Overexpression of the Barley Nicotianamine Synthase Gene HvNAS1 Increases Iron and Zinc Concentrations in Rice Grains
© The Author(s) 2009
Received: 27 April 2009
Accepted: 8 September 2009
Published: 30 September 2009
In humans, iron (Fe) and zinc (Zn) deficiencies result in major worldwide health problems. Transgenic technologies to produce Fe- and Zn-biofortified rice varieties offer a promising potential solution. Nicotianamine, the precursor of phytosiderophores, chelates Fe2+ and Zn2+ and plays an important role in transporting these metals to both vegetative and reproductive organs within the plant. Our objective was to increase Fe and Zn contents in rice grains by overexpressing the barley nicotianamine synthase gene HvNAS1. HvNAS1-overexpressing transgenic rice showed increased HvNAS1 expression and subsequent increases in endogenous nicotianamine and phytosiderophore content in shoots, roots, and seeds. Fe and Zn concentrations in polished T1 seeds from transgenic plants increased more than three and twofold, respectively; Fe and Zn concentrations also increased in both polished and brown T2 seeds. These results suggest that the overproduction of nicotianamine enhances the translocation of Fe and Zn into rice grains.
Iron (Fe) and zinc (Zn) are essential micronutrients in both plants and animals. Shortages in Fe and Zn are among the most prevalent human micronutrient deficiencies in the world, each affecting an estimated one third of the world’s population (Sandstead 1991; Stoltzfus and Dreyfuss 1998) and resulting, separately, in 0.8 million deaths per year worldwide (WHO 2002). Fe and Zn deficiencies are the sixth and fifth highest health risk factors, respectively, in developing countries with high mortality rates (WHO 2002). Several approaches have been proposed to decrease the prevalence of micronutrient deficiencies. Biofortification (i.e., breeding micronutrient-fortified crops) is particularly advantageous for people living in rural areas because it does not require specific processing techniques after harvest or specialized infrastructure (Lucca et al. 2006). In addition, transgenic techniques offer the possibility of producing micronutrient-fortified rice varieties.
Transgenic Fe fortification has been attempted using ferritin, a protein that stores approximately 4,500 Fe atoms in a complex. Goto et al. (1999) generated transgenic rice plants that expressed soybean ferritin in the endosperm using the endosperm-specific rice GluB1 promoter; the transformants showed Fe accumulation in brown rice seeds. However, further enhancement of ferritin expression did not produce further increases in seed Fe content (Qu et al. 2005). In addition to increased Fe storage in seeds, enhanced Fe uptake from the soil and translocation within the plant are thought to be required to further improve Fe fortification in seeds.
Nicotianamine (NA), a chelator of metal cations such as Fe(II) and Zn(II), is biosynthesized from S-adenosyl methionine via NA synthase (NAS; Higuchi et al. 1994). All higher plants synthesize and utilize NA for the internal transport of Fe and other metals (Hell and Stephan 2003; Takahashi et al. 2003). Indeed, the NA-defective tomato mutant chloronerva (Rudolph et al. 1985) presents a phenotype indicative of Fe deficiency (Pich and Scholz 1996; Stephan et al. 1996). Takahashi et al. (2003) produced NA-deficient transgenic tobacco plants by constitutively expressing the barley NA aminotransferase (NAAT) gene, HvNAAT-A. The transformants showed young leaves with interveinal chlorosis, and as a result of disrupted internal metal transport, Fe and Zn concentrations in the leaves and flowers decreased. Conversely, overexpressing the barley NAS gene, HvNAS1, led to increased Fe and Zn concentrations in the leaves, flowers, and seeds of tobacco plants (Takahashi et al. 2003). Similarly, in Arabidopsis AtNAS quadruple mutant, Fe concentration in seeds was found to decrease (Klatte et al. 2009). These reports suggest that NA plays an essential role for Fe translocation to seeds.
In addition, Koike et al. (2004) identified the rice NA-Fe(II) transporter gene OsYSL2, which is preferentially expressed in leaf phloem cells and the vascular bundles of flowers and developing seeds, suggesting a role in internal Fe transport.
In graminaceous plants, NA is the precursor of mugineic acid family phytosiderophores (MAs), which are natural Fe(III) chelators used in Fe acquisition from the rhizosphere (Mihashi and Mori 1989; Takagi 1976). Under Fe-deficient conditions, graminaceous plants synthesize and secrete MAs into the rhizosphere where they form Fe(III)–MAs complexes that are absorbed into the root via YS1-like transporters (Curie et al. 2001). Rice produces 2′-deoxymugineic acid (DMA), which chelates Fe(III) and contributes to Fe uptake from the soil and internal transport (Kobayashi et al. 2008). Therefore, increased NA concentrations would also be expected to increase DMA concentrations in rice plants, with subsequent improvements in Fe uptake and transport.
We hypothesized that the overexpression of the NAS gene in rice would enhance the synthesis of NA and DMA, and thus increase Fe and Zn concentrations in seeds. Higuchi et al. (2001a) introduced the HvNAS1 gene into rice under the control of the 35S promoter. The resultant transgenic lines produced two to threefold greater concentrations of NA compared to non-transgenic lines. DMA production and secretion also increased under Fe-sufficient conditions. In this paper, we describe new transgenic rice lines that overexpress the HvNAS1 gene under the control of the rice Actin1 promoter. Using these lines and previously generated 35S-HvNAS1 transformants (Higuchi et al. 2001a), we analyzed HvNAS1 gene expression, endogenous NA and DMA concentrations, and Fe and Zn contents in seeds. NA concentrations in the shoots, roots, and seeds of transgenic rice, and Fe and Zn concentrations in polished T1 rice seeds, increased compared to non-transgenic plants. Fe and Zn concentrations also increased in polished and brown T2 seeds in proportion to the expression of the HvNAS1 transgene in T1 plants.
Fe and Zn concentrations in polished T1 seeds from OsActin1-HvNAS1 lines
Transgene expression in HvNAS1 overexpression lines
Endogenous NA and DMA concentrations in T1 plants
Fe and Zn contents in T2 seeds
NA and DMA contents in T2 seeds
Fe and Zn concentration and localization in T2 seeds of soil culture
In rice, NA is thought to function as an Fe(II) chelator in phloem and to participate in long-distance Fe transport within the plant (Inoue et al. 2003; Koike et al. 2004). We cultivated transgenic rice lines carrying OsActin1promoter-HvNAS1 (AN lines) and 35S promoter-HvNAS1 (35S line), which overexpress the HvNAS1 transgene (Fig. 2). In the AN lines, Fe and Zn concentrations increased in polished T1 and T2 seeds cultivated via two distinct methods (Figs. 1, 4, and 7). We observed a positive correlation between the expression of HvNAS1 in the shoots and Fe and Zn concentrations in polished and brown rice seeds (Figs. 2 and 4). In addition, a positive correlation was observed between Fe or Zn concentration and NA or DMA concentration in seeds (Fig. 6). These results suggest that enhanced NA production increases the translocation of Fe and Zn into seeds. Bashir et al. (2006) suggested that DMA-Fe is transported from the roots to shoots in Fe-sufficient rice plants. In Zn-deficient rice plants, Suzuki et al. (2008b) showed that DMA promotes increased Zn translocation. We observed increased DMA concentrations in the 35S and AN1 lines (Fig. 3b), which may also have contributed to increased Fe and Zn concentrations in these plants.
NA concentration in T2 seeds of the 35S line and AN lines also increased compared to the NT line (Fig. 6). Usuda et al. (2009) produced transgenic rice that expressed HvNAS1 under the control of an endosperm-specific promoter, OsGluB-1. NA concentration in brown seeds of this transgenic line increased threefold compared to the NT line, and Fe concentration in polished seeds also increased 1.5-fold (data not shown). Higher NA productivity in the endosperm of this transgenic rice might have improved Fe accumulation in seeds. Thus, enhanced NA synthesis in the 35S and AN lines may have improved not only Fe translocation to polished seeds but also Fe accumulation in polished seeds.
In contrast to polished and brown rice seeds, Fe and Zn contents decreased in hulls per seed (Fig. 4). The vascular bundle inside a seed is divided and runs in two directions: one that reaches the ovary, which develops into the bran and endosperm, and another that reaches the hull in a rachilla (Takane and Hoshikawa 1993). NA-Fe(II) and NA-Zn(II) may be more easily transported into the bran and endosperm than into the hull.
Cheng et al. (2007) produced a rice mutant with increased endogenous NA concentrations via knockout of the OsNAAT1 gene, which is the sole Fe deficiency-induced NAAT gene in rice (Inoue et al. 2008). The OsNAAT1 knockout mutant accumulated greater amounts of Fe, but not Zn, in polished rice seeds when cultivated under waterlogged conditions. Because the mutant expressed a wide range of Fe deficiency-responsive genes, the OsNAAT1 knockout mutant appears to induce physiological Fe deficiency. In contrast, we found that the overexpression of the NAS gene alone is sufficient to increase Fe and Zn concentrations in seeds.
Higuchi et al. (2001a, b) produced rice lines expressing a genomic fragment of HvNAS1, resulting in increased NA content in the roots and leaves under Fe-sufficient conditions. However, Fe and Zn concentrations did not increase in seeds collected from HvNAS1 genome insertion rice plants when grown under Fe-deficient conditions in calcareous paddy fields or under Fe-sufficient conditions in andosol paddy fields (Masuda et al. 2008; Suzuki et al. 2008a). This difference in Fe accumulation in rice seeds is thought to be related to the expression of the NAS transgene and subsequent NA content. Under Fe-sufficient conditions, NA concentrations in 35S-HvNAS1 lines were higher compared to HvNAS1 genome insertion lines (Higuchi et al. 2001a). Moreover, in 35S-HvNAS1 lines, the NAS transgene is presumably expressed throughout the rice plant, which would promote widespread Fe circulation and bioavailability.
In addition to seed Fe and Zn concentration, Fe and Zn content per seed in T2 seeds of AN lines also increased compared to those of the NT line (Fig. 5). There was no significant correlation between yield and Fe or Zn concentration in polished seeds in each T2 plant of NT, AN1, and AN3 lines (data not shown). Therefore, it can be postulated that increased Fe and Zn concentration in seeds of transgenic lines was not due to decreased grain size or yield. Fe and Zn concentration in leaves of AN lines were not significantly increased compared to NT (Fig. 7e, f). Overexpression of HvNAS1 might not lead to Fe and Zn overaccumulation in leaves. We observed no symptom of Fe toxicity in transgenic rice lines.
Actin promoter is reported to induce generally stronger expression than 35S promoter when introduced into graminaceous plants such as rice (McElroy et al. 1990). Therefore, we produced not only 35S lines but also AN lines in order to obtain new lines with higher Fe and Zn concentrations. However, 35S line accumulated more NA, DMA than AN lines (Fig. 3). Fe and Zn concentration in seeds were also higher in 35S line than AN lines (Fig. 4). Nevertheless, AN lines also showed higher Fe and Zn concentrations in seeds than NT line.
In lines AN1, AN2, and AN3, Fe accumulation was relatively low in T2 polished rice seeds compared to T1 seeds (Figs. 1, 4, and 7). However, marked increases in Fe and Zn concentrations, similar to T1 seeds, were achieved in some T2 sublines (Fig. 7). These results may reflect differences in the genotype of each subline. Fe concentrations in the 35S line were consistently high because of its stable homozygous genotype. Further selection of AN sublines with high Fe and Zn content in T2 seeds such as AN1-1 and AN3-6 would produce stable increases in micronutrient content.
Our results clarified the effect of overexpression of NAS to increase Fe and Zn concentrations in rice seeds. In previous studies, we generated a variety of transgenic rice lines bearing one or more of the genes involved in MA biosynthesis (Suzuki et al. 2008a). Among these, a transgenic rice line carrying a barley genomic fragment containing the IDS3 gene showed increased Fe and Zn contents when grown in calcareous or andosol paddy fields (Masuda et al. 2008; Suzuki et al. 2008a). Therefore, we assume that introduction of barley IDS3 genome fragments into rice also has high potential to increase Fe concentration in rice seeds. Vasconcelos et al. (2003) produced transgenic rice plants with introduced Ferritin genes into high Fe variety IR68144, which resulted in increase of both Fe and Zn concentration in polished rice up to 3.7 or 1.4 times. Wirth et al. (2009) also reported that constitutive overexpression of AtNAS1 in combination with endosperm-specific expression of Ferritin is an effective method to increase Fe concentration in polished rice seeds. Because the IDS3 genome fragment might enhance Fe and Zn uptake from the soil, concomitant NAS overexpression might also increase Fe and Zn transport to the seeds, while endosperm-specific expression of Ferritin might further enhance Fe storage ability in seeds. Thus, we postulate that combination of these three genes may allow further improvement in Fe and Zn accumulation in seeds.
Japonica rice (Oryza sativa L.), cultivar Tsukinohikari, was used as the NT (non-transgenic) control and for transformation. The transgenic rice line bearing 35S-HvNAS1 (35S) was originally produced by Higuchi et al. (2001a) and designated S11. This S11 line was cultured through several generations to select a homozygous line, which was then used in all analyses.
Construction of OsActin1-HvNAS1 and rice transformation
The OsActin1-HvNAS1-nopaline synthase gene 3′-UTR (tNOS) cassette was constructed as follows. The OsActin1 promoter was amplified using rice genomic DNA and the primers 5′-AAG CTT CGA GGT CAT TCA TAT GCT TG-3′ and 5′-GTC GAC CTT CTA CCT ACA AAA AAG CT-3′ and then subcloned using the HindIII and SalI sites in the pUC18 vector (TaKaRa Bio, Shiga, Japan). This promoter sequence was excised from the vector using HindIII and XbaI and then ligated into pBluescript SK(+) (Stratagene, La Jolla, CA, USA). The HvNAS1 open reading frame (ORF) -tNOS sequence was amplified via polymerase chain reaction using the 35S-HvNAS1-tNOS cassette (Higuchi et al. 2001a) as a template and the primers 5′-GCA CTA GTA GCC ATG GAT GCC CAG AAC AGG AGG-3′ and 5′-GCA CTA GTA TTC CCG ATC TAG TAA CAT AGA TG-3′, both of which contained a SpeI site at the 5′-end. After sequence verification, the HvNAS1-tNOS fragment was digested with SpeI, whereas the OsActin1 promoter in the SK(+) vector was digested with XbaI. These two fragments were then ligated together to produce the OsActin1-HvNAS1-tNOS cassette. To create transgenic lines, this cassette was excised using NotI and KpnI and then cloned into the same sites in the pBIGRZ1 vector (Akiyama et al. 1997). Agrobacterium tumefaciens (C58) was used to carry this construct into O. sativa L. cv. Tsukinohikari using the method developed by Hiei et al. (1994) and modified by Akiyama et al. (1997). Twenty OsActin1-HvNAS1-tNOS transgenic (AN) lines were obtained as T0 transgenic plants.
Plant growth conditions
Twenty T0 AN lines carrying OsActin1-HvNAS1 and control NT plants were cultivated in 3.5-CL pots (1,000-ml volume; Kaneya, Aichi, Japan) containing artificial soil composed of a 2:1 mixture of bonsol-ichigou (Sumitomo Kagaku, Tokyo, Japan) and vermiculite (Green Tec, Tochigi, Japan). The soil was evenly fertilized with 3.5 g of a slow-release fertilizer, LongTotal-70 (Chisso-Asahi, Tokyo, Japan; N/P/K, 13:11:13), and 3.5 g of Long Total-140 (Chisso-Asahi; N/P/K, 13:11:13) per plant. Plants were grown in a greenhouse under natural light conditions, with 14 h of light at 30 °C and 10 h of dark at 25 °C. Plants were transplanted on 18 December 2006 and seeds harvested on 4 June 2007. Twelve lines were subjected to metal concentration analysis.
NT, T1 AN, and 35S seeds were germinated on Murashige and Skoog (MS) medium. After 3 weeks, six seedlings from each line were transferred into nutrient solution [2 mM Ca(NO3)2, 0.5 mM MgSO4, 0.7 mM K2SO4, 0.1 mM KCl, 0.1 mM KH2PO4, 0.1 mM Fe(III)-EDTA, 10 mM H3BO3, 0.5 μM MnSO4, 0.5 μM ZnSO4, 0.2 μM CuSO4, and 0.01 μM (NH4)6Mo7O25] and grown in a greenhouse under the conditions described above. The pH of the culture solution was adjusted daily to between 5.5 and 5.8 with 1 N HCl. Twenty-three days after transplanting, leaves and roots were harvested for Northern blot analysis and the determination of NA and DMA concentrations.
For the metal concentration analysis, T1 transgenic and NT lines were grown using two different methods. In the first method, three lines of T1 AN seeds (AN1, AN2, and AN3), 35S line seeds, and NT line seeds were germinated on MS medium for 3 weeks, and then seedlings were transplanted into small ceramic pots (250-ml volume) filled with bonsol-ichigou, but no additional fertilizer. Six pots containing three seedlings each were prepared for each line, and each pot was submerged in nutrient solution. The plants were settled on 6 August 2007 and harvested on 28 November 2007. T2 seeds were used in the metal concentration analysis.
In the second method, NT seeds and T1 seeds from lines AN1, AN2, and AN3 were germinated on MS medium on 17 August 2007. On 3 September 2007, six T1 seedlings from each line were settled in 3.5-CL pots containing artificial soil (2:1 mixture of bonsol-ichigou and vermiculite). The soil was fertilized evenly with 3.5 g of LongTotal-70 and 3.5 g of Long Total-140 per plant. Plants were grown in a greenhouse under natural light conditions, with 14 h of light at 30 °C and 10 h of dark at 25 °C. The plants were harvested on 5 January 2008 and the seeds used in the metal concentration analysis.
The ORF sequence for HvNAS1 (AB010086) was used to create an HvNAS1 probe (Suzuki et al. 2006). This fragment was labeled with [a-32P]-dATP using the random labeling method; labeled DNA was purified using a ProbeQuant G-50 micro-column (Pharmacia, Uppsala, Sweden). Total RNA in shoots was extracted using the SDS-phenol method. Total RNA (20 µg per lane) was separated on 1.4% (w/v) agarose gels. Blotting, hybridization and radioactive detection, and quantification were performed as previously described (Ogo et al. 2006).
Determination of NA and DMA concentrations
For determination of endogenous NA and DMA concentrations in the T1 plant body, approximately 5 g of frozen shoots and roots from each transgenic and NT line was ground and homogenized with a mortar and pestle and then suspended in 100 ml of deionized hot water (80 °C) for 20 min. After suspension, the samples were centrifuged at 8,000×g for 15 min and the supernatants collected. These extraction steps were the same for both NA and DMA measurement. For HPLC analysis of NA, the supernatants were subjected to ion-exchange purification, evaporated, and filtered as previously described (Wada et al. 2006). For HPLC analysis of DMA, the supernatants were subjected to ion-exchange purification, evaporated, and filtered under different conditions as previously described (Suzuki et al. 2006). All steps for DMA measurement were performed in the dark. One sample was prepared from each line and analyzed in triplicate to determine the concentrations of NA and DMA.
For determination of NA and DMA concentrations in T2 seeds, ten brown seeds from each of three lines of 35S, AN1, and AN3, whose Fe concentrations in T2 polished seeds were higher than in the other three lines, were selected randomly. These seeds were crushed in a Multi-Beads Shocker (Yasui Kikai, Osaka, Japan). Samples were mixed with 400 µl of 80% ethanol, centrifuged at 12,000×g for 5 min, and the supernatants were collected for measurement. This step was repeated three times. One hundred microliters of supernatant was filtered in a 10 K Nanosep centrifugal device (Pall, Ann Arbor, MI, USA). One sample was prepared from each line and the concentrations of NA and DMA were analyzed by LC/ESI-TOF-MS (JEOL, Tokyo, Japan) as previously described (Kakei et al. 2007; Wada et al. 2007).
Metal concentration analysis
Brown seeds were collected randomly from the ear of the main tiller (the tiller in the center or the largest among all tillers in one plant). Ten T1 seeds from each T0 transgenic line or ten T2 seeds from each T1 transgenic lines were dried overnight at 80 °C in a heat-dry machine. After determining the dry weight of each sample, the seeds were digested in 1 ml of 13 M HNO3 and 1 ml of 8.8 M H2O2 (Wako, Osaka, Japan) at 200 °C for 20 min with MARS Xpress (CEM, Matthews, NC, USA). After digestion, the samples were diluted to a volume of 5 ml and analyzed via inductively coupled plasma atomic emission spectrometry (SPS1200VR; Seiko, Tokyo, Japan).
To polish seeds, 30 brown seeds from the ear of the main tiller were placed into a 2-ml tube and shaken vigorously for 150 s at 2,500 rpm for at least four cycles using a Multi-Beads Shocker (Yasui Kikai). Ten well-polished seeds from each plant were selected. After removing the embryos, polished seeds were dried overnight, weighed, digested, and then analyzed as described above.
Rice bran (approximately 30–50 mg) was obtained as a by-product of the polishing process. Rice bran was dried overnight, weighed, and then digested at 220 °C for 20 min in 1 ml of 13 M HNO3 and 1 ml of 8.8 M H2O2. Metal analysis was performed as described above. Rice hulls (approximately 80–160 mg) were also dried overnight, weighed, digested, and subjected to metal analysis as described above. Leaf sheaths and leaf blades were harvested, dried, and crashed by homogenizer (Fujiwara Seisakusyo, Tokyo, Japan). About 200 mg of crashed leaves was then digested and subjected to metal analysis as described above.
Fe localization by prussian blue staining
T2 brown seeds were soaked in water overnight and were cut by a razor. Then, the samples were soaked in 2% HCl (Wako) and 2% potassium hexacyanoferrate(II) trihydrate (Wako) for 1.5 h. Stained samples were washed with distilled water and observed by stereoscopic microscope (AX10 canHRP, ZEISS).
Analysis of variance with the Student–Newman–Keuls test was used to compare data. The level of significance was set at p < 0.05. The statistical software package JMP7 (SAS Institute, Cary, NC, USA) was used in all analyses.
We thank Mr. T. Maruyama for cultivating rice in the greenhouse. We also thank Ms. R.N. Itai for technical advice regarding Northern blot analysis.
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