Constitutive overexpression of the OsNAS gene family reveals single-gene strategies for effective iron- and zinc-biofortification of rice endosperm

Detection of quantitative trait loci controlling grain zinc concentration using Australian wild rice, Oryza meridionalis, a potential genetic resource for biofortification of rice

Zinc (Zn) is one of the essential mineral elements for both plants and humans. Zn deficiency in human is one of the major causes of hidden hunger, a serious health problem observed in many developing countries. Therefore, increasing Zn concentration in edible part is an important issue for improving human Zn nutrition. Here, we found that an Australian wild rice O. meridionalis showed higher grain Zn concentrations compared with cultivated and other wild rice species. The quantitative trait loci (QTL) analysis was then performed to identify the genomic regions controlling grain Zn levels using backcross recombinant inbred lines derived from O. sativa 'Nipponbare' and O. meridionalis W1627. Four QTLs responsible for high grain Zn were detected on chromosomes 2, 9, and 10. The QTL on the chromosome 9 (named qGZn9), which showed the largest effect on grain Zn concentration was confirmed with the introgression line, which had a W1627 chromosomal segment covering the qGZn9 region in the genetic background of O. sativa 'Nipponbare'. Fine mapping of this QTL resulted in identification of two tightly linked loci, qGZn9a and qGZn9b. The candidate regions of qGZn9a and qGZn9b were estimated to be 190 and 950 kb, respectively. Furthermore, we also found that plants having a wild chromosomal segment covering qGZn9a, but not qGZn9b, is associated with fertility reduction. qGZn9b, therefore, provides a valuable allele for breeding rice with high Zn in the grains.

Phytosiderophores determine thresholds for iron and zinc accumulation in biofortified rice endosperm while inhibiting the accumulation of cadmium

Nicotianamine (NA) and 2'-deoxymugenic acid (DMA) are metal-chelating ligands that promote the accumulation of metals in rice endosperm, but it is unclear how these phytosiderophores regulate the levels of different metals and limit their accumulation. In this study, transgenic rice plants producing high levels of NA and DMA accumulated up to 4-fold more iron (Fe) and 2-fold more zinc (Zn) in the endosperm compared with wild-type plants. The distribution of Fe and Zn in vegetative tissues suggested that both metals are sequestered as a buffering mechanism to avoid overloading the seeds. The buffering mechanism involves the modulation of genes encoding metal transporters in the roots and aboveground vegetative tissues. As well as accumulating more Fe and Zn, the endosperm of the transgenic plants accumulated less cadmium (Cd), suggesting that higher levels of Fe and Zn competitively inhibit Cd accumulation. Our data show that although there is a strict upper limit for Fe (~22.5 µg g-1 dry weight) and Zn (~84 µg g-1 dry weight) accumulation in the endosperm, the careful selection of strategies to increase endosperm loading with essential minerals can also limit the accumulation of toxic metals such as Cd, thus further increasing the nutritional value of rice.

Engineering crop nutrient efficiency for sustainable agriculture

Increasing crop yields can provide food, animal feed, bioenergy feedstocks and biomaterials to meet increasing global demand; however, the methods used to increase yield can negatively affect sustainability. For example, application of excess fertilizer can generate and maintain high yields but also increases input costs and contributes to environmental damage through eutrophication, soil acidification and air pollution. Improving crop nutrient efficiency can improve agricultural sustainability by increasing yield while decreasing input costs and harmful environmental effects. Here, we review the mechanisms of nutrient efficiency (primarily for nitrogen, phosphorus, potassium and iron) and breeding strategies for improving this trait, along with the role of regulation of gene expression in enhancing crop nutrient efficiency to increase yields. We focus on the importance of root system architecture to improve nutrient acquisition efficiency, as well as the contributions of mineral translocation, remobilization and metabolic efficiency to nutrient utilization efficiency.

The Arabidopsis MTP8 transporter determines the localization of manganese and iron in seeds

Understanding how seeds obtain and store nutrients is key to developing crops with higher agronomic and nutritional value. We have uncovered unique patterns of micronutrient localization in seeds using synchrotron X-ray fluorescence (SXRF). Although all four members of the Arabidopsis thaliana Mn-CDF family can transport Mn, here we show that only mtp8-2 has an altered Mn distribution pattern in seeds. In an mtp8-2 mutant, Mn no longer accumulates in hypocotyl cortex cells and sub-epidermal cells of the embryonic cotyledons, but rather accumulates with Fe in the cells surrounding the vasculature, a pattern previously shown to be determined by the vacuolar transporter VIT1. We also show that MTP8, unlike the other three Mn-CDF family members, can transport Fe and is responsible for localization of Fe to the same cells that store Mn. When both the VIT1 and MTP8 transporters are non-functional, there is no accumulation of Fe or Mn in specific cell types; rather these elements are distributed amongst all cell types in the seed. Disruption of the putative Fe binding sites in MTP8 resulted in loss of ability to transport Fe but did not affect the ability to transport Mn.

Copper and ectopic expression of the Arabidopsis transport protein COPT1 alter iron homeostasis in rice (Oryza sativa L.)

Copper deficiency and excess differentially affect iron homeostasis in rice and overexpression of the Arabidopsis high-affinity copper transporter COPT1 slightly increases endogenous iron concentration in rice grains. Higher plants have developed sophisticated mechanisms to efficiently acquire and use micronutrients such as copper and iron. However, the molecular mechanisms underlying the interaction between both metals remain poorly understood. In the present work, we study the effects produced on iron homeostasis by a wide range of copper concentrations in the growth media and by altered copper transport in Oryza sativa plants. Gene expression profiles in rice seedlings grown under copper excess show an altered expression of genes involved in iron homeostasis compared to standard control conditions. Thus, ferritin OsFER2 and ferredoxin OsFd1 mRNAs are down-regulated whereas the transcriptional iron regulator OsIRO2 and the nicotianamine synthase OsNAS2 mRNAs rise under copper excess. As expected, the expression of OsCOPT1, which encodes a high-affinity copper transport protein, as well as other copper-deficiency markers are down-regulated by copper. Furthermore, we show that Arabidopsis COPT1 overexpression (C1 ^OE ) in rice causes root shortening in high copper conditions and under iron deficiency. C1 ^OE rice plants modify the expression of the putative iron-sensing factors OsHRZ1 and OsHRZ2 and enhance the expression of OsIRO2 under copper excess, which suggests a role of copper transport in iron signaling. Importantly, the C1 ^OE rice plants grown on soil contain higher endogenous iron concentration than wild-type plants in both brown and white grains. Collectively, these results highlight the effects of rice copper status on iron homeostasis, which should be considered to obtain crops with optimized nutrient concentrations in edible parts.

Single genetic locus improvement of iron, zinc and β-carotene content in rice grains

Nearly half of the world's population obtains its daily calories from rice grains, which lack or have insufficient levels of essential micronutrients. The deficiency of micronutrients vital for normal growth is a global health problem, and iron, zinc and vitamin A deficiencies are the most prevalent ones. We developed rice lines expressing Arabidopsis NICOTIANAMINE SYNTHASE 1 (AtNAS1), bean FERRITIN (PvFERRITIN), bacterial CAROTENE DESATURASE (CRTI) and maize PHYTOENE SYNTHASE (ZmPSY) in a single genetic locus in order to increase iron, zinc and β-carotene content in the rice endosperm. NAS catalyzes the synthesis of nicotianamine (NA), which is a precursor of deoxymugeneic acid (DMA) iron and zinc chelators, and also chelate iron and zinc for long distance transport. FERRITIN provides efficient storage of up to 4500 iron ions. PSY catalyzes the conversion of GGDP to phytoene, and CRTI performs the function of desaturases required for the synthesis of β-carotene from phytoene. All transgenic rice lines have significantly increased β-carotene, iron, and zinc content in the polished rice grains. Our results establish a proof-of-concept for multi-nutrient enrichment of rice grains from a single genetic locus, thus offering a sustainable and effective approach to address different micronutrient deficiencies at once.

Iron homeostasis in plants - a brief overview

Iron plays a crucial role in biochemistry and is an essential micronutrient for plants and humans alike. Although plentiful in the Earth's crust it is not usually found in a form readily accessible for plants to use. They must therefore sense and interact with their environment, and have evolved two different molecular strategies to take up iron in the root. Once inside, iron is complexed with chelators and distributed to sink tissues where it is used predominantly in the production of enzyme cofactors or components of electron transport chains. The processes of iron uptake, distribution and metabolism are overseen by tight regulatory mechanisms, at the transcriptional and post-transcriptional level, to avoid iron concentrations building to toxic excess. Iron is also loaded into seeds, where it is stored in vacuoles or in ferritin. This is important for human nutrition as seeds form the edible parts of many crop species. As such, increasing iron in seeds and other tissues is a major goal for biofortification efforts by both traditional breeding and biotechnological approaches.

Metabolite Profiling of a Zinc-Accumulating Rice Mutant

Breeding crops with high zinc (Zn) density is an effective way to alleviate human dietary Zn deficiencies. We characterized a mutant Lilizhi (LLZ) accumulating at least 35% higher Zn concentration in grain than the wild type (WT) in hydroponic experiments. The mutant stored less Zn content in the root and transported more Zn to the grain. Metabolite profiling demonstrated that, with high Zn treatment, the contents of proline, asparagine, citric acid, and malic acid were enhanced in both LLZ and the WT, which were thought to be involved in Zn transport in rice. Furthermore, the contents of cysteine, allothreonine, alanine, tyrosine, homoserine, β-alanine, and nicotianamine required for the production of many metal-binding proteins were specifically increased in LLZ. LLZ had higher capability of amino acid biosynthesis and metal cation transportation. The current research extends our understanding on the physiological mechanisms of Zn uploading into grain and provides references for further Zn biofortification breeding in rice.

Dealing with iron metabolism in rice: from breeding for stress tolerance to biofortification

Iron is a well-known metal. Used by humankind since ancient times in many different ways, this element is present in all living organisms, where, unfortunately, it represents a two-way problem. Being an essential block in the composition of different proteins and metabolic pathways, iron is a vital component for animals and plants. That is why iron deficiency has a severe impact on the lives of different organisms, including humans, becoming a major concern, especially in developing countries where access to adequate nutrition is still difficult. On the other hand, this metal is also capable of causing damage when present in excess, becoming toxic to cells and affecting the whole organism. Because of its importance, iron absorption, transport and storage mechanisms have been extensively investigated in order to design alternatives that may solve this problem. As the understanding of the strategies that plants use to control iron homeostasis is an important step in the generation of improved plants that meet both human agricultural and nutritional needs, here we discuss some of the most important points about this topic.

Paralogs and mutants show that one DMA synthase functions in iron homeostasis in rice

Rice (Oryza sativa) secretes 2'-deoxymugineic acid (DMA) to acquire insoluble iron (Fe) from the rhizosphere. In rice, DMA is synthesized by DMA synthase 1 (OsDMAS1), a member of the aldo-keto reductase super family. We screened OsDMAS1 paralogs for DMA synthesis. None of these paralogs displayed in vitro DMA synthesis activity, suggesting that rice only harbors one functional DMAS. We further characterized OsDMAS1 mutant plants. We failed to screen homozygous knock-out plants (dmas-1), so we characterized DMAS knock-down plants (dmas-kd1 and dmas-kd2). Under Fe-deficient conditions, dmas-kd1 plants were more chlorotic compared to the wild-type (WT) plants, and the expression of OsNAS3, OsYSL2, OsIRT1, and OsIRO2 was significantly up-regulated in the dmas-kd1 mutant, indicating that metal homeostasis was significantly disturbed. The secretion of DMA in dmas-kd1 was not significantly reduced. The dmas-kd1 plants accumulated less Fe in their roots compared to WT plants when grown with 10 μM FeSO4. The dmas-kd1 plants accumulated more Zn in their roots compared to WT plants under Fe-deficient, Fe-EDTA, and FeSO4 conditions. In both dehusked rice seeds (brown rice) and polished rice, no differences were observed for Fe, Cu, or Mn accumulation, whereas dmas-kd1 seeds significantly accumulated more Zn in brown rice. Our data suggests that rice only harbors one functional gene for DMA synthesis. In addition, the knock-down of OsDMAS1 significantly up-regulates the genes involved in Fe uptake and homeostasis.

Rice NICOTIANAMINE SYNTHASE 2 expression improves dietary iron and zinc levels in wheat

KEY MESSAGE

Iron and zinc deficiencies negatively impact human health worldwide. We developed wheat lines that meet or exceed recommended dietary target levels for iron and zinc in the grains. These lines represent useful germplasm for breeding new wheat varieties that can reduce iron and zinc deficiency-associated health burdens in the affected populations. Micronutrient deficiencies, including iron and zinc deficiencies, have negative impacts on human health globally. Iron-deficiency; anemia affects nearly two billion people worldwide and is the cause of reduced cognitive development, fatigue and overall low productivity. Similarly, zinc deficiency causes stunted growth, decreased immunity and increased risk of respiratory infections. Biofortification of staple crops is a sustainable and effective approach to reduce the burden of health problems associated with micronutrient deficiencies. Here, we developed wheat lines expressing rice NICOTIANAMINE SYNTHASE 2 (OsNAS2) and bean FERRITIN (PvFERRITIN) as single genes as well as in combination. NAS catalyzes the biosynthesis of nicotianamine (NA), which is a precursor of the iron chelator deoxymugeneic acid (DMA) required for long distance iron translocation. FERRITIN is important for iron storage in plants because it can store up to 4500 iron ions. We obtained significant increases of iron and zinc content in wheat grains of plants expressing either OsNAS2 or PvFERRTIN, or both genes. In particular, wheat lines expressing OsNAS2 greatly surpass the HarvestPlus recommended target level of 30 % dietary estimated average requirement (EAR) for iron, and 40 % of EAR for zinc, with lines containing 93.1 µg/g of iron and 140.6 µg/g of zinc in the grains. These wheat lines with dietary significant levels of iron and zinc represent useful germplasm for breeding new wheat varieties that can reduce micronutrient deficiencies in affected populations.

Dynamic Subcellular Localization of Iron during Embryo Development in Brassicaceae Seeds

Iron is an essential micronutrient for plants. Little is know about how iron is loaded in embryo during seed development. In this article we used Perls/DAB staining in order to reveal iron localization at the cellular and subcellular levels in different Brassicaceae seed species. In dry seeds of Brassica napus, Nasturtium officinale, Lepidium sativum, Camelina sativa, and Brassica oleracea iron localizes in vacuoles of cells surrounding provasculature in cotyledons and hypocotyl. Using B. napus and N. officinale as model plants we determined where iron localizes during seed development. Our results indicate that iron is not detectable by Perls/DAB staining in heart stage embryo cells. Interestingly, at torpedo development stage iron localizes in nuclei of different cells type, including integument, free cell endosperm and almost all embryo cells. Later, iron is detected in cytoplasmic structures in different embryo cell types. Our results indicate that iron accumulates in nuclei in specific stages of embryo maturation before to be localized in vacuoles of cells surrounding provasculature in mature seeds.

Finger on the Pulse: Pumping Iron into Chickpea

Iron deficiency is a major problem in both developing and developed countries, and much of this can be attributed to insufficient dietary intake. Over the past decades several measures, such as supplementation and food fortification, have helped to alleviate this problem. However, their associated costs limit their accessibility and effectiveness, particularly amongst the financially constrained. A more affordable and sustainable option that can be implemented alongside existing measures is biofortification. To date, much work has been invested into staples like cereals and root crops-this has culminated in the successful generation of high iron-accumulating lines in rice and pearl millet. More recently, pulses have gained attention as targets for biofortification. Being secondary staples rich in protein, they are a nutritional complement to the traditional starchy staples. Despite the relative youth of this interest, considerable advances have already been made concerning the biofortification of pulses. Several studies have been conducted in bean, chickpea, lentil, and pea to assess existing germplasm for high iron-accumulating traits. However, little is known about the molecular workings behind these traits, particularly in a leguminous context, and biofortification via genetic modification (GM) remains to be attempted. This review examines the current state of the iron biofortification in pulses, particularly chickpea. The challenges concerning biofortification in pulses are also discussed. Specifically, the potential application of transgenic technology is explored, with focus on the genes that have been successfully used in biofortification efforts in rice.

Enhanced Grain Iron Levels in Rice Expressing an IRON-REGULATED METAL TRANSPORTER, NICOTIANAMINE SYNTHASE, and FERRITIN Gene Cassette

Micronutrient malnutrition is widespread, especially in poor populations across the globe, and iron deficiency anemia is one of the most prevalent forms of micronutrient deficiencies. Iron deficiency anemia has severe consequences for human health, working ability, and quality of life. Several interventions including iron supplementation and food fortification have been attempted and met with varied degrees of success. Rice, which is a staple food for over half of the world's population, is an important target crop for iron biofortification. The genetic variability of iron content in the rice germplasm is very narrow, and thus, conventional breeding has not been successful in developing high iron rice varieties. Therefore, genetic engineering approaches have targeted at increasing iron uptake, translocation, and storage in the rice endosperm. We previously reported that AtIRT1, when expressed together with AtNAS1 and PvFERRITIN (PvFER) in high-iron (NFP) rice, has a synergistic effect of further increasing the iron concentration of polished rice grains. We have now engineered rice expressing AtIRT1, AtNAS1, and PvFER as a single locus gene cassette and compared the resulting lines with transgenic lines expressing AtIRT1 and PvFER gene cassettes. We also evaluated the efficacies of the MsENOD12B and native AtIRT1 promoters for the expression of AtIRT1 in rice in both types of gene cassettes, and found the native AtIRT1 promoter to be a better choice for driving the AtIRT1 expression in our biofortification strategy. All the single insertion transgenic lines have significant increases of iron concentration, both in polished and unpolished grains, but the concerted expression of AtIRT1, AtNAS1, and PvFER resulted to be a more effective strategy in achieving the highest iron increases of up to 10.46 μg/g dry weight. Furthermore, the transformed high iron lines grew better under iron deficiency growth conditions and also have significantly increased grain zinc concentration. Together, these rice lines have nutritionally relevant increases in polished grain iron and zinc concentration necessary to support human health.

Rice grain nutritional traits and their enhancement using relevant genes and QTLs through advanced approaches

BACKGROUND

Rice breeding program needs to focus on development of nutrient dense rice for value addition and helping in reducing malnutrition. Mineral and vitamin deficiency related problems are common in the majority of the population and more specific to developing countries as their staple food is rice.

RESULTS

Genes and QTLs are recently known for the nutritional quality of rice. By comprehensive literature survey and public domain database, we provided a critical review on nutritional aspects like grain protein and amino acid content, vitamins and minerals, glycemic index value, phenolic and flavonoid compounds, phytic acid, zinc and iron content along with QTLs linked to these traits. In addition, achievements through transgenic and advanced genomic approaches have been discussed. The information available on genes and/or QTLs involved in enhancement of micronutrient element and amino acids are summarized with graphical representation.

CONCLUSION

Compatible QTLs/genes may be combined together to design a desirable genotype with superior in multiple grain quality traits. The comprehensive review will be helpful to develop nutrient dense rice cultivars by integrating molecular markers and transgenic assisted breeding approaches with classical breeding.

Advances in breeding for high grain Zinc in Rice

Zinc (Zn) is one of the most essential micronutrients required for the growth and development of human beings. More than one billion people, particularly children and pregnant women suffer from Zn deficiency related health problems in Asia. Rice is the major staple food for Asians, but the presently grown popular high yielding rice varieties are poor supplier of Zn in their polished form. Breeding rice varieties with high grain Zn has been suggested to be a sustainable, targeted, food-based and cost effective approach in alleviating Zn deficiency. The physiological, genetic and molecular mechanisms of Zn homeostasis have been well studied, but these mechanisms need to be characterized from a biofortification perspective and should be well integrated with the breeding processes. There is a significant variation for grain Zn in rice germplasm and efforts are being directed at exploiting this variation through breeding to develop high Zn rice varieties. Several QTLs and gene specific markers have been identified for grain Zn and there is a great potential to use them in Marker-Assisted Breeding. A thorough characterization of genotype and environmental interactions is essential to identify key environmental factors influencing grain Zn. Agronomic biofortification has shown inconsistent results, but a combination of genetic and agronomic biofortification strategies may be more effective. Significant progress has been made in developing high Zn rice lines for release in target countries. A holistic breeding approach involving high Zn trait development, high Zn product development, product testing and release, including bioefficacy and bioavailability studies is essential for successful Zn biofortification.

The Combined Application of the Caco-2 Cell Bioassay Coupled with In Vivo (Gallus gallus) Feeding Trial Represents an Effective Approach to Predicting Fe Bioavailability in Humans

Research methods that predict Fe bioavailability for humans can be extremely useful in evaluating food fortification strategies, developing Fe-biofortified enhanced staple food crops and assessing the Fe bioavailability of meal plans that include such crops. In this review, research from four recent poultry (Gallus gallus) feeding trials coupled with in vitro analyses of Fe-biofortified crops will be compared to the parallel human efficacy studies which used the same varieties and harvests of the Fe-biofortified crops. Similar to the human studies, these trials were aimed to assess the potential effects of regular consumption of these enhanced staple crops on maintenance or improvement of iron status. The results demonstrate a strong agreement between the in vitro/in vivo screening approach and the parallel human studies. These observations therefore indicate that the in vitro/Caco-2 cell and Gallus gallus models can be integral tools to develop varieties of staple food crops and predict their effect on iron status in humans. The cost-effectiveness of this approach also means that it can be used to monitor the nutritional stability of the Fe-biofortified crop once a variety has released and integrated into the food system. These screening tools therefore represent a significant advancement to the field for crop development and can be applied to ensure the sustainability of the biofortification approach.

Iron biofortification in the 21st century: setting realistic targets, overcoming obstacles, and new strategies for healthy nutrition

Plant-based foods offer a wide range of nutrients that are essential for human and animal health. Among these nutrients, iron stands out as one of the most important micronutrients. Increasing the iron content in many staple and non-staple plant foods continues to be a goal of many scientists around the world. However, the success of such initiatives has sometimes fallen short of their expected targets. In this review we highlight the most recent and promising results that have contributed to increasing the iron content in different crops. We also discuss methods that to date have been used to reach iron biofortification goals and new strategies that we believe are most promising for crop biofortification in the future. Plant anatomical, physiological and metabolic hurdles still need to be tackled for making progress on further increasing currently reached levels of micronutrient improvements. New strategies need to take into account growing environmental challenges that may constrain biofortification efforts.

Genetically modified (GM) crops: milestones and new advances in crop improvement

New advances in crop genetic engineering can significantly pace up the development of genetically improved varieties with enhanced yield, nutrition and tolerance to biotic and abiotic stresses. Genetically modified (GM) crops can act as powerful complement to the crops produced by laborious and time consuming conventional breeding methods to meet the worldwide demand for quality foods. GM crops can help fight malnutrition due to enhanced yield, nutritional quality and increased resistance to various biotic and abiotic stresses. However, several biosafety issues and public concerns are associated with cultivation of GM crops developed by transgenesis, i.e., introduction of genes from distantly related organism. To meet these concerns, researchers have developed alternative concepts of cisgenesis and intragenesis which involve transformation of plants with genetic material derived from the species itself or from closely related species capable of sexual hybridization, respectively. Recombinase technology aimed at site-specific integration of transgene can help to overcome limitations of traditional genetic engineering methods based on random integration of multiple copy of transgene into plant genome leading to gene silencing and unpredictable expression pattern. Besides, recently developed technology of genome editing using engineered nucleases, permit the modification or mutation of genes of interest without involving foreign DNA, and as a result, plants developed with this technology might be considered as non-transgenic genetically altered plants. This would open the doors for the development and commercialization of transgenic plants with superior phenotypes even in countries where GM crops are poorly accepted. This review is an attempt to summarize various past achievements of GM technology in crop improvement, recent progress and new advances in the field to develop improved varieties aimed for better consumer acceptance.

Inventory of metal complexes circulating in plant fluids: a reliable method based on HPLC coupled with dual elemental and high-resolution molecular mass spectrometric detection

Description of metal species in plant fluids such as xylem, phloem or related saps remains a complex challenge usually addressed either by liquid chromatography-mass spectrometry, X-ray analysis or computational prediction. To date, none of these techniques has achieved a complete and true picture of metal-containing species in plant fluids, especially for the least concentrated complexes. Here, we present a generic analytical methodology for a large-scale (> 10 metals, > 50 metal complexes) detection, identification and semiquantitative determination of metal complexes in the xylem and embryo sac liquid of the green pea, Pisum sativum. The procedure is based on direct injection using hydrophilic interaction chromatography with dual detection by elemental (inductively coupled plasma mass spectrometry) and molecular (high-resolution electrospray mass spectrometry) mass spectrometric detection. Numerous and novel complexes of iron(II), iron(III), copper(II), zinc, manganese, cobalt(II), cobalt(III), magnesium, calcium, nickel and molybdenum(IV) with several ligands including nicotianamine, citrate, malate, histidine, glutamine, aspartic acid, asparagine, phenylalanine and others are observed in pea fluids and discussed. This methodology provides a large inventory of various types of metal complexes, which is a significant asset for future biochemical and genetic studies into metal transport/homeostasis.

Improving Rice Zinc Biofortification Success Rates Through Genetic and Crop Management Approaches in a Changing Environment

Though rice is the predominant source of energy and micronutrients for more than half of the world population, it does not provide enough zinc (Zn) to match human nutritional requirements. Moreover, climate change, particularly rising atmospheric carbon dioxide concentration, reduces the grain Zn concentration. Therefore, rice biofortification has been recognized as a key target to increase the grain Zn concentration to address global Zn malnutrition. Major bottlenecks for Zn biofortification in rice are identified as low Zn uptake, transport and loading into the grain; however, environmental and genetic contributions to grain Zn accumulation in rice have not been fully explored. In this review, we critically analyze the key genetic, physiological and environmental factors that determine Zn uptake, transport and utilization in rice. We also explore the genetic diversity of rice germplasm to develop new genetic tools for Zn biofortification. Lastly, we discuss the strategic use of Zn fertilizer for developing biofortified rice.

Engineering the Rhizosphere

All components of the rhizosphere can be engineered to promote plant health and growth, two features that strongly depend upon the interactions of living organisms with their environment. This review describes the progress in plant and microbial molecular genetics and ecology that has led to a wealth of potential applications. Recent efforts especially deal with the plant defense machinery that is instrumental in engineering plant resistance to biotic stresses. Another approach involves microbial population engineering rather than single strain engineering. More generally, the plants (and the associated microbes) are no longer seen as 'individual' but rather as a holobiont, in other words a unit of selection in evolution, a concept that holds great promise for future plant breeding programs.

NOD promoter-controlled AtIRT1 expression functions synergistically with NAS and FERRITIN genes to increase iron in rice grains

Rice is a staple food for over half of the world's population, but it contains only low amounts of bioavailable micronutrients for human nutrition. Consequently, micronutrient deficiency is a widespread health problem among people who depend primarily on rice as their staple food. Iron deficiency anemia is one of the most serious forms of malnutrition. Biofortification of rice grains for increased iron content is an effective strategy to reduce iron deficiency. Unlike other grass species, rice takes up iron as Fe(II) via the IRON REGULATED TRANSPORTER (IRT) in addition to Fe(III)-phytosiderophore chelates. We expressed Arabidopsis IRT1 (AtIRT1) under control of the Medicago sativa EARLY NODULIN 12B promoter in our previously developed high-iron NFP rice lines expressing NICOTIANAMINE SYNTHASE (AtNAS1) and FERRITIN. Transgenic rice lines expressing AtIRT1 alone had significant increases in iron and combined with NAS and FERRITIN increased iron to 9.6 µg/g DW in the polished grains that is 2.2-fold higher as compared to NFP lines. The grains of AtIRT1 lines also accumulated more copper and zinc but not manganese. Our results demonstrate that the concerted expression of AtIRT1, AtNAS1 and PvFERRITIN synergistically increases iron in both polished and unpolished rice grains. AtIRT1 is therefore a valuable transporter for iron biofortification programs when used in combination with other genes encoding iron transporters and/or storage proteins.

Biofortified indica rice attains iron and zinc nutrition dietary targets in the field

More than two billion people are micronutrient deficient. Polished grains of popular rice varieties have concentration of approximately 2 μg g(-1) iron (Fe) and 16 μg g(-1) zinc (Zn). The HarvestPlus breeding programs for biofortified rice target 13 μg g(-1) Fe and 28 μg g(-1) Zn to reach approximately 30% of the estimated average requirement (EAR). Reports on engineering Fe content in rice have shown an increase up to 18 μg g(-1) in glasshouse settings; in contrast, under field conditions, 4 μg g(-1) was the highest reported concentration. Here, we report on selected transgenic events, field evaluated in two countries, showing 15 μg g(-1) Fe and 45.7 μg g(-1) Zn in polished grain. Rigorous selection was applied to 1,689 IR64 transgenic events for insert cleanliness and, trait and agronomic performances. Event NASFer-274 containing rice nicotianamine synthase (OsNAS2) and soybean ferritin (SferH-1) genes showed a single locus insertion without a yield penalty or altered grain quality. Endosperm Fe and Zn enrichment was visualized by X-ray fluorescence imaging. The Caco-2 cell assay indicated that Fe is bioavailable. No harmful heavy metals were detected in the grain. The trait remained stable in different genotype backgrounds.

Vacuolar Iron Transporter BnMEB2 Is Involved in Enhancing Iron Tolerance of Brassica napus

Iron toxicity is a nutrient disorder that severely affects crop development and yield in some soil conditions. Vacuolar detoxification of metal stress is an important strategy for plants to survive and adapt to this adverse environment. Vacuolar iron transporter (VIT) members are involved in this process and play essential roles in iron storage and transport. In this study, we identified a rapeseed VIT gene BnMEB2 (BnaC07g30170D) homologs to Arabidopsis MEB2 (At5g24290). Transient expression analysis revealed that BnMEB2 was localized to the vacuolar membrane. Q-PCR detection showed a high expression of BnMEB2 in mature (60-day-old) leaves and could be obviously induced by exogenous iron stress in both roots and leaves. Over-expressed BnMEB2 in both Arabidopsis wild type and meb2 mutant seedlings resulted in greatly improved iron tolerability with no significant changes in the expression level of other VIT genes. The mutant meb2 grew slowly and its root hair elongation was inhibited under high iron concentration condition while BnMEB2 over-expressed transgenic plants of the mutant restored the phenotypes with apparently higher iron storage in roots and dramatically increased iron content in the whole plant. Taken together, these results suggested that BnMEB2 was a VIT gene in rapeseed which was necessary for safe storage and vacuole detoxification function of excess iron to enhance the tolerance of iron toxicity. This research sheds light on a potentially new strategy for attenuating hazardous metal stress from environment and improving iron biofortification in Brassicaceae crops.

Association of Increased Grain Iron and Zinc Concentrations with Agro-morphological Traits of Biofortified Rice

Biofortification of rice (Oryza sativa L.) with micronutrients is widely recognized as a sustainable strategy to alleviate human iron (Fe) and zinc (Zn) deficiencies in developing countries where rice is the staple food. Constitutive overexpression of the rice nicotianamine synthase (OsNAS) genes has been successfully implemented to increase Fe and Zn concentrations in unpolished and polished rice grain. Intensive research is now needed to couple this high-micronutrient trait with high grain yields. We investigated associations of increased grain Fe and Zn concentrations with agro-morphological traits of backcross twice second filial (BC₂F₂) transgenic progeny carrying OsNAS1 or OsNAS2 overexpression constructs under indica/japonica and japonica/japonica genetic backgrounds. Thirteen agro-morphological traits were evaluated in BC₂F₂ transgenic progeny grown under hydroponic conditions. Concentrations of eight mineral nutrients (Fe, Zn, copper, manganese, calcium, magnesium, potassium, and phosphorus) in roots, stems/sheaths, non-flag leaves, flag leaves, panicles, and grain were also determined. A distance-based linear model (DistLM) was utilized to extract plant tissue nutrient predictors accounting for the largest variation in agro-morphological traits differing between transgenic and non-transgenic progeny. Overall, the BC₂F₂ transgenic progeny contained up to 148% higher Fe and 336% higher Zn concentrations in unpolished grain compared to non-transgenic progeny. However, unpolished grain concentrations surpassing 23 μg Fe g^-1 and 40 μg Zn g^-1 in BC₂F₂indica/japonica progeny, and 36 μg Fe g^-1 and 56 μg Zn g¹ in BC₂F₂japonica/japonica progeny, were associated with significant reductions in grain yield. DistLM analyses identified grain-Zn and panicle-magnesium as the primary nutrient predictors associated with grain yield reductions in the indica/japonica and japonica/japonica background, respectively. We subsequently produced polished grain from high-yield BC₂F₂ transgenic progeny carrying either the OsNAS1 or OsNAS2 overexpression constructs. The OsNAS2 overexpressing progeny had higher percentages of Fe and Zn in polished rice grain compared to the OsNAS1 overexpressing progeny. Results from this study demonstrate that genetic background has a major effect on the development of Fe and Zn biofortified rice. Moreover, our study shows that high-yielding rice lines with Fe and Zn biofortified polished grain can be developed by OsNAS2 overexpression and monitoring for Zn overaccumulation in the grain.

Vacuolar Iron Transporter BnMEB2 Is Involved in Enhancing Iron Tolerance of Brassica napus

Iron toxicity is a nutrient disorder that severely affects crop development and yield in some soil conditions. Vacuolar detoxification of metal stress is an important strategy for plants to survive and adapt to this adverse environment. Vacuolar iron transporter (VIT) members are involved in this process and play essential roles in iron storage and transport. In this study, we identified a rapeseed VIT gene BnMEB2 (BnaC07g30170D) homologs to Arabidopsis MEB2 (At5g24290). Transient expression analysis revealed that BnMEB2 was localized to the vacuolar membrane. Q-PCR detection showed a high expression of BnMEB2 in mature (60-day-old) leaves and could be obviously induced by exogenous iron stress in both roots and leaves. Over-expressed BnMEB2 in both Arabidopsis wild type and meb2 mutant seedlings resulted in greatly improved iron tolerability with no significant changes in the expression level of other VIT genes. The mutant meb2 grew slowly and its root hair elongation was inhibited under high iron concentration condition while BnMEB2 over-expressed transgenic plants of the mutant restored the phenotypes with apparently higher iron storage in roots and dramatically increased iron content in the whole plant. Taken together, these results suggested that BnMEB2 was a VIT gene in rapeseed which was necessary for safe storage and vacuole detoxification function of excess iron to enhance the tolerance of iron toxicity. This research sheds light on a potentially new strategy for attenuating hazardous metal stress from environment and improving iron biofortification in Brassicaceae crops.

Review of functional markers for improving cooking, eating, and the nutritional qualities of rice

After yield, quality is one of the most important aspects of rice breeding. Preference for rice quality varies among cultures and regions; therefore, rice breeders have to tailor the quality according to the preferences of local consumers. Rice quality assessment requires routine chemical analysis procedures. The advancement of molecular marker technology has revolutionized the strategy in breeding programs. The availability of rice genome sequences and the use of forward and reverse genetics approaches facilitate gene discovery and the deciphering of gene functions. A well-characterized gene is the basis for the development of functional markers, which play an important role in plant genotyping and, in particular, marker-assisted breeding. In addition, functional markers offer advantages that counteract the limitations of random DNA markers. Some functional markers have been applied in marker-assisted breeding programs and have successfully improved rice quality to meet local consumers' preferences. Although functional markers offer a plethora of advantages over random genetic markers, the development and application of functional markers should be conducted with care. The decreasing cost of sequencing will enable more functional markers for rice quality improvement to be developed, and application of these markers in rice quality breeding programs is highly anticipated.

Genome-wide Association Mapping of Quantitative Trait Loci (QTLs) for Contents of Eight Elements in Brown Rice (Oryza sativa L.)

An association mapping of quantitative trait loci (QTLs) regulating the concentrations of eight elements in brown rice (Oryza sativa L.) was performed using USDA mini-core subset cultivated in two different environments. In addition, correlation between the grain elemental concentrations was also studied. A total of 60 marker loci associated with 8 grain elemental concentrations were identified, and these loci were clustered into 37 genomic regions. Twenty new QTLs were found to be associated with important elements such as Zn, Fe, and P, along with others. Fe concentration was associated with the greatest number of markers in two environments. In addition, several important elemental/metal transporter genes were identified in a few mapped regions. Positive correlation was observed within all grain elemental concentrations. In summary, the results provide insight into the genetic basis of rice grain element accumulation and may help in the identification of genes associated with the accumulation of Zn, Fe, and other essential elements in rice.

Bypassing Iron Storage in Endodermal Vacuoles Rescues the Iron Mobilization Defect in the natural resistance associated-macrophage protein3natural resistance associated-macrophage protein4 Double Mutant

To improve seed iron (Fe) content and bioavailability, it is crucial to decipher the mechanisms that control Fe storage during seed development. In Arabidopsis (Arabidopsis thaliana) seeds, most Fe is concentrated in insoluble precipitates, with phytate in the vacuoles of cells surrounding the vasculature of the embryo. NATURAL RESISTANCE ASSOCIATED-MACROPHAGE PROTEIN3 (AtNRAMP3) and AtNRAMP4 function redundantly in Fe retrieval from vacuoles during germination. When germinated under Fe-deficient conditions, development of the nramp3nramp4 double mutant is arrested as a consequence of impaired Fe mobilization. To identify novel genes involved in seed Fe homeostasis, we screened an ethyl methanesulfonate-mutagenized population of nramp3nramp4 seedlings for mutations suppressing their phenotypes on low Fe. Here, we report that, among the suppressors, two independent mutations in the VACUOLAR IRON TRANSPORTER1 (AtVIT1) gene caused the suppressor phenotype. The AtVIT1 transporter is involved in Fe influx into vacuoles of endodermal and bundle sheath cells. This result establishes a functional link between Fe loading in vacuoles by AtVIT1 and its remobilization by AtNRAMP3 and AtNRAMP4. Moreover, analysis of subcellular Fe localization indicates that simultaneous disruption of AtVIT1, AtNRAMP3, and AtNRAMP4 limits Fe accumulation in vacuolar globoids.

Got to hide your Zn away: Molecular control of Zn accumulation and biotechnological applications

Zinc (Zn) is an essential micronutrient for all organisms, with key catalytic and structural functions. Zn deficiency in plants, common in alkaline soils, results in growth arrest and sterility. On the other hand, Zn can become toxic at elevated concentrations. Several studies revealed molecules involved with metal acquisition in roots, distribution within the plant and translocation to seeds. Transmembrane Zn transport proteins and Zn chelators are involved in avoiding its toxic effects. Plant species with the capacity to hyperaccumulate and hypertolerate Zn have been characterized. Plants that accumulate and tolerate high amounts of Zn and produce abundant biomass may be useful for phytoremediation, allowing cleaning of metal-contaminated soils. The study of Zn hyperaccumulators may provide indications of genes and processes useful for biofortification, for developing crops with high amounts of nutrients in edible tissues. Future research needs to focus on functional characterization of Zn transporters in planta, elucidation of Zn uptake and sensing mechanisms, and on understanding the cross-talk between Zn homeostasis and other physiological processes. For this, new research should use multidisciplinary approaches, combining traditional and emerging techniques, such as genome-encoded metal sensors and multi-element imaging, quantification and speciation using synchrotron-based methods.

Contrasting effects of nicotianamine synthase knockdown on zinc and nickel tolerance and accumulation in the zinc/cadmium hyperaccumulator Arabidopsis halleri

Elevated nicotianamine synthesis in roots of Arabidopsis halleri has been established as a zinc (Zn) hyperaccumulation factor. The main objective of this study was to elucidate the mechanism of nicotianamine-dependent root-to-shoot translocation of metals. Metal tolerance and accumulation in wild-type (WT) and AhNAS2-RNA interference (RNAi) plants were analysed. Xylem exudates were subjected to speciation analysis and metabolite profiling. Suppression of root nicotianamine synthesis had no effect on Zn and cadmium (Cd) tolerance but rendered plants nickel (Ni)-hypersensitive. It also led to a reduction of Zn root-to-shoot translocation, yet had the opposite effect on Ni mobility, even though both metals form coordination complexes of similar stability with nicotianamine. Xylem Zn concentrations were positively, yet nonstoichiometrically, correlated with nicotianamine concentrations. Two fractions containing Zn coordination complexes were detected in WT xylem. One of them was strongly reduced in AhNAS2-suppressed plants and coeluted with (67) Zn-labelled organic acid complexes. Organic acid concentrations were not responsive to nicotianamine concentrations and sufficiently high to account for complexing the coordinated Zn. We propose a key role for nicotianamine in controlling the efficiency of Zn xylem loading and thereby the formation of Zn coordination complexes with organic acids, which are the main Zn ligands in the xylem but are not rate-limiting for Zn translocation.

Plant ferritin--a source of iron to prevent its deficiency

Iron deficiency anemia affects a significant part of the human population. Due to the unique properties of plant ferritin, food enrichment with ferritin iron seems to be a promising strategy to prevent this malnutrition problem. This protein captures huge amounts of iron ions inside the apoferritin shell and isolates them from the environment. Thus, this iron form does not induce oxidative change in food and reduces the risk of gastric problems in consumers. Bioavailability of ferritin in human and animal studies is high and the mechanism of absorption via endocytosis has been confirmed in cultured cells. Legume seeds are a traditional source of plant ferritin. However, even if the percentage of ferritin iron in these seeds is high, its concentration is not sufficient for food fortification. Thus, edible plants have been biofortified in iron for many years. Plants overexpressing ferritin may find applications in the development of bioactive food. A crucial achievement would be to develop technologies warranting stability of ferritin in food and the digestive tract.

Next biotech plants: new traits, crops, developers and technologies for addressing global challenges

Most of the genetically modified (GM) plants currently commercialized encompass a handful of crop species (soybean, corn, cotton and canola) with agronomic characters (traits) directed against some biotic stresses (pest resistance, herbicide tolerance or both) and created by multinational companies. The same crops with agronomic traits already on the market today will continue to be commercialized, but there will be also a wider range of species with combined traits. The timeframe anticipated for market release of the next biotech plants will not only depend on science progress in research and development (R&D) in laboratories and fields, but also primarily on how demanding regulatory requirements are in countries where marketing approvals are pending. Regulatory constraints, including environmental and health impact assessments, have increased significantly in the past decades, delaying approvals and increasing their costs. This has sometimes discouraged public research entities and small and medium size plant breeding companies from using biotechnology and given preference to other technologies, not as stringently regulated. Nevertheless, R&D programs are flourishing in developing countries, boosted by the necessity to meet the global challenges that are food security of a booming world population while mitigating climate change impacts. Biotechnology is an instrument at the service of these imperatives and a wide variety of plants are currently tested for their high yield despite biotic and abiotic stresses. Many plants with higher water or nitrogen use efficiency, tolerant to cold, salinity or water submergence are being developed. Food security is not only a question of quantity but also of quality of agricultural and food products, to be available and accessible for the ones who need it the most. Many biotech plants (especially staple food) are therefore being developed with nutritional traits, such as biofortification in vitamins and metals. The main international seed companies continue to be the largest investors in plant biotechnology R&D, and often collaborate in the developing world with public institutions, private entities and philanthropic organizations. These partnerships are particularly present in Africa. In developed countries, plant biotechnology is also used for non-food purposes, such as the pharmaceutical, biofuel, starch, paper and textile industries. For example, plants are modified to specifically produce molecules with therapeutic uses, or with an improved biomass conversion efficiency, or producing larger volumes of feedstocks for biofuels. Various plant breeding technologies are now used in the entire spectrum of plant biotechnology: transgenesis producing proteins or RNAi. Cisgenesis (transgenes isolated from a crossable donor plant) and intragenesis (transgenes originate from the same species or a crossable species), null segregants are also used. To date, the next generation precision gene editing tools are developed in basic research. They include: clustered regularly interspaced short palindromic repeats (CRISPR), oligonucleotide-directed mutagenesis (ODM), transcription activator-like effects nucleases (TALENs) and zinc-finger nuclease (ZFN).

Green biotechnology, nanotechnology and bio-fortification: perspectives on novel environment-friendly crop improvement strategies

Food insecurity and malnutrition are prominent issues for this century. As the world's population continues to increase, ensuring that the earth has enough food that is nutritious too will be a difficult task. Today one billion people of the world are undernourished and more than a third are malnourished. Moreover, the looming threat of climate change is exasperating the situation even further. At the same time, the total acreage of arable land that could support agricultural use is already near its limits, and may even decrease over the next few years due to salination and desertification patterns resulting from climate change. Clearly, changing the way we think about crop production must take place on multiple levels. New varieties of crops must be developed which can produce higher crop yields with less water and fewer agricultural inputs. Besides this, the crops themselves must have improved nutritional qualities or become biofortified in order to reduce the chances of 'hidden hunger' resulting from malnourishment. It is difficult to envision the optimum way to increase crop production using a single uniform strategy. Instead, a variety of approaches must be employed and tailored for any particular agricultural setting. New high-impact technologies such as green biotechnology, biofortification, and nanotechnology offer opportunities for boosting agricultural productivity and enhancing food quality and nutritional value with eco-friendly manner. These agricultural technologies currently under development will renovate our world to one that can comfortably address the new directions, our planet will take as a result of climate change.

The use of metabolomics in the study of metals in biological systems

Metabolomics and systems biology/toxicology can elucidate novel pathways and mechanistic networks of metals and metalloids in biological systems, as well as providing useful biomarkers of the metal status of organisms.

Enriching rice with Zn and Fe while minimizing Cd risk

Enriching iron (Fe) and zinc (Zn) content in rice grains, while minimizing cadmium (Cd) levels, is important for human health and nutrition. Natural genetic variation in rice grain Zn enables Zn-biofortification through conventional breeding, but limited natural Fe variation has led to a need for genetic modification approaches, including over-expressing genes responsible for Fe storage, chelators, and transporters. Generally, Cd uptake and allocation is associated with divalent metal cations (including Fe and Zn) transporters, but the details of this process are still unknown in rice. In addition to genetic variation, metal uptake is sometimes limited by its bioavailability in the soil. The availability of Fe, Zn, and Cd for plant uptake varies widely depending on soil redox potential. The typical practice of flooding rice increases Fe while decreasing Zn and Cd availability. On the other hand, moderate soil drying improves Zn uptake but also increases Cd and decreases Fe uptake. Use of Zn- or Fe-containing fertilizers complements breeding efforts by providing sufficient metals for plant uptake. In addition, the timing of nitrogen fertilization has also been shown to affect metal accumulation in grains. The purpose of this mini-review is to identify knowledge gaps and prioritize strategies for improving the nutritional value and safety of rice.

From the concept of totipotency to biofortified cereals

I was a college teacher when opportunity opened a path into academia. A fascination with totipotency channeled me into research on tissue culture. As I was more interested in contributions to food security than in scientific novelty, I turned my attention to the development of genetic modification technology for cereals. From my cell culture experience, I had reasons not to trust Agrobacterium for that purpose, and I developed direct gene transfer instead. In the early 1990s, I became aware of the problem of micronutrient deficiency, particularly vitamin A deficiency in rice-eating populations. Golden Rice, which contains increased amounts of provitamin A, was probably instrumental for the concept of biofortification to take off. I realized that this rice would remain an academic exercise if product development and product registration were not addressed, and this is what I focused on after my retirement. Although progress is slowly being made, had I known what this pursuit would entail, perhaps I would not have started. Hopefully Golden Rice will reach the needy during my lifetime.

Phosphate and zinc transport and signalling in plants: toward a better understanding of their homeostasis interaction

Inorganic phosphate (Pi) and zinc (Zn) are two essential nutrients for plant growth. In soils, these two minerals are either present in low amounts or are poorly available to plants. Consequently, worldwide agriculture has become dependent on external sources of Pi and Zn fertilizers to increase crop yields. However, this strategy is neither economically nor ecologically sustainable in the long term, particularly for Pi, which is a non-renewable resource. To date, research has emphasized the analysis of mineral nutrition considering each nutrient individually, and showed that Pi and Zn homeostasis is highly regulated in a complex process. Interestingly, numerous observations point to an unexpected interconnection between the homeostasis of the two nutrients. Nevertheless, despite their fundamental importance, the molecular bases and biological significance of these interactions remain largely unknown. Such interconnections can account for shortcomings of current agronomic models that typically focus on improving the assimilation of individual elements. Here, current knowledge on the regulation of the transport and signalling of Pi and Zn individually is reviewed, and then insights are provided on the recent progress made towards a better understanding of the Zn-Pi homeostasis interaction in plants.

Zn and Fe biofortification: the right chemical environment for human bioavailability

A considerable fraction of global disease burden and child mortality is attributed to Fe and Zn deficiencies. Biofortification, i.e. the development of plants with more bioavailable Zn and Fe, is widely seen as the most sustainable solution, provided suitable crops can be generated. In a cereal-dominated diet availability of Fe and Zn for absorption by the human gut is generally low and influenced by a highly complex chemistry. This complexity has mostly been attributed to the inhibitory effect of Fe and Zn binding by phytate, the principal phosphorus storage compound in cereal and legume seeds. However, phytate is only part of the answer to the multifaceted bioavailability question, albeit an important one. Recent analyses addressing elemental distribution and micronutrient speciation in seeds strongly suggest the existence of different Fe and Zn pools. Exploration of natural variation in maize showed partial separation of phytate levels and Fe bioavailability. Observations made with transgenic plants engineered for biofortification lend further support to this view. From a series of studies the metal chelator nicotianamine is emerging as a key molecule. Importantly, nicotianamine levels have been found to not only increase the loading of Fe and Zn into grains. Bioavailability assays indicate a strong activity of nicotianamine also as an enhancer of intestinal Fe and Zn absorption.

Expression patterns of QTL based and other candidate genes in Madhukar × Swarna RILs with contrasting levels of iron and zinc in unpolished rice grains

BACKGROUND

Identifying QTLs/genes for iron and zinc in rice grains can help in biofortification programs. Genome wide mapping showed 14 QTLs for iron and zinc concentration in unpolished rice grains of F7 RILs derived from Madhukar × Swarna. One line (HL) with high Fe and Zn and one line (LL) with low Fe and Zn in unpolished rice were compared with each other for gene expression using qPCR. 7 day old seedlings were grown in Fe+ and Fe- medium for 10 days and RNA extracted from roots and shoots to determine the response of 15 genes in Fe- conditions.

RESULTS

HL showed higher upregulation than LL in shoots but LL showed higher upregulation than HL in roots. YSL2 was upregulated only in HL roots and YSL15 only in HL shoots and both up to 60 fold under Fe- condition. IRT2 and DMAS1 were upregulated 100 fold and NAS2 1000 fold in HL shoot. NAS2, IRT1, IRT2 and DMAS1 were upregulated 40 to 100 fold in LL roots. OsZIP8, OsNAS3, OsYSL1 and OsNRAMP1 which underlie major Fe QTL showed clear allelic differences between HL and LL for markers flanking QTL. The presence of iron increasing QTL allele in HL was clearly correlated with high expression of the underlying gene. OsZIP8 and OsNAS3 which were within major QTL with increasing effect from Madhukar were 8 fold and 4 fold more expressed in HL shoot than in LL shoot. OsNAS1, OsNAS2, OsNAS3, OsYSL2 and OsYSL15 showed 1.5 to 2.5 fold upregulation in flag leaf of HL when compared with flag leaf of Swarna.

CONCLUSION

HL and LL differed in root length, Fe concentration and expression of several genes under Fe deficiency. The major distinguishing genes were NAS2, IRT2, DMAS1, and YSL15 in shoot and NAS2, IRT1, IRT2, YSL2, and ZIP8 in roots. The presence of iron increasing QTL allele in HL at marker locus close to genes also increased upregulation in HL.