH100 were expressed in roots in the root hair and mature root zones and in leaves, where they were induced upon iron deWciency.…”

“…At low iron supply FRO2 and IRT1 genes are induced whereas upon suYcient iron supply the genes are down regulated (Eide et al 1996;Robinson et al 1999;Vert et al 2002). A central transcriptional regulator in this process in tomato is the basic helix-loop-helix (bHLH) factor FER; the A. thaliana ortholog is encoded by the gene FIT (= fer-like iron deWciency-induced transcription factor, previously known as FRU and FIT1, gene number At2 g28160; Bauer et al 2004;Colangelo and Guerinot 2004;Jakoby et al 2004;Yuan et al 2005; FIT was renamed by Bauer, Guerinot and Ling). FER and FIT share higher sequence similarity with each other rather than with any other known bHLH protein from the two species.…”

“…FER is required for induction of iron reductase and iron transporter genes in tomato (Ling et al 2002;Bereczky et al 2003;Bauer et al 2004;Li et al 2004). FIT was found essential for the induction of FRO2 upon iron deWciency in Arabidopsis (Colangelo and Guerinot 2004;Jakoby et al 2004) as well as for induction of IRT1 (Jakoby et al 2004). In our previous studies we utilised two loss of function Wt alleles that were not capable of producing any functional FIT protein.…”

“…One allele has a T-DNA insertion in an exon (Wt-3 = fru-G108), the other allele contains a premature EMS-induced stop codon in an exon (Jakoby et al 2004). Loss of function of FER and FIT resulted in lethal leaf chlorosis due to insuYcient iron uptake in tomato and in Arabidopsis (Ling et al 2002;Colangelo and Guerinot 2004;Jakoby et al 2004). Expression of FER and FIT was induced by low iron supply compared to suYcient and/or generous iron supply suggesting that iron mobilization is controlled by a cascade of transcription factors with FER/FIT genes being part of it.…”