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. 2014 Dec;7(1):27.
doi: 10.1186/s12284-014-0027-0. Epub 2014 Oct 7.

Iron deficiency responses in rice roots

Affiliations

Iron deficiency responses in rice roots

Takanori Kobayashi et al. Rice (N Y). 2014 Dec.

Abstract

Iron (Fe) is an essential element for most living organisms. To acquire sparingly soluble Fe from the rhizosphere, rice roots rely on two Fe acquisition pathways. The first of these pathways involves Fe(III) chelators specific to graminaceous plants, the mugineic acid family phytosiderophores, and the second involves absorption of Fe(2+). Key components in this response include enzymes involved in the biosynthesis of deoxymugineic acid (OsNAS1, OsNAS2, OsNAAT1, and OsDMAS1), the deoxymugineic acid efflux transporter (TOM1), the Fe(III)-deoxymugineic acid transporter (OsYSL15), and Fe(2+) transporters (OsIRT1, OsIRT2, and OsNRAMP1). In whole roots, these proteins are expressed in a coordinated manner with strong transcriptional induction in response to Fe deficiency. Radial transport of Fe to xylem and phloem is also mediated by the mugineic acid family phytosiderophores, as well as other chelators and their transporters, including Fe(II)-nicotianamine transporter (OsYSL2), phenolics efflux transporters (PEZ1 and PEZ2), and citrate efflux transporter (OsFRDL1). Among these, OsYSL2 is strongly induced under conditions of Fe deficiency. Both transcriptional induction and potential feedback repression mediate the expressional regulation of the genes involved in Fe uptake and translocation in response to Fe deficiency. The transcription factors IDEF1, IDEF2, and OsIRO2 are responsible for transcriptional induction, whereas the ubiquitin ligases OsHRZ1 and OsHRZ2, as well as the transcription factors OsIRO3 and OsbHLH133, are thought to mediate negative regulation. Furthermore, IDEF1 and OsHRZs bind Fe and other metals, and are therefore candidate Fe sensors. The interacting functions of these regulators are thought to fine tune the expression of proteins involved in Fe uptake and translocation.

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Figures

Figure 1
Figure 1
Fe acquisition systems in rice roots. A) Strategy II system. B) Partial Strategy I system. Ovals represent transporters and enzymes that play central roles in Fe uptake from the rhizosphere. All the indicated transporters and enzymes except PEZ2 are strongly induced in response to Fe deficiency. Broken lines indicate putative pathways. DMA, 2′-deoxymugineic acid, NA, nicotianamine, SAM, S-adenosyl-L-methionine; PCA, protocatechuic acid; CA, caffeic acid.
Figure 2
Figure 2
Fe translocation in vascular cells of rice roots. Molecules involved in xylem and phloem loading of Fe. Ovals represent transporters. Putative involvement of transporters and Fe-chelates in Fe translocation is indicated by question marks. Red arrows with broken lines indicate translocation of Fe-chelates.
Figure 3
Figure 3
Regulation of Fe deficiency responses in rice roots. Ovals indicate regulatory proteins. Boxes indicate proteins responsible for Fe uptake and translocation. All depicted proteins except IDEF1, IDEF2, and OsHORZ1 are transcriptionally induced in response to Fe deficiency. Broken lines indicate putative pathways. Line colors indicate the type of regulation: black lines, transcriptional regulation; pink lines, IDEF1 protein degradation and its inhibition; red lines, unknown mechanism of regulation occurring primarily under Fe sufficiency, which may involve protein ubiquitination by OsHRZs; blue lines, putative Fe sensing by IDEF1 and OsHRZs via direct binding of Fe and other metals; green line, putative Fe sensing by IDEF2 through an unknown mechanism.

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