FRU (BHLH029) is required for induction of iron mobilization genes in Arabidopsis thaliana

Abstract

Iron mobilization responses are induced by low iron supply at transcriptional level. In tomato, the basic helix-loophelix gene FER is required for induction of iron mobilization. Using molecular-genetic techniques, we analyzed the function of BHLH029, named FRU (FER-like regulator of iron uptake), the Arabidopsis thaliana homolog of the tomato FER gene. The FRU gene was mainly expressed in roots in a cell-specific pattern and induced by iron deficiency. FRU mutant plants were chlorotic, and the FRU gene was found necessary for induction of the essential iron mobilization genes FRO2 (ferric chelate reductase gene) and IRT1 (iron-regulated transporter gene). Overexpression of FRU resulted in an increase of iron mobilization responses at low iron supply. Thus, the FRU gene is a mediator in induction of iron mobilization responses in Arabidopsis, indicating that regulation of iron uptake is conserved in dicot species.

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

“…Arabidopsis thaliana seeds were from wild type ecotype Col-0 and the mutant GABI-Kat Wt-3 = formerly fru-G108 T-DNA insertion line (further description in Jakoby et al 2004). Seeds of homozygous Wt-3 mutant plants could be obtained after germination on high iron Hoagland medium under low light conditions and continuous spraying with iron fertilizer sequestrene after transfer into soil.…”

“…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.…”

“…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).…”

“…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.…”