Tomoko Nozoye

Rice OsYSL15 Is an Iron-regulated Iron(III)-Deoxymugineic Acid Transporter Expressed in the Roots and Is Essential for Iron Uptake in Early Growth of the Seedlings

Graminaceous plants take up iron through YS1 (yellow stripe 1) and YS1-like (YSL) transporters using iron-chelating compounds known as mugineic acid family phytosiderophores. We examined the expression of 18 rice (Oryza sativa L.) YSL genes (OsYSL1-18) in the epidermis/exodermis, cortex, and stele of rice roots. Expression of OsYSL15 in root epidermis and stele was induced by iron deficiency and showed daily fluctuation. OsYSL15 restored a yeast mutant defective in iron uptake when supplied with iron(III)-deoxymugineic acid and transported iron(III)-deoxymugineic acid in Xenopus laevis oocytes. An OsYSL15-green fluorescent protein fusion was localized to the plasma membrane when transiently expressed in onion epidermal cells. OsYSL15 promoter-β-glucuronidase analysis revealed that OsYSL15 expression in roots was dominant in the epidermis/exodermis and phloem cells under conditions of iron deficiency and was detected only in phloem under iron sufficiency. These results strongly suggest that OsYSL15 is the dominant iron(III)-deoxymugineic acid transporter responsible for iron uptake from the rhizosphere and is also responsible for phloem transport of iron. OsYSL15 was also expressed in flowers, developing seeds, and in the embryonic scutellar epithelial cells during seed germination. OsYSL15 knockdown seedlings showed severe arrest in germination and early growth and were rescued by high iron supply. These results demonstrate that rice OsYSL15 plays a crucial role in iron homeostasis during the early stages of growth.

Phytosiderophore Efflux Transporters Are Crucial for Iron Acquisition in Graminaceous Plants

Eukaryotic organisms have developed diverse mechanisms for the acquisition of iron, which is required for their survival. Graminaceous plants use a chelation strategy. They secrete phytosiderophore compounds, which solubilize iron in the soil, and then take up the resulting iron-phytosiderophore complexes. Bacteria and mammals also secrete siderophores to acquire iron. Although phytosiderophore secretion is crucial for plant growth, its molecular mechanism remains unknown. Here, we show that the efflux of deoxymugineic acid, the primary phytosiderophore from rice and barley, involves the TOM1 and HvTOM1 genes, respectively. Xenopus laevis oocytes expressing TOM1 or HvTOM1 released 14C-labeled deoxymugineic acid but not 14C-labeled nicotianamine, a structural analog and biosynthetic precursor of deoxymugineic acid, indicating that the TOM1 and HvTOM1 proteins are the phytosiderophore efflux transporters. Under conditions of iron deficiency, rice and barley roots express high levels of TOM1 and HvTOM1, respectively, and the overexpression of these genes increased tolerance to iron deficiency. In rice roots, the efficiency of deoxymugineic acid secretion was enhanced by overexpression of TOM1 and decreased by its repression, providing further evidence that TOM1 encodes the efflux transporter of deoxymugineic acid. We have also identified two genes encoding efflux transporters of nicotianamine, ENA1 and ENA2. Our identification of phytosiderophore efflux transporters has revealed the final piece in the molecular machinery of iron acquisition in graminaceous plants.

In vivo analysis of metal distribution and expression of metal transporters in rice seed during germination process by microarray and X-ray Fluorescence Imaging of Fe, Zn, Mn, and Cu

To investigate the flow of the metal nutrients iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu) during rice seed germination, we performed microarray analysis to examine the expression of genes involved in metal transport. Many kinds of metal transporter genes were strongly expressed and their expression levels changed during rice seed germination. We found that metal transporter genes such as ZIP family has tendency to decrease in their expressions during seed germination. Furthermore, imaging of the distribution of elements (Fe, Mn, Zn, and Cu) was carried out using Synchrotron-based X-ray microfluorescence at the Super Photon ring-8 GeV (SPring-8) facility. The change in the distribution of each element in the seeds following germination was observed by in vivo monitoring. Iron, Mn, Zn, and Cu accumulated in the endosperm and embryos of rice seeds, and their distribution changed during rice seed germination. The change in the patterns of mineral localization during germination was different among the elements observed.

Diurnal changes in the expression of genes that participate im phytosiderophore synthesis in rice

Abstract The roots of graminaceous plants secrete mugineic acid family phytosiderophores (MAs), allowing the acquisition of sparingly soluble iron (Fe) from the soil. In barley, the secretion of MAs occurs in a distinct diurnal rhythm. Rice also secretes deoxymugineic acid (DMA) in response to Fe deficiency, but it is unclear whether the secretion also occurs in a diurnal rhythm. A time course analysis with Northern blots was used to examine the expression of genes encoding enzymes in the MA biosynthetic pathway. The transcriptional levels of the NAS and NAAT genes fluctuated diurnally in Fe-deficient rice roots. These results suggest that, as in barley, the secretion of DMA in rice changes in a diurnal rhythm. Furthermore, the transcriptional levels of genes encoding translation initiation factor 4A2 (elF4A2), ras-related small GTP-binding protein (GTPase), and ADP-ribosylation factor 1 (ARF1), which fluctuate diurnally in barley and are thought to be involved in the diurnal changes in MA secretion, also changed diurnally. As these genes are involved in intracellular vesicular transport, it is possible that secretion of DMA in rice is under the control of vesicular transport, as in barley root cells.

Characterizing the crucial components of iron homeostasis in the maize mutants ys1 and ys3.

To acquire iron (Fe), graminaceous plants secrete mugineic acid family phytosiderophores through the phytosiderophore efflux transporter TOM1 and take up Fe in the form of Fe(III)–phytosiderophore complexes. Yellow stripe 1 (ys1) and ys3 are recessive mutants of maize (Zea mays L.) that show typical symptoms of Fe deficiency, i.e., interveinal chlorosis of the leaves. The ys1 mutant is defective in the Fe(III)–phytosiderophore transporter YS1 and is therefore unable to take up Fe(III)–phytosiderophore complexes. While the ys3 mutant has been shown to be defective in phytosiderophores release, the causative gene has not been identified. The present study was performed to characterize the expression profiles of the genes in ys1 and ys3 mutants to extend our understanding of Fe homeostasis in maize. Using quantitative real-time polymerase chain reaction, we assessed changes in the levels of gene expression in response to Fe deficiency of genes involved in Fe homeostasis, such as those related to phytosiderophore biosynthesis and Fe transport. As with other crops, these Fe deficiency-inducible genes were also upregulated in maize. In addition, these Fe deficiency-inducible genes were upregulated in both the ys1 and ys3 mutants, even under Fe-sufficient conditions. Indeed, the Fe concentrations in the roots of ys1 and ys3 plants were lower than that of wild-type controls. These results suggest that ys1 and ys3 are Fe-deficient during growth in the presence of Fe. In agreement with previous reports, the level of YS1 expression decreased in the ys1 mutant. Moreover, the expression level of a homolog of TOM1 in maize decreased significantly in the ys3 mutant. Unspliced introns of ZmTOM1 were detected only in ys3, and not in YS1YS3 or ys1, suggesting that ZmTOM1 may be involved in the ys3 phenotype.

The spatial expression and regulation of transcription factors IDEF1 and IDEF2

BACKGROUND AND AIMS Under conditions of low iron availability, rice plants induce genes involved in iron uptake and utilization. The iron deficiency-responsive cis-acting element binding factors 1 and 2 (IDEF1 and IDEF2) regulate transcriptional response to iron deficiency in rice roots. Clarification of the functions of IDEF1 and IDEF2 could uncover the gene regulation mechanism. METHODS Spatial patterns of IDEF1 and IDEF2 expression were analysed by histochemical staining of IDEF1 and IDEF2 promoter-GUS transgenic rice lines. Expression patterns of the target genes of IDEF1 and IDEF2 were analysed using transformants with induced or repressed expression of IDEF1 or IDEF2 grown in iron-rich or in iron-deficient solutions for 1 d. KEY RESULTS IDEF1 and IDEF2 were highly expressed in the basal parts of the lateral roots and vascular bundles. IDEF1 and IDEF2 expression was dominant in leaf mesophyll and vascular cells, respectively. These expression patterns were similar under both iron-deficient and iron-sufficient conditions. IDEF1 was strongly expressed in pollen, ovaries, the aleurone layer and embryo. IDEF2 was expressed in pollen, ovaries and the dorsal vascular region of the endosperm. During seed germination, IDEF1 and IDEF2 were expressed in the endosperm and embryo. Expression of IDEF1 target genes was regulated in iron-rich roots similar to early iron-deficiency stages. In addition, the expression patterns of IDEF2 target genes were similar between iron-rich conditions and early or subsequent iron deficiency. CONCLUSIONS IDEF1 and IDEF2 are constitutively expressed during both vegetative and reproductive stages. The spatial expression patterns of IDEF1 and IDEF2 overlap with their target genes in restricted cell types, but not in all cells. The spatial expression patterns and gene regulation of IDEF1 and IDEF2 in roots are generally conserved under conditions of iron sufficiency and deficiency, suggesting complicated interactions with unknown factors for sensing and transmitting iron-deficiency signals.

Nicotianamine synthase 2 localizes to the vesicles of iron‐deficient rice roots, and its mutation in the YXXφ or LL motif causes the disruption of vesicle formation or movement in rice

Graminaceous plants release mugineic acid family phytosiderophores (MAs) to acquire iron from the soil. Here, we show that deoxymugineic acid (DMA) secretion from rice roots fluctuates throughout the day, and that vesicles accumulate in roots before MAs secretion. We developed transgenic rice plants that express rice nicotianamine (NA) synthase (NAS) 2 (OsNAS2) fused to synthetic green fluorescent protein (sGFP) under the control of its own promoter. In root cells, OsNAS2-sGFP fluorescence was observed in a dot-like pattern, moving dynamically within the cell. This suggests that these vesicles are involved in NA and DMA biosynthesis. A tyrosine motif and a di-leucine motif, which have been reported to be involved in cellular transport, are conserved in all identified NAS proteins in plants. OsNAS2 mutated in the tyrosine motif showed NAS activity and was localized to the vesicles; however, these vesicles stuck together and did not move. On the other hand, OsNAS2 mutated in the di-leucine motif lost NAS activity and did not localize to these vesicles. The amounts of NA and DMA produced and the amount of DMA secreted by OsNAS2-sGFP plants were significantly higher than in non-transformants and domain-mutated lines, suggesting that OsNAS2-sGFP, but not the mutated forms, was functional in vivo. Overall, the localization of NAS to vesicles and the transport of these vesicles are crucial steps in NA synthesis, leading to DMA synthesis and secretion in rice.

Exploiting new tools for iron bio-fortification of rice.

Rice is one of the most important staple crops and efficient iron (Fe) adsorption during growth not only improves rice yield, but also enriches this essential micronutrient in rice grains to address Fe deficiency in humans. In this article, we review updates on research into the molecular mechanisms regulating Fe uptake from soil and its transport from roots to shoots to seeds in rice plants. Understanding the regulation and expression of genes involved in Fe homeostasis will benefit the development of variants with enhanced Fe utilization to improve rice output and quality.

Expression of peanut Iron Regulated Transporter 1 in tobacco and rice plants confers improved iron nutrition

Iron (Fe) limitation is a widespread agricultural problem in calcareous soils and severely limits crop production. Iron Regulated Transporter 1 (IRT1) is a key component for Fe uptake from the soil in dicot plants. In this study, the peanut (Arachis hypogaea L.) AhIRT1 was introduced into tobacco and rice plants using an Fe-deficiency-inducible artificial promoter. Induced expression of AhIRT1 in tobacco plants resulted in accumulation of Fe in young leaves under Fe deficient conditions. Even under Fe-excess conditions, the Fe concentration was also markedly enhanced, suggesting that the Fe status did not affect the uptake and translocation of Fe by AhIRT1 in the transgenic plants. Most importantly, the transgenic tobacco plants showed improved tolerance to Fe limitation in culture in two types of calcareous soils. Additionally, the induced expression of AhIRT1 in rice plants also resulted in high tolerance to low Fe availability in calcareous soils.

Strigolactone Regulates Anthocyanin Accumulation, Acid Phosphatases Production and Plant Growth under Low Phosphate Condition in Arabidopsis

Phosphate is an essential macronutrient in plant growth and development; however, the concentration of inorganic phosphate (Pi) in soil is often suboptimal for crop performance. Accordingly, plants have developed physiological strategies to adapt to low Pi availability. Here, we report that typical Pi starvation responses in Arabidopsis are partially dependent on the strigolactone (SL) signaling pathway. SL treatment induced root hair elongation, anthocyanin accumulation, activation of acid phosphatase, and reduced plant weight, which are characteristic responses to phosphate starvation. Furthermore, the expression profile of SL-response genes correlated with the expression of genes induced by Pi starvation. These results suggest a potential overlap between SL signaling and Pi starvation signaling pathways in plants.