Recent insights into iron homeostasis and their application in graminaceous crops

Abstract

Higher plants utilize various mechanisms to maintain iron homeostasis. To acquire sparingly soluble iron from the rhizosphere, graminaceous plants synthesize natural iron (III) chelators known as mugineic acid family phytosiderophores (MAs). Recent research has uncovered various genes involved in iron uptake and translocation, as well as factors regulating the expression of these genes, especially in rice. Manipulation of these molecular components is used to produce transgenic crops with enhanced tolerance to iron deficiency, or with a high seed iron content. Since iron homeostasis is closely linked to that of other mineral elements, an understanding of this phenomenon will serve as the basis for the production of crops with low concentrations of toxic metals and transgenic plants for phytoremediation.

Figure 1.

The biosynthetic pathway of MAs in graminaceous plants. SAMS, S-adenosylmethionine synthetase; NAS, nicotianamine synthase; NAAT, nicotianamine aminotransferase; DMAS, deoxymugineic acid synthase; IDS2, iron-deficiency-specific clone no. 2; IDS3, iron-deficiency-specific clone no. 3; NA, nicotianamine; DMA, 2′-deoxymugineic acid; MA, mugineic acid; HMA, 3-hydroxymugineic acid; epiHDMA, 3-epihydroxy-2′-deoxymugineic acid; epiHMA, 3-epihydroxymugineic acid. To date, four other MAs have been identified.

Figure 2.

A simplified scheme of the uptake and translocation of iron in rice. Ovals represent transporters. Iron flow is depicted in red arrows. Xylem loading and phloem unloading of iron would require efflux-type transporters, which are little characterized.

Figure 3.

Localization of transporter gene expression in germinating seeds, as observed by histochemical staining of GUS activity in the OsYSL2 (A), OsIRT1 (B) or OsYSL15 (C) promoter-GUS transgenic rice lines in fully mature seeds (0 d) and seeds 1–3 days after sowing.

Figure 4.

Model of the gene regulatory network mediated by IDEF1, IDEF2, and OsIRO2 in response to iron deficiency. Dotted lines indicate possible regulation. Ovals indicate transcription factors. All of the indicated genes except IDEF1 and IDEF2 are transcriptionally induced in response to iron deficiency.

Figure 5.

Iron deficiency tolerance of transgenic rice plants in a calcareous paddy field. (A) Photograph of the entire field at 35 days after transplanting. The rice lines tested are those transformed with barley genome fragment(s) containing (i) IDS3, (ii) HvNAS1, and (iii) HvNAS1, HvNAAT-A and -B, as well as non-transformants. (B) Visual comparison between IDS3-transformed line (right) and non-transformants (left) at 44 days after transplanting, as illustrated in the red box in (A).