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Deoxymugineic acid (DMA) is a member of the mugineic acid family phytosiderophores (MAs), which are natural metal chelators produced by graminaceous plants. Rice secretes DMA in response to Fe deficiency to take up Fe in the form of Fe(III)-MAs complex. In contrast with barley, the roots of which secrete MAs in response to Zn deficiency, the amount of DMA secreted by rice roots was slightly decreased under conditions of low Zn supply. There was a concomitant increase in endogenous DMA in rice shoots, suggesting that DMA plays a role in the translocation of Zn within Zn-deficient rice plants. The expression of OsNAS1 and OsNAS2 was not increased in Zn-deficient roots but that of OsNAS3 was increased in Zn-deficient roots and shoots. The expression of OsNAAT1 was also increased in Zn-deficient roots and dramatically increased in shoots; correspondingly, HPLC analysis was unable to detect nicotianamine in Zn-deficient shoots. The expression of OsDMAS1 was increased in Zn-deficient shoots. Analyses using the positron-emitting tracer imaging system (PETIS) showed that Zn-deficient rice roots absorbed less (62)Zn-DMA than (62)Zn(2+). Importantly, supply of (62)Zn-DMA rather than (62)Zn(2+) increased the translocation of (62)Zn into the leaves of Zn-deficient plants. This was especially evident in the discrimination center (DC). These results suggest that DMA in Zn-deficient rice plants has an important role in the distribution of Zn within the plant rather than in the absorption of Zn from the soil.

Zinc (Zn), an essential nutrient in cells, plays a vital role in controlling cellular processes such as growth, development, and differentiation. Although the mechanisms of Zn translocation in rice plants (Oryza sativa) are not fully understood, it has recently received increased interest. OsZIP4 is a Zn transporter that localizes to apical cells. Transgenic rice plants overexpressing the OsZIP4 gene under the control of the cauliflower mosaic virus (CaMV) 35S promoter were produced. The Zn concentration in roots of 35S-OsZIP4 transgenic plants was 10 times higher than in those of vector controls, but it was five times lower in shoots. The Zn concentration in seeds of 35S-OsZIP4 plants was four times lower compared with vector controls. Northern blot analysis and quantitative real-time reverse transcription-PCR revealed transcripts of OsZIP4 expression driven by the CaMV 35S promoter in roots and shoots of 35S-OsZIP4 plants, but levels of endogenous OsZIP4 transcripts were low in roots and high in shoots compared with vector controls. Microarray analysis revealed that the genes expressed in shoots of 35S-OsZIP4 plants coincided with those induced in shoots of Zn-deficient plants. These results indicate that constitutive expression of OsZIP4 changes the Zn distribution within rice plants, and that OsZIP4 is a critical Zn transporter that must be strictly regulated.

Mugineic acid family phytosiderophores (MAs) are metal chelators that are produced in graminaceous plants in response to iron (Fe) deficiency, but current evidence regarding secretion of MAs during zinc (Zn) deficiency is contradictory. Our studies using HPLC analysis showed that Zn deficiency induces the synthesis and secretion of MAs in barley plants. The levels of the HvNAS1, HvNAAT-A, HvNAAT-B, HvIDS2 and HvIDS3 transcripts, which encode the enzymes involved in the synthesis of MAs, were increased in Zn-deficient roots. Studies of the genes involved in the methionine cycle using microarray analysis showed that the transcripts of these genes were increased in both Zn-deficient and Fe-deficient barley roots, probably allowing the plant to meet its demand for methionine, a precursor in the synthesis of MAs. In addition, HvNAAT-B transcripts were detected in Zn-deficient shoots, but not in those that were deficient in Fe. Increased synthesis of MAs in Zn-deficient barley was not due to a deficiency of Fe, because Zn-deficient barley accumulated more Fe than did the control plants, ferritin transcripts were increased in Zn-deficient plants, and Zn deficiency promoted Fe transport from root to shoot. Moreover, analysis using the positron-emitting tracer imaging system (PETIS) confirmed that more 62Zn(II)-MAs than 62Zn2+ were absorbed by the roots of Zn-deficient barley plants. These data suggest that the increased biosynthesis and secretion of MAs arising from a shortage of Zn are not due to an induced Fe deficiency, and that secreted MAs are effective in absorbing Zn from the soil.

Iron deficiency is a serious problem around the world, especially in developing countries. The production of iron-biofortified rice will help ameliorate this problem. Previously, expression of the iron storage protein, ferritin, in rice using an endosperm-specific promoter resulted in a two-fold increase in iron concentration in the resultant transgenic seeds. However, further over expression of ferritin did not produce an additional increase in the seed iron concentration, and symptoms of iron deficiency were noted in the leaves of the transgenic plants. In the present study, we aimed to further increase the iron concentration in rice seeds without increasing the sensitivity to iron deficiency by enhancing the uptake and transport of iron via a ferric iron chelator, mugineic acid. To this end, we introduced the soybean ferritin gene (SoyferH2) driven by two endosperm-specific promoters, along with the barley nicotianamine synthase gene (HvNAS'), two nicotianamine aminotransferase genes (HvNAAT-A and -B), and a mugineic acid synthase gene (IDS3) to enhance mugineic acid production in rice plants. A marker-free vector was utilized as a means of increasing public acceptance. Representative lines were selected from 102 transformants based on the iron concentration in polished seeds and ferritin accumulation in the seeds. These lines were grown in both commercially supplied soil (iron-sufficient conditions) and calcareous soil (iron-deficient conditions). Lines expressing both ferritin and mugineic acid biosynthetic genes showed signs of iron-deficiency tolerance in calcareous soil. The iron concentration in polished T3 seeds was increased by 4 and 2.5 times, as compared to that in non-transgenic lines grown in normal and calcareous soil, respectively. These results indicate that the concomitant introduction of the ferritin gene and mugineic acid biosynthetic genes effectively increased the seed iron level without causing iron sensitivity under iron-limited conditions. [ABSTRACT FROM AUTHOR]

Zinc (Zn) excess has significant toxicity to biological systems through metal-based cytotoxic reactions. Nicotianamine (NA) and deoxymugineic acid (DMA) are low-molecular-weight, high-affinity transition metal chelators. Studies have shown that NA may have a role in the tolerance of excess Zn. We show that a gene coding the iron (Fe)-regulated DNA-binding transcription factor (OsIRO2) and the downstream genes of OsIRO2, such as NA synthase, DMA synthase and the DMA-Fe3+ transporter, were induced in rice roots by excess Zn. Consistent with the expression of these genes, the amounts of endogenous NA, endogenous DMA and DMA secretion increased in the excess Zn roots. Although the Fe concentration in the excess Zn roots was much higher than that in the control, rice ferritin gene, OsFer1, was downregulated in Zn excess roots. OsIRT1, which is upregulated by Fe deficiency, was not induced in Zn excess roots, suggesting that OsIRO2 may not be induced simply by the Fe deficiency caused by excess Zn. The data indicate that the induction of OsIRO2 by excess Zn is responsible for the production of NA and DMA, which may play a role in maintaining cellular Zn availability. [ABSTRACT FROM AUTHOR]