Search

Iron deficiency is a worldwide agricultural problem on calcareous soils with low iron (Fe) availability and also poses a major human nutritional problem. Plants induce Fe-acquisition systems under conditions of low Fe availability. Previously, we reported that an Fe deficiency-inducible bHLH transcription factor, OsIRO2, is responsible for regulating the genes involved in Fe homeostasis in rice. Here, we show that OsIRO2 overexpression results in improved tolerance to low Fe availability in calcareous soil. In addition to increased Fe content in shoots, rice overexpressing OsIRO2 accumulates more Fe in seeds than non-transformant rice when grown on calcareous soil. OsIRO2 promoter–GUS analysis revealed that OsIRO2 expression could be detected in developing seeds. These results suggest that OsIRO2 plays a crucial role in regulating both Fe uptake from soil and Fe translocation to grain. Improved uptake and translocation of Fe in the OsIRO2-overexpressing rice under low Fe availability provides an approach for producing graminaceous crops that are tolerant to Fe deficiency and have Fe-rich seeds to enhance production and the nutritional quality of food.

Iron (Fe) is essential for most living organisms, and thus Fe deficiency poses a major abiotic stress in crop production. Plants induce Fe utilization systems under conditions of low-Fe availability, but the molecular mechanisms of gene regulation under Fe deficiency remain largely unknown. We identified a novel transcription factor in rice and barley, IDEF2, which specifically binds to the Fe deficiency-responsive cis-acting element 2 (IDE2), using yeast one-hybrid screening. IDEF2 belongs to an uncharacterized branch of the NAC transcription factor family and exhibits novel properties of sequence recognition. An electrophoretic mobility shift assay and cyclic amplification and selection of targets (CASTing) experiment revealed that IDEF2 predominantly recognizes CA[A/C]G[T/C][T/C/A][T/C/A] within IDE2 as the core binding site. IDEF2 transcripts are constitutively present in rice roots and leaves. Repression of the function of IDEF2 by RNA interference (RNAi) and chimeric repressor gene-silencing technology (CRES-T) caused aberrant Fe distribution between shoots and roots in rice grown hydroponically under Fe-sufficient or deficient conditions. These results reveal novel cis-element/trans-factor interactions that are functionally associated with iron homeostasis.

Although the genes involved in metal homeostasis have been investigated over the past few decades, many genes related to metal homeostasis remain uncharacterized, and a comprehensive analysis of the expression of these genes is required., In the present study, we investigated the spatial gene expression profile of iron (Fe)-deficient and cadmium (Cd)-stressed Oryza sativa (rice) using laser microdissection and microarray analysis. Roots of Fe-deficient and Cd-stressed rice were separated into the vascular bundle, cortex, and epidermis plus exodermis. In addition, vascular bundles from new and old leaves at the lowest node, which are important for metal distribution, were analyzed separately., The spatial expression patterns were distinct in each tissue type. Fe deficiency and Cd stress also had significant effects on the transcriptomes, although these were less pronounced than the spatial effects. Genes encoding transporters involved in metal homeostasis, proteins associated with heavy metal detoxification, and phytohormone-related proteins were comprehensively investigated. Additionally, cis motifs involved in the regulation of these diverse expression changes in various tissue types were predicted., The spatial transcriptomes presented here provide novel insight into the molecular mechanisms of metal homeostasis. [ABSTRACT FROM AUTHOR]

To clarify the molecular mechanism that regulates iron (Fe) acquisition in graminaceous plants, a time-course analysis of gene expression during Fe deficiency stress was conducted using a rice 22K oligo-DNA microarray. Twenty-one genes for proteins that function in gene regulation were induced by Fe deficiency. Of these genes, a putative basic helix–loop–helix (bHLH) transcription factor gene, named OsIRO2, was strongly expressed in both roots and shoots during Fe deficiency stress. The expression of OsIRO2 was induced exclusively by Fe deficiency, and not by deficiencies in other metals. Expression of the barley HvIRO2 gene, which is a homologue of OsIRO2, was also induced by Fe deficiency. An in silico search revealed that IRO2 is highly conserved among graminaceous plants, which include wheat, sorghum, and maize. The cyclic amplification and selection of targets (CASTing) technique revealed that OsIRO2 bound preferentially to the sequence 5′-ACCACGTGGTTTT-3′, and the electrophoretic mobility shift assay revealed 5′-CACGTGG-3′ as the core sequence for OsIRO2 binding. Sequences similar to the OsIRO2-binding sequence were found upstream of several genes that are involved in Fe acquisition, such as OsNAS1, OsNAS3, OsIRT1, OsFDH, OsAPT1, and IDS3. The core sequence of the OsIRO2-binding sequence occurred more frequently in the upstream regions of Fe deficiency-inducible genes than in the corresponding regions of non-inducible genes. These results suggest that IRO2 is involved in the regulation of gene expression under Fe-deficient conditions. [ABSTRACT FROM PUBLISHER]

Iron is essential for most living organisms, and thus iron deficiency poses a major abiotic stress in crop production. Plants induce iron utilization systems under conditions of low iron availability, but the molecular mechanisms of gene regulation under iron deficiency remain largely unknown. We identified a novel transcription factor of rice and barley, IDEF2, which specifically binds to the iron deficiency-responsive cis-acting element 2 (IDE2) by yeast one-hybrid screening. IDEF2 belongs to an uncharacterized branch of the NAC transcription factor family and exhibits novel properties of sequence recognition. An electrophoretic mobility shift assay and cyclic amplification and selection of targets experiment revealed that IDEF2 predominantly recognized CA(A/C)G(T/C)(T/C/A)(T/C/A) within IDE2 as the core-binding site. IDEF2 transcripts are constitutively present in rice roots and leaves. Repression of the function of IDEF2 by the RNA interference (RNAi) technique and chimeric repressor gene-silencing technology (CRES-T) caused aberrant iron homeostasis in rice. Several genes up-regulated by iron deficiency, including the Fe(II)-nicotianamine transporter gene OsYSL2, were less induced by iron deficiency in the RNAi rice of IDEF2, suggesting that IDEF2 is involved in the regulation of these genes. Many genes with repressed expression in IDEF2 RNAi rice possessed the IDEF2-binding core sites in their promoters, and the flanking sequences were also highly homologous to IDE2. IDEF2 bound to OsYSL2 promoter region containing the binding core site, suggesting direct regulation of OsYSL2 expression. These results reveal novel cis-element/trans-factor interactions functionally associated with iron homeostasis. [ABSTRACT FROM AUTHOR]

Iron (Fe) deficiency is a major abiotic stress in crop production. Although responses to Fe deficiency in graminaceous plants, such as increased production and secretion of mugineic acid family phytosiderophores (MAs), have been described, the gene regulation mechanisms related to these responses are largely unknown. To elucidate the regulation mechanisms of the genes related to Fe acquisition in graminaceous plants, we characterized the Fe-deficiency-inducible basic helix-loop-helix transcription factor OsIRO2 in rice. In yeast cells, OsIRO2 functioned as a transcriptional activator. In rice, overexpression of OsIRO2 resulted in increased MAs secretion, whereas repression of OsIRO2 resulted in lower MAs secretion and hypersensitivity to Fe deficiency. Northern blots revealed that the expression of the genes involved in the Fe(III)-MAs transport system was dependent on OsIRO2. The expression of the genes for nicotianamine synthase, a key enzyme in MAs synthesis, was notably affected by the level of OsIRO2 expression. Microarray analysis demonstrated that OsIRO2 regulates 59 Fe-deficiency-induced genes in roots. Some of these genes, including two transcription factors upregulated by Fe deficiency, possessed the OsIRO2 binding sequence in their upstream regions. OsIRO2 possesses a homologous sequence of the Fe-deficiency-responsive cis-acting elements (IDEs) in its upstream region. We propose a novel gene regulation network for Fe-deficiency responses, including OsIRO2, IDEs and the two transcription factors.